Interfacial engineering via dipolar fullerene derivative for efficient tin halide perovskite indoor photovoltaics

Domain: physics
Published: 2026-01-21
Editorial outcome: minor_revision

Abstract
Modulating hot carrier dynamics is crucial in tin halide perovskite photovoltaics, particularly under indoor illumination with limited photon flux. Herein, a fullerene derivative bearing four piperazine groups (denoted as TPPC) is synthesized to engineer the perovskite/C60 interface. The TPPC molecule exhibits a dipole moment of 1.97 Debye, leading to enhanced adsorption energy on perovskite surface and robust interfacial interaction. The newly formed surface dipole optimizes the interfacial energy level alignment via a cascade gradient, enabling modulation of interfacial hot carrier dynamics. Consequently, TPPC-treated photovoltaic devices achieve a champion power conversion efficiency (PCE) of 22.49% and a maximum output power density (Pout) of 64.1 μW cm-2 under white light-emitting diode illumination (3000 K, 1000 lux, 285 μW cm-2). Large-area (1.21 cm2) devices attain a PCE of 17.94% (certified: 15.93%) and a maximum Pout of 51.2 μW cm-2 under the same illumination conditions.

Introduction
Perovskite solar cells (PSCs) have attracted considerable attention owing to the exceptional characteristics of perovskite, including large adsorption coefficient, tunable bandgap, high charge carrier mobility, solution processibility, etc1–5. The power conversion efficiency (PCE) of multiple-junction PSCs has achieved 33.9% thus far6, indicating great potential for commercial applications. Currently, owing to the emergency of a technology known as Internet of Things worldwide, it is imperative to fabricate indoor photovoltaic (IPV) devices to serve as power sources for electric appliances within a smart home system7–11. To date, the PCE of lead-based PSCs has reached 44.72% for IPV applications12. However, since the lead leakage is harmful to human health, especially for indoor applications, it is desirable to fabricate non-toxic and lead-free PSCs for IPV applications.

Reported lead-free PSCs targeting IPV applications are predominantly categorized into antimony (Sb)-based, bismuth (Bi)-based, and tin (Sn)-based devices13–25. Since tin and lead belong to the same group in the periodic table, tin halide perovskites share similar optoelectronic properties with their lead-based counterparts. This similarity has made them a promising alternative for IPV applications. Moreover, their bandgaps can be adjusted to exceed 1.65 eV through halide composition engineering to better match the indoor light spectrum and the theoretical photovoltaic efficiency limit can reach a remarkable 55%26–28. Therefore, tin halide perovskites exhibit great potential for IPV applications. Liao et al. first report the use of tin halide perovskites for IPV applications with a PCE of 10.45%21. They attribute such a low PCE to the oxidation of tin within perovskites. To solve this issue, they introduce catechin into tin halide perovskites as an antioxidant. Ultimately, they achieve a maximum PCE approaching 12.81% and a maximum output power density (Pout) of 52.53 μW cm−2 under 1000 lux. The modification of hole-transport layer (HTL) is another effective strategy to increase the PCE of IPV devices. For example, Wang et al. demonstrate a PCE of 17.57% and a maximum Pout of 58.75 μW cm−2 for tin halide PSCs under the white light-emitting diode (WLED) illumination (2,956 K, 1062 lux, 334.41 μW cm−2) by modifying poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with potassium thiocyanate (KSCN)19. They reveal that the KSCN interlayer not only optimizes the energy level alignment, but also improves the perovskite crystallization. To regulate the nucleation kinetics of tin halide perovskites, our group has previously incorporated cesium fluoride (CsF) into the PEDOT:PSS layer29. Results show that this method can enhance the adhesion of perovskites at the buried interface, thereby facilitating the heterogeneous nucleation. The CsF-incorporated PSCs demonstrate an efficiency of 20.12% and a maximum Pout of 57.32 μW cm-2 under the WLED illumination (3000 K, 1000 lux, 285 μW cm−2).

As is well established, when a semiconductor is excited by incident photons with energies exceeding its bandgap, hot carriers are generated30–32. Their excess energy is then dissipated through phonon emission until the carriers eventually relax to the bottom of the conduction band. This hot-carrier thermalization represents a major energy loss channel in photovoltaics, posing a fundamental limitation to the efficiency of PSCs as described by the Shockley–Queisser limit33–35. In principle, this limitation could be mitigated by extracting hot-carrier energy through the perovskite/electron transport layer (ETL) or perovskite/HTL interface prior to thermalization. Since low-light indoor illumination exhibits a substantially lower photon flux than one-sun illumination, the modulation of hot carrier dynamics is critically important to obtain the maximal photon utilization efficiency for IPV applications. It is noteworthy that the hot carrier cooling in tin halide perovskites typically takes place within nanosecond scale, which is significantly slower than that in conventional lead halide perovskites and silicon (within picosecond scale)30,36–40. Such a long cooling time provides a window of opportunity for efficient hot carrier extraction in tin halide perovskites. Effective modulations enable the extraction of high-energy hot carriers prior to their excess energy dissipation via lattice vibrations, thus improving the device performance. In the past few years, considerable progress has been attained in controlling hot carrier dynamics in perovskites, which mainly includes the use of quantum confinement effect in nanostructured perovskites41,42, the incorporation of plasmonic nanoparticles into perovskite films43,44, and the engineering of exciton-phonon coupling to slow down the hot carrier relaxation in perovskites37,45. However, the efficient extraction of hot carriers in perovskite photovoltaic device under low-light indoor illumination still remains a big challenge for IPV applications.

Herein, a multifunctional fullerene (C60) derivative with four piperazine groups (denoted as TPPC) is synthesized and used to modulate hot carrier dynamics at the perovskite/C60 (PVK/C60) interface. The TPPC molecule exhibits a dipole moment as high as 1.97 Debye, indicating its interaction with perovskite to form surface dipoles. This strong interaction is confirmed by the increased adsorption energy of TPPC on the perovskite surface, along with the distinct chemical shift observed in the 1H nuclear magnetic resonance (1H-NMR) spectrum after the incorporation of tin (II) iodide (SnI2) into TPPC. The TPPC treatment also optimizes the interfacial energy level alignment by constructing a gradient, resulting in an extra back-surface electric field (Eextra). Since the Eextra is in the same direction as the built-in potential of the PVK/C60 interface, electron charge extraction at the interface is significantly improved. In addition, the TPPC-treated photovoltaic devices exhibit higher charge mobility, accelerated electron extraction and elevated charge collection efficiency. As a result, the modified IPV devices obtain a champion PCE of 22.49%. The certificated PCE for the large-area (1 cm2) photovoltaic devices is as high as 15.93%. More importantly, the encapsulated devices exhibit enhanced operational stability, where ~91% of its initial efficiency is maintained after 2000 h of continuous maximum power point (MPP) tracking under simulated 1000 lux illuminance (ISOS-L-3 protocol) in ambient air (humidity: 40 ± 10%, temperature: 25 ± 5 °C). The strategy of modulating interfacial hot carrier dynamics with fullerene derivatives provides valuable guidance for improving the performance of IPV devices.

Results
Interaction between fullerene derivative and perovskite
The synthetic route for the TPPC molecule is illustrated in Fig. 1a. First, chlorofullerene (C60Cl6) was prepared by reacting C60 with excess iodine monochloride (ICl). Subsequently, tert-butoxycarbonyl (Boc) protected TPPC (TPPC-Boc) was produced by reacting C60Cl6 with piperazine in the presence of cumene hydroperoxide (CHP). Finally, TPPC molecules were obtained by replacing the Boc groups in TPPC-Boc with trifluoroacetic acid (TFA) in dichloromethane (CH2Cl2). Supplementary Figs. 1–4 depict the high-performance liquid chromatography (HPLC) analysis, electrospray ionization mass spectrometry mass spectra (ESI-MS), 1H-NMR spectroscopy and 13C-NMR spectroscopy of the prepared TPPC-Boc and TPPC molecules, respectively. These results demonstrated the successful syntheses of the fullerene derivatives with high purity. The electrostatic potential distribution of TPPC is shown in Fig. 1b. Obviously, the positive potential primarily localizes around the carbon (C) and hydrogen (H) atoms. While the negative potential concentrates around the nitrogen (N) atoms, suggesting the potential interaction with the positive Sn2+ of the tin halide perovskite. Furthermore, the TPPC molecule exhibits a dipole moment as high as 1.97 Debye, which further validates its interaction with perovskite to form the surface dipole.

To verify the potential interaction between TPPC and perovskite, density functional theory (DFT) calculations were performed. For comparison purposes, we first calculated the adsorption energy (Eads) of C60 on the perovskite surface. As expected, the obtained Eads was as low as −0.02 eV due to the weak physical interaction between C60 and perovskite (Fig. 1c). In contrast, the Eads value (−1.06 eV) of TPPC on the perovskite surface was significantly increased compared to C60, as depicted in Fig. 1d and Supplementary Fig. 5. It is noteworthy that the Eads value of TPPC on the SnI2-terminated perovskite surface (−1.06 eV) was significantly higher than that on the formamidinium bromide (FABr)-terminated surface (−0.18 eV). This suggested that TPPC molecules preferred to adsorb on the SnI2-terminated perovskite surface due to the robust Lewis base-acid interaction between TPPC and SnI2. Furthermore, the Eads value (−0.21 eV) of C60 on the TPPC-treated perovskite was significantly higher than that on the untreated perovskite (−0.02 eV) because of the strong π-π interactions between fullerenes (Supplementary Fig. 6). This is advantageous for establishing an effective contact between perovskite and C60, which facilitates electron transfer and prevents non-radiative recombination.

The Sn-N interaction could be validated by comparing the 1H-NMR spectra of TPPC and TPPC+SnI2. As depicted in Fig. 1e, the spectrum of TPPC exhibited a characteristic peak at the chemical shift of 9.02 ppm, while the peak showed a distinct shift of 0.22 ppm for TPPC+SnI2. This pronounced shift clearly indicated a strong interaction between TPPC and SnI2. Furthermore, the shift in the high-resolution X-ray photoelectron spectroscopy (XPS) spectrum of iodine (I) 3d also confirmed the existence of this interaction (Fig. 1f). In addition, the TPPC molecule features four piperazine units in its skeleton, giving it strong basicity. This basicity effectively suppressed the oxidation of Sn2+ in the perovskite film. As revealed by the high-resolution XPS spectrum of Sn 3d (Fig. 1g), the Sn2+ content in the perovskite film showed a notable rise from 63% to 76% upon the incorporation of TPPC.

The robust interaction between TPPC and perovskite could modulate the properties of perovskite films. According to the scanning electron microscope (SEM) images (Supplementary Fig. 7), it was evident that the TPPC treatment with an optimal concentration (0.1 mg mL-1) suppressed the pinhole formation, thereby enhancing the perovskite film quality. This was supported by the photoluminescence (PL) mapping (Supplementary Fig. 8), where the modified film exhibited a more uniform and stronger PL intensity distribution than the pristine film. Furthermore, the atomic force microscopy (AFM) images and their corresponding line-scan height profiles also suggested the formation of a smoother perovskite film subsequent to TPPC treatment (Supplementary Fig. 9). X-ray diffraction (XRD) patterns were also measured to examine the crystallinity variation of perovskite film after TPPC treatment. The improved crystallinity and enlarged grain size of the TPPC-treated perovskite film were evidenced by the stronger and sharper diffraction peaks at the (100) and (200) planes (Supplementary Fig. 10). Specifically, the full width at half-maximum (FWHM) narrowed from 0.08° to 0.06° after TPPC treatment. According to the grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns and the azimuthally integrated plots of the scattering vector (q)-dependent intensity (Supplementary Fig. 11), we could observe multiple well-defined diffraction peaks (e.g., (100), (011), (022), (211), (122), (120), and (208)), unequivocally confirming the formation of tin halide perovskite5. The GIWAXS patterns also validated the enhancement of the film crystallinity after TPPC treatment (Supplementary Fig. 11a, b), particularly as evidenced by the (100) diffraction plane. Additionally, according to the intensity-q plots (Supplementary Fig. 11c-h), the characteristic peaks showed no obvious shifts after TPPC treatment, demonstrating that the TPPC molecules did not incorporate into the lattice or alter the bulk crystal structure of perovskite. In summary, it can be concluded that the strong interaction between TPPC and the perovskite can suppress the oxidation of Sn2+, leading to reduced defect density and enhanced film crystallinity. Consequently, the overall quality of the perovskite film is improved after TPPC treatment.

Optimization of the perovskite/fullerene interface
In order to demonstrate the optimization of the PVK/C60 interface, we examined the energy level alignment by analyzing the bandgap, work function (WF), conduction band and valence band of pristine perovskite film, TPPC-modified perovskite film and C60 film. These parameters were determined from the Tauc plot of the ultraviolet-visible (UV-Vis) absorption spectrum (Supplementary Fig. 12) and ultraviolet photoelectron spectroscopy (UPS) (Supplementary Fig. 13). Severe charge carrier recombination was observed at the untreated PVK/C60 interface, as depicted in Supplementary Fig. 14. In contrast, an energy level gradient was established at the modified interface owing to the downward band bending of the TPPC-treated perovskite surface (Fig. 2a), which reduced the interfacial energy loss and suppressed interfacial charge carrier recombination by inhibiting the backflow of holes. It was noteworthy that the conduction band energy difference (ΔEc) between perovskite and C60 decreased from 0.51 to 0.34 eV after TPPC treatment. Moreover, since the Fermi level (Ef) of the modified perovskite surface (Ef2 = −4.97 eV) was higher than that of the bulk (Ef1 = −5.01 eV), electrons would spontaneously flow from the perovskite surface to the bulk. This charge redistribution resulted in the formation of surface dipoles until the Fermi level alignment was reached. In such a case, the extra back-surface electric field Eextra pointing towards the substrate was established. Since the Eextra was in the same direction as the built-in potential of the PVK/C60 interface, electron charge extraction at the interface was significantly improved. The enhancement of the built-in electric field at the PVK/C60 interface could be further demonstrated by Kelvin probe force microscopy (KPFM) measurements (Fig. 2b). By analyzing the potential distribution along the blue dished line in KPFM images, it was observed that the potential difference of the TPPC-treated interface (0.25 V) was much larger than that of the untreated interface (0.09 V) (Fig. 2c), indicating an elevated electric field and agreeing well with the energy level analysis. To explore the reasons for the accelerated electron extraction at the modified interface, KPFM measurements were conducted to examine the impact of TPPC on the modulation of the work function of perovskite film. In contrast to pristine perovskite film, the TPPC-treated film exhibited a smaller contact potential difference (CPD, 0.65 vs 0.77 V) along a certain direction of the KPFM images (Supplementary Fig. 15), suggesting a reduced WF owing to the formation of the surface dipole according to the following equation46

WF = Wtip + e × CPD (1)

where Wtip is the work function of the measuring probe and e is the unit charge, respectively. The modulation of work function was also confirmed through DFT calculations, where the calculated WF of perovskite decreased from 4.87 eV to 4.41 eV after TPPC treatment (Supplementary Fig. 16). This decrease in WF indicated a more n-type character of the perovskite surface, thereby facilitating electron extraction.

The optical properties of the PVK/C60 interface by TPPC treatment were also experimentally investigated. Compared with the pristine PVK/C60 film, the TPPC-treated film showed not only reduced PL intensity (Fig. 2d) but also a shorter lifetime in the time-resolved photoluminescence (TRPL) measurements (Fig. 2e), indicating the accelerated electron extraction at the modified interface. Additionally, the photoluminescence quantum yield (PLQY) of pristine perovskite film showed a decrease of 1.4% subsequent to the incorporation of C60 (Fig. 2f), indicating a substantial energy loss at the PVK/C60 interface. In contrast, the PLQY of the TPPC-treated perovskite film decreased by only 0.9% after the deposition of C60, suggesting a reduced energy loss at the modified interface.

