Heteroaryl derivatives for hole-transport layers improve thermal stability of perovskite solar cells

Domain: neuroscience
Published: 2026-02-13
Editorial outcome: major_revision

Abstract
The thermal stability of perovskite solar cells (PSCs) remains a critical challenge for their integration into power grid applications. Here, we report the thermally stable PSCs by employing heteroaryl additives in the Spiro-OMeTAD. These additives effectively control void formation in the hole-transport layer and minimize reactivity with the perovskite layer, significantly improving the thermal stability of PSCs at 85 °C. As a result, PSCs with 3-phenylpyridine and 2-phenylpyridine maintained 101% and 104% of their initial photoconversion efficiency after 2400 hours of 85 °C test, respectively. As an added benefit, photovoltaic performance achieved a photoconversion efficiency of 25%. Also, the outdoor test shows 90% of the initial power point voltage after 1570 hours with maximum-power point tracking, showing remarkable light and cycle stability. We revealed the mechanism of how the additive can improve the thermal stability of PSCs by comparing 36 heteroaryl derivatives and 60 additive combinations.

Introduction
Perovskite solar cells (PSCs) have undergone remarkable advancements since their initial conception, achieving photovoltaic conversion efficiencies exceeding 26%1–8. Metal halide perovskites developed by incorporating low-dimensional perovskite9–18, dopant modification19–27, passivation technology28–39, and self-assembling monolayer40–44. Exceptional optoelectronic properties, including long carrier lifetimes45, flexibility11, and compatibility with facile solution-based fabrication processes, are additional advantages of the PSCs. These characteristics enable PSCs to be a promising candidate for integration into power grid applications as a next-generation solar cell technology after solving stability issue46–50.

Thermal instability of the PSCs remains a critical challenge for their commercialization. Additives such as 4-tert-butylpyridine (tBP) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been commonly introduced into 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)−9,9′-spirobifluorene (Spiro-OMeTAD) of the hole-transport layer (HTL), where tBP plays an essential role in improving the film morphology and conductivity due to the coordination of tBP and lithium in Spiro-OMeTAD. This improvement from additives allows for increased HTL thickness (>200 nm), which is important for scalable fabrication techniques such as blade coating for roll-to-roll processing51,52. However, under high-temperature conditions, tBP leads to the formation of voids in the HTL due to notorious volatilization, and also causes the reaction with the perovskite layer and compromises the stability of PSCs53,54. To improve the stability, several additives and methods were incorporated into the Spiro-OMeTAD layer, such as lithium alternatives55–64, oxide dopants65–68, additive engineering69–82, different doping system83–86, and stabilization methods87,88. Only a few additives were introduced as alternative additives for tBP89–92, limiting improvement in the stability of PSCs. To date, suitable alternative additives for tBP that ensure thermal durability have yet to be developed.

Heteroaryl derivatives are chemical compounds derived from heteroaryl groups, which are a subclass of aromatic compounds93–95. These compounds consist of an aromatic ring structure that includes at least one heteroatom, such as nitrogen, sulfur, and oxygen, within the ring. Derivatives are formed by substituting hydrogen atoms on the heteroaryl ring with other chemical groups, such as alkyl, hydroxyl, halogen, or functional groups like a phenyl ring. The presence of heteroatoms and these substitutions can significantly alter the chemical properties, including volatilization and reactivity with the perovskite layer. Therefore, by tailoring the chemical structure, heteroaryl derivatives can potentially be designed as alternative additives for tBP to enhance the thermal durability of PSCs.

In this study, we identified heteroaryl derivatives, namely—4-phenylpyridine, 3-phenylpyridine, and 2-phenylpyridine—, as key contributors to the thermal stability of PSCs. The impact of additives on thermal stability at 85 °C was systematically evaluated by comparing 36 heteroaryl derivatives and 60 additive combinations. Additionally, the heteroaryl derivatives enhanced charge extraction from the perovskite layer to the hole-transport layer (HTL), resulting in improved photovoltaic performance of the PSCs. This work provides a promising pathway for developing highly thermally stable PSCs for practical applications.

Results
The heteroaryl derivatives examined in this study are presented in Fig. 1 and Supplementary Table 1. These derivatives were categorized based on their structural features: pyridine derivatives substituted at the para position (Fig. 1a), pyridine derivatives substituted at the meta and/or ortho positions (Fig. 1b), para-substituted pyridine derivatives containing a phenyl or hetero ring (Fig. 1c), derivatives featuring phenyl groups substituted at the para, meta, or ortho positions (Fig. 1d) and their substitution (Fig. 1e), and other heteroaryl derivatives, including thiophene, imidazole, and unique structures such as bipyridine (Fig. 1f). For identification purposes, these additives were assigned code names ranging from K1 to K36. The materials were selected and screened to investigate the overall effect of para and ortho/meta substitutions on thermal durability. Phenyl and heterocyclic rings were included to explore their influence on volatilization and reactivity with the perovskite layer, while thiophene and imidazole derivatives were chosen to evaluate the impact of different heteroatoms on the thermal stability of PSCs. These additives were incorporated into Spiro-OMeTAD along with LiTFSI and/or tris(2-(1H-pyrazol-1-yl)−4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (Co(III)TFSI, FK209). Since there are few examples of the additives other than tBP, screening of such materials will be useful for comprehensively understanding the effect of additives on thermal durability. It is desirable to examine the selection of heteroaryl derivatives and their impact on durability from the following perspectives. For the volatility, some literature insists on the volatility of the tBP, which can be the reason for the weak durability of the Spiro-OMeTAD96. Thus, it is important to evaluate heteroaryl derivatives with different boiling points to investigate the effects of volatility on thermal stability. For example, the boiling points of 2-phenylpyridine, 3-phenylpyridine, and 4-phenylpyridine correspond to 270 °C, 274 °C, and 281 °C, respectively, which is higher than tBP (197 °C). Also, derivatives with lower boiling points should be selected for comparison, e.g., 4-ethylpyridine (165 °C), 4-aminopyridine (157~162 °C). Regarding steric hindrance, Yue et al. introduced 2-amylpyridine instead of tBP, which is substituted with an amyl group at ortho position91. They explained that the steric hindrance from the ortho-substitution of the pyridine ring may prevent the formation of a complex with additives and perovskite, which could increase the durability of perovskite solar cells. Therefore, we can select ortho, meta, and para-substituted heteroaryl derivatives to investigate the effect of the steric hinderance on the durability. As for the electronic properties, they can be tuned by introducing various functional groups and heteroatoms onto the hetero ring. These functional groups exhibit electron-donating or electron-withdrawing effects, which influence the electronegativity of the pyridine ring and its interaction with lithium. Evaluating these effects is important to determine whether the electronic state influences thermal stability. Therefore, a variety of functional groups with different electron-donating or -withdrawing characteristics, as well as heteroatoms, need to be verified.

The thermal stability results are presented in Fig. 2, Supplementary Figs. 1-67, and Tables 2-61. In these figures and tables, LiTFSI and Co(III)TFSI are abbreviated as Li and Co, respectively. Heteroaryl derivatives were incorporated into Spiro-OMeTAD with LiTFSI (e.g., Li + K1) or co-doped with both LiTFSI and Co(III)TFSI (e.g., Li, Co+K1). Importantly, n-octylammonium iodide (OAI) passivation was not used in this study to isolate the intrinsic effects of the additives on thermal stability, avoiding any confounding influences from passivation97. While OAI passivation increases the photoconversion efficiency (PCE) to 21.6% and decreases the hysteresis (Li + K29: Supplementary Figs. 68, 69), the unpassivated devices—despite their lower efficiency—offer clearer insights into how additives directly impact thermal stability of PSCs. This approach can be vital for effective materials screening and evaluation. Figure 2a shows stability results compared to initial efficiency and after 85 °C thermal stability tests after 1000 hours. Note that if the stability test was finished before 1000 hours, the final efficiency was recorded as described in the supplemental information.

Regarding the pyridine derivatives substituted at the para-position (Fig. 1a), the categorized summary is presented in Supplementary Fig. 63. For the reference device (Li+tBP; Supplementary Figs. 1, 58, 63, Table 2), the initial PCE was 15.6%, but it exhibited a sharp decline to 0.7% after 72 hours of thermal stability testing at 85 °C. This degradation is attributed primarily to reductions in current density (JSC) from 24.78 mA/cm² to 2.96 mA/cm² and fill factor (FF) from 0.626 to 0.238, while the open-circuit voltage (VOC) remained relatively unchanged. In the case of devices co-doped with Co(III)TFSI, LiTFSI, and tBP (Co, Li+tBP; Supplementary Figs. 2, 59, 63, Table 3) exhibited slightly improved stability, but the overall trend remain similar. This instability is linked to the presence of tBP, which promotes void formation in the Spiro-OMeTAD layer, leading to substantial declines in FF and PCE96. Interestingly, certain pyridine derivatives demonstrated enhanced thermal stability, including allyl isonicotinate (Li, Co+K10), 4-propylpyridine (Li+K2), methyl isonicotinate (Li+K8), and 4-ethylpyridine (Li+K1) (Supplementary Fig. 63). Among these, the device doped with allyl isonicotinate (Li, Co+K10, Supplementary Fig. 21, Supplementary Table 22) retained 89% of its initial PCE after 1104 hours in the thermal stability test. Several factors may contribute to these improvements. Regarding additive volatility, the vapor pressure of 4-ethylpyridine (K1) is expected to be lower than that of tBP due to the size of its hydrocarbon group and Antoine constants, indicating that the thermal stability of PSCs with tBP is higher than 4-ethylpyridine (K1)98. However, the thermal stability of PSCs with tBP was significantly lower than that of 4-ethylpyridine (Li+K1), suggesting that additional factors influence the thermal stability of these devices.

As for the electronic effects of substituents, substituents of 4-propylpyridine (K2) and 4-tert-butylpyridine (tBP) exhibit weak electron-donating properties, while 4-aminopyridine (K6) and 4-acetylpyridine (K7) display strong electron-donating and electron-withdrawing effects, respectively99. While methyl isonicotinate (K8) is a weak electron-withdrawing. These electronic properties may locally alter the electron density within the pyridine ring. However, no clear trend correlating these effects with the thermal stability of PSCs was observed.

