Tailored PEDOT:PSS phase segregation for high-efficiency flexible all-perovskite tandem solar cells and mini-modules

Domain: physics
Published: 2025-12-11
Editorial outcome: major_revision

Abstract
Flexible all-perovskite tandem solar cells (TSCs) promise lightweight power for portable and aerospace uses, yet performance remains limited by interfacial losses in narrow-bandgap (NBG) sub-cells, particularly arising from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Here, we uncover a previously unrecognized vertical phase segregation in PEDOT:PSS films, with an insulating PSS-rich surface layer atop a conductive PEDOT-rich base. This architecture induces interfacial electric dipoles that hinder efficient hole extraction. We find that the inclusion of Triton X-100 disrupts this segregation by modulating polymer interactions, thereby suppressing surface dipole formation. NBG perovskite cells incorporating this modified interface exhibit enhanced performance and reproducibility. This strategy enables flexible TSCs with a power conversion efficiency (PCE) of 25.4% and a proof-of-concept flexible tandem solar mini-module reaches 19.7%, with theoretical modeling projecting module efficiencies exceeding 24%. These results provide key insights into PEDOT:PSS interfacial phenomena and elucidate design principles for optimizing interfaces for next-generation flexible perovskite tandems.

Introduction
All-perovskite tandem solar cells (TSCs) offer a promising route to surpass the efficiency limits of single-junction devices1,2, with certified power conversion efficiencies (PCEs) exceeding 30% on rigid substrates3. Their flexible counterparts offer additional advantages, especially a high power-to-weight ratio, making them particularly attractive for applications in portable electronics, building-integrated photovoltaics, and aerospace systems4,5. Despite these benefits, flexible TSCs still lag behind rigid devices, with reported PCEs below 25%6,7. One critical barrier lies in the fabrication of efficient narrow-bandgap (NBG) perovskite solar cells (PSCs) on non-planar, flexible substrates. To achieve high-performance flexible TSCs, both the wide-bandgap (WBG) and NBG PSCs require conformal and high-quality hole transport layers (HTLs). While recent advances—such as self-assembled monolayers (SAMs) and NiO interlayers—have improved HTL interfaces in WBG PSCs6–8, the NBG PSCs still rely heavily on poly(3,4-ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS).

Despite its compatibility with solution processing, PEDOT:PSS presents two major challenges in flexible devices: difficulty in forming uniform ultrathin coatings and phase segregation that leads to insulating, hygroscopic PSS accumulation9,10. These limitations compromise film uniformity, interfacial quality, and ultimately, device reproducibility and performance.

Although PEDOT:PSS has often been assumed to adopt a “pancake” morphology composed of PEDOT-rich cores surrounded by PSS-rich shells, this structural model—supported mainly by morphological studies—does not fully account for the film’s interfacial behavior11,12. Due to the insulating and hygroscopic nature of PSS, many efforts have sought to remove the PSS shell within the pancake-like PEDOT:PSS film through post-deposition treatments (PDTs), such as the application of solvents (e.g., dimethyl sulfoxide (DMSO))13,14, ionic liquids15, surfactants16, or acids17. Most of these studies include a washing process for the as-deposited PEDOT:PSS film and observed a reduced PSS phase and enhanced conductivity in the film. However, the underlying mechanisms driving these improvements remain unclear, and the structural properties of PEDOT:PSS films are still not fully understood10,13. Furthermore, processing challenges for PEDOT:PSS films on flexible WBG PSCs persist, limiting progress in high-performance flexible TSCs.

Here, we present an additive engineering strategy to overcome these limitations by incorporating the non-ionic surfactant Triton X-100 directly into the PEDOT:PSS dispersion. Our characterization reveals a vertically segregated double-layer structure in pristine PEDOT:PSS films, with an insulating PSS-rich surface atop a conductive PEDOT base. We hypothesize that this structure induces interfacial electric dipoles that impede charge extraction. The incorporation of Triton X-100 suppresses this surface segregation, resulting in significant enhancements in both performance and reproducibility of NBG PSCs—from an average PCE of ~14.0% to 19.9%. Integrating this strategy into flexible tandem architectures, we achieve a high PCE of 25.4%. Furthermore, leveraging simulation-assisted laser scribing, we demonstrate a proof-of-concept mini-module with 19.7% efficiency. Theoretical modeling suggests that module efficiencies above 24% are achievable on 10 cm × 10 cm flexible substrates, marking a critical step toward scalable, high-performance flexible tandem photovoltaics.

Results
Challenges in NBG PSCs and tandems
Achieving high-performance NBG PSCs with excellent uniformity is essential for the development of efficient flexible TSCs and, subsequently, tandem solar modules. However, our observations show that the processing of NBG PSCs suffers from poor reproducibility and uniformity. This issue is reflected in the PV performance of small-area devices (0.09 cm2), as shown in Fig. 1a–d. A significant spread in PV performance is observed, primarily attributed to variations in JSC and FF.

During the J-V measurements, we noticed that the JSC and FF of devices on the same substrate are highly dependent on the measurement sequence. The first measured cell (Cell #1) consistently exhibited higher JSC and FF, whereas the last measured cell (Cell #4) showed significantly lower values. When the first cell (Cell #1) was remeasured after the measurement of Cell #4, it also displayed reduced JSC and FF. However, the integrated JSC derived from EQE measurement is much higher than that obtained from the J-V measurement (Supplementary Fig. 1). To investigate this observation further, we examined the growth of NBG perovskite film using scanning electron microscopy (SEM). The SEM images revealed no pinholes within the perovskite absorber (Supplementary Fig. 2). Based on these results, we assume photocurrent under the light-soaking conditions of J-V measurements with AM1.5 illumination to be limited by the insufficient rate of thermionic emission over interfacial barriers, while no such limitation applies for the low current density during a quantum efficiency measurement18. We hypothesize that the perovskite/PEDOT:PSS buried interface is mainly responsible for this phenomenon and, therefore, a significant spread in PV performance for our NBG PSCs.

Based on these NBG cells, flexible all-perovskite TSCs were also fabricated, using an architecture consisting of a polyethylene naphthalate (PEN) substrate with the following layer stack: ITO/(4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (4PADCB)/WBG perovskite/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/SnOx/gold (Au)/PEDOT:PSS/NBG perovskite/C60/SnOx/copper (Cu), of which the WBG cell is based on our previous work with optimized uniformity7,19. Another challenge encountered is associated with the processing of the PEDOT:PSS film, which is relatively straightforward in single-junction devices but becomes problematic in TSCs, especially on flexible substrates. As illustrated in Fig. 1e, we found that spin-coating PEDOT:PSS (Clevios™ PVP Al 4083) onto flexible WBG PSCs presents significant challenges in realizing conformal coating. The spin-coating process often results in non-conformal coatings (green dashed lines) due to the low wettability of the PEDOT:PSS dispersion or damage to the underlying WBG PSCs (white dashed lines), caused by the long interaction with the aqueous dispersion. With this non-conformal layer, the overall PV performance uniformity of the flexible TSCs was unsatisfactory, with an average PCE of around 21 to 22%, as shown in the histogram in Fig. 1f.

Triton X-100 additives in PEDOT:PSS
To optimize the coating process of PEDOT:PSS and improve the PV performance of the NBG PSCs and therefore TSCs, we developed an additive engineering strategy by introducing a non-ionic surfactant, Triton X-100, into the PEDOT:PSS dispersion. The chemical structures of PEDOT:PSS and Triton X-100 are shown in Fig. 2a. Triton X-100 features a hydrophilic polyethylene oxide chain and a hydrophobic aromatic hydrocarbon group. The hydrophilic chain interacts with PSS, while the hydrophobic group interacts with PEDOT, promoting a well-dispersed system20. We first compared the wettability of PEDOT:PSS on glass/ITO substrates. Contact angle measurements, shown in Fig. 2b, revealed that adding 0.5 wt% Triton X-100 to PEDOT:PSS significantly reduced the contact angle of droplets on the glass/ITO surface, confirming improved wettability. Moreover, on flexible PEN/ITO and PEN/WBG cell surfaces, pristine PEDOT:PSS exhibits much larger contact angles, reflecting poorer wetting. The introduction of Triton X-100 effectively reduces these contact angles as well, as shown in Supplementary Fig. 3, confirming its beneficial effect on wettability across both rigid and flexible substrates. Supplementary Note 1 presents a schematic of the mechanism by which Triton X-100 enhances aqueous dispersion, along with the corresponding experimental data.

To examine its influence on perovskite growth, we compared top-view SEM images of perovskite films grown on PEDOT:PSS with and without Triton X-100, shown in Fig. 2c (see additional images in Supplementary Fig. 4). It is calculated that the perovskite film based on PEDOT:PSS with Triton X-100 exhibited an average grain size of 0.45 μm2, significantly larger than the 0.18 μm2 obtained in the reference group (Supplementary Fig. 5). This increase in grain size indicates a reduced concentration of perovskite seeds, which is most likely due to the modified morphology of PEDOT:PSS21,22. Furthermore, the films showed more pronounced columnar growth (Fig. 2d and Supplementary Fig. 6), which is beneficial for charge transport and collection23. Grazing-incidence wide-angle X-ray scattering (GIWAXS) confirmed that the NBG perovskites grown on both types of PEDOT:PSS films preserved their polycrystalline structure and maintained the same crystal orientation (Supplementary Fig. 7). We then measured the time-resolved photoluminescence (TrPL) of the perovskite films deposited on different substrates. As shown in Fig. 2e, the TrPL decay curve of the film grown on PEDOT:PSS modified with Triton X-100 exhibited a faster decay compared to the reference sample, indicating improved charge carrier extraction at the PEDOT:PSS/perovskite interface (lifetime fitted in Supplementary Table 1).

We fabricated NBG PSCs using PEDOT:PSS with various concentrations of Triton X-100 (0 to 1 wt%). As shown in Supplementary Fig. 8, device performance improved across a wide concentration range (0.1–0.75 wt%), while higher concentrations (>0.75 wt%) led to excessive spreading and lower performance. This observation confirms that Triton X-100 not only enhances wettability but also contributes to improved device performance and narrowed spreading. Furthermore, the previously observed drop in JSC and FF during sequential J–V measurements has been eliminated by incorporating Triton X-100, as shown in Supplementary Fig. 9. This further supports our earlier assumption that the issue with our NBG PSCs originates from the PEDOT:PSS layer. With Triton X-100 additive in PEDOT:PSS, the champion device exhibited a PCE of 21.5% as the J-V results shown in Fig. 2f, with the measured JSC closely matching the integrated JSC obtained from EQE measurements (Fig. 2g). Devices with Triton X-100 also show enhanced shelf stability: under N2-glovebox storage they retain 93.8% of the initial PCE after 1375 h, while reference devices retain only 80.7% after 100 h (Fig. 2h). The operational stability of the device was checked by maximum power point (MPP) tracking as shown in Supplementary Fig. 10, with a steady-state efficiency of ~20.7%.

