MXene-driven nanoscale field-effect junction for advanced 4-terminal perovskite/silicon tandem solar panels

Domain: chemistry
Published: 2026-03-06
Editorial outcome: major_revision

Abstract
The commercialization of perovskite/silicon tandem solar cells hinges on achieving high efficiency and stability
while maintaining scalability. This study demonstrates an original approach for inducing the formation of a field
effect junction within the perovskite active layer for efficient semi-transparent top modules to be integrated in
four-terminal perovskite/silicon tandem panels. A synergy of MXene-based doping and surface gradient
passivation enabled semi-transparent perovskite modules with efficiencies surpassing 16% on 60 cm² active area.
These were integrated into a four terminal tandem panel (0.2 m2) with a power conversion efficiency of 19.45%,
further enhanced by bifacial silicon heterojunction cells to reach a power generation density exceeding 23 mWcm-
² under 30% ground albedo conditions. The tandem panel, installed in Crete, retained over 95% of its initial
delivered power after three months, showcasing robust real-world stability. This work provides a significant step
toward industrial adoption, presenting a scalable, high-efficiency solution for next-generation photovoltaics with
minimal modifications to silicon production lines.

Introduction
The need to shift from fossil fuels to renewable energy has inspired the scientific community to innovate
efficient and affordable photovoltaic (PV) technologies with low Levelized Cost of Electricity (LCOE). A promising
strategy involves integrating in tandem configuration emergiPng technologies such as perovskite solar cells (PSCs)
(recently showing impressive power conversion efficiency ( PCE) above 27%) with established solutions like silicon
(Si) PV, now close to its theoretical PCE limit of 29.56%.1,2 Small-area perovskite/Si cells have achieved efficiencies
above 35%,3 yet replicating this success in larger deIvices has been challenging. Thus, developing and analysing
large-area perovskite devices that align with standard silicon wafer dimensions is crucial for addressing
commercialization challenges.4 Indeed, numerous advancements in large-area tandem devices have been
announced but scientific report and publications are few, indicating a competitive landscape for scaling these
technologies.

Scaling up tandem architecture requires overcoming efficiency losses related to material processing, electrical
interconnection, and stability. Among various tandem configurations,5 four-terminal (4T) architectures offer
significant advantages, including independent maximum power point (MPP) tracking for the top and bottom cells,
minimal modifications to existing Si fabrication lines,6 and compatibility with bifacial Si cells, which can capture
reflected sunlight to enhance energy yield, although independent MPP trackers adds system-level cost.7,8
Moreover, developing high-efficiency, scalable, and stable 4T perovskite/Si tandem modules presents several
critical challenges. Firstly, a certain loss in PCE is inevitable when the typical opaque metal electrode used in top
PSCs is replaced by a semi-transparent top electrode. Among the best-efficient semi-transparent PSCs (ST-PSCs),
Yu and coworkers9 succeeded to fabricate ST-PSCs with PCE of 20.25% by introducing AZO/SnO as a sputtering
x
buffer layer in inverted configuration. Additionally, the absence of the metal contact acting as a back-reflector
reduces the light harvesting efficiency of ST-PSCs.10 Moreover, optimizing the perovskite composition for tandem
configurations often compromises light harvesting efficiency and current generation.11,12 As a second challenge,
the minimization of PCE losses when moving from small area ST-PSCs to semi-transparent perovskite solar
modules (ST-PSMs) remains a key target for the practical realization of efficient tandem devices. Encouragingly, a
25.8% PCE has been reported for a large-area 4-terminal (4T) silicon/perovskite tandem module (2054 cm²)
fabricated by LONGi Green Energy Technologies, where the top perovskite sub cells contribute 15.9% absolute to
the overall efficiency.13 Finally, addressing electrical connections, encapsulation, and lamination processes is
crucial, as these have only been demonstrated with opaque devices.14 Therefore, enhancing the PCE of ST-PSCs
and ensuring their dimension scaling are pivotal for advancing perovskite/Si technology.

Commonly in 4T tandem devices perovskite absorber, wide-bandgap (WBG)15 formulations are employed, but
the resulting PCE of the ST-PSCs is still hampered by a significant open-circuit voltage (V ) deficit. This issue is
OC
mainly caused by non-radiative recombination due to deep-level acceptor defects that can be strongly hampered
by the use of ammonium salts,16 such as phenylethyl ammonium iodide (PEAI) derivatives via anti-solvent additive
engineering.17,18 Among them, owing to its elevated electro-positivity, para-fluorinated PEAI (4-FPEAI)
demonstrated stronger interaction with critical defects (iodide-lead (I-Pb) and iodide-cation antisite defects)
present in WBG perovskites, resulting in V losses mitigation. Here, a surface gradient passivation (SGP) can be
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achieved when 4-FPEAI is introduced into the perovskite film during the anti-solvent step, resulting in a
concentration gradient decreasing from the surface toward the perovskite bulk.19 Moreover, 4-FPEAI confers a
gradual p-type character to the film by favouring the energy level alignment across the PSC structure.19 Indeed,
tailoring the perovskite work function (WF) has emerged as a key strategy, alongside the design of homojunctions
to modulate the absorber’s electronic structure. 20

However, creating p-n homojunctions generally requires the deposition of two successive perovskite layers,
increasing fabrication complexity. In our previous work, we demonstrated the use of two-dimensional (2D)
materials, in particular MXenes, to dope the perovskite precursor solution, obtaining its WF tunability in a broad
range without altering its optoelectronic properties.21

Here we propose a original approach to fabricate perovskite field-effect junctions arising from a modified
interfacial electronic environment with a local n-type character at the buried perovskite layer, induced by
chlorine-based MXene doping,21 and a surface-localized p-type shift prSomoted by 4-FPEAI based SGP. This
intentional spatial asymmetry induces dipole-driven band bending at the perovskite interfaces, enabling more
efficient carrier extraction and reduced recombination. A 2D perovskite overlayer further strengthens the p-type
character of 3D perovskite surface, thereby minimizing charge recombination at perovskite/hole transport layer
interface. Notably, this strategy is fully scalable, enabling the fabrication of 4T large-area (44x44 cm²)
perovskite/silicon tandem PV panels, incorporating bifacial Si technology and developing panel lamination
protocols compatible with silicon industry standards.

Finally, the performance of tandem panels have been evaluated under real-world conditions in Crete, as
laboratory results often differ from actual outdoor p I erformance,22 underscoring the importance of field testing.
The results demonstrated durability, with the Etandem panel retaining over 95% of its initial maximum power
output, primarily showing a slow fill factor (FF) degradation of 1.27% per month, expressed as a relative decrease
respect to the initial FF. Additionally, levCeraging the bifacial nature of the silicon heterojunction (Si-HJT) bottom
cell, the system efficiently harnessed ground-reflected light, further enhancing power generation density (PGD)
up to 23.6 mWcm-². These findings Thighlight the real-world viability of the proposed technology, bridging the gap
between laboratory efficiency and large-scale deployment in diverse environmental conditions.

Results
Small area cells
Our reference n-i-p perovskite top cell (Fig. 1a) employs an electron transporting layer (ETL) based on graphene
nanoflakes (G) to improve both charge transfer at the interface and cell stability.23,24 Additional strategies
implemented to further enhance the performance of the ST-PSCs include: i) tuning the perovskite energy band
gap; ii) replacing hazardous anti-solvent with environmentally friendly alternatives and iii) utilizing tailored 2D
materials to enhance the absorber’s optoelectronic properties. A detailed description of the fabrication process
for the four structures of tested PSCs (Fig. 1) is reported in the experimental section.

The proposed Cs FA Pb(I Br ) perovskite formula tion features an energy gap (E ) of 1.68 eV (see
0.18 0.82 0.8 0.2 3 g
Supplementary Information (S.I.) Supplementary Fig. 1) which is particular crucial for the performance of
perovskite/silicon tandem devices operating at temperature above T > 55°C.25 This consideration is especially
relevant in our case, given the installation condEitions on Crete Island. To reduce the environmental impact and
human health risk, ethyl acetate (EA) is seLlected as green anti-solvent replacing chlorobenzene (CB) for the
perovskite production.26,27 The fabricateCd cells (Supplementary Fig. 2a) incorporating graphene nanoflakes and
EA (G/EA) demonstrated comparable pIerformance achieving a +2% increase of PCE compared to those utilizing
CB (G/CB, as evidenced by Fig. 2 and Tab. 1). Furthermore, replacing CB with to EA, the perovskite post-deposition
annealing time was reduced by half (30 minutes), effectively halving the energy required for perovskite absorber
realization. Surface gradienAt passivation (SGP) strategy19 of the perovskite absorber was implemented by
dissolving an optimized amount of 2D perovskite precursor (4-FPEAI) within EA anti-solvent step during perovskite
deposition. SGP permits i) to passivate deep-level trap states in Br-enriched WBG perovskite films caused by faster
crystallization rate during perovskite solution process28 and ii) to reduce the energy-level mismatch between the
WBG perovskite layer and charge transporting layers (CTLs).19 The SGP-based small area (0.1 cm2) opaque devices
(G/4-FPEAI) showed superior PCE of +8% (+5.8%) with respect to G/CB (G/EA) cells (Fig. 2d), mainly due to
improved V (+1.6% and +2% in G/EA and G/CB, respectively, Fig. 2a.
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Finally, PCE up to 22.12% (+A9.6% versus G/4-FPEAI best cell, Fig. 1d was obtained by doping perovskite/4-PEAI
active layer with Ti C Cl MXenes (MX-Cl) by carefully tuning the MXene dopant amount (Supplementary Fig. 2b).
3 2 2
MX-Cl synthesis is reported in the experimental section while their morphological and chemical characterizations
are reported in S.I. section S.I. 3 (Supplementary Figs. 3- 6). The high performance of MX-Cl cells (G/MX/4-FPEAI)
can be related to high V (> 1.19 V), significant enhancement in short circuit current density (J ) about +5% and
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+8% compared with G/4-FPEAI or G/EA respectively and minimized hysteresis in the I-V characteristics
(Supplementary Figs. 7a-d) reflecting in a stabilization of the maximum power point (MPP) under prolonged
illumination (Supplementary Fig. 7e).

The MX-Cl role on perovskite films morphology was clarified by scanning electron microscopy (SEM). SGP has
minimal impact on domain size and surface morphology (Supplementary Fig. 8), whereas MX-Cl doping, resulted
in a noticeable increase in perovskite domains compared to the layer without MX-Cl addition (Figs. 3a and 3d).
The presence of small domains around the enlarged ones in the MX-Cl-doped film indicated that MXene flakes
locally influenced perovskite morphology (see Figs. 3b and 3e for the statistical analysis of domain’s dimensions).

This is confirmed by the cross-section SEM images acquired on complete devices stacks for G/MX/4-FPEAI (Fig.
3f) and G/4-FPEAI (Fig. 3c). As shoTwn by Liu et al.,29 the strong coordination between Cl atoms and Pb2+ ions
reduces metallic lead clusters (PRb0), which are known to induce deep defects and trap free charge carriers in the
perovskite films, leading to nAon-radiative recombination.

X-Ray Diffraction (XRD) analysis (Fig. 3g) reveals improved crystallinity in MX-Cl/4-FPEAI films, evidenced by
stronger diffraction peaks for the (110), (220), and (310) lattice planes of cubic perovskite structure, reflecting in
reduces lattice distortion, thereby lowering defect density and minimizing trap states. The introduction of MX-Cl
improves perovskite crystallinity and promotes preferred orientation growth (section S.I. 5, Supplementary Tab.
1) due to the interaction between the Cl terminations and the Pb2+ ions forming adducts act as heterogeneous
nucleation site for the perovskite film.29 Complementarily, TRPL analysis (Fig. 3h) shows a marked increase in
average carrier lifetime in MXene-modified films, pointing to efficient trap-state passivation, likely due to both
improved crystallinity and reduced interfacial defect density (see Supplementary Tab. 2 in the S.I. for full fitting
parameters).

The impact of both MX-Cl and 4-FPEAI on the trap assisted recombination and on perovskite crystallinity was
assessed by transient photovoltage (TPV), Photoluminescence (PL), V vs Light intensity, XRD and UV-Vis
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measurements as reported in S.I. (section S.I. 5). TPV measurements (Supplementary Fig. 9a) reveal that G/MX/4-
FPEAI significantly extends charge recombination lifetime versus G/EA and G/4-FPEAI. This indicates that, beyond
creating a p-doping gradient from ETL to HTL, SGP also provides residual passivation, enhancing J (Fig. 2b).
SC
Additionally, devices with MX-Cl exhibit higher External Quantum Efficiency (EQE) values across entire spectral
range versus 4-FPEAI devices with enhance EQE-calculated J (from 21.01 mA cm-² for 4-FPEAI to 22.3 mA cm-²
SC
for G/MX/4-FPEAI), reflecting enhanced charge dynamics (Supplementary Fig. 9b) and absorbance
(Supplementary Fig. 10a). Furthermore, MX-doping reduces defect state density by enlarging perovskite domains
and mitigating grain boundary-induced trap states. Steady-state PL measurements (Supplementary Fig. 10b)
confirm the impact of MX-Cl, showing a 75% increase in PL intensity and a slight red shift (from 763 nm to 758
nm) for MX-Cl/4-FPEAI films, assessing the reduced impact of the non-radiative recombination. Trap-assisted
recombination at mTiO /perovskite interface was also studied via V vs. incident light power (P ) measurements
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(Supplementary Fig. 9d) and dark-J-V analysis (Supplementary Fig. 10c) Adding MX-Cl in G/MX/4-FPEAI lowers the
V (P ) slope, reducing ETL/perovskite recombination at this interface, and boosting J ,30 while the decrease of
OC inc SC
both reverse leakage current and ideality factor in dark J-V curves is in line with the longer TRPL lifetimes and
higher PL intensity.

The reduction of trap states in MX-Cl/4-FPEAI-modified perovskites can be attributed to the strong interaction
between Cl terminations of MX-Cl and Pb²⁺ ions, which mitigates non-radiative recombination caused by
uncoordinated lead sites. This interaction, stronger than that one between carbonyl-groups and Pb2+ as in case of
Ti C T -based MXenes, makes its effect on crystallization and domain size here more pronounced, compared to
3 2 x
our previous work,21 yielding a more stable perovskite lattice.

From SEM images of Fig. 3f, clearly MX-Cl flakes are mainly distribSuted at bottom interface between
perovskite/mTiO
2
layer. The presence of MX-Cl at the ETL/perovskite iSnterface calls for a possible tuning of the
buried perovskite interface WF.21

Building on this, ultraviolet photoelectron spectroscopy (UPS), was employed to elucidate the progressive
modulation of the perovskite energy levels depending on the different treatments. The measurements were
carried out on ad-hoc fabricated samples (see section S.I. 6 in S.I.). Notably, the combination of MXene and 4-
FPEAI induces a graded shift in WF and valence band mNaximum (VBM) position across the series, consistent with
dipole-induced band bending localized at the peroIvskite interfaces. A full discussion and corresponding UPS
spectra and energy level diagram reconstructed from the UPS data are provided in section S.I. 6 in S.I.,
Supplementary Fig. 11, while the extracted values of WF and VBM are summarized in Tab. 2, clearly highlighting
the energetic alignment changes induced by each modification To exclude charging or chemical artefacts, XPS
analyses were performed, confirming that the observed WF and VBM shifts arise from genuine interfacial
electronic modifications. (see Supplementary Fig. 12 for detailed discussion).

To confirm this effect, Kelvin probe force microscopy (KPFM) measurements were carried out on the same
samples, allowing us to extract WF values (Supplementary Tab. 3) and to distinguish the contributions of MXene
doping and SGP (see Supplementary Fig. 13)

Density functional theory (DFT) simulations of the MX-Cl/perovskite/4-FPEAI system can elucidate the passivation
capability of 4-FPEAI and the impact of MX-Cl on the electronic characteristics of perovskite. As a trade-off
between accuracy and computability, single cation MAPbI perovskite was considered, as discussed in section S.I.
3
7 in S.I. along with computational details about DFT simulations (see Supplementary Fig. 14). The structure of MX-
Cl/perovskite/4-FPEAI (Fig. 4a), as obtained by structural optimization, shows that no covalent bonds are involved
at perovskite/MXenes interface. However, the interaction results in a reduction of the WF versus undoped
perovskite (-0.39 eV, Supplementary Tab. 4), as highlighted by the slope of the vacuum potential, indicated with
red arrow in the figure. The WF reduction results from band bending induced by the surface dipole not from a
shift of the Fermi level, here assumed to be in the middle of the perovskite gap. Moreover, the projected density
of states (PDOS) analysis underlines perovskite E remains mostly unaffected, and MX-Cl additive introduces
g
donor states near perovskite conduction band edge (Supplementary Fig. 15), confirming the locally induced
perovskite n-character at the MX-Cl/perovskite interface. Notably, same qualitative outcomes result for MAPbBr
3
interacting with MX-Cl (Supplementary Tab. 4, Supplementary Fig. 16). Conversely, the interaction of perovskite
with 4-FPEAI results in the formation of covalent bonds between iodine atoms of the additive molecules and lead
atoms on perovskite surface, while the intercalation of 4-FPEA acts as a cation for the perovskite inorganic cage.
The WF shift caused by 4-FPEAI (0.14 eV, Supplementary Tab. 4) is opposite to that of the MX-Cl additive, as shown
by the opposite slopes of the vacuum level and red arrow directions. Finally, the PDOS analysis of perovskite/4-
FPEAI interface (Supplementary Fig. 15) shows that the passivation of the states contributed by the Pb ions on
the perovskite surface results in an up-shift of the conduction band edge of perovskite, giving a p-type behaviour
of the doped perovskite with respect to the pristine material.

Although enlarging the domain size leads to a slight increase in UV–Vis absorption (Supplementary Fig. 10a), it
only partially explains the J enhancement observed in G/MX/4-FPEAI devices. Indeed, the dipole induced by the
SC
MX-Cl addition at the mTiO /perovskite interface improves charge-transfer efficiency and consequently J . Thus,
2 SC
the reduction of J-V curve hysteresis (Supplementary Fig. 7) through MX-Cl engineering is mainly attributed to the
decrease of charge accumulation at the interface.

We made use of physical device simulations to account for the WF shift, grading and charge recombination
changes solar induced by MXenes and 4-FPEAI in the cell electro-optical properties (Section S.I. 8 in S.I.,
Supplementary Tab. 5). Figure 4b shows the simulated J-V curves for the three PSC typologies: reference, 4-FPEAI
and 4-FPEAI+MX-Cl. The reference curve nicely reproduces the experimental J and V , (insets Fig. 4b). In
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agreement with measurements, the 4-FPEAI additive boosts V to 1.19 eV, and MX-Cl further increases it to 1.20
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eV due to dipole-induced electron accIumulation at the buried interface and reduced recombination. This also
improves FF, evidenced in Fig. 4b from blue J-V characteristic versus the green and black ones. The field-effect
junctions arising from spatially separated interfacial dipoles improve V , especially with 4-FPEAI additive,
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enhancing hole extraction, as demonstrated by calculated hole density at the p-side perovskite interface
(Supplementary Fig. 17).

Four-terminal tandem panels
The ST-PSCs for 4T tandem were obtained replacing the opaque top-electrode with an ultra-thin gold layer and a
sputtered indium thin-oxide (ITO, Supplementary Fig. 18). By employing the same 2D material-engineering
strategy, MXene doping and 4-FPEAI-based surface gradient passivation, previously validated in opaque devices,
the ST-PSCs used in the 4T tandem architecture demonstrated enhanced PCEs up to 18.3% (av. PCE = 17.63%) due
to a higher V (av. V = 1.2 V) and J (av. J = 19.92 mA cm-²) compared to the REF sample (see Supplementary
OC OC SC SC
Tab. 6, Supplementary Figs. 19–22a, 20 and 21 for a detailed analysis), confirming the effectiveness of 2D material-
engineering approach even for ST structures.31,32 Moreover, the combination among the MX-Cl addition and the
4-FPEAI SGP strategy not only translated into superior device performance but also conferred superior robustness
on the perovskite film against light-driven degradation, as demonstrated by prolonged light cycling test
(Supplementary Fig. 22c) and combined heating + light soaking test (Supplementary Fig. 22d).

ST-PSCs were scaled up to module and to 4T PSK/Si panel detailed as follows. The module layout (Fig. 5a) consists
of 24 series connected solar cells on a 9.5x9.5 cm2 substrate area (cell width of 3.1 mm). Laser ablation parameters
(P1-P2-P3 processes) have been optimized for maximizing the module active area by ensuring a geometrical
FF>96% (Supplementary Figs. 23-24).33,34

Four parallel-connected ST-PSMs covering oneE M2 Si-HJT bottom cell were employed to realize a first PSK/Si
tandem panel demonstrator (DEM1) (Fig. L5 and Supplementary Fig. 25). The total active area of the four
perovskite top modules was matched to C the Si cell active area (240 cm2). The four ST-PSMs and the silicon cell
were laminated together by employingI an industrial laminator (S.I., section S.I. 12, Supplementary Fig. 26).
Fig. 6 shows the picture of the fabricated 4T PSK/Silicon tandem together with the I-V characteristics for the best
ST-PSM, obtained with optimized 2D material strategy, as well as the filtered Si-HJT single cell and the perovskite
top ST panel. This confirms the scalability and effectiveness of the approach at module level (section S.I. 11 in
S.I.).

The best efficient 2D material-engineered ST-PSM reached a PCE>16%, surpassing the state of art for ST-PSCs
used in record 4T large-area tandem cells,13 while strongly limiting the PCE drop (<13%) usually observed in scaling
the device size (see S.I. section S.I. 11 and Supplementary Tab. 7 for detailed discussion). Meanwhile, the 4T-
DEM1 device reached a PCE of approximately 21%, underscoring the effectiveness of both the 2D materials and
the tailored lamination procedure. To demonstrate the high reproducibility of the developed ST-PSM fabrication
protocol, we produced 150 2D material-engineered ST-PSMs, with their PCE statistics shown in Fig. 7a. Ultimately,
incorporating 2D materials yielded high-performing ST-PSMs, reduced performance variability, and enabled
reproducible production of large area PSK/Si tandem panels, thereby advancing industrially oriented production
of the perovskite/Si tandem technology.35

As a final demonstrator, a 4T tandem panel (DEM2, Figs. 7b-c) was fabricated by connecting in parallel 16 ST-
PSMs, while 4 bifacial Si-HJT cells from 3SUN company were series connected. Both were laminated in “one-step”
process, creating a single tandem object electrically accessible via the 4 external terminals (S.I. section S.I. 12).
The tandem panel (DEM2) were measured outdoor in Rome (41.85371; 12.63508) in a clear sky day (06/08/2023,
Supplementary Fig. 27) and the IV-characteristics are reported in Figs. 7d-e for perovskite top panel and Si-HJT
bottom module, respectively. The top semi-transparent panel (ST-PSP) achieved a PCE ~ 12.5%, with negligible
relative PCE reduction (-4.5%) compared to PCE of ST-PSP for DEM1 (+13.1%), despite quadrupling the number of
parallel connected ST-PSMs. Thus, the lamination approach developed in this work (S.I., section S.I. 13) minimally
impacts the perovskite panel (PSP) performance once the dimensions of the panel are scaled up. The filtered Si-
HJT bottom panel measured at 1 SUN approached 7%, contributing to a final PCE of DEM2 reaching 19.45%. We
should point out that, due to the manual stringing (electrical connection among adjacent cells), the bottom Si-HJT
module has a reduced efficiency compared to those obtained on an automatized industrial line (S.I., section S.I.
13). DEM2 was finally installed in Crete Island and its performance were monitored in outdoor conditions by
following the ISOS-O-2 protocol.

The tandem panel's electrical chRaracteristics were studied under varying irradiance and temperature (S.I., section
S.I. 14). Outdoor measuremAents performed using a metallic mounting base were carried out with the module
installed at a fixed inclination of 30° and a height of approximately 100 cm above ground (Fig. 7g). From the
outdoor performance monitoring at open circuit conditions, the I-V curves for top ST-PSP were acquired during a
3-months period (from August to November 2023) and the main electrical parameters were normalized in STC
(1000 W m-2 irradiance and 25oC temperature) considering both the temperature and irradiance dependency of
the main electrical parameters (S.I., section S.I. 14). Notably, the ST-PSP constituting DEM2 retained over 95% of
its initial P value (Fig. 7f) during outdoor exposure, while mainly its FF was showing a slow decreasing trend
MPP
over time (-1.27%/month). No evident performance losses were observed for the Si-HJT bottom module. Despite
degradation was not the focus of this study, the gradual decline in performance for ST-PSP can be ascribed to the
well-known intrinsic degradation mechanisms affecting ST-PSM,36 since no evident lamination failure of the
tandem panel was detected upon dismounting.

The bifaciality of the Si-HTJ bottom panel allowed us to estimate the final PGD of the 4T tandem
perovskite/bifacial Si-HJT under varying reflective irradiance (albedo) from different ground materials (S.I., section
S.I. 14). To date, the natural albedo of soil in solar installations ranges from 10% to 30%, the PGD for DEM2 was
estimated to range from 21.15 mW cm-2 to 23.66 mW cm-2 (Tab.3).

This study explores the optimization and scalability of 4T perovskite/silicon tandem devices. A field-effect junction
within the perovskite layer by combining the SGP strategy with Cl-based MXene doping yielded to ST 60 cm2 active
area modules with top PCE>16% following integrated into a 0.2 m² 4T perovskite/silicon tandem panel. The as-
developed all-in-one lamination process is compatible with the existing Si-HJT production lines, marking a
significant step toward commercialization. Moreover, outdoor panel performance at STC (PCE of 19.45%) and in
presence of 30% ground albedo conditions (PGD>23 mW cm-2), showed remarkable stability during three
operative months.

Crucially, this work introduces a perovskite field-effect junctions, enabled by the spatially resolved integration of
Cl-terminated MXenes and 4-FPEAI, representing an advanced approach to interface and energy landscape
engineering in perovskite photovoltaics.

Future efforts will focus on device cost reduction (i.e. Au/ITO transparent electrode replacement with
graphene37), refining production methods to enhance scalability and economic viability. Optimizations like 2T
voltage-matched architecture could further reduce costs and improve spectral resilience. Post-operation
diagnostic analysis of field-deployed modules could offer valuable insight into the intrinsic degradation pathways
and stability limitations of the proposed device architecture. Overall, this study demonstrates significant progress
toward efficient, scalable, and commercially viable tandem solar solutions, underscoring the transformative
potential of integrating 2D materials into photovoltaic technology.

Methods
Materials
All the materials were used as received, unless specified otherwise. All the following materials, including
titanium(IV) isopropoxide (TTIP), diisopropoxytitanium bis(acetylacetonate) (Ti(AcAc) ), acetyl acetone (AcAc),
2
ethyl acetate (EA), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), ethanol (EtOH), isopropanol (IPA),
acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile (ACN), tert-butylpyridine (tBP),
chlorobenzene (CB), toluene (T), N-methyl-2–pyrrolidone (NMP), 3-mercaptopropionic acid (MPA) and graphite
flakes were purchased from Sigma-Aldrich. Mesoporous transparent titania paste (30 NR-D), formamidinium
iodide (FAI) and methylammonium bromide (MABr) were purchased from GreatCell Solar. Lead(II) iodide (PbI ),
2
lead(II) bromide (PbBr ) were purchased from TCI, while caesium iodide (CsI) from GmbH. Poly (triarylamine)
2
(PTAA) Medium Mw: 20 – 75 kDa (SOL2426M) from Solaris Chem. The graphene flakes ink in EtOH (≥99.8%)
(concentration of 0.9 mg mL-1) was prepared by liquid phase exfoliation (LPE) of graphite flakes (Sigma–Aldrich)
in NMP and exchanged into EtOH (see our previous paper for further details)38

Ti C Cl MXene synthesis: Ti C Cl MXenes were synthesized by etching of Al layers from Ti AlC MAX phase
3 2 2 3 2 2 3 2
precursor using Lewis acid. In detail, Ti AlC MAX phase powder was mixed with anhydrous salts of ZnCl , NaCl,
3 2 2
and KCl in a molar ratio of 1 : 3 : 2 : 2 under argon atmosphere. The mixture was placed in an alumina crucible
covered with a lid and heated to 650oC for 5 h. After synthesis, the mixture was washed with deionized (DI) water
to dissolve and remove the residual salts, and then with HCl to remove elemental Zn formed by the reduction of
Zn2+. Delamination of multilayer Ti C Cl MXenes was carried out under Ar atmosphere by intercalation of LiCl salt
3 2 2
dissolved in anhydrous polar organic solvents as described in detail in.39 The delaminated suspension was
collected as a film by vacuum filtration followed by drying at 80oC under vacuum.

Ti C Cl MXene structural analysis: The MAX phase precursor and resulted delaminated film were studied by XRD
3 2 2
using a D8 Bruker diffractometer (Bruker, Massachusetts, USA) operating under the Bragg-Brentano geometry
and equipped with Cu anode providing X-rays with a wavelength of 1.5406 Å. The morphology and chemical
composition of the synthesized Ti C Cl multilayer powder was investigated using a field-emission scanning
3 2 2
electron microscopy (SEM) (ZEISS Gemini 300, Carl Zeiss AG, Oberkochen, Germany) equipped with a Si(Li)
detector (Oxford Instruments) for (Energy Dispersive Spectroscopy) EDS. The combined
photoemission/microscopy Omicron setup ESCA-STM was used to carry out XPS and UPS of Ti C Cl MXenes.
3 2 2
For the measurements, the delaminated suspension of Ti C Cl was drop-casted on ITO substrates. XPS was
3 2 2
performed using monochromatic Al-Kα radiation (1486.6 eV) and deconvolution of the high-resolution (HR) XPS
spectra was performed in CasaXPS software. UPS measurements were performed using a He I source (21.22 eV).
During the measurements, a bias of 9 V was applied between the sample and the analyzer.