Modulations of interfacial hot carrier dynamics
The femtosecond transient absorption (fs-TA) spectrum was utilized to explore the influences of TPPC on the modulations of hot carrier dynamics at the PVK/C60 interface. It is well-known that the hot carrier cooling time is dependent on the phonon scattering rate under low and middle pump fluence, and the variation of cooling time can reflect the carrier extraction rate37,47,48. In the following, we first investigated the hot carrier dynamics under a low pump fluence of 1.05 μJ cm−2. As depicted in the 2D pseudo-color images (Supplementary Fig. 17) and fs-TA spectra (Fig. 3a, b) within a short-time scale (1.2–81.9 ps), the photobleaching peaks occurred around 1.7 eV, due to the band edge filling effect. By normalizing the TA spectra near the band edge (Fig. 3c, d), the photobleaching signals were broadened in the high-energy region owing to the Fermi–Dirac statistics of hot carriers. To further investigate the decay dynamics of hot carriers, the high-energy tail (1.75–1.83 eV) of the spectrum was fitted using the Maxwell-Boltzmann distribution function as follows46,49

ΔA(E)/e(E−Ef)/kBTc = k (2)

where E is photo energy, kB is Bolzman constant and Tc is hot carrier temperature, respectively. After extracting Tc from the fitting, the time-dependent carrier temperature was obtained (Fig. 3e). It was observed that the TPPC-treated PVK/C60 interface exhibited a shorter hot carrier cooling time (6.29 ps) than the untreated interface (6.46 ps). To further quantify the degree of error for the hot carrier cooling time, we made 4 samples without TPPC treatment and 4 samples with TPPC treatment. As shown in Supplementary Fig. 18, the hot carrier cooling time was 6.70 ± 0.19 ps and 6.00 ± 0.23 ps for the samples without and with TPPC treatment, respectively, suggesting that TPPC treatment could possibly facilitate hot carrier extractions at the interface and minimize thermalization losses of interfacial hot carriers. Note that we cannot definitely attribute the decrease in hot carrier cooling time to the accelerated hot carrier extraction at this stage, since a change in cold carrier extraction is another possibility. The final Tc value (224 K) of the treated interface was significantly lower than that of the untreated interface (288 K), indicating that the final formations of hot carriers in the modified perovskite exhibited a narrower Boltzman distribution and a broader range of charge carrier extraction. In addition, the hot carrier energy loss rate (Jr) was calculated by the following equation46,49

Jr = −1.5kB dT/dt (3)

As depicted in Fig. 3f, the TPPC-treated interface exhibited a higher energy loss rate than the original one, which also indicated the possibly accelerated hot carrier extraction at the modified interface. We also investigated the fs-TA spectra of the modified interface under a larger pump fluence of 2.63 μJ cm−2 (Supplementary Fig. 19). As shown in Supplementary Fig. 20, the TPPC-treated PVK/C60 interface showed a longer hot carrier cooling time (3.61 ± 0.32 ps) than the untreated interface (2.70 ± 0.35 ps). The prolonged hot carrier cooling time in the TPPC-treated interface at high fluence was probably ascribed to the reduction of defect density and suppression of Auger recombination and carrier-carrier scattering33,50. Compared to the hot carrier cooling time under the low pump fluence (1.05 μJ cm−2), the shortening of hot carrier cooling time under high pump fluence (2.63 μJ cm−2) primarily reflected the weakening of the hot phonon bottleneck effect and the intensification of recombination losses. In sum, we could draw the conclusion that owing to the suppressed Sn2+ oxidation, reduced trap density and optimized energy level alignment, the hot carrier extraction was possibly enhanced and the Auger recombination was efficiently mitigated at the TPPC-modified PVK/C60 interface.

Indoor photovoltaic performances
As TPPC treatment was able to accelerate hot carrier extraction at the PVK/C60 interface, TPPC-treated IPV devices were fabricated. Figure 4a shows a cross-sectional SEM image of the fabricated photovoltaic device, which consists of indium tin oxide (ITO)/PEDOT:PSS/PVK/C60/bathocuproine (BCP)/silver (Ag). Supplementary Fig. 21 shows the schematic diagram of the experimental setup for evaluating the performance characteristics of IPV devices. Fig. 4b shows the current density-voltage (J-V) curves of the IPV devices based on pristine and TPPC-treated perovskite under the simulated indoor illumination (Supplementary Fig. 22, 3000 K, 1000 lux, 285 μW cm−2). The J-V curves for devices treated with different concentrations of TPPC were also examined, as depicted in Supplementary Fig. 23. The control IPV devices exhibited a champion PCE of 15.19% with a maximum Pout of 43.3 μW cm−2 under the simulated indoor illumination, whereas the TPPC-treated devices achieved a remarkable champion PCE of 22.49% with an open circuit voltage (Voc) of 0.72 V, a short-circuit current density (Jsc) of 140.0 μA cm-2, a fill factor (FF) of 63.6% and a maximum Pout of 64.1 μW cm−2 (Supplementary Figs. 24, 25, Supplementary Tables 1, 2). Furthermore, we investigated the dependence of photovoltaic performances on incident light intensity and light color temperature (Supplementary Figs. 26, 27, Supplementary Tables 3, 4). As expected, the PCE of the device increased with light intensity and reached the maximum under 3000 K. The IPV devices with a large effective area of 1.21 cm2 were also fabricated. As depicted in Fig. 4c and Supplementary Table 5, the control devices with large effective areas obtained a champion PCE of 10.3% with a maximum Pout of 29.3 μW cm−2 under the simulated indoor illumination (Supplementary Fig. 22, 3000 K, 1000 lux, 285 μW cm−2), while the TPPC-treated devices delivered a PCE as high as 17.94% with a maximum Pout of 51.2 μW cm-2. Impressively, a certificated PCE of 15.93% was obtained for the TPPC-treated devices with an effective area of 1 cm2 (Supplementary Fig. 28). It should be noted that since the severe shadow effect induced by the square mask (thickness: 200 μm) reduced the light intensity under the indoor light illumination, the certified PCE (15.93%) for the device with a mask is lower than that (17.94%) measured in our lab without masks. The integrated current densities calculated from external quantum efficiency (EQE) of the IPV devices were consistent with the measured J-V characteristics (Fig. 4d). It was noted that since the absorption of perovskite film did not show significant enhancement after TPPC treatment, the notable increase in Jsc of the TPPC-treated devices was attributed to the improvement of charge collection efficiency. For comparison purposes, we summarized the performance distributions of 51 control and TPPC-treated devices (Fig. 4e). The statistical analysis demonstrated excellent reproducibility of the IPV devices and significant enhancements in PCE, Voc and Jsc due to the TPPC modification. Furthermore, based on the recent advancements in PCE of lead-free perovskite-based IPV devices (Fig. 4f, Supplementary Table 6)13–19, 21–25, 31, it is evident that our measured PCE (22.49%) without a light mask is higher than that in the previous reports. The Pout of 64.1 μW cm−2 also ranks among the highest reported values thus far (Supplementary Table 6). It should be noted that since all devices reported in the literatures are measured under different light sources, we cannot make a 1:1 comparison across all data points, but this provides a general indication of progress. We should also note that the measurements without a mask may cause several issues, including inaccurate definition of the device’s active area and the inclusion of peripheral leakage currents. On the other side, since the severe shadow effect of the square mask (thickness: 200 μm) makes the active area of the devices smaller than the mask’s aperture area under the diffuse indoor illumination (Supplementary Fig. 29), the measured PCE with a mask is underestimated. Given the more pronounced impact of the shadow effect on the small-area devices, the photovoltaic performance of small-area devices was measured without a mask and not certificated, while that of large-area devices was certificated with a mask in this study.

Additionally, the stability of IPV devices was investigated according to the International Summit on Organic Photovoltaic Stability (ISOS) protocols51. The measured devices were encapsulated with the cover glass and UV-curing resin (Supplementary Fig. 30). The long-term operation stability was evaluated by MPP tracking under constant simulated 1000 lux light illumination (285 μW cm−2, ISOS-L-3 protocol), as depicted in Fig. 4g. The TPPC-treated device maintained ~91% of its initial efficiency after 2000 h of continuous tests, whereas the pristine device retained only ~63% of the initial efficiency after the same duration. We also investigated the long-term operation stability of the devices by MPP tracking under constant simulated air mass 1.5 G illumination (100 mW cm−2, ISOS-L-3 protocol), as shown in Supplementary Fig. 31. As expected, the TPPC-treated device preserved over 98% of its initial efficiency after 600 h of continuous tests (~60 °C), while the pristine device kept only 77% of the initial efficiency after the same duration. In the studies of the thermal stability of the IPV devices, it was found that the TPPC-treated device preserved 90% of the initial efficiency after 462 h (T0.9) of thermal aging (85 °C) in nitrogen atmosphere (ISOS-D-2 protocol), surpassing the 252 h for the control device (Fig. 4h). The enhanced stability of the modified device primarily resulted from the reinforced PVK/C60 interface, which might suppress ion migration and perovskite degradation. To verify this, voltage-dependent PL intensity was investigated52, as shown in Fig. 4i. In contrast to the control device, the TPPC-treated device presented a smaller PL-V hysteresis under the forward and revserse scans, indicating the mitigated ion migration. Water contact angle was also carried out to probe the TPPC-induced changes in the hydrophobicity of the perovskite film. The TPPC-treated perovskite film showed a larger water contact angle than that on the pristine one and it also showed a smaller variation with time. Therefore, the increased hydrophobicity of the modified perovskite film might be another reason for the improvement of the device stability.

To gain deeper insights into the modulation of charge carrier dynamics of the TPPC-treated IPV devices, we employed a series of complementary optoelectronic measurements, including photo-generated charge extraction by linearly increasing voltage (photo-CELIV), transient photocurrent (TPC) decay, intensity‑modulated photocurrent spectroscopy (IMPS) and intensity‑modulated photovoltage spectroscopy (IMVS). As the incident illumination scaled from 2% to 100%, the current density of the TPPC-treated device consistently exhibited a more rapid rise and reached its maximum much earlier than that of the control device (Fig. 5a, b). The charge mobility (μ) is determined by employing the following equation53

μ = 2d2 / 3Atmax2 (1 + 0.36Δj/j0) (4)

where d is the film thickness, A is the voltage ramp rate, tmax is the time to reach the maximum current density, and j0 and Δj are the initial current density and its variation, respectively. It was observed that the charge mobility of the TPPC-treated device was always higher than that of the control device (Fig. 5c). Since TPC allows to investigate the time-dependent extraction of charge carriers in solar cells, we further carried out TPC measurements to calculate the extracted charge carrier density (nCE) by the following equation54–56

nCE = 1 / SLq ∫0t e j(t)dt (5)

where S is the device area, L is the perovskite thickness, q is the unit charge, and te is the charge extraction time, respectively. As shown in Fig. 5f, the nCE of the TPPC-treated device consistently outperformed that of the control device at varying light intensities. In addition, we employed IMPS and IMVS to probe the electron transport, recombination and charge collection dynamics in the IPV devices. As depicted in Fig. 5g, the electron transport time (τtr) of control and TPPC-treated devices was determined to be 1.3 and 0.9 μs, respectively, by using the following equation57

τtr = 1 / 2πfIMPS (6)

where fIMPS is the peak frequency of the IMPS spectrum. The electron recombination time (τrec) of control and TPPC-treated devices under 10% light intensity was calculated to be 22.7 and 93.6 μs (Fig. 5h), respectively, by using the following equation57

τrec = 1 / 2πfIMVS (7)

where fIMVS represents the peak frequency of the IMVS spectrum. Finally, under 10% light intensity, the charge collection efficiency (ηcc) was calculated to be 94.3% for the control device and 99.0% for the TPPC-treated device (Fig. 5i), according to the following equation58

ηcc = 1 − τtr / τrec (8)

Therefore, the performance improvements in the TPPC-treated devices were attributed to the modulated carrier dynamics at the PVK/C60 interface, as evidenced by the enhanced charge mobility, improved charge extraction, and elevated charge collection efficiency.

In addition to modulating the carrier dynamics at the PVK/C60 interface, it was discovered that the TPPC treatment was capable of achieving surface passivation and suppressing nonradiative recombination of the IPV devices. As shown in Supplementary Fig. 33, the lower trap-filling limit voltage (VTFL) observed in the electron-only device upon TPPC treatment indicated a reduced trap density in the modified perovskite film. This was validated by the trap energy-dependent trap density of state (tDOS) (Supplementary Fig. 34)59, where the TPPC-treated device showed a lower tDOS within the trap energy range of 0.4–0.55 eV. Moreover, according to the dark J-V plots (Supplementary Fig. 35) and admittance spectra (Supplementary Fig. 36), the decreased dark current and capacitance both indicated that the modified perovskite film featured a lower defect density. The electrochemical impedance spectroscopy (EIS) was also measured under dark condition to analyze the nonradiative recombination in the devices. Supplementary Fig. 37 shows an increased recombination resistance after TPPC treatment, confirming the effective suppression of nonradiative recombination. The TPPC-treated device also showed an extended charge recombination lifetime according to the normalized transient photovoltage (TPV) decay profiles (Supplementary Fig. 38), which demonstrated a lower energy loss from charge recombination. Additionally, the built-in potential (Vbi) of the device was estimated by Mott–Schottky curves. As shown in Supplementary Fig. 39, the Vbi exhibited an increase from 0.66 V to 0.75 V after TPPC treatment. This improvement implied the enhancement of the inner electric field at the PVK/C60 interface, contributing to the prominently enhanced Voc of the modified devices.

Methods
Materials
Unless otherwise noted, all chemicals were used in their as-received state, including tin (Sn) powder (99.8%, Sigma-Aldrich), iodine (I2) powder (99.8%, Sigma-Aldrich), tin fluoride (SnF2, 99%, Sigma-Aldrich), formamidinium bromide (FABr, 99.9%, TCI), 2-phenylethylamine hydrobromide (PEABr, 98%, aladdin), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (Mn=15,000–25,000, Xi’an P-OLED Co. Ltd.), fullerene (C60, 99.9%, Sigma-Aldrich), bathocuproine (BCP, 99%, TCI), dimethylsulfoxide (DMSO, anhydrous, >99.9%, Sigma-Aldrich), chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich), and indium tin oxides (ITO) (Advanced Eletion Technology Co. Ltd.).

Synthesis of tin iodide solution
The SnI2 solution was synthesized by the reaction of Sn and I2 in DMSO. First, 0.85 mmol I2 powder was dissolved in 1 mL DMSO. Subsequently, an excess of Sn powder (0.92 mmol) was added to the I2 solution. Finally, the mixture solution was stirred overnight in a glovebox to obtain the SnI2 solution.

Synthesis of fullerene derivative
First, C60Cl6 was prepared by reacting C60 with sufficient ICl. Following this, C60Cl6 was reacted with 7.5 folds of 4-(tert-butoxycarbonylaminoethyl)piperidine and 3 folds of 80% CHP in toluene for 1 hour. The crude product was first washed with a saturated aqueous solution of NH4Cl and NaHCO3, and then purified on a silica gel chromatography column with a toluene–ethyl acetate combination to obtain the solid TPPC-Boc (purity >98%). Lastly, TPPC (purity >97%) was prepared by eliminating the Boc groups with 10% TFA in CH2Cl2 for 6 h at room temperature.