In terms of pyridine substituted at the meta and/or ortho positions (Fig. 1b, Supplementary Fig. 64), devices incorporating 2,4,6-trimethylpyridine (Li+K14), 2,6-dimethoxypyridine (Li+K13), and 3,5-lutidine (Li+K16) demonstrated improved thermal stability compared to the device with tBP (Supplementary Figs. 26, 28, 32, Supplementary Tables 27, 29, 33). Notably, dopants with simple chemical structures, such as 2,4,6-trimethylpyridine (K14), exhibited greater durability than tBP. The overall trend observed for substituents at the ortho, meta, and para positions indicated minimal changes in JSC and VOC across devices during the thermal stability test, except in cases with poor stability. The FF appeared to be the primary parameter associated with the thermal stability of PSCs in these systems.

As for para-substituted pyridine derivatives containing a phenyl or hetero ring (Fig. 1c, Supplementary Fig. 65), devices incorporating 4-phenylpyridine (Li+K27) demonstrated significantly enhanced thermal stability (Fig. 2b, Supplementary Figs. 45, 60, Supplementary Table 49). PCE retained 87% of the initial value (PCE_Initial = 17.8%) after 1,104 hours of thermal test at 85 °C. In contrast, the thermal stability of devices with other additives was markedly low (Supplementary Fig. 65). Interestingly, despite 4-benzylpyridine (Li+K21) having only one additional alkyl chain linking the pyridine and phenyl rings compared to 4-phenylpyridine (Li+K27), its thermal stability was significantly reduced (Supplementary Fig. 36, Supplementary Table 39). This suggests that the steric hindrance of the additive may play a critical role in thermal durability. Moreover, in the case of the co-doping with LiTFSI, Co(III)TFSI, and 4-phenylpyridine (Li, Co+K27), thermal stability was significantly decreased. This can also be related to the steric hindrance influenced by the cobalt complex which is much larger than lithium. These findings highlight the importance of interaction between additives and counterparts of TFSI in the HTL.

Regarding phenyl-substituted pyridines at the para, meta, or ortho positions (Fig. 1d) and their substitution (Fig. 1e), remarkable improvements in thermal stability were observed (Fig. 2a, b, Supplementary Fig. 66). Notably, the PCE of devices containing 3-phenylpyridine (Li+K28) and 2-phenylpyridine (Li+K29) were 101% and 104% of their initial PCE after 2,424 hours of 85 °C test, respectively (Supplementary Fig. 47, 48, 62, 63, 66, Supplementary Tables 51, 52), demonstrating a slight improvement in the PCE. Both VOC and FF remained virtually unchanged during the 85 °C test. For 2-(p-tolyl)pyridine (Li+K30), the stability was comparable to that of 2-phenylpyridine (Li+K29), indicating that small substituents like a methyl group do not adversely affect the thermal stability of PSCs (Supplementary Fig. 49, Supplementary Table 53). Thermal stability further improved when the phenyl group was substituted from the para to the meta or ortho position (Supplementary Fig. 70). However, 4-Phenylpyridine showed higher PCE than 2-phenylpyridine and 3-phenylpyridine for approximately 1,100 hours from the start of the 85 °C stability test. This suggests that 4-phenylpyridine has an advantage in initial PCE, which may contribute to greater overall energy output during the degradation period. For example, if 1000 hours accelerated test at 85 °C is sufficient to ensure practical outdoor durability, 4-phenylpyridine could be more advantageous in practical use compared to 2-phenylpyridine and 3-phenylpyridine. Therefore, it is premature to conclude which molecular orientation is the most superior. We also provided stability comparison data of dispersed-solution-SnO2 (SnO2_DS) vs chemical bath deposition (SnO2_CBD) as well as CsFAPbI3 vs RbCsFAPbI3 (Supplementary Fig. 71). To provide high-efficiency PSCs, we optimized the recipe for perovskite fabrication for each condition, which is explained at the experimental part. In terms of dispersed-solution SnO2 (SnO2_DS) vs. CBD SnO2 (SnO2_CBD), the initial efficiency of SnO2_CBD with 2-phenylpyridine was 20.1% and remained at 18.7% after 936 hours of testing at 85 °C. In contrast, SnO2_DS with 2-phenylpyridine showed an initial efficiency of 14.9% and retained 16.2% after 936 hours at 85 °C. In both cases, 2-phenylpyridine provided higher stability compared with 4-tert-butylpyridine. As for CsFAPbI3 vs RbCsFAPbI3, rubidium-doped perovskite (RbCsFAPbI3) achieved higher efficiency (24.2%) than CsFAPbI3 (20.1%), however, stability with RbCsFAPbI3 showed a significant drop after 87 hours of 85 °C test. From this result, It is possible that rubidium-doped perovskite itself has problems with thermal durability. Correa-Baena et al. indicated that a dosage of 1% Rb was enough for causing segregation, which is a different kind of issue from those associated with additives100. These results suggest that optimization of perovskite composition is also important to achieve high thermal stability. A detailed analysis of this phenomenon is provided in the characterization section.

For other heteroaryl additives (Fig. 1f, Supplementary Fig. 67), 1-phenylimidazole (Li+K32) and 3-phenylthiophene (Li+K31) exhibited superior thermal stability compared to 4-tert-butylpyridine (Li+tBP) (Supplementary Figs. 50, 51, Supplementary Tables 54, 55). 1-Phenylimidazole (K32) and 3-phenylthiophene (K31) can interact with lithium via the lone pairs on the nitrogen at the 3-position and sulfur at the 1-position, respectively. Given sulfur’s lower electronegativity compared to nitrogen, its interaction with lithium is expected to be weaker. Despite these differences in interaction strength, no notable differences in photovoltaic performance or thermal stability were observed between the two additives.

Overall, these results suggest that various heteroaryl derivatives can function as effective additives for Spiro-OMeTAD, enhancing thermal stability relative to tBP (Fig. 2c). Notably, phenyl groups exhibited exceptional improvements in thermal stability, highlighting the need for further investigation into their specific role, as detailed in the following section.

To examine the impact of additives on void formation in the HTL, SEM measurements were conducted on both as-fabricated devices and those after the 85 °C thermal stability test (Fig. 3). Cross-sectional SEM images of the as-fabricated devices with 4-tert-butylpyridine (Li+tBP), 4-phenylpyridine (Li+K27), and 2-phenylpyridine (Li+K29) are presented in Fig. 3a–c, respectively. In the case of the device containing 4-tert-butylpyridine (Li+tBP), small voids were observed within the Spiro-OMeTAD layer (Fig. 3a). These voids are likely attributed to the post-annealing process temperature (85 °C, 10 min) following HTL deposition, as described in the experimental section. Conversely, the devices containing 4-phenylpyridine (Li+K27) and 2-phenylpyridine (Li+K29) exhibited void-free HTLs, as shown in Fig. 3b and 3c. Cross-sectional SEM images of the devices after 400 hours of thermal stability test at 85 °C are shown in Fig. 3d-f. For the device with 4-tert-butylpyridine (Li+tBP), significantly larger voids were observed within the Spiro-OMeTAD layer (Fig. 3d, Li, Co+tBP, Supplementary Fig. 72). These voids appeared to expand during thermal testing, extending to the HTL/perovskite and HTL/Au interfaces. This extensive structural transformation likely explain the observed decrease in the FF during the thermal stability test, leading to the poor thermal stability of PSCs. In contrast, the device containing 4-phenylpyridine (Li+K27) exhibited reduced void formation, though several small voids persisted within the Spiro-OMeTAD layer (Fig. 3e). Additionally, a thin layer of by-products may have formed at the interface between the Spiro-OMeTAD and perovskite layers. These small voids and/or by-products may contribute to the gradual decline in FF and PCE observed during the thermal stability test (Supplementary Fig. 45). Notably, the device with 2-phenylpyridine (Li+K29, HTL thickness > 200 nm) demonstrated significant suppression of void formation in the Spiro-OMeTAD layer (Fig. 3f). Clear interfaces between the HTL/perovskite and HTL/Au layers were also observed. The boiling points are 274 °C for 3-phenylpyridine, 270 °C for 2-phenylpyridine, and 197 °C for 4-tert-butylpyridine, respectively. These values are consistent with the trends observed in Fig. 3. This void-free HTL facilitated by 2-phenylpyridine (Li+K29) can be responsible for the exceptional thermal stability of PSCs under prolonged thermal testing (Fig. 2b). Furthermore, we performed optical microscopy and AFM measurements of the HTLs before and after thermal aging (85 °C for 100 hours, Supplementary Figs. 73, 74). After thermal aging, the HTL with 4-tert-butylpyridine exhibited pin-hole formation and cracks originating from crystalline-like regions. These results are consistent with previous reports96. The depths of the pin-holes and cracks in the crystalline-like regions were in the range of 50–150 nm. Such defects are expected to deteriorate the electrical properties of the HTLs as well as the interfaces between HTL/Au and Perovskite/HTL. In contrast, the HTLs with 4-phenylpyridine and 2-phenylpyridine showed no significant morphological changes after the 85 °C test. These observations suggest that 4-phenylpyridine and 2-phenylpyridine effectively suppress pin-hole and crack formation in crystalline-like regions, which account for their higher thermal stability. We also examined the dissolution effect of the additives on lithium salts. Since the additives promote the dissolution of lithium salts and prevent the agglomeration of lithium salts, this may be a key factor affecting device performance and thermal stability. Lithium salts were dissolved in Spiro-OMeTAD solutions containing each additive. Supplementary Fig. 75 presents the relationship between thermal stability (PCE after 1000 h) and the solubility of lithium salts. tBP and 3-phenylpyridine (K28) dissolved 62.5 mg/mL of lithium salts, whereas 4-phenylpyridine (K27) and 2-phenylpyridine (K29) dissolved 55.6 mg/mL and 11.4 mg/mL, respectively. These results show no clear correlation between thermal stability and lithium salt solubility, suggesting that other mechanisms contribute to thermal stability.