Electro-optical characterizations: SEM, UV-Vis, PL, XRD, IPCE, TransEient measurements, I-V characteristics, XPS,
UPS, stability tests.
SEM images were performed by using a TESCAN MIRA equ ipment. SEM images are acquired through an in-beam
secondary electrons (in beam-SE) detector by using a SNchottky field electron emitter providing an electron beam
with an energy of 5Kev and a probe current of 100pAI.

The UV-Vis absorption spectra of the perovskite layer were recorded using UV-Vis 2550 spectrophotometer from
Shimadzu. The spectra were collected using a scan rate of 480 nm min-1 and a resolution of 1 nm. Steady state PL
measurements were performed with a commercial apparatus (Arkeo – Cicci Research s.r.l.) composed of a 0.3 m
focal length spectrograph with a photon counting unit. The substrates were excited by a green (532 nm) laser at
45° of incidence with a circular spTot diameter of 1 mm. The optical coupling system is composed of a lens
condenser and a long pass filter.R The PL spectra were elaborated by an in-house developed Matlab script. In detail,
perovskite emission spectra Awere fitted by employing the best fitting between single, double, or triple exponential
Gaussian line-shape by minimizing the RMSE (root mean square error).

XRD measurements were collected with a Rigaku SmartLab SE 1D Type diffractometer working in Bragg−Brentano
geometry equipped with a Cu Kα source and a D/teX Ultra250 detector. Current-Voltage (I-V) characteristics of
masked and encapsulated devices were acquired in air by using a solar simulator (ABET Sun 2000, class A)
calibrated at AM1.5 and 100 mW cm-2 illumination with a certified reference Si Cell (RERA Solutions RR-1002).
Incident power was measured with a Skye SKS 1110 sensor. The class was measured with a BLACK-Comet UV–vis
spectrometer. Both reverse and forward I-V scans were performed by using a scan rate of 20 mV s-1 and 100 mVs-
1 for masked small-area PSCs and unmasked large-area ST-PSMs, respectively.

Combined heating+light soaking stress test was performed using a class-B solar simulator (SolarConstant 1200,
K.H. Steuernagel Lichttechnik GmbH, Germany). The system employs a xenon arc lamp coupled with an optical
AM 1.5G filter to reproduce the standard solar spectrum over a broad spectral range (300–2500 nm). According
to manufacturer specifications and IEC 60904-9 classification, the simulator exhibits class-B performance in
spectral match, irradiance uniformity, and temporal stability. In particular, the spectral distribution in the near-
infrared region shows the characteristic enhancement associated with xenon-based sources, consistent with the
class-B spectral mismatch factor. The output irradiance is stabilized and set to 1000 W m⁻² at the sample plane
under ambient laboratory conditions, and the lamp intensity is calibrated before each measurement session using
a certified silicon reference cell.

The relatively broad IR output, combined with continuous illumination, results in a device operating temperature
representative of realistic stress conditions, while remaining fully compliant with the IEC tolerances for
photovoltaic testing.

Illumination intensity dependence of V and dark I-V measurements were performed with a modular testing
OC
platform (Arkeo – Cicci Research s.r.l.) composed by a white LED array (4200 K) tunable up to 200 mW cm-2 of
optical power density and a high-speed source meter unit (600,00 samples s-1) in a four-wire configuration. A
spring contact-based sample holder was used to improve the repeatability of the experiments. TPV measurements
were performed in a high perturbation configuration by acquiring the entire V rise profile after switching the
OC
light intensity from 0 to 1 Sun. Incident Photon to current Conversion Efficiency (IPCE) spectra acquisitions were
carried out by means of an Arkeo system (Cicci Research s.r.l.) with a 150 W xenon lamp and a double grating
(300 to1400 nm). A Si photodiode was used for incident light calibration prior to the IPCE measurement.

Photoelectron spectroscopy measurements were performed in a ThermoScientific ESCALAB QXi XPS apparatus in
ultra-high vacuum conditions (10-10 mbar base pressure). The valence band Sregion of the samples was investigated
by UPS using the 21.2 eV radiation of the HeI line and biasing the sampSles at -5 V, in order to determine both the
work function and the position of the valence band edge.40 The electronic band structure of the perovskites was
characterized by X-ray photoemission spectroscopy (XPS) using monochromatic 1486.8 eV photons of the Al Kα
line and analysed with the Thermo-Fischer Avantage Software. XPS measurements were performed on small
devices (14x14 mm) applying the system charge compensation, while the work function was measured without
charge compensation, by providing an additional contaNct to an edge of the device front surface.

Device fabrication
A preliminary patterning step on the glass/FLTO substrate (Pilkington, 8 Ω/□, 110 × 110 mm²) is carried out using
a raster-scanning Nd:YVO₄ laser (pulsed a C t 80 kHz, average fluence 700 mJ cm-2, λ = 350 nm) to electrically isolate
adjacent cells on the same substrate. InI the case of modules, this step (P1 process) is used to isolate the individual
cells that compose the final device. For both cells and modules, after a deep cleaning sequence, consisting in a
triple step of ultra sonic bath with cleaning liquid dissolved in deionized water, acetone and 2-propanol for 10
min each one, a patterned f A luorine-doped tin oxide (FTO) coated glasses 2.5x2.5 cm2 (9.5 x 9.5 cm2 for modules)
are spray-pyrolised with a solution of acetylacetone (1 mL), titanium diisopropoxide (1.5 mL) and ethanol (22.5
mL) at 460 °C.

The subsequent step of small area (and large area) device fabrication consists in a thin mesoporous TiO (mTiO )
2 2
film (~130 nm) deposited by spin coating. The TiO paste (Dyesol 30 NRD paste diluted in ethanol 1:6 in wt.) was
2
spun at 3000 rpm for 20 s (2000 rpm for 20 s) and followed by the subsequent sintering at 480 ◦C for 30 min in
air. The graphene doped solutions, cTiO +G and mTiO +G, are obtained by doping with 1 vol% of graphene ink
2 2
(from IIT) the pristine solution. Then the substrates were transferred into a N glove box.
2

The perovskite films were deposited combining a one-step deposition with anti-solvent method. The perovskite
layer was deposited in a N -filled glove box after cooling down the samples to room temperature. The perovskite
2
solution was obtained by mixing FAI, PbI , PbBr and CsI in a mixture of anhydrous DMF/DMSO (4:1 vol/vol) in a
2 2
proper ratio to obtain Cs FA Pb(I Br ) . After 30 min of stirring at room temperature, the perovskite (100 μl
0.18 0.82 0.8 0.2 3
for small area devices, 1 ml for large area PSMs) was spin coated onto the samples, using an anti-solvent method.
When using CB as anti-solvent, a two-step program at 1,000 r.p.m. for 10 s and 5,000 r.p.m. for 30 s was used to
deposit the perovskite precursor solution.

During the second step, 150 μl of CB was poured on the spinning substrate 7 s before the end of the program.
Immediately after spin coating, the substrates were annealed at 100 °C for 1 h to form a perovskite crystal
structure resulting in a compact perovskite layer with a thickness of 450 nm.

Alternatively, in the case of EA anti-solvent a two-step program at 1,000 r.p.m. for 10 s and 4,000 r.p.m. for 45 s
(same ramp used in the case of ST-PSMs) was used for the deposition of the perovskite precursor solution. During
the second step, 250 μl of EA (2.5 ml in the case of ST-PSMs) was poured on the spinning substrate 17 s before
the end of the program. The deposited perovskite film was annealed at 100°C for 30 min, in the case of both small
area cells and modules. It is important to note that, given its lower boiling point and optimal polarity, ethyl acetate
promotes faster crystallization and more efficient solvent removal during film formation, which enables shorter
annealing times compared to chlorobenzene under identical thermal conditions.41

For device structure G/4-FPEAI and G/MX/4-FPEAI the 4-FPEAI was dissolved in the EA anti-solvent in a
concentration of 0.13 mg ml−1 and shaken prior the use. Once 3D perovskite-absorber was ready, a 3.73 mg ml-1
solution of 2-phenylethylammonium iodide (PEAI) in 2-propanol was prepared in glovebox and deposited by spin-
coater atop the perovskite-absorber at 5000 rpm for 30 s (4000 rpm for 30 s for modules). For the incorporation
of chlorine-terminated MXenes (MX-Cl) into the perovskite layer, a stock solution of MX-Cl is first prepared by
dispersing the desired amount of material in anhydrous acetonitrile (ACN) uSsing 1 hour of sonication, followed by
vigorous stirring to ensure a well-dispersed suspension. This solution is Sthen added to the perovskite precursor to
achieve a final MX-Cl concentration of 0.016 mg mL-1. The resulting mixture is deposited onto the substrate using
the same spin-coating and annealing parameters as the pristine perovskite formulation. To complete the 2D
perovskite layer formation, the samples were annealed at 100 °C for 10 minutes. Subsequently, the PE was
covered by a PTAA (10 mg ml−1) solution in toluene, doped with tBP 7 μl ml−1 and Li-TFSI salt (170 mg ml−1 in ACN)
10 μl ml−1 and spin coated at 3700 r.p.m. for 30 s (same spin parameters for modules).

At this stage, in the case of ST-PSMs, a second laser pIrocess (P2) was carried out to clean the FTO interconnection
areas.42 Finally, a high vacuum chamber (10−6 m bar) was used to thermally evaporate the 150 nm Au back
electrode in case of opaque device or alternatively 3 nm gold buffer layer in case of semi-transparent (ST) devices.
In this latter case, about 100 nm of indium tin oxide (ITO) was deposited onto PTAA layers in a linear magnetron
radio frequency (RF) sputtering system (Kenosistek).

Lastly, in case of ST-PSMs, adjacent cell isolation from the counter-electrode (CE) side was achieved by a third
laser ablation (P3 process). The detailed optimization of the P1, P2, P3 laser step together with the description
and the optimization of ST-PSM layout are reported in S.I. section S.I. 10.

Regarding the bottom module, commercial c-Si HJT solar cells were provided by 3Sun company embedding the
silver metal grids on both sides. The cell was produced in standard industrial conditions and used without further
modifications.