Structure of PEDOT:PSS
To elucidate the mechanism by which Triton X-100 enhances the quality of PEDOT:PSS films and the performance of NBG PSCs, we conducted time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling on films with 0.5 wt% Triton X-100 (hereafter refer as with Triton X-100) or without Triton X-100 (hereafter refer as reference). Depth profiling of ethylene oxide fragments (C2H4O) characteristic of Triton X-100, revealed their localization predominantly at the bottom of the PEDOT:PSS film (Fig. 3a). In addition, depth profiles of sulfur (S) (Fig. 3b) revealed a reduced S at the surface of the film containing Triton X-100. While ToF-SIMS cannot differentiate between S originating from PEDOT or PSS, the observed reduction in S suggests a strong interaction between Triton X-100 and PEDOT or PSS, changing the film composition. To further quantify this compositional shift, we performed X-ray photoelectron spectroscopy (XPS) measurements on PEDOT:PSS films. As shown in Fig. 3c, d, raw XPS data were acquired for PEDOT:PSS films in the range of 160–174 eV (S2p), and the fitted peaks represent sulfur atoms from the styrenesulfonate (170–166 eV, red gradients) and the EDOT thiophene (166–161 eV, navy gradients). Given that both PEDOT and PSS contain one sulfur atom per repeat unit, the surface PSS/PEDOT ratio can be directly obtained from these spectral areas24. The reference film exhibited a surface PSS/PEDOT ratio of 5.28:1, which decreased significantly to 2.38:1 upon Triton X-100 incorporation. This reduction indicates effective suppression of surface-enriched PSS, which is beneficial for improving film morphology, electrical conductivity, and the growth of high-quality perovskite layers25. In addition to enhancing film quality, the reduced PSS content also addresses its intrinsic hygroscopicity, which is known to induce water uptake and interfacial instability during device fabrication26. In this work, the detrimental effect of water uptake is further confirmed, as NBG PSCs based on PEDOT:PSS exhibit degraded performance after prolonged exposure to ambient air following annealing, as shown in Supplementary Fig. 11.

To further investigate the PSS/PEDOT ratio within the bulk of the film, we performed XPS depth profiling on PEDOT:PSS films. Using an Ar ion gun, the PEDOT:PSS (~20 nm) film deposited on ITO was sputtered layer by layer, and XPS analysis was performed at each depth. The evolution of the S2p region with sputtering time is presented in Fig. 3e. Notably, the results reveal that PSS accumulates only near the top surface of the PEDOT:PSS film, while the bulk region is dominated by PEDOT. Our observation indicates a distinct structural arrangement for PEDOT:PSS that differs from the previously assumed “pancake” structure model reported in the literature27. When Triton X-100 is introduced, as shown in Fig. 3f, a substantial reduction in the PSS phase is observed at the top surface of the film. To quantify the overall PSS/PEDOT ratio across the films, we integrated the intensities of the S2p peaks corresponding to the binding energy ranges for styrene sulfonate (170–166 eV) and EDOT thiophene (166–161 eV). The atomic ratio of sulfur from PSS decreased from 13.4% in the reference film to 5.4% in the film with Triton X-100. This significant reduction highlights the ability of Triton X-100 to alter the bulk composition of PEDOT:PSS by weakening the ionic interactions between PEDOT and PSS28. Such modulation facilitates the removal or redistribution of excess PSS, ultimately improving the interfacial properties and overall quality of the hole transport layer. XPS depth profiling for the PEDOT:PSS films on PEN/ITO shows a similar trend as shown in Supplementary Fig. 12.

Double-layer structure and dipoles
Through our in-depth investigation of PEDOT:PSS, we uncovered a previously overlooked double-layer structure in ultra-thin PEDOT:PSS films. The addition of Triton X-100 was found to significantly reducing the PSS content in the resulting film, thereby modifying both the interface properties and the subsequent perovskite growth. The formation and impact of this double-layer structure on device performance are explained as follows:

As depicted in Fig. 4a, spin-coating the PEDOT:PSS dispersion led to the formation of a double-layer structure after drying, consisting of a thin PSS layer at the top surface and a thick PEDOT-rich base layer at the bottom. We attribute this phase segregation to the preferential lateral interactions between PEDOT units, which drive PSS segregation toward the surface. When applied to NBG solar cells, the averaged PCE was merely ~13.9%, with poor uniformity across multiple fabrication batches (Fig. 4b). We suspect that the PSS layer poses a current barrier at the buried interface during light soaking and thus leads to decreased performance for the solar cells. By incorporating Triton X-100 into the PEDOT:PSS dispersion prior to the spin-coating process, we observed a significant reduction in the PSS phase amount (as illustrated in Fig. 4c), changing the overall interfacial properties of the PEDOT:PSS layer. As shown in the box chart (Fig. 4d), Triton X-100 enhanced the uniformity of photovoltaic parameters, with an average PCE of ~19.9%, a substantial improvement over the reference group. The improved uniformity was primarily from the improvements in JSC and FF (Supplementary Fig. 13). Electroluminescence mapping of the 1 cm2 NBG PSCs further confirmed the improved uniformity of the cells across the substrate (Supplementary Fig. 14). During the formation of the double-layer structure, Triton X-100 likely accelerates the phase segregation by weakening ionic interactions between PEDOT and PSS, thereby reducing the insulating PSS phase in the resultant film28. As detailed in Supplementary Note 2, Triton X-100 associates with PSS, loosening the core–shell structure and increasing the transmittance of the PEDOT:PSS dispersion. It is worth noting that the resulted double-layer structure may represent a unique case in ultra-thin films where the thickness is comparable to or smaller than the core-shell dimensions, whereas, in thicker films (>100 nm), previous studies have reported PSS distributed throughout the bulk29.

Although the addition of Triton X-100 to PEDOT:PSS has resulted in improved perovskite growth, it remains unclear whether the enhanced device performance is primarily due to the improved perovskite quality or the modified buried interface properties, specifically the reduced PSS content. To elucidate the role of surface PSS, we conducted post-deposition treatments (PDTs) on reference PEDOT:PSS films using deionized water and Triton X-100, respectively, as illustrated in Fig. 4e. Solar cells processed with these films exhibited treatment-dependent behavior. With the water-washed films, the devices showed a modest improvement in PCE (Fig. 4f), likely resulting from the partial removal of the insulating PSS phase at the surface. Intriguingly, Triton X-100-treated films demonstrated significantly enhanced PCE and improved uniformity (Fig. 4g), primarily due to an increase in JSC (Supplementary Fig. 15). This underscores not only the critical role of this interface but also that Triton X-100 more effectively removes the insulating PSS phase due to its stronger interaction with PSS. To check the effect of Triton X-100 at the ITO/perovskite interface, solar cells were also fabricated using Triton X-100-treated ITO substrates prior to PEDOT:PSS deposition. As the results shown in Supplementary Fig. 16, their performance remained statistically similar to that of the reference group, indicating that the ITO/perovskite interface plays a negligible role. Instead, the buried perovskite/PEDOT:PSS interface emerges as the dominant loss pathway of the devices.

In the traditional “pancake” model, improvements in PEDOT:PSS film quality are typically ascribed to enhanced conductivity resulting from reduced PSS content30. However, with our identification of a vertically segregated double-layer structure in PEDOT:PSS and the observation of abnormal J-V behavior under light soaking, we propose an additional mechanism involving the intrinsic charge characteristics of the material. PEDOT:PSS is a polyelectrolyte complex composed of positively charged PEDOT chains and negatively charged PSS counterions, bound through electrostatic interactions. We hypothesize that vertical phase segregation gives rise to localized electric dipoles at the interface between the PSS-rich, negatively charged surface layer and the PEDOT-rich, positively charged bottom layer (Fig. 4h). To evaluate this mechanism, we performed drift–diffusion simulations using SETFOS (Fluxim AG), modeling dipole-induced energy-level shifts at the PEDOT:PSS/perovskite interface; the simulations predict reductions in JSC and fill factor when interfacial dipoles are included, closely matching our experimental trends (Supplementary Note 3). During J–V characterization, this asymmetric charge distribution may become further polarized, enhancing dipole formation at the buried interface. These interfacial dipoles are likely to hinder vertical charge transport—particularly hole extraction from the perovskite into the PEDOT:PSS layer—thereby limiting device performance. Treatment with Triton X-100 mitigates this issue by reducing surface PSS accumulation, weakening dipole formation and thus facilitating more efficient charge extraction (Fig. 4i). This mechanism accounts for the observed enhancement in device performance and uniformity. Collectively, these findings reveal a previously overlooked microstructural and electrostatic aspect of PEDOT:PSS, offering a conceptual framework for optimizing its role in optoelectronic applications.

Flexible tandem solar cells and mini-modules
Leveraging the improved conformal coating of PEDOT:PSS with Triton X-100 and the resulting enhanced performance in single junction devices, we progressed to the fabrication of high-performance flexible solar cells and mini-modules.

During the processing of flexible TSCs, our finding, illustrated in Supplementary Fig. 17, confirms that the incorporation of Triton X-100 significantly enhances the conformality of the PEDOT:PSS coating on flexible WBG perovskite cells. This improvement provides a robust foundation for the fabrication of flexible tandem devices. With Triton X-100 in PEDOT:PSS, we first fabricated flexible TSCs, and the cross-sectional SEM image is presented in Fig. 5a. We note that the devices processed with Triton X-100 show markedly improved uniformity in PV performance compared to that of the reference devices. Our best-performing device achieves a PCE of 25.4%, with a VOC of 2.08 V, a JSC of 15.27 mA cm−2, and a FF of 80.2% (Fig. 5b). This is one of the highest efficiencies reported to date for flexible all-perovskite TSCs, summarized in the efficiency progress shown in Fig. 5c. The EQE spectra for our flexible TSC are shown in Supplementary Fig. 18.