Preparation of tin halide perovskite films
The precursor for the tin halide perovskite (FASnI2Br) film was obtained by mixing 0.12 mmol PEABr, 0.68 mmol FABr and 0.12 mmol SnF2 in 1 mL SnI2 solution (0.8 M). The mixture solution was then subjected to magnetic stirring at 50 °C over a period of 3 h. Following this, the precursor solution was filtered with 0.22 μm polytetrafluoroethylene filters. After that, the perovskite precursor was deposited onto the ITO substrate by spin-coating (2000 rpm for 12 s, 5000 rpm for 60 s). At the 40 second after the speed attained 5000 rpm, 135 μL of chlorobenzene was dispensed onto the substrate. Finally, the perovskite film was annealed at 70 °C for 30 min. For TPPC-treated perovskite film, 100 μL of TPPC dissolved in 2,2,2-Trifluoroethanol was deposited onto the perovskite film via spin-coating at 5000 rpm for 30 seconds.

Fabrication of perovskite solar cells
First, a sequential cleaning protocol was applied to the patterned ITO glass substrates, comprising 15-minute ultrasonic treatments in detergent, deionized water, acetone, and ethanol. Then, the substrates were subjected to a 20-minute ultraviolet ozone treatment. After that, 200 μL of PEDOT:PSS solution was deposited onto the ITO substrate via spin-coating (4000 rpm, 30 s), followed by annealing at 150 °C for 20 minutes. Following this, the ITO substrates were placed into the glovebox and the tin halide perovskite films were fabricated by the above method. Subsequently, 50 nm C60 layer and 5 nm BCP layer were deposited on the substrate via successive thermal evaporation. Finally, the Ag electrode (70 nm) was thermally evaporated through a mask (area: 0.1225 cm2). To eliminate the pronounced shadow effect introduced by masks under indoor illumination, the performances of the small-area (0.1225 cm2) devices were measured without masks. For the stability test, the devices were encapsulated with the cover glass (23×11.5 mm2, internal rectangular groove: 19×7.5 mm2) and UV curing resin (Lot NO. A20J01, 3014, Three Bond). Specifically, a uniform layer of UV-curing resin was first applied to the cover glass. The cover glass was then immediately positioned onto the device. Subsequently, the device was placed in the chamber of the UV-curing lamp and exposed to UV light for 40 s. Finally, the device was transferred to a transition chamber and subjected to vacuum for 1 minute, resulting in a well-sealed encapsulated device.

Characterization
The purity of material was tested by HPLC (LC2030C3D, Shimadzu). ESI-MS was measured using a mass spectrometer (Orbitrap Fusion LUMOs, Thermo Scientific). UV-Vis absorption spectra were recorded with a spectrophotometer (V-JASCO). PL, PLQY and TRPL measurements were carried out on an FLS1000 spectrometer. PL mappings were acquired by using the Raman image-scanning electron microscope. The film morphologies and cross-sections of the devices were characterized by SEM (JSM-6500F). XRD patterns were measured with a Rigaku SmartLab X-ray diffractometer (CuKα, λ=1.54050 Å). 1H-NMR spectra were acquired on a JEOL JNM ECZ 500R spectrometer (500 MHz). XPS measurements were conducted on a Thermo Scientific ESCALAB 250Xi spectrometer. FTIR spectra were acquired using a Thermo Scientific NICOLET iS50 spectrometer. GIWAXS data were collected at the 1W1A station of Beijing Synchrotron Radiation Facility. KPFM tests were conducted on a Bruker Bio-FastScan atomic force microscope. Contact angle tests were conducted using an Attension Theta Optical Tensiometer (Biolin Scientific, Sweden).

Femtosecond transient absorption spectrum
The fs-TA spectrometer consisted of an amplified Ti:sapphire laser system (central wavelength: 800 nm, pulse width: 25 fs, pulse repetition rate: 1 kHz, maximum pulse energy: 4 mJ) integrated with a Helios pump–probe system. The 800 nm fundamental beam was split into two paths. The majority was directed into an optical parametric amplifier to generate the 840 nm pump light, modulated by a 500 Hz mechanical chopper. A minor portion passed through a motorized delay line before being focused onto a sapphire to produce broadband probe pulses (430–820 nm or 800–1400 nm). The delay line controlled the optical path difference between the pump and probe lights for time-resolved measurements. Pump and probe beams were spatially and temporally overlapped on the sample.

Device measurements
The performances of the fabricated IPV devices were measured with an indoor light simulator (WLED, model: ILS-30) and a Keithley 2400 source meter. To evaluate the uniformity (U) of the light source, we mapped the spatial distribution of measured light power (P) across the sample stage using a power meter. Measurements were taken across a grid with 1 cm spacing. Their radiation uniformity was calculated by

U = 1 − (Pmax − Pmin) / (Pmax + Pmin) × 100% (9)

where Pmax is the maximum power, and Pmin is the minimum power. Results showed that the WLED used in our experimental setup achieved a high uniformity of 98.1% over a 5×5 cm2 area (Supplementary Fig. 40), confirming excellent irradiation homogeneity. To further evaluate the temporal stability (S) of the light source, we monitored the irradiance (I) of the WLED at its operating intensity for over 1 hour using the spectrometer. The value of S was then calculated by

S = 1 − (Imax − Imin) / (Imax + Imin) × 100% (10)

where Imax represents the maximum irradiance, and Imin represents the minimum irradiance. As a result, the light source demonstrated excellent stability (Supplementary Fig. 41), with a S of 99.5% over the measurement period. During the J-V measurements, a consistent light source-to-substrate distance was maintained: 34.2 cm in our lab and 40.5 cm in the certification laboratory. These distances were chosen to ensure a sufficiently large and uniform illumination area. The spectrometer (Model: HS-IL) used in our IPV measurement setup has a cosine corrector. The purpose of using a cosine corrector on the spectrometer is to ensure the instrument accurately measures light incident from all angles, providing a response that is proportional to the cosine of the angle of incidence. This achieves accurate measurements of diffuse or oblique light. But it should be noted that while a cosine corrector was used for the spectrometer, a slight difference in the angular response between the spectrometer and the photovoltaic device could still persist, representing a potential source of error in the measured irradiance and consequent current density. The spectrometer has been calibrated by the manufacturer using a NIST-traceable standard light source prior to this study (Supplementary Fig. 42). EQE spectra were acquired with a QE-R3011 system. The measurements of TPC, TPV, capacitance-frequency spectra, Mott-Schottky plots, photo-CELIV, IMPS and IMVS were all performed using a Paios 4.1 instrument. The space-charge-limited current was characterized using a Keithley 2400 source meter to obtain the dark-state J-V curves. For all in-lab measurements, the light source, the device under test (DUT), and the spectrometer were all arranged in a normal incidence (0°) configuration. The active layer of the DUT and the detector of the spectrometer were placed parallel to the light-emitting surface of the light source. The measurements at the certification laboratory (FUJIAN METROLOGY INSTITUTE) were performed following their standard procedures, and a normal incidence geometry was also employed.

Density functional theory calculations
The electrostatic potential profiles, molecular dipole moment and geometry of TPPC were calculated using Gaussian 09 at the B3LYP/6-31G* level. All DFT calculations were performed using the Vienna Ab initio Simulation Package. The generalized gradient approximation of Perdew–Burke–Ernzerhof was adopted for the exchange–correlation potential. The projector augmented-wave method was used to describe the ion core/valence electron interactions. A plane-wave cutoff energy of 450 eV was employed. The models were geometrically optimized until the Hellmann–Feynman forces were less than 0.02 eV/Å and the energy change was below 10−5 eV. For the adsorption models, the DFT-D3 correction was employed for the dispersion interactions across all atoms. The adsorption energy of an additive molecule on the perovskite surface was calculated by

Eads = Epero+addi − Epero − Eaddi (11)