Tandem Panel fabrication
An ultra-clear 4 mm thick tempered glass was used to give robustness to full panel keeping a good transmission
rate. Atop the primary glass, a 100 µm ionomer foil as primary sealer was posed followed by the positioning of
the ST-PSMs one next to other (4 column x 4 lines in DEM2). The parallel electrical connection among the ST-PSMs
composing the perovskite top panel were realized employing charge collector tapes from 3M + tabbing ribbons
(see S.I. section S.I. 12-a for further details). As following step, a 400 µm ionomer foil as insulator was positioned
atop the backside (ITO side) of the ST-PSMs as an electrical insulator avoiding electrical shunt formation between
the perovskite top panel and the silicon bottom module once laminated together. Then, the Si-HJT cells are
positioned over the isomer foil and electrically connected by using 6 mm silver charge collector tapes.
Subsequently, a 300 µm ionomer foil as primary sealer was deposited atop the device stack while an edge sealer
was placed all around the ST-PSM area. (see S.I. section S.I. 12-b for further details). Finally, a 150 um polymeric
transparent foil was posed atop the tandem device stack as a back sheet (from 3M, WVTR < 6*10-5 g/m2/day, 90%
transparency in the 400–1400 nm range) prior lamination procedure carried out by an industrial hot vacuum
laminator (see SI section S.I. 12-c for further details).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript reports progress in MXene-driven nanoscale graded homojunctions for perovskite top cell in 4-terminal perovskite/silicon tandem solar panels.\nThe manuscript is technically well written and sound. However, I have one major and several minor remarks that need to be considered before I can recommend the manuscript for publication.\nMAJOR remark:\n1) l. 303: \"monitored in outdoor conditions\" must be specified in detail. Were top cells and bottom cells in 4-T tandems in MPP or open-circuit or short-circuit? You need to show the behaviour for the 4 months in Suppelement!\nHow did determine data in Fig. 7f? Did you bring tandems indoor and measure them under STC? Fig. 7f shows only period of 3-months (and not 4 months!).\nIn line 318 you state FF degradation -1.05%/month, which in 4 months end in 4,20% which means a 8% degradation from initial FF value of ~51% (estimated from Fig. 7f, since there is no table of exact values – add a table in Supplement!) In Fig. 7f also Isc degradation is seen, so the total degradation of 7% (over 93% retained after 4 months) is undermined and UNDER QUESTION.\nNo metastability behaviour of perovskite cells on day-night basis is nor mentioned not discussed, but data should be recorded and such analysis would clarify the stability of the MXene-driven nanoscale graded homojunction perovskite solar cells.\nMinor remarks:\n1) l. 36: \"under albedo conditions\" needs to be more specific, e.g. \"under 30% ground albedo conditions\".\n2) l. 36: \"retained over 93%\" should be corrected according to l. 96 and l. 318 (see below remark l.318)\n3) l.53 \"significant advantages, including indpendent MPPT\" brings also additional cost that must be added to this too optimistic statement\n3) l. 54 I miss reference to 3-terminal architecture of perovskite/silicon tandem cells that also exhibits flexibility to bifaciality\n4) l. 57: \"typical opaque electrode\" - not clear for which cell (top PK or bottom Si)?\n5) l. 63: \"the scaling-up PCE losses\" is not a challenge, but rather minimation of PCE losses upon area up-scaling\n6) l. 96: degradation of -1.05%/month * 4 months = -4,20% is this absolute or relative value of FF change?\n7) l. 120: comma is missing between ref 28 and 29 (28,29)\n8) l. 135: 8 samples in Fig. 2 deserve to have a photo of a glass substrate with the 8 cells in Supplement.\n9) l. 137: you show only results in reverse scan, although in Supplement you show both reverse and forward scan. Since there is quite a difference between reverse and forward scan, you should at least mention this difference (not only FF, but even Voc is quite different).\n10) l. 312: “optimal distance” deserves an argumentation WHY? As Fig. 7g reveals the ground albedo is very high and the module under test is nor surrounded by neighboring modules or shades in plane-of-array.", "reviewer_2": "The manuscript concisely reports the steps taken to work on the efficiency of perovskite cells by manipulating both interfaces of the absorber layer. The working principles on how this leads to improved device performance are well backed by a thorough analysis of additional experimental results trying to identify the opto-electronic build-up of the energetics in the layer stack.\nAdditionally, fabrication of perovskite semi-transparent minimodules, 4T tandem with a Si solar cell and finally also a larger area perovskite multi-minimodule/Si minimodule 4T tandem assembly is demonstrated, with outdoor operating data for few months.\nWhile these results can have high relevance to proof the progress made in this research domain of perovskite photovoltaics, it remains a too fragmented report overall. There's no clear link, with for example a cell-to-module loss analysis, between the advancements seen in the (opaque) perovskite cell developments and the (ST) minimodules. Also, no stability results on cell level are presented to support any early interpretation on the potential degradation mechanisms that can play in the outdoor tested samples.\nMoreover, claims repeatedly being made in the manuscript that this work shows scalability of the perovskite and tandem PV technology, even alluding on the low LCoE that can be obtained, are not well placed here. The process is still very lab scale oriented (still spin coating based), highly manual - both on minimodule manufacturing as on tandem assembly - and at sizes still well below m2 dimensions.", "reviewer_3": "The manuscript titled “MXene-driven nanoscale graded homojunctions for advanced 4-terminal perovskite/silicon tandem solar panels” by Agresti et al. demonstrates and studies chlorine-terminated MXene (Ti3C2Cl2) ETL and PEAI passivated HTL contacts for NIP perovskite solar cells and 4T silicon/perovskite tandems. The authors claim that this architecture creates a graded homojunction within the perovskite bulk that suppresses non-radiative recombination and facilitates carrier extraction for high-efficiency devices. That strategy is scaled to a 19.45% efficient 0.2m2 tandem modules and fielded outdoors in Crete for 4 months. The module fabrication steps and performance characterization were well documented with the inclusion of temperature and irradiance dependent data on reasonably sized tandem modules as well as details on module scribing, interconnects, and lamination processes. The scalability of this approach and field data could be of high interest to the field, however, I have some significant concerns limiting the potential impact of this work.\nFirst, I do not find sufficient evidence to support the claim for the formation of a graded homojunction. Second, while the authors clearly advance device demonstration of halogen MXene doping for tuning perovskite interfaces, this concept has been reported before (e.g. interface calculations of Ti3C2Cl2 DOI: 10.1016/j.vacuum.2023.112381; MXene work function tuning for perovskite solar cells DOI: 10.1039/D2NR02799B). Third, the reported tandem module efficiency of 19.45% is not above the efficiency of an average single junction silicon module, even though the tandem pairing resulted in a net 1.05% absolute efficiency increase over the Si HJT cells used in this study and the reported field stability is not at the state-of-the-art of other public reports for perovskite-based tandem modules (https://pvpact.sandia.gov/results-and-data/). I have included additional comments below, but unfortunately cannot recommend publication of this manuscript in nature communications in its current form.\nAdditional comments:\n1) The authors report UPS and Kelvin probe CPD data on Ti3C2Cl2 MXenes in Fig S6 and additional CPD measurements in Fig S13 and half-stacks in Tab. 2. These experiments appear to be film surface measurements but not a depth profile of an actual perovskite film. As far as I can tell, the perovskite work function and band structure primarily comes from DFT simulations and TiberCAD simulations in Figs S15-S17. A work function change due to doping of a contact layer can induce an electric field at a contact interface, but that is insufficient to claim the formation of a graded homojunction. Is there any experimental evidence to suggest that there are doping changes within the perovskite layer as a result of the new contact strategy that are aiding in the suppression of non-radiative recombination and carrier collection?\n2) The authors should be careful about making claims of grain sizes based on SEM methods. It has been shown in the literature that these techniques cannot identify true grain size or grain boundaries (DOI: 10.1021/acsenergylett.8b01704, DOI: 10.1016/j.joule.2018.12.011). It is reasonable to say “apparent grains”, “perovskite domain”, or to describe the “morphology”, but this should be taken into consideration when attributing film properties to SEM images.\n3) The authors claim “Moreover, the hydrophobic nature of MX-Cl further boosts both device efficiency and operational stability, as confirmed by Maximum Power Point (MPP) tracking of the best-performing cell (Fig. S12).” However, figure S12 only shows 180s of maximum power point data. 180s is not sufficient to claim operational stability and does not compare against a control. Furthermore, I do not follow the reasoning as to how the MPP data is related to the hydrophobic nature of MX-Cl.\n4) In the SI, it is claimed that drawing from Table S5, the 4T tandem pairing resulted in a 2.74% PCE improvement over the silicon HJT module alone. However, it appears that DEM2 is a 19.45% 4T module fabricated with 3SUN. HJT silicon (18.4%; Table S5). Wouldn’t this indicate an efficiency improvement of 1.05% over the single junction silicon?\n5) The authors should provide accelerated stability data (with at a minimum light + heat, preferably 85C or higher) for the 4F-PEAI/MX-Cl architecture vs. control.\n6) The authors should consider extending the outdoor field test and providing a post-mortem characterization of the fielded module to identify apparent degradation modes of this architecture.", "reviewer_4": "In this manuscript, the authors demonstrate a synergistic approach of doping a two-dimensional material, MXene Ti3C2Cl2, in the perovskite absorber layer and adding 4-FPEAI to the antisolvent in anticipation of forming a n-p junction, to enhance the performance of perovskite solar cells. And the method is extended to semi-transparent cells and four-terminal perovskite/silicon tandem devices. Finally PCE of 19.45% was achieved on a 0.2 m2 4T tandem device and field tested for stability. However, I don't see much correlation with the methodology described in this manuscript in the section of the tandem devices, and it doesn't adequately describe the characterization of tandem devices. In addition, there are some problems with the article as follows, so I don't recommend this manuscript for publication in Nature Communications.\nQ1) Researchers have already revealed that 4-FPEAI and MXene play important roles in improving the performance of perovskite solar cells. The highlights and novelty of this work should be highlighted.\nQ2) In Fig. 2 and Fig.S19, the statistics of the electrical parameters are all obtained from 8 samples, which is not a sufficient amount of data to support the experimental conclusions. For example, in lines 122 “...and exhibiting reduced PCE dispersion ...” and lines 123 “... Improvements highlights enhance device reproducibility ...”, 8 samples are clearly not enough to account for these.\nQ3) In lines 124, “Furthermore, replacing CB with to EA, the perovskite post-deposition annealing time was reduced by half (30 minutes), effectively halving the energy required for perovskite absorber realization.” Is there any experiment or reference that proves it, I don't understand.\nQ4) Lines 140 “...with Ti3C2Cl2 MXenes (MX-Cl) by carefully tuning the MXene dopant amount (Fig. S2).” In Fig. S2, what are the units of MX-Cl concentration? The interval between 0.016 and 0.15 is too large, are there conditions for better concentrations in between?\nQ5) Lines 146 and 147, “SGP has minimal impact on grain size and surface morphology (Fig. S8), whereas MX-Cl doping, resulted in a noticeable increase in perovskite grain size compared to the pristine layer (Fig. 3a and 3c).” Statistical distribution and comparison of average grain size should be given.\nQ6) Lines 162, “XRD analysis (Fig. 3e) reveals ... and minimizing trap states.” Enhancement of the XRD peak intensity does not necessarily directly indicate a reduction in defects in the film, and enhancement of the diffraction peaks may reflect increased crystallinity or increased grain size, which corresponds to SEM. Increased grain size results in sharper peaks and increased intensity, but this may be directly related to a reduction in grain boundaries rather than point defects. A reduction in point defects or dislocations may reduce lattice distortion, resulting in increased peak intensity, but changes in defect density need to be confirmed in conjunction with other means such as TEM and SCLC.\nQ7) The peak intensity ratios in the XRD pattern in Fig.3e clearly do not correspond to the values in section SI 5, Tab. S1, e.g., the ratio of 4-FPEAI (110)/(310) is clearly greater than 1 in the figure, but 0.92 in the table.\nQ8) In lines 179, “...slight blue shift (767 nm vs. 763 nm) for MX-Cl/4-FPEAI films...”, the optical band gap of Cs0.18FA0.82Pb(I0.8Br0.2)3 perovskite was 1.68 eV in Fig.S1, and the band gap is calculated to be 1.62 eV (~760 nm) based on the emission peaks of PL. Does the introduction of 4-FPEAI and MX-Cl change the band gap of perovskite?\nQ9) In lines 189, “...the hydrophobic nature of MX-Cl further boosts both device efficiency and operational stability, ...” There's no pristine group here in Fig.S12. And a 3-minute mpp tracking is too short to indicate stability. Contact angle and surface tension calculations are needed to show that MXene doping leads to an increase in the hydrophobicity of perovskite films.\nQ10) In lines 206 and 207, “...could be concluded that the combination of 4-FPEAI and MX-Cl doping forms a graded n-p junction along the thickness of the perovskite absorber from ETL to HTL side.” The variation of the work function of CPD is not a sufficient proof of the existence of n-p junction, which can be further demonstrated by calculating the energy level structure based on UPS of different films.\nQ11) In lines 258, “...confirming the effectiveness of 2D material-engineering approach even for semi-transparent structures.” The cell structure changes from opaque to semi-transparent, where the only change seems to be the change of the top electrode from Au to ultrathin Au/ITO. Whereas the effect of MX-Cl/4-FPEAI is significantly diminished (the average increase of 30 mV is diminished to 11 mV), and is it possible that due to the change in work function of electrode that leads to a diminution of the effect of the n-p junction as described in the manuscript."}, "author_response": "Answers to the Reviewers' comments:\nWe would like to sincerely thank the reviewers for their thorough and constructive evaluation of our\nmanuscript. We are grateful for their recognition of the technical quality, clarity, and overall structure of our\nwork. We particularly appreciate the positive comments highlighting the scientific soundness of our study,\nthe well-documented fabrication and characterization of the tandem modules, and the relevance of the\nscalability and outdoor stability data presented.\nWe are pleased that the reviewers acknowledged the novelty of our synergistic interface engineering\napproach, combining chlorine-terminated MXene doping and 4-FPEAI passivation, to form a graded\nhomojunction within the perovskite absorber. We also thank them for appreciating the demonstration of\nhigh-efficiency four-terminal perovskite/silicon tandem devices, including large-area modules tested under\nreal outdoor conditions.\nBelow, we provide detailed point-by-point responses to the reviewers’ comments.\nReviewer #1 (Remarks to the Author):\nThe manuscript reports progress in MXene-driven nanoscale graded homojunctions for perovskite top cell in\n4-terminal perovskite/silicon tandem solar panels. The manuscript is technically well written and sound.\nHowever, I have one major and several minor remarks that need to be considered before I can recommend\nthe manuscript for publication.\nMAJOR remark:\n1) l. 303: \"monitored in outdoor conditions\" must be specified in detail. Were top cells and bottom\ncells in 4-T tandems in MPP or open-circuit or short-circuit? You need to show the behaviour for the\n4 months in Supplement! How did determine data in Fig. 7f? Did you bring tandems indoor and\nmeasure them under STC?\nWe thank the reviewer for underlining this point that indeed requires some clarifications. Regarding the\nprolonged outdoor testing, we decided to test our DEM2 panel at open circuit condition, following the\nISOS-O-2 protocol. Thus, the panel was not “continuously monitored” in a strict sense, but was left\nmounted on the rack in the operative position at open-circuit and measured once per month. The\nparameters reported in Fig. 7f were extracted from the I-V curves, acquired for the perovskite top panel\nafter a stabilization time of 10 minutes, since we observed an increase in the delivered power till reaching\na stabilized value after 10 minutes (see the new Fig. S29a in SI). Since the outdoor characterizations are\ntypically subjected to the instantaneous variation of the irradiance and the panel temperature, we\nselected a clear sky day in the first half of the month performing the measurements when the irradiance\nwas supposed to be maximum (at 12:30 am). The irradiance daily curves for Hellenic Mediterranean\nUniversity site in Heraklion (35.31959; 25.10244) recorded during the day when the outdoor I-V curve\nwere acquired, are reported in Fig. S29b. Once acquired the I-V curves, the extracted electrical\nparameters were normalized in STC (1000 W/m2 irradiance and 25°C temperature) considering i) the\npanel temperature and the irradiance values recorded at the measurement time (at 12:30 am, see values\nreported in the new Tab. S10) ii) both temperature and irradiance dependency of the main electrical\nparameters reported in Fig. S28. What is stated above is now included in SI section S.I. 14 and following\nreported:\nThe prolonged outdoor testing was performed on DEM2 panel at open circuit condition, following the ISOS-\nO-2 protocol. Thus, the panel was not “continuously monitored” in a strict sense, but was left mounted on\nthe rack in the operative position at open-circuit and measured once per month. The parameters reported in\nFig.7f were extracted from the I-V curves, acquired for the perovskite top panel after a stabilization time of\n10 minutes, since we observed an increase in the delivered power till reaching a stabilized value after 10\nminutes (see Fig. S29a). Since the outdoor characterizations are typically subjected to the instantaneous\nvariation of the irradiance and the panel temperature, we selected a clear sky day in the first half of the month\nperforming the measurements when the irradiance was supposed to be maximum (at 12:30 am). The\nirradiance daily curves for Hellenic Mediterranean University site in Heraklion (35.31959; 25.10244) recorded\nduring the day when the outdoor I-V curve were acquired, are reported in Fig. S29b. Once acquired the I-V\ncurves, the extracted electrical parameters were normalized in STC (1000 W/m2 irradiance and 25°C\ntemperature) considering i) the panel temperature and the irradiance values recorded at the measurement\ntime (at 12:30 am, see values reported in Tab. S10) ii) both temperature and irradiance dependency of the\nmain electrical parameters reported in Fig. S28.\nFig. S29: a) Normalized power at maximum power point (PMPP) stabilization prior the acquisition of the monthly I-V curves for\nthe DEM2 left at V in outdoor conditions; b) daily irradiance for the days selected during the outdoor test for the acquisition\nOC\nof the I-V curves.\n2) Fig. 7f shows only period of 3-months (and not 4 months!). In line 318 you state FF degradation -\n1.05%/month, which in 4 months end in 4,20% which means a 8% degradation from initial FF value\nof ~51% (estimated from Fig. 7f, since there is no table of exact values x 2013; add a table in\nSupplement!) In Fig. 7f also Isc degradation is seen, so the total degradation of 7% (over 93%\nretained after 4 months) is undermined and UNDER QUESTION.\nWe thank the reviewer for this careful observation. The reviewer is right, the stability test shown in Fig. 7f\ncovers a period of three months rather than four. We have corrected the text accordingly.\nThanks to the suggestion of the reviewer, with the aim to ensure full data transparency and to avoid potential\nambiguities in data interpretation, in the new version of the SI we have included a table named Tab. S10\nreporting the electrical parameters (for the perovskite top panel of DEM2) related to Fig. 7f, allowing a\nquantitative assessment of the parameter degradation over the testing period. Considering a three-months\nperiod, the extracted parameter values (Tab. S10) indicate that FF decreases from 52.2% to 50.2% (≈-3.83%\nrelative degradation over three months), I decreases from 0.76 A to 0.74 A (≈-2.6% relative degradation),\nSC\nwhile V remains largely stable (≈+0.8%), resulting in an overall panel power decreases from 10.5 W to 10.0\nOC\nW (≈-4.8% relative degradation). Thus, the perovskite top panel retained approximately 94–95% of its initial\nperformance over the three-months period. Notably, the observed modest degradation is consistent with the\nmetastability behaviour (now reported in SI, section S.I. 9) of graded homojunction perovskite cells subjected\nto accelerated light-cycling tests, showing limited performance loss (T =600 hours at MPP) over time. We\n91\nadded a discussion in SI, section S.I. 14, that now sounds as follows reported:\nTab. S10: Photovoltaic parameters of the perovskite top panel (DEM2) during the three-month outdoor stability test and quantitative\nevaluation of performance degradation over time. The electrical data were obtained by acquiring the I-V characteristics in outdoor\ncondition in selected days (one per each stress month; panel temperature and irradiance values at the I-V curve acquisition time are\nreported in the first two columns) at 12:30 am and subsequently normalized in STC (1000 W/m2 irradiance and 25°C temperature).\nPanel T (°C) Irradiance V (V) I (A) FF(%) Power(W)\nOC SC\n(W/m2)\n8th August 2023 61.3 986 26.4 0.76 52.2 10.5\n9th September 2023 53.9 829 26.6 0.76 51.6 10.5\n9th October 2023 50.4 782 27.1 0.75 50.9 10.4\n9th November 2023 43.5 608 26.6 0.74 50.2 10.0\nTotal degradation over three months 0.76% -2.63 -3.83% -4.76%\nTab. S10 summarizes the photovoltaic parameters of the perovskite top panel (DEM2) during outdoor testing\nfrom August to November 2023. The results show a gradual decrease in FF (≈-3.1%) and I (≈-2.6%) over the\nSC\nthree-month period, while V remains nearly constant. The overall power output decreases by ≈-4.8%,\nOC\ncorresponding to a retained performance of about 95% of the initial value.\n3) No metastability behaviour of perovskite cells on day-night basis is nor mentioned not discussed,\nbut data should be recorded and such analysis would clarify the stability of the MXene-driven\nnanoscale graded homojunction perovskite solar cells.\nWe thank the reviewer for his insightful suggestion that pushed us to add the analysis of the metastable\nbehavior of our perovskite semi-transparent top cell. With the aim of investigating the metastability of our\ngraded homojunction cell, we performed light-cycling tests now inserted in SI as follows reported.\nIndoor photo-stability under MPP tracking of semi-transparent encapsulated cells upon exposure to cycled\nlight. As recently discussed in literature, the most effective way to perform a reliable stability analysis consists\nin light cycling test at constant temperature, representing a good trade-off between simplicity of execution in\nlab conditions while taking into account phenomena observed only in case of outdoor testing (that cannot be\nconsidered in case of constant light experiments).15 Indeed, transient behavior occurs in PSCs as a result of\nthe coexistence of several dynamics with characteristic times spanning from time scales of seconds to hours.\nThus, day–night cycling of devices can consider such slow dynamics as they occur on similar timescales.\nFollowing this idea, we performed a light–dark cycling test as recommended by the ISOS-LC-1 protocol,\nexposing the cells to simulated sunlight turned on and off with cycle periods of 24 h and duty cycles\n(light:dark) of 1:2, mimic an accelerated diurnal sun cycle (the solar cell is maintained at ambient conditions\nwhile the temperature is monitored and RH is maintained at 30%).16 The recorded maximum power (P ) is\nMPP\nreported in Fig. S7e for both G/EA and fully optimized G/MX/4-FPEAI semi-transparent cells, normalized at\nthe initial value. The combined stabilization effect of the 4-FPEAI based SGP together with the presence of\nMX-Cl results in a T =600 hours vs a T =574 hours of the untreated device, demonstrating the superior\n91 80\nstability (T 6.5 times larger) of the device employing the proposed graded homojunction approach. Despite\n91\nmore in depth analyses are required to elucidate the role of MX-Cl and 4-FPEAI SGP, we can confidently ascribe\nthe improved stability to a synergistic effect of the two treatments. On one side, the addition of MX-Cl resulted\nin a perovskite film with improved crystallinity eventually reducing lattice distortion, lowering defect density\nand minimizing trap states, as demonstrated by XRD, transient PL and SEM characterizations. This, combined\nwith the enlarged perovskite domains and mitigated grain boundary-induced trap states, makes the trapped\ncharge driven degradation less effective,17 resulting in an overall enlargement of device lifetime. We cannot\nexclude a side role of the Cl in reducing the tensile strain within the perovskite layer, recently pointed out as\none of the main degradation channel when light-dark cycling is applied.18 On the other side, the\nphenylalkylammonium compounds as 4-FPEAI has been proven to stabilize the perovskite through\nsuppression of iodide ion migration, likely responsible for metastable perovskite behavior under light cycling\nobserved for the G/EA device, while passivating the surface defects.19 In addition, the 4-FPEAI SGP strategy\nhas been already proven to improve the long-term operational stability of the encapsulated devices under\nthe continuous illumination in ambient condition.12 Thus, the superior light cycling stability showed by the\ngraded homojunction cell can be confidently ascribed to a combined stabilizing effect from 4-FPEAI and MX-\nCl, definitively reducing the device metastable behavior.\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1).\n(12) Yan, N.; Gao, Y.; Yang, J.; Fang, Z.; Feng, J.; Wu, X.; Chen, T.; Liu, S. F. Wide-Bandgap Perovskite Solar Cell Using a Fluoride-\nAssisted Surface Gradient Passivation Strategy. Angew. Chem. Int. Ed. 2023, 62, e202216668.\nhttps://doi.org/10.1002/anie.202216668.\n(15) Khenkin, M.; Köbler, H.; Remec, M.; Roy, R.; Erdil, U.; Li, J.; Phung, N.; Adwan, G.; Paramasivam, G.; Emery, Q.; et al. Light\nCycling as a Key to Understanding the Outdoor Behaviour of Perovskite Solar Cells. Energy Environ. Sci. 2023, 17 (2), 602–\n610. https://doi.org/10.1039/d3ee03508e.\n(16) Khenkin, M. V.; Katz, E. A.; Abate, A.; Bardizza, G.; Berry, J. J.; Brabec, C.; Brunetti, F.; Bulović, V.; Burlingame, Q.; Di Carlo,\nA.; et al. Consensus Statement for Stability Assessment and Reporting for Perovskite Photovoltaics Based on ISOS\nProcedures. Nat. Energy 2020, 5 (1), 35–49. https://doi.org/10.1038/s41560-019-0529-5.\n(17) Ahn, N.; Kwak, K.; Jang, M. S.; Yoon, H.; Lee, B. Y.; Lee, J.; Pikhitsa, P. V; Byun, J.; Choi, M. Trapped Charge-Driven\nDegradation of Perovskite Solar Cells. Nat. Commun. 2016, 8 (May), 1–9. https://doi.org/10.1038/ncomms13422.\n(18) Shen, Y.; Zhang, T.; Xu, G.; Steele, J. A.; Chen, X.; Chen, W.; Zheng, G.; Li, J.; Guo, B.; Yang, H.; et al. Strain Regulation\nRetards Natural Operation Decay of Perovskite Solar Cells. Nature 2024, 635 (8040), 882–889.\nhttps://doi.org/10.1038/s41586-024-08161-x.\n(19) Guo, Y.; Apergi, S.; Li, N.; Chen, M.; Yin, C.; Yuan, Z.; Gao, F.; Xie, F.; Brocks, G.; Tao, S.; et al. Phenylalkylammonium\nPassivation Enables Perovskite Light Emitting Diodes with Record High-Radiance Operational Lifetime: The Chain Length\nMatters. Nat. Commun. 2021, 12 (1). https://doi.org/10.1038/s41467-021-20970-6.\nMoreover, we modified the main text as follows:\n“Moreover, the combination among the MX-Cl addition and the 4-FPEAI SGP strategy not only translated into\nsuperior device performance but also conferred superior robustness to the perovskite film against light-driven\ndegradation, as demonstrated by prolonged light cycling test (Fig. S22c).”\nMinor remarks:\n1) l. 36: \"under albedo conditions\" needs to be more specific, e.g. \"under 30% ground albedo conditions\".\nWe appreciate the reviewer’s comment regarding the need for clarity in the phrase “under albedo\nconditions.” We agree that specifying the exact albedo value improves the precision and readability of the\nmanuscript.\nAccordingly, we have revised the text to read “under 30% ground albedo conditions” wherever applicable.\nThis change has been implemented throughout the manuscript to ensure consistency and to accurately\nreflect the simulation setup used in our study.\n2) l. 36: \"retained over 93%\" should be corrected according to l. 96 and l. 318 (see below remark l.318)\nThank you for your observation. We acknowledge the inconsistency between the statement in line 36 and the\nmore precise data provided in lines 96 and 318. We have now revised the sentence in the abstract to reflect\nthe exact performance metric, specifying that the perovskite top panel of DEM2 retained over 95% of its\ninitial P after three months at open circuit, with a slow FF degradation of 1.27% per month. This ensures\nMPP\nconsistency and accuracy throughout the manuscript.\nWe change the sentence that now sounds as: “The tandem panel, installed in Crete, retained over 95% of its\ninitial P after three months.”\nMPP\n3) l.53 \"significant advantages, including indpendent MPPT\" brings also additional cost that must be\nadded to this too optimistic statement\nWe appreciate the reviewer’s valuable comment. We agree that the implementation of independent MPPT\nin 4-terminal architecture entails additional costs related to the required power electronics and system\ncomplexity. However, these added costs are often justified by the performance benefits, especially in non-\nuniform irradiance conditions or bifacial configurations, where independent MPPT allows for optimized\nenergy harvesting from each sub-cell.\nTo acknowledge this important aspect, we have revised the manuscript by adding in the main text the\nfollowing sentence, to clarify that while independent MPPT offers significant operational advantages, it also\nintroduces added costs at the system level, which must be considered in techno-economic evaluations:\n“and compatibility with bifacial Si cells, which can capture reflected sunlight to enhance energy yield,\nalthough independent MPPT adds system-level cost.6,7\n(6) Robles-Algarín, C., Olivero-Ortíz, V., & Restrepo-Leal, D. (2022). Techno-Economic Analysis of MPPT and PWM Controllers\nPerformance in Off-Grid PV Systems. International Journal of Energy Economics and Policy, 12(6), 370–376.\n(7) Faranda, R. S., Leva, S., & Maugeri, V. (2008). MPPT Techniques for PV Systems: Energetic and Cost Comparison.\n4) l. 54 I miss reference to 3-terminal architecture of perovskite/silicon tandem cells that also exhibits\nflexibility to bifaciality\nThanks for your observation. We agree that the 3-terminal perovskite/silicon tandem architecture also allows\nfor bifacial operation and represents a promising configuration. However, the partial electrical coupling\nbetween the subcells still requires precise current balancing and complex interconnection schemes, which\ncan hinder independent energy yield optimization and large-scale integration compared to fully decoupled 4-\nterminal configurations. We have now included a reference and a brief discussion on this architecture at the\nrelevant point in the manuscript (previous line 54) to provide a more complete overview of tandem options.\n\"Additionally, 3-terminal architectures have also been proposed as a compromise solution, combining\nreduced complexity with bifacial compatibility,8 despite their implementation still requires careful current\nmatching and interconnection design, which may limit scalability compared to 4-terminal configurations.\"\n(8) Gota, F.; Langenhorst, M.; Schmager, R.; Lehr, J.; Paetzold, U. W. Energy Yield Advantages of Three-Terminal Perovskite-Silicon\nTandem Photovoltaics. Joule 2020, 4 (11), 2387–2403. https://doi.org/10.1016/j.joule.2020.08.021.\n5) l. 57: \"typical opaque electrode\" - not clear for which cell (top PK or bottom Si)?\nThank you for your comment. We acknowledge that the sentence “typical opaque electrode” could be\nambiguous. We confirm that it refers specifically to the top perovskite solar cell, which typically uses a\nreflective metal back electrode (e.g., Au or Ag) in its standard opaque configuration. To avoid confusion, we\nhave revised the sentence to make this clear in the updated manuscript. The new sentence is now:\n“a certain loss in PCE is inevitable when the typical opaque metal electrode used in top PSCs is replaced by a\nsemi-transparent top electrode.”\n6) l. 63: \"the scaling-up PCE losses\" is not a challenge, but rather minimation of PCE losses upon area\nup-scaling\nWe thank the reviewer for this valuable clarification. We agree that the correct formulation should emphasize\nthe minimization of PCE losses upon area upscaling as the actual challenge, rather than describing the losses\nthemselves as the challenge. We have accordingly revised the sentence in the manuscript to better reflect\nthis point. The modified new sentence is:\n“As a second challenge, the minimization of PCE losses when moving from small area ST-PSCs to semi-\ntransparent perovskite solar modules (ST-PSM) remains a key target for the practical realization of efficient\ntandem devices.”\nMoreover, with the aim of providing further inside the PCE losses in the scaling from opaque small area cells\nto semi-transparent large area modules, we added a discussion in the SI section S.I: 11 that now sounds as\nfollows reported:\nCell-to-module performance gap analysis.