As a further step toward practical applications, we developed a proof-of-concept flexible all-perovskite tandem solar mini-module. Tandems based on blade-coated 5 × 5 cm2 flexible WBG cells showed improved performance uniformity with Triton X-100, but the baseline performance is relatively poor, and the uniformity remained insufficient for reliable mini-module fabrication (Supplementary Fig. 19). We therefore used spin-coated 2.5 × 2.5 cm2 flexible WBG cells for mini-module development. We adapted the established thin-film module interconnection approach, utilizing three laser-scribed lines to interconnect cells, as depicted in Fig. 5d. While this interconnection schemes were successfully implemented in rigid all-perovskite tandem solar mini-modules in recent years9,31–33, its adaptation to flexible substrates poses significant challenges due to substrate deformation, uneven thermal distribution, and mechanical fragility during laser processing. In this work, laser scribing techniques for flexible tandem solar mini-modules were thoroughly investigated, with the optimized results presented in Fig. 5e. A proof-of-concept flexible all-perovskite tandem solar mini-module achieved a PCE of 19.7%, with a FF of 75%, a VOC of 2.085 V per cell, and an aperture area of ~1.3 cm2 (comprising three cell strips with an ~83% geometric fill factor), as shown in Fig. 5f.

To assess mechanical and operational stability, we performed bending and lifetime tests (Supplementary Fig. 20). The mini-module retained ~100% of its initial PCE after 2000 manual bending cycles at a 15 mm radius and maintained >83% after 700 cycles during in situ MPP tracking on a motorized bender. Under continuous MPP tracking, performance showed a rapid initial drop over ~30 h, indicating that their operational stability remains limited and requires further optimization.

During the development of laser scribing techniques for flexible all-perovskite tandem solar modules, we employed optoelectronic simulation using Laoss v4.2 by Fluxim AG (www.fluxim.com/laoss) to analyze loss mechanisms and to guide process optimization. As shown in Supplementary Fig. 21, experimentally obtained J–V curves were accurately reproduced by simulations incorporating multiple loss channels. In non-optimized mini-modules, the dominant losses were attributed to non-ohmic active area shunting (~18.4%) and dead area losses (~20.6%). The former originated primarily from non-uniform deposition of functional layers, while the latter was mainly due to suboptimal laser scribing quality and layout design (Fig. 5g). Further simulations revealed that poor P2 scribing significantly impacts the FF, while shunting behavior—linked to layer inhomogeneities and interfacial defects—limits the VOC (Supplementary Figs. 22 and 23). After improving film uniformity and optimizing P2 scribing parameters, the active-area shunting loss was significantly mitigated. In the optimized mini-module, the primary remaining loss was associated with the dead area (~17.2%) (Fig. 5h).

Based on our layer design and laser scribing techniques, we simulated the performance of a flexible tandem solar module on a 10 cm × 10 cm substrate with minimized active shunting losses, reduced laser scribing losses, and optimized stripe widths, as shown in Fig. 5i. Our simulation predicts that, with improved uniformity across a large area, a PCE exceeding 24% can be achieved using the laser scribing and layer stacks developed in this study, with a small cell-to-module loss less than 1.5%, showing the great potential of this technology. Concurrently, a related study reported a 23% PCE on a 20.26 cm2 flexible all-perovskite tandem mini-module, underscoring the rapid pace of progress in this area34.

Methods
Materials
Pre-patterned indium tin oxide (ITO) coated glass (15 Ω/sq) or polyethylenenaphthalate (PEN) (12 Ω/sq) were purchased from Advanced Election Technology Co., Ltd. Lead(II) iodide (PbI2, 99.99%), cesium iodide (CsI, 99%), methylammonium iodide (MAI, 98%), methylammonium bromide (MABr, ≥98%), were purchased from Tokyo Chemical Industry Co., Ltd. 4PADCB was purchased from Luminescence Technology Corp. Formamidinium iodide (FAI, >99.99%), 2-thiopheneethylammonium chloride (TEACl) was purchased from Greatcell Solar Materials Pty Ltd. Dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, anhydrous, ≥99.9%), chlorobenzene (CB, anhydrous, 99.8%), chloroform (CF, anhydrous, 99.8%), isopropanol (IPA, anhydrous, ≥99.9%), toluene (anhydrous, 99.8%), lead (II) bromide (PbBr2, 99.999%), tin (II) iodide (SnI2, 99,99%), ammonium thiocyanate (NH4SCN, 99.99%), tin (II) fluoride (SnF2, 99%), copper (Cu, 99.99%), Glycin-hydrochlorid (GlyHCl, ≥99.9%), Triton™ X-100 (~10% in H2O), Ethylenediammonium diiodide (EADI2, ≥98.0%) were purchased from Sigma-Aldrich Pty Ltd. Ethanol (anhydrous, ≥99.9%) was purchased from VWR International, LLC. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM), fullerene-C60 and bathocuproine (BCP) were purchased from Xi’an Yuri Solar Co., Ltd. PEDOT:PSS (Clevios PVP Al 4083) was purchased from Heraeus Epurio LLC. All the materials were used as received.

Solar cells fabrication
Device fabrication for NBG PSCs: The pre-patterned ITO substrates were soaked in detergent (1% water solution of Hellmanex® III) and then cleaned by sonication for 10 min with deionized water and isopropanol sequentially. After drying with N2 flow, the substrates were further cleaned by UV/Ozone treatment (Jelight Company Inc.) for 30 min. PEDOT:PSS dispersion (Triton X-100 were added to the dispersion according to different weight ratios) was filtered with a 0.45 μm polyvinylidene difluoride (PVDF) filter, then coated on the cleaned ITO substrate at 4000 rpm for 50 s and annealed at 140 °C for 20 min. The substrates were then transferred to an N2-filled glovebox for perovskite film fabrication. 1.8 M Cs0.1FA0.6MA0.3Sn0.5Pb0.53I perovskite precursor were prepared by dissolving NH4SCN (5.46 mg), GlyHCl (8.03 mg), SnF2 (28.21 mg), CsI (93.53 mg), FAI (371.46 mg), MAI (171.69 mg), SnI2 (670.53 mg), and PbI2 (829.81 mg) into a mixed solvent of DMF (1500 μL) and DMSO (500 μL). The molar ratio of NH4SCN:GlyHCl:SnF2:CsI:FAI:MAI:SnI2:PbI2 is 0.036:0.036:0.09:0.18:1.08:0.54:0.9:0.9. Before spin-coating, the precursor was filtered with a 0.22 μm polytetrafluoroethylene (PTFE) filter. The precursor was spin-coated onto ITO/PEDOT:PSS by a two-step spin-coating. The first step is 1000 rpm for 10 s with a ramp-up of 200 rpm s−1. The second step is 4000 rpm for 40 s with a ramp-up of 2000 rpm s−1. CB (400 μL) was dropped onto the spinning substrate at the 20 s of the second step. The substrate was then annealed at 100 °C for 10 min and 65 °C for 5 min. For a PDT with EDAI235, 1.0 mg EDAI2 was added to 1.0 mL IPA and 1.0 mL toluene. The mixed solvent solution was stirred overnight and then filtered through a 0.22 μm PTFE filter. EDAI2 solution was spin-coated onto the perovskite film at 4000 rpm for 20 s with a ramp-up of 1333 rpm s−1, followed by an annealing at 100 °C for 2 min. All the substrates were then transferred into a vacuum chamber for thermal evaporation. C60 (23 nm)/BCP (7 nm)/Cu (100 nm) were sequentially deposited on the substrates to complete the solar cell. For each substrate, there are four cells. The designed contact area of each device is 0.1024 cm2. The illuminated area of the device was defined with a patterned mask (0.09 cm2). Devices were encapsulated with cover glass and UV-curable epoxy. All the procedures were carried out in a N2-filled glove box except the spin-coating and annealing of PEDOT:PSS.

Device fabrication for WBG PSCs: Pre-patterned PEN/ITO substrates were first fixed onto rigid glass substrates with UV epoxy and then cleaned with ethanol and dried with N2 flow. Before device fabrication, the substrates were further cleaned by UV/Ozone treatment for 30 min. 4PADCB precursor (0.3 mg mL−1 in ethanol, preheated at 40 °C) was spin-coated onto the cleaned ITO substrates at 3000 rpm for 30 s after 30 s resting on the substrate, followed by an annealing at 100 °C for 2 min. 1.2 M Cs0.12FA0.8MA0.08PbI1.8Br1.2 precursor was prepared by dissolving MABr (21.50 mg), CsI (74.83 mg), FABr (95.98 mg), FAI (198.11 mg), PbBr2 (352.33 mg) and PbI2 (663.85 mg) into a mixed solvent of DMF (1600 μL) and DMSO (400 μL). The molar ratio of MABr:CsI:FABr:FAI:PbBr2:PbI2 is 0.08:0.12:0.32:0.48:0.4:0.6. Before spin-coating, the precursor was filtered with a 0.22 μm PTFE filter. The precursor was spin-coated onto ITO/SAM by a two-step spin-coating. The first step is 2000 rpm for 10 s with a ramp-up of 200 rpm s−1. The second step is 6000 rpm for 40 s with a ramp-up of 2000 rpm s−1. CB (300 μL) was dropped onto the spinning substrate at the 20 s of the second step. The substrates were then annealed at 60 °C for 2 min and 100 °C for 7 min. For TEACl PDT7, TEACl in IPA (0.5 mg mL−1) is spin-coated onto the perovskite film at 3000 rpm for 30 s, followed by an annealing at 100 °C for 3 min. After cooling, PCBM in CF (20 mg mL−1) right after filtering with a 0.22 μm PTFE filter was spin-coated at 3000 rpm for 50 s, followed by annealing at 100 °C for 5 min. The substrates were then transferred to an atomic layer deposition (ALD) chamber for SnOx deposition (~20 nm) at relatively low temperature (100 °C) by periodic pulse of tetrakis(dimethylamino) tin(IV) (99.9999%, Nanjing AiMouYuan Scientific Equipment Co., Ltd) and deionized water. Thereafter, the substrates were then transferred into vacuum chamber for the thermal evaporation of Cu (100 nm). For each substrate, there are four cells. The designed contact area of each device is 0.1024 cm2. The illuminated area of the cell was defined with a patterned mask (0.09 cm2). All spin coatings were carried out in an N2-filled glove box.