The XRD patterns (Fig. 4a, b) were analyzed to evaluate the reactivity between the additives and the perovskite layer, comparing before and after 85 °C tests. Samples were prepared same as the device structure of FTO glass/SnO2/perovskite (PVK)/Spiro-OMeTAD (+ additives)/Au (Supplementary Fig. 76). Au layer was peeled out just before XRD measurement. Additives of 4-tert-butylpyridine (tBP), 4-phenylpyridine (K27), 3-phenylpyridine (K28), or 2-phenylpyridine (K29) were used for this experiment. For all the samples, peaks appeared at 13.9°, 19.8° and 24.3°, which are associated with (100), (110), and (111) of the perovskite crystal, respectively. Before 85 °C test (Fig. 4a), the sample with 4-phenylpyridine (K27) showed a small peak at 6.9° that could be from PbI2-additives complex91. After 85 °C tests, samples with 4-tert-butylypyridine (tBP) and 4-phenylpyridine (K27) exhibit a clear peak at 6.9° that could be from PbI2-additives complex. These are possibly harmful by-products leading to the instability of PSCs. It seem that these by-products themselves are not the primary reason for poor thermal stability, but it can be the secondary reason. Interestingly, samples with 2-phenylpyridine (K29) and 3-phenylpyridine (K28) appear no peak at 6.9°, but small peaks were observed at 12.6 corresponding to PbI2. These results indicate that 2-phenylpyridine and 3-phenylpyridine have low reactivity with the perovskite layer. This lack of reactivity is attributed to the steric hindrance introduced by the meta and ortho substitutions, which can prevent coordination with PbI2, thereby contributing to their excellent thermal stability (Fig. 2b).

The impact of additives on carrier dynamics was further examined using steady-state PL and time-resolved photoluminescence (trPL), as shown in Figs. 4c and 4d, respectively. Samples were prepared as glass/perovskite/Spiro-OMeTAD with each additive. The PL spectrum of the perovskite-only sample exhibited a prominent peak, indicating minimal non-radiative pathways, consistent with the extended carrier lifetime measured by trPL (τb = 5806.1 ns, Supplementary Table 62). In contrast, the sample with 4-tert-butylpyridine (PVK/tBP) showed quenched PL emissions and a reduced carrier lifetime (τb = 43.7 ns), attributed to charge transfer from the perovskite layer to the Spiro-OMeTAD layer. Notably, this quenching was significantly accelerated in samples incorporating 4-phenylpyridine (PVK/K27, τb = 18.5 ns) and 2-phenylpyridine (PVK/K29, τb = 13.2 ns). These results suggest enhanced charge transfer facilitated by these additives, potentially leading to improved photovoltaic performance in PSCs compared to tBP. The influence of this accelerated charge transfer on PSC performance is discussed in subsequent sections.

The interaction between additives and lithium was analyzed using DFT calculations (Fig. 4e-l). Figure 4e-h illustrates the coordination of 4-tert-butylpyridine (tBP) with lithium, 4-phenylpyridine (K27) with lithium, 3-phenylpyridine (K28) with lithium 2-phenylpyridine (K29), respectively, and Fig. 4i-l are corresponding structures viewed from different angles. All additives were found to form a tetrahedral arrangement as the most stable configuration, all with point group S4. 4-tert-Butylpyridine (tBP) and 4-phenylpyridine (K27) were the similar geometries. Interestingly, 3-phenylpyridine exhibited a slightly more compact structure than 4-phenylpyridine (K27) due to geometric distortion induced by its molecular orientation. Furthermore, 2-phenylpyridine (K29) formed a significantly more compact coordination structure compared to the other additives, in which the lithium ion appeared to be tightly surrounded. The minimum edge lengths of the smallest boxes that could enclose each molecule were determined to be 10.93 Å for 4-tert-butylpyridine (tBP), 12.16 Å for 4-phenylpyridine (K27), 8.01 Å for 3-phenylpyridine (K28), and 9.06 Å for 2-phenylpyridine (K29), respectively. This difference in coordination may be related to the lower volatility and absence of voids observed in Spiro-OMeTAD with 2-phenylpyridine under the 85 °C test conditions. When three molecules of each additive were coordinated with lithium, an energetically favorable state of 2-phenylpyridine, 3-phenylpyridine, and 4-phenylpyridine was point group C3, while tBP was D3, respectively. The desorption energy required to remove a coordinated molecule from lithium was calculated to be 0.667 eV for tBP, 0.659 eV for K27, 0.680 eV for K28, and 0.100 eV for K29, indicating that there is no relationship between thermal stability and ease of molecular desorption. Supplementary Figs. 77a, b depict the interactions between the coordinated additive-lithium complexes and Spiro-OMeTAD. The influence of π-interactions between the methoxyphenylethylamine group in Spiro-OMeTAD and the pyridine rings of the additives was evaluated. Calculations show that a positive formation energy is required for interaction between tBP and K27 and Spiro-OMeTAD to happen because steric effects require a large energy to deform Spiro-OMeTAD to allow such interactions to happen. These results are consistent with the X-ray photoelectron spectroscopy, which shows no significant chemical shift (Supplementary Fig. 78). These results suggest that π-interactions between the additives and Spiro-OMeTAD do not affect the overall stability or performance of the system.

We measured Fourier-transform infrared spectroscopy (FTIR) to investigate the molecular interactions (Supplementary Fig. 79). Peaks at 1061 cm⁻¹ and 1349 cm⁻¹ were observed, which correspond to the S=O stretching vibration of TFSI63,101. Interestingly, these peaks were shifted to higher wavenumber in the cases of 4-phenylpyridine and 2-phenylpyridine. This shift could be attributed to the stronger coordination of 4‑phenylpyridine and 2‑phenylpyridine with lithium, which reduces TFSI coordination and thereby strengthens the S=O bond.

For further understanding of the mechanism of the void-formation, we investigated the effect of additives on elemental distribution using ToF-SIMS (Supplementary Fig. 80). The measured samples were prepared with the configuration FTO/SnO2/Perovskite/Spiro-OMeTAD (with additives)/Au, following the same procedure as device fabrication. The Au layer was peeled off immediately before the ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-phenylpyridine before and after thermal aging at 85 °C for 100 hours. Lithium migrated toward the ETL side and slightly accumulated at the surface, which is consistent with a previous report89. The lithium distribution was not significantly affected by the choice of dopant, nor by the thermal test. From these results, 2-phenylpyridine does not appear to promote a more homogeneous lithium distribution. For the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-OMeTAD layer (Supplementary Fig. 80a). After the 85 °C test, however, 4-tert-butylpyridine had migrated substantially into the perovskite layer (Supplementary Fig. 80b). This migration is likely driven by its reaction with the perovskite, which is consistent with the by-product confirmed by XRD (Fig. 4b). In contrast, 2-phenylpyridine was mainly localized in the Spiro-OMeTAD layer, and its distribution showed no significant change after the 85 °C test (Supplementary Figs. 80c, d). This stability may contribute to its beneficial effect on thermal stability, probably because 2-phenylpyridine does not react with the perovskite and therefore does not diffuse into the perovskite layer. These ToF-SIMS results may explain the absence of voids in the 2-phenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-butylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have contributed to void formation within Spiro-OMeTAD (Supplementary Figs. 81a, b). By contrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the stability of its distribution (Supplementary Figs. 81c, d).

Additionally, to evaluate the impact of additives on photovoltaic performance, PSCs were optimized for high efficiency (Fig. 5a-e, Supplementary Figs. 82, 83, Supplementary Table 63). These optimizations included OAI passivation and a SnO2 layer deposited via chemical bath deposition to enhance device performance. The PCE of devices incorporated with 4-phenylpyridine (Li+K27) was 23.2% which was higher than that of the tBP (22.4%), primarily due to improvements in both VOC and FF. Notably, devices incorporating 2-phenylpyridine (Li+K29) achieved a remarkable PCE of 25%, accompanied by an enhanced VOC of 1.165 V and FF of 0.82, with reduced hysteresis. These improvements are consistent with the fast-charge extraction measured by trPL (Fig. 4d). The exited carrier was extracted from the perovskite to HTL before undergoing carrier recombination, resulting in higher VOC, FF, and PCE. This demonstrates the potential for achieving both high durability and conversion efficiency in the Spiro-OMeTAD system. We also examined whether the concentration of Spiro-OMeTAD influences device performance by comparing 30 mg/ml and 120 mg/ml solutions (Supplementary Fig. 84, 85). The results indicate that the higher concentration (120 mg/ml) improves reproducibility without significantly increasing resistance. UV–vis absorption spectroscopy was performed to evaluate the potential influence of the additives on the oxidation of the hole-transport layer (Supplementary Fig. 86). The UV–vis spectra showed no appreciable differences among the samples, suggesting that neither 4-phenylpyridine nor 2-phenylpyridine has a measurable effect on the oxidation process. Furthermore, energy level diagrams of Spiro-OMeTAD with additives were constructed based on photoelectron yield spectroscopy and Kelvin probe force microscopy (KPFM) (Supplementary Figs. 87–89). The valence band edge (Ev) of Spiro-OMeTAD with 4-tert-butylpyridine was −5.58 eV. In contrast, the Ev values of Spiro-OMeTAD with 4-phenylpyridine, 3-phenylpyridine, and 2-phenylpyridine were −5.76 eV, −5.76 eV, and −5.83 eV, respectively, which are closer to that of perovskite (−5.89 eV) than the value obtained with 4-tert-butylpyridine. This alignment is likely to facilitate more efficient charge transfer, thereby contributing to improved photoconversion efficiency. The resistivity of Spiro-OMeTAD layers containing LiTFSI and each additive was 19,100 Ω cm for 4-tert-butylpyridine, 8540 Ω cm for 4-phenylpyridine, 6050 Ω cm for 3-phenylpyridine, and 2680 Ω cm for 2-phenylpyridine, respectively (Supplementary Fig. 90). These results suggest that 2,3,4-phenylpyridine has an advantage in the resistivity compared to 4-tert-butylpyridine, resulting in improved FF and PCE. Furthermore, mini modules incorporating 4-phenylpyridine achieved a PCE of 21.2% (PCE_active area = 18.3% with 1.664 cm2 of aperture area), indicating that this strategy is scalable to larger device areas. Outdoor performance testing (Fig. 5f and 5g) revealed stable device operation with 2-phenylpyridine (Li+K29), with minor distortions in performance curves on e.g. 3rd of January due to intermittent cloud cover (Supplementary Fig. 91). In the case of tBP, the maximum power point voltage of the device continuously decreased to 81% of its initial value after 1,248 hours (Supplementary Fig. 92). As for 2-phenylpyridine, maximum power point voltage dropped to about 90% of its initial value within 300 hours from the start of the experiment, and no further voltage drop occurred for the till 1570 hours (90% of maximum power point voltage at 1570 hours). Importantly, 94% of output power was observed from the initial power after 1570 hours of outdoor testing, which exhibits remarkable light and cycle stability. From the above experiment results, it was found that additives that improve the thermal stability of PSCs have superior characteristics to conventional materials in terms of conversion efficiency and outdoor durability.