\nTo clarify the link between small-area opaque devices and the performance of large-area semi-transparent\n(ST) modules, we carried out a loss analysis identifying the main contributing factors. The best-performing\nopaque device (G/MX/4-FPEAI) exhibited a PCE of 22.12%, while the corresponding 60 cm² ST module\nreached 16.2%, resulting in an absolute performance drop of ~5.96 percentage points.\nThis gap can be attributed to three main sources:\n(i) Optical losses due to the absence of the reflective metal electrode in ST configurations, which reduces\nphoton recycling and light harvesting. To quantify the effect of transparency on photovoltaic performance,\nwe compared our optimized opaque device (G/MX/4-FPEAI) with its semi-transparent (ST) counterpart\nfeaturing an Au/ITO top electrode. The opaque device exhibited a PCE of 22.12%, while the ST cell reached\n18.3%, under identical illumination and processing conditions (see Tab. S7). This ~17.2 % relative efficiency\ndrop (≈3.82 percentage points absolute) can be attributed primarily to optical losses due to the removal of\nthe reflective metal electrode, which otherwise enhances internal light trapping and photon recycling. With\nthe aim to quantify the impact of the ST electrode onto the device photocurrent, we performed IPCE\nmeasurements on both the best performing opaque and semi-transparent cell, engineered with graded\nhomojunction strategy. By computing the integrated current density, we can conclude that the replacement\nof opaque electrode with a ST one (gold/ITO) contributed to a J loss of ~2.0 mA/cm². This is in line with what\nSC\nobserved by De Wolf, S. and co-workers,20,21 which employed optical modeling (Transfer Matrix Method\n(TMM)) of the device stack to predict a 10–12% of incident photons loss due to front-surface reflection and\nincomplete absorption in the perovskite because of the reduced optical path length. These factors jointly\nreduce the short-circuit current density (J ) by ~ 1.8–2.2 mA/cm². In our case the reflection at the glass\nSC\nsubstrate is the same for both the ST and opaque devices, concluding that the observed J drop can be\nSC\nattributed to the absence of photon recycling in the 580-730 nm region of the visible spectra.\n(ii) Electrical losses from the transparent electrode stack (Au/ITO), including increased sheet resistance and\ncontact resistance, resulting in an FF drop of ~6.32%. Nonetheless, the retained high V (~1.22 V) and FF\nOC\n(~75%) in the ST cell confirm that our band alignment strategy remains effective under reduced optical\nconfinement.\n(iii) Scaling-related effects, including increased surface non-uniformity, edge shunting, and series resistance\ndue to larger interconnection paths, contributing to a further loss in FF of -7.5% moving from the ST-cell to\nthe ST-module. FF loss is the main responsible for the observed drop in PCE moving from ST-cell to ST-\nmodules, since V and J for each large area cells (LAC, with an active area of 2.5 cm2) composing the module\nOC SC\ndid not undergo a significant drop (V ~ 1.25 V and J ~ 20.8 mA/cm2, see Tab. S7)\nOC_LAC SC_LAC\nDespite these inevitable losses, the use of a vertically graded homojunction via Cl-MXene doping and 4-FPEAI\npassivation proved critical in maintaining high V (1.226 V) and FF (67-69%) at the module level. This\nOC\nhighlights the effectiveness of the interface and band alignment engineering in mitigating efficiency\ndegradation upon scaling and semi-transparency.\nTab. S7:Electrical parameters for best performing opaque cell, ST cell and ST module.\nFF (%)\nDevice Type Area (cm²) PCE (%) V (V) J (mA/cm²)\nOC SC\nOpaque cell\n81.82\n0.1 22.12 1.21 22.5\n(G/MX/4-FPEAI)\n75.5\nST cell (Au/ITO) 0.1 18.3 1.22 20.5\nFF (%)\nArea (cm²) PCE (%) V (V) I (mA)\nOC SC\n67.9\nST module 60.0 16.16 26.98 52.7\nAnd the following statement in the main text:\n“while strongly limiting the PCE drop (<13%) usually observed in scaling the device size (S.I. section S.I. 11\nand Tab. S7 for detailed discussion).”\n7) l. 96: degradation of -1.05%/month * 4 months = -4,20% is this absolute or relative value of FF\nchange?\nWe thank the reviewer for this pertinent observation. The reported fill factor degradation rate of 1.27% per\nmonth refers to a relative decrease respect to the initial FF value. Moreover, as we point out in the answer to\nthe second question in the major remarks, we underline that the stability test shown in Fig. 7f covers a period\nof three months rather than four, thus the total degradation for FF is -3.8%. We have revised the text to clarify\nthis point accordingly. The new sentence in the main text now sounds as:\n“primarily showing a slow fill factor (FF) degradation of 1.27% per month, expressed as a relative decrease\nrespect to the initial FF.”\nThis\n8) l. 120: comma is missing between ref 28 and 29 (28,29)\nThank you for pointing this out. We have corrected the citation by adding the missing comma between\nreferences 28 and 29, which now correctly appears as (31, 32) in the revised manuscript.\n9) l. 135: 8 samples in Fig. 2 deserve to have a photo of a glass substrate with the 8 cells in Supplement.\nThank you for the suggestion. We agree that including a photograph of the substrate containing the tested\ncells enhances the clarity of the experimental section. We would like to clarify that in the new version of the\nmanuscript we extended the device statistics to 12 samples fabricated on three glass substrates, each hosting\n4 perovskite solar cells. As an example, we have now added a photograph of one of these substrates to the\nSupporting Information shown as panel a) in Fig. S2. This integration provides visual context for the sample\nlayout used in the statistical analysis.\nFig. S2: a) Photograph of one of the two glass substrates used for device fabrication, each hosting four small-area perovskite solar\ncells. A total of 12 devices were fabricated for the statistical evaluation of photovoltaic parameters (Fig. 2,) while 8 devices were used\nfor the optimization of the Mxene-Cl doping concentration, b) PCE of PSCs realized by varying the amount of MXene-Cl added to 1\nml of perovskite precursor solution.\n10) l. 137: you show only results in reverse scan, although in Supplement you show both reverse and\nforward scan. Since there is quite a difference between reverse and forward scan, you should at least\nmention this difference (not only FF, but even Voc is quite different).\nWe thank the reviewer for the valuable observation. As correctly pointed out, in the main manuscript we\nreported only the reverse scan results for the opaque devices shown in Fig. 2. However, we agree that the\ndifference between forward and reverse scans should be explicitly addressed, especially for key parameters\nsuch as V and FF. To this end, we now clarified in the revised manuscript that forward and reverse J–V\nOC\ncharacteristics for the opaque devices are provided in Fig. S7 of the Supporting Information by adding the\nfollowing sentence in the caption of the Fig. 2.\n“ Forward and reverse scan J–V curves for all four opaque device architectures are provided in the Supporting Information (Fig. S7 a),\nb), c), d)). The minimized hysteresis in the G/MX/4-FPEAI device highlights the improved stability and reduced interfacial\nrecombination enabled by the device engineering via graded homojunction strategy. “\nThese measurements confirm the presence of scan direction-dependent hysteresis in the reference devices,\nwhich is progressively reduced in the engineered devices thanks to MXene doping and surface gradient\npassivation (SGP). We added in the SI the following text:\nFrom Figs. S7 a), b), c), d), we extracted the forward and reverse scan values of V , FF, J and PCE for each\nOC SC\nof the four opaque PSC structures. As shown by the data, the difference in open-circuit voltage between\nreverse and forward scan (ΔV ) ranges from 0.04 V in the G/CB reference device down to 0.002 V in the\nOC\noptimized G/MX/4-FPEAI device. Similarly, the fill factor (ΔFF) decreases from 6.3% in G/CB to 1.4% in\nG/MX/4-FPEAI. This clearly demonstrates that the combined MXene and 4-FPEAI strategy significantly\nreduces hysteresis, in terms of PCE between forward and reverse scan directions. The ΔPCE decreases from\n1.94% in G/CB devices to just 0.4% in G/MX/4-FPEAI.\nThis trend supports the conclusion that our dual strategy (SGP + MX-Cl) effectively mitigates interfacial\nrecombination and ionic migration, leading to more stable and symmetric device performance under J–V\nscanning.\n11) l. 312”optimal distance” deserves an argumentation WHY? As Fig. 7g reveals the ground albedo is\nvery high and the module under test is nor surrounded by neighboring modules or shades in plane-of-\narray.\nWe acknowledge the reviewer’s comment and agree that the term “optimal” may be misleading in this\ncontext. To avoid confusion, we have rephrased the sentence as follows:\n“Outdoor measurements performed using a metallic mounting base were carried out with the module\ninstalled at a fixed inclination of 30° and a height of approximately 100 cm above ground (see Fig. 7g).”\nThe choice of these parameters follows internationally recognized practices for outdoor photovoltaic\nperformance measurements. In our setup, the module under test was mounted at ~1 m above ground and\ntilted by 30°, in accordance with the procedures described in IEC 61853-21 and IEC 612152. These standards\nprovide representative mounting conditions but do not prescribe a unique “optimal” distance, leaving the\nconfiguration to be adapted to the specific experimental environment (see also: NREL Best Practices\nHandbook for the Collection and Use of Solar Resource Data, 2017;3 IEA-PVPS Task 13 Report on Outdoor\nPerformance Testing, 20204). The mounting height of ~1 m was selected to ensure a representative exposure\nto ground albedo while still avoiding possible shading effects from nearby structures, which are present in\nthe experimental field even if only partially visible in the Fig. 7g. Furthermore, the inclination of 30° is\nconsistent with the optimal yearly tilt for Heraklion, Crete (latitude ≈ 35°N), where the optimum fixed-tilt\nangle is generally close to the site latitude. Thus, the chosen configuration not only aligns with international\ntesting practice but also provides realistic operating conditions for the geographical location and device\narchitecture under study.\n1. IEC 61853-2:2016. Photovoltaic (PV) Module Performance Testing and Energy Rating – Part 2: Spectral\nResponse, Incidence Angle and Module Operating Temperature Measurements; International\nElectrotechnical Commission, Geneva, Switzerland, 2016.\n2. IEC 61215-1:2021. Terrestrial Photovoltaic (PV) Modules – Design Qualification and Type Approval –\nPart 1: Test Requirements; International Electrotechnical Commission, Geneva, Switzerland, 2021.\n3. Sengupta, M.; Habte, A.; Lopez, A.; Gueymard, C.; Wilbert, S. Best Practices Handbook for the\nCollection and Use of Solar Resource Data for Solar Energy Applications; NREL/TP-5D00-78489;\nNational Renewable Energy Laboratory: Golden, CO, 2017.\n4. IEA-PVPS Task 13. Outdoor Performance Analysis of Photovoltaic Systems: Report IEA-PVPS T13-\n20:2020; International Energy Agency Photovoltaic Power Systems Programme, 2020.\nReviewer #2 (Remarks to the Author):\nThe manuscript concisely reports the steps taken to work on the efficiency of perovskite cells by manipulating\nboth interfaces of the absorber layer. The working principles on how this leads to improved device\nperformance are well backed by a thorough analysis of additional experimental results trying to identify the\nopto-electronic build-up of the energetics in the layer stack.\nAdditionally, fabrication of perovskite semi-transparent modules, 4T tandem with a Si solar cell and finally\nalso a larger area perovskite multi-module/Si module 4T tandem assembly is demonstrated, with outdoor\noperating data for few months.\n1. While these results can have high relevance to proof the progress made in this research domain of\nperovskite photovoltaics, it remains a too fragmented report overall. There's no clear link, with for\nexample a cell-to-module loss analysis, between the advancements seen in the (opaque) perovskite\ncell developments and the (ST) minimodules.\nWe thank the Reviewer for these valuable observations. We partially agree with the Reviewer that the\nmanuscript may appear somewhat fragmented at first glance, due to the broad scope of the study. However,\nwe respectfully believe that this work represents a comprehensive and methodologically coherent\nprogression, from the optimization of opaque single-junction devices, through the development of semi-\ntransparent architectures, to the fabrication of large-area modules fully integrated in a 4-terminal tandem\npanel. This stepwise approach reflects the practical route required to translate laboratory-scale advances into\nscalable and commercially relevant photovoltaic technologies. Furthermore, we believe that the strength of\nour approach lies in the careful co-optimization of materials and processing techniques across multiple device\narchitectures. By tailoring interfacial properties and energy alignment at the material level, first in small-area\nopaque devices and then in semi-transparent configurations, we were able to ensure compatibility and\nreproducibility at the module and panel scale. This integrated development strategy is essential to bridge the\ngap between material innovation and scalable device implementation in the perovskite/Si tandem field.\nNevertheless, we agree that clearly establishing the link between the opaque cell developments and the final\nperformance of the semi-transparent (ST) modules is crucial to better frame the progress achieved.\nTo address this, we have now included in the SI of the revised manuscript a more explicit cell-to-module loss\nanalysis. This discussion outlines the main factors contributing to the performance gap between lab-scale\nopaque devices and ST modules, including: i) the expected optical losses due to the absence of the back metal\nreflector, ii) the impact of the transparent electrode stack on parasitic absorption and series resistance,\nand\niii) the influence of scaling-related non-uniformities in film morphology and interface quality.\nWe further highlight how the graded homojunction architecture, developed and validated at the cell level,\ncontributes to maintaining high open-circuit voltage and fill factor even in the ST configuration, thereby\nmitigating the typical losses associated with transparency and upscaling.\nWe added the following comments and Fig. S22 b) in the SI in Section S.I. 9\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1).\nExternal quantum efficiency (IPCE) spectra of opaque and semi-transparent perovskite solar cells. The\nopaque device exhibits a higher IPCE across the entire spectral range, particularly between 580 and 730 nm,\nindicating enhanced photon harvesting due to increased optical path length and efficient photon recycling. In\ncontrast, the semi-transparent cell, incorporating a gold/ITO rear electrode, shows a reduced IPCE response\nin this region, consistent with optical losses from the transparent electrode and limited back reflection. This\nreduction contributes to J loss of approximately 2.0 mA/cm², in case of the optimized G/MX/4-FPEAI device\nSC\nstructure.\nAnd the following discussion in section S.I. 11\nCell-to-module performance gap analysis.\nTo clarify the link between small-area opaque devices and the performance of large-area semi-transparent\n(ST) modules, we carried out a loss analysis identifying the main contributing factors. The best-performing\nopaque device (G/MX/4-FPEAI) exhibited a PCE of 22.12%, while the corresponding 60 cm² ST module\nreached 16.2%, resulting in an absolute performance drop of ~5.96 percentage points.\nThis gap can be attributed to three main sources:\n(i) Optical losses due to the absence of the reflective metal electrode in ST configurations, which reduces\nphoton recycling and light harvesting. To quantify the effect of ST electrode on photovoltaic performance, we\ncompared our optimized opaque device (G/MX/4-FPEAI) with its ST counterpart featuring an Au/ITO top\nelectrode. The opaque device exhibited a PCE of 22.12%, while the ST cell reached 18.3%, under identical\nillumination and processing conditions (see Tab. S7). This ~17.2 % relative efficiency drop (≈3.82 absolute\npercentage points) can be attributed primarily to optical losses due to the removal of the reflective metal\nelectrode, which otherwise enhances internal light trapping and photon recycling. With the aim to quantify\nthe impact of the ST electrode onto the device photocurrent, we performed IPCE measurements on both the\nbest performing opaque and semi-transparent cell, engineered with graded homojunction strategy. By\ncomputing the integrated current density, we can conclude that the replacement of opaque electrode with a\nST one (gold/ITO) contributed to a J loss of ~2.0 mA/cm². This is in line with what observed by De Wolf, S.\nSC\nand co-workers,20,21 which employed optical modeling (Transfer Matrix Method (TMM)) of the device stack to\npredict a 10–12% of incident photons loss due to front-surface reflection and incomplete absorption in the\nperovskite because of the reduced optical path length. These factors jointly reduce the short-circuit current\ndensity (J ) by ~ 1.8–2.2 mA/cm². In our case the reflection at the glass substrate is the same for both the ST\nSC\nand opaque devices, concluding that the observed J drop can be attributed to the absence of photon\nSC\nrecycling in the 580-730 nm region of the visible spectra.\n(ii) Electrical losses from the transparent electrode stack (Au/ITO), including increased sheet resistance and\ncontact resistance, resulting in an FF drop of ~6.32%. Nonetheless, the retained high V (~1.22 V) and FF\nOC\n(~75%) in the ST cell confirm that our band alignment strategy remains effective under reduced optical\nconfinement.\n(iii) Scaling-related effects, including increased surface non-uniformity, edge shunting, and series resistance\ndue to larger interconnection paths, contributing to a further loss in FF of -7.5% moving from the ST-cell to\nthe ST-module. FF loss is the main responsible for the observed drop in PCE moving from ST-cell to ST-\nmodules, since V and J for each large area cells (LAC, with an active area of 2.5 cm2) composing the module\nOC SC\ndid not undergo a significant drop (V ~ 1.25 V and J ~ 20.8 mA/cm2, see Tab. S7)\nOC_LAC SC_LAC\nDespite these inevitable losses, the use of a vertically graded homojunction via Cl-MXene doping and 4-FPEAI\npassivation proved critical in maintaining high V (1.226 V) and FF (67-69%) at the module level. This\nOC\nhighlights the effectiveness of the interface and band alignment engineering in mitigating efficiency\ndegradation upon scaling and semi-transparency.\nTab. S7:Electrical parameters for best performing opaque cell, ST cell and ST module.\nFF (%)\nDevice Type Area (cm²) PCE (%) V (V) J (mA/cm²)\nOC SC\nOpaque cell\n81.82\n0.1 22.12 1.21 22.5\n(G/MX/4-FPEAI)\n75.5\nST cell (Au/ITO) 0.1 18.3 1.22 20.5\nFF (%)\nArea (cm²) PCE (%) V (V) I (mA)\nOC SC\nFF (%)\nDevice Type Area (cm²) PCE (%) V (V) J (mA/cm²)\nOC SC\n67.9\nST module 60.0 16.16 26.98 52.7\nAnd the following statement in the main text:\n“while strongly limiting the PCE drop (<13%) usually observed in scaling the device size (S.I. section S.I. 11\nand Tab. S7 for detailed discussion).”\n2. Also, no stability results on cell level are presented to support any early interpretation on the potential\ndegradation mechanisms that can play in the outdoor tested samples.\nWe fully agreed with the concern raised by reviewer about the possibility to support an early interpretation\nof the outdoor stability behavior by performing indoor stress test. On this basis, following what was recently\nsuggested by authoritative works in the literature17,18, we performed light-cycling tests that are now inserted\nand commented in SI, Fig. S22c, as follows reported:\nIndoor photo-stability under MPP tracking of semi-transparent encapsulated cells upon exposure to cycled\nlight. As recently discussed in literature, the most effective way to perform a reliable stability analysis consists\nin light cycling test at constant temperature, representing a good trade-off between simplicity of execution in\nlab conditions while taking into account phenomena observed only in case of outdoor testing (that cannot be\nconsidered in case of constant light experiments).15 Indeed, transient behavior occurs in PSCs as a result of\nthe coexistence of several dynamics with characteristic times spanning from time scales of seconds to hours.\nThus, day–night cycling of devices can consider such slow dynamics as they occur on similar timescales.\nFollowing this idea, we performed a light–dark cycling test as recommended by the ISOS-LC-1 protocol,\nexposing the cells to simulated sunlight turned on and off with cycle periods of 24 h and duty cycles\n(light:dark) of 1:2, mimic an accelerated diurnal sun cycle (the solar cell is maintained at ambient conditions\nwhile the temperature is monitored and RH is maintained at 30%).16 The recorded maximum power (P ) is\nMPP\nreported in Fig. S7e for both G/EA and fully optimized G/MX/4-FPEAI semi-transparent cells, normalized at\nthe initial value. The combined stabilization effect of the 4-FPEAI based SGP together with the presence of\nMX-Cl results in a T =600 hours vs a T =574 hours of the untreated device, demonstrating the superior\n91 80\nstability (T 6.5 times larger) of the device employing the proposed graded homojunction approach. Despite\n91\nmore in depth analyses are required to elucidate the role of MX-Cl and 4-FPEAI SGP, we can confidently ascribe\nthe improved stability to a synergistic effect of the two treatments. On one side, the addition of MX-Cl resulted\nin a perovskite film with improved crystallinity eventually reducing lattice distortion, lowering defect density\nand minimizing trap states, as demonstrated by XRD, transient PL and SEM characterizations. This, combined\nwith the enlarged perovskite domains and mitigated grain boundary-induced trap states, makes the trapped\ncharge driven degradation less effective,17 resulting in an overall enlargement of device lifetime. We cannot\nexclude a side role of the Cl in reducing the tensile strain within the perovskite layer, recently pointed out as\none of the main degradation channel when light-dark cycling is applied.18 On the other side, the\nphenylalkylammonium compounds as 4-FPEAI has been proven to stabilize the perovskite through\nsuppression of iodide ion migration, likely responsible for metastable perovskite behavior under light cycling\nobserved for the G/EA device, while passivating the surface defects.19 In addition, the 4-FPEAI SGP strategy\nhas been already proven to improve the long-term operational stability of the encapsulated devices under\nthe continuous illumination in ambient condition.12 Thus, the superior light cycling stability showed by the\ngraded homojunction cell can be confidently ascribed to a combined stabilizing effect from 4-FPEAI and MX-\nCl, definitively reducing the device metastable behavior.\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1).\n(12) Yan, N.; Gao, Y.; Yang, J.; Fang, Z.; Feng, J.; Wu, X.; Chen, T.; Liu, S. F. Wide-Bandgap Perovskite Solar Cell Using a Fluoride-\nAssisted Surface Gradient Passivation Strategy. Angew. Chem. Int. Ed. 2023, 62, e202216668.\nhttps://doi.org/10.1002/anie.202216668.\n(15) Khenkin, M.; Köbler, H.; Remec, M.; Roy, R.; Erdil, U.; Li, J.; Phung, N.; Adwan, G.; Paramasivam, G.; Emery, Q.; et al. Light\nCycling as a Key to Understanding the Outdoor Behaviour of Perovskite Solar Cells. Energy Environ. Sci. 2023, 17 (2), 602–\n610. https://doi.org/10.1039/d3ee03508e.\n(16) Khenkin, M. V.; Katz, E. A.; Abate, A.; Bardizza, G.; Berry, J. J.; Brabec, C.; Brunetti, F.; Bulović, V.; Burlingame, Q.; Di Carlo,\nA.; et al. Consensus Statement for Stability Assessment and Reporting for Perovskite Photovoltaics Based on ISOS\nProcedures. Nat. Energy 2020, 5 (1), 35–49. https://doi.org/10.1038/s41560-019-0529-5.\n(17) Ahn, N.; Kwak, K.; Jang, M. S.; Yoon, H.; Lee, B. Y.; Lee, J.; Pikhitsa, P. V; Byun, J.; Choi, M. Trapped Charge-Driven\nDegradation of Perovskite Solar Cells. Nat. Commun. 2016, 8 (May), 1–9. https://doi.org/10.1038/ncomms13422.\n(18) Shen, Y.; Zhang, T.; Xu, G.; Steele, J. A.; Chen, X.; Chen, W.; Zheng, G.; Li, J.; Guo, B.; Yang, H.; et al. Strain Regulation\nRetards Natural Operation Decay of Perovskite Solar Cells. Nature 2024, 635 (8040), 882–889.\nhttps://doi.org/10.1038/s41586-024-08161-x.\n(19) Guo, Y.; Apergi, S.; Li, N.; Chen, M.; Yin, C.; Yuan, Z.; Gao, F.; Xie, F.; Brocks, G.; Tao, S.; et al. Phenylalkylammonium\nPassivation Enables Perovskite Light Emitting Diodes with Record High-Radiance Operational Lifetime: The Chain Length\nMatters. Nat. Commun. 2021, 12 (1). https://doi.org/10.1038/s41467-021-20970-6.\nMoreover, we modified the main text as follows:\n“Moreover, the combination among the MX-Cl addition and the 4-FPEAI SGP strategy not only translated into\nsuperior device performance but also conferred superior robustness to the perovskite film against light-driven\ndegradation, as demonstrated by prolonged light cycling test (Fig. S22c).”\n3. Moreover, claims repeatedly being made in the manuscript that this work shows scalability of the\nperovskite and tandem PV technology, even alluding on the low LCoE that can be obtained, are not\nwell placed here. The process is still very lab scale oriented (still spin coating based), highly manual -\nboth on minimodule manufacturing as on tandem assembly - and at sizes still well below m2\ndimensions.\nWe agree with the reviewer in arguing that a new photovoltaic technology aiming to enter in the market must\nshow a low LCOE. Achieving low LCOE is possible if industrially compatible and low-cost large-area deposition\nmethods are proposed, such as large-area printing methods. Therefore, there are 2 levels of technology\ndevelopment to be considered. The first, intended as “scalability” of the device dimensions, consists in\nproposing a cell/module/panel structure that can minimize the performance decrease and the device\nprocessability issues when moving from a lab size to a dimension compatible with industrial standards. This\ncan also be done using spin-coating techniques once optimizing the deposition of the cell constituting layers\non the desired substrate size. The second level consists in trying to reduce the costs associated with the\nproduction of the device, therefore using printing techniques that reduce material waste as much as possible\nand that are as compatible as possible with a high-yield production line. Both development levels contribute\nto a low LCOE of the new technology that is intended to be proposed on the market. In our work, we aim to\nprovide a manufacturing strategy for semi-transparent perovskite devices that can be used as top cells in the\n4-terminal tandem architecture while demonstrating great resilience to the performance losses usually\nexperienced in the size scaling process. The manufacturing processes are lab-oriented because they are\ncarried out in a research laboratory, which is currently not equipped with a pilot production line for perovskite\nmodules. For this reason, a massive production of semi-transparent perovskite modules was reported in the\nmanuscript, demonstrating a reasonable module performance repeatability not guaranteed when using the\nspin-coater, and obtained in our case thanks to the use of two-dimensional materials. Moreover, we already\ndemonstrated that 2D materials-based approach combined with the spin-coating technique is effective in\nguaranteeing excellent performance repeatability at module and panel level, realizing the first ever showed\nin literature perovskite solar farm of 4.5 m2, composed by 9 panels of 0.5 m2, each of them realized by\nelectrically connecting 40 opaque perovskite modules (10x10 cm2 each one).1 Thus, in our opinion, the use\nof spin-coater does not preclude the \"scalability\" of the device dimensions. As further confirmation, our\nresearch group has already demonstrated the scalability of spin-coated perovskite monolithic modules\nbeyond the 9.5X9.5 cm2 size, covering a surface of about 300 cm2 (15.7x15.7 cm2) and demonstrating\nefficiencies above 17% for opaque devices.2 While these dimensions are exactly compatible with those of the\nM2 silicon wafer size, in tandem technology it is necessary to consider the presence of the semi-transparent\nelectrode, usually made with sputtered ITO, whose series resistance significantly limits the maximum\ndimensions of the module. In fact, in our work, we proposed to use module dimensions limited to 9.5x9.5\ncm2, size also strongly compatible with more “industrially oriented” deposition techniques such as blade-\ncoating or slot dye coating, as extensively demonstrated in literature.3 However, such “out of glove box”\ntechniques require a careful control of the environmental conditions during and after the deposition process,\nwhose variability could preclude acceptable device manufacture and performance reproducibility. For these\nreasons, our choice felt on the spin-coating technique in inert environment, using an industrial spin-coater\nwith the possibility to spin up to 25x25 cm2 substrate size. We therefore believe that the use of spin-coating\ndoes not compromise in any way the scalability of our tandem technology, that have been designed with a\n“modular approach” for further extending the dimension of the tandem in the next future (up to several m2\nsize), without compromise too much the final performance, while being compatible also with other bottom\ncell technologies available in the market, such as CIGS or CdTe PV. For these reasons, we strongly believe that\nour work provides a step forward the industrialization of the tandem technology, proposing a modular\napproach where perovskite top panel based on graded homojunction concept, can be realized employing\nboth lab-scale or industrially oriented deposition techniques. This provides also a way for broadening the\naudience of our work in accordance with the broad audience of Nature Communication Journal, giving the\nchance to scientists and researchers to repeat and verify the proposed approach in their lab, without the\nneed to have industrial facilities. We agree instead on the impact of the spin-coating technique can have on\nproduction costs, and therefore on the overall LCOE.\n(1) Pescetelli, S.; Agresti, A.; Viskadouros, G.; Razza, S.; Rogdakis, K.; Kalogerakis, I.; Spiliarotis, E.; Leonardi, E.; Mariani, P.; Sorbello,\nL.; et al. Integration of Two-Dimensional Materials-Based Perovskite Solar Panels into a Stand-Alone Solar Farm. Nat. Energy 2022, 7,\n597–607.\n(2) H. Nikbakht, P. Mariani, L. Vesce, F. Di Giacomo, E. Leonardi, G. Viskadouros, E. Spiliarotis, K. Rogdakis, S.Pescetelli, A. Agresti, S.\nBellani, F. Bonaccorso, E. Kymakis, . Di Carlo Upscaling Perovskite Photovoltaics: from 156 cm2 Modules to 0.73 M2 Panels. Advanced\nScience 2025, 22,12, 2416316.\n(3) Geistert, K., Ternes, S., Ritzer, D. B. & Paetzold, U. W. Controlling Thin Film Morphology Formation during Gas Quenching of Slot-Die\nCoated Perovskite Solar Modules. ACS Appl. Mater. Interfaces 15, 52519-52529 (2023)\nFor this reason, following the reviewer's suggestion, we have modified the text in the main as follows:\n“Ultimately, incorporating 2D materials yielded high-performing ST-PSMs, reduced performance variability,\nand enabled reproducible production of large area PSK/Si tandem panels, thereby advancing industrially\noriented production of the perovskite/Si tandem technology.37”\n(37) Čulík, P.; Brooks, K.; Momblona, C.; Adams, M.; Kinge, S.; Maréchal, F.; Dyson, P. J.; Nazeeruddin, M. K. Design and Cost Analysis\nof 100 MW Perovskite Solar Panel Manufacturing Process in Different Locations. ACS Energy Lett. 2022, 3039–3044.\nRegarding the “manual” techniques for manufacturing the tandem panel, these were dictated by the fact that\nthe panels were assembled in the laboratory, without the possibility to employ very expensive and\ncustomized tools, typical of a PV industry. As an example, the stringer machine used to make the series\nconnections between silicon cells costs from a few hundred thousand euros up to several million euros for\nthe most advanced and automated production lines, making their use impossible in a standard research lab.\nIn our revised work we have now underlined this aspect, also making considerations in terms of efficiency\nloss when using manual rather than industrial stringing methods (see SI, SECTION S.I. 13). However, we\nbelieve that, looking at the results shown in this work obtained using lab-scale production processes, these\nare really promising and surly improvable once transferred to an industrial environment. In this view, our\nwork represents one of the very few demonstrations present in the literature on the possibility to scale the\n4T tandem technology at panel level, without asking for any modifications of the silicon production line. We\nare pretty confident that, with all of this in mind, the reviewer could appreciate the tremendous efforts\ndisplayed in this work for providing one of the first demonstration of a working large-area perovskite/silicon\ntandem panel.\nReviewer #3 (Remarks to the Author):\nThe manuscript titled “MXene-driven nanoscale graded homojunctions for advanced 4-terminal\nperovskite/silicon tandem solar panels” by Agresti et al. demonstrates and studies chlorine-terminated\nMXene (Ti C Cl ) ETL and PEAI passivated HTL contacts for NIP perovskite solar cells and 4T silicon/perovskite\n3 2 2\ntandems. The authors claim that this architecture creates a graded homojunction within the perovskite bulk\nthat suppresses non-radiative recombination and facilitates carrier extraction for high-efficiency devices. That\nstrategy is scaled to a 19.45% efficient 0.2m2 tandem modules and fielded outdoors in Crete for 4 months.\nThe module fabrication steps, and performance characterization were well documented with the inclusion of\ntemperature and irradiance dependent data on reasonably sized tandem modules as well as details on\nmodule scribing, interconnects, and lamination processes. The scalability of this approach and field data could\nbe of high interest to the field; however, I have some significant concerns limiting the potential impact of this\nwork.\nFirst, I do not find sufficient evidence to support the claim for the formation of a graded homojunction.\nThank you for your valuable comment. We appreciate the opportunity to clarify the experimental basis for\nour conclusions regarding the electronic structure of the perovskite films and the evidence supporting the\ngraded homojunction formation. To address this point, we performed UPS measurements on four perovskite\nfilms specifically fabricated for this analysis, each with a reduced thickness of approximately 200 nm. This\nthickness is comparable to the lateral size of the MXene flakes (~200 nm) and was selected to ensure more\nuniform incorporation of the additives and to allow UPS signals to better reflect the bulk of the perovskite\nfilm, not just the topmost surface. In this way, the UPS measurements can provide useful inside on these four\ndistinct thin perovskite films, each designed to isolate the effect of individual and combined additive\ntreatments:\n1. REF: untreated reference perovskite;\n2. MX-Cl: perovskite containing Ti₃C₂Cl₂ MXenes;\n3. 