Device fabrication for monolithic TSCs and TSMs: For TSCs, all the procedures are the same for the flexible WBG cell until ALD-SnOx is finished. The substrates were then transferred to the magnetron sputtering system to sputter 100 nm IZO at a 30 W power under an Ar pressure of 0.4 Pa. Then, PEDOT:PSS diluted with IPA (volume ratio 1:1) was spin-coated onto the sputtered IZO substrates at 4000 rpm for 50 s and then annealed at 100 °C for 5 min in air. The other steps to fabricate monolithic TSCs were consistent with those of the fabrication of NBG PSCs. For TSMs, the substrates were first processed with P1 laser scribing before cleaning. When the ALD-SnOx for WBG cell was finished, the substrates were transferred into a vacuum chamber for thermal evaporation of Au (1.5 nm). In addition, the BCP layer in the NBG cell was replaced by ALD-SnOx for better protection, followed by a P2 laser scribing. And P3 laser scribing was performed after Cu deposition. The other steps to fabricate monolithic tandem solar modules were consistent with those of the fabrication of NBG PSCs. All the laser scribing processes were done in the ambient air.

Laser scribing for the modules: Laser scribing was performed on a TruMark Station 5000, using a nanosecond pulsed Nd:YVO4 laser with a wavelength of 355 nm.

Characterizations
Contact angle measurements, SEM and TrPL: Contact angle measurements were carried out on a drop shape analyzer (Krüss, DSA30E). The SEM images were taken with a Hitachi S-4800 Scanning Electron Microscope using 5–10 kV acceleration voltage. TrPL was measured using the MicroTime 100 system from PicoQuant. A 639 nm pulsed laser diode (pulse width at ~100 ps) was used. The laser pulse frequency was 0.5 MHz. For the fitting procedure, a biexponential decay was used.

ToF-SIMS: Element depth profiles were obtained with a time-of-flight secondary ion mass spectrometer (ToF-SIMS V system, ION-TOF). The primary beam was 25 keV Bi3+ with a total current of 0.41 pA and a raster size of 50 × 50 μm2. Cs+ ions were used with 1000 eV ion energy, 15 nA pulse current on a 400 × 400 μm2 raster size to bombard and etch the film. The data were plotted with the intensity for each signal normalized to the total counts of the signal.

XPS: XPS were performed using the ULVAC-PHI PHI 5000 Versaprobe III system. The analyses were conducted under ultra-high vacuum conditions at a pressure of 10−7 to 10−8 Pa. A LaB6 filament served as the source for the electron beam, which was subsequently focused by an electrostatic lens and directed onto an aluminum anode by modulating the voltages on the scanning plates. The interaction between the electron beam and the aluminum anode at a power of 50 W and a voltage of 15 kV generated a monochromatic X-ray beam. This beam, with a spot size of 20 μm, was reflected and used as the source beam for sample analysis.

The system’s 180° spherical capacitor energy analyzer, equipped with a multichannel detector, provided an energy resolution of ≤0.5 eV at the Ag 3d5/2 peak. Electron and ion charge neutralizers were employed to mitigate surface charging effects during XPS measurements. Depth profiling was achieved by etching the sample surface with a 2 kV Ar+ ion beam over a 2 mm × 2 mm area between measurements. Spectrum curve fitting was carried out using MultiPak software, applying Shirley-background subtraction and fitting the data with a Gaussian-Lorentzian function.

FIB-SEM: SEM (Thermo Fisher Scientific, Scios 2 DualBeam) studied the microstructure of the flexible all-perovskite TSC stack. In a high vacuum (~10−6 mbar), a cross-section was roughly milled with an ion beam current of 1 nA, followed by a cleaning cut with an ion beam current of 100 pA at 30 kV.

Electroluminescence: Device spatial homogeneity was characterized by electron luminescence measurements using a CCD camera (300-1000 nm spectra response, ORCA-ER C4742-80-12AG, HAMAMASU) and a Keithley 2450 source meter by applying a constant voltage of 0.9 V through the devices.

J-V and EQE characteristics: Current density-voltage (J−V) characteristics were measured in four-contact mode at standard test conditions (100 mW cm−2) using a Keithley 2400 source meter. A solar simulator (ABA class, LOT-Quantum Design) was calibrated to AM1.5G one sun illumination using a certified monocrystalline silicon solar cell (RS-ID-5, Fraunhofer-ISE). The solar cells were measured with an aperture mask with an aperture area of 0.09 cm2 (1.3 cm2 for the modules). The J–V measurements were performed in forward and reverse directions. The cells were measured with a scan rate of 100 mV/s (using an integration time of 0.1 s and a delay of 0.1 s for each data point, the voltage step was 0.02 V). The steady-state efficiency as a function of time was recorded using an MPP tracker, which adjusts the applied voltage to reach the maximum power point (perturb and observe algorithm). The external quantum efficiency of the solar cells was measured with a lock-in amplifier. The probing beam was generated by a chopped white source (900 W, halogen lamp, 280 Hz) and a dual grating monochromator. The beam size was adjusted to ensure an illumination area within the cell area. A certified single-crystalline silicon solar cell was used as a reference cell. White bias light was applied during the measurement with an intensity of ~0.1 sun. The EQE characterizations of the TSCs were conducted using an EQE setup (QE-R, Enlitech). The bias illumination from a 150 W white lamp was filtered with 550 and 850 nm optical filters for the measurement of bottom and top subcells’ responses, respectively. Both J-V and EQE characteristics are measured under ~25 °C in ambient air. The shelf stability of the encapsulated NBG PSCs was evaluated via repeated J–V measurements over time, and the devices were stored in an N2-filled glovebox between measurements.

Bending test: The mechanical bending test is done by manually conforming the mini-moudules to the mold with a radius of 15 mm. For the MPP tracking during the bending, the sample is fixed onto the stages of a customized bending machine, and as the distance between the two stages becomes shorter, bending stress is applied to the device fixed to the stages during the MPP tracking. To preserve flexibility and protect the devices, the flexible tandem mini-modules were encapsulated using a desktop laminator (Fellowes Laminiergerät Caliber) with double sided PET foil (80 micrometers for each) at a temperature of 125 °C. (Supplementary Note 4 details the bending test and its visualization).