In conclusion, we demonstrated that incorporating additives into Spiro-OMeTAD can effectively address the thermal stability issue. Our key findings are that the heteroaryl derivatives with phenyl group control void formation in the Spiro-OMeTAD layer and minimize reactivity with the perovskite layer, significantly enhancing the thermal stability of PSCs under 85 °C conditions. The PSCs incorporated with 3-phenylpyridine (Li+K28) and 2-phenylpyridine (Li+K29) maintained 101% and 104% of their initial PCE after 2,400 hours of 85 °C test, respectively, showcasing their remarkable durability. Additionally, the additives improved charge extraction, contributing to enhanced photovoltaic performance with a PCE of 25%. Also, the outdoor test shows 90% of the initial power point voltage after 1570 hours with continuous MPPT measurement, showing remarkable light and cycle stability. A detailed investigation of 38 heteroaryl derivatives and 60 additive combinations reveals the underlying mechanisms that enable these improvements, offering a pathway toward thermally stable, light-stable, and high-performance PSCs with thick HTL (>200 nm), which is suitable for scalable fabrication with blade coating.

Methods
Materials
Cesium iodide, lead iodide, formamidinium iodide, heteroaryl derivatives, tin(IV) chloride, and thioglycolic acid were purchased from TCI. n-octylammonium iodide was purchased from GreatCell Solar. Urea was purchased from Fujifilm Wako. The FTO glass substrates of TEC-10 and VU-glass were purchased from Nippon Sheet Glass and AGC, respectively. Chlorobenzene, bis(trifluoromethane)sulfonimide lithium salt, rubidium iodide, and the cobalt-complex (FK209) were purchased from Aldrich. DMF, DMSO, isopropanol, and acetonitrile were purchased from Aldrich. Methylammonium chloride was purchased from Xi’an Yuri Solar. SnO2 nanoparticle dispersed solution (S-8) was purchased from Taki Chemical. Anti-reflection film (Mosmite) was provided by Mitsubishi Chemical Group. All chemicals were used as received without further purification.

Device fabrication
Encapsulated PSCs were fabricated by the following steps. Devices were fabricated under ambient dry air (dew point = −25~−40 °C). The FTO glass (TEC-10) was UV/O3 treated for 30 minutes. The FTO glass was etched by laser. Then, 200 µL of the SnO2 dispersed solution (S-8) was spin-coated on the top of the FTO substrate at 3000 rpm for 20 s. These substrates were annealed at 100 °C for 5 min and 150 °C for 30 min sequentially. For the perovskite deposition, a perovskite precursor composed of CsFAPbI3 as prepared by dissolving 556.7 mg of lead iodide, 180.6 mg of formamidinium iodide, 13.6 mg of cesium iodide and 7.1 mg of methylammonium chloride in a solution of DMSO (300 μL) and DMF (700 μL). The Spiro-OMeTAD precursor solution was prepared by dissolving 120 mg of Spiro-OMeTAD in 1000 µL chlorobenzene, 70.2 µL 4-tert-butylpyridine, 41.4 µL of bis(trifluoromethane)sulfonimide lithium in acetonitrile solution (99 mg of LiTFSI + 263 µL of acetonitrile) and/or 32.4 µL of tris(2(1H-pyrazol-1-yl)−4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] in acetonitrile solution (196 mg of LiTFSI + 379 µL of acetonitrile). In the case of other heteroaryl derivatives, the same molar amount was added to the Spiro-OMeTAD solution instead of 4-tert-butylpyridine. Thereafter, two-step spin-coating of perovskite precursor was accomplished through a two-step program running at 0 rpm (loading for solution), 1000 rpm, and 6000 rpm for 10 s, 10 s (+2 s acceleration), and 30 s (+2 s acceleration), respectively. At 48 s of the spin-coating program, 200 μL of chlorobenzene was dispensed. The substrates were then transferred for annealing at 140 °C for 5 min. Then, hole transport layer was spin-coated (4000 rpm, 20 s) onto the perovskite layer then transferred for annealing at 85 °C for 10 min for dying. Laser scribing was performed to remove the unnecessary perovskite layer and to make contact. Finally, a 70 nm gold contact layer was deposited by a thermal evaporation method. The devices were encapsulated with a glass cap, UV-glue, and getter seal.

For high-efficiency PSCs, VU glass was used instead of the TEC-10. The chemical bath deposition (CBD) method was used for SnO2 deposition instead of SnO2 nano-particle (S-8). CBD solution was prepared with 1100 mg of tin(II) chloride, 5000 mg of urea, 100 µL of thioglycolic acid, 5000 µL of hydrochloric acid and 400 mL of deionized water. Substrates were dipped in the CBD solution and annealed at 90 °C for 5 hours, followed by sonification with deionized water for 5 min and isopropanol for 5 min. Substrates were annealed at 170 °C for 1 hour, then UV/O3 cleaned for 30 min. For the perovskite deposition, a perovskite precursor composed of CsFAPbI3 as described before or RbCsFAPbI3 prepared by dissolving 594.0 mg of lead iodide, 180.6 mg of formamidinium iodide, 13.6 mg of cesium iodide, 7.1 mg of methylammonium chloride, 6.7 mg of rubidium iodide in a solution of DMSO (300 μL) and DMF (700 μL). After perovskite deposition, 9.8 mg of n-octylammonium iodide (OAI) in 1 mL of isopropanol was spin-coated dynamically on the perovskite for 20 seconds at 4000 rpm. IPA (50 µL) was spun on samples (4000 rpm, dynamic) for stability measurement devices. Then samples were annealed at 100 °C for 5 min. After Spiro-OMeTAD deposition, samples were annealed at 50 °C. After Au deposition, an anti-reflection film was attached to the devices. Other procedures (e.g., Spiro-OMeTAD solution, spin-coating program) are the same as encapsulated PSCs fabrication as described above. Note that regarding devices with tBP, the MACl concentration of the perovskite layer was increased from 10% to 20%, which is an optimized condition for tBP to achieve high efficiency.

For the mini-module fabrication, a laser scribing system was used for P1, P2, and P3 scribing. The width of P1, P2, and P3 was 100 μm, and the margin between P1-P2 and P2-P3 was 100 μm. Pulse width was 1 picosecond. The wavelength of the laser was 1030 nm. Butylammonium iodide + octylammonium iodide (BAI+OAI) passivation was incorporated according to a previous report97. 4-Phenylpyridine was used as an additive in Spiro-OMeTAD.

Devices for the outside were prepared with encapsulated PSCs. 2-Phenylpyridine was used as an additive in Spiro-OMeTAD. OAI passivation was incorporated. After the fabrication of PSCs, devices were stored at 85 °C for 40–50 hours for stabilization purposes. Butyl seal was used for additional encapsulation.

Characterization
AM1.5 pseudo-solar light (100 mW cm−2) was generated using a solar simulator equipped with a xenon lamp (Bunkou Keiki). Standard silicon solar cells (BS-521BK, Bunkou Keiki) were used to calibrate the incident light intensity. The I–V measurements were performed by using a solar simulator, carried out from 1.2 to 0 V and from 0 to 1.2 V as reverse and forward scans, respectively, using a mask of 0.0887 cm2. The scanning step and speed for solar cells were 10 mV and 100 mV s−1, respectively. The scanning step and speed for mini modules were 50 mV and 500 mV s−1, respectively. IV curves of devices with or without encapsulation were measured at room temperature in air. Thermal stability tests were performed for encapsulated devices in dark in air without voltage bias. Samples were kept in the electrical oven at 85 °C. Preconditioning of the device was not applied before the measurement. The active area and aperture area for mini modules were 1.44 cm2 and 1.664 cm2, respectively. The active and aperture area for mini modules was determined by the active area excluding or including P1, P2, and P3 area, respectively, and mini modules were measured without a mask. SEM images were obtained using a field emission scanning electron microscope (HITACH corporation, SU9000). X-ray diffraction (XRD) analysis was carried out using a D8 Advance diffractometer (Rigaku, SmartLab) with Cu Kα radiation (λ=1.5418 Å). Samples for PL measurement were prepared with glass/perovskite/additives. Additives were deposited by spin-coating (4000 rpm for 20 s) with the additive solution (70.2 μL of 4-tert-butylpyridine in 300 μL of chlorobenzene) on the perovskite layer. In the case of 4-phenylpyridine, 3-phenylpyridine, or 2-phenylpyridine samples, the same molar amount of 4-tert-butylpyridine was added in 300 μL of chlorobenzene. Substrates were annealed at 85 °C for 10 min. The PL lifetimes were measured using a time-correlated single photon counting system (Fluorolog-QM, Horiba) with an excitation at 634 nm. The PL emission was measured with an excitation at 640 nm. Samples for PL measurement were prepared with glass/perovskite/Spiro-OMeTAD with additives. Samples were annealed at 85 °C for 10 min. X-ray photoelectron spectroscopy (XPS) was performed by ULVAC-PHI PHI5000 VersaProbe. Samples for XPS were prepared as FTO/SnO2/perovskite/Spiro-OMeTAD with additives. Samples were annealed at 85 °C for 10 min. IPCE measurement was performed by CPE-2000 (Bunkoukeiki Co. Ltd). Samples were prepared with anti-reflection film. Outside performance was measured by the maximum-power point tracking system (24-channel μMPPT, Laboratory of Photovoltaics and Optoelectronics (LPVO)). The active area of the device was 5 mm × 5 mm. Samples were set on the outside stage without the mask. The angle of the outside stage was 20 degrees and faced south. Samples were connected by a 4-terminal-method. Short-term MPPT under the solar simulator was measured by the MPPT system by SYSTEM HOUSE SUNRISE. UV-vis was measured by MSV-5800 Microscopic Spectrophotometer (JASCO). Depth profiles of ToF-SIMS were performed by Toray Research Center with a GCIB beam with sample cooling. Resistivity was measured with the van der Pauw method. The Spiro-OMeTAD layers were deposited on glass substrates by spin-coating. Spiro-OMeTAD solutions were prepared using the same procedure as in the device fabrication. FTIR was measured by IRT-5200 Irtron Infrared Microscope (JASCO). Photoelectron yield spectroscopy (PYS) was measured by an ionization energy measurement system (BIP-KV100-WB, Bunkoukeiki). AFM/KPFM measurements were performed using an AFM100 (HITACHI). A rhodium-coated cantilever tip was employed, with an associated work function of 5.4 eV. The optical microscope was measured by the laser microscope VK-X3100 (KEYENCE).