4-FPEAI: perovskite post-treated with 4-FPEAI;\n4. MX-Cl / 4-FPEAI: perovskite containing both additives.\nTab. 2. Extracted values of work function (WF), valence band maximum (VBM) with respect to the Fermi level, and ionization energy\n(IE = WF + VBM) for the four analyzed perovskite film configurations, measured by UPS. The results demonstrate distinct electronic\neffects induced by MXene and 4-FPEAI treatments, supporting the formation of dipoles and doping gradients.\nSample WF (eV) VBM (eV) (IE) [eV]\n5.65\nREF 4.75 0.9\n5.45\nMX-Cl 4.55 0.9\n5.50\nMX-Cl / 4-FPEAI 4.70 0.8\n5.55\n4-FPEAI 4.85 0.7\nFrom these results, we observe two different behaviors:\n In the MX-Cl sample, the VBM position remains unchanged with respect to the REF sample (0.9 eV\nfrom the Fermi energy level-EF), while the WF decreases from 4.75 eV to 4.55 eV. This suggests the\nformation of a surface dipole at the interface, which modifies the vacuum level without altering the\nvalence band position—a signature of interface dipole formation. This is consistent with our DFT\ncalculation, which shows a vacuum level shift of -0.39 eV due to MXene-induced dipole formation.\n Conversely, in the 4-FPEAI sample, both the WF increases (to 4.85 eV) and the VBM shifts closer to E\nF\n(from 0.9 to 0.7 eV). This implies a more complex interaction, where not only vacuum level bending\noccurs, but also an actual shift in the electronic band structure, possibly due to surface passivation\nand local electronic doping. DFT results support this observation, indicating a +0.14 eV vacuum level\nshift with 4-FPEAI relative to the REF.\n The MX-Cl/4-FPEAI sample shows an intermediate behavior, further supporting the scenario of a\ngraded energy alignment across the film thickness, where the MXene dominates the bottom interface\nand 4-FPEAI modifies the top surface.\nThese findings strongly support the graded electronic structure scenario. The selective dipole formation and\nVBM modulation induced by the two additives at opposite sides of the perovskite layer result in a quasi-\nhomojunction or gradient doping profile, which facilitates carrier separation and extraction, thereby reducing\nrecombination.\nWe added the following text and table in the main text and in the experimental section, while we moved the\ndiscussion on Kelvin probe (KP) measurement in the SI.\nFrom SEM images of Fig. 3f, clearly MX-Cl flakes are mainly distributed at bottom interface between\nperovskite/mTiO layer. The presence of MX-Cl at the ETL/perovskite interface calls for a possible tuning of\n2\nthe buried perovskite interface WF.21\nBuilding on this, ultraviolet photoelectron spectroscopy (UPS), was employed to elucidate the progressive\nmodulation of the perovskite energy levels depending on the different treatments. Notably, the combination\nof MXene and 4-FPEAI induces a graded shift in work function and valence band position across the series,\nconsistent with the formation of a built-in electric field within the absorber. A full discussion and\ncorresponding UPS spectra and energy level diagram reconstructed from the UPS data are provided in section\nS.I. 6 in SI Fig. S11 and Fig. S12 respectively, while the extracted values of work function and valence band\nmaximum (VBM) are summarized in Tab. 2, clearly highlighting the energetic alignment changes induced by\neach modification.\nTab. 2. Extracted values of work function (WF), valence band maximum (VBM) with respect to the Fermi level, and ionization energy\n(IE = WF + VBM) for the four analyzed perovskite film configurations, measured by UPS. The results demonstrate distinct electronic\neffects induced by MXene and 4-FPEAI treatments, supporting the formation of dipoles and doping gradients.\nSample WF (eV) VBM (eV) (IE) [eV]\nREF\n4.75±0.1 0.9±0.1 5.65±0.1\nMX-Cl\n4.55±0.1 0.9±0.1 5.45±0.1\nMX-Cl / 4-FPEAI\n4.70±0.1 0.8±0.1 5.50±0.1\n4-FPEAI\n4.85±0.1 0.7±0.1 5.55±0.1\nTo confirm this effect, Kelvin probe force microscopy (KPFM) measurements were carried out on dedicated\nhalf-cell samples (FTO/G+cTiO₂/G+mTiO₂/perovskite), allowing us to extract WF values (Tab. S3) and to\ndistinguish the contributions of MXene doping and SGP (see Fig. S13)\nIn the experimental section we added:\nPhotoelectron spectroscopy measurements were performed in a ThermoScientific ESCALAB QXi XPS\napparatus in ultra-high vacuum conditions (10-10 mbar base pressure). The valence band region of the samples\nwas investigated by UPS using the 21.2 eV radiation of the HeI line and biasing the samples at -5 V, in order\nto determine both the work function and the position of the valence band edge. The electronic band structure\nof the perovskites was characterized by X-ray photoemission spectroscopy (XPS) using monochromatic 1486.8\neV photons of the Al Kα line and analysed with the Thermo-Fischer Avantage Software. XPS measurements\nwere performed on small devices (14x14 mm) applying the system charge compensation, while the work\nfunction was measured without charge compensation, by providing an additional contact to an edge of the\ndevice front surface.\nMoreover, the complete discussion on UPS results with the relative figures (Figs: S11 and S12) are reported\nin SI\nUltraviolet photoelectron spectroscopy (UPS) measurements\nTo gain insight into the energetic effects induced by the surface gradient passivation and MXene doping\nengineering strategies, we performed ultraviolet photoelectron spectroscopy (UPS) measurements on a set\nof ad-hoc fabricated perovskite thin films with reduced thickness (comparable to the MXene flake size, ~200\nnm) and varied interfacial treatments (REF, MX-Cl, 4-FPEAI, and MX-Cl/4-FPEAI). The extracted values of work\nfunction (WF) and valence band maximum (VBM) for each cell configuration are summarized in Tab. 2 of the\nmain text.\nAs shown in Fig. S12, the energy level diagram reconstructed from the UPS data (Fig. S11) clearly reveals a\nsystematic shift in both WF and VBM across the series.\nThe reference (REF) sample exhibits a WF of 4.75 eV and a VBM located 0.9 eV below the Fermi level (EF),\ncorresponding to an ionization energy (IE) of 5.65 eV. Upon incorporation of Ti₃C₂Cl₂ MXenes (MX-Cl), the WF\ndecreases to 4.55 eV while the VBM remains unchanged at 0.9 eV. This behavior indicates that the addition\nof MXenes enhances the n-type character of the perovskite, consistent with the formation of a surface dipole\nat the perovskite/MXene interface. Notably, this interface dipole shifts the vacuum level without affecting the\nperovskite band structure. Conversely, the use of 4-FPEAI as SGP induces a WF increase to 4.85 eV and a shift\nin the VBM to 0.7 eV below EF. These results suggest a clear modification of the perovskite electronic\nstructure, indicating a p-type doping effect, in addition to surface dipole formation. The combined MX-Cl/4-\nFPEAI treatment results in intermediate values (WF = 4.70 eV, VBM = 0.8 eV), consistent with a graded band\nalignment across the film thickness.\nThese findings are corroborated by DFT simulations, which show a vacuum level shift of -0.39 eV for the\nMXene interface and +0.14 eV for 4-FPEAI, supporting the observed WF modulation. Overall, the UPS data\nconfirm the formation of a vertical energy level gradient across the perovskite layer, driven by the two\ntreatments acting at opposite interfaces.\nFig. S11. UPS spectra for perovskite films modified with different interface strategies: reference (a–b), MX-Cl (c–d), MX-Cl/4-FPEAI (e–\nf), and 4-FPEAI (g–h). Panels (a, c, e, g) show the secondary electron cutoff region used to extract the WF, while panels (b, d, f, h)\ndisplay the valence band region used to determine the VBM with respect to the EF. A gradual modulation of the WF and VBM is\nobserved across the series, highlighting the impact of dipole formation and band structure tuning induced by MXene doping and\nsurface gradient passivation.\nFig. S12: Schematic energy level alignment of perovskite films extracted from UPS measurements for the four configurations: REF\n(untreated), MX-Cl, 4-FPEAI, and combined MX-Cl/4-FPEAI. The WF and VBM positions are plotted relative to the vacuum level. A\nprogressive modulation of the energy levels is observed, with MX-Cl inducing a downward shift in WF (consistent with dipole-induced\nvacuum level bending) and 4-FPEAI causing an upward shift in both WF and VBM. The intermediate alignment in the combined\ntreatment suggests the formation of a graded homojunction across the perovskite layer, promoting internal electric field formation\nand enhanced charge extraction.\nSecond, while the authors clearly advance device demonstration of halogen MXene doping for tuning\nperovskite interfaces, this concept has been reported before (e.g. interface calculations of Ti C Cl DOI:\n3 2 2\n10.1016/j.vacuum.2023.112381; MXene work function tuning for perovskite solar cells DOI:\n10.1039/D2NR02799B).\nWe thank the Reviewer for highlighting key prior work in the field, including our own earlier study (Titanium-\ncarbide MXenes for work function and interface engineering in perovskite solar cells). Indeed, that work\nintroduced the concept of interface dipole formation via MXenes and demonstrated that WF tuning without\nvalence band distortion could be achieved through selected surface terminations (T ), confirming that\nx\nMXenes can effectively modulate energy alignment at the perovskite/ETL interface. However, the current\nstudy builds significantly on that foundation and introduces several key innovations.\nUnlike our previous work that employed mixed-termination Ti₃C₂T MXenes, here we use MXenes synthesized\nx\nwith exclusively Cl terminations. This pure halogen termination introduces stronger and more defined\ninterfacial dipoles (confirmed by DFT with a -0.39 eV vacuum shift) and improves film crystallinity and defect\npassivation, as supported by SEM, XRD, and TRPL.\nMoreover, while both MXenes and 4-FPEAI have been studied individually in prior literature, their\nsimultaneous and spatially controlled incorporation is novel. In our architecture, MXenes accumulate at the\nbottom interface (near the ETL), while 4-FPEAI is applied at the top surface during anti-solvent dripping,\nresulting in a graded modulation of the perovskite energy landscape along its thickness.\nThis dual-modifier strategy leads to the formation of an internal n–p gradient across the perovskite layer, a\nconcept not previously demonstrated. UPS measurements clearly show a progression in WF and VBM values\nacross REF, MX-Cl, 4-FPEAI, and MX-Cl/4-FPEAI samples, consistent with a built-in electric field spanning from\nETL to HTL interfaces. This experimental validation of the internal band alignment further differentiates our\nwork from prior surface-only studies.\nLastly, beyond materials development, we implement this strategy in large-area modules and semi-\ntransparent tandem top cells, achieving over 16% PCE and high stability under outdoor conditions,\ndemonstrating its practical relevance and scalability.\nTaking together, these elements introduce a novel functional architecture that goes beyond the concepts\nexplored in our previous publication and distinguish this work from earlier MXene-related studies.\nFor greater clarity and to better highlight the novelty of our approach, we have added the following sentence\nto the main text of the manuscript:\nCrucially, this work introduces a vertically graded perovskite homojunction, enabled by the spatially resolved\nintegration of Cl-terminated MXenes and 4-FPEAI, a concept not previously demonstrated, representing a\nnew class of interface and energy landscape engineering in perovskite photovoltaics.\nThird, the reported tandem module efficiency of 19.45% is not above the efficiency of an average\nsingle junction silicon module, even though the tandem pairing resulted in a net 1.05% absolute\nefficiency increase over the Si HJT cells used in this study and the reported field stability is not at the\nstate-of-the-art of other public reports for perovskite-based tandem modules\n(https://urldefense.com/v3/__https://pvpact.sandia.gov/results-and-\ndata/__;!!O5Bi4QcV!AhStXVfRb4M0c2PywTXMfB_lGLzEuLZkOnh8pur96Sf74vNpdeoJ0-\n16p1095RgyfvE6JrfjxE842t00SPCy7Xv3OcM$ ).I have included additional comments below but\nunfortunately cannot recommend publication of this manuscript in nature communications in its\ncurrent form.\nWe believe that the main message of our paper does not consist in claiming record efficiency. Such claims\nshould be supported by certified efficiency measurements (as those indicated by the Reviewer). Instead, our\nstudy aims to introduce a novel design concept for semi-transparent perovskite top cells in 4-terminal\nperovskite/Si tandem architectures.\nSpecifically, this study advances the state of the art in interface and energy landscape engineering by\nintroducing a vertically graded homojunction within the perovskite absorber, realized through the spatially\nselective integration of chlorine-terminated MXenes and 4-FPEAI. This synergistic combination enables\nsimultaneous work function modulation and defect passivation across the perovskite thickness, establishing\nan internal electric field that promotes efficient charge extraction and suppressed recombination.\nFollowing your suggestion, to further emphasize this point we added the corresponding statement to the\nmain text of the revised manuscript, as detailed in the previous point.\nWe would like also to underline to the reviewer that the bifacial Si-HJT technology used in this work is a state\nof art silicon technology, produced in the industrial facilities of 3SUN company, located in Catania, south of\nItaly. 3SUN, is the largest high-performance photovoltaic cell and module production factory in Europe, with\na production up to 3GW of photovoltaic module per year based on silicon heterojunction (HJT) technology\n(Investment of 1B€ of which around 200 M€ from EU and national funding; 150,000 m2 facility). 3SUN is\nworking on the next perovskite/Si-HJT technology to reach more than 30% efficiency over the next few years\nand thanks to the demonstration of potentialities carried out during our joint work, the fab hosts now a pilot\nline for 2D material-based perovskite/silicon tandem panels, aiming to launch tandem panels to the market\nin the next coming years (see in Fig. R1 reported below the roadmap released by 3Sun at\nhttps://www.enelgreenpower.com/media/news/2022/09/3sun-gigafactory-evolution-solar-technologies).\nFig.R1: Roadmap released by 3Sun at https://www.enelgreenpower.com/media/news/2022/09/3sun-gigafactory-evolution-solar-\ntechnologies, clearly including the PSK/Si technology as expected to be fully up and running at the 3Sun Gigafactory in 2025, promises\nto further boost the efficiency of photovoltaic modules up to over 30%, while also aiming to ensure a life span of at least 35 years.\nWe would kindly like to clarify a possible misunderstanding regarding the absolute efficiency gain\ndemonstrated by our “lab-fabricated” tandem panel. In the manuscript, we report a PCE of 19.45% for the\ntandem panel, compared to 16.7% for the manually stringed silicon bottom cell, resulting in an absolute\nimprovement of approximately 2.74 percentage points. It seems that the reviewer may have instead\ncompared our tandem panel to an industrially fabricated silicon cell (PCE = 18.4%), leading to a reported\ndifference of 1.05 percentage points.\nHowever, this comparison may not reflect the actual experimental setup used in our work. This point was\nalready addressed in the submitted version of the manuscript, with further clarification provided in the\nSupporting Information, Section S.I. 13. For your convenience, we report below the relevant paragraph where\nthis aspect is discussed in more detail.\nMAIN TEXT: “We should point out that, due to the manual stringing (electrical connection among adjacent\ncells), the bottom Si-HJT module have a reduce efficiency compared to those obtained on an automatized\nindustrial line (see SI Section 13).”\nSI: “In this way, we fabricated a Si-HJT module having the same electrical connections and same lamination\nprocess as those employed for Si bottom module composing the tandem all-in-one DEM2. In the as-realized\nDEM3, the cell stringing process is manually realized using 6 mm width charger collector tapes from 3M.\nConversely, in an industrially relevant production environment, Si-HJT cells are usually stringed and laminated\nusing industrial tools (such as an industrial stringing machine), material specifically design for Si-HJT\nlamination (such as external glass texturized 3 mm thick, polyolefin foil) and higher temperature hot vacuum\nlamination procedure (performed at 180°C). All these aspects strongly affect the final Si-HJT module\nperformance. Indeed, when considering the lab-produced module (DEM3) it showed maximum PCE of 16.7%\nonce tested at STC while the Si-HJT realized in the 3SUN industrial facilities showed PCE approaching 18.5%\n(see Tab. S5). The main PCE losses at 1SUN conditions in the case of lab-made DEM3 have to be imputed to\nseries path introduced during the manual cell stringing procedure, eventually penalizing FF and I .\nSC\nConversely, when the irradiance is set to 400 W/m2 (nearby the irradiance experienced from a filtered Si-HJT\nmodule in a tandem panel) the FF gap between the lab-made DEM3 and the 3SUN demonstrator is strongly\nreduced with respect to the case of 1 SUN irradiation, leading to a PCE gap of around 1 percentage point.\nIndeed, the lower generated current under reduced irradiance level minimizes the impact of the series\nresistive path introduced in case of a manual cell stringing process, leading to a more comparable\nperformance among the lab and industry made Si-HJT module.\nTab. S5: Electrical parameters for both lab-made and 3SUN made Si-HJT module (4 series connected cells) in case of\n1SUN and 400 W/m2 irradiance level.\nModule (4 cells) V (V) I (A) FF (%) PCE (%)\noc sc\nSi-HJT (lab-made) 2.86 8.18 57.57 16.71\n@ STC\nSi-HJT (3SUN) 2.81 9.16 74.8 18.4\n@ STC\nSi-HJT (lab-made) 2.76 3.26 69.8 7.29\n@ 400 W/m2\nSi-HJT (3SUN) 2.81 3.66 77.5 8.33\n@ 400 W/m2\nThus, for a fair comparison between the proposed 2D materials-engineered tandem panel (DEM2) and the\nSi-HJT based single junction technology, DEM2 and DEM3 should be compared. From Tab. S5, we can estimate\nan overall PCE increase of 2.74 percentage points when moving from the Si-HJT to tandem technology.”\nAdditional comments:\n1) The authors report UPS and Kelvin probe CPD data on Ti C Cl MXenes in Fig S6 and additional CPD\n3 2 2\nmeasurements in Fig S13 and half-stacks in Tab. 2. These experiments appear to be film surface\nmeasurements but not a depth profile of an actual perovskite film. As far as I can tell, the perovskite\nwork function and band structure primarily come from DFT simulations and TiberCAD simulations in\nFigs S15-S17. A work function changes due to doping of a contact layer can induce an electric field at\na contact interface, but that is insufficient to claim the formation of a graded homojunction. Is there\nany experimental evidence to suggest that there are doping changes within the perovskite layer as a\nresult of the new contact strategy that aids in the suppression of non-radiative recombination and\ncarrier collection?\nWe sincerely thank the Reviewer for the thoughtful comment. We acknowledge that in the original\nsubmission, the interpretation regarding the formation of a graded homojunction was mainly based on CPD\ntrends and simulations, which alone were not sufficient to provide direct experimental evidence of energy\nlevel modulation within the perovskite bulk. In response to the Reviewer’s suggestion, we have now included\nnew Ultraviolet Photoelectron Spectroscopy (UPS) measurements (see Tab. 2 and Supporting Figs. S11 and\nS12), carried out on a series of carefully designed perovskite films incorporating individual and combined\nsurface/contact passivation strategies (MX-Cl and/or 4-FPEAI). These films were realized with reduced\nthickness to ensure that the UPS signal was representative of the electronic structure of the perovskite\nabsorber and sensitive to vertical compositional and electronic modulation induced by the additive\ndistribution. The UPS results show a systematic and progressive shift of both the work function and the\nvalence band maximum (VBM) across the sample series, with MX-Cl leading to a downward vacuum level shift\n(dipole formation and increased n-type character), while 4-FPEAI induces a larger upward shift and clear band\nbending (toward p-type). Notably, the sample with both MX-Cl and 4-FPEAI exhibits intermediate energy\nlevels, suggesting the formation of a graded band structure consistent with a vertical n-to-p transition within\nthe perovskite. These experimental observations, now integrated into the main text and SI, support our\nhypothesis of a graded homojunction induced by the spatial distribution of the interfacial modifiers, which is\nalso corroborated by the enhanced photovoltaic performance and reduced non-radiative recombination\nlosses. In the revised form of our article, we reported these achievements as detailed in the answer to your\nabove “first” comment.\n2) The authors should be careful about making claims of grain sizes based on SEM methods. It has been\nshown in the literature that these techniques cannot identify true grain size or grain boundaries (DOI:\n10.1021/acsenergylett.8b01704, DOI:10.1016/j.joule.2018.12.011). It is reasonable to say “apparent\ngrains”, “perovskite domain”, or to describe the “morphology”, but this should be taken into\nconsideration when attributing film properties to SEM images.\nThank you for pointing this out. We have revised the text in the main text and Supporting Info to avoid\noverinterpreting SEM images and now refer to 'perovskite domains' as reported in the following changed\nversion:\n“SGP has a minimal impact on grain size and surface morphology (Fig. S8), whereas MX-Cl doping, resulted in\na noticeable increase in perovskite domains compared to the pristine layer (Fig. 3a and 3c). The presence of\nsmall domains around the enlarged ones in the MX-Cl-doped film indicated that MXene flakes locally\ninfluenced perovskite morphology.”\nAnd in SI:\n“On the other side, MX-doping is strongly effective in reducing the defect states density thanks to an\nenlargement in perovskite domains where MXenes are present, by reducing the impact of the grain\nboundaries that are well known to insert deep trap state levels in the perovskite gap.”\n3) The authors claim “Moreover, the hydrophobic nature of MX-Cl further boosts both device efficiency\nand operational stability, as confirmed by Maximum Power Point (MPP) tracking of the best-\nperforming cell (Fig. S12).” However, figure S12 only shows 180s of maximum power point data. 180s\nis not sufficient to claim operational stability and does not compare against control. Furthermore, I\ndo not follow the reasoning as to how the MPP data is related to the hydrophobic nature of MX-Cl.\nWe fully agree with the concern raised by the reviewer about the need to further support our claim on\nimproved operational stability of our graded homojunction cell against control. On this basis, following what\nwas recently suggested by authoritative works in the literature17,18, we performed light-cycling tests that are\nnow inserted and commented in SI, Fig. S22c, as follows reported:\nIndoor photo-stability under MPP tracking of semi-transparent encapsulated cells upon exposure to cycled\nlight. As recently discussed in literature, the most effective way to perform a reliable stability analysis consists\nin light cycling test at constant temperature, representing a good trade-off between simplicity of execution in\nlab conditions while taking into account phenomena observed only in case of outdoor testing (that cannot be\nconsidered in case of constant light experiments).15 Indeed, transient behavior occurs in PSCs as a result of\nthe coexistence of several dynamics with characteristic times spanning from time scales of seconds to hours.\nThus, day–night cycling of devices can consider such slow dynamics as they occur on similar timescales.\nFollowing this idea, we performed a light–dark cycling test as recommended by the ISOS-LC-1 protocol,\nexposing the cells to simulated sunlight turned on and off with cycle periods of 24 h and duty cycles\n(light:dark) of 1:2, mimic an accelerated diurnal sun cycle (the solar cell is maintained at ambient conditions\nwhile the temperature is monitored and RH is maintained at 30%).16 The recorded maximum power (P ) is\nMPP\nreported in Fig. S7e for both G/EA and fully optimized G/MX/4-FPEAI semi-transparent cells, normalized at\nthe initial value. The combined stabilization effect of the 4-FPEAI based SGP together with the presence of\nMX-Cl results in a T =600 hours vs a T =574 hours of the untreated device, demonstrating the superior\n91 80\nstability (T 6.5 times larger) of the device employing the proposed graded homojunction approach. Despite\n91\nmore in depth analyses are required to elucidate the role of MX-Cl and 4-FPEAI SGP, we can confidently ascribe\nthe improved stability to a synergistic effect of the two treatments. On one side, the addition of MX-Cl resulted\nin a perovskite film with improved crystallinity eventually reducing lattice distortion, lowering defect density\nand minimizing trap states, as demonstrated by XRD, transient PL and SEM characterizations. This, combined\nwith the enlarged perovskite domains and mitigated grain boundary-induced trap states, makes the trapped\ncharge driven degradation less effective,17 resulting in an overall enlargement of device lifetime. We cannot\nexclude a side role of the Cl in reducing the tensile strain within the perovskite layer, recently pointed out as\none of the main degradation channel when light-dark cycling is applied.18 On the other side, the\nphenylalkylammonium compounds as 4-FPEAI has been proven to stabilize the perovskite through\nsuppression of iodide ion migration, likely responsible for metastable perovskite behavior under light cycling\nobserved for the G/EA device, while passivating the surface defects.19 In addition, the 4-FPEAI SGP strategy\nhas been already proven to improve the long-term operational stability of the encapsulated devices under\nthe continuous illumination in ambient condition.12 Thus, the superior light cycling stability showed by the\ngraded homojunction cell can be confidently ascribed to a combined stabilizing effect from 4-FPEAI and MX-\nCl, definitively reducing the device metastable behavior.\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1).\n(12) Yan, N.; Gao, Y.; Yang, J.; Fang, Z.; Feng, J.; Wu, X.; Chen, T.; Liu, S. F. Wide-Bandgap Perovskite Solar Cell Using a Fluoride-\nAssisted Surface Gradient Passivation Strategy. Angew. Chem. Int. Ed. 2023, 62, e202216668.\nhttps://doi.org/10.1002/anie.202216668.\n(15) Khenkin, M.; Köbler, H.; Remec, M.; Roy, R.; Erdil, U.; Li, J.; Phung, N.; Adwan, G.; Paramasivam, G.; Emery, Q.; et al. Light\nCycling as a Key to Understanding the Outdoor Behaviour of Perovskite Solar Cells. Energy Environ. Sci. 2023, 17 (2), 602–\n610. https://doi.org/10.1039/d3ee03508e.\n(16) Khenkin, M. V.; Katz, E. A.; Abate, A.; Bardizza, G.; Berry, J. J.; Brabec, C.; Brunetti, F.; Bulović, V.; Burlingame, Q.; Di Carlo,\nA.; et al. Consensus Statement for Stability Assessment and Reporting for Perovskite Photovoltaics Based on ISOS\nProcedures. Nat. Energy 2020, 5 (1), 35–49. https://doi.org/10.1038/s41560-019-0529-5.\n(17) Ahn, N.; Kwak, K.; Jang, M. S.; Yoon, H.; Lee, B. Y.; Lee, J.; Pikhitsa, P. V; Byun, J.; Choi, M. Trapped Charge-Driven\nDegradation of Perovskite Solar Cells. Nat. Commun. 2016, 8 (May), 1–9. https://doi.org/10.1038/ncomms13422.\n(18) Shen, Y.; Zhang, T.; Xu, G.; Steele, J. A.; Chen, X.; Chen, W.; Zheng, G.; Li, J.; Guo, B.; Yang, H.; et al. Strain Regulation\nRetards Natural Operation Decay of Perovskite Solar Cells. Nature 2024, 635 (8040), 882–889.\nhttps://doi.org/10.1038/s41586-024-08161-x.\n(19) Guo, Y.; Apergi, S.; Li, N.; Chen, M.; Yin, C.; Yuan, Z.; Gao, F.; Xie, F.; Brocks, G.; Tao, S.; et al. Phenylalkylammonium\nPassivation Enables Perovskite Light Emitting Diodes with Record High-Radiance Operational Lifetime: The Chain Length\nMatters. Nat. Commun. 2021, 12 (1). https://doi.org/10.1038/s41467-021-20970-6.\nWe also modified the main text by briefly recalling the abovementioned stability analysis as in the following:\n“Moreover, the combination among the MX-Cl addition and the 4-FPEAI SGP strategy not only translated into\nsuperior device performance but also conferred superior robustness to the perovskite film against light-driven\ndegradation, as demonstrated by prolonged light cycling test (Fig. S22c).”\nFollowing the reviewer suggestion, we removed from the main text the sentence related to the superior\nstability of the graded homojunction due to the hydrophobicity of the employed MXenes, since it could be\nmisleading. Indeed, the hydrophobicity of the chlorine-based MXenes can have a role only in slowing down\nthe mTiO /perovskite interface degradation, since they are sitting at this device interface, as demonstrated\n2\nby the cross-sectional SEM images reported in Fig. 3d. In the revised version of the SI we discussed more in\ndetail the possible stabilization effects of MX-Cl once the graded homojunction undergoes prolonged light-\ncycling stress test. Moreover, as suggested by the referee we also modified the normalized MPP tracking by\nadding the same plot for the reference device and following reported\nFig. S7: Density of Current-Voltage (J-V) characteristics, acquired in forward and reverse voltage scan directions for the best efficient\na) G/CB, b) G/EA, c) G/4-FPEAI, d) G/MX/4-FPEAI cells. e) Normalized Maximum Power Point (MPP) tracking for the best-performing\nopaque cell employing the 2D material-engineered structure (G/MX/4-FPEAI) vs reference structure (G/EA).\n4) In the SI, it is claimed that drawing from Table S5, the 4T tandem pairing resulted in a 2.74% PCE\nimprovement over the silicon HJT module alone. However, it appears that DEM2 is a 19.45% 4T\nmodule fabricated with 3SUN. HJT silicon (18.4%; Table S5). Wouldn’t this indicate an efficiency\nimprovement of 1.05% over the single junction silicon?\nWe would like to point out to the reviewer that absolute efficiency improvement demonstrated by our “lab-\nfabricated” tandem panel (showing a PCE of 19.45%) is calculated versus the “manually stringed” silicon\nbottom module (showing a PCE of 16.7%), which is around 2.74 percentage points (and not 1.05 percentage\npoints, obtained by comparing the tandem panel realized in our research lab with the industrially made silicon\nbottom module showing PCE=18.4%). This aspect has been discussed in the original version of the manuscript\nwith a more in depth discussion reported in SI. We are now reporting in the following for your convenience\nthe paragraph where this aspect is discussed.\nMAIN TEXT: “We should point out that, due to the manual stringing (electrical connection among adjacent\ncells), the bottom Si-HJT module have a reduce efficiency compared to those obtained on an automatized\nindustrial line (SI, S.I. Section 13).”\nSI: “In this way, we fabricated a Si-HJT module having the same electrical connections and same lamination\nprocess as those employed for Si bottom module composing the tandem all-in-one DEM2. In the as-realized\nDEM3, the cell stringing process is manually realized using 6 mm width charger collector tapes from 3M.\nConversely, in an industrially relevant production environment, Si-HJT cells are usually stringed and laminated\nusing industrial tools (such as an industrial stringing machine), material specifically design for Si-HJT\nlamination (such as external glass texturized 3 mm thick, polyolefin foil) and higher temperature hot vacuum\nlamination procedure (performed at 180°C). All these aspects strongly affect the final Si-HJT module\nperformance. Indeed, when considering the lab-produced module (DEM3) it showed maximum PCE of 16.7%\nonce tested at STC while the Si-HJT realized in the 3SUN industrial facilities showed PCE approaching 18.5%\n(see Tab. S5). The main PCE losses at 1SUN conditions in the case of lab-made DEM3 have to be imputed to\nseries path introduced during the manual cell stringing procedure, eventually penalizing FF and I .\nSC\nConversely, when the irradiance is set to 400 W/m2 (nearby the irradiance experienced from a filtered Si-HJT\nmodule in a tandem panel) the FF gap between the lab-made DEM3 and the 3SUN demonstrator is strongly\nreduced with respect to the case of 1 SUN irradiation, leading to a PCE gap of around 1 percentage point.\nIndeed, the lower generated current under reduced irradiance level minimizes the impact of the series\nresistive path introduced in case of a manual cell stringing process, leading to a more comparable\nperformance among the lab and industry made Si-HJT module.\nTab. S5: Electrical parameters for both lab-made and 3SUN made Si-HJT module (4 series connected cells) in case of\n1SUN and 400 W/m2 irradiance level.\nModule (4 cells) V (V) I (A) FF (%) PCE (%)\noc sc\nSi-HJT (lab-made) 2.86 8.18 57.57 16.71\n@ STC\nSi-HJT (3SUN) 2.81 9.16 74.8 18.4\n@ STC\nSi-HJT (lab-made) 2.76 3.26 69.8 7.29\n@ 400 W/m2\nSi-HJT (3SUN) 2.81 3.66 77.5 8.33\n@ 400 W/m2\nThus, for a fair comparison between the proposed 2D materials-engineered tandem panel (DEM2) and the\nSi-HJT based single junction technology, DEM2 and DEM3 should be compared. From Tab. S5, we can estimate\nan overall PCE increase of 2.74 percentage points when moving from the Si-HJT to tandem technology.”\n5) The authors should provide accelerated stability data (with at a minimum light + heat, preferably 85C\nor higher) for the 4F-PEAI/MX-Cl architecture vs. control.\nWe fully agree with the concern raised by the reviewer about the need to further support our claim on\nimproved operational stability of our graded homojunction cell against control. On this basis, following what\nrecently suggested by authoritative works in the literature17,18, we performed light-cycling tests that are now\ninserted and commented in SI, Fig. S22c, reported in this document as answer to question 3.\n6) The authors should consider extending the outdoor field test and providing a post-mortem\ncharacterization of the fielded module to identify apparent degradation modes of this architecture.