Operational stability: Light-soaking stability test was done by continuous MPP tracking of the encapsulated tandem solar mini-modules under simulated 1 sun illumination (using white light-emitting diode (LED) solar simulator with intensity equivalent to 100 mW cm−2) in a 300 mbar N2 chamber (purged for three times with N2). No active cooling is applied, and the device temperature rose to ~42 °C during operation due to the heating effect of light illumination. Because the current encapsulation is not yet sufficiently robust for moisture- and oxygen-sensitive devices under prolonged light/thermal stress, these MPP data should be regarded as preliminary; improved encapsulation will be implemented to enable systematic, long-term operational-stability testing.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the manuscript \"Tailored PEDOT:PSS Phase Segregation for High-Efficiency Flexible All-Perovskite Tandem Solar Cells and Mini-Modules\", the authors propose an additive engineering strategy to overcome the limitations of PEDOT:PSS HTLs, aiming to enhance the performance of flexible all-perovskite tandem solar cells. Specifically, the authors found vertical phase separation in PEDOT:PSS films, and the addition of the surfactant Triton X-100 suppressed surface PSS accumulation, mitigated dipole formation, thereby improving hole extraction efficiency, device performance, and reproducibility. Overall, the presented concept and results are promising, but several issues require further consideration.\n1. The authors attribute the vertical phase separation of PEDOT:PSS to preferential lateral interactions between PEDOT units, which promotes PSS segregation towards the surface. However, the current manuscript lacks sufficient evidence to confirm the claimed bilayer structure and phase separation distribution. Furthermore, compelling data is missing on how the addition of Triton X-100 alleviates the phase separation in PEDOT:PSS.\n2. The authors suggest that PEDOT:PSS phase separation generates interfacial dipoles, which hinder effective hole extraction, but this critical mechanistic claim remains inadequately substantiated. Even if dipoles are present, their negative impact on hole extraction is still speculative.\n3. In line 49, the authors claim that PEDOT:PSS faces two major challenges in flexible devices: difficulty in forming uniform ultrathin coatings and phase separation. Is the observed hygroscopic-driven phase separation unique to flexible substrates? A comparative study with rigid substrates under identical processing conditions could clarify whether substrate flexibility intrinsically exacerbates these effects.\n4. In line 85, the author noticed that the JSC and FF of devices on the same substrate are highly dependent on the measurement sequence. Is this phenomenon consistently observed across the broader literature where PEDOT:PSS is employed as the HTL or just this study? Furthermore, the conclusion regarding interfacial issues at the PEDOT:PSS/perovskite interface appears inadequately supported, as the deduction based solely on the sequence-dependent J-V measurements and SEM morphological analysis lacks scientific rigor.\n5. In line 106, the author found that spin-coating PEDOT:PSS onto flexible WBG PSCs often results in non-conformal coatings or damage to the underlying WBG PSCs, caused by the long interaction with the aqueous dispersion. I would like to know if Triton X-100 can improve the aforementioned issues without changing the solvent of aqueous dispersion.\n6. The theoretical basis for the improvement of wettability after adding Triton X-100 to PEDOT:PSS should be explained, and contact angle tests with higher concentrations (＞0.75 wt%) should be added to prove its excessive spreading.", "reviewer_2": "Flexible perovskite solar cells hold significant application potential, yet research progress in this field remains largely unexplored, particularly concerning the bottom cell development (narrow-bandgap materials). This work addresses conformal coating challenges on flexible stacks and mitigates inherent vertical phase separation in PEDOT:PSS through surfactant additive modification, demonstrating flexible tandem devices and proof-of-concept flexible tandem modules. While the strategy appears effective for narrow-bandgap systems, the following critical enhancements are required: More appropriate experimental data and mechanistic analyses to elucidate key issues in flexible tandem cells/modules and clarify the specific efficacy of this approach; higher efficiency results of flexible tandem modules (aperture area >10 cm² with third-party certification); demonstration of mechanical and operational stability improvements attributed to such interfacial optimization. In summary, while the study addresses a meaningful topic with a well-defined methodology, insufficient validation undermines the demonstration of the strategy's full potential. Substantial revisions incorporating the above improvement are necessary to meet this journal's stringent standards prior to considering its publication.\n1. In Figure. 1e, authors show the failed samples with damage to the underlying WBG PSCs, caused by the long interaction with the aqueous dispersion while spin-coating PEDOT:PSS in tandem devices. This process has no distinct difference between rigid and flexible substrates. Why this phenomenon became a significant challenge in flexible tandem device? The Triton X-100 can solve which kind of unique issues of flexible device?\n2. In Figure. S3, the grain sizes firstly increase and then decrease as the concentration of Triton X-100 increases. Sometimes, the large perovskite grain sizes have a direct relation to the improved wettability and reduced contact angles. How authors attribute the specific causes for enlarged grain sizes?\n3. Authors emphasize the uniformity across fabrication batches of PCSs. For large area flexible tandem modules, the horizontal uniformity should be explored.\n4. The authors emphasize in Section 1 that conformal coating of PEDOT:PSS on flexible stacks is critical, noting issues of non-conformal coatings and aqueous damage, while acknowledging the effectiveness of SAMs layers for conformal coating on flexible substrates (PEN/ITO):\n(1) The article provides minimal explicit data and targeted characterization regarding the surface characteristics of flexible wide-bandgap bottom cell stacks. Specifically, there is insufficient evidence to determine whether these surfaces possess comparable complex morphologies or other distinctive features to flexible substrates.\n(2) Previous work (Zhu et al., Nat Commun. 16, 240 (2025)) demonstrates SAMs' successful application in narrow-bandgap devices with effective prevention of aqueous damage to buried layers. Given their potential advantages for flexible tandem devices, did the authors conduct comparative studies between these two coating approaches?\n5. There appears to be a contradiction between the poor wettability phenomenon and the prolonged aqueous solvent exposure causing bottom cell damage. How does the author explain the correlation between these opposing mechanisms?\n6. While the authors attribute batch-to-batch reproducibility issues to PEDOT:PSS hygroscopicity: Please provide more concrete data demonstrating clear performance variations directly attributable to this property and clarify how this hygroscopic nature impacts operational stability of devices?\n7. For comprehensive evaluation of tailored PEDOT:PSS layer's role in device modification, TRPL measurements should be presented for both ITO/HTL/perovskite and HTL/perovskite stacks (Figure. 2e) in parallel.\n8. The article's focus on addressing uniformity in narrow-bandgap perovskite devices requires stronger characterization support:\n(1) While multi-batch J-V statistics (Figures. 1a-d, f and 4b, d, f, g) demonstrate improved reproducibility, they don't establish enhanced performance uniformity across sub-cells on the same substrate - the crucial metric for uniformity assessment\n(2) Lack of direct uniformity characterization (e.g., PL/EL mapping, spatially resolved analysis)\n(3) Figure S4 shows target sample has larger average grain size with more dispersed distribution. Is this dispersion region-dependent? If so, please explain the origin of such regional heterogeneity.\n9. Flexible all-perovskite tandem modules have attracted growing attention for their higher efficiency potential. However, current certified efficiencies for flexible single-junction modules have already exceed 20% with areas >10 cm² (21.82%-12.8 cm² - Zhu et al., Adv. Mater. 2025; 20.3%-27.9 cm² - Zhang et al., Small 2024; 20.2-12.9 cm²-Zhang et al. Adv. Funct. Mater. etc). 19.7% efficiency for 1.3 cm² flexible tandem module fails to demonstrate scalable efficiency advantages over existing single-junction devices.\n10. The schematic of tandem module in Figure. 5d omits several critical functional layers. The figure requires revision with clear labeling of all essential component.\n11. The mechanical stability and operational stability data of flexible tandem devices/modules hold critical importance for evaluating their potential application scenarios. Could the authors provide explicit experimental data demonstrating the impact of modified PEDOT:PSS on these stability metrics? Quantitative evidence in this regard would advance the development of flexible tandem modules."}, "author_response": "We thank the reviewer for careful reading of our manuscript and for recognizing the potential of our strategy to improve flexible all-perovskite tandem solar cells. We greatly appreciate the reviewer’s positive assessment of our concept and results, as well as the constructive feedback. We have carefully addressed all the specific points raised.\n1. The authors attribute the vertical phase separation of PEDOT:PSS to preferential lateral interactions between PEDOT units, which promotes PSS segregation towards the surface. However, the current manuscript lacks sufficient evidence to confirm the claimed bilayer structure and phase separation distribution. Furthermore, compelling data is missing on how the addition of Triton X-100 alleviates the phase separation in PEDOT:PSS.\nResponse:\nWe sincerely thank the reviewer for this insightful comment.\nWe fully agree that clearly demonstrating vertical phase separation within PEDOT:PSS films is inherently challenging due to the chemical similarity between PEDOT and PSS moieties, both of which predominantly contain C, H, O, N, and S atoms. Conventional analytical techniques often lack the sensitivity required to unambiguously distinguish PEDOT-rich and PSS-rich regions within the films.\nIn the literature, phase segregation in spin-coated PEDOT:PSS films has typically been inferred through indirect methods such as atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and surface-sensitive X-ray photoelectron spectroscopy (XPS). These approaches primarily provide information limited to the surface or near-surface regions, thus offering insufficient insights into the bulk composition or vertical distribution.1\nIn our study, we addressed this limitation by utilizing depth-resolved XPS analysis. Specifically, we leveraged the subtle yet distinguishable chemical shifts in sulfur atoms associated with PEDOT and PSS units, which enabled us to perform XPS depth profiling to investigate the vertical compositional distribution explicitly.2 Using this method, we successfully tracked changes in the S 2p signals with sputtering depth, thereby providing direct and robust evidence of surface accumulation of PSS and demonstrating its clear suppression upon adding Triton X-100. We believe this approach provides more definitive evidence for the vertical phase segregation compared to conventional methods.\nRegarding the mechanism by which Triton X-100 alleviates phase separation in PEDOT:PSS, we appreciate the reviewer’s request for clarification and have further strengthened this part as follows:\nTriton X-100, a nonionic amphiphilic surfactant containing an aromatic hydrophobic group and a hydrophilic poly(ethylene oxide) chain, interacts preferentially with the insulating PSS chains with in the core-shell structure.3 This interaction disrupts the typical core-shell structure wherein conductive PEDOT cores are encapsulated by PSS-rich shells (Figrue R 1a, top).4 Consequently, excess insulating PSS is effectively removed during spin-coating with the addition of Triton X-100 (Figrue R 1a, bottom).\nTo substantiate this mechanism experimentally, we performed UV-Vis spectroscopy measurements on PEDOT:PSS aqueous dispersions with and without Triton X-100. As shown in Figure R1b, the addition of 1 wt% Triton X-100 led to a significant increase in transmittance across the 400–950 nm range. This observation strongly indicates reduced particle scattering due to the effective disruption of PEDOT:PSS core-shell structures by Triton X-100.\nWe have incorporated these findings and detailed discussions into the revised manuscript and Supplementary Note 1, aiming to provide a comprehensive and coherent explanation that directly addresses the reviewer’s valuable concerns.\n2. The authors suggest that PEDOT:PSS phase separation generates interfacial dipoles, which hinder effective hole extraction, but this critical mechanistic claim remains inadequately substantiated. Even if dipoles are present, their negative impact on hole extraction is still speculative.