where Epero+addi, Epero and Eaddi denote the energies of the passivated perovskite, pristine perovskite and additive molecule, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have made a big effort to address all of the detailed sets of reviewer comments raised. Many of these questions were satisfactorily answered. However, one of the main claims of the paper is that hot carrier extraction is improving device performance. After reading the authors’ response, and their revised paper, I still do not agree that this conclusion can be reached with complete certainty. I find the authors to have jumped to the conclusion that this is happening and are simply fitting every observation to this conclusion without due consideration for alternative possibilities. For example, in performing the requested fluence-dependent measurements, they obtained the opposite result at higher fluence, but argued this to be due to Auger effects when they present no proof of this. The authors try to argue that the difference in hot carrier cooling at the low fluence with and without TPPC is significant, but then go on to say that they cannot exactly quantify exactly what factors are playing a role on allowing improved hot carrier extraction. I agree that this is very complex, and if the authors cannot pin down the mechanism, they should not be presenting their preferred conclusion with such certainty. The authors have also not attempted to quantify the degree of error. For example, if the authors made 10 samples without TPPC and 10 samples with TPPC and measured the hot carrier cooling fairly using exactly the same conditions, do they see a statistically significant shortening (i.e., p value < 0.01 if they were to run a t-test) of the hot carrier cooling time with TPPC? To me an alternative explanation is that the TPPC is simply improving interface passivation, such that more cold carriers are extracted, which is a very ordinary phenomenon. From the results shown, I don’t think it is possible to quantify how much of the improvement in Jsc is from hot carriers vs. cold carriers that are not lost due to interface recombination. We should also note that the hot carrier cooling measurements were made on film stacks (i.e., open-circuit conditions), whereas the Jsc change is under short-circuit conditions. Hot carrier cooling dynamics could change significantly in going from open- to short-circuit. Ideally, the authors would have done pump-push-probe spectroscopy measurements on devices in-operando under short-circuit conditions or at the maximum power point.\nIf this paper were to tone down the discussion around hot carrier cooling and remove it from the title, I think the paper would still be suitable for Nature Communications because the TPPC is novel (as the authors explain) and the performance is currently the record value for Sn perovskites. These materials are also well characterized.\nOther technical comments (major and minor):\n• It should be illuminance not illumination. Please be precise in the terminology between illuminance (units: lux) and irradiance (units: mW/cm^2)\n• Authors claim from Suppl fig. 10 that there is greater crystallinity. We cannot conclude this without any analysis conducted on the XRD patterns, e.g., crystalline size after correcting for instrument broadening\n• The authors claim the differences between their lab efficiencies and certified efficiencies is because they did not use a mask. This makes me concerned that they are overestimating their efficiencies due to edge effects. EQE and measured Jsc do match though, but this does put the claim for a “record” PCE in doubt\n• The authors used a commercial indoor PV simulator that has a window. The authors need to confirm whether or not confirm whether or not they covered up this window during measurements. If not, they could again be overestimating their efficiencies because of extra stray light from outside increasing the illuminance on the device and therefore increasing the power output\n• Can the authors confirm that they used the spectrometer in the setup to measure the light source spectrum during measurements? How did the authors obtain the absolute spectral irradiance? This could be quite different to that on the device due to angular response differences between the device and spectrometer.\n• The authors compare their light spectra with the absorbance to explain why 3000 K worked the best. They should compare with EQE, since that is what is directly relevant to the device", "reviewer_2": "The authors have addressed several points raised in the previous round of review. However, beyond the ongoing concerns regarding the overall novelty of the study, several specific issues remain:\n1. While the authors point out that their illumination source (3000 K LED) differs from that in the cited literature (DOI: 10.1039/C9TA11894B, 3328 K LED), the reported incident power density in this study (0.285 mW cm-2) is slightly lower than that in the reference (0.298 mW cm-2). Given that the perovskite absorber (Eg = 1.63 eV) fully covers the emission spectra of both light sources, the theoretical Jsc limit in this work should not exceed the 138.2 μA cm-2 reported for the 3328 K light source.\nIn fact, when estimating the Jsc limit under indoor illumination, the total incident power is the dominant factor, and while spectral shape matters to some extent, small shifts in color temperature do not significantly impact the theoretical Jsc for a fixed bandgap.\nTherefore, I recommend that the authors calculate the theoretical Jsc limit using the measured emission spectrum of their actual light source, to ensure the reported performance is not overestimated.\nAdditionally, the certification was carried out using a fluorescent lamp, rather than an LED source. While this is acceptable in the absence of standardized LED testing protocols, it prevents direct comparison of Jsc values (certification and lab measurement). By the way, the Jsc in the certification is lower than the theoretical limit, which is reasonable.\n2. The explanation of the shadow effect on Jsc appears questionable because Jsc=Isc/mask size. The authors mentioned that no mask was used in their measurements. This could raise concerns about light leakage or over-illumination of the device area. For example, in the 1.21 cm2 device, Jsc obtained without a mask is higher than that obtained with a 1.0 cm2 mask. This can be caused by light leakage or imprecise illumination area control. Anyway, the authors are suggested to calculate the Jsc limitation depending on the actual incident light intensity and define the accurate illuminated area by a mask to ensure reproducible and credible data.\n3. Given the limited number of studies reporting MPP tracking under 1000 lux indoor conditions, the authors are encouraged to include the raw MPP tracking data in the supplementary materials.\n4. The efficiencies under lower light intensities are wrongly calculated. For example, the light power intensity at 500 lux, which is 50% of 1000 lux (0.285 mW cm-2), should be 0.1425 mW cm-2.\nPCE under 500 lux should be 71.4 uA cm-2 * 0.659 V * 0.5657/142.5 uW cm-2 = 18.42%, not 9.34%.\nThe PCEs under 250, 500, and 750 lux should be corrected.", "reviewer_3": "This manuscript by Meng et al. was extensively revised after the first revision, according to reviewers' suggestions. The quality of the work is now significantly enhanced, and the results are more convincingly discussed. The manuscript could be accepted in publication in the current form."}, "author_response": "Response Letter to Reviewers’ Comments\nReviewer #1\nThe authors have made a big effort to address all of the detailed sets of reviewer\ncomments raised. Many of these questions were satisfactorily answered. However,\none of the main claims of the paper is that hot carrier extraction is improving device\nperformance. After reading the authors’ response, and their revised paper, I still do not\nagree that this conclusion can be reached with complete certainty. I find the authors to\nhave jumped to the conclusion that this is happening and are simply fitting every\nobservation to this conclusion without due consideration for alternative possibilities.\nFor example, in performing the requested fluence-dependent measurements, they\nobtained the opposite result at higher fluence, but argued this to be due to Auger\neffects when they present no proof of this. The authors try to argue that the difference\nin hot carrier cooling at the low fluence with and without TPPC is significant, but then\ngo on to say that they cannot exactly quantify exactly what factors are playing a role\non allowing improved hot carrier extraction. I agree that this is very complex, and if\nthe authors cannot pin down the mechanism, they should not be presenting their\npreferred conclusion with such certainty. The authors have also not attempted to\nquantify the degree of error. For example, if the authors made 10 samples without\nTPPC and 10 samples with TPPC and measured the hot carrier cooling fairly using\nexactly the same conditions, do they see a statistically significant shortening (i.e., p\nvalue < 0.01 if they were to run a t-test) of the hot carrier cooling time with TPPC? To\nme an alternative explanation is that the TPPC is simply improving interface\npassivation, such that more cold carriers are extracted, which is a very ordinary\nphenomenon. From the results shown, I don’t think it is possible to quantify how\nmuch of the improvement in J is from hot carriers vs. cold carriers that are not lost\nsc\ndue to interface recombination. We should also note that the hot carrier cooling\nmeasurements were made on film stacks (i.e., open-circuit conditions), whereas the J\nsc\nchange is under short-circuit conditions. Hot carrier cooling dynamics could change\n1\nsignificantly in going from open- to short-circuit. Ideally, the authors would have\ndone pump-push-probe spectroscopy measurements on devices in-operando under\nshort-circuit conditions or at the maximum power point.\nIf this paper were to tone down the discussion around hot carrier cooling and remove\nit from the title, I think the paper would still be suitable for Nature Communications\nbecause the TPPC is novel (as the authors explain) and the performance is currently\nthe record value for Sn perovskites. These materials are also well characterized.\nReply: We sincerely thank the reviewer for their thoughtful and constructive\ncomments regarding the interpretation of hot carrier extraction in our manuscript. We\nfully agree that the dynamics of hot carrier extraction at perovskite/ETL interfaces are\nhighly complex, and that our current experimental evidence cannot completely\nexclude alternative explanations, such as improved interface passivation. Specifically,\nwe will explain the concerns raised by the reviewer through the following five\naspects:\n(1) Tone down the discussion about hot carrier cooling and remove it from the title:\nAccording to the reviewer’s comments, we have toned down the discussion about hot\ncarrier cooling and removed it from the title. The new title of the manuscript is\n“Interfacial Engineering via Dipolar Fullerene Derivative for Efficient Tin Halide\nPerovskite Indoor Photovoltaics”. Furthermore, we have deleted some relevant\nstatements in the Abstract, Introduction, and Conclusion sections of the revised\nmanuscript. The removed discussions from the original version are provided below\nfor clarity：\nPage 1 line 24 in the original manuscript: The significantly reduced hot carrier\ntemperature from 288 K to 224 K indicated a broader range of carrier extraction at the\nmodified interface.\nPage 4 line 4 in the original manuscript: The significantly reduced hot carrier\ntemperature from 288 K to 224 K indicated a broader range of carrier extraction at the\nmodified interface.\nrevealed a broader range of charge carrier extraction at the treated interface.\n(2) Avoid presenting the conclusion with complete certainty for the claim that hot\ncarrier extraction can improve device performance: According to the reviewer’s\nvaluable comments, we have toned down the discussion about hot carrier cooling and\nremoved it from the title to avoid presenting the conclusion with complete certainty.\nThe detailed corrections have been shown above.\n(3) Quantify the degree of error for hot carrier cooling time: As suggested, we have\nquantified the degree of error for hot carrier cooling time in the revised manuscript.\nWe have made 4 samples without TPPC and 4 samples with TPPC. The measured\nresults showed that the hot carrier cooling time is 6.70±0.19 ps and 6.00±0.23 ps for\nthe samples without and with TPPC-treatment, respectively (Supplementary Figure 18,\nalso shown below). We also run a t-test for the measurement data and the probability\n(p) value is 0.003 (＜0.01), demonstrating a statistically significant shorting of the hot\ncarrier cooling time after TPPC treatment. The added figure and descriptions in the\nrevised manuscript are shown below:\nPage 10 line 9: To further quantify the degree of error for the hot carrier cooling time,\nwe made 4 samples without TPPC and 4 samples with TPPC. As shown in\nSupplementary Figure 18, the hot carrier cooling time was 6.70±0.19 ps and\n6.00±0.23 ps for the samples without and with TPPC-treatment, respectively,\nconfirming that TPPC-treatment could facilitate hot carrier extractions at the interface\nand minimize thermalization losses of interfacial hot carriers.\nPVK/C interfaces under the pump fluence of 1.05 μJ cm-2. The hot carrier cooling\n60\ntime was 6.70±0.19 ps and 6.00±0.23 ps for the samples without and with\nTPPC-treatment, respectively.\n(4) Explanation for fluence-dependent fs-TA measurements: In the revised manuscript\n(page 9 line 11), we have introduced that “It is well-known that the hot carrier cooling\ntime is dependent on phonon scattering rate under low and middle pump fluence, and\nthe variation of cooling time can reflect the hot carrier extraction rate46,49,50. While the\nhot carrier cooling time is controlled by Auger recombination under high pump\nfluence, and the variation of cooling time represents the recombination losses51,52.”\nWe have cited several literatures (ref [46,49-52]) in the revised manuscript to support\nthe opinion that hot carriers show different cooling time variation under low and high\npump fluences. To further emphasize this, we have also added citations of literatures\nfor the descriptions of the fs-TA measurement results under high pump fluence, the\ncorrections in the revised manuscript are shown below:\nPage 10 line 23: As shown in Supplementary Fig. 20, the TPPC-treated PVK/C\n60\ninterface showed a longer hot carrier cooling time (3.61±0.32 ps) than the untreated\ninterface (2.70±0.35 ps). This was because that the hot carrier cooling time was\ncontrolled by Auger recombination under high pump fluence and the TPPC-treatment\n4\nindirectly mitigated Auger losses by boosting hot carrier extraction efficiency and\nsuppressing defect recombination51,52, which collectively reduced the local carrier\nconcentration to extend the hot carrier cooling time.\n(5) Hot carrier cooling dynamics could change significantly in going from open- to\nshort-circuit: We completely agree with the reviewer that the hot carrier cooling\ndynamics are different in film stacks and device. The ideal condition, like the\nreviewer pointed out, is to do pump-push-probe spectroscopy measurements on\ndevices under short-circuit conditions or at the maximum power point. But since the\ntransient adsorption (TA) setup cannot be properly coupled with the solar cell\nmeasurement setup currently, we were unable to do the TA measurements on devices\nunder short-circuit conditions or at the maximum power point at this stage.We plan to\npursue such experiments in future work to further validate these dynamics under\noperating conditions. In such a case, we tried to investigate the carrier dynamics of the\nfilm stacks via TA measurements and analyzed the electron extraction at the interface.\nActually, some researchers also utilized the method of investigating the carrier\ndynamics of film stacks via TA measurements to explore the electron extraction at the\nperovskite/electron transport layer (ETL) interface[1-5] (ref: [1] Tian C, et al. Adv.\nMater. 2024, 36(36): 2404797. [2] Zheng Y, et al. Energ. Environ. Sci. 2024, 17(3):\n1153-1162. [3] Sun C, et al. Adv. Mater. 2024, 36(44): 2410248. [4] Yan W, et al. Adv.\nMater. 2024, 36(18): 2312170. [5] Wen L, et al. Adv. Funct. Mater. 2025, 35(7):\n2410457.).\nQuestion 1: It should be illuminance not illumination. Please be precise in the\nterminology between illuminance (units: lux) and irradiance (units: mW/cm2).\nReply: We sincerely appreciate the reviewer’s helpful comments. As suggested, we\nhave corrected the “illumination” to “illuminance” in the following positions of the\nrevised manuscript:\nPage 2 line 26, Page 3 line 1, Page 4 line 12, Page 14 line 5 and 6, Page 17 line 32.\nQuestion 2: Authors claim from Suppl fig. 10 that there is greater crystallinity. We\n5\ncannot conclude this without any analysis conducted on the XRD patterns, e.g.,\ncrystalline size after correcting for instrument broadening.\nReply: We sincerely appreciate the reviewer’s professional comments. As suggested,\nwe have measured the full width at half maxima (FWHM) of the XRD peaks and\nadded some analysis for Suppl fig. 10 to conclude the greater crystallinity. The added\nfigure and descriptions in the revised manuscript are shown below:\nPage 6 line 17: X-ray diffraction (XRD) patterns were also measured to investigate\nthe crystallinity variation of perovskite film after TPPC-treatment. According to\nSupplementary Fig.10, the characteristic peaks corresponding to the (100) and (200)\nplanes of the TPPC-treated perovskite film exhibited higher intensity with reduced\nfull width at half-maximum (FWHM) (from 0.08° to 0.06°) compared to the pristine\nperovskite film, indicating the increased crystalline size and enhanced crystallinity.\nThe grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns\n(Supplementary Fig. 11) could further validate the enhancement of perovskite film\ncrystallinity after TPPC-treatment, especially the (100) plane.\nSupplementary Figure 10. (a) XRD patterns for pristine and TPPC-treated\nperovskite films. (b) Full width at half-maximum (FWHM) of XRD peaks for pristine\nand TPPC-treated perovskite films.\nQuestion 3: The authors claim the differences between their lab efficiencies and\ncertified efficiencies is because they did not use a mask. This makes me concerned\nthat they are overestimating their efficiencies due to edge effects. EQE and measured\nJ do match though, but this does put the claim for a “record” PCE in doubt.