\nWe thank the reviewer for this thoughtful and insightful suggestion. We fully agree that post-mortem\ncharacterization of the fielded module could provide valuable information on the degradation mechanisms\nspecific to this architecture. However, such an in-depth analysis falls outside the scope and objectives of the\npresent work, which focuses primarily on the implementation and initial outdoor validation of the graded\nhomojunction strategy in 4T tandem modules. Given the substantial amount of experimental work and data\nanalysis required, we believe that a comprehensive post-mortem study would be more appropriately\naddressed in a dedicated follow-up publication. A similar approach was taken in our previous work on all-\nperovskite solar farms, where degradation analysis was reported in a separate study [E. Spiliarotis et al., 2025,\nEES Solar, DOI: 10.1039/d5el00042d]. Nonetheless, we appreciate the reviewer’s comment and have included\na note in the Conclusions to acknowledge this as an important direction for future research.\n“Post-operation diagnostic analysis of field-deployed modules could offer valuable insight into the intrinsic\ndegradation pathways and stability limitations of the proposed device architecture.”\nReviewer #4 (Remarks to the Author):\nIn this manuscript, the authors demonstrate a synergistic approach of doping a two-dimensional material,\nMXene Ti C Cl , in the perovskite absorber layer and adding 4-FPEAI to the antisolvent in anticipation of\n3 2 2\nforming a n-p junction, to enhance the performance of perovskite solar cells. And the method is extended to\nsemi-transparent cells and four-terminal perovskite/silicon tandem devices. Finally, PCE of 19.45% was\nachieved on a 0.2 m2 4T tandem device and field tested for stability. However, I don't see much correlation\nwith the methodology described in this manuscript in the section of the tandem devices, and it doesn't\nadequately describe the characterization of tandem devices.\nWe thank the reviewer for the insightful comments. We respectfully address the two main points raised:\n1. On the correlation between the methodology and the tandem device section:\nWe would like to clarify that the same graded homojunction engineering strategy — combining MXene doping\nand 4-FPEAI-based surface gradient passivation — was employed in both the small-area opaque perovskite\nsolar cells and the semi-transparent top cells used in the four-terminal (4T) tandem architecture. This is\nexplicitly stated in the Results section and detailed in the Supporting Information (Section 10). The improved\nperformance of the ST-PSMs and the resulting 4T tandem modules confirms the scalability and transferability\nof the strategy. Nevertheless, we have revised the manuscript to better highlight the methodological\ncontinuity between the cell-level and tandem-level implementations. The modified text is following reported:\n“By employing the same 2D material-engineering strategies, MXene doping and 4-FPEAI-based surface\ngradient passivation, previously validated in opaque devices, the semi-transparent perovskite top cells used\nin the 4T tandem architecture demonstrated enhanced PCEs up to 18.3% (av. PCE = 17.63%) due to a higher\nV (av. V = 1.2 V) and J (av. J = 19.92 mA/cm²) compared to the REF sample (see Tab. S6, Figs. S19–22a,\nOC OC SC SC\nS20 and S21 for a detailed analysis), confirming the effectiveness of 2D material-engineering approach even\nfor semi-transparent structures.”\nand\n“This confirms the scalability and effectiveness of the approach at module level.”\nMoreover, we carried out a detailed analysis onto the performance losses occurring moving from small-area\nopaque cells to large-area ST modules, highlighting the crucial role our graded homojuction strategy had in\nlimiting the scaling losses. We underlined this aspect in section S.I: 11, as follows reported:\nCell-to-module performance gap analysis.\nTo clarify the link between small-area opaque devices and the performance of large-area semi-transparent\n(ST) modules, we carried out a loss analysis identifying the main contributing factors. The best-performing\nopaque device (G/MX/4-FPEAI) exhibited a PCE of 22.12%, while the corresponding 60 cm² ST module\nreached 16.2%, resulting in an absolute performance drop of ~5.96 percentage points.\nThis gap can be attributed to three main sources:\n(i) Optical losses due to the absence of the reflective metal electrode in ST configurations, which reduces\nphoton recycling and light harvesting. To quantify the effect of ST electrode on photovoltaic performance, we\ncompared our optimized opaque device (G/MX/4-FPEAI) with its ST counterpart featuring an Au/ITO top\nelectrode. The opaque device exhibited a PCE of 22.12%, while the ST cell reached 18.3%, under identical\nillumination and processing conditions (see Tab. S7). This ~17.2 % relative efficiency drop (≈3.82 absolute\npercentage points) can be attributed primarily to optical losses due to the removal of the reflective metal\nelectrode, which otherwise enhances internal light trapping and photon recycling. With the aim to quantify\nthe impact of the ST electrode onto the device photocurrent, we performed IPCE measurements on both the\nbest performing opaque and semi-transparent cell, engineered with graded homojunction strategy. By\ncomputing the integrated current density, we can conclude that the replacement of opaque electrode with a\nST one (gold/ITO) contributed to a J loss of ~2.0 mA/cm². This is in line with what observed by De Wolf, S.\nSC\nand co-workers,20,21 which employed optical modeling (Transfer Matrix Method (TMM)) of the device stack to\npredict a 10–12% of incident photons loss due to front-surface reflection and incomplete absorption in the\nperovskite because of the reduced optical path length. These factors jointly reduce the short-circuit current\ndensity (J ) by ~ 1.8–2.2 mA/cm². In our case the reflection at the glass substrate is the same for both the ST\nSC\nand opaque devices, concluding that the observed J drop can be attributed to the absence of photon\nSC\nrecycling in the 580-730 nm region of the visible spectra.\n(ii) Electrical losses from the transparent electrode stack (Au/ITO), including increased sheet resistance and\ncontact resistance, resulting in an FF drop of ~6.32%. Nonetheless, the retained high V (~1.22 V) and FF\nOC\n(~75%) in the ST cell confirm that our band alignment strategy remains effective under reduced optical\nconfinement.\n(iii) Scaling-related effects, including increased surface non-uniformity, edge shunting, and series resistance\ndue to larger interconnection paths, contributing to a further loss in FF of -7.5% moving from the ST-cell to\nthe ST-module. FF loss is the main responsible for the observed drop in PCE moving from ST-cell to ST-\nmodules, since V and J for each large area cells (LAC, with an active area of 2.5 cm2) composing the module\nOC SC\ndid not undergo a significant drop (V ~ 1.25 V and J ~ 20.8 mA/cm2, see Tab. S7)\nOC_LAC SC_LAC\nDespite these inevitable losses, the use of a vertically graded homojunction via Cl-MXene doping and 4-FPEAI\npassivation proved critical in maintaining high V (1.226 V) and FF (67-69%) at the module level. This\nOC\nhighlights the effectiveness of the interface and band alignment engineering in mitigating efficiency\ndegradation upon scaling and semi-transparency.\nTab. S7:Electrical parameters for best performing opaque cell, ST cell and ST module.\nFF (%)\nDevice Type Area (cm²) PCE (%) V (V) J (mA/cm²)\nOC SC\nOpaque cell\n0.1 22.12 1.21 22.5 81.82\n(G/MX/4-FPEAI)\n75.5\nST cell (Au/ITO) 0.1 18.3 1.22 20.5\nFF (%)\nDevice Type Area (cm²) PCE (%) V (V) J (mA/cm²)\nOC SC\nFF (%)\nArea (cm²) PCE (%) V (V) I (mA)\nOC SC\n67.9\nST module 60.0 16.16 26.98 52.7\nWe also added the following statement in the main text:\n“while strongly limiting the PCE drop (<13%) usually observed in scaling the device size (see in S.I. section S.I.\n11 and Tab. S7 for detailed discussion).”\n2. On the adequacy of tandem device characterization:\nWe acknowledge that a more detailed description of the tandem characterization could further strengthen\nthe manuscript. We note, however, that the characterization of the semi-transparent top perovskite module\nand the filtered Si-HJT bottom cell, including IV curves, optical filtering effects, and bifacial power generation\ndensity (PGD), are provided in both the main text (Fig. 6 and Fig. 7) and the Supporting Information (Sections\n12–14). To address the reviewer’s concern, we have further clarified the methodology used to evaluate the\noutdoor performance and stability and explicitly referred to the relevant SI sections in the revised manuscript.\nIn the following the relevant changes we made following the reviewer’s comments:\nMAIN TEXT: Outdoor measurements performed using a metallic mounting base were carried out with the\nmodule installed at a fixed inclination of 30° and a height of approximately 100 cm above ground (see Fig.\n7g). From the outdoor performance monitoring at open circuit conditions, the I-V curves for top ST-PSP were\nacquired during a 3-months period (from August to November 2023) and the main electrical parameters were\nnormalized in STC (1000 W/m2 irradiance and 25°C temperature) considering both the temperature and\nirradiance dependency of the main electrical parameters (SI section 14).\nSI: The prolonged outdoor testing was performed on DEM2 panel at open circuit condition, following the\nISOS-O-2 protocol. Thus, the panel was not “continuously monitored” in a strict sense, but was left mounted\non the rack in the operative position at open-circuit and measured once per month. The parameters reported\nin Fig.7f were extracted from the I-V curves, acquired for the perovskite top panel after a stabilization time of\n10 minutes, since we observed an increase in the delivered power till reaching a stabilized value after 10\nminutes (see Fig. S29a). Since the outdoor characterizations are typically subjected to the instantaneous\nvariation of the irradiance and the panel temperature, we selected a clear sky day in the first half of the month\nperforming the measurements when the irradiance was supposed to be maximum (at 12:30 am). The\nirradiance daily curves for Hellenic Mediterranean University site in Heraklion (35.31959; 25.10244) recorded\nduring the day when the outdoor I-V curve were acquired, are reported in Fig. S29b. Once acquired the I-V\ncurves, the extracted electrical parameters were normalized in STC (1000 W/m2 irradiance and 25°C\ntemperature) considering i) the panel temperature and the irradiance values recorded at the measurement\ntime (at 12:30 am, see values reported in Tab. S10) ii) both temperature and irradiance dependency of the\nmain electrical parameters reported in Fig. S28.\nFig. S29: a) Normalized power at maximum power point (PMPP) stabilization prior the acquisition of the monthly I-V curves for the\nDEM2 left at V in outdoor conditions; b) daily irradiance for the days selected during the outdoor test for the acquisition of the I-V\nOC\ncurves.\nIn addition, there are some problems with the article as follows, so I don't recommend this manuscript\nfor publication in Nature Communications.\nQ1) Researchers have already revealed that 4-FPEAI and MXene play important roles in improving the\nperformance of perovskite solar cells. The highlights and novelty of this work should be highlighted.\nWe thank the reviewer for this important observation. We fully agree that both 4-FPEAI and MXene materials\nhave been individually explored in previous studies and shown to improve the performance of perovskite\nsolar cells through passivation and work function tuning, respectively.\nHowever, to the best of our knowledge, this is the first work that strategically combines both 4-FPEAI and\nMXene within the same perovskite absorber to intentionally create a graded homojunction with a built-in n-\nto-p electronic gradient. This approach allows for spatial modulation of the energetic landscape across the\nabsorber thickness, thereby reducing recombination and enhancing charge transport — a mechanism that\ngoes beyond the individual contributions of each additive.\nWe have now revised the manuscript to more clearly emphasize this point, both in the Introduction and the\nConclusions, and to better articulate how our integration of materials and device architecture represents a\nstep forward in perovskite solar cell engineering.\n“Here we propose a novel approach to fabricate graded perovskite homojunctions from an n-type character\nat the buried perovskite layer, induced by chlorine-based MXene doping,2 to a p-type character at the surface,\nenhanced by 4-FPEAI based SGP. This intentional spatial gradient forms a built-in n-to-p homojunction across\nthe absorber, enabling more efficient carrier extraction and reduced recombination.”\n“Crucially, this work introduces a vertically graded perovskite homojunction, enabled by the spatially resolved\nintegration of Cl-terminated MXenes and 4-FPEAI, a concept not previously demonstrated, representing a\nnew class of interface and energy landscape engineering in perovskite photovoltaics.”\nQ2) In Fig. 2 and Fig.S19, the statistics of the electrical parameters are all obtained from 8 samples,\nwhich is not a sufficient amount of data to support the experimental conclusions. For example, in lines\n“...and exhibiting reduced PCE dispersion “ and lines 123 “... Improvements highlights enhance device\nreproducibility ...”, 8 samples are clearly not enough to account for these.\nWe appreciate the reviewer’s insightful comment regarding the statistical validity of our data set. We agree\nthat a larger number of samples would provide stronger statistical power to support claims related to device\nreproducibility and PCE dispersion. Our initial set of 8 devices was chosen based on the typical standard in\npreliminary studies of this nature, aiming to balance experimental throughput with material and time\nconstraints. Nevertheless, we expanded our dataset to twelve samples and included these extended statistics\nin the revised manuscript accordingly by substituting the figures Fig. 2 and Fig. S19 and Tab. 1 and Tab. S6\nrelated to opaque and semitransparent devices respectively to avoid any overgeneralized conclusions.\nMoreover, we have changed in the entire manuscript and SI the discussion on the results obtained\naccordingly.\nIn the main text we changed the following Fig. 2, Tab.1\nFig. 2: Electrical parameter statistics (12 samples) a) open circuit voltage (V ); b) short circuit current density (J ); c) fill factor (FF);\nOC SC\nd) power conversion efficiency (PCE), for the four investigated opaque PSC structures extracted by the current-voltage (I-V)\ncharacteristics acquired under 1 SUN irradiation. Forward and reverse scan J–V curves for all four opaque device architectures are\nprovided in the Supporting Information (Fig. S7 a), b), c), d)). The minimized hysteresis in the G/MX/4-FPEAI device highlights the\nimproved stability and reduced interfacial recombination enabled by the device engineering via graded homojunction strategy.\nTab. 1: Electrical photovoltaic parameters for the four investigated opaque PSCs structures extracted by the I-V characteristics\nacquired under 1 SUN irradiation in reverse scan reported as averaged values ±standard error obtained on 12 samples for each cell\ntypology and for the best performing devices.\nDevice V (V) J (mA/cm2) FF(%) PCE(%)\nOC SC\nCB Champion 1.17 20.55 79.16 18.87\nAverage 1.166±0.001 20.22±0.08 78.53±0.11 18.51±0.07\nEA Champion 1.176 20.79 79.76 19.19\nAverage 1.170±0.001 20.36±0.08 79.23±0.09 18.87±0.08\n4-FPEAI Champion 1.193 21.30 80.05 20.18\nAverage 1.190±0.0006 21.03±0.04 79.78±0.09 19.95±0.04\nMX_4-FPEAI Champion 1.205 22.5 81.82 22.12\nAverage 1.197±0.001 22.05±0.09 80.86±0.19 21.34±0.13\nWhile in the supporting Info the following Fig. S19, Tab. S6:\nFig. S19: Electrical parameter statistics (12 samples) for the four investigated ST-PSC structures extracted by the current-voltage (I-V)\ncharacteristics acquired under 1 SUN irradiation.\nTab. S6: Electrical photovoltaic parameters for the four investigated ST-PSCs structures extracted by the I-V characteristics acquired\nunder 1 SUN irradiation in reverse scan reported as averaged values ± standard error obtained on 12 samples for each cell typology\nand for the best performing devices.\nDevice V (V) J (mA/cm2) FF(%) PCE(%)\nOC SC\nCB Champion 1.191 19.5 74.04 16.77\nAverage 1.179±0.0019 18.81±0.11 72.5±0.33 16.09±0.10\nEA Champion 1.199 19.42 74.45 17.15\nAverage 1.191±0.0017 19.01±0.092 72.93±0.28 16.50±0.10\n4-FPEAI Champion 1.21 20.42 74.4 17.60\nAverage 1.196±0.003 19.54±0.10 73.55±0.16 17.19±0.04\nMX_4-FPEAI Champion 1.222 20.51 75.47 18.29\nAverage 1.203±0.002 19.92±0.08 73.60±0.29 17.63±0.067\nQ3) In lines 124, “;Furthermore, replacing CB with to EA, the perovskite post-deposition annealing\ntime was reduced by half (30 minutes), effectively halving the energy required for perovskite absorber\nrealization” Is there any experiment or reference that proves it, I don't understand.\nWe thank the reviewer for this question. We would like to clarify that the statement refers to a direct and\nexperimentally observed difference in the annealing time required to fully crystallize the perovskite film,\ndepending on the anti-solvent used during deposition.\nIn our fabrication process, as specified in device fabrication procedure reported in the experimental section,\nwhen chlorobenzene (CB) is used as anti-solvent, a post-deposition annealing time of 60 minutes at 100°C is\nnecessary to achieve complete perovskite crystallization. However, when ethyl acetate (EA) is used, the same\ncrystallization quality is achieved within only 30 minutes of annealing under identical conditions. This\nreduction in processing time was reported usually in literature considering EA’s lower boiling point (77 °C)\nand moderate polarity lead to faster solvent evaporation and more rapid film formation compared to heavier\naromatic anti-solvents like CB,44 and consistently observed and confirmed across multiple samples, as\nreported in our manuscript.\nOur intent was to highlight this clear and repeatable experimental outcome, which has a direct implication\non process efficiency and energy consumption, since halving the annealing time reduces the energy required\nduring this thermal step. We considered this a straightforward consequence of the processing condition\nchange, but we appreciate the opportunity to make it clearer in the revised text and have added in the\nexperimental section the following sentence for improved clarity.\nIt is important to note that, given its lower boiling point and optimal polarity, ethyl acetate promotes faster\ncrystallization and more efficient solvent removal during film formation, which enables shorter annealing\ntimes compared to chlorobenzene under identical thermal conditions.44\n(44) Podapangi, S. K.; Jafarzadeh, F.; Mattiello, S.; Korukonda, T. B.; Singh, A.; Beverina, A.; Beverina, L.; Brown, T. M. Green Solvents,\nMaterials, and Lead-Free Semiconductors for Sustainable Fabrication of Perovskite Solar Cells. RSC Adv. 2023, 13 (27), 18165–18206.\nhttps://doi.org/10.1039/d3ra01692g.\nQ4) Lines 140 “...with Ti C Cl MXenes (MX-Cl) by carefully tuning the MXene dopant amount (Fig.\n3 2 2\nS2).”In Fig. S2, what are the units of MX-Cl concentration? The interval between 0.016 and 0.15 is too\nlarge, are there conditions for better concentrations in between?\nThank you for your valuable feedback. Unfortunately, the concentration units for MX-Cl were inadvertently\nomitted due to image scaling, and we have now reinserted them in the updated version of Fig. S2.\nAdditionally, in response to your comment about the large interval between 0.016mg and 0.15mg, we have\nperformed a new experiment to include two new data points at 0.02mg and 0.05mg respectively to better\ncapture the trend in that range. The figure and corresponding caption have been revised accordingly in SI and\nfollowing reported.\nFig. S2: a) Photograph of one of the two glass substrates used for device fabrication, each hosting four small-area perovskite solar\ncells. A total of 12 devices were fabricated for the statistical evaluation of photovoltaic parameters (Fig. 2,) while 8 devices were used\nfor the optimization of the Mxene-Cl doping concentration, b) PCE of PSCs realized by varying the amount of MXene-Cl added to 1\nml of perovskite precursor solution.\nQ5) Lines 146 and 147, “SGP has minimal impact on grain size and surface morphology (Fig. S8),\nwhereas MX-Cl doping, resulted in a noticeable increase in perovskite grain size compared to the\npristine layer (Fig. 3a and 3c)”. Statistical distribution and comparison of average grain size should be\ngiven.\nWe thank the reviewer for his constructive suggestion. In response, we added a detailed statistical analysis of\nthe domain size distribution for both the pristine 4-FPEAI-modified perovskite film and the film incorporating\nCl-functionalized MXenes, as now shown in the updated Fig. 3 (panels b and e). This analysis clearly\ndemonstrates a significant increase in average domain size from 0.125 µm (σ = 0.047 µm) in the 4-FPEAI only\nsample to 0.286 µm (σ = 0.070 µm) in the MXene/4FPEAI sample. Furthermore, the maximum domain size\nincreases from 0.242 µm to 0.452 µm, indicating more pronounced domain growth induced by the presence\nof MXenes. These findings support our claim that MX-Cl doping leads to noticeable domain enlargement and\nmorphological evolution, consistently with enhanced crystallization and improved film quality. The domain\nsize statistics provide quantitative confirmation of the morphological differences initially observed in the SEM\nimages (panels a and d) and reinforce the interpretation of improved film properties upon MXene\nincorporation.\nThe modified Fig. 2 with the new panel b) and e) and the relative caption are reported in the following\nFig. 3: a) SEM image of a) perovskite layer without MX-Cl addition and d) MX-Cl doped perovskite film surface obtained by applying\nthe SGP strategy during the anti-solvent step. The Cross-section SEM images for the same samples are reported in panel c) and f)\nrespectively. In the colored part of panel f) the incorporation of MX-Cl flakes is highlighted with a light-red color showing that MX\nflakes are sitting at the mTiO /perovskite interface, while they are not present in the case of pristine perovskite film in panel c). The\n2\npresence of small domains still appearing around the enlarged domains in the case of MX-Cl-doped film elucidated that the presence\nof MXene flakes induced a localized effect on the perovskite morphology, acting as a template for the perovskite crystals growing on\ntop of them. The domain size distribution reveals significant differences between the perovskite modified with only 4FPEAI and that\none treated with both 4FPEAI and MXenes. In the sample containing only 4FPEAI, the average domain size is 0.125 µm, with a standard\ndeviation of 0.047 µm, while in the MXene/4FPEAI sample the mean increases to 0.286 µm, with a standard deviation of 0.070 µm.\nFurthermore, the maximum domain size rises from 0.242 µm (4FPEAI) to 0.452 µm (MXene/4FPEAI), indicating a more pronounced\ndomain growth. These results suggest that the incorporation of MXenes promotes more efficient crystal growth and greater grain\ncoalescence, which is consistent with a reduction in grain boundary defect density and potentially improved charge transport within\nthe film. g) XRD patterns for the perovskite films with and without MX-Cl, h) Normalized transient photoluminescence (TRPL) decay\ncurves of the 4-FPEAI perovskite film and the film incorporating Cl-functionalized MXene. The decay profiles were fitted with a bi-\nexponential model. The MXene-treated film exhibits a significantly longer average carrier lifetime (τₐᵥ = 1.11 × 10⁻⁷ s) compared to\ng\nthe 4-FPEAI (τₐᵥ = 6.88 × 10⁻⁸ s), indicating reduced trap-assisted recombination and improved defect passivation.\ng\nQ6) Lines 162 “XRD analysis (Fig. 3e) reveals ... and minimizing trap states” Enhancement of the XRD\npeak intensity does not necessarily directly indicate a reduction in defects in the film, and\nenhancement of the diffraction peaks may reflect increased crystallinity or increased grain size, which\ncorresponds to SEM. Increased grain size results in sharper peaks and increased intensity, but this may\nbe directly related to a reduction in grain boundaries rather than point defects. A reduction in point\ndefects or dislocations may reduce lattice distortion, resulting in increased peak intensity, but changes\nin defect density need to be confirmed in conjunction with other means such as TEM and SCLC.\nWe agree with the reviewer that an increase in XRD peak intensity, while indicative of enhanced crystallinity\nor increased domain size, does not alone confirm a reduction in defect density, especially point defects. To\nfurther validate our interpretation regarding defect passivation, we have complemented the XRD analysis\nwith transient photoluminescence (TRPL) measurements, which provide insights into charge carrier dynamics\nand trap-assisted recombination processes. The TRPL decay curves, reported in the new Fig. 3h, were fitted\nusing a bi-exponential model, and the extracted lifetimes are summarized in the table below reported in SI as\nTab. S2. For the sample incorporating 4-FPEAI/MX-Cl, the average carrier lifetime (τ ) is 1.11 × 10⁻⁷ s, notably\navg\nlonger than the 6.88 × 10⁻⁸ s observed for the sample with only 4-FPEAI. The fast decay component (τ₁), often\nassociated with trap-assisted recombination, also shows a significant increase from 4.16 × 10⁻⁸ s (REF) to 7.57\n× 10⁻⁸ s (4-FPEAI/MX-Cl), accompanied by a larger amplitude (A₁), suggesting that traps are being effectively\npassivated in the Mxene-modified film. These results support the hypothesis that the incorporation of Cl-\nfunctionalized Mxenes leads to a reduction in non-radiative recombination pathways, likely due to trap state\npassivation. This improvement in carrier dynamics corroborates the increased crystallinity observed in XRD\nand the enhanced morphology seen in SEM, thereby offering a more holistic understanding of the film quality\nimprovements.\nWe added the panel h) in Fig. 3 by adapting the caption and the following text in the main:\nComplementarily, TRPL analysis (Fig. 3h) shows a marked increase in average carrier lifetime in MXene-\nmodified films, pointing to efficient trap-state passivation, likely due to both improved crystallinity and\nreduced interfacial defect density (see Tab. S2 in the SI for full fitting parameters).\nFig. 3: a) SEM image of a) perovskite layer without MX-Cl addition and d) MX-Cl doped perovskite film surface obtained by applying\nthe SGP strategy during the anti-solvent step. The Cross-section SEM images for the same samples are reported in panel c) and f)\nrespectively. In the colored part of panel f) the incorporation of MX-Cl flakes is highlighted with a light-red color showing that MX\nflakes are sitting at the mTiO /perovskite interface, while they are not present in the case of pristine perovskite film in panel c). The\n2\npresence of small domains still appearing around the enlarged domains in the case of MX-Cl-doped film elucidated that the presence\nof MXene flakes induced a localized effect on the perovskite morphology, acting as a template for the perovskite crystals growing on\ntop of them. The domain size distribution reveals significant differences between the perovskite modified with only 4FPEAI and that\none treated with both 4FPEAI and MXenes. In the sample containing only 4FPEAI, the average domain size is 0.125 µm, with a standard\ndeviation of 0.047 µm, while in the MXene/4FPEAI sample the mean increases to 0.286 µm, with a standard deviation of 0.070 µm.\nFurthermore, the maximum domain size rises from 0.242 µm (4FPEAI) to 0.452 µm (MXene/4FPEAI), indicating a more pronounced\ndomain growth. These results suggest that the incorporation of MXenes promotes more efficient crystal growth and greater grain\ncoalescence, which is consistent with a reduction in grain boundary defect density and potentially improved charge transport within\nthe film. g) XRD patterns for the perovskite films with and without MX-Cl, h) Normalized transient photoluminescence (TRPL) decay\ncurves of the 4-FPEAI perovskite film and the film incorporating Cl-functionalized MXene. The decay profiles were fitted with a bi-\nexponential model. The MXene-treated film exhibits a significantly longer average carrier lifetime (τₐᵥ = 1.11 × 10⁻⁷ s) compared to\ng\nthe 4-FPEAI (τₐᵥ = 6.88 × 10⁻⁸ s), indicating reduced trap-assisted recombination and improved defect passivation.\ng\nWhile in the supporting info in SECTION S.I. 5 we added the following text and table:\nTo further evaluate the influence of Cl-functionalized MXenes on the optoelectronic properties of the\nperovskite films, we performed time-resolved photoluminescence (TRPL) measurements. The decay profiles\nof the reference and MXene-treated films were fitted using a bi-exponential decay model:\n𝐹(𝑡) = 𝐴\n1\n𝑒−𝑡⁄𝜏1\n+𝐴\n2\n𝑒−𝑡⁄𝜏2\nwhere τ and τ represent the fast and slow decay lifetimes, respectively, and A , A are the corresponding\n1 2 1 2\namplitudes. The fast decay component (τ ) is commonly associated with trap-assisted recombination\n1\nprocesses, while the slow component (τ ) reflects radiative recombination of free carriers.\ng\nto the reference (68.76 ns). The fast decay time τ also increases from 41.63 ns to 75.67 ns, indicating fewer\n1\nshallow trap states. This reduction is likely due to larger domain sizes induced by MXene flakes acting as\nlocalized crystallization templates, which decrease the overall grain boundary density and associated trap\nsites. The amplitude A also increases significantly (from 3.16 to 9.127), further suggesting a dominant role\n1\nof non-radiative pathways in the control film that are effectively passivated by the MXene layer.\nThese findings corroborate the beneficial role of Cl-functionalized MXenes in reducing trap-assisted\nrecombination by passivating defect sites—likely through interaction between Cl⁻ terminations and\nundercoordinated Pb²⁺ ions. The TRPL results align with structural improvements observed via XRD and SEM,\nfurther confirming that the incorporation of MXenes enhances the film’s crystallinity, reduces trap densities,\nand improves charge carrier dynamics.\nTab. S2: Fitting parameters extracted from the bi-exponential fitting of TRPL decay curves for the reference and MX-Cl modified\nperovskite films. A and A are the amplitudes of the fast and slow decay components, respectively; τ corresponds to trap-assisted\n1 2 1\n(non-radiative) recombination, and τ to radiative recombination. τ is the average carrier lifetime calculated as\n2 avg\nτ =(A τ 2+A τ 2)/(A τ +A τ ). The increased τ and τ in the MXene-modified film suggest reduced trap density and suppressed non-\navg 1 1 2 2 1 1 2 2 1 avg\nradiative recombination.\nSample A τ (ns) A τ (ns) τ (ns)\n1 1 2 2 avg\n4-FPEAI 3.617 41.63 0.157 199.3 68.76\nMX-Cl/4-FPEAI 9.127 75.67 1.327 203.2 111.5\nQ7) The peak intensity ratios in the XRD pattern in Fig.3e clearly do not correspond to the values in\nsection SI 5, Tab. S1, e.g., the ratio of 4-FPEAI (110)/(310) is clearly greater than 1 in the figure, but\n0.92 in the table.\nThank you for your careful observation. You are absolutely right, the peak intensity ratios reported in Tab. S1\n(Section SI 5) were affected by a formatting error, which led to the inversion of some values. Specifically, the\n(110)/(310) ratio for 4-FPEAI was mistakenly listed as lower than 1. We have now corrected the table with the\naccurate values, which are consistent with the XRD pattern shown in Fig. 3e. We appreciate your attention to\ndetail and apologize for the oversight.\n(110)/(310) (220)/(310)\n4-FPEAI 1,62 0.92\nMX-Cl/4-FPEAI 2.29 1.31\nQ8) In lines 179, “...slight blue shift (767 nm vs. 763 nm) for MX-Cl/4-FPEAI films...”, the optical band gap of Cs FA Pb(I Br ) perovskite was 1.68 eV in Fig.S1, and the band gap is\n0.18 0.82 0.8 0.2 3\ncalculated to be 1.62 eV (~760 nm) based on the emission peaks of PL. Does the introduction of 4-\nFPEAI and MX-Cl change the band gap of perovskite?\nWe are grateful to the reviewer for underlining this aspect that gives us the opportunity to repeat both\nabsorption and PL spectra analysis, newly acquired on fresh perovskite layers on glass. In particular, we made\n4-FPEAI treated perovskite film (named 4-FPEAI) and MXene-doped perovskite film treated with 4-FPEAI\n(named MX-Cl/4-FPEAI). The band gap (E ) extracted by the Tauc plots reported in Fig. S10a was 1.67 eV for\ng\n4-FPEAI film while 1.667 eV for MX-Cl/4-FPEAI film. The slight blue-shift induced by the MXene doping can be\nascribed to the enlarged dimension of the perovskite domain size.3,4 Moreover, by comparing the Tauc plot\nof the bare perovskite film reported in SI and that one of 4-FPEAI sample, it’s quite evident a slight E blue\ng\nshift induced by the 4-FPEAI treatment, shifting the E from 1.68 to 1.67 eV. A similar effect has been already\ng\nobserved in previous publications when using fluorinated aromatic cation-based treatment as passivation\nstrategy for 3D perovskite film.5–7 On the same samples, we acquired PL spectra on three different points of\nthe films. The PL spectra reported in Fig. S10b showed averaged emission peaks centered at 758 nm and 763\nnm for 4-FPEAI and MX-Cl/4-FPEAI samples respectively. The red-shift in PL observed upon the incorporation\nof MXenes can be attributed to the growth of larger perovskite domains. This morphological evolution\nreduces the density of trap states at grain boundaries, thereby significantly enhancing the PL intensity.\n(3) Ummadisingu, A.; Meloni, S.; Mattoni, A.; Tress, W.; Grätzel, M. Crystal-Size-Induced Band Gap Tuning in Perovskite Films.\nAngew. Chemie - Int. Ed. 2021, 60 (39), 21368–21376. https://doi.org/10.1002/anie.202106394.\n(4) Chen, L.; Paillard, C.; Zhao, H. J.; Íñiguez, J.; Yang, Y.; Bellaiche, L. Tailoring Properties of Hybrid Perovskites by Domain-\nWidth Engineering with Charged Walls. npj Comput. Mater. 2018, 4 (1). https://doi.org/10.1038/s41524-018-0134-3.\n(5) Zhou, Q.; Liang, L.; Hu, J.; Cao, B.; Yang, L.; Wu, T.; Li, X.; Zhang, B.; Gao, P. High-Performance Perovskite Solar Cells with\nEnhanced Environmental Stability Based on a (p-FC 6 H 4 C 2 H 4 NH 3 ) 2 [PbI 4 ] Capping Layer. Adv. Energy Mater. 2019,\n9 (12), 1–11. https://doi.org/10.1002/aenm.201802595.\n(6) Zhou, L.; Su, J.; Lin, Z.; Guo, X.; Ma, J.; Li, T.; Zhang, J.; Chang, J.; Hao, Y. Synergistic Interface Layer Optimization and\nSurface Passivation with Fluorocarbon Molecules toward Efficient and Stable Inverted Planar Perovskite Solar Cells.\nResearch 2021, 2021. https://doi.org/10.34133/2021/9836752.\n(7) Xu, J.; Boyd, C. C.; Yu, Z. J.; Palmstrom, A. F.; Witter, D. J.; Larson, B. W.; France, R. M.; Werner, J.; Harvey, S. P.; Wolf, E. J.;\net al. Triple-Halide Wide-Band Gap Perovskites with Suppressed Phase Segregation for Efficient Tandems. Science (80-. ).\n2020, 367 (6482), 1097–1104. https://doi.org/10.1126/science.aaz4639.\n(8) Campanari, V.; Martelli, F.; Agresti, A.; Pescetelli, S.; Nia, N. Y.; Giacomo, F. Di; Catone, D.; Keeffe, P. O.; Turchini, S.; Yang,\nB.; et al. Reevaluation of Photoluminescence Intensity as an Indicator of Efficiency in Perovskite Solar Cells. Sol. RRL 2022, 6\n(8), 2200049–2200059. https://doi.org/10.1002/solr.202200049.