\nResponse:\nWe sincerely thank the reviewer for this valuable comment.\nWe agree that the impact of interfacial dipoles at the PEDOT:PSS/perovskite interface on hole extraction requires better substantiation.\nTo thoroughly address this concern, we conducted drift-diffusion simulations using SETFOS software developed by Fluxim AG.5,6\nSpecifically, we modeled the effect of dipole-induced energy level shifts at the PEDOT:PSS/perovskite interface (Figure R 2a and b). Our simulations demonstrated a notable reduction in photovoltaic performance parameters, including short-circuit current density (J ) and fill factor (FF), when interfacial dipoles were introduced (Figure R 2c). Importantly, the simulated device performance closely matched with our experimental observations, thereby reinforcing the conclusion that such dipoles detrimentally affect hole extraction efficiency.\nWe have incorporated detailed descriptions of the simulation methodology and corresponding results into the revised manuscript and Supplementary Note 2, providing rigorous evidence supporting our mechanistic claim that phase-separation-induced interfacial dipoles hinder effective hole extraction.\nWe greatly appreciate the reviewer's insights, which have helped us strengthen the manuscript.\n3. In line 49, the authors claim that PEDOT:PSS faces two major challenges in flexible devices: difficulty in forming uniform ultrathin coatings and phase separation. Is the observed hygroscopic-driven phase separation unique to flexible substrates? A comparative study with rigid substrates under identical processing conditions could clarify whether substrate flexibility intrinsically exacerbates these effects.\nResponse:\nWe thank the reviewer for the insightful comment.\nIn our experiments, we found that substrate flexibility mainly exacerbates the non-uniform coating of the PEDOT:PSS layer. To directly address the reviewer’s suggestion, we conducted a comparative study using rigid (glass/ITO) and flexible (PEN/ITO) substrates under identical processing conditions.\nAs shown in Figure R 3, both the pristine PEDOT:PSS dispersion (reference) and those containing 0.5 wt% Triton X-100 were spin-coated onto rigid and flexible ITO substrates. On rigid substrates, the films displayed comparable uniformity in both cases (Figure R 3a and b). In contrast, on flexible substrates, the reference sample exhibits pronounced non-uniformities with characteristic “comet-like” regions (circled in Figure R 3c), which we attribute to the lower flatness of the flexible substrate. The addition of Triton X-100 improves the wettability of the dispersion, resulting in a more uniform coating on flexible substrates (Figure R 3d).\nRegarding the phase separation of PEDOT:PSS, previous studies have reported that this phenomenon is intrinsic to the PEDOT:PSS material system and can occur independently of the substrate type.7 To further substantiate this, we performed XPS depth profiling of PEDOT:PSS films spin-coated onto flexible substrates Figure R 4). The results reveal similar vertical phase segregation of PEDOT and PSS, confirming that this behavior originates from the material system itself rather than from substrate flexibility.\nThese results have been incorporated into the revised manuscript as Supplementary Figure 12. We thank the reviewer for the suggestion, which strengthens the experimental evidence.\n4. In line 85, the author noticed that the JSC and FF of devices on the same substrate are highly dependent on the measurement sequence. Is this phenomenon consistently observed across the broader literature where PEDOT:PSS is employed as the HTL or just this study? Furthermore, the conclusion regarding interfacial issues at the PEDOT:PSS/perovskite interface appears inadequately supported, as the deduction based solely on the sequence-dependent J-V measurements and SEM morphological analysis lacks scientific rigor.\nResponse:\nWe thank the reviewer for the comment.\nTo our knowledge, the phenomenon of sequence-dependent J-V characteristics on the same substrate has not been widely reported in the existing literature on PEDOT:PSS-based perovskite solar cells, making this a potentially underexplored phenomenon. Given the significant cell-to-cell variation in photovoltaic performance (Figure 1a–d), we hypothesized that the losses could arise from either bulk material quality or interfacial effects.\nSEM analysis of our perovskite films revealed dense and uniform morphology, suggesting that bulk film quality may not be the dominant limiting factor.8 In contrast, many previous studies have highlighted that PEDOT:PSS—particularly in NBG perovskites—can suffer from interfacial challenges such as phase separation, and energetic mismatch.9,10 While we acknowledge that attributing the sequence-dependent behavior solely to the PEDOT:PSS/perovskite interface is a strong claim, our subsequent modification of PEDOT:PSS using Triton X-100 provides further evidence supporting our hypothesis. After this modification, the sequence-dependent J-V behavior was eliminated, and the device performance became more uniform (Figure 4b and d, Supplementary Figure 9 after revision). These results suggest that our additive engineering for PEDOT:PSS is an effective strategy to mitigate the performance variability observed in the baseline devices.\n5. In line 106, the author found that spin-coating PEDOT:PSS onto flexible WBG PSCs often results in non-conformal coatings or damage to the underlying WBG PSCs, caused by the long interaction with the aqueous dispersion. I would like to know if Triton X-100 can improve the aforementioned issues without changing the solvent of aqueous dispersion.\nResponse:\nWe thank the reviewer for raising this question.\nIndeed, Triton X-100 can effectively address the mentioned issues without altering the solvent composition of the aqueous PEDOT:PSS dispersion. Specifically, as a nonionic surfactant, Triton X-100 significantly reduces the surface tension of the PEDOT:PSS dispersion, thereby improving its wettability on flexible wide-bandgap perovskite solar cells. Enhanced wettability enables faster and more uniform film coverage, which shortens the interaction time between the aqueous dispersion and the underlying perovskite layer. Consequently, the likelihood of damage (physical or chemical) due to prolonged exposure to water-based solvents is substantially reduced.\n6. The theoretical basis for the improvement of wettability after adding Triton X-100 to PEDOT:PSS should be explained, and contact angle tests with higher concentrations (＞0.75 wt%) should be added to prove its excessive spreading.\nResponse:\nWe thank the reviewer for the suggestion. We have now added the theoretical basis for the improved wettability (i.e., reduced contact angle) upon the addition of Triton X-100, as follows:\nThe relationship between water contact angle and surface energies is described by Young's equation, a formula that describes the wetting characteristics of a smooth or flat surface:\n𝛾𝛾𝑆𝑆𝑆𝑆−𝛾𝛾𝑆𝑆𝑆𝑆\n𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = (1)\nWhere represents the solid-vapor surface tension, 𝛾𝛾 i𝑆𝑆s𝑆𝑆 the solid-liquid interfacial tension, and is the liquid-v\n𝛾𝛾\na𝑆𝑆p𝑆𝑆or surface tension.11 The contact angle meas\n𝛾𝛾\nu𝑆𝑆r𝑆𝑆ement is illustrated in Figure R 5a. In the aq\n𝛾𝛾\nu𝑆𝑆𝑆𝑆e ous\nPEDOT:PSS dispersion, Triton X-100 improves the wettability mainly by adsorbing at the liquid–air interface and disrupting hydrogen-bonding networks among water molecules, thereby reducing .12 According to equation (1), a decrease in reduces the denominator, increasing and thus decre\n𝛾𝛾\na𝑆𝑆s𝑆𝑆ing (for 0\n< <90 ), consistent with t\n𝛾𝛾\nh𝑆𝑆e𝑆𝑆 enhanced spreading observed experim\n𝑐𝑐𝑐𝑐\ne\n𝑐𝑐\nn\n𝑐𝑐\ntally.In response to the\n𝑐𝑐\nreviewe r’s re-quest, we have performed additional contact angle measurements using PEDOT:PSS dispersions containing a\n° 𝑐𝑐 °\nhigher Triton X-100 concentration (2 wt%). As shown in Figure R 5b, this higher concentration results in pronounced over-spreading, confirming the strong wetting effect of excessive surfactant. These results have been added to the revised manuscript and Supplementary Note 3.\nWe appreciate the reviewer’s suggestion, which has strengthened both the mechanistic discussion and the experimental evidence.\nWe sincerely thank the reviewer for recognizing the significance of our work and for the constructive and thoughtful summary of our study. We greatly appreciate the reviewer’s acknowledgment that our methodology is well-defined and addresses an important challenge in the development of flexible narrow-bandgap perovskite solar cells and tandem devices.\nWe have carefully considered the reviewer’s suggestions regarding the need for additional experimental data and mechanistic analyses to further elucidate the key issues in flexible tandem cells/modules. In the revised manuscript, we have incorporated additional experiments, detailed mechanistic discussions analyses and discussions to address these points in detail. We believe these substantial revisions significantly strengthen the manuscript and the efficacy and potential of our strategy for advancing flexible perovskite tandem solar cells and modules.\n1. In Figure. 1e, authors show the failed samples with damage to the underlying WBG PSCs, caused by the long interaction with the aqueous dispersion while spin-coating PEDOT:PSS in tandem devices. This process has no distinct difference between rigid and flexible substrates. Why this phenomenon became a significant challenge in flexible tandem device? The Triton X-100 can solve which kind of unique issues of flexible device?\nResponse:\nWe thank the reviewer for this thoughtful question.\nAlthough the interaction between the aqueous PEDOT:PSS dispersion and the underlying WBG subcell occurs for both rigid and flexible substrates, achieving a uniform coating is significantly more challenging in flexible tandem devices. This is primarily due to two factors:\n1. Poorer wettability of PEDOT:PSS on flexible WBG cells compared to rigid glass-based cells.\n2. Non-flatness of flexible substrates, which hinders uniform liquid spreading before drying.\nAs shown in Figure R 6a–c, the contact angle of the pristine PEDOT:PSS dispersion is significantly higher on a flexible WBG cell (77.1 °) compared with bare PEN/ITO or glass/ITO substrates. This reduced wettability slows spreading, causing the dispersion to remain in contact with the underlying WBG subcell for a longer period during spin-coating. Prolonged exposure to the aqueous dispersion increases the risk of damaging the WBG subcell, as illustrated in Figure 1e.\nThe addition of Triton X-100, a nonionic surfactant, effectively mitigates this problem. As shown in Figure R 6d–f, 0.5 wt% Triton X-100 substantially improves the wettability of the PEDOT:PSS dispersion on all tested substrates, including flexible WBG cells. Enhanced wettability promotes rapid and uniform spreading, allowing the PEDOT:PSS layer to form uniformly even when the coating time is rather short. This minimizes the interaction time between the aqueous dispersion and the underlying flexible WBG subcell, thereby mitigating the damage observed in Figure 1e.\nWe have included the additional results in Supplementary Figure S3 and believe they fully address the reviewer’s question.\n2. In Figure. S3, the grain sizes firstly increase and then decrease as the concentration of Triton X-100 increases. Sometimes, the large perovskite grain sizes have a direct relation to the improved wettability and reduced contact angles. How authors attribute the specific causes for enlarged grain sizes?\nResponse:\nWe thank the reviewer for this valuable comment.\nWe agree that improved wettability and reduced contact angles can influence perovskite crystallization. In our case, we attribute the initial enlargement of perovskite grain sizes primarily to changes in the surface states of the PEDOT:PSS layer induced by the addition of Triton X-100. Specifically, XPS analysis (Figure 3c-f) shows that Triton X-100 partially removes the insulating PSS component at the PEDOT:PSS surface. We suspect that this modification likely reduces the density of nucleation sites, leading to a lower nucleation density during perovskite film formation. Consequently, the grains have more space to grow laterally, resulting in larger average grain sizes at moderate Triton concentrations.\nAt higher Triton X-100 concentrations (>0.75 wt%), excessive surfactant can accumulate at the surface of PEDOT:PSSe, which may increase the nucleation density and consequently leads to smaller grains.\n3. Authors emphasize the uniformity across fabrication batches of PCSs. For large area flexible tandem modules, the horizontal uniformity should be explored.\nResponse:\nTo investigate horizontal uniformity, we fabricated additional flexible tandem solar cells on 5 cm × 5 cm PEN/ITO substrates and compared the lateral uniformity of device performance with and without Triton X-100. As shown in Figure R 7, the incorporation of Triton X-100 significantly improves uniformity across the entire substrate area, consistent with its role in enhancing wettability and promoting more homogeneous film formation. We believe these supplementary experimental results address the reviewer’s concern regarding horizontal uniformity in larger-area flexible tandem devices. The corresponding data have been included in the revised manuscript and Supplementary Figure S18 to further strengthen the manuscript.