Calculations
Calculations were conducted using the Gaussian 16 code. The B3LYP/6-31++G(d,p) basis set was used with the Opt=Tight convergence criterion. The net electric charge was +1, and the spin multiplicity was 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript presents a comprehensive study on the incorporation of heteroaryl derivatives as additives in Spiro-OMeTAD to enhance the thermal stability and photovoltaic performance of perovskite solar cells (PSCs). The work is well-structured and tackles a crucial challenge in the field of PSCs, namely, thermal instability. The experimental findings are promising, particularly regarding the remarkable thermal stability achieved with 3-phenylpyridine and 2-phenylpyridine additives. However, several critical issues must be addressed to improve the clarity, rigor, and overall impact of the study. I recommend major revisions before the manuscript can be considered for publication.\n1. Selection Criteria for Heteroaryl Derivatives\nThe study investigates 38 heteroaryl derivatives; however, the rationale behind the selection of these specific compounds is not clearly justified. A discussion on the selection criteria—such as volatility, steric hindrance, electronic properties, or previous literature reports—would provide valuable context and strengthen the study’s foundation.\n2. Fill Factor (FF) Discrepancy\nThe device with a passivation layer and 4-tert-butylpyridine (tBP) as an additive exhibits an FF below 70%, which appears significantly lower than commonly reported values. The authors should provide an explanation for this discrepancy.\n3. Comparison of Stability Among Additives\nThe claim that A81 (3-Phenylpyridine) and A82 (2-Phenylpyridine) offer superior stability compared to A15 (4-Phenylpyridine) is not convincingly demonstrated, as the initial efficiencies of these devices differ significantly. From an absolute efficiency standpoint, A15 may contribute to greater overall energy output during aging. This needs to be addressed with additional discussion or data normalization.\n4. Void Formation in the HTL\nThe manuscript attributes void formation in the hole transport layer (HTL) to the use of low-boiling-point additives such as 4-tert-butylpyridine, which is reasonable. However, the observation that A82 (2-Phenylpyridine) does not induce void formation, while A15 (4-Phenylpyridine) does, requires further explanation. The authors should clarify whether they have investigated the thermodynamic properties of complexes formed between these additives and LiTFSI. Additionally, have they measured the glass transition temperature (Tg) of HTLs containing different additives? This data could provide insight into differences in void formation behavior.\n5. Reactivity Between Perovskite and Pyridine Derivatives\nThe study reports that spin-coating pyridine derivatives directly onto perovskite films leads to degradation. However, in standard device fabrication, spin-coating HTL solutions containing 4-tert-butylpyridine does not appear to significantly impact device performance. The authors should clarify whether they observed the emergence of new XRD diffraction peaks in aged devices, similar to those found in direct pyridine spin-coating experiments. Furthermore, the claim that steric hindrance differences between 4-phenylpyridine and 3-phenylpyridine significantly impact coordination behavior is not well-supported. The authors should provide further discussion or computational evidence to substantiate this point.\n6. Typographical Errors\nMinor typographical errors exist throughout the manuscript, such as \"phyenylpyridine\" instead of \"phenylpyridine\" in the Figure 3 captions. A thorough proofreading is recommended to improve readability and ensure accuracy.", "reviewer_2": "The authors have done a substantial amount of work comparing the effects of different tBP substitutes on improving the thermal stability of doped Spiro-OMeTAD. This is a commendable effort. Although the author provides extensive thermal stability test data demonstrating the enhancement of device thermal stability with 3-phenylpyridine and 2-phenylpyridine, the explanation of the mechanism is quite lacking. More characterisation and analysis are needed to substantiate the role of 3-phenylpyridine and 2-phenylpyridine. Hope the following questions can be addressed in the revised manuscript.\n1. For the thermal stability, SnO2 dispersed solution was used to fabricate SnO2 carrier-transport-layer and CsFAPbI3 was used to fabricate perovskite, whereas for the efficiency part, the carrier-transport-layer was changed to CBD SnO2, and the perovskite was changed to RbCsFAPbI3 with excess PbI2 in precursor. It is necessary to provide the stability comparison data of highly efficient PSCs (dispersed-solution-SnO2 vs CBD, CsFAPbI3 vs RbCsFAPbI3 with excess PbI2).\n2. It is necessary to provide the IPCE data and integrated current density of highly efficient PSCs shown in Fig.5c.\n3. Solely using DFT to confirm the interaction between additives, lithium and Spiro-OMeTAD is not sufficient. Fourier-transform infrared spectroscopy (FTIR) is needed to evidence the presence of interaction.\n4. Traditional Spiro-OMeTAD HTL suffers crystallization after thermal aging which worsens the electrical properties of HTL. Therefore, it is necessary to provide AFM and microscope photographs of the HTLs before and after thermal aging.\n5. Additives could influence the oxidation of hole-transport layer. In this work, does the best two additives influence the oxidation/doping progress of hole-transport layer? Accelerate or postpone it? It is necessary to provide UV–vis absorption data to analyse the HTL before and after oxidation. UPS/KPFM is also necessary to investigate the associated working function/energy level changes.\n6. Lithium salt was observed to accumulate at interfaces (Perovskite solar cells based on spiro-OMeTAD stabilized with an alkylthiol additive Nature Photonics, 2023, 17, 96-105). Are the additives proposed in this work helpful to obtain homogenous dopant distribution? Also, it is important to locate the position of additives. Is it homogenously distributed throughout the HTL? Therefore, time-of-flight secondary-ion mass spectrometry (TOF-SIMS) or other characterization methods which can clearly demonstrate the out-of-plane component distribution is important to help readers understand the benefits of new the additives proposed in this work.\n7. The poor thermal stability of doped Spiro-OMeTAD is not solely attributed to the additives (such as Li, tBP); it may also be related to intrinsic factors of Spiro-OMeTAD itself. For instance, its low Tg makes it susceptible to molecular segmental motion, and non-crystalline Spiro-OMeTAD can undergo local recrystallization at high temperatures. Additionally, the diffusion of I- from the perovskite into HTL can also lead to the failure of Spiro-OMeTAD. Given this, why does the substitution of tBP with 3-phenylpyridine and 2-phenylpyridine result in a significant improvement in thermal stability? Please provide more evidence to determine whether 3-phenylpyridine and 2-phenylpyridine influence the factors mentioned above.\n8. What are the boiling points of 3-phenylpyridine, 2-phenylpyridine, and tBP? Could the differences in the results shown in Figure 3 be due to the different boiling points of the three materials?\n9. Please provide the AFM images of Spiro-OMeTAD doped with the three additives, Li+3-phenylpyridine, Li+2-phenylpyridine, and Li+tBP, before and after heating, in order to observe the behavior of Li+ before and after heating.\n10. For stability, the thickness of Spiro-OMeTAD (120 mg/ml) has been increased in this manuscript. However, thicker Spiro-OMeTAD tends to increase the resistance of the HTL layer. Any trade-off benefit here?", "reviewer_3": "In this manuscript, the authors consider tBP as the key factor for the instability of Spiro-OMeTAD-based devices at high temperatures. They employed a large number of heteroaryl derivatives to verify which materials could improve thermal stability. Finally, the authors proposed that three types of derivatives have a positive effect on thermal stability, and based on this, devices were fabricated, which showed good stability in outdoor stability tests. Overall, the authors are thanked for their extensive work and experiments in screening suitable derivatives, and for providing reliable efficiency aging data instead of the normalized data in most literature. The authors also provided some mechanistic explanations to better understand the impact of such additives on the stability of Spiro-OMeTAD. However, despite the numerous experiments and tests conducted by the authors, the research results are not persuasive enough, and the mechanistic explanations lack new ideas, failing to arouse greater interest among readers. In addition, there are some logical issues in this research. Therefore, I do not recommend further consideration of this manuscript for publication in Nature Communications. The specific comments are as follows:\nMajor comments:\n1. The authors believe that the thermal instability of tBP stems from its volatilization at high temperatures, which causes pore problems in Spiro-OMeTAD, thereby leading to the thermal instability of the devices. However, for Spiro-OMeTAD-based devices, the interaction between tbp and Li salts cannot be ignored. The core function of tbp doping is to promote the dissolution of lithium salts and prevent the agglomeration of lithium salts. Nevertheless, in this study, the authors used a large number of additives without considering the dissolution effect of the materials on lithium salts. This may be a key factor affecting device performance and thermal stability, rather than the effect brought about by the structure of the derivatives themselves. The authors did not rule out this key factor in their own research.\n2. Although the author provided a lot of device aging data for different additives in the supporting information, the device performance indicated by these data is really too low. Five years ago, it was very easy for devices using TBP to achieve an efficiency of over 20%, and it was not difficult to reach an efficiency of over 23% after passivation. However, the author only obtained an efficiency of around 15%, which indicates that there are significant problems with the perovskite thin film itself. The initial efficiency of the additives A80 and A81 proposed by the author is only above 14%, and using such data to measure the thermal stability of the devices lacks sufficient persuasiveness. In addition, after the modification of the electron transport layer (ETL) and passivation, the efficiency can be increased to 25%, but the final efficiency of the device modified with TBP is only 20.5%, which is hardly convincing.