\nWe modified the SI accordingly, that now sounds as follows reported:\nThe enlarged perovskite domain size translates in a slight reduction of the perovskite band gap, shifting from\n1.67 eV for perovskite films with SGP (named 4-FPEAI) to 1.667 in case of same film but with the addition of\nMX-Cl (named MX-Cl/4-FPEAI), as reported in Fig. S10a. Moreover, the beneficial role of MX-Cl in reducing\nthe charge recombination is confirmed by the steady state photoluminescence (PL) measurements performed\non both 4-FPEAI and MX-Cl/4-FPEAI films (Fig. S10b). Regarding PL spectra, generally a reduced PL emission\nis imputed to the non-radiative recombination acting in the film, while the presence of a red-shift emission\ncan be ascribed to an enlargement of the averaged perovskite domain size.8 Both phenomena can be\nobserved in Fig. S10b where in the case of optimized MX-Cl/4-FPEAI a higher PL emission intensity (+75.1%)\nis recorded together with a slightly red shift of the emission peak (763 nm) once compared with 4-FPEAI\ndevice (758nm).\nFig. S10: a) Tauc plots and b) steady-state PL spectra for perovskite films with SGP (named 4-FPEAI) and same film but\nwith the addition of MX-Cl (named MX-Cl/4-FPEAI). PL spectra were acquired on three different points of the samples\n(named _1,_2,_3).\nQ9) In lines 189, “...the hydrophobic nature of MX-Cl further boosts both device efficiency\nand operational stability, ...” There's no pristine group here in Fig.S12. And a 3-minute mpp\ntracking is too short to indicate stability. Contact angle and surface tension calculations are needed\nto show that MXene doping leads to an increase in the hydrophobicity of perovskite films.\nWe thank the reviewer for his insightful observation. Verifying the effect of MX-Cl on the hydrophobic nature\nof the perovskite surface is not a trivial task, since MXenes flakes are sitting at the mTiO /perovskite interface.\nfor UPS now reported in the revised version of the SI. As evident from Fig. R2, two bands are detected, one\naround 532.8 eV, related to the oxygen present in the adsorbed water, and one around 532.4 eV, very close\nto the first, which can be attributed to C=O groups.1 In particular, the measurements were performed on\nsample named 4-FPEAI (4-FPEAI treated perovskite film with a reduced thickness of around 200 nm growth\non mTiO scaffold) and sample named MX-Cl/4-FPEAI (4-FPEAI treated perovskite film with MX-Cl with a\n2\nreduced thickness around 200 nm growth on mTiO ). The two samples showed pretty similar percentage of\n2\nadsorbed water onto the surface, (see Tab.R1) confirming the minor role of the MX-Cl in improving the\nhydrophobicity of the perovskite film surface.\nFig. R2: XPS measurements O1s peaks for a) sample A1 (4-FPEAI treated perovskite film with a reduced thickness of around 200 nm\ngrowth on mTiO scaffold) and b) sample B1 (4-FPEAI treated perovskite film with MX-Cl with a reduced thickness around 200 nm\n2\ngrowth on mTiO .\nPosition (eV) / relative amount\n4-FPEAI 532.80/82.2% 532.47/17.8%\nMX-Cl/4-FPEAI 533.14/81.5% 532.43/18.5%\nTab. R1 Adsorbed water values a) sample named 4-FPEAI (4-FPEAI treated perovskite film with a reduced thickness of around 200 nm\ngrowth on mTiO scaffold) and b) sample named MX-Cl/4-FPEAI (4-FPEAI treated perovskite film with MX-Cl with a reduced thickness\n2\naround 200 nm growth on mTiO ).\n2\n(1) Larciprete, R.; Agresti, A.; Pescetelli, S.; Pazniak, H.; Liedl, A.; Lacovig, P.; Lizzit, D.; Tosi, E.; Lizzit, S.; Carlo, A. Di. Mixed\nCation Halide Perovskite under Environmental and Physical Stress. Materials (Basel). 2021, 14, 3954 (1-20)\nFor this reason, on one side we modify the main text by removing the sentence related to the “hydrophobic\nnature of MX-Cl”. On the other side, with the aim to deeply investigate the improved stability of our graded\nhomojunction, we performed cycling light test (ISOS-C1), following the recently established stressing\nprotocol.18 The results are reported and commented on SI. The main modifications carried out on both the\nmain text of the manuscript and the SI are reported following for the reviewer convenience.\nIn SI: Indoor photo-stability under MPP tracking of semi-transparent encapsulated cells upon exposure to\ncycled light. As recently discussed in literature, the most effective way to perform a reliable stability analysis\nconsists in light cycling test at constant temperature, representing a good trade-off between simplicity of\nexecution in lab conditions while taking into account phenomena observed only in case of outdoor testing\n(that cannot be considered in case of constant light experiments).15 Indeed, transient behavior occurs in PSCs\nas a result of the coexistence of several dynamics with characteristic times spanning from time scales of\nseconds to hours. Thus, day–night cycling of devices can consider such slow dynamics as they occur on similar\ntimescales. Following this idea, we performed a light–dark cycling test as recommended by the ISOS-LC-1\nprotocol, exposing the cells to simulated sunlight turned on and off with cycle periods of 24 h and duty cycles\n(light:dark) of 1:2, mimic an accelerated diurnal sun cycle (the solar cell is maintained at ambient conditions\nwhile the temperature is monitored and RH is maintained at 30%).16 The recorded maximum power (P ) is\nMPP\nreported in Fig. S7e for both G/EA and fully optimized G/MX/4-FPEAI semi-transparent cells, normalized at\nthe initial value. The combined stabilization effect of the 4-FPEAI based SGP together with the presence of\nMX-Cl results in a T =600 hours vs a T =574 hours of the untreated device, demonstrating the superior\n91 80\nstability (T 6.5 times larger) of the device employing the proposed graded homojunction approach. Despite\n91\nmore in depth analyses are required to elucidate the role of MX-Cl and 4-FPEAI SGP, we can confidently ascribe\nthe improved stability to a synergistic effect of the two treatments. On one side, the addition of MX-Cl resulted\nin a perovskite film with improved crystallinity eventually reducing lattice distortion, lowering defect density\nand minimizing trap states, as demonstrated by XRD, transient PL and SEM characterizations. This, combined\nwith the enlarged perovskite domains and mitigated grain boundary-induced trap states, makes the trapped\ncharge driven degradation less effective,17 resulting in an overall enlargement of device lifetime. We cannot\nexclude a side role of the Cl in reducing the tensile strain within the perovskite layer, recently pointed out as\none of the main degradation channel when light-dark cycling is applied.18 On the other side, the\nphenylalkylammonium compounds as 4-FPEAI has been proven to stabilize the perovskite through\nsuppression of iodide ion migration, likely responsible for metastable perovskite behavior under light cycling\nobserved for the G/EA device, while passivating the surface defects.19 In addition, the 4-FPEAI SGP strategy\nhas been already proven to improve the long-term operational stability of the encapsulated devices under\nthe continuous illumination in ambient condition.12 Thus, the superior light cycling stability showed by the\ngraded homojunction cell can be confidently ascribed to a combined stabilizing effect from 4-FPEAI and MX-\nCl, definitively reducing the device metastable behavior.\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1).\n(12) Yan, N.; Gao, Y.; Yang, J.; Fang, Z.; Feng, J.; Wu, X.; Chen, T.; Liu, S. F. Wide-Bandgap Perovskite Solar Cell Using a Fluoride-\nAssisted Surface Gradient Passivation Strategy. Angew. Chem. Int. Ed. 2023, 62, e202216668.\nhttps://doi.org/10.1002/anie.202216668.\n(15) Khenkin, M.; Köbler, H.; Remec, M.; Roy, R.; Erdil, U.; Li, J.; Phung, N.; Adwan, G.; Paramasivam, G.; Emery, Q.; et al. Light\nCycling as a Key to Understanding the Outdoor Behaviour of Perovskite Solar Cells. Energy Environ. Sci. 2023, 17 (2), 602–\n610. https://doi.org/10.1039/d3ee03508e.\n(16) Khenkin, M. V.; Katz, E. A.; Abate, A.; Bardizza, G.; Berry, J. J.; Brabec, C.; Brunetti, F.; Bulović, V.; Burlingame, Q.; Di Carlo,\nA.; et al. Consensus Statement for Stability Assessment and Reporting for Perovskite Photovoltaics Based on ISOS\nProcedures. Nat. Energy 2020, 5 (1), 35–49. https://doi.org/10.1038/s41560-019-0529-5.\n(17) Ahn, N.; Kwak, K.; Jang, M. S.; Yoon, H.; Lee, B. Y.; Lee, J.; Pikhitsa, P. V; Byun, J.; Choi, M. Trapped Charge-Driven\nDegradation of Perovskite Solar Cells. Nat. Commun. 2016, 8 (May), 1–9. https://doi.org/10.1038/ncomms13422.\n(18) Shen, Y.; Zhang, T.; Xu, G.; Steele, J. A.; Chen, X.; Chen, W.; Zheng, G.; Li, J.; Guo, B.; Yang, H.; et al. Strain Regulation\nRetards Natural Operation Decay of Perovskite Solar Cells. Nature 2024, 635 (8040), 882–889.\nhttps://doi.org/10.1038/s41586-024-08161-x.\n(19) Guo, Y.; Apergi, S.; Li, N.; Chen, M.; Yin, C.; Yuan, Z.; Gao, F.; Xie, F.; Brocks, G.; Tao, S.; et al. Phenylalkylammonium\nPassivation Enables Perovskite Light Emitting Diodes with Record High-Radiance Operational Lifetime: The Chain Length\nMatters. Nat. Commun. 2021, 12 (1). https://doi.org/10.1038/s41467-021-20970-6.\nWe also modified the main text by briefly recalling the abovementioned stability analysis as in the following:\n“Moreover, the combination among the MX-Cl addition and the 4-FPEAI SGP strategy not only translated in\nsuperior device performance but also conferred superior robustness to the perovskite film against light-driven\ndegradation, as demonstrated by prolonged light cycling test (Fig. S22c).”\nFinally, we inserted in ex Fig. S12 (now Fig. S7e) the stabilized MPP plot for the G/EA cells, as requested by\nthe reviewer.\nQ10) In lines 206 and 207, “...could be concluded that the combination of 4-FPEAI and MX-Cl doping\nforms a graded n-p junction along the thickness of the perovskite absorber from ETL to HTL side.” The\nvariation of the work function of CPD is not a sufficient proof of the existence of n-p junction, which\ncan be further demonstrated by calculating the energy level structure based on UPS of different films.\nWe thank the Reviewer for the insightful comment. We agree that the variation of the contact potential\ndifference (CPD) alone is not sufficient to unequivocally demonstrate the formation of a graded n–p junction\nacross the perovskite layer. Following the Reviewer’s suggestion, we have complemented our analysis with\nUltraviolet Photoelectron Spectroscopy (UPS) measurements on ad hoc prepared films, allowing for a more\naccurate evaluation of the energy level structure and its modulation upon MX-Cl doping and 4-FPEAI\nincorporation.\nThe revised main text now includes a discussion of the UPS results, which clearly show a progressive tuning\nof both work function and valence band maximum values across the series of modified films. This supports\nthe presence of an internal electric field and energy level bending consistent with a graded n–p junction\nconfiguration. Full UPS spectra and extracted values are now reported in the Supporting Information (Figs.\nS11, S12, and Tab. 2 in main text), and relevant references to these data have been added to the main\nmanuscript.\nBelow we provide the specific additions made to the main text and the SI to address this point more\ncomprehensively.\nFrom SEM images of Fig. 3f, clearly MX-Cl flakes are mainly distributed at bottom interface between\nperovskite/mTiO layer. The presence of MX-Cl at the ETL/perovskite interface calls for a possible tuning of\n2\nthe buried perovskite interface WF.21\nBuilding on this, ultraviolet photoelectron spectroscopy (UPS), was employed to elucidate the progressive\nmodulation of the perovskite energy levels depending on the different treatments. Notably, the combination\nof MXene and 4-FPEAI induces a graded shift in WF and valence band maximum (VBM) position across the\nseries, consistent with the formation of a built-in electric field within the absorber. A full discussion and\ncorresponding UPS spectra and energy level diagram reconstructed from the UPS data are provided in section\nS6 in SI Fig. S11 and Fig. S12 respectively, while the extracted values of WF and VBM are summarized in Tab. 2,\nclearly highlighting the energetic alignment changes induced by each modification.\nTab. 2. Extracted values of work function (WF), valence band maximum (VBM) with respect to the Fermi level, and ionization energy\n(IE = WF + VBM) for the four analyzed perovskite film configurations, measured by UPS. The results demonstrate distinct electronic\neffects induced by MXene and 4-FPEAI treatments, supporting the formation of dipoles and doping gradients.\nSample WF (eV) VBM (eV) (IE) [eV]\nREF\n4.75±0.1 0.9±0.1 5.65±0.1\nMX-Cl\n4.55±0.1 0.9±0.1 5.45±0.1\nMX-Cl / 4-FPEAI\n4.70±0.1 0.8±0.1 5.50±0.1\n4-FPEAI\n4.85±0.1 0.7±0.1 5.55±0.1\nTo confirm this effect, Kelvin probe force microscopy (KPFM) measurements were carried out on dedicated\nhalf-cell samples (FTO/G+cTiO₂/G+mTiO₂/perovskite), allowing us to extract WF values (Tab. S3) and\ndistinguish the contributions of MXene doping and SGP (see Fig. S13)\nIn the experimental section we added:\nPhotoelectron spectroscopy measurements were performed in a ThermoScientific ESCALAB QXi XPS\napparatus in ultra-high vacuum conditions (10-10 mbar base pressure). The valence band region of the samples\nwas investigated by UPS using the 21.2 eV radiation of the HeI line and biasing the samples at -5 V, in order\nto determine both the work function and the position of the valence band edge. The electronic band structure\nof the perovskites was characterized by X-ray photoemission spectroscopy (XPS) using monochromatic 1486.8\neV photons of the Al Kα line and analysed with the Thermo-Fischer Avantage Software. XPS measurements\nwere performed on small devices (14x14 mm) applying the system charge compensation, while the work\nfunction was measured without charge compensation, by providing an additional contact to an edge of the\ndevice front surface.\nMoreover, the complete discussion on UPS results with the relative figures (Fig. S11 and S12) are reported in\nSI\nUltraviolet photoelectron spectroscopy (UPS) measurements\nTo gain insight into the energetic effects induced by the surface and contact engineering strategies, we\nperformed ultraviolet photoelectron spectroscopy (UPS) measurements on a set of ad-hoc fabricated\nperovskite thin films with reduced thickness (comparable to the MXene flake size, ~200 nm) and varied\ninterfacial treatments (REF, MX-Cl, 4-FPEAI, and MX-Cl/4-FPEAI). The extracted values of work function (WF)\nand valence band maximum (VBM) for each cell configuration are summarized in Tab. 2 of the main text.\nAs shown in Fig. S12, the energy level diagram reconstructed from the UPS data Fig. S11 clearly reveals a\nsystematic shift in both WF and VBM across the series.\nThe reference (REF) sample exhibits a WF of 4.75 eV and a VBM located 0.9 eV below the Fermi level (EF),\ncorresponding to an ionization energy (IE) of 5.65 eV. Upon incorporation of Ti₃C₂Cl₂ MXenes (MX-Cl), the WF\ndecreases to 4.55 eV while the VBM remains unchanged at 0.9 eV. This behavior indicates that the addition\nof MXenes enhances the n-type character of the perovskite, consistent with the formation of a surface dipole\nat the perovskite/MXene interface. Notably, this interface dipole shifts the vacuum level without affecting the\nperovskite band structure. Conversely, the use of 4-FPEAI as SGP induces a WF increase to 4.85 eV and a shift\nin the VBM to 0.7 eV below EF. These results suggest a clear modification of the perovskite electronic\nstructure, indicating a p-type doping effect, in addition to surface dipole formation. The combined MX-Cl/4-\nFPEAI treatment results in intermediate values (WF = 4.70 eV, VBM = 0.8 eV), consistent with a graded band\nalignment across the film thickness.\nThese findings are corroborated by DFT simulations, which show a vacuum level shift of –0.39 eV for the\nMXene interface and +0.14 eV for 4-FPEAI, supporting the observed WF modulation. Overall, the UPS data\nconfirm the formation of a vertical energy level gradient across the perovskite layer, driven by the two\ntreatments acting at opposite interfaces.\nFig. S11. UPS spectra for perovskite films modified with different interface strategies: reference (a–b), MX-Cl (c–d), MX-Cl/4-FPEAI (e–\nf), and 4-FPEAI (g–h). Panels (a, c, e, g) show the secondary electron cutoff region used to extract the WF, while panels (b, d, f, h)\ndisplay the valence band region used to determine the VBM with respect to the EF. A gradual modulation of the WF and VBM is\nobserved across the series, highlighting the impact of dipole formation and band structure tuning induced by MXene doping and\nsurface gradient passivation.\nFig. S12: Schematic energy level alignment of perovskite films extracted from UPS measurements for the four configurations: REF\n(untreated), MX-Cl, 4-FPEAI, and combined MX-Cl/4-FPEAI. The WF and VBM positions are plotted relative to the vacuum level. A\nprogressive modulation of the energy levels is observed, with MX-Cl inducing a downward shift in WF (consistent with dipole-induced\nvacuum level bending) and 4-FPEAI causing an upward shift in both WF and VBM. The intermediate alignment in the combined\ntreatment suggests the formation of a graded homojunction across the perovskite layer, promoting internal electric field formation\nand enhanced charge extraction.\nQ11) In lines “;...confirming the effectiveness of 2D material-engineering approach even for semi-\ntransparent structures” The cell structure changes from opaque to semi-transparent, where the only\nchange seems to be the change of the top electrode from Au to ultrathin Au/ITO. Whereas the effect\nof MX-Cl/4-FPEAI is significantly diminished (the average increase of 30 mV is diminished to 11 mV),\nand is it possible that due to the change in work function of electrode that leads to a diminution of\nthe effect of the n-p junction as described in the manuscript.\nWe thank the reviewer for this relevant observation. To clarify this point, we performed a series of numerical\nsimulations to assess the possible impact of the work function of the top electrode and unintentional effects\nduring ITO deposition.\nFirstly, we simulated the effect of a slightly lower work function for the Au/ITO semi-transparent electrode\n(5.18 eV) compared to Au (5.2 eV). The results indicate that this small variation primarily affects the fill factor\n(FF), while the values of V and J remain essentially unchanged. Therefore, we believe that the change in\nOC SC\nWF alone cannot explain the reduced ΔV .\nOC\nSecond, we considered the possible impact of unintentional doping of the PTAA hole transport layer during\nITO sputtering. By simulating an increased acceptor doping density in the semi-transparent devices, we\nobserved that V in the reference cell is significantly improved, while the gain in V in the optimized (MX-\nOC OC\nCl/4-FPEAI-treated) structure is less pronounced. This can explain the smaller ΔV observed for the semi-\nOC\ntransparent devices, which results from a raised baseline V in the untreated sample, rather than a reduced\nOC\neffectiveness of the MX-Cl/4-FPEAI interface.\nMoreover, we analyzed the statistical distribution of ΔV and ΔJ values across multiple devices. The broader\nOC SC\nspread in ΔV observed for the semi-transparent devices is correlated with a broader ΔJ distribution, as\nOC SC\nexpected from the logarithmic relationship between these quantities. For this reason, comparing average\nΔV values may be misleading. A more reliable metric is the direct comparison between the top-performing\nOC\ndevices, which still shows a consistent ΔV improvement (29 mV for opaque vs. 23 mV for semi-transparent).\nOC\nIn conclusion, the observed variation in ΔV does not indicate a diminished effectiveness of the n–p junction\nOC\nstrategy. Rather, it reflects the marginal influence of fabrication-related effects—such as unintentional doping\nand electrode modification—on device performance. We have added these insights and updated Figs. S20–\nS21 in the revised Supporting Information. For the reviewer’s convenience, we also report the discussion and\nfigures directly below.\nTo further understand the observed variation in V between reference and optimized ST devices, we provide\nOC\nhere an extended discussion based on simulation results and interface analysis. The Fig. S19 shows the\nstatistical distribution of the main photovoltaic parameters, highlighting a reduced ΔV improvement in ST\nOC\nstructures compared to opaque ones. In the following, we examine possible physical origins of this behaviour,\nincluding the effect of the top contact work function, unintentional doping of the PTAA layer during ITO\ndeposition, and the role of interfacial engineering. The analysis supports the hypothesis that the reduced\nΔV is not due to a diminished effectiveness of the MX-Cl/4-FPEAI treatment, but rather to changes induced\nOC\nby the device architecture and processing conditions.\nEffect of the work function: due to the use of Au/ITO semitransparent contact instead of Au, the effective\nwork function (WF) of the anode could be slightly lower for the semitransparent cell with respect to the\nopaque cell. In order to investigate this effect, we repeated the simulations for the reference and optimized\ndevices varying only the effective WF value employed for the p-contact, namely, WF = 5.2 eV for the opaque\ncell and WF = 5.18 eV for the semitransparent structure. The results are shown in Fig. S20a), where the cyan\nsolid and dashed lines represent respectively the J-V characteristics of the opaque reference and optimized\ndevices, while the red solid and dashed lines depict the J-V curves of the semitransparent reference and\noptimized devices, respectively. We can see that the role of the work function is reflected by the fill factor\n(FF) features, giving a smaller FF when a smaller WF is considered, while J and V are not affected.\nSC OC\nEffect of an unintentional doping: a possible effect induced by the sputtering process employed to deposit\nthe ITO layer at the p-contact is an unintentional doping of the PTAA hole transport layer (HTL). We analyzed\nthe impact of such unintentional doping comparing the simulation results of the reference and optimized\ndevices with the behavior obtained for the same structures but with a higher acceptor doping density in the\nHTL, namely, N = 1x1017 cm-3 for the opaque cell and N = 1.5x1017 cm-3 for the semitransparent cell.\nA,PTAA A,PTAA\nThe derived J-V characteristics are depicted in Fig. S20b), where the cyan solid and dashed lines represent\nrespectively the J-V characteristics of the opaque reference and optimized devices, while the green solid and\ndashed lines depict the J-V curves of the semitransparent reference and optimized devices, respectively.\nWe can see that V value of the reference device is significantly increased by the unintentional doping while the\nOC\nincrement of V in the optimized structures is less pronounced, resulting in a smaller improvement of V for\nOC OC\nthe semitransparent cell with respect to the opaque cell, namely, ΔV = 32 mV for the opaque contact and\nOC\nΔV = 25 mV for the semitransparent contact.\nOC\nThe more pronounced effect of unintentional PTAA doping on the reference device can be attributed to the\ninitial less favorable energy level alignment and interfacial contact properties at the perovskite/HTL interface.\nIn the absence of interfacial modifiers such as MX-Cl/4-FPEAI, the reference structure exhibits a higher energy\noffset for hole extraction and a greater density of interfacial recombination pathways. Increasing the acceptor\nconcentration in PTAA enhances the built-in electric field and improves hole-transporting efficiency, which\nfacilitates charge extraction and reduces interfacial recombination losses, resulting in a notable V increase.\nOC\nIn contrast, the optimized device incorporating MX-Cl and 4-FPEAI, already features tailored interfacial\nenergetics and improved chemical passivation. These interlayers contribute to more efficient charge\nextraction and reduced defect-mediated recombination. As a result, additional doping of PTAA has a\ncomparatively minor effect on V , since the contact is already near optimal.\nOC\na) b)\nFig. S20: a) effect of a smaller effective work function value of the p-contact. b) effect of a higher acceptor doping density in the hole\ntransport layer.\nDiscussion on the statistics of ΔV and ΔJ : we want to point out that, if the top cells are compared, the\nOC SC\nimprovement in V of the semitransparent cell is comparable with the value obtained for the opaque cell,\nOC\nnamely, ΔV = 29 mV for the opaque structure and ΔV = 23 mV for the semitransparent structure.\nOC OC\nConsidering the known relation existing between ΔV and ΔJ , i.e., V/V ~ ln(J/J ), the fluctuations in the\nOC SC 0 0\nobserved values of ΔV are related to the fluctuations in the increment of J , as suggested by the figure\nOC SC\nbelow (Fig. S21), where the distributions of the improvement of V (Fig. S21a)) and J (Fig. S21b)) features\nOC SC\nfor the opaque and semitransparent structures are shown. So, in our opinion, it may be misleading the\ncomparison of the average improvement in V , since the width of the ΔV distribution of the\nOC OC\nsemitransparent device is larger with respect to the opaque device, due to the related broader ΔJ\nSC\ndistribution, and the specific correlation between ΔV and ΔJ would be lost. We think that, at this stage,\nOC SC\nthe specific comparison between the top cells in the two sets of samples is more reliable. However, this\nanalysis highlights the importance of achieving major control on the deposition process of the ITO layer.\nSuch a strategy could be addressed in our future work.\na) b)\nFig. S21: distribution of a) ΔV = V - V and b) ΔJ = J - J for the opaque (blue) and semitransparent\nOC OC_optimized OC_eference SC SC_optimized SC_reference\n(red) structures.\nAnd we recall this discussion in the main text as in the following indicate:\nBy employing the same 2D material-engineering strategies, MXene doping and 4-FPEAI-based surface\ngradient passivation, previously validated in opaque devices, the semi-transparent perovskite top cells used\nin the 4T tandem architecture demonstrated enhanced PCEs up to 18.3% (av. PCE = 17.63%) due to a higher\nV (av. V = 1.2 V) and J (av. J = 19.92 mA/cm²) compared to the REF sample (see Tab. S6, Figs. S19–22a,\nOC OC SC SC\nS20 and S21 for a detailed analysis), confirming the effectiveness of 2D material-engineering approach even\nfor semi-transparent structures. Moreover, the combination among the MX-Cl addition and the 4-FPEAI SGP\nstrategy not only translated into superior device performance but also conferred superior robustness on the\nperovskite film against light-driven degradation, as demonstrated by prolonged light cycling test (Fig. S22c).\nReviewer’s Comments:\nReviewer #2 (Remarks to the Author)\nThe authors have addressed all previous comments and therefore publication of the manuscript is\nagreed.\nWe are grateful for the Reviewer’s positive recommendation and for the thoughtful suggestions that\nstrengthened our work.\nReviewer #3 (Remarks to the Author)\nThe authors have made a significant effort to revise the manuscript with additional experiment and\ndiscussion. While there have been improvements in many areas, I still have serious concerns\nabout the graded homojunction claim. Considering how this is central to the proposed mechanisms\nin this paper, I cannot support publication this claim included. I’ll discuss in more detail below.\nResponse:\nWe thank the Reviewer for the careful re-evaluation of our revised manuscript and for the constructive\ncomments. Below we provide a detailed point-by-point response by further clarifying the key experimental\nevidences supporting the formation of a vertically graded homojunction. In the revised version, we have\nstrengthened the discussion, expanded the Supporting Information, and clearly articulated the mechanistic\nrole of MX-Cl and 4-FPEAI at their respective interfaces.\nConcern 1: UPS is highly surface sensitive (probe depth ~1-2nm), but there is no discussion on how\nsamples were handled or if they ever saw air. These details are important to the interpretation of the\nresults. For example, differences in hydrophobicity of samples surfaces (e.g. with 4-FPEAI\ntreatment) can alter water uptake. In addition, the 4-FPEAI treatment is additive to the surface and\ntherefore the measurement may not be representative of the perovskite band structure.\nAnswer 1: We thank the Reviewer for raising this important point, which gives us the opportunity to further\nclarify our experimental procedure. All UPS samples were handled under strictly controlled inert conditions.\nAll samples were transported using a sealed transfer container to move the sample from the glovebox to the\nUPS laboratory without exposure to ambient atmosphere. In particular, for each specimen the sealed\ncontainer was opened only immediately before mounting the sample on the manipulator holder, and the\nsample was then rapidly introduced into the analysis chamber through a fast-entry load lock, ensuring less\nthan 10 s exposure to ambient air. Under these conditions, surface contamination or water adsorption is\nminimized and cannot significantly affect the measured work-function and VBM values.\nAs already discussed in our previous revision, assessing the effect of MX-Cl on the hydrophobic character of\nthe perovskite surface is not straightforward, since MXene flakes are located at the buried m-TiO₂/perovskite\ninterface. For this reason, we performed dedicated XPS measurements of the O 1s region for the same\nsamples ad-hoc fabricated (4-FPEAI and MX-Cl/4-FPEAI, both fabricated with ~200 nm perovskite thickness\non m-TiO₂).\nAs shown in the revised Fig. S12, all samples exhibit two contributions in the O 1s region:\n• ~532.8–532.9 eV, corresponding to adsorbed water;\n• ~532.4–532.5 eV, assigned to C=O groups.\nImportantly, the relative amount of adsorbed water is essentially identical for the 4-FPEAI sample (84.0%) and\nthe MX-Cl/4-FPEAI sample (86.0%), indicating that MX-Cl does not measurably modify the hydrophobicity of\nthe perovskite surface and therefore cannot introduce artefacts in the UPS measurements related to\ndifferential water uptake.\nTo further strengthen this point, we have now extended the XPS O 1s analysis to the pristine perovskite\nreference (without MX-Cl and without 4-FPEAI). As reported in the updated Fig. S12, the reference film\nexhibits an adsorbed water contribution of 82.2%, fully comparable to that of the 4-FPEAI (84.0%) and MX-\nCl/4-FPEAI (86.0%) samples. These differences fall well within the intrinsic variability of multi-component O\n1s XPS fitting (typically ±3–4%) and cannot be regarded as indicative of any meaningful change in surface\nhydrophilicity.\nFig. S12: Panel a) XPS measurements O1s peaks and panel b) Pb 4f core-level spectra for reference perovskite film, 4-FPEAI treated\nperovskite film, 4-FPEAI treated perovskite film with MX-Cl. The samples are fabricated with a reduced thickness around 200 nm\ngrown on mTiO .\nsurface adsorption of water, and that all UPS/XPS samples share a comparable hydrophobic character.\nConsequently, the observed energy-level shifts cannot originate from differences in environmental exposure\nor water uptake. Instead, they result from the intended interfacial chemical modifications, namely, the MX-\nCl-induced dipole at the buried perovskite/m-TiO₂ interface conferring a localized n-type character and the\np-type surface shift produced by 4-FPEAI at the top interface.\nWe added in SI SECTION S.I. 6 the figure Fig. S12 and the following paragraph to comment on it\nNotably, Fig. S12 panel a) shows that the O 1s XPS spectra of the pristine perovskite, 4-FPEAI-treated, and\nMX-Cl/4-FPEAI samples exhibit nearly identical contributions from adsorbed water (82.2%, 84.0%, and 86.0%,\nrespectively). These values lie well within the intrinsic ±3–4% variability of multi-component O 1s peak fitting,\nindicating that neither MX-Cl nor 4-FPEAI significantly alter the hydrophilicity of the perovskite surface. This\nconfirms that the energy-level shifts observed in UPS/XPS arise from the intended interfacial chemical\nmodifications, at the buried m-TiO₂/perovskite interface and at the top perovskite surface, rather than from\ndifferences in environmental exposure or water uptake.\nWhile in the main text the following sentence:\nTo exclude charging or chemical artefacts, XPS analyses were performed, confirming that the observed WF\nand VBM shifts arise from genuine interfacial electronic modifications. (see Fig. S12 for detailed discussion).\nConcern 2: Doping of a semiconductor inherently moves the fermi level relative the valence band\nand conduction band of the material. There is no characterization of the material bandgaps provided.\nIn order to interpret where the fermi level lies within the bandgap, one must know two of the three:\nVBM, conduction band minimum (CBM), and bandgap. The VBM has been measured, but subtle\nbandgap changes from MX-Cl and/or 4-FPEAI processes could impact the interpretation of the\ndoping of the semiconductor.\nAnswer 2: We thank the Reviewer for this important point. We would like to point out that the optical\nbandgap of the films was measured from the Tauc analysis reported in Fig. S10a for perovskite films with SGP\n(named 4-FPEAI) and same film but with the addition of MX-Cl (named MX-Cl/4-FPEAI). Here we used the\ndirect-transition Tauc relation [𝑛=2]. However, the determination of the absorption coefficient α of halide-\nperovskite films can be challenging because these materials typically suffer from high surface roughness.\nConsequently, light scattering at the surface of the perovskite is commonly a dominant phenomenon. An\nalternative approach to determine the band gap of perovskites is to derive it from the external quantum\nefficiency (EQE or IPCE) of perovskite solar cells. In particular, as suggested by R. Carron and co-workers\n[Carron 2019_Thin Solid Films 669 (2019) 482–486], the peak energy of the derivative (or inflection point) of\nthe EQE curve can be interpreted as the bandgap. The result is independent from the user and is trustable if\nonly a clearly distinguishable peak is present in the dIPCE/dE plot (in case of a double peak the as-discussed\nmethodology for extracting the optical band-gap cannot be considered reliable due to the presence of\ninterference fringes while the use of IQE is recommended). In order to provide further inside the optical band-\ngap of the investigated perovskite absorber modification, we carried out the extrapolation of the E for\ng,opt\nG/EA (reference), G/4-FPEAI and G/MX/4-FPEAI from the IPCE spectra reported in Fig. S9a. The dIPCE/dE plots\nand the respective peak (E ) value have now been inserted in Fig. S9 as new panel “b”. Notably, the samples\ng,opt\nshowed very similar E values, confirming that the MXenes addition and/or the SGP did not significantly\ng,opt\naffect the optical band-gap of the perovskite absorber. Once obtained the E , the conduction band\ng,opt\nminimum (CBM) can be estimated as: CBM≈VBM +E .