\n4. The authors emphasize in Section 1 that conformal coating of PEDOT:PSS on flexible stacks is critical, noting issues of non-conformal coatings and aqueous damage, while acknowledging the effectiveness of SAMs layers for conformal coating on flexible substrates (PEN/ITO):\n(1) The article provides minimal explicit data and targeted characterization regarding the surface characteristics of flexible wide-bandgap bottom cell stacks. Specifically, there is insufficient evidence to determine whether these surfaces possess comparable complex morphologies or other distinctive features to flexible substrates.\nResponse:\nWe thank the reviewer for the comment.\nIn our previous work,13 We have extensively investigated and optimized the surface morphology and uniformity of flexible WBG bottom cell stacks. PLQY mapping and improved PV uniformity demonstrated that better uniformity can be achieved by optimizing the coating of self-assembled monolayers (Figure S4, S6, S7 in the mentioned work).\nIn the present study, our focus is on further improving the narrow-bandgap (NBG) cell part of the flexible tandem stack. It should be noted that the deposition of WBG layers inevitably worsens the surface morphology of flexible substrates, due to the imperfect coating of certain layers, making it even more challenging to deposit uniform layers in the subsequent NBG stack. This further underscores the importance of optimizing the PEDOT:PSS coating process to ensure conformal coverage and to minimize aqueous damage to the WBG subcell. We have clarified this point and cited our previous work in the revised manuscript.\n(2) Previous work (Zhu et al., Nat Commun. 16, 240 (2025)) demonstrates SAMs' successful application in narrow-bandgap devices with effective prevention of aqueous damage to buried layers. Given their potential advantages for flexible tandem devices, did the authors conduct comparative studies between these two coating approaches?\nResponse:\nWe thank the reviewer for this pertinent question.\nAlthough in the work by Zhu et al. (Nat. Commun. 16, 240 (2025)), self-assembled monolayers (SAMs) were successfully applied in NBG single-junction devices to prevent aqueous damage to buried layers, we have found that SAMs alone are not sufficiently compact or thick to fully protect the underlying layers when subsequently depositing the NBG perovskite layer on top. This is why, in the same referenced study, the tandem devices were fabricated using a PEDOT:PSS/SAM bilayer structure rather than SAMs alone. The same observation was also reported in another previous work.14\n5. There appears to be a contradiction between the poor wettability phenomenon and the prolonged aqueous solvent exposure causing bottom cell damage. How does the author explain the correlation between these opposing mechanisms?\nResponse:\nWe thank the reviewer for this question.\nWe do not review these as opposing mechanisms, but rather two interconnected consequences of poor wettability. To minimize damage to the flexible WBG subcell, spin-coating should ideally begin immediately after dispensing the PEDOT:PSS dispersion. However, when wettability is poor, the dispersion does not spread quickly to cover the entire substrate, leading to non-uniform film formation if spinning is started too early (Figure R 8a). Conversely, one could wait longer for the PEDOT:PSS to spread uniformly before spinning, but in this case, the central area remains in prolonged contact with the aqueous dispersion, which causes damage to the underlying WBG subcell (Figure R 8b). This interplay underscores the importance of achieving both rapid and uniform spreading, which is precisely the role that Triton X-100 plays in our process.\n6. While the authors attribute batch-to-batch reproducibility issues to PEDOT:PSS hygroscopicity: Please provide more concrete data demonstrating clear performance variations directly attributable to this property and clarify how this hygroscopic nature impacts operational stability of devices?\nResponse:\nWe thank the reviewer for this comment.\nThe hygroscopic nature of PEDOT:PSS and its detrimental impact on solar cell performance have been well documented in the literature.15,16 In particular, for NBG perovskite solar cells, it is standard practice to transfer PEDOT:PSS-coated substrates immediately into a nitrogen glovebox after annealing to avoid ambient exposure.17\nTo directly assess the impact of PEDOT:PSS hygroscopicity in our devices, we performed controlled experiments on NBG PSCs by comparing freshly prepared PEDOT:PSS layers with those intentionally exposed to a humid environment prior to perovskite deposition. As shown in Figure R 9, humidity-exposed samples exhibited both a lower average efficiency and a larger performance spread compared to the freshly prepared controls. We attribute this to water uptake by the surface PSS, which causes swelling, reduces the electrical conductivity of the PEDOT:PSS layer,16 and promotes interfacial degradation with the perovskite layer. These factors collectively lead to poorer photovoltaic performance and diminished long-term operational stability.\nThe detailed experimental results and discussion have been included in the revised manuscript (Supplementary Figure 11) to provide concrete evidence directly linking PEDOT:PSS hygroscopicity to device performance variations and stability loss.\n7. For comprehensive evaluation of tailored PEDOT:PSS layer's role in device modification, TRPL measurements should be presented for both ITO/HTL/perovskite and HTL/perovskite stacks (Figure. 2e) in parallel.\nResponse:\nWe appreciate the constructive suggestion. Following the reviewer’s advice, we have performed the requested TRPL measurements for both ITO/HTL/perovskite and HTL/perovskite stacks. The results are now presented in Figure R 10 below, and the corresponding Figure 2e in the manuscript has been updated accordingly. The related discussion in the text has also been revised to reflect these new results.\n8. The article's focus on addressing uniformity in narrow-bandgap perovskite devices requires stronger characterization support:\n(1) While multi-batch J-V statistics (Figures. 1a-d, f and 4b, d, f, g) demonstrate improved reproducibility, they don't establish enhanced performance uniformity across sub-cells on the same substrate - the crucial metric for uniformity assessment\nResponse:\nWe thank the reviewer for the constructive suggestion.\nIn response, we have evaluated the performance uniformity of sub-cells on the same substrate, as shown in Figure R 11. The PV performance distribution maps reveal that devices based on pristine PEDOT:PSS exhibit a larger spread in all PV parameters, with a particularly pronounced decrease in J when measured sequentially from cell #1 to #4. In contrast, the cells prepared with Triton X-100 show markedly improved uniformity across the substrate for all parameters. These results have been added to the revised manuscript and Supplementary Figure 9 to strengthen our assessment of performance uniformity.\n(2) Lack of direct uniformity characterization (e.g., PL/EL mapping, spatially resolved analysis)\nResponse:\nWe appreciate the reviewer’s comment.\nTo address this point, we performed supplementary experiments using 1 cm2 cells to image the electroluminescence (EL) and assess spatial uniformity, as shown in Figure R 12a and b. The EL images clearly show that the device prepared with Triton X-100 exhibits more homogeneous emission compared to the reference device, confirming improved uniformity. These results have been included in the revised manuscript and supplementary Figure S13 to further strengthen the manuscript.\n3) Figure S4 shows target sample has larger average grain size with more dispersed distribution. Is this dispersion region-dependent? If so, please explain the origin of such regional heterogeneity.\nResponse:\nWe appreciate the reviewer’s question. In our measurements, we did not observe any distinct region-specific variations in grain size across the imaged areas. Rather, the broader distribution in the “With Triton X-100” sample arises from the coexistence of moderately larger grains (relative to the reference) and occasional significantly larger grains dispersed throughout the film. We attribute this to the modified nucleation and growth dynamics of the perovskite induced by the Triton X-100–modified PEDOT:PSS morphology, which promotes more heterogeneous grain growth and thereby results in a wider grain size distribution.\n9. Flexible all-perovskite tandem modules have attracted growing attention for their higher efficiency potential. However, current certified efficiencies for flexible single-junction modules have already exceed 20% with areas >10 cm² (21.82%-12.8 cm² - Zhu et al., Adv. Mater. 2025; 20.3%-27.9 cm² - Zhang et al., Small 2024; 20.2-12.9 cm²-Zhang et al. Adv. Funct. Mater. etc). 19.7% efficiency for 1.3 cm² flexible tandem module fails to demonstrate scalable efficiency advantages over existing single-junction devices.\nResponse:\nWe thank the reviewer for this comment.\nWe acknowledge recent reports demonstrating certified efficiencies above 20% for flexible single-junction perovskite modules with areas exceeding 10 cm2. However, as this is the first work on flexible all-perovskite tandem solar mini-modules, we would like to highlight the following points:\n1. First demonstration in this field: To the best of our knowledge, our work presents the first successful demonstration of flexible all-perovskite tandem solar cells, which represents a key milestone for advancing this emerging technology.\n2. Significance despite smaller area: Although our current flexible tandem module (1.3 cm2) is smaller than the single-junction examples cited, achieving 19.7% efficiency at the mini-module level in a tandem configuration demonstrates the feasibility of integrating two perovskite sub-cells on a flexible substrate with compatible module-level processing. Given the complexity of tandem architecture and laser scribing on flexible substrates, we believe these results represent a significant step forward.\n3. Foundation for upscaling: In this work, we developed a complete set of laser-scribing techniques optimized for flexible tandem modules. These methods are directly transferable to larger-area fabrication and are critical for future scaling efforts.\n4. Potential for competitive large-area performance: We have performed module-level simulations predicting that efficiencies over 24% could be achieved for 100 cm2 flexible tandem modules (Figure 5i), indicating strong potential to surpass current single-junction records in the near future.\nWe believe these points underscore the novelty and importance of our study and its relevance for the development of high-efficiency, lightweight, and scalable tandem photovoltaic technology.\n10. The schematic of tandem module in Figure. 5d omits several critical functional layers. The figure requires revision with clear labeling of all essential component.\nResponse:\nWe thank the reviewer for pointing this out.\nIn the revised manuscript, Figure 5d has been updated to include clear labeling of all essential functional layers, ensuring the schematic accurately represents the full tandem module structure.\n11. The mechanical stability and operational stability data of flexible tandem devices/modules hold critical importance for evaluating their potential application scenarios. Could the authors provide explicit experimental data demonstrating the impact of modified PEDOT:PSS on these stability metrics? Quantitative evidence in this regard would advance the development of flexible tandem modules.\nResponse:\nWe thank the reviewer for highlighting this important point. We fully agree that mechanical and operational stability are critical for evaluating the practical application potential of flexible tandem modules.\nIn response, we have conducted additional experiments to quantitatively assess the impact of the modified PEDOT:PSS layer on both stability metrics of our flexible all-perovskite tandem mini-modules.\nMechanical stability:\nAs shown in Figure R 13a, a manual bending test with a radius of 15 mm revealed that the flexible tandem mini-module retained nearly 100% of its initial performance after 2000 bending cycles, indicating excellent flexibility. To further evaluate operational stability under mechanical stress, we performed bending tests using a homemade bending machine while the device was under MPP tracking. As shown in Figure R 13b, the device maintained over 83% of its initial PCE after 700 bending cycles. Although the exact bending radius in this setup is difficult to determine, the results clearly demonstrate robust mechanical endurance of the flexible tandem architecture. We note that device degradation during MPP operation under bending could also be affected by the very basic encapsulation used in this work — a simple household lamination rather than an industrial-grade encapsulation — which likely allows significant water or oxygen ingress. The bending direction may also play a role and merits further investigation.