\n-- It is insufficient to merely use TRPL to characterize the performance of the Spiro-OMeTAD thin films after the addition of different additives. If the modified Spiro-OMeTAD has sufficient performance advantages, the author should provide more characteristics of the optimized Spiro-OMeTAD, including the energy band, electrical conductivity, and so on.\n3. The author spent a great deal of space in the previous part of the manuscript on using additives to address the thermal instability of perovskite devices. However, in the subsequent characterization of the optimized devices, the author tested the operational stability under actual outdoor ambient. Although this outdoor stability is crucial for perovskite devices, I fail to find sufficient logical coherence between the two parts. According to the temperature diagram provided by the author in the SI, it seems that the stability at high temperatures is not covered. Instead, the author emphasizes the performance under diurnal cycles. Therefore, I believe there are significant logical issues in this section. The author should focus on and provide the thermal stability performance when the efficiency is optimized to over 25%.\nOther comments:\n--Table 1 shows the code of different additives, which is not necessary to present it in main text.\n--What is the author's numbering criterion for the additives? It seems rather chaotic in the article, as there is no specific order and some of the numbers are missing.\n--In fig.3, the SEM images of perovskite film based on different spiro-OMeTAD show different thickness, the authors should confirm this.\n--TRPL curves are very strange compared to other literature, the authors should re-test them.\n--From the EQE curves and integrated Jsc in Fig.S69, the PCE of devices in Fig.5a-c should not obtain such high Jsc. The performance of these devices is overestimated.\n--It is rarely that 9.8 mg/mL OAI was used for passivating the perovskite surface because it is hydrophobic to influence the deposition of Spiro-OMeTAD, the authors should confirm this.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We appreciate so much for the effort and dedicated work of the editor and the reviewers. We are grateful for offering the opportunity to improve our work as major revision. We have judiciously taken all comments into account following the comments raised.\nComments on the reviewers: in red color\nOur response: in blue color\nRevised manuscript: in yellow highlighted\nReviewer #1 (Remarks to the Author):\nRecommendation: Major revisions\nThe manuscript presents a comprehensive study on the incorporation of heteroaryl derivatives as\nadditives in Spiro-OMeTAD to enhance the thermal stability and photovoltaic performance of\nperovskite solar cells (PSCs). The work is well-structured and tackles a crucial challenge in the\nfield of PSCs, namely, thermal instability. The experimental findings are promising, particularly\nregarding the remarkable thermal stability achieved with 3-phenylpyridine and 2-phenylpyridine\nadditives. However, several critical issues must be addressed to improve the clarity, rigor, and\noverall impact of the study. I recommend major revisions before the manuscript can be considered\nfor publication.\nAnswer: Thank you so much for your valuable comments and recommendations for major\nrevision. We revised manuscript according to reviewer’s pointing.\n1\n1-1. Selection Criteria for Heteroaryl Derivatives\nThe study investigates 38 heteroaryl derivatives; however, the rationale behind the selection of\nthese specific compounds is not clearly justified. A discussion on the selection criteria—such as\nvolatility, steric hindrance, electronic properties, or previous literature reports—would provide\nvaluable context and strengthen the study’s foundation.\nAnswer: Thank you for your valuable comments. We revised manuscript by adding an explanation\nof the rationale behind of the selection of these specific compounds referring previous literature\nreports.\nSentence below was added on page 6.\nIt is desirable to examine the selection of heteroaryl derivatives and their impact on durability\nfrom the following perspectives.\nFor the volatility, some literature insists on the volatility of the tBP which can be the reason\nfor the weak durability of the Spiro-OMeTAD96. Thus, it is important to evaluate heteroaryl\nderivatives with different boiling points to investigate the effects of volatility on the thermal\nstability. For example, boiling points of 2-phenylpyridine, 3-phenylpyridine, and 4-\nphenylpyridine corresponds to 270 ℃, 274 ℃, and 281 ℃, respectively, which is higher than tBP\n(197 ℃). Also, derivatives with lower boiling point should be selected for comparison e.g. 4-\nethylpyridine (165 ℃), 4-aminopyridine (157~162 ℃).\nRegarding steric hindrance, Y. Yue et al., introduced 2-amylpyridine instead of tBP, which is\nsubstituted with an amyl group at ortho position91. They explained that the steric hindrance from\nthe ortho-substitution of pyridine ring may prevent the formation of complex with additives and\nperovskite, which could increase the durability of perovskite solar cells. Therefore, we can select\northo, meta, and para substituted heteroaryl derivatives to investigate the effect of the steric\nhinderance on the durability.\nAs for the electronic properties, they can be tuned by introducing various functional groups\nand heteroatoms onto the hetero ring. These functional groups exhibit electron-donating or\nelectron-withdrawing effects, which influence the electronegativity of the pyridine ring and its\n2\ninteraction with lithium. Evaluating these effects is important to determine whether the electronic\nstate influences thermal stability. Therefore, a variety of functional groups with different electron-\ndonating or -withdrawing characteristics, as well as heteroatoms, need to be verified.\n1-2. Fill Factor (FF) Discrepancy\nThe device with a passivation layer and 4-tert-butylpyridine (tBP) as an additive exhibits an FF\nbelow 70%, which appears significantly lower than commonly reported values. The authors\nshould provide an explanation for this discrepancy.\nAnswer: Thank you for your pointing. The reason for the discrepancy is that the optimal condition\nof the perovskite solar cells can be different by different additives. To improve the FF of the\nperovskite solar cells with tBP, we optimized device perovskite recipe by increasing MACl\nconcentration from 10% to 20%. By this optimization, devices with tBP achieved 0.771 of FF and\n22.4% of PCE. These results were added to the manuscript.\nFig. 5 a I-V scans of the perovskite solar cells with 4-tert-butylpyridine (Li+tBP). Note that MACl\nconcentration increased from 10% to 20% to improve efficiency for devices with tBP.\nincreased from 10% to 20% to improve efficiency.\nCell Jsc Jsc Voc (V) Voc (V) FF (-) FF (-) PCE (%) PCE (%)\n(mA/sq) (mA/sq)\nForward Reverse Forward Reverse Forward Reverse Forward Reverse\n1 26.17 26.17 1.058 1.085 0.549 0.726 15.21 20.63\n2 26.13 26.13 1.087 1.113 0.612 0.771 17.37 22.43\n3 26.20 26.19 1.103 1.123 0.541 0.717 15.66 21.11\n4 26.21 26.21 1.107 1.125 0.455 0.707 13.20 20.86\nSentence below was added on page 32.\nNote that regarding devices with tBP, MACl concentration of the perovskite layer was\nincreased from 10% to 20%, which is optimized condition for tBP to achieve high efficiency.\n4\n1-3. Comparison of Stability Among Additives\nThe claim that A81 (3-Phenylpyridine) and A82 (2-Phenylpyridine) offer superior stability\ncompared to A15 (4-Phenylpyridine) is not convincingly demonstrated, as the initial efficiencies\nof these devices differ significantly. From an absolute efficiency standpoint, A15 may contribute\nto greater overall energy output during aging. This needs to be addressed with additional\ndiscussion or data normalization.\nAnswer: Thank you for your valuable comments. We agree that claiming 3-phenylpyridine and\n2-phenylpyridine are superior stability compared to 4-phenylpyridine is an overstatement. We\nadded discussion considering energy output during aging as well as added normalized data.\nThe sentence below was added on page 12.\n4-Phenylpyridine showed higher PCE than 2-phenylpyridine and 3-phenylpyridine for\napproximately 1100 hours from the start of the 85 ℃ stability test. This suggests that 4-\nphenylpyridine has an advantage in initial PCE, which may contribute to greater overall energy\noutput during the degradation period. For example, if 1000 hours accelerated test at 85°C is\nsufficient to ensure practical outdoor durability, 4-phenylpyridine could be more advantageous in\npractical use compared to 2-phenylpyridine and 3-phenylpyridine. Therefore, it is premature to\nconclude which molecular orientation is the most superior.\nFigure S70. Normalized 85 ℃ thermal stability result with 4-phenylpyridine (Li+A15), 3-\nphenylpyridine (Li+A81), 2-phenylpyridine (Li+A82), and 4-tert-butylpyridine (Li+tBP).\n5\n1-4. Void Formation in the HTL\nThe manuscript attributes void formation in the hole transport layer (HTL) to the use of low-\nboiling-point additives such as 4-tert-butylpyridine, which is reasonable. However, the\nobservation that A82 (2-Phenylpyridine) does not induce void formation, while A15 (4-\nPhenylpyridine) does, requires further explanation. The authors should clarify whether they have\ninvestigated the thermodynamic properties of complexes formed between these additives and\nLiTFSI. Additionally, have they measured the glass transition temperature (Tg) of HTLs\ncontaining different additives? This data could provide insight into differences in void formation\nbehavior.\nAnswer: Thank you for your pointing. To investigate the thermodynamic properties and to\nunderstand why 2-phenylpyridine does not induce void formation while 4-phenylpyridine does,\nwe conducted differential scanning calorimetry (DSC). Through this measurement, we aimed to\ndetermine the glass transition temperature (Tg) of HTLs containing different additives. However,\ndue to poor reproducibility, it was not possible to clearly identify the Tg. This may be attributed\nto residual solvents in Spiro-OMeTAD causing low reproducibility. Although we attempted\nsolvent removal via pre-heating and/or vacuum treatment, reproducibility did not improve.\nTherefore, we would like to present these results only for the reviewers’ reference.\nOnly for reviewers. Reproducibility check for DSC measurement of Spiro-OMeTAD containing\ncondition and prepared for checking reproducibility. Spiro-OMeTAD solution was spin-coated\nonto the glass substrate and then peeled off to prepare the samples.\nAs an alternative aspect, density functional theory (DFT) calculations of the most stable\nconfigurations when the additives coordinate with lithium revealed significantly different\ngeometries between 2-phenylpyridine and 4-phenylpyridine. Notably, 2-phenylpyridine formed a\nmore compact coordination structure than 4-phenylpyridine. This difference in coordination may\naffect to thermodynamic properties, resulting in the lower volatility and absence of voids observed\nin Spiro-OMeTAD with 2-phenylpyridine under the 85°C test conditions. We noted this\nspeculation in the manuscript. (see Comment 5 for more details)\nFigure 4. DFT calculated chemical structure with a, 4-tert-butylpyridine + lithium b, 2-\nphenylpyridine + lithium, and viewed from different angles of c, 4-tert-butylpyridine + lithium\n7\nand d, 2-phenylpyridine + lithium.