\nUPS g,opt\nWe explicitly report the VBM measured by UPS in Fig.S11. Therefore, by combining the VBM with the\nUPS\nindependently determined E , we obtained the CBM position and hence the Fermi level position within the\ng,opt\nband gap.\nSince no systematic change in E has been observed over the investigated sample typologies, we can\ng,opt\nconclude that the observed work-function shifts (observed by both UPS and KPFM measurements) reflect\nchanges in the Fermi level (doping/charge density) rather than changes in the band edges.\nFollowing this discussion, we have now included in the SI the following paragraph commenting the new added\nFig. S9b while also modifying the energy band diagram schematics by inserting the CBM position in Fig. S11i.\n“To verify that these recombination improvements are not associated with bandgap variations, we\nadditionally extracted the optical bandgap (E ) of the G/EA, G/4-FPEAI and G/MX/4-FPEAI devices from the\ng,opt\nderivative of the IPCE spectra, following the methodology proposed by Carron et al.10 (Thin Solid Films 669,\n482–486, 2019). In this approach, the energy position of the maximum in the d(IPCE)/d(E) curve corresponds\nto the optical bandgap, provided that a single, well-defined peak is present and interference fringes are\nnegligible.\nAs shown in Fig. S9c, all samples exhibit nearly identical peak energies (E ≈ 1.67–1.68 eV), indicating that\ng,opt\nneither the 4-FPEAI surface treatment nor the MX-Cl incorporation produces any measurable change in the\noptical bandgap of the perovskite absorber. This confirms that the electronic modifications observed by UPS\ndo not arise from variations in the band-edge positions.\nUsing these E values, the conduction band minimum (CBM) was then estimated as:\ng,opt\nCBM ≈ VBM +𝐸 .\nUPS g,opt\nThe corresponding CBM levels have been included in the updated energy-level diagrams in Fig. S11. Since the\nCBM values remain essentially unchanged across all sample configurations, the observed work-function shifts\nmust arise from Fermi-level shift (i.e., changes in interfacial charge density) rather than modifications of the\nfundamental bandgap. This validates the interpretation provided in the main text and supports the formation\nof a dipole-induced interfacial band bending rather than any bulk-doping-driven shift of the band edges.\nGiven the absence of measurable bandgap changes, the influence of MX-Cl on structural quality and\nrecombination pathways was subsequently examined.”\nFig. S9. a) Charge carrier lifetime extracted by the small-signal transient photo-voltage (TPV) decay profiles; b) Incident Photon to\ncurrent Conversion Efficiency (IPCE) spectra with the integrated photocurrent density (Integrated J ) related to the best efficient cell\nSC\nfor each proposed device structure. c) Derivative d(IPCE)/d(E) curves for G/EA, G/4-FPEAI, and G/MX/4-FPEAI devices. The energy\nposition of the maximum corresponds to the optical bandgap (E ). All samples exhibit nearly identical peak energies (≈1.67–1.68\ng,opt\neV), confirming that neither MX-Cl nor 4-FPEAI induces measurable changes in the perovskite bandgap. d) V light intensity-\nOC\ndependence [V (P )] of the investigated PSC structures. Linear fitting of the curves has been performed for P > 0.2 SUN, and the\nOC inc inc\nrespective slope values are reported on the plot.\nFig. S11. UPS spectra for perovskite films modified with different interface strategies: reference (a–b), MX-Cl (c–d), MX-Cl/4-FPEAI\n(e–f), and 4-FPEAI (g–h). Panels (a, c, e, g) show the secondary electron cutoff region used to extract the WF, while panels (b, d, f, h)\ndisplay the valence band region used to determine the VBM with respect to the EF. A gradual modulation of the WF and VBM is\nobserved across the series, highlighting the impact of dipole formation and band structure tuning induced by MXene doping and\nsurface gradient passivation; i) Schematic energy level alignment of perovskite films extracted from UPS measurements for the four\nconfigurations: REF (untreated), MX-Cl, 4-FPEAI, and combined MX-Cl/4-FPEAI. The WF, CBM and VBM positions are plotted relative\nto the vacuum level. A progressive modulation of the energy levels is observed, with MX-Cl inducing a downward shift in WF\n(consistent with dipole-induced vacuum level bending) and 4-FPEAI causing an upward shift in both WF and VBM. The intermediate alignment in the combined\ntreatment suggests the formation of a graded homojunction across the perovskite layer, promoting internal electric field formation\nand enhanced charge extraction.\nConcern 3: Assuming that the bandgap remains constant for all tested samples (REF, MX-Cl,\nMX-Cl/4-FPEAI, 4-FPEAI), there is no change in the doping level of the perovskite between the REF and\nMX-Cl samples (0.9eV between the VBM and the Fermi level for both samples). There is no\nenhancement of n-type character with MX-Cl addition (stated in the discussion), but it is consistent\nwith the MX-Cl dipole that the authors mention. However, that implies that the homojunction grading;\nof the band structure arises purely from the 4-FPEAI surface treatment. This would theoretically be\ntrue for both the 4-FPEAI and MX-Cl/4-FPEAI samples and the homojunction does not arise from a\nsynergy of MX-Cl and 4-FPEAI.\nAnswer 3: We thank the Reviewer for this constructive remark which gives us the opportunity to clarify the\nfundamental concept underlying the graded homojunction mechanism. We agree that the MX-Cl treatment\ndoes not introduce bulk n-type doping, as reflected by the unchanged EF–VBM values. This is consistent with\nits role as an interfacial dipole, as correctly noted by the Reviewer. However, the absence of an EF–VBM shift\ndoes not imply the absence of electronic modification at the buried interface. As reported in interface and\ndipole-layer studies on halide perovskites (Lim K.-G., Ahn S., Lee T.-W. Energy level alignment of dipolar\ninterface layer in organic and hybrid perovskite solar cells. J. Mater. Chem. C, 2018, 6, 3871-3881 and Basera\nP., Traoré B., Even J., Katan C. Interfacial engineering to modulate surface dipoles, work functions and dielectric\nconfinement of halide perovskites. Nanoscale, 2023, 15, 11884-11897.), EF–VBM is not the appropriate metric\nfor identifying interfacial dipoles or interface-specific band bending.\nIn perovskite systems where interfacial dipoles govern the band alignment, the relevant experimental\nfingerprint is the work-function shift (ΔWF), not the absolute EF–VBM value. When the VBM and bandgap\nremain unchanged, as in our samples, a reduction of the work function reflects a downward shift of the\nvacuum level induced by an interface dipole, rather than bulk doping. This behaviour is exactly what we\nobserve for MX-Cl based samples. Both DFT and UPS consistently show a vacuum-level shift at the\nperovskite/m-TiO₂ interface while EF–VBM remains unchanged, demonstrating that MX-Cl induces a dipole-\ndriven local electron accumulation layer, not a modification of the bulk perovskite DOS.\nThe Reviewer’s statement that “there is no enhancement of n-type character” is therefore correct for the\nbulk perovskite, but does not apply to the interfacial region, where DFT-PDOS reveals additional donor-like\nstates near the CBM and localized charge redistribution consistent with an effective n-type interfacial\ncharacter. These are widely recognized signatures of dipole-induced bands bending at hybrid perovskite\ninterfaces.\nWe would underline regarding the origin of the graded homojunction that:\n MX-Cl modifies the bottom interface, introducing a downward vacuum-level shift and interfacial\nelectron accumulation.\n 4-FPEAI modifies the top interface, inducing a p-type surface shift.\nThese two effects occur in distinct spatial regions of the perovskite layer and therefore do not replicate one\nanother. Their combination produces a vertical energy-level asymmetry, which cannot be obtained with 4-\nFPEAI alone. This interpretation is reinforced by the full set of complementary measurements: surface-\npotential changes in KPFM, vacuum-level shifts in UPS, interfacial states and dipoles in DFT, reduced\nrecombination in TRPL/TPV, and improved V and hysteresis reduction at the device level. SEM cross-sections\nOC\nfurther confirm that MX-Cl flakes reside exclusively at the bottom interface, enabling the spatial separation\nnecessary for a true graded architecture.\nIn summary, the graded homojunction arises from the synergy of two spatially separated interfacial\nmechanisms: the interfacial dipole induced by MX-Cl at the buried perovskite/m-TiO₂ interface (a localized n-\ntype character) and the p-type surface shift induced by 4-FPEAI at the top interface. The constancy of EF–\nVBM reflects the absence of bulk doping, not the absence of interfacial band bending, and is fully consistent\nwith the dipole-driven mechanism we propose. Accordingly, we have revised the corresponding sentences in\nthe Main Text and Supporting Information to avoid misunderstandings and reported below for convenience.\nIn the main text:\n“Indeed, tailoring the perovskite work function (WF) has emerged as a key strategy, alongside the design of\nhomojunctions to modulate the absorber’s electronic structure.19 “\n“Here we propose a novel approach to fabricate graded perovskite homojunctions arising from a modified\ninterfacial electronic environment with a local n-type character at the buried perovskite layer, induced by\nchlorine-based MXene doping,21 and a surface-localized p-type shift promoted by 4-FPEAI based SGP. This\nintentional spatial asymmetry establishes an effective n-to-p energy gradient across the absorber, enabling\nmore efficient carrier extraction and reduced recombination.”\n“Moreover, the projected density of states (PDOS) analysis underlines perovskite E remains mostly\ng\nunaffected, and MX-Cl additive introduces donor states near perovskite conduction band edge (Fig. S15),\nconfirming the locally induced perovskite n-character at the MX-Cl/perovskite interface”\nAnd in the S.I:\n“The reference (REF) sample exhibits a WF of 4.75 eV and a VBM located 0.9 eV below the Fermi level (EF),\ncorresponding to an ionization energy (IE) of 5.65 eV. Upon incorporation of Ti₃C₂Cl₂ MXenes (MX-Cl), the WF\ndecreases to 4.55 eV while the VBM remains unchanged at 0.9 eV below EF. This behavior indicates that the\naddition of MXenes enhances the localized n-type character of the perovskite, consistent with the formation\nof a surface dipole at the perovskite/MXene interface.”\n“Since WF of MX-Cl is 4.19 eV (Fig. S5f), the MX-Cl doping of perovskite enhances its localized n-type\ncharacter, as confirmed by decreased WF value of MX-Cl versus REF sample (see Tab. S3). Indeed, the MXene\naddition into the perovskite film translates in the formation of a dipole at perovskite/MXene interface,\nwithout altering the perovskite bandgap, while shifting the perovskite WF.11”\nMoreover, as further supporting experimental evidence, dark J-V curves for G/4-FPEAI and G/MX/4-FPEAI\nopaque devices have now been included and commented in SI (see Fig. S10c). Here, the reverse leakage\ncurrent and the ideality factor (n) of the G/MX/4-FPEAI cell are smaller than those of the G/4-FPEAI device\nsuggesting suppressed non-radiative recombination, in accordance with the increased τ in TRPL\n1\nmeasurements (see S.I. Section S.I. 5) and the higher PL emission intensity (see S.I., Fig. S10b). Moreover, the\nhigher forward injection current recorded for G/MX/4-FPEAI device implies an enhanced electron injection at\nthe buried perovskite/mTiO interface, attributed to the locally-enhanced n-type character (EF being closer\n2\nto the conduction band) of the MXene-containing perovskite film.\nWe now inserted dark I-V characterization and the related discussion in S.I. Section S.I. 5 that now sounds as\nfollowing reported:\n“Dark J–V measurements (Fig. S10c) further confirm the reduced non-radiative recombination in G/MX/4-\nFPEAI devices. The lower reverse leakage current and reduced ideality factor compared to G/4-FPEAI are\nconsistent with the longer TRPL lifetimes (main text, Fig. 3h) and higher PL intensity (Fig. S10b), while the\nenhanced forward current indicates more efficient electron injection at the buried perovskite/m-TiO₂\ninterface, arising from the locally strengthened n-type character induced by MX-Cl.”\nFig. S10: a) Tauc plots and b) steady-state PL spectra for perovskite films with SGP (named 4-FPEAI) and same film but with the addition\nof MX-Cl (named MX-Cl/4-FPEAI). PL spectra were acquired on three different points of the samples (named _1,_2,_3); c) Dark current\ndensity–voltage (J-V) curves comparing G/4-FPEAI and G/MX/4-FPEAI opaque devices. The ideality factor (n) is extracted from the\n𝑘𝑇 𝑑(𝑙𝑛𝐽)\ndiffusion-dominated current region according to the equation: 𝑛= ( ).\n𝑞 𝑑𝑉\nand we also recall the discussion in the main text by adding the followed sentence:\n“Adding MX-Cl in G/MX/4-FPEAI lowers the V (P ) slope, reducing ETL/perovskite recombination at this\nOC inc\ninterface, and boosting J ,36 while the decrease of both reverse leakage current and ideality factor in dark J-\nSC\nV curves is in line with the longer TRPL lifetimes and higher PL intensity.”\nConcern 4: The CPD and UPS measurements are only measuring surface properties of individual\nfilms. None of these methods directly measure the graded depth dependence that is being claimed.\nIt would be more appropriate to characterize the claimed effect with a method such as cross-\nsectional scanning KPFM, for example DOI: 10.1038/s41560-018-0324-8.\nAnswer 4: We thank the Reviewer for this comment, which allows us to clarify the methodological\nlimitations of cross-sectional KPFM on halide perovskites, and to justify why this technique is not required in\nthe context of our study. While cross-sectional KPFM can in principle provide depth-resolved information,\nits applicability to perovskite absorbers is severely constrained by well-documented artefacts arising from\nsurface preparation, environmental sensitivity and charge trapping.\nAs demonstrated by Lanzoni et al. (Nano Energy 88, 105941, 2021) even the widely used AM-KPFM mode is\nnot suitable to quantify work-function variations on rough perovskite films, and is highly susceptible to\nenvironmental conditions, which can lead to erroneous interpretations of the electronic landscape. These\nlimitations become even more critical in cross-sectional configurations, where mechanical cutting or FIB\npreparation introduces uncontrolled defects, surface charges, and local variations that dominate the\nmeasured contact potential.\nIn fact, even in the seminal work of Jiang et al. (Nature Communications, 6, 8397, 2015), one of the most\nadvanced demonstrations of cross-sectional KPFM in perovskite devices, the authors explicitly state that the\ntechnique is highly surface-sensitive and that “any modification of the cross-sectional surface region may\ninduce significant artefacts”. They further show that potential maps are often dominated by nonuniform\nsurface charges introduced during cleaving, making the interpretation of the junction potential unreliable\nunless a custom measurement protocol and specialized KPFM setup are developed to suppress these\nartefacts. The same work reports that previous cross-sectional KPFM studies yielded inconsistent or incorrect\njunction assignment precisely because of these issues.\nThis underscores that cross-sectional KPFM in perovskites is not a standard, routine, nor universally reliable\ntechnique, and that meaningful measurements require a level of instrumental modification (shielded\nglovebox operation, high-frequency dual-mode detection, bias-sequencing to cancel trapped charges) that\ngoes well beyond conventional AFM/KPFM capabilities.\nMoreover, the reviewer’s comment implicitly assumes that a graded homojunction must arise from a\ncontinuous bulk-doping profile, which would require a depth-resolved measurement. However, our system\ndoes not rely on bulk doping but on two spatially separated induced p or n character:\n MX-Cl creates a dipole and downward vacuum-level shift at the buried perovskite/m-TiO₂ interface,\nproducing localized n-type interfacial character and electron accumulation.\n 4-FPEAI induces a p-type shift at the top perovskite surface, modifying the electronic landscape only\nat the upper interface.\nBecause these two mechanisms act at opposite interfaces, the result is an asymmetric vertical band structure\nacross the film, even though the bulk perovskite electronic states remain unchanged. Cross-sectional KPFM\nwould not selectively resolve these two localized effects, because the bottom dipole is buried below the cross-\nsectional surface plane while the p-type shift is already directly measured by conventional KPFM/UPS.\nIn contrast, our interpretation is supported by a coherent and converging multi-technique dataset, including\nUPS, surface KPFM, DFT calculations, TRPL, TPV, SEM cross-sections, and device-level photovoltaic trends,\nwhich collectively provide a consistent and physically grounded picture of the interfacial electronic landscape.\nThis multipronged approach aligns with standard practice in the highest-impact literature for establishing\ninterface-induced band modulation in perovskite optoelectronics.\nFor these reasons, while cross-sectional KPFM may be of conceptual interest, it would not yield more\nconclusive evidence than the suite of complementary techniques already presented, nor it is required to\nsubstantiate the conclusions of this work.\nMoreover, as correctly noted by the Reviewer in the first comment, UPS is indeed the preferable technique\nwithin the scientific community for determining the work function, but it remains intrinsically surface\nsensitive. Given the limitations of cross-sectional KPFM, which in perovskite systems often suffers from\npreparation-induced artefacts, instabilities, and poor quantitative reliability, we adopted a different strategy\nto emulate depth-resolved UPS information in a controlled and reproducible manner.\nSpecifically, we fabricated the four representative films (reference, MX-Cl, 4-FPEAI, and MX-Cl/4-FPEAI) as ad-\nhoc samples with a reduced thickness of approximately 200 nm, a value deliberately chosen because it\nmatches the lateral size of the MXene flakes (see the picture in the following now added in Fig. S4 panel c).\nFig. S4: SEM images with corresponding chemical composition of a) multilayer Ti C Cl and b) delaminated Ti C Cl ; c) AFM topography\n3 2 2 3 2 2\nof the Cl-terminated Ti₃C₂ MXene flakes.\nUnder these conditions, the UPS probing depth effectively samples electronic properties that correspond to\ndistinct regions of the full device stack without requiring destructive cross-sectional preparation. In practice,\neach tailored thin film acts as a “depth-selected proxy,” enabling us to probe the electronic influence of MX-\nCl and 4-FPEAI at different vertical positions of the actual perovskite layer, while fully preserving sample\nintegrity.\nThis approach overcomes the intrinsic z-direction limitations of UPS while avoiding the well-known artefacts\nassociated with cross-sectional KPFM on halide perovskites. The resulting dataset therefore provides a robust\nand internally consistent picture of the vertical electronic landscape across the device.\nWe hope this clarification addresses the Reviewer’s concern. In the new version of the main text, we included\na clear reference to the ad-hoc sample fabrication for UPS measurements as followed reported:\n“ad-hoc fabricated samples (see section S.I. 6 in SI)”.\nConcern 5: In Tab. 2. the authors provide an error of +/- 0.1eV on the VBM-work function energy\noffsets. The VBM position relative to the Fermi level is 0.9 +/-0.1 eV, 0.9 +/-0.1eV, and 0.8 +/-0.1eV\nfor the REF, MX-Cl, and MX-Cl/4F-PEAI samples, respectively. Therefore, all of these measurements\nare within error and there is no discernable difference in doping between these three samples and\nthe graded homojunction conclusion should not be made.\nAnswer 5: We thank the Reviewer for this constructive observation. We agree that the EF–VBM values do\nnot indicate bulk n-type doping in the MX-Cl sample, which is fully consistent with the role of MX-Cl as an\ninterfacial dipole rather than a bulk dopant, as we have discussed at extensively in the responses to concerns\n3 and 4. However, the absence of an EF–VBM shift does not preclude the presence of interfacial electronic\nmodifications, and the interpretation of the UPS results must consider the distinction between absolute and\nrelative energy uncertainties.\nThe distinction between absolute and relative uncertainties in UPS/XPS is well documented in the\nphotoemission literature (see Seah, M. P. & Dench, W. A. \"Quantitative electron spectroscopy of surfaces: A\nstandard data base for electron inelastic mean free paths in solids.\" Surf. Interface Anal. 1979, 1, 2–11; Hufner,\nS. Photoelectron Spectroscopy: Principles and Applications, 3rd Ed. Springer, 2003.; Greczynski, G. & Hultman,\nL. \"Compromising science by ignorance: guidelines to minimize false discoveries in XPS analysis.\" Prog. Mater.\nSci. 2020, 100, 100766; Kraut, E. A. et al. \"Precise determination of the valence-band edge in XPS.\" Phys. Rev.\nLett. 1980, 44, 1620), the typical ±0.1 eV uncertainty reflects the absolute determination of the VBM and\nwork function. This error budget is dominated by spectrometer calibration, secondary-electron cutoff fitting,\nand the VBM extrapolation procedure. Crucially, these components behave largely as systematic offsets, while\nthe relative shifts between samples measured in the same UPS session are substantially more precise\n(typically ±0.03–0.05 eV). Therefore, even shifts of a few ten millielectronvolts are considered meaningful\nwhen comparing a series of samples evaluated under identical conditions.\nMoreover, in halide perovskites a Fermi-level shift as small as 40–80 meV corresponds to a large relative\nchange in carrier concentration, due to the low density of states near the band edges (Meggiolaro & De\nAngelis, ACS Energy Lett. 2018; Tress, Adv. Energy Mater. 2019). Thus, modest energy shifts can have\nsubstantial implications for built-in electric fields, interfacial band bending, and junction formation.\nIn our system, MX-Cl induces a significant work-function reduction, while maintaining EF–VBM unchanged\nwithin the expected range for an undoped perovskite. This behaviour is fully consistent with an interfacial\ndipole and a downward vacuum-level shift, as also predicted by DFT and seen in the layer-resolved PDOS,\nwhich shows donor-like interfacial states near the perovskite CBM and local electron accumulation. \nAccordingly, EF–VBM is not the correct metric to diagnose dipole-induced band alignment changes.\nThis interpretation is also supported by our XPS core-level analysis. The Pb 4f peaks remain essentially\nunchanged across REF, MX-Cl, and MX-Cl/4F-PEAI samples (Fig. S12), indicating the absence of chemical shifts\nor charging artefacts. As shown by Cui et al. (Nat. Energy, 2019), paper also suggested by the reviewer, the\ninvariance of Pb and I core levels is a strong indication that observed changes in the valence region and work\nfunction originate from genuine modifications of the electronic structure rather than instrumental or fitting\nartefacts. Our data exhibit the same behaviour: core-level peaks remain stable, whereas ΔWF and VB spectral\nchanges follow the expected trend for dipole formation at the buried MX-Cl interface and a surface p-shift\ninduced by 4-FPEAI.\nFinally, the graded homojunction does not originate solely from 4-FPEAI, as the Reviewer suggests. MX-Cl\nmodifies locally the buried bottom interface (interfacial dipole, vacuum-level offset, electron accumulation),\nwhereas 4-FPEAI modifies the top surface (p-type shift). These two spatially separated effects produce a\nvertical asymmetry in the energy landscape that neither additive can generate alone. The constancy of EF–\nVBM is thus fully consistent with a dipole-driven mechanism and does not contradict the graded\nhomojunction interpretation.\nWe added in S.I SI SECTION S.I. 6 the Fig. S12 b) and the relative comment as in the following reported:\n“The XPS Pb 4f core-level spectra of all samples show in Fig.S12 panel b) (REF, MX-Cl, 4-FPEAI, and MX-Cl/4-\nFPEAI) exhibit nearly identical binding energies and relative component ratios (Pb in perovskite ≈ 97–98%, Pb\nin PbO ≈ 2–3%), with no evidence of chemical shifts or peak broadening. This invariance confirms that the\nobserved UPS valence-band and work-function shifts are not affected by charging, surface deterioration, or\nmeasurement artefacts, but rather arise from genuine electronic modifications induced by the different\nstrategies: doping with MX-Cl and interfacial treatment with 4-FPEAI.”\nFig. S12: Panel a) XPS measurements O1s peaks and panel b) Pb 4f core-level spectra respectively, for reference perovskite film, 4-\nFPEAI treated perovskite film, 4-FPEAI treated perovskite film with MX-Cl. The samples are fabricated with a reduced thickness around\n200 nm grown on mTiO .\n“To exclude charging or chemical artefacts, XPS analyses were performed, confirming that the observed WF\nand VBM shifts arise from genuine interfacial electronic modifications. (see Fig. S12 for detailed discussion).”\nConcern 6: In addition to my above concerns, I do not believe that MPP under cycled light is a\nsufficient alternative to the combined stressors of light + heat and reiterate my comment that\naccelerated testing under 85C and 1 sun illumination should be included.\nAs stated in the manuscript, accelerated stability testing, while unquestionably relevant, is beyond the scope\nof this work and not central to our investigation. Nevertheless, we already included a prolonged light cycling\nMPP test, which is widely regarded as a reliable indicator of device operational stability. Moreover, our\noutdoor monitoring of the tandem panel installed in Heraklion (Crete) from August to November 2023\nshowed that the top-surface temperature never exceeded ~65 °C, even under peak irradiance (see revised\nFig. S29c in S.I.). These real-world data already provide a meaningful context for evaluating the device\nrobustness. However, we agree that the LED-based light-cycling setups may underestimate the combined\neffect of temperature and light soaking when the cell is working at MPP, as their operating temperature\ntypically remains in the 35–40 °C range. For this reason, and to directly address the reviewer’s concern, we\nperformed an additional stability experiment under a class B solar simulator (SolarConstant 1200, K.H.\nSteuernagel Lichttechnik GmbH, Germany) calibrated at 1000 W/m2, continuously tracking both the P and\nMPP\nfront surface temperature. Small area opaque devices were subjected to 100 h of 1 Sun illumination, during\nwhich the cell temperature exceeded 85°C within the first hour (see SI, section S.I. 9, Fig. S22d). Under these\nharsher conditions, the graded-homojunction device demonstrated excellent stability, with a P loss below\nMPP\n5% over the entire stress period (100 hours).\nThis new 1-Sun + 85 °C stability test is now reported and discussed in Section S.I. 9 of the Supporting\nInformation as following reported:\nFig. S22: a) J-V curve in forward and reverse scan for the best-performing 2D material-engineered ST-PSC (G/MX/4-FPEAI structure);\nb) IPCE spectra with the integrated photocurrent density (Integrated J ) related to the best efficient optimized cell (G/MX/4-FPEAI)\nSC\nfor both opaque and ST proposed device structure. c) Indoor photo-stability under MPP tracking of G/EA and G/MX/4-FPEAI semi-\ntransparent encapsulated cells upon exposure to cycled light (ISOS-LC-1); d) Normalised power at maximum power point (P )\nMPP\ncontinuously acquired during a light soaking test performed under a solar simulator class B @ 1 Sun (circle dotted red curve). Substrate\ntemperature has been monitored by a thermocouple (star dotted dark cyan curve) fixed on the device top surface, showing an\nincreasing value up to 87°C during the first stress test hour, while retaining a constant value during the rest of the accelerated test.\n“Although light-cycling test at MPP is widely recognized as a reliable indicator of device operational stability,\nthis test may underestimate the coupled effects of light soaking and temperature, since the typically\nemployed white LED illumination sources maintain device temperature in the 35–40 °C range. To directly\naccount for these limitations, we carried out an additional stability test under a class-B sun simulator (1000\nW m⁻², see experimental section for further details about the apparatus), simultaneously tracking P and\nMPP\nthe front-surface temperature. Under this configuration, the cell temperature overcame 85 °C within the first\nhour, and the device was continuously stressed for 100 hours. As shown in Fig. S22d, the graded-\nhomojunction device retained more than 95% of its initial P , demonstrating excellent thermal and photo-\nMPP\noperational stability under harsh conditions.”\nAnd modify Fig.S29 by adding panel c) and Fig. S22 by adding panel d) and relative captions.\nFig. S29: a) Normalized power at maximum power point (P ) stabilization prior the acquisition of the monthly I-V curves for the\nMPP\nDEM2 left at V in outdoor conditions; b) daily irradiance for the days selected during the outdoor test for the acquisition of the I-V\nOC\ncurves. c) Top-surface temperature of the tandem panel recorded under real operating conditions between August and November\n2023. Even during peak irradiance at midday under clear-sky conditions, the module temperature did not exceed ~65 °C, providing a\nrealistic reference for assessing the operational thermal load experienced by the devices.\nThe discussion is also recalled in the main text by adding the following sentence:\n“…and combined heating + light soaking test (Fig. S22d).”\nand the following paragraph in the experimental section:\n“Combined heating+light soaking stress test was performed using a class-B solar simulator (SolarConstant\n1200, K.H. Steuernagel Lichttechnik GmbH, Germany). The system employs a xenon arc lamp coupled with an\noptical AM 1.5G filter to reproduce the standard solar spectrum over a broad spectral range (300–2500 nm).\nAccording to manufacturer specifications and IEC 60904-9 classification, the simulator exhibits class-B\nperformance in spectral match, irradiance uniformity, and temporal stability. In particular, the spectral\ndistribution in the near-infrared region shows the characteristic enhancement associated with xenon-based\nsources, consistent with the class-B spectral mismatch factor. The output irradiance is stabilized and set to\nbefore each measurement session using a certified silicon reference cell.\nThe relatively broad IR output, combined with continuous illumination, results in a device operating\ntemperature representative of realistic stress conditions, while remaining fully compliant with the IEC\ntolerances for photovoltaic testing.”\nReviewer #4 (Remarks to the Author)\nI have carefully reviewed the revised manuscript and am pleased to inform you that I am satisfied\nwith the revisions. The manuscript has addressed the concerns raised previously, and I believe it is\nnow suitable for publication in Nature Communications. I recommend its acceptance.\nThank you for the opportunity to review this work.\nWe thank the Reviewer for the positive evaluation and for acknowledging the revisions made to the\nmanuscript. We are grateful for the insightful comments that helped us improve the clarity and robustness of\nour work.\nReviewer’s Comments:\nReviewer #3 (Remarks to the Author)\nI appreciate the authors detailed responses to the concerns that have been raised. The\nauthors included the following statement in their response:\n\"However, our system does not rely on bulk doping but on two spatially separated induced\np or n character:\n“MX-Cl creates a dipole and downward vacuum-level shift at the buried perovskite/m-TiO2;\ninterface, producing localized n-type interfacial character and electron accumulation.”; “ 4-\nFPEAI induces a p-type shift at the top perovskite surface, modifying the electronic\nlandscape only at the upper interface.\""}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed all previous comments and therefore publication of the manuscript is agreed.", "reviewer_3": "I appreciate the authors’ detailed responses to the concerns that have been raised. The authors included the following statement in their response:\n\"However, our system does not rely on bulk doping but on two spatially separated induced p or n character:\n• MX-Cl creates a dipole and downward vacuum-level shift at the buried perovskite/m-TiO₂ interface, producing localized n-type interfacial character and electron accumulation.\n• 4-FPEAI induces a p-type shift at the top perovskite surface, modifying the electronic landscape only at the upper interface.\"\n\"This validates the interpretation provided in the main text and supports the formation of a dipole-induced interfacial band bending rather than any bulk-doping-driven shift of the band edges.\"\nI generally agree with the above proposed mechanisms by the authors and believe the data are consistent with the conclusion that the observed effects are independent surface dipoles generated by the MX-Cl at the buried interface and 4-FPEAI at the top surface. The specifics of the 4-FPEAI passivation in these statements are a bit vague, but the accepted mechanisms in the field are i) surface passivation of undercoordinated lead sites, ii) the formation of a 2D/3D heterostructure, and iii) surface dipole passivation.\nA graded homojunction and field-effect passivation are not the same mechanism. A graded homojunction implies an internal electric field generated by a doping profile within the material. Field-effect passivation is achieved through the application of an external electric field or fixed charges/dipoles from an adjacent layer. I believe the authors and I are in agreement that a bulk doping mechanism is not supported by the data and therefore field-effect passivation is a more appropriate description of the mechanism.\nI would support this manuscript for publication if the claim of a graded homojunction is removed and is instead described as field-effect or surface dipole passivation.", "reviewer_4": "I have carefully reviewed the revised manuscript and am pleased to inform you that I am satisfied with the revisions. The manuscript has addressed the concerns raised previously, and I believe it is now suitable for publication in Nature Communications. I recommend its acceptance.\nThank you for the opportunity to review this work."}, "author_response": "Answers to the Reviewers' comments:\nWe would like to sincerely thank the reviewers for their thorough and constructive evaluation of our\nmanuscript. We are grateful for their recognition of the technical quality"}]

Decision letter