\nOperational stability:\nWe also performed continuous MPP tracking on flexible tandem mini-modules. As shown in Figure R 13c, the devices exhibited a rapid initial drop in performance during the first 30 hours. While recent reports on rigid all-perovskite tandems show T90 values exceeding several hundreds of hours, similar long-term operational stability data for flexible tandem mini-modules are currently lacking in the literature.17 Our findings suggest that while the flexible tandem mini-modules remain stable under mechanical stress, their long-term operational stability is considerably limited at the moment, likely due to the inadequate encapsulation—prone to water and oxygen ingress—and deformation of the flexible substrates under thermal stress.\nOverall, these results demonstrate that the flexible all-perovskite tandem mini-modules exhibit decent mechanical durability and reasonable operational stability, which can be further enhanced through improved encapsulation and other optimization strategies. We have incorporated these new results and discussion into the revised manuscript (Supplementary Figure 20) to advance understanding and development of this technology."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors'response has adequately addressed the reviewers'concerns. I'm pleased to recommend acceptance of the manuscript without further revision.", "reviewer_2": "The authors and their team have provided detailed responses and supplementary data addressing our previous concerns regarding the mechanistic aspects, which is commendable. Recently, significant progress has been reported for flexible tandem modules with areas exceeding 20 cm² (https://doi.org/10.1038/s41566-025-01746-6), indicating growing interest in this research direction. As a representative study, this manuscript would benefit from additional detailed data and information to enhance its guidance and demonstrative value:\nBoth Figure R7 and Figure R11 present performance comparisons among multiple sub-cells on the same substrate. The dimensions of the substrate and the sub-cells should be clearly indicated in both figures. Additionally, a photographic image of the sample corresponding to Figure R7 should be provided.\nNarrow-bandgap devices and corresponding tandem devices based on SAM-only, PEDOT:PSS/SAM bilayer, and PEDOT:PSS-only structures are of great interest in the field. It would be highly beneficial if the authors could provide comparative results combining or contrasting the modified PEDOT:PSS with these three structures. Such comparisons would significantly contribute to advancing flexible tandem device research.\nThe fitting parameters obtained from the TRPL measurements should be explicitly listed in a table in the Supplementary Information.\nThe authors should update the relevant descriptions in the manuscript to reflect the latest advancements in flexible tandem devices and modules.\nThe revised manuscript includes additional data on mechanical and operational stability, which is appreciated. However, we find these results not fully convincing and recommend the following improvements:\n(1) The methods used for mechanical stability testing, including details of the bending setup (if applicable), should be thoroughly described in the Methods section or main text. This is crucial for the systematic evaluation of progress in the mechanical stability of flexible tandems in the further.\n(2) We anticipate that the modified HTL should contribute to improved stability. Data and discussion supporting this point should be provided in an appropriate section, as this would offer important insights into the understanding of the modification strategy in the further.\n(3) Regarding the MPP stability test under bending mentioned in Figure R13b, the use of \"simple household lamination\" for encapsulation is noted. The preparation method for encapsulated samples used in mechanical stability tests should be detailed in the Methods section. This is essential for a systematic assessment, as sample thickness influences the neutral mechanical plane position and thereby affects mechanical stability.\n(4) The MPP stability results presented under nitrogen atmosphere are not ideal. It should be noted that MPP tests for rigid tandem devices are typically performed under encapsulated conditions. Therefore, comparative MPP stability results for encapsulated flexible tandem samples should be provided."}, "author_response": "We appreciate Reviewer 1’s positive assessment and recommendation for acceptance. The reviewers’ feedback has strengthened the manuscript, and we are ready to proceed to production.\nWe appreciate the reviewer’s thoughtful assessment and the constructive suggestions, especially regarding flexible tandem modules and the recent progress on >20 cm2 devices (Nature Photonics, 2025, doi: 10.1038/s41566-025-01746-6). In the revised manuscript, we have added new data, clarified methods, and updated context to increase the guidance and demonstrative value of our work. Our changes are summarized point-by-point below.\nBoth Figure R7 and Figure R11 present performance comparisons among multiple sub-cells on the same substrate. The dimensions of the substrate and the sub-cells should be clearly indicated in both figures. Additionally, a photographic image of the sample corresponding to Figure R7 should be provided.\nResponse:\nWe thank the reviewer for the reminder. We now clearly annotate the substrate and sub-cell dimensions for the previous Figure R7 (here Figure 1). Specifically, the substrate size is 50 mm × 50 mm, and for each cell the active area is 0.0855 cm2 with an aperture mask of 0.0616 cm2. A photographic image of the sample is also provided (Figure 1a). The above information is provided in the revised Figure S19.\nWe now also annotate, in the caption of the revised Figure S9, the substrate and sub-cell dimensions for the sample previously shown in Figure R11: substrate 25 mm × 25 mm; for each cell the active area is 0.144 cm2 and the aperture mask is 0.09 cm2.\nNarrow-bandgap devices and corresponding tandem devices based on SAM-only, PEDOT:PSS/SAM bilayer, and PEDOT:PSS-only structures are of great interest in the field. It would be highly beneficial if the authors could provide comparative results combining or contrasting the modified PEDOT:PSS with these three structures. Such comparisons would significantly contribute to advancing flexible tandem device research.\nResponse:\nWe appreciate the constructive suggestion and agree that cross-architecture comparisons would benefit the field. However, this study is deliberately focused on PEDOT:PSS and its additive engineering, so we limit comparisons to PEDOT:PSS baselines vs. our modified PEDOT:PSS. Incorporating SAM-only and PEDOT:PSS/SAM bilayer stacks would require full, architecture-specific re-optimization, which lies outside the scope of this work.\nIn future studies, we will benchmark SAM-only and bilayer architectures under unified processing and test protocols to provide the comprehensive comparison the reviewer suggests.\nThe fitting parameters obtained from the TRPL measurements should be explicitly listed in a table in the Supplementary Information.\nResponse:\nWe thank the reviewer for the helpful suggestion. In the revised Supplementary Information we have added Table S1, which lists the TRPL fitting parameters for all samples.\nThe authors should update the relevant descriptions in the manuscript to reflect the latest advancements in flexible tandem devices and modules.\nResponse:\nWe thank the reviewer for this valuable suggestion. We have substantially updated the manuscript to reflect the latest advances in flexible tandem devices and modules (as of September 2025). We also updated Figure 5c (here below as Figure 2) adding the new report. To highlight the rapid recent progress, we added the following sentence in the revised manuscript in the section Flexible tandem solar cells and mini-modules with the citation of the mentioned work:\n\"Concurrently, a related study reported a 23% PCE on a 20.26 cm2 flexible all-perovskite tandem mini-module, underscoring the rapid pace of progress in this area.\"\nThe revised manuscript includes additional data on mechanical and operational stability, which is appreciated. However, we find these results not fully convincing and recommend the following improvements:\n(1) The methods used for mechanical stability testing, including details of the bending setup (if applicable), should be thoroughly described in the Methods section or main text. This is crucial for the systematic evaluation of progress in the mechanical stability of flexible tandems in the further.\nResponse:\nWe thank the reviewer for this helpful suggestion.\nWe have described the bending setup in the Methods section in revised manuscript as below:\n\"Bending test: The mechanical bending test is done by manually conforming the mini-moudules to the mould with a radius of 15 mm. For the MPP tracking during the bending, the sample is fixed onto the stages of a customized bending machine, and as the distance between the two stages becomes shorter, bending stress is applied to the device fixed to the stages during the MPP tracking. \"\nAnd we also provided a detailed explanation on the bending tests with visualization in the Supplementary Note 4.\n(2) We anticipate that the modified HTL should contribute to improved stability. Data and discussion supporting this point should be provided in an appropriate section, as this would offer important insights into the understanding of the modification strategy in the further.\nResponse:\nWe thank the reviewer for this suggestion.\nWe have added shelf-stability tracking (N₂-filled glovebox storage) in Figure 2h (also below in Figure 3). Devices with the modified HTL (with Triton X-100) retain 93.8% of the initial PCE after 1375 h, whereas reference devices retain 80.7% after 100 h. We opted for shelf-stability rather than MPP tracking at this stage because our current encapsulation is not yet sufficiently robust for long term stability tracking, since our J-V/MPP setup is outside of the glove box, MPP results would be dominated by moisture/oxygen ingress and encapsulation failure, obscuring the intrinsic HTL effect.\n(3) Regarding the MPP stability test under bending mentioned in Figure R13b, the use of \"simple household lamination\" for encapsulation is noted. The preparation method for encapsulated samples used in mechanical stability tests should be detailed in the Methods section. This is essential for a systematic assessment, as sample thickness influences the neutral mechanical plane position and thereby affects mechanical stability.\nResponse:\nWe thank the reviewer for the constructive suggestion.\nWe have added full details of the encapsulation used for the bending tests in the Methods (Section: Bending test) and clarified thickness and neutral-plane considerations in Supplementary Note 4. Specifically, the flexible tandem mini-modules were encapsulated using a desktop laminator (Fellowes Laminiergerät Calibre) with double sided PET foil (80 micrometers for each) at a temperature of 125 °C. The total laminated stack thickness is 285 µm, and the device stack lies near the laminate mid-plane due to the symmetric PET/PET layup.\nWe have also noted in Supplementary Note 4 the calculation of the strain with total thickness of the sample. It is also clarified that with the double-sided PET encapsulation, the device can thus be near the neutral-strain position, which reduces the strain value applied to the device according to the literature (Nature Energy 9.11 (2024): 1335-1343.).\n(4) The MPP stability results presented under nitrogen atmosphere are not ideal. It should be noted that MPP tests for rigid tandem devices are typically performed under encapsulated conditions. Therefore, comparative MPP stability results for encapsulated flexible tandem samples should be provided.\nResponse:\nWe thank the reviewer for the comment.\nWe clarify that the reported MPP stability measurements for the flexible tandem mini-modules were performed on encapsulated samples (cover glass + UV-curable epoxy). We agree that the MPP stability is not yet ideal. Our assessment is that the decay is dominated by encapsulation limitations rather than intrinsic stack instability: (i) the present flexible encapsulation is not sufficiently robust under prolonged light/thermal stress; (ii) laser-scribing lines are likely ingress pathways for residual moisture/oxygen in a purged (but not fully sealed) N environment; and (iii) thermal deformation of flexible substrates can exacerbate these effects.\n\"Because the current encapsulation is not yet sufficiently robust for moisture- and oxygen-sensitive devices under prolonged light/thermal stress, these MPP data should be regarded as preliminary; improved encapsulation will be implemented to enable systematic, long-term operational-stability testing.\""}, {"round": 3, "reviewer_comments": {"reviewer_2": "The author has successfully addressed all my concerns through their revisions, and I believe the manuscript now meets the high standards of Nature Communications; I recommend it for publication."}, "author_response": ""}]

Decision letter