\nsc\nReply: We sincerely thank the reviewer for raising this important point concerning\nthe reliability of our efficiency measurements. We will explain the concerns through\nthe following two aspects:\n(1) The efficiency of our large-area device was certificated with a mask: As shown in\nSupplementary Figure 28 of the manuscript, the efficiency of our large-area (1.21 cm2)\ndevice is certificated with a 1 cm2 mask in the third-party certification authority\n(FUJIAN METROLOGY INSTITUTE, National PV Industry Measurement and\nTesting Center), which demonstrates that our results are convincing. Moreover, the\ncertificated PCE of 15.93% is also a record efficiency for 1 cm2 devices. For the\nsmall-area (0.1225 cm2) device, since we did not certificate its efficiency in a\nthird-party certification authority due to severe shadow effect brought by masks, we\nhave avoided describing its efficiency as a record in the revised manuscript. The\ndetailed corrections in the revised manuscript are shown below:\nPage 1 (Abstract): The original sentence “With these merits, the champion power\nconversion efficiency (PCE) of the TPPC-treated photovoltaic devices reached 22.49%\nunder 1000 lux, ranking among the highest values of lead-free perovskites for indoor\nphotovoltaics thus far.” has been changed to “With these merits, the champion power\nconversion efficiency (PCE) of the TPPC-treated photovoltaic devices reached 22.49%\nunder 1000 lux.”.\nPage 4 line 4: The original sentence “As a result, the champion PCE of the modified\nphotovoltaic devices achieved 22.49%, ranking among the highest values of lead-free\nperovskites for IPV applications to date.” has been changed to “As a result, the\nchampion PCE of the modified photovoltaic devices achieved 22.49%.”.\nPage 12 line 23: The original sentence “Furthermore, based on the recent\nadvancements in PCE of lead-free perovskite-based indoor photovoltaic devices (Fig.\n4f, Supplementary Table 6)13-19,21-25,31, it is evident that we have achieved a record\nadvancements in PCE of lead-free perovskite-based indoor photovoltaic devices (Fig.\n4f, Supplementary Table 6)13-19,21-25,31, it is evident that we have achieved a significant\nhigh PCE (22.49%) thus far.”.\nPage 17 line 30: The original sentence “As a result, the champion PCE of the\nTPPC-treated photovoltaic devices achieved 22.49% under the indoor illuminance of\n1000 lux, ranking among the highest values of lead-free perovskites for IPV\napplications to date.” has been changed to “As a result, the champion PCE of the\nTPPC-treated photovoltaic devices achieved 22.49% under the indoor illuminance of\n1000 lux.”.\n(2) Severe shadow effect brought by masks during the PCE measurement under\nindoor illumination: We fully understand the reviewer’s concern that measuring the\nefficiency of devices without a mask could potentially lead to an overestimation of\nPCE due to edge effects. The measurement with a mask is the standard way to\nevaluate the PCEs of solar cells. However, for indoor photovoltaics, the use of a mask\nwill bring severe shadow effect under indoor illumination during the PCE\nmeasurement, leading to an underestimation of PCE. Shadow effect refers to the\nreduction in actual light irradiance reaching the device caused by the physical\nthickness of the mask blocking a portion of the incident light, especially at\nnon-perpendicular angles. It should be noted that the shadow effect of mask is quite\nweak under 1-sun (AM 1.5G) illumination and it can be ignored in the efficiency\nmeasurement. However, since the indoor illumination is much weaker than 1-sun\nillumination (0.285 vs 100 mW cm-2), the shadow effect exhibits a significant\ninfluence on PCE under indoor illumination. Specifically, owing to the physical\nthickness of the mask blocking a portion of the incident light, the actual irradiance on\nthe device is significantly reduced, resulting in a decrease in PCE. We also\ndemonstrated the significant influence of shadow effect brought by the mask on\nindoor photovoltaics. As shown in the below Table 1, for the large-area (1.21 cm2)\ndevice, the measured PCE with a mask is reduced from 17.94% to 15.94% compared\n8\nwith that without a mask. To demonstrate the accuracy of the measurement in our lab,\nwe have also certificated the efficiency of the large-area device. According to the\ncomparison in the table, the device performance measured in our lab aligns well with\nthe certificated results measured in the third-party certification authority (FUJIAN\nMETROLOGY INSTITUTE, National PV Industry Measurement and Testing Center).\nThis demonstrates that the measurement results in our lab are believable and the\nsevere shadow effect of mask do exist under indoor illumination. More importantly,\nwe find that the shadow effect shows a greater influence on the small-area device than\nthe large-area device, this is also the reason why we only certificate the PCE of the\nlarge-area device. In sum, for small-area device measurement under indoor\nillumination, the measurement without a mask will bring in edge effect (overestimate\nPCE) and the measurement with a mask will bring in severe shadow effect\n(underestimate PCE). It is hard to address this issue at this moment for researchers\nworking on indoor photovoltaics. But we will try to make ultra-thin masks to weaken\nthe shadow effect in our future work plan. To avoid misunderstanding readers, we\nhave emphasized the measurement condition (with or without mask) in the Methods\nsection of the revised manuscript. The detailed corrections are shown below:\nPage 23 line 17: To eliminate the pronounced shadow effect introduced by masks\nunder indoor illumination, the performance of the small-area (0.1225 cm2) devices\nwas evaluated without the uses of masks.\nthird-party certification authority (FUJIAN MRTROLOGY INSTITUTE, National PV\nIndustry Measurement and Testing Center).\nLight\nMeasurement Mask source Scans V (V) J sc (mA FF (%) PCE (%)\norganization (irradiance oc cm-2)\n)\nILS-30\nWithout mask\nOur lab (0.2850 Forward 0.650 0.1141 69.1 17.94\n(1.21 cm2)\nmW cm-2)\nReverse 0.652 0.1139 68.63 17.90\nILS-30\nWith a 1 cm2\nOur lab (0.2850 Forward 0.635 0.1040 68.88 15.94\nmask\nmW cm-2)\nReverse 0.632 0.1040 68.90 15.90\nThird-party\ncertification U30\nWith a 1 cm2\nauthority (FUJIAN (0.3396 Forward 0.6352 0.1228 69.10 15.87\nmask\nMETROLOGY mW cm-2)\nINSTITUTE)\nReverse 0.6294 0.1232 69.77 15.93\nQuestion 4: The authors used a commercial indoor PV simulator that has a window.\nThe authors need to confirm whether or not confirm whether or not they covered up this window during\nmeasurements. If not, they could again be overestimating their efficiencies because of\nextra stray light from outside increasing the illuminance on the device and therefore\nincreasing the power output.\nReply: We sincerely appreciate the reviewer’s valuable comments. When measuring\nthe power conversion efficiency (PCE) of the devices, an aluminum foil was always\nused to shield the glass window of the measurement apparatus, as shown in the below\nFigure R1. This prevented the light intensity within the chamber from being affected,\nthus ensuring that the measured PCE was not overestimated. To avoid unnecessary\nmisunderstanding, we have emphasized this in the figure caption of Supplementary\nFig.21 of the revised Supplementary information, as shown below:\nan aluminum foil was always used to shield the glass window of the measurement\napparatus to prevent the light intensity within the chamber from being affected, thus\nensuring the measured PCE was not overestimated.\nFigure R1. Picture of the efficiency measurement setup. When measuring the power\nconversion efficiency (PCE) of the devices, an aluminum foil was always used to\nshield the glass window of the measurement apparatus.\nQuestion 5: Can the authors confirm that they used the spectrometer in the setup to\nmeasure the light source spectrum during measurements? How did the authors obtain\nthe absolute spectral irradiance? This could be quite different to that on the device due\nto angular response differences between the device and spectrometer.\nReply: We sincerely appreciate the reviewer’s constructive and valuable comments.\nWe will explain the concerns through the following two aspects.\n(1) Confirm the use of spectrometer in the setup to measure the light source spectrum\nduring measurements: Even though the spectrometer in our experimental setup\ncan measure the spectrum of the light source during measurements, we do not use\nthis spectrum to calculate the irradiance on the device since the relatively long\ndistance between the spectrometer and device will lead to different irradiances at\nthese two positions. But this real-time spectrum is used to monitor the stability of\n11\nthe LED source. The specific method to measure the spectrum of the light source\nirradiated on the device is detailedly described in the following.\n(2) The method to obtain the absolute spectral irradiance: Before the measurement of\nthe device performance, we first moved away the sample stage (the position is\nmarked as position A) and then measured the light spectrum at the position A\nusing spectrometer. Since the spectrometer is placed at the same height with the\ndevice, the measured light spectrum by spectrometer at position A is completely\nthe same with that irradiated on the device. This method avoids angular response\ndifferences between the device and spectrometer. It should be also noted that the\nirradiation uniformity of the used LED source (model: ILS-30) in our\nexperimental setup is over 95% within 5×5 cm2 area and over 90% within 10×\n10 cm2 area, thus ensuring uniform irradiation on the device and accurate\nmeasurement of the light spectrum irradiated on the device. We also placed a\nportable spectral illuminance meter (as shown in below Figure R2a) in close\nproximity to the device on the sample stage to compare the measured light\nspectrum with that measured by the spectrometer. As shown in Figure R2b, the\nmeasured spectrum by the portable spectral illuminance meter agrees well with\nthat measured by the spectrometer. This result demonstrates that we successfully\nobtain the absolute spectral irradiance on the device. After obtaining the light\nspectrum, the absolute spectral irradiance (I ) is obtained by\ntotal\n= (λ)dλ\n900\nthe irradiance illuminated on the device. (b) Comparison of the light spectra measured\nby spectrometer at the position A (the place that the device is placed on the sample\nstage) and portable spectral illuminance meter in close proximity to the device.\nQuestion 6: The authors compare their light spectra with the absorbance to explain\nwhy 3000 K worked the best. They should compare with EQE, since that is what is\ndirectly relevant to the device.\nReply: We sincerely appreciate the reviewer’s constructive and valuable comments.\nAs suggested, we first checked the EQE and we found that the EQE spectra of the\ndevice are the same under different light temperatures. In the following, we will\nexplain why the device shows the highest PCE under 3000 K compared with 4000 K\nand 5500 K from the considerations of J , V and FF:\nsc oc\n(1) J : J is directly proportional to the number of photons that the device can absorb\nsc sc\nand convert.\nranging from 500-700 nm (as shown in the below Figure R3). The energy of these\nphotons (1.77-2.48 eV) is just slightly higher than the band gap of perovskite (E\ng\n=1.62 eV), enabling them to be efficiently absorbed and converted into electrical\nenergy with almost no thermal loss.\n13\nhigh-energy photons), a considerable portion of them is high-energy blue-violet light\n(400-500 nm). The energy of these photons (2.48-3.1 eV) is much higher than the\nband gap of perovskite. After being absorbed, the excess energy is dissipated in the\nform of heat (thermal relaxation) rather than being used to generate more charges.\nThis reduces the energy utilization efficiency.\nsc\nTherefore, even though the J of the 3000 K may not be the highest, the power\nsc\nconversion efficiency (PCE) is highest since the energy of the photons is fully\nabsorbed and converted into electrical energy with almost no thermal loss.\nFigure R3. The UV-Vis absorptance spectrum of perovskite film and the spectra of the\nwhite LED under the light color temperatures of 3000, 4000 and 5500 K. The light\nillumination is 1000 lux.\n(2) V : V is directly related to the quasi-Fermi level splitting value of the device,\noc oc\nwhich is theoretically determined by the concentration and recombination of\nphotogenerated carriers.\nband gap extremely well. The carriers generated by these photons have lower excess\nkinetic energy, so the lattice thermal vibration of the device is smaller, and\nnon-radiative recombination (such as Shockley-Read-Hall recombination) is relatively\n14\nless. This helps maintain a higher carrier concentration and a larger quasi-Fermi level\nsplitting, thus exhibiting a higher V (0.72 V).\noc\nabsorbed, these photons generate hot carriers, which convert the excess energy into\nheat in an extremely short time through phonon scattering, intensifying the local\ntemperature rise and lattice vibration of the device, thereby promoting various\nrecombination processes. More severe recombination leads to a reduction in\nquasi-Fermi level splitting, so V decreases (5500 K: 0.70 V; 4000 K: 0.69 V).\noc\n(3) FF: FF is affected by series resistance (R ) and shunt resistance (R ), and R is\ns sh sh\nclosely related to recombination.\nsh\ncurrent. Therefore, the J-V curve is more \"square\", with the highest FF (63.6%).\nrecombination lead to a decrease in R (50 Ω) and an increase in leakage current.\nsh\nMeanwhile, the thermal effect may slightly alter the performance of the charge\ntransport layer, increasing the R (10543 Ω). It is noted that the R under the 3000 K\ns s\nlight illumination is 7494 Ω. The combined effect results in a significant reduction in\nFF (58.0%).\n4000K light source: It is in an intermediate state (FF= 60.1%, R =3643 Ω, R\nsh s\n=10051 Ω).\nThe authors have addressed several points raised in the previous round of review.\nHowever, beyond the ongoing concerns regarding the overall novelty of the study,\nseveral specific issues remain.\nReply: We sincerely appreciate the reviewer’s very valuable comments on our work.\nIn the revised manuscript, we have thoroughly considered these helpful comments and\nmade necessary revisions to address the concerns raised. The detailed point-to-point\nresponses are attached below. The novelty of this study is primarily reflected in the\nfollowing two aspects:\n(i) For the first time, a multifunctional fullerene derivative with four piperazidine\ngroups (TPPC) has been synthesized and used to accelerate hot carrier extractions at\nthe perovskite/C interface under low-light indoor illumination.\n60\n(ii) The obtained record-high efficiency (certificated efficiency of 15.93% for 1 cm2\ndevices) for tin halide perovskite indoor photovoltaic devices marks a significant\nmilestone in sustainable energy harvesting, which validates tin halide perovskites as a\ncredible alternative for indoor photovoltaics, combining high efficiency and\nsustainability.\nQuestion 1: While the authors point out that their illumination source (3000 K LED)\ndiffers from that in the cited literature (DOI: 10.1039/C9TA11894B, 3328 K LED),\nthe reported incident power density in this study (0.285 mW cm-2) is slightly lower\nthan that in the reference (0.298 mW cm-2). Given that the perovskite absorber (E =\ng\n1.63 eV) fully covers the emission spectra of both light sources, the theoretical J\nsc\nlimit in this work should not exceed the 138.2 μA cm-2 reported for the 3328 K light\nsource. In fact, when estimating the J limit under indoor illumination, the total\nsc\nincident power is the dominant factor, and while spectral shape matters to some extent,\nsmall shifts in color temperature do not significantly impact the theoretical J for a\nsc\nfixed bandgap. Therefore, I recommend that the authors calculate the theoretical J\nsc\n16\nlimit using the measured emission spectrum of their actual light source, to ensure the\nreported performance is not overestimated. Additionally, the certification was carried\nout using a fluorescent lamp, rather than an LED source. While this is acceptable in\nthe absence of standardized LED testing protocols, it prevents direct comparison of J\nsc\nvalues (certification and lab measurement). By the way, the J in the certification is\nsc\nlower than the theoretical limit, which is reasonable.\nReply: We sincerely appreciate the reviewer’s constructive and valuable comments.\nAs suggested，we have calculated the theoretical J limit (J ) using the measured\nsc sc,limit\nemission spectrum of the actual light source and the bandgap of perovskite. As a\nresult, the calculated J is 152.1 μA/cm2. Therefore, the measured J in our\nsc,limit sc\nexperiment (140 μA/cm2) did not exceed the J , and the J is not overestimated.\nsc,limit sc\nThe specific calculation process for J is shown below：\nsc,limit\nJ =q\nλgI(λ)\nEQE(λ)dλ,\nsc,limit 0 E\n∫\nWhere q is electron charge, λ is wavelength, λ is the cutoff wavelength corresponding\ng\nto the bandgap of perovskite, I(λ) is the spectral irradiance of light source, E is the\nphoton energy, EQE(λ) is the external quantum efficiency.\nSince E=hv=hc/λ, where h is plank constant (h=4.136×10-15 eV·s), v is light\nfrequency, and c is light speed in vacuum (c=3×108 m s),\n⁄\nq λg\nJ = λ·I(λ)EQE(λ)dλ\nsc,limit hc\n0\n∫\nSince EQE λ = 1 for J ,\nsc,limit\n( )\nq λg\nJ = λ·I(λ)dλ\nsc,limit hc\n0\n∫\nhc 1240 eV·nm\nFor our case, E = 1.62 eV, λ = = =765 nm\ng g\nEg 1.62 eV\n.4\nq λg\nJ = λ·I λ dλ\nsc,limit hc\n0\n∫ ( )\n17\nq 765\nλ·I λ dλ\nhc .4\n0\n= ∫ ( )\ne\n× nm·W m2\n1240 eV·nm\n= 1885.8 ⁄\n1\n= × A m2\n1240\n1885.8 ⁄\n=1. A m2=1 μA cm2\n521 ⁄ 52.1 ⁄\nIt is important to note that although the irradiance of the 3000 K LED source used in\nour work (0.285 mW cm-2) is slightly lower than that of the 3328 K source (0.298\nmW cm-2) reported in the reference literature, the perovskite material employed in our\nstudy has a narrower bandgap (1.62 eV vs. 1.83 eV). This means that our device\nexhibits a longer absorption cutoff edge (765.4 nm vs. 677.6 nm), enabling it to more\neffectively capture the abundant long-wavelength photons present in the\nlow-color-temperature light source, thereby leading to a relatively higher J . This\nsc,limit\nrepresents a superior \"spectral match.