\nThe sentence below was added on page 19.\nFurthermore, 2-phenylpyridine (A82) formed a significantly more compact coordination\nstructure compared to the other additives, in which the lithium ion appeared to be tightly\nsurrounded. This difference in coordination may be related to the lower volatility and absence of\nvoids observed in Spiro-OMeTAD with 2-phenylpyridine under the 85°C test conditions.\nAnother possible mechanism for void formation can be attributed to the migration of the\ndopant from spiro-OMeTAD to the perovskite layer (see Reviewer 2, Comment 6).\n8\n1-5. Reactivity Between Perovskite and Pyridine Derivatives\nThe study reports that spin-coating pyridine derivatives directly onto perovskite films leads to\ndegradation. However, in standard device fabrication, spin-coating HTL solutions containing 4-\ntert-butylpyridine does not appear to significantly impact device performance. The authors should\nclarify whether they observed the emergence of new XRD diffraction peaks in aged devices,\nsimilar to those found in direct pyridine spin-coating experiments. Furthermore, the claim that\nsteric hindrance differences between 4-phenylpyridine and 3-phenylpyridine significantly impact\ncoordination behavior is not well-supported. The authors should provide further discussion or\ncomputational evidence to substantiate this point.\nAnswer: Thank you for your valuable comments. We attempted to observe new XRD diffraction\npeaks in aged devices.\nSupplementary Fig. S76 shows the sample structure used for XRD measurements, Figure 4a\nis the XRD patterns before aging, and Figure 4b is those after 100 hours of aging at 85(cid:3031)°C. The\nsamples were prepared using the same recipe of device fabrication, incorporating 4-tert-\nbutylpyridine, 4-phenylpyridine, 3-phenylpyridine, or 2-phenylpyridine.\nBefore aging (Figure 4a), the sample containing 4-phenylpyridine showed a small peak at 6.9°,\nwhich may be attributed to a PbI₂–additive complex.[ref] After 85(cid:3031)°C aging (Figure 4b), both the\n4-tert-butylpyridine and 4-phenylpyridine samples exhibited a clear peak at 6.9°, suggesting the\nformation of a PbI₂–additive complex. In contrast, the samples containing 3-phenylpyridine and\n2-phenylpyridine showed a peak at 12.7°, corresponding to PbI₂ generated during aging.\nThese results suggest that 4-tert-butylpyridine and 4-phenylpyridine may react with PbI₂\nduring aging, resulting in the formation of a PbI₂–additive complex. Although it is unclear how\nmuch this compound induces degradation, it may have some influence on the device durability.\nThe sentences were revised on page 17.\nThe XRD patterns (Fig. 4a,b) were analyzed to evaluate the reactivity between the additives\nand the perovskite layer comparing before and after 85℃ tests. Samples were prepared same as\ndevice structure of FTO glass/SnO₂/perovskite (PVK)/Spiro-OMeTAD( + additives)/Au\n9\n(Supplementary Fig. S76). Au layer was pealed out just before XRD measurement. Additives of\n4-tert-butylpyridine (tBP), 4-phenylpyridine (A15), 3-phenylpyridine (A81), or 2-phenylpyridine\n(A82) were used for this experiment. For all the samples appeared peaks at 13.9°, 19.8° and 24.3°,\nwhich are associated with (100), (110), and (111) of perovskite crystal, respectively. Before 85℃\ntest (Fig. 4a), the sample with 4-phenylpyridine (A15) showed a small peak at 6.9° that could be\nfrom PbI -additives complex.91 After 85 ℃ tests, samples with 4-tert-butylypyridine (tBP) and 4-\n2\nphenylpyridine (A15) exhibit clear peak at 6.9° that could be from PbI -additives complex.\nproducts themselves are not the primary reason for poor thermal stability, but it can be the\nsecondary reason. Interestingly, samples with 2-phenylpyridine (A82) and 3-phenylpyridine\n(A81) appear no peak at 6.9°, but small peaks were observed at 12.6 corresponding to PbI . These\n2\nresults indicate that 2-phenylpyridine and 3-phenylpyridine are low reactivity with the perovskite\nlayer. This lack of reactivity is attributed to the steric hindrance introduced by the meta and ortho\nsubstitutions, which can prevent coordination with PbI₂, thereby contributing to their excellent\nthermal stability (Fig. 2b).\nSupplementary Figure S76. Sample structure for XRD measurement. Each layer was deposited\nusing the same recipe for device fabrication. Au layer was pealed out by using tape just before\nXRD measurement.\nbutylpyridine (tBP), 4-phenylpyridine (A15), 3-phenylpyridine (A81), and 2-phenylpyridine\n(A82). b, those of XRD patterns after 85 ℃ aging for 100 hours.\nTo investigate the effect of the steric hindrance differences on coordination behavior, we\ncalculated the most stable configurations when the additives coordinate with lithium, which\nrevealed different geometries. Figure 4e-l shows the DFT calculated chemical structure with 4-\ntert-butylpyridine (tBP), 4-phenylpyridine (A15), 3-phenylpyridine (A81), and 2-phenylpyridine\n(A82). All additives were found to form a tetrahedral arrangement as the most stable configuration,\nwith point group S4. 4-tert-Butylpyridine and 4-phenylpyridine were the similar geometries.\nInterestingly, 3-phenylpyridine exhibited a slightly more compact structure than 4-phenylpyridine\ndue to geometric distortion induced by its molecular orientation. Furthermore, 2-phenylpyridine\nformed a significantly more compact coordination structure compared to the other additives, in\nwhich the lithium ion appeared to be tightly surrounded. This result was explained in the\nmanuscript.\nThe sentences were revised on page 21.\nThe interaction between additives and lithium was analyzed using DFT calculations (Fig. 4e-\nl). Figures 4e-h illustrate the coordination of 4-tert-butylpyridine (tBP) with lithium, 4-\nphenylpyridine (A15) with lithium, 3-phenylpyridine (A81) with lithium 2-phenylpyridine (A82),\nrespectively, and Figures 4i-l are corresponding structures viewed from different angles. All\nadditives were found to form a tetrahedral arrangement as the most stable configuration, all with\n11\npoint group S . 4-tert-Butylpyridine (tBP) and 4-phenylpyridine (A15) were the similar\n4\ngeometries. Interestingly, 3-phenylpyridine exhibited a slightly more compact structure than 4-\nphenylpyridine (A15) due to geometric distortion induced by its molecular orientation.\nFurthermore, 2-phenylpyridine (A82) formed a significantly more compact coordination structure\ncompared to the other additives, in which the lithium ion appeared to be tightly surrounded. This\ndifference in coordination may be related to the lower volatility and absence of voids observed in\nSpiro-OMeTAD with 2-phenylpyridine under the 85°C test conditions. When three molecules of\neach additive were coordinated with lithium, an energetically favorable state of 2-phenylpyridine,\n3-phenylpyridine, and 4-phenylpyridine was point group C while tBP was D , respectively. The\n3 3\ndesorption energy required to remove a coordinated molecule from lithium was calculated to be\n0.667 eV for tBP, 0.659 eV for A15, 0.680 eV for A81, and 0.100 eV for A82, indicating that there\nis no relationship between thermal stability and ease of molecular desorption. Supplementary\nFigures S78a,b depict the interactions between the coordinated additive-lithium complexes and\nSpiro-OMeTAD. The influence of π-interactions between the methoxyphenylethylamine group in\nSpiro-OMeTAD and the pyridine rings of the additives was evaluated. Calculations show that a\npositive formation energy is required for interaction between tBP and A15 and Spiro-OMeTAD\nto happen because steric effects require a large energy to deform Spiro-OMeTAD to allow such\ninteractions to happen. These results are consistent with the X-ray photoelectron spectroscopy,\nwhich shows no significant chemical shift (Supplementary Fig. S71). These results suggest that\nπ-interactions between the additives and Spiro-OMeTAD do not affect the overall stability or\nperformance of the system.\nPhenylpyridine (A15) and lithium. g, 3-Phenylpyridine (A81) and lithium. h, 2-Phenylpyridine\n(A82) and lithium. i-l, Corresponding structures viewed from different angles.\n1-6. Typographical Errors\nMinor typographical errors exist throughout the manuscript, such as \"phyenylpyridine\" instead of\n\"phenylpyridine\" in the Figure 3 captions. A thorough proofreading is recommended to improve\nreadability and ensure accuracy.\nAnswer: Thank you for pointing out. We corrected typographical errors.\nThe authors have done a substantial amount of work comparing the effects of different tBP\nsubstitutes on improving the thermal stability of doped Spiro-OMeTAD. This is a commendable\neffort. Although the author provides extensive thermal stability test data demonstrating the\nenhancement of device thermal stability with 3-phenylpyridine and 2-phenylpyridine, the\nexplanation of the mechanism is quite lacking. More characterisation and analysis are needed to\nsubstantiate the role of 3-phenylpyridine and 2-phenylpyridine. Hope the following questions can\nbe addressed in the revised manuscript.\nAnswer: Thank you so much for your valuable comments. We revised manuscript according to\ncomments.\n2-1. For the thermal stability, SnO2 dispersed solution was used to fabricate SnO2 carrier-\ntransport-layer and CsFAPbI3 was used to fabricate perovskite, whereas for the efficiency part,\nthe carrier-transport-layer was changed to CBD SnO2, and the perovskite was changed to\nRbCsFAPbI3 with excess PbI2 in precursor. It is necessary to provide the stability comparison\ndata of highly efficient PSCs (dispersed-solution-SnO2 vs CBD, CsFAPbI3 vs RbCsFAPbI3 with\nexcess PbI2).\nAnswer: Thank you for your pointing out. We provided stability comparison data of dispersed-\nsolution-SnO vs CBD and CsFAPbI vs RbCsFAPbI . To provide high efficiency PSCs, we\n2 3 3\noptimized the recipe for perovskite fabrication for each condition, which is explained at\nexperimental part. These results indicate the way to approach both high efficiency and high\nstability.\nIn terms of dispersed-solution SnO₂ (SnO₂_DS) vs. CBD SnO₂ (SnO₂_CBD), the initial\nefficiency of SnO₂_CBD with 2-phenylpyridine was 20.1% and remained at 18.7% after 936\nhours of testing at 85 ℃. In contrast, SnO₂_DS with 2-phenylpyridine showed an initial efficiency\nof 14.9% and retained 16.2% after 936 hours at 85 ℃. In both cases, 2-phenylpyridine provided\nhigher stability compared with 4-tert-butylpyridine. From these results, SnO₂_CBD exhibited\nhigher initial efficiency and may also offer excellent stability, note that, it is difficult to draw a\n14\ndefinitive conclusion based on this data alone.