\" Thus, even though the total optical power in\nour study is slightly lower than that in the reference literature, the number of\neffectively utilizable photons is greater, ultimately resulting in a higher J .\nsc,limit\nQuestion 2: The explanation of the shadow effect on J appears questionable because\nsc\nJ =I /mask size. The authors mentioned that no mask was used in their\nsc sc\nmeasurements. This could raise concerns about light leakage or over-illumination of\nthe device area. For example, in the 1.21 cm2 device, J obtained without a mask is\nsc\nhigher than that obtained with a 1.0 cm2 mask. This can be caused by light leakage or\nimprecise illumination area control. Anyway, the authors are suggested to calculate\nthe J limitation depending on the actual incident light intensity and define the\nsc\naccurate illuminated area by a mask to ensure reproducible and credible data.\nReply: We sincerely appreciate the reviewer’s constructive comments. We will\nexplain the concerns through the following four aspects.\n(1) The efficiency of our large-area device was certificated with a mask: As shown in\ndevice is certificated with a 1 cm2 mask in the third-party certification authority\n(FUJIAN METROLOGY INSTITUTE, National PV Industry Measurement and\nTesting Center), which demonstrates that our results are convincing.\n(2) Severe shadow effect brought by masks during the PCE measurement under\nindoor illumination: We fully understand the reviewer’s concern that measuring the\nefficiency of devices without a mask could potentially lead to an overestimation of\nPCE due to edge effects. The measurement with a mask is the standard way to\nevaluate the PCEs of solar cells. However, for indoor photovoltaics, the use of a mask\nwill bring severe shadow effect under indoor illumination during the PCE\nmeasurement, leading to an underestimation of PCE. Shadow effect refers to the\nreduction in actual light irradiance reaching the device caused by the physical\nthickness of the mask blocking a portion of the incident light, especially at\nnon-perpendicular angles. It should be noted that the shadow effect of mask is quite\nweak under 1-sun (AM 1.5G) illumination and it can be ignored in the efficiency\nmeasurement. However, since the indoor illumination is much weaker than 1-sun\nillumination (0.285 vs 100 mW cm-2), the shadow effect exhibits a significant\ninfluence on PCE under indoor illumination. Specifically, owing to the physical\nthickness of the mask blocking a portion of the incident light, the actual irradiance on\nthe device is significantly reduced, resulting in a decrease in PCE. We also\ndemonstrated the significant influence of shadow effect brought by the mask on\nindoor photovoltaics. As shown in the below Table 1, for the large-area (1.21 cm2)\ndevice, the measured PCE with a mask is reduced from 17.94% to 15.94% compared\nwith that without a mask. To demonstrate the accuracy of the measurement in our lab,\nwe have also certificated the efficiency of the large-area device. According to the\ncomparison in the table, the device performance measured in our lab aligns well with\nthe certificated results measured in the third-party certification authority (FUJIAN\nMETROLOGY INSTITUTE, National PV Industry Measurement and Testing Center).\nThis demonstrates that the measurement results in our lab are believable and the\n19\nsevere shadow effect of mask do exist under indoor illumination. More importantly,\nwe find that the shadow effect shows a greater influence on the small-area device than\nthe large-area device, this is also the reason why we only certificate the PCE of the\nlarge-area device. In sum, for small-area device measurement under indoor\nillumination, the measurement without a mask will bring in edge effect (overestimate\nPCE) and the measurement with a mask will bring in severe shadow effect\n(underestimate PCE). It is hard to address this issue at this moment for researchers\nworking on indoor photovoltaics. But we will try to make ultra-thin masks to weaken\nthe shadow effect in our future work plan. To avoid misunderstanding readers, we\nhave emphasized the measurement condition (with or without mask) in the Methods\nsection of the revised manuscript. The detailed corrections are shown below:\nPage 23 line 17: To eliminate the pronounced shadow effect introduced by masks\nunder indoor illumination, the performance of the small-area (0.1225 cm2) devices\nwas evaluated without the uses of masks.\nTable 1. Comparison between the device performances measured in our lab and in the\nthird-party certification authority (FUJIAN MRTROLOGY INSTITUTE, National PV\nIndustry Measurement and Testing Center).\nLight\nMeasurement Mask source Scans V (V) J sc (mA FF (%) PCE (%)\norganization (irradiance oc cm-2)\n)\nILS-30\nWithout mask\nOur lab (0.2850 Forward 0.650 0.1141 69.1 17.94\n(1.21 cm2)\nmW cm-2)\nReverse 0.652 0.1139 68.63 17.90\nILS-30\nWith a 1 cm2\nOur lab (0.2850 Forward 0.635 0.1040 68.88 15.94\nmask\nmW cm-2)\nReverse 0.632 0.1040 68.90 15.90\nThird-party U30\nWith a 1 cm2\ncertification (0.3396 Forward 0.6352 0.1228 69.10 15.87\nmask\nauthority mW cm-2)\nReverse 0.6294 0.1232 69.77 15.93\n20\n(3) The explanation of the shadow effect on J : If the I is unchanged, the obtained\nsc sc\nJ without a mask (1.21 cm2) should be lower than that obtained with a 1.0 cm2 mask\nsc\naccording to J =I /A, where I is short-circuit current, and A is the effective area of\nsc sc sc\nthe device. However, it should be noted that the actual I is reduced after the\nsc\nplacement of a mask on the device due to the severe shadow effect under low-light\nindoor illumination. Therefore, owing to the simultaneous decrease of I and A, it is\nsc\nreasonable to observe that the J obtained with a 1.0 cm2 mask is slightly lower than\nsc\nthat obtained without a mask.\n(4) Calculate the J limitation depending on the actual incident light intensity: As\nsc\nsuggested, we have calculated the theoretical J limit using the measured emission\nsc\nspectrum of the actual light source and the bandgap of perovskite. As a result, the\ncalculated J is 152.1 μA/cm2 (the calculation process has been shown in reply to\nsc,limit\nQuestion 1). Therefore, the measured J in our experiment (140 μA/cm2) did not\nsc\nexceed the J . It should be also noted that the effective area of the small-area\nsc,limit\n(0.1225 cm2) device is strictly defined by the area of the Ag electrode in our\nmeasurements, and it is kept consistent across devices to avoid overestimation.\nQuestion 3: Given the limited number of studies reporting MPP tracking under 1000\nlux indoor conditions, the authors are encouraged to include the raw MPP tracking\ndata in the supplementary materials.\nReply: We sincerely appreciate the reviewer’s helpful comments. As suggested, we\nhave included the raw MPP tracking data in the submitted source data (Figure 4g) of\nthe revised manuscript.\nQuestion 4: The efficiencies under lower light intensities are wrongly calculated. For\nexample, the light power intensity at 500 lux, which is 50% of 1000 lux (0.285 mW\ncm-2), should be 0.1425 mW cm-2. PCE under 500 lux should be 71.4 uA cm-2 *\n0.659 V * 0.5657/142.5 uW cm-2 = 18.42%, not 9.34%. The PCEs under 250, 500,\nand 750 lux should be corrected.\nReply: We sincerely appreciate the reviewer’s very helpful and careful comments. As\n21\npointed out by the reviewer, we did make a mistake when recording the irradiances\nfor the illuminances of 250, 500 and 750 lux. We have corrected the irradiances and\nrecalculated the PCEs under the illuminances of 250, 500 and 750 lux in the revised\nmanuscript. The detailed corrections are shown below:\nPage 43 line 1 (Supporting information): Supplementary Table 3. Photovoltaic\nparameters of the TPPC-treated photovoltaic devices under different indoor\nilluminations.\nLight\nLight\nirradiance J (uA cm-2) V (V) FF (%) PCE (%)\nSC OC\nilluminance\n(mW cm-2)\n250 lux 0.067 35.2 0.588 53.68 16.58\n500 lux 0.141 71.4 0.659 56.57 18.88\n750 lux 0.210 105.9 0.687 60.04 20.80\n1000 lux 0.285 140.0 0.720 63.6 22.49\nThis manuscript by Meng et al. was extensively revised after the first revision,\naccording to reviewers' suggestions. The quality of the work is now significantly\nenhanced, and the results are more convincingly discussed. The manuscript could be\naccepted in publication in the current form.\nReply: We sincerely appreciate the reviewer’s acknowledgement of our work and\nthanks again for his/her very valuable comments.\n23"}, {"round": 2, "reviewer_comments": {"reviewer_1": "There are still major issues with how this paper is written. Although the authors wrote in their rebuttal letter that they have softened their conclusion of enhanced hot carrier extraction, there are no changes made to the abstract or conclusions, which still misleadingly presents this conclusion, which as the authors admitted is still conjecture. If the authors are not willing to write a balanced paper, then this should be rejected. Far better than present misleading conclusions and waste future researchers time trying to reproduce these results, or, even worse, would try to justify similarly unproven conclusions by citing this work.\nWe are still not convinced by the explanation from the authors of Auger recombination accounting for their fluence-dependent measurements. Under low fluence, the hot carrier lifetime is shorter for the TPPC treated film, which they attribute to faster hot carrier extraction. But at higher fluence, the opposite trend is observed where the TPPC treated film show longer hot carrier lifetime, and the overall hot carrier lifetime for both films is reduced. They explained this as an Auger effect. For hot carrier cooling process, Auger induced lifetime reduction usually happens when the carrier population is over 10^19 cm-3. Before entering into this regime, we would expect to see an increase in HC lifetime due to the hot phonon bottleneck effect (carrier density around 10^16-10^19 cm-3), which is not reported here. Increasing the fluence from 1.05 uJ/cm2 to 2.63 uJ/cm2 seems insufficient to raise the carrier population by a few orders of magnitude. Hence, the assertion the authors still make that Auger recombination controlling hot carrier kinetics at high fluence is not well founded, and it is insufficient to merely cite a few references without further critical evaluation.\nBeyond this, further details are needed for the reporting of indoor PV performance.\nThe authors need to quote the thickness of the light mask used. Since the authors argue that the diffuse nature of their light source is causing discrepancies in performance without and with the light mask, the mask thickness is an essential detail.\nFor certification, was the fluorescent light source collimated or diffuse? Can the authors quote the angle of divergence of the light source?\nIn the abstract, the authors quote a PCE value and the illuminance. I would argue that the field should move away from reporting IPV performance in such a poorly specified way. The irradiance is needed, as is the CCT of the light source, and whether it is from a WLED or fluorescent light source. The power density produced by the device should also be quoted, ideally that measured under maximum power point tracking.\nSimilarly, in the introduction, the authors talk about progress in PCE, but there is no mention of how comparable these PCEs are if all reports did their measurements under different conditions (different light sources, illuminances, etc). Further qualification is needed in this discussion. Again, quoting power density would be helpful.\nThe phase \"...we have achieved a significant high PCE (22.49%) thus far.\" is not grammatically correct. What does significant high mean? It would be better if the authors just plainly say that their measured PCE without a light mask is higher than previous reports, and then add a short discussion of how significant this really is, i.e., how similar or different was their light source to those of previous record tin perovskite indoor PV? Can the authors compare power density of their champion devices vs. the previous champion tin perovskite indoor PV devices? The authors need to plainly say here the problems with not using a light mask, and the errors introduced if they did under diffuse light. Add in the thickness of the light mask.\nReferring to a sketch of the setup and illustrating how using the light mask leads to errors is also helpful. This will help future researchers.\nIn the abstract, the authors refer to adsorption energy, but it is not clear what this means? Binding energy of molecules to perovskite surface?\nFig. 4f - please add the caveat in the figure caption that all devices were measured under different light sources. We cannot make a 1:1 comparison across all data points because of this, but this provides a general indication of progress\nFig. 4e - restate the light source used. WLED? Irradiance?\nFig. 2f should have error bars added. Caption should quote the power density used for these measurements. Presumably cw laser illumination? What wavelength?\nFig. 2e needs to have the fluence used, excitation wavelength and repetition rate.\nThe author should fit the integrated diffraction patterns of their GIWAXS measurements. Bragg-Brentano XRD only shows two peaks. Hardly useful for proving that they have achieved the material they thought they had. Integrating the GIWAXS measurements should give them more peaks to fit.\nFig. S13 - I am concerned that the authors are fitting the lower energy edge of their excitonic peak. Did they try Elliott model fitting? Why did adding C60 give rise to a peak near their primary edge?\nThe light spectrum of their indoor light simulator needs to be much more prominently features. When they quote their PCE, they should refer to exactly where the plot of their light source spectrum is.\nPlease add the following to the Methods:\nThe authors need to specify the angle of their light source and device under test for all measurements, both within their lab and in the certification laboratory.\nHow did they calibrate the spectrometer used? I think there is a misunderstanding with the authors regarding the angular response. Even if the spectroradiometer is exactly in the position the DUT is, this spectroradiometer can have a different response to light coming in from different angles than the DUT, leading to errors in the measurement. It is OK to still use this, since there is no proof so far that using reference cells is better, but the authors need to make it clear that this is a potential source of error. Did the authors use a cosine corrector on their spectrometer?\nSpecify the tests the authors did to measure the uniformity and temporal stability of their light source. This data should be provided in the SI and quoted in the Methods.\nThe authors should specify the source-substrate distance used.", "reviewer_2": "Thanks for the authors' effort in revising the manuscript.\nThe concerns of reviewer have been addressed and the results are now more convincing."}, "author_response": "Response Letter to Reviewers’ Comments\nReviewer #1\nThere are still major issues with how this paper is written. Although the authors wrote\nin their rebuttal letter that they have softened their conclusion of enhanced hot carrier\nextraction, there are no changes made to the abstract or conclusions, which still\nmisleadingly presents this conclusion, which as the authors admitted is still conjecture.\nIf the authors are not willing to write a balanced paper, then this should be rejected.\nFar better than present misleading conclusions and waste future researchers time\ntrying to reproduce these results, or, even worse, would try to justify similarly\nunproven conclusions by citing this work.\nWe are still not convinced by the explanation from the authors of Auger\nrecombination accounting for their fluence-dependent measurements. Under low\nfluence, the hot carrier lifetime is shorter for the TPPC treated film, which they\nattribute to faster hot carrier extraction. But at higher fluence, the opposite trend is\nobserved where the TPPC treated film show longer hot carrier lifetime, and the\noverall hot carrier lifetime for both films is reduced. They explained this as an Auger\neffect. For hot carrier cooling process, Auger induced lifetime reduction usually\nhappens when the carrier population is over 1019 cm-3. Before entering into this\nregime, we would expect to see an increase in HC lifetime due to the hot phonon\nbottleneck effect (carrier density around 1016-1019 cm-3), which is not reported here.\nIncreasing the fluence from 1.05 μJ/cm2 to 2.63 μJ/cm2 seems insufficient to raise the\ncarrier population by a few orders of magnitude. Hence, the assertion the authors still\nmake that Auger recombination controlling hot carrier kinetics at high fluence is not\nwell founded, and it is insufficient to merely cite a few references without further\ncritical evaluation.\nReply: We sincerely thank the reviewer for these very valuable and professional\ncomments, which will definitely help enhance the quality of our manuscript. In\nresponse to these comments, we will explain the concerns through the following two\n1\naspects:\n(1) Make sure to soften the conclusion of enhanced hot carrier extraction in the\nabstract and conclusions\nWe really apologize for not strictly softening the statement of enhanced hot carrier\nextraction in the previous round of revision. In the current edition, we have made sure\nto delete all the statement about enhanced hot carrier extraction in the Abstract,\nIntroduction and Conclusions sections. The specific corrections in the revised\nmanuscript are shown below:\ni. Statement of hot carrier extraction before correction (Page 1, Abstract): The newly\nformed surface dipole at the interface optimized the interfacial energy level alignment\nby constructing a cascaded energy gradient, enabling facilitated extraction of hot\nelectrons before thermalization. Furthermore, by investigating the interfacial hot\ncarrier dynamics, the TPPC-treated interface exhibited a short hot carrier cooling time\n(6.29 ps), thereby confirming the facilitated hot carrier extractions and minimized\nthermalization losses.\nStatement of hot carrier after correction (Page 1, Abstract): The newly formed surface\ndipole at the interface optimized the interfacial energy level alignment by constructing\na cascaded energy gradient, enabling the modulation of hot carrier dynamics at the\ninterface.\nii. Statement of hot carrier extraction in the Introduction section (Page 4) has been\ndeleted in the revised manuscript. The deleted sentence is shown below: Furthermore,\nby analyzing the femtosecond transient absorption (fs-TA) spectrum, it was observed\nthat the TPPC-treated interface exhibited a shorter hot carrier cooling time (6.29 ps),\nconfirming that TPPC-treatment could facilitate hot carrier extractions at the interface.\niii. Statement of hot carrier extraction in the Conclusion section (Page 17) has been\ndeleted in the revised manuscript. The deleted sentence is shown below: Furthermore,\nby investigating the fs-TA spectrum, it was discovered that the hot carrier at the\nTPPC-treated interface exhibited a shorter cooling time than that at the untreated\n2\ninterface, validating that TPPC-treatment could facilitate charge carrier extractions at\nthe interface.\n(2) Explanation for the fluence-dependent hot carrier cooling time\nWe are grateful to the reviewer for this insightful comment regarding the\nfluence-dependent hot carrier cooling time, which prompts us to re-evaluate our\ninterpretation. The reviewer points out that the typical regime for strong Auger\nrecombination is above 1019 cm-3. Upon careful re-evaluation, we have calculated the\ninitial carrier density (n ) in our experiments using the following formula:\n0\nn = (1 -R) × F × α / E ,\n0 photon\nwhere R represents reflectance, F represents pump fluence, α represents absorption\ncoefficient, and E represents photon energy, respectively. The value of α can be\nphoton\n2.303A\ncalculated by α= , where A represents absorbance, and d represents film\nd\nthickness, respectively. For pristine perovskite film, since R = 0.1,\nα= 2.303A = 2.303×2.31 cm-1=2.7×105cm-1, E = hc = 6.626×10-34×3×108 J=5.37×10-19J, the\nd 200 photon λ 370×10-9\ncalculated n is:\nAt the high fluence of 2.63 μJ/cm2: n₀ = 1.2 × 1018 cm-3\nAt the fluence of 1.05 μJ/cm2, the n₀ of 4.8 × 1017 cm-3 is obviously below the\nthreshold for dominant Auger recombination. However, since the measured hot carrier\ncooling time (Control: 2.70±0.35 ps, TPPC-treatment: 3.61±0.32 ps) at the fluence of\n2.63 μJ/cm2 is obviously lower than that (Control: 6.70±0.19 ps, TPPC-treatment:\n6.00±0.23 ps) at the fluence of 1.05 μJ/cm2, it suggests that the n₀ of 1.2 × 1018 cm-3\nenters a regime where Auger recombination begins to influence carrier kinetics. The\nemergence of Auger recombination in our tin-based perovskite films at a fluence of\n2.63 μJ/cm2 (n =1.2 × 1018 cm-3) can be attributed to the material's inherent properties.\nbackground doping due to defects like Sn4+, which not only pre-fills electronic states\n3\nbut also intensifies Coulomb interactions. These combined effects can substantially\nlower the carrier density threshold for Auger recombination compared to conventional\nsemiconductors like silicon (Ref: [1] Treglia A, et al. Materials Horizons, 2022, 9(6):\n1763-1773. [2] Shen J X, et al. Adv. Energy Mater. 2018, 8(30): 1801027. [3] Berger F\nJ, et al. ACS Energy Lett. 2023, 8(9): 3876-3882. [4] Scajev P, et al. J. Phys. Chem. C,\n2019, 123(32): 19275-19281.).\nIn the revised manuscript, we have deleted the definitive claim of \"Auger\nrecombination controlling hot carrier kinetics at high fluence\" and reframed it as \"The\nprolonged cooling time in the TPPC-treated interface at high fluence could be\nprobably ascribed to the reduction of defect density and suppression of Auger\nrecombination and carrier-carrier scattering.\" We believe this interpretation is\nconsistent with both our experimental data and the well-established carrier density\nthreshold for Auger recombination. The specific corrections in the revised manuscript\nare shown below:\ni. Statement of “Auger recombination controlling hot carrier kinetics at high fluence”\n(Page 9) has been deleted in the revised manuscript. The deleted sentence is shown\nbelow: While the hot carrier cooling time is controlled by Auger recombination under\nhigh pump fluence, and the variation of cooling time represents the recombination\nlosses.\nii. The statement of hot carrier cooling time under the high fluence of 2.63 μJ/cm2 has\nbeen corrected as follows:\nPage 10 line 24: As shown in Supplementary Fig. 20, the TPPC-treated PVK/C\n60\ninterface showed a longer hot carrier cooling time (3.61±0.32 ps) than the untreated\ninterface (2.70±0.35 ps). The prolonged cooling time in the TPPC-treated interface at\nhigh fluence could be probably ascribed to the reduction of defect density and\nsuppression of Auger recombination and carrier-carrier scattering51,52. Compared to\nthe hot carrier cooling time under the low pump fluence (1.05 μJ cm-2), the shortening\nof hot carrier cooling time under high pump fluence (2.63 μJ cm-2) primarily reflected\nthe weakening of the hot phonon bottleneck effect and the intensification of\n4\nrecombination losses.\nQuestion 1: Beyond this, further details are needed for the reporting of indoor PV\nperformance. The authors need to quote the thickness of the light mask used. Since\nthe authors argue that the diffuse nature of their light source is causing discrepancies\nin performance without and with the light mask, the mask thickness is an essential\ndetail.\nReply: We sincerely appreciate the reviewer’s very valuable comments. As suggested,\nwe have added the thickness (200 μm) of used light mask in the revised manuscript.\nThe specific corrections in the revised manuscript are shown below:\nPage 12 line 13: It should be noted that since the severe shadow effect induced by the\nsquare mask (mask thickness: 200 μm) reduced the light intensity under the indoor\nlight illumination, the certified PCE (15.93%) for the device with a mask is lower than\nthat (17.94%) measured in our lab without masks.\nQuestion 2: For certification, was the fluorescent light source collimated or diffuse?\nCan the authors quote the angle of divergence of the light source?\nReply: We sincerely appreciate the reviewer’s professional comments. For\ncertification, the fluorescent light source is diffuse. For diffuse light source, the angle\nof divergence does not exist. As suggested, we have added this information in the\nSupplementary information of the revised manuscript, as shown below:\nPage 29 line 2 (Supplementary information): Supplementary Figure 28. Picture of\ncertification for the fabricated indoor photovoltaic device (effective area: 1 cm2) based\non TPPC-treated perovskite film. The used fluorescent light source is diffuse.\nQuestion 3: In the abstract, the authors quote a PCE value and the illuminance. I\nwould argue that the field should move away from reporting IPV performance in such\na poorly specified way. The irradiance is needed, as is the CCT of the light source,\nand whether it is from a WLED or fluorescent light source. The power density\nproduced by the device should also be quoted, ideally that measured under maximum\n5\npower point tracking.\nReply: We sincerely appreciate the reviewer’s very helpful and professional\ncomments. As suggested, we have added the specific details when reporting the IPV\nperformance in the abstract of the revised manuscript. The specific corrections in the\nrevised manuscript are shown below:\nPage 1 line 24 (Abstract): With these merits, the TPPC-treated photovoltaic devices\ndelivered a champion power conversion efficiency (PCE) of 22.49% and a maximum\noutput power density (P ) of 64.1 μW·cm-2 under the white light-emitting diode\nout\nillumination (3000 K, 1000 lux, 285 μW·cm-2). The large-area (1.21 cm2)\nphotovoltaic devices showed a record PCE of 17.94% (certified: 15.93%) and a\nmaximum P of 51.1 μW·cm-2 under the same illumination condition.\nout\nQuestion 4: Similarly, in the introduction, the authors talk about progress in PCE,\nbut there is no mention of how comparable these PCEs are if all reports did their\nmeasurements under different conditions (different light sources, illuminances, etc).\nFurther qualification is needed in this discussion. Again, quoting power density would\nbe helpful.\nReply: We sincerely appreciate the reviewer’s very helpful comments. As suggested,\nwe have added the details of light source and quoted the power density of the devices\nwhen introducing the progress in PCE in the Introduction section of the revised\nmanuscript. The specific corrections in the revised manuscript are shown below:\nPage 2 line 23: Ultimately, they achieved a maximum PCE approaching 12.81% and a\nmaximum output power density (P ) of 52.53 μW cm-2 under 1000 lux.\nout\n(Note that since this referenced literature does not specify the light source, CCT and\nirradiance, the more illumination details cannot be added here.)\nPage 2 line 26: For example, Wang et al. demonstrated a PCE of 17.57% and a\nmaximum P of 58.75 μW cm-2 for tin halide PSCs under the white light-emitting\nout\ndiode (WLED) illumination (2956 K, 1062 lux, 334.41 μW cm-2) by modifying\npoly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) with\n6\npotassium thiocyanate (KSCN).\nPage 3 line 2: The CsF-incorporated PSCs demonstrated an efficiency of 20.12% and\na maximum P of 57.32 μW cm-2 under the indoor WLED illumination (3000 K,\nout\n1000 lux, 285 μW cm-2).\nQuestion 5: The phase \"...we have achieved a significant high PCE (22.49%) thus\nfar.\" is not grammatically correct. What does significant high mean? It would be\nbetter if the authors just plainly say that their measured PCE without a light mask is\nhigher than previous reports, and then add a short discussion of how significant this\nreally is, i.e., how similar or different was their light source to those of previous\nrecord tin perovskite indoor PV? Can the authors compare power density of their\nchampion devices vs. the previous champion tin perovskite indoor PV devices? The\nauthors need to plainly say here the problems with not using a light mask, and the\nerrors introduced if they did under diffuse light. Add in the thickness of the light mask.\nReferring to a sketch of the setup and illustrating how using the light mask leads to\nerrors is also helpful. This will help future researchers.\nReply: We sincerely appreciate the reviewer’s very valuable and professional\ncomments. In response to these comments, we will explain the concerns through the\nfollowing two aspects:\n(1) Grammar correction and plain statement of the device PCE\nWe apologize for the use of unidiomatic expression \"significant high\" and we have\ndeleted it in the current edition. As suggested, we have rephrased the sentence and add\na short discussion to describe the device performance. The used light sources in the\nliteratures and this work are compared in the below Supplementary Table 6. The\nvalues of the output power density (P ) are also compared in the table. The specific\nout\ncorrections in the revised manuscript are shown below:\nPage 12 line 24: Furthermore, based on the recent advancements in PCE of lead-free\nperovskite-based indoor photovoltaic devices (Fig. 4f, Supplementary Table\n6)13-19,21-25,31, it is evident that our measured PCE (22.49%) without a light mask is\n7\nhigher than that in the previous reports. The P of 64.1 μW cm-2 also ranks among\nout\nthe highest reported values thus far (Supplementary Table 6). It should be noted that\nsince all devices are measured under different light sources, we cannot make a 1:1\ncomparison across all data points, but this provides a general indication of progress.\nSupplementary Table 6. Summarization of indoor photovoltaic performances for\nlead-free PSCs.\nLight source V J FF PCE P\noc sc out\nMaterial[ref]\n(LED) (V) (μA cm-2) (%) (%) (μW cm-2)\n1000 lux,\nAg BiI [1] 0.70 39.2 48.0 5.02 13.0\n2 5\n262 μW cm-2\nCs MA FA Sb I 4000 K, 1000 lux,\n2.4 0.5 0.1 2 8.5\n0.55 102.0 52.0 6.40 29.2\nCl [2] 460 μW cm-2\n0.5\nCu AgBiI [3] 0.53 72.0 67.0 5.52 25.6\n2 6\n460 μW cm-2\nCu AgBiI [4] 0.54 62.0 64.6 4.70 21.6\n2 6\n460 μW cm-2\n1000 lux,\nCs AgBiBr [5] 0.67 41.9 76.9 7.18 21.5\n2 6\n300 μW cm-2\nATBiI [6] 0.71 4.4 50.0 0.52 1.5\n4\n301 μW cm-2\nFA MA SnI Br[7] 0.57 162.0 64.0 17.57 58.8\n0.75 0.25 2\n334 μW cm-2\n1000 lux,\nFA MA SnI Br[8] 0.56 123.0 70.0 14.40 48.2\n0.75 0.25 2\n335 μW cm-2\nCs Sb I [9] 0.74 47.7 69.0 9.20 24.4\n3 2 9\n280 μW cm-2\n1000 lux,\nFA MA SnI Br[10] 0.28 280.0 67.0 12.81 52.5\n0.75 0.25 2\n410 μW cm-2\n1000 lux,\nAgBiI [11] 0.55 5.7 56.0 5.17 1.8\n4\n34 μW cm-2\n1000 lux,\nCs Sb Cl I [12] 0.47 76.0 40.0 4.40 14.3\n3 2 x 9-x\n320 μW cm-2\nFASnI Br-CsF[13] 0.70 120.5 68.4 20.12 57.3\n2\n285 μW cm-2\nFASnI Br-TPPC (this 3000 K, 1000 lux,\n2\n0.72 140.0 63.6 22.49 64.1\nwork) 285 μW cm-2\n(2) Clarify the problems with/without using a light mask\nsketch of the setup illustrating how using a light mask will lead to errors has also been\nadded in Supplementary Fig. 29 (also shown below). The specific corrections in the\nrevised manuscript are shown below:\nPage 12 line 30: We should also note that the measurements without a mask may\ncause several issues, including inaccurate definition of the device's active area and the\ninclusion of peripheral leakage currents. On the other side, since the severe shadow\neffect of a square mask (thickness: 200 μm) makes the active area of the device\nsmaller than the mask's aperture area under the diffuse indoor illumination\n(Supplementary Fig. 29), the measured PCE with a mask is underestimated. Given the\nmore pronounced impact of the shadow effect on small-area devices, the photovoltaic\nperformance of small-area devices was measured without a mask and not certificated,\nwhile that of large-area devices was certificated with a mask in this study.\nSupplementary Figure 29. Schematic illustration of the active area for solar cell device under\n(a) standard AM 1.5 G simulated sunlight and (b) diffuse indoor light. Under collimated 1-sun\nillumination, the light is incident perpendicularly, resulting in an active area that is equal to\nthe mask's aperture area. However, under diffuse indoor illumination, the finite thickness of\nthe mask casts a shadow, restricting the active area to a region smaller than the mask's\naperture area. In the following calculation of J with J =I S, where I represents\nSC SC SC SC\nshort-circuit current and S represents active area, since the actual v⁄alue of S is overestimated\n9\nwhen using a mask, the calculated J is underestimated. Consequently, this underestimated\nSC\nJ leads to a corresponding underestimation of PCE.\nSC\nQuestion 6: In the abstract, the authors refer to adsorption energy, but it is not clear\nwhat this means? Binding energy of molecules to perovskite surface?\nReply: We sincerely appreciate the reviewer’s valuable comments. Adsorption energy\nis the energy released when a molecule attaches to a surface, typically a negative\nvalue indicating stability. Binding energy is the energy required to separate that\nmolecule from the surface, a positive value representing the bond strength. In surface\nscience, they often describe the same phenomenon but with opposite signs: Binding\nEnergy ≈ -Adsorption Energy. The term of “adsorption energy” has been widely\nutilized in surface science of many literatures (Ref: [1] Liu S, et al. Nature, 2024,\n632(8025): 536-542.; [2] Yang S, et al. Nat. Energy, 2016, 1(2): 1-7.; [3] Shi C, et al.\nNat. Commun. 2025, 16(1): 3029.; [4] Xu X, et al. Adv. Funct. Mater. 2024, 34(44):\n2408512.).\nQuestion 7: Fig. 4f - please add the caveat in the figure caption that all devices were\nmeasured under different light sources. We cannot make a 1:1 comparison across all\ndata points because of this, but this provides a general indication of progress.\nReply: We sincerely appreciate the reviewer’s very valuable and professional\ncomments. As suggested, we have added the recommended caveat in the figure\ncaption of Fig. 4f. The specific corrections in the revised manuscript are shown\nbelow:\nPage 14 line 7: f, Development of PCE for lead-free perovskite-based indoor\nphotovoltaic devices reported in literatures and this work to date. It should be noted\nthat since all devices are measured under different light sources, we cannot make a 1:1\ncomparison across all data points, but this provides a general indication of progress.\nQuestion 8: Fig. 4e - restate the light source used. WLED? Irradiance?\nReply: We sincerely appreciate the reviewer’s very valuable and professional\n10\ncomments. As suggested, we have added the details of the used light source in the\nfigure caption of Fig. 4e. The specific corrections in the revised manuscript are shown\nbelow:\nPage 14 line 5: e, Statistical photovoltaic parameters of PCE, FF, V and J for 51\noc SC\ndevices under the WLED illumination (3000 K, 1000 lux, 285 μW cm-2).\nQuestion 9: Fig. 2f should have error bars added. Caption should quote the power\ndensity used for these measurements. Presumably cw laser illumination? What\nwavelength?\nReply: We sincerely appreciate the reviewer’s valuable and professional comments.\nAs suggested, we first added the error bars in Fig.2f (as shown below). Then we\nadded the power density (measured power: 158.9 μW, light spot area: 0.07 cm2, power\ndensity: 2.27 mW cm2), the light source (Xenon lamp) and illumination wavelength\n(450 nm) for the measurements. The specific corrections in the revised manuscript are\nshown below:\nPage 9 line 9: f, PLQYs for PVK, PVK/C , PVK+TPPC and PVK+TPPC/C layers\n60 60\nunder the Xenon lamp illumination (wavelength: 450 nm, power density: 2.27 mW\ncm-2).\nFig. 2f, PLQYs for PVK, PVK/C , PVK+TPPC and PVK+TPPC/C layers under the Xenon\n60 60\nlamp illumination (wavelength: 450 nm, power density: 2.27 mW cm-2).\nQuestion 10: Fig. 2e needs to have the fluence used, excitation wavelength and\n11\nrepetition rate.\nReply: We sincerely appreciate the reviewer’s valuable and professional comments.\nAs suggested, we have added the used fluence (12.74 nJ cm-2), excitation wavelength\n(450 nm) and repetition rate (1 MHz) for the TRPL measurements. The fluence (F) is\ncalculated by F=\nPaver,\nwhere P represents average power, f represents repetition\naver\nfS\nrate, and S represents laser spot area. Since P = 140.1 μW, f= 1 MHz, S = 0.011\naver\ncm2 in our measurements, the value of F can be calculated by F = Paver= 140.1 nJ\nfS 1000×0.011\ncm-2=12.74 nJ cm-2. The specific corrections in the revised manuscript are shown\nbelow:\nPage 9 line 7: e, Measured TRPL spectra under the 450-nm laser illumination (fluence:\n12.74 nJ cm-2, repetition rate: 1 MHz).\nQuestion 11: The author should fit the integrated diffraction patterns of their\nGIWAXS measurements. Bragg-Brentano XRD only shows two peaks. Hardly useful\nfor proving that they have achieved the material they thought they had. Integrating the\nGIWAXS measurements should give them more peaks to fit.