\nAs for CsFAPbI vs RbCsFAPbI , rubidium doped perovskite (RbCsFAPbI ) achieved higher\n3 3 3\nefficiency (24.2%) than CsFAPbI (20.1%), however, stability with RbCsFAPbI showed\n3 3\nsignificantly dropped after 87 hours of 85 ℃ test. From this result, It is possible that\nrubidium doped perovskite itself has problems with thermal durability. Correa-Baena et al.\nindicated that a dosage of 1% Rb was enough for causing segregation, which is a different kind\nof issue from those associated with additives100. These results suggest that optimization of\nperovskite composition is also important to achieve high thermal stability.\nFigure S71. Stability comparison of perovskite solar cells. (a) dispersed-solution-SnO\n2\n(SnO₂_DS) vs chemical bath deposition (SnO₂_CBD). (b) CsFAPbI vs RbCsFAPbI .\n3 3\nThe sentences were added to page 13.\nWe also provided stability comparison data of dispersed-solution-SnO (SnO₂_DS) vs\n2\n15\nchemical bath deposition (SnO₂_CBD) as well as CsFAPbI vs RbCsFAPbI (Supplementary Fig.\n3 3\nS71). To provide high efficiency PSCs, we optimized the recipe for perovskite fabrication for each\ncondition, which is explained at experimental part. In terms of dispersed-solution SnO₂\n(SnO₂_DS) vs. CBD SnO₂ (SnO₂_CBD), the initial efficiency of SnO₂_CBD with 2-\nphenylpyridine was 20.1% and remained at 18.7% after 936 hours of testing at 85 ℃. In contrast,\nSnO₂_DS with 2-phenylpyridine showed an initial efficiency of 14.9% and retained 16.2% after\n936 hours at 85 ℃. In both cases, 2-phenylpyridine provided higher stability compared with 4-\ntert-butylpyridine. As for CsFAPbI vs RbCsFAPbI , rubidium doped perovskite (RbCsFAPbI )\n3 3 3\nachieved higher efficiency (24.2%) than CsFAPbI (20.1%), however, stability with RbCsFAPbI\n3 3\nshowed significantly dropped after 87 hours of 85 ℃ test. From this result, It is possible that\nrubidium doped perovskite itself has problems with thermal durability. Correa-Baena et al.\nindicated that a dosage of 1% Rb was enough for causing segregation, which is a different kind\nof issue from those associated with additives100. These results suggest that optimization of\nperovskite composition is also important to achieve high thermal stability.\n16\n2-2. It is necessary to provide the IPCE data and integrated current density of highly efficient\nPSCs shown in Fig.5c.\nAnswer: Thank you for your suggestions. We have provided the IPCE data and the integrated\ncurrent density of the highly efficient PCSs in Fig. 5c. The integrated current density was 25.14\nmA/cm² for 4-tert-butylpyridine, 25.33 mA/cm² for 4-phenylpyridine, and 25.39 mA/cm² for 2-\nphenylpyridine. The difference between Jsc obtained from the IV curves and the integrated current\ndensity was within 4%, which is acceptable considering recent publications in Nature\nCommunications, Nature, etc. These results have been included in the supplementary information.\nFigure S83. IPCE of perovskite solar cells with 4-tert-butylpyridine (tBP), 4-phenylpyridine\n(A15), and 2-phenylpyridine (A82).\n17\n2-3. Solely using DFT to confirm the interaction between additives, lithium and Spiro-OMeTAD\nis not sufficient. Fourier-transform infrared spectroscopy (FTIR) is needed to evidence the\npresence of interaction.\nAnswer: Thank you for your valuable comments. We measured FTIR spectra to investigate the\nmolecular interactions. The hole-transport layer (Spiro-OMeTAD + LiTFSI + dopant) was\nexamined by comparing 4-tert-butylpyridine, 4-phenylpyridine, and 2-phenylpyridine. Overall,\nthe spectra of 4-phenylpyridine and 2-phenylpyridine were similar to that of 4-tert-butylpyridine.\nPeaks at 1061 cm⁻¹ and 1349 cm⁻¹ were observed, which correspond to the S=O stretching\nvibration of TFSI. Interestingly, these peaks were shifted to higher wavenumber in the cases of 4-\nphenylpyridine and 2-phenylpyridine. This shift could be attributed to the stronger coordination\nof 4‑phenylpyridine and 2‑phenylpyridine with lithium, which reduces TFSI coordination and\nthereby strengthens the S=O bond. These results could relate to the interaction between additives\nconfirmed by DFT.\nThe sentences were revised on page 19.\nWe measured Fourier-transform infrared spectroscopy (FTIR) to investigate the molecular\ninteractions (Supplementary Fig. S79). Peaks at 1061 cm⁻¹ and 1349 cm⁻¹ were observed, which\ncorrespond to the S=O stretching vibration of TFSI101,102. Interestingly, these peaks were shifted\nto higher wavenumber in the cases of 4-phenylpyridine and 2-phenylpyridine. This shift could be\nattributed to the stronger coordination of 4‑phenylpyridine and 2‑phenylpyridine with lithium,\nwhich reduces TFSI coordination and thereby strengthens the S=O bond.\nFigure S79. FTIR spectra of Spiro-OMeTAD with LiTFSI and different dopants: (a) full spectrum\nand expanded views of (b) 1055–1066 cm⁻¹ and (c) 1345–1355 cm⁻¹.\n18\n2-4. Traditional Spiro-OMeTAD HTL suffers crystallization after thermal aging which worsens\nthe electrical properties of HTL. Therefore, it is necessary to provide AFM and microscope\nphotographs of the HTLs before and after thermal aging.\nAnswer: Thank you for your valuable comments. We performed AFM and optical microscopy\nmeasurements of the HTLs before and after thermal aging (85 ℃ for 100 hours). Sample structure\nwas FTO glass/SnO /Perovskite/Sprio-OMeTAD/Au. Spiro-OMeTAD layer contains LiTFSI and\n2\neach additive same as device fabrication. Au layer was pealed out by tape just before measurement.\nAfter the 85 ℃ test, the HTL with 4-tert-butylpyridine exhibited pin-hole formation\n(Supplementary Fig. S74d) and cracks originating from crystalline-like regions (Figure S74e).\nThese results are consistent with previous reports97. The depths of the pin-holes and cracks in the\ncrystalline-like regions were in the range of 50–150 nm. Such defects are expected to deteriorate\nthe electrical properties of the HTLs as well as the interfaces between HTL/Au and\nPerovskite/HTL. In contrast, the HTLs with 4-phenylpyridine and 2-phenylpyridine showed no\nsignificant morphological changes after the 85 ℃ test. These observations suggest that 4-\nphenylpyridine and 2-phenylpyridine effectively suppress pin-hole and crack formation in\ncrystalline-like regions, which account for their higher thermal stability.\nThe sentences were revised on page 17.\nFurthermore, we performed optical microscopy and AFM measurements of the HTLs before\nand after thermal aging (85 ℃ for 100 hours, Supplementary Fig. S73, S74). After thermal aging,\nthe HTL with 4-tert-butylpyridine exhibited pin-hole formation and cracks originating from\ncrystalline-like regions. These results are consistent with previous reports96. The depths of the\npin-holes and cracks in the crystalline-like regions were in the range of 50–150 nm. Such defects\nare expected to deteriorate the electrical properties of the HTLs as well as the interfaces between\nHTL/Au and Perovskite/HTL. In contrast, the HTLs with 4-phenylpyridine and 2-phenylpyridine\nshowed no significant morphological changes after the 85 ℃ test. These observations suggest that\n4-phenylpyridine and 2-phenylpyridine effectively suppress pin-hole and crack formation in\ncrystalline-like regions, which account for their higher thermal stability.\n(a) 4-tert-butylpyridine, (b) 4-phenylpyridine, (c) 2-phenylpyridine and (d-f) corresponding\nadditives after 85 ℃ test for 100 hours. Sample structure was FTO glass/SnO /Perovskite/Sprio-\nfabrication. Au layer was pealed out by tape just before measurement. Scale bar corresponds to\n30 μm.\nbutylpyridine, (b) 4-phenylpyridine, (c) 2-phenylpyridine, and (d-g) corresponding additives after\n85 ℃ test for 100 hours. Sample structure was FTO glass/SnO /Perovskite/Sprio-OMeTAD/Au.\nwas pealed out by tape just before measurement. Scale bar corresponds to 5 μm.\n21\n2-5. Additives could influence the oxidation of hole-transport layer. In this work, does the best\ntwo additives influence the oxidation/doping progress of hole-transport layer? Accelerate or\npostpone it? It is necessary to provide UV–vis absorption data to analyse the HTL before and after\noxidation. UPS/KPFM is also necessary to investigate the associated working function/energy\nlevel changes.\nAnswer: Thank you for your valuable comments.\nUV–vis absorption spectroscopy was conducted to assess the potential influence of the\nadditives on the oxidation of the hole-transport layer. The spiro-OMeTAD films were prepared\nwith LiTFSI, 4-tert-butylpyridine, 4-phenylpyridine, and 2-phenylpyridine, following the\nstandard device fabrication protocol. The UV–vis spectra exhibited no appreciable differences\namong the samples, and no characteristic oxidation-related absorption peaks were detected. These\nfindings indicate that neither 4-phenylpyridine nor 2-phenylpyridine exerts a measurable effect\non the oxidation process.\nFigure S86. (a) UV-vis spectra and (b) bandgap of Spiro-OMeTAD with additives. Bandgap was\n3.02 eV.\nPYS/KPFM is also measured to investigate the associated working function/energy level\nchanges. Photoelectron yield spectroscopy (PYS) was used to measure the valence band instead\nof UPS. From these data, energy diagrams were calculated as shown supplementary Figrure S89.\nValence band edges (E ) of Spiro-OMeTAD with 4-tert-butylpyridine was -5.58 eV. Interestingly,\nv\nv\neV, -5.76 eV, and -5.83 eV, which is closer to perovskite (-5.89 eV) than 4-tert-butylpyridine. This\ncould be the reason for effective charge transfer and improved photoconversion efficiency.\nThe sentences were revised on page 23.\nUV–vis absorption spectroscopy was performed to evaluate the potential influence of the\nadditives on the oxidation of the hole-transport layer (Supplementary Figure S86). The UV–vis\nspectra showed no appreciable differences among the samples, suggesting that neither 4-\nphenylpyridine nor 2-phenylpyridine has a measurable effect on the oxidation process.\nFurthermore, energy level diagrams of Spiro-OMeTAD with additives were constructed based on\nphotoelectron yield spectroscopy and Kelvin probe force microscopy (KPFM) (Supplementary\nFig. S87-89). The valence band edge (Ev) of Spiro-OMeTAD with 4-tert-butylpyridine was -5.58\neV. In contrast, the Ev values of Spiro-OMeTAD with 4-phenylpyridine, 3-phenylpyridine, and\n2-phenylpyridine were -5.76 eV, -5.76 eV, and -5.83 eV, respectively, which are closer to that of\nperovskite (–5.89 eV) than the value obtained with 4-tert-butylpyridine. This alignment is likely\nto facilitate more efficient charge transfer, thereby contributing to improved photoconversion\nefficiency.\npassivation, (c) spiro-OMeTAD with LiTFSI and 4-tert-butylpyridine, (d) spiro-OMeTAD with\nLiTFSI and 4-phenylpyridine, (e) spiro-OMeTAD with LiTFSI and 3-phenylpyridine, (f) spiro-\nOMeTAD with LiTFSI and 2-phenylpyridine.\nFigure S88. KPFM mapping of (a) perovskite, (b