\nReply: We sincerely appreciate the reviewer’s very professional and valuable\ncomments. As suggested, we have integrated the 2D GIWAXS patterns to obtain the\nscattering vector (q)-dependent intensity plots, as shown in the below Supplementary\nFigure 11. Obviously, these intensity-q profiles reveal multiple well-defined\ndiffraction peaks (e.g., (100), (011), (022), (211), (122), (120), and (208)), which have\nbeen indexed to the reference pattern of the target tin halide perovskite (Ref: [1] Wang\nJ, et al. Angew. Chem. Int. Ed. 2024, 136(17): e202317794.; [2] Liu T, et al. Angew.\nChem. Int. Ed. 2025, 64, e202418470.). The presence of these diffraction peaks\nprovides definitive evidence for the successful formation of the tin halide perovskite\nwith high purity. The figures of the integrated diffraction patterns of the GIWAXS\nmeasurements and corresponding analysis have been added in the revised manuscript.\nThe specific corrections in the revised manuscript are shown below:\n(GIWAXS) patterns and the azimuthally integrated plots of the scattering vector\n(q)-dependent intensity (Supplementary Fig. 11), we could observe multiple\nwell-defined diffraction peaks (e.g., (100), (011), (022), (211), (122), (120), and\n(208)), unequivocally confirming the successful formation of tin halide perovskite5.\nThe GIWAXS patterns could also validate the enhancement of the film crystallinity\nafter TPPC-treatment (Supplementary Fig. 11a,b), especially the (100) plane.\nAdditionally, according to the intensity-q plots (Supplementary Fig. 11c-h), the\ncharacteristic peaks showed no obvious shifts after TPPC-treatment, demonstrating\nthat the TPPC-treatment did not incorporate into the lattice or alter the bulk crystal\nstructure of the perovskite material.\nSupplementary Figure 11. GIWAX patterns for (a) pristine and (b) TPPC-treated perovskite\nfilms, respectively. The incident angle is 0.2°. The azimuthally integrated plots of the\nscattering vector (q)-dependent intensity for (c) (100) plane, (d) (011) plane, (e) (022) plane,\n13\n(f) (211) plane, (g) (120) and (122) planes, and (h) (208) plane, respectively.\nQuestion 12: Fig. S13 - I am concerned that the authors are fitting the lower energy\nedge of their excitonic peak. Did they try Elliott model fitting? Why did adding C\n60\ngive rise to a peak near their primary edge?\nReply: We sincerely appreciate the reviewer’s very professional and valuable\ncomments. We will explain the concerns through the following three aspects:\n(1) Concerns of fitting the lower energy edge of the excitonic peak\nWe acknowledge the concern that the value of 1.63 eV might originate from fitting an\nexcitonic feature. Upon re-examination and further consideration, we provide the\nfollowing clarification to affirm the validity of our analysis.\nThe linear extrapolation yielding 1.63 eV (line 1 in the below Figure R1) is applied to\nthe initial, steeply rising portion of the absorption edge. This region is unequivocally\nassociated with the onset of the fundamental band-to-band transition in\nsemiconductors, and its extrapolation is the standard method for determining the\noptical bandgap (E ). The reviewer's concern will be valid if a distinct excitonic peak\ng\nis present at the low-energy side of this edge. However, in our material, any excitonic\nfeature is weak and does not form a discrete shoulder that can be mistakenly\nextrapolated. The higher-energy linear region (line 2 in the below Figure R1) that the\nreviewer might be referring to corresponds to the absorption saturation regime at\nelevated energies, and its extrapolation does not define a bandgap but is related to\nhigher-energy optical transitions. To further strengthen our assignment, we would like\nto point out that if the value of 1.63 eV corresponds to the exciton energy (E ), the\nexciton\nE would have to be at a higher energy according to E = E + E (E represents\ng g exciton b b\nexciton binding energy). In that case, a second and even steeper linear region\ncorresponding to this higher bandgap should be clearly visible to the right of line 1 in\nthe plot. However, it is obvious to see that no such a secondary and even steeper\nlinear region is observed in the below Figure R1. This provides definitive evidence\nthat the linear extrapolation of line 1 represents the bandgap of the material. We are\n14\ngrateful for the reviewer's scrutiny, which has prompted us to provide a more detailed\njustification for our methodology.\nFigure R1. Tauc plot for pristine perovskite film.\n(2) Try to use Elliott model fitting\nWe thank the reviewer for the insightful suggestion regarding the Elliott model fitting.\nWe have carefully followed this advice and performed extensive attempts to fit our\nabsorption spectrum using the Elliott model. As shown in the Figure R2 below, the\nfitting results were unfortunately very bad, with a very low goodness-of-fit. The\nextracted parameters, including the exciton binding energy (R), exciton peak\namplitude (A), exciton peak broadening (Gamma), and continuous-state amplitude\n(B), were physically unreasonable and unstable across different fitting attempts. We\nattribute this to the fact that the absorption edge of our tin-based perovskite film is\nsmooth (the absorption spectrum exhibits a continuous rise) and lacks a distinctly\nresolved excitonic shoulder, which is a prerequisite for reliable application of the\nstandard Elliott model. This also indicates that excitonic effects are either extremely\nweak or completely masked by the continuum absorption. Forcibly incorporating an\nexcitonic term would introduce redundant parameters, leading to fit divergence.\nTherefore, in such material systems where the excitonic feature is weak and merged\n15\nwith the band-edge absorption, the Tauc plot method remains a robust and widely\nadopted approach for determining the optical bandgap, as consistently reported in the\nliterature for perovskites (Ref: [1] Liang Z, et al. Nature, 2023, 624(7992): 557-563.\n[2] Lanzetta L, et al. Nature Communications, 2021, 12(1): 2853. [3] Zhao B, et al.\nAdvanced materials, 2017, 29(2): 1604744. [4] Kuan C H, et al. ACS Energy Letters,\n2024, 9(5): 2351-2357. [5] Kim S Y, et al. Advanced Energy Materials, 2024, 14(48):\n2402433.).\nFigure R2. Elliott model fittings for the absorption spectrum of the pristine perovskite film\nunder different fitting parameters.\n(3) Explanation for the peak near the primary edge after adding C\nconsidered two possible explanations for the origin of this spectral feature as follows:\n①Partial surface oxidation of the C layer: Although the C deposition was\n60 60\nperformed in a vacuumed chamber, the subsequent transfer for UPS measurement\nmight have involved minimal exposure to ambient atmosphere. C is known to be\n60\nsusceptible to oxidation due to its highly strained spherical structure, electron-rich\nproperty and the presence of reactive double bonds. The incorporation of oxygen\n16\natoms with C can introduce new electronic states within the bandgap, which\n60\nmanifest as a shoulder on the higher-binding-energy side of the primary HOMO\nfeature.\n②Interfacial interaction with the TPPC-treated substrate: The presence of the TPPC\ninterlayer could induce specific interfacial interactions with the deposited C . This\n60\ncould generate charge rearrangement or the formation of an interface dipole, which\ncould modify the local electronic structure of the C molecules at the interface and\n60\ngive rise to new electronic states detectable by UPS as a distinct feature adjacent to\nthe main HOMO position.\nQuestion 13: The light spectrum of their indoor light simulator needs to be much\nmore prominently features. When they quote their PCE, they should refer to exactly\nwhere the plot of their light source spectrum is.\nReply: We sincerely appreciate the reviewer’s very professional comments. As\nsuggested, we have referred to the place of the light source spectrum when quoting\nthe PCE. The specific corrections in the revised manuscript are shown below:\nPage 11 line 19: The control photovoltaic devices exhibited a champion PCE of 15.19%\nwith a maximum P of 43.3 μW cm-2 under the simulated indoor illumination\nout\n(Supplementary Fig. 22, 3000 K, 1000 lux, 285 μW cm-2), whereas the TPPC-treated\ndevices achieved a remarkable champion PCE of 22.49% with an open circuit voltage\n(V ) of 0.72 V, a short-circuit current density (J ) of 140.0 μA cm-2, a fill factor (FF)\noc sc\nof 63.6% and a maximum P of 64.1 μW cm-2 (Supplementary Figs. 24,25,\nout\nSupplementary Tables 1 and 2).\nPage 12 line 7: As depicted in Fig. 4c and Supplementary Table 5, the control devices\nwith large effective areas obtained a champion PCE of 10.3% with a maximum P of\nout\n29.3 μW cm-2 under the simulated indoor illumination (Supplementary Fig. 22, 3000\nK, 1000 lux, 285 μW cm-2), while the TPPC-treated devices delivered a PCE as high\nas 17.94% with a maximum P of 51.2 μW cm-2.\nout\nWLED under the illuminance of 1000 lux. The integrated power density of the light source\nwithin the wavelength range of 300-1100 nm is 285 μW cm-2.\nQuestion 14: Please add the following to the Methods:\nThe authors need to specify the angle of their light source and device under test for all\nmeasurements, both within their lab and in the certification laboratory.\nHow did they calibrate the spectrometer used? I think there is a misunderstanding\nwith the authors regarding the angular response. Even if the spectroradiometer is\nexactly in the position the DUT is, this spectroradiometer can have a different\nresponse to light coming in from different angles than the DUT, leading to errors in\nthe measurement. It is OK to still use this, since there is no proof so far that using\nreference cells is better, but the authors need to make it clear that this is a potential\nsource of error. Did the authors use a cosine corrector on their spectrometer?\nSpecify the tests the authors did to measure the uniformity and temporal stability of\ntheir light source. This data should be provided in the SI and quoted in the Methods.\nThe authors should specify the source-substrate distance used.\nReply: We greatly appreciate the reviewer for highlighting these essential details\nregarding the measurement setup, which are crucial for the reproducibility and\n18\naccuracy of indoor photovoltaic characterization. We apologize for the lack of clarity\nin our previous manuscript and have now comprehensively addressed these points as\nfollows:\n(1) The angle between the light source and the device under test (DUT)\nLab Measurements: For all in-lab measurements, the light source, the DUT, and the\nspectroradiometer were all arranged in a normal incidence (0°) configuration. The\nactive layer of the DUT and the detector of the spectroradiometer were placed parallel\nto the light-emitting surface of the light source.\nCertification Laboratory: The measurements at the certification laboratory (FUJIAN\nMETROLOGY INSTITUTE, National PV Industry Measurement and Testing Center)\nwere performed following their standard procedures. According to their measurement,\na normal incidence (0°) geometry was also employed.\nWe have now specified all these geometric details in the \"Methods\" section of the\nrevised manuscript as shown below:\nPage 25 line 33: For all in-lab measurements, the light source, the device under test\n(DUT), and the spectrometer were all arranged in a normal incidence (0°)\nconfiguration. The active layer of the DUT and the detector of the spectrometer were\nplaced parallel to the light-emitting surface of the light source. The measurements at\nthe certification laboratory (FUJIAN METROLOGY INSTITUTE, National PV\nIndustry Measurement and Testing Center) were performed following their standard\nprocedures, and a normal incidence (0°) geometry was also employed.\n(2) The way to calibrate the used spectrometer: The used spectrometer (Model: HS-IL)\nwas calibrated by the manufacturer using a NIST-traceable standard light source prior\nto this study. The calibration certificate is now provided as Supplementary Figure 42\nin the Supplementary Information. We have also clarified the calibration of the used\nspectrometer in the \"Methods\" section of the revised manuscript as shown below:\nPage 25 line 25: The used spectrometer has been calibrated by the manufacturer using\n19\na NIST-traceable standard light source prior to this study (Supplementary Fig. 42).\nSupplementary Figure 42. The calibration certificate of the used spectrometer (Model:\nHS-IL).\n(3)Clarify the potential source of error due to different angular responses\nWe fully agree with the reviewer that even the spectrometer is in the same position as\nthe DUT, a difference in the angular response between them could be a potential\nsource of error. Following the reviewer's professional and valuable suggestion, we\nhave now explicitly stated this as a potential source of error in the revised manuscript\nas shown below:\nPage 25 line 22: But it should be noted that while a cosine corrector was used for the\nspectrometer, a slight difference in the angular response between the spectrometer and\nthe photovoltaic device could still persist, representing a potential source of error in\nthe measured irradiance and consequent current density.\n(4) Confirm the use of cosine corrector on the spectrometer\nThe spectrometer (Model: HS-IL) used in our IPV measurement setup has a cosine\n20\ncorrector. The purpose of using a cosine corrector on the spectrometer is to ensure the\ninstrument accurately measures light incident from all angles, providing a response\nthat is proportional to the cosine of the angle of incidence. This achieves accurate\nmeasurements of diffuse or oblique light. We have specified this detail in the\n\"Methods\" section of the revised manuscript as shown below:\nPage 25 line 18: The spectrometer (Model: HS-IL) used in our IPV measurement\nsetup has a cosine corrector. The purpose of using a cosine corrector on the\nspectrometer is to ensure the instrument accurately measures light incident from all\nangles, providing a response that is proportional to the cosine of the angle of\nincidence. This achieves accurate measurements of diffuse or oblique light.\n(5) Specify the tests to measure the uniformity and temporal stability of the light\nsource\nUniformity Test: We mapped the measured light power over the sample plane (an\narea larger than our device active area) using a calibrated power meter. The\nmeasurement was performed on a grid with a spacing of 1 cm. The results show that\nthe irradiation uniformity (U), which is calculated by U=1-\nPmax-P\nmin ×100%, of the\nPmax+P\nmin\n86.8-83.6\nused WLED in our experimental setup is 98.1% (U=1- ×100%=98.1%) within\n86.8+83.6\na 5×5 cm2 area (see the Supplementary Figure 40 below), thus ensuring uniform\nirradiation on the device and accurate measurement of the light spectrum irradiated on\nthe device. The measured light power distribution over the sample stage plane is now\nprovided as Supplementary Figure 40 in the revised Supplementary Information. We\nhave also specified the uniformity test of the light source in the \"Methods\" section of\nthe revised manuscript as shown below:\nPage 25 line 3: To evaluate the uniformity (U) of the light source, we mapped the\nspatial distribution of measured light power (P) across the sample stage using a power\nmeter. Measurements were taken across a grid with 1 cm spacing. The irradiation\nuniformity was calculated by U=1-\nPmax-P\nmin ×100%, where P represents the\nmax\nPmax+P\nmin\n21\nmaximum power, and P represents the minimum power. Results showed that the\nmin\nWLED used in our experimental setup achieved a high uniformity of 98.1% over a 5\n× 5 cm2 area (Supplementary Figure 40), confirming excellent irradiation\nhomogeneity.\nSupplementary Figure 40. The measured light power distribution over the sample stage plane\n(5×5 cm2) using a power meter. The measurement was performed on a grid with a spacing of\n1 cm. The measured light wavelength is 600 nm.\nTemporal stability test: To evaluate the temporal stability (S) of the light source, we\nmonitored the irradiance (I) of the used WLED at its operating intensity for over 1\nhour using the spectrometer. The value of S was then calculated by\nS=1-\nmax-I\nmin ×100%, where I represents the maximum irradiance, and I\nmax min\nmax+I\nmin\nI\nrepreIsents the minimum irradiance. The measured time-dependent irradiance of the\nWLED demonstrates excellent stability (see the Supplementary Figure 41 below),\nwith a S of 99.5% over the measurement period. The stability curve is now provided\nas Supplementary Figure 41 in the Supplementary Information. We have also\nspecified the temporal stability test of the light source in the \"Methods\" section of\nthe revised manuscript as shown below:\nPage 25 line 10: To further evaluate the temporal stability (S) of the light source, we\n22\nmonitored the irradiance (I) of the WLED at its operating intensity for over 1 hour\nusing the spectrometer. The value of S was then calculated by S=1-\nmax-I\nmin ×100%,\nmax+I\nmin\nI\nwhere I represents the maximum irradiance, and I representIs the minimum\nmax min\nirradiance. As a result, the light source demonstrated excellent stability\n(Supplementary Fig. 41), with a S of 99.5% over the measurement period.\nSupplementary Figure 41. The temporal stability test of the 3000 K WLED under the\nilluminance of 1000 lux.\n(6)Specify the source-substrate distance\nDuring the J-V measurements, the distances between the light source and the device\nsubstrate surface were fixed at 34.2 cm and 40.5 cm in our lab and in the certification\nlaboratory, respectively. These distances were chosen to ensure a sufficiently large and\nuniform illumination area. We have specified this detail in the \"Methods\" section of\nthe revised manuscript as shown below:\nPage 25 line 15: During the J-V measurements, a consistent light source-to-substrate\ndistance was maintained: 34.2 cm in our lab and 40.5 cm in the certification\nlaboratory. These distances were chosen to ensure a sufficiently large and uniform\nillumination area.\nThanks for the authors' effort in revising the manuscript. The concerns of reviewer\nhave been addressed and the results are now more convincing.\nReply: We would like to express our sincere gratitude to the reviewer for his/her\nthoughtful and constructive comments, which have been instrumental in improving\nthe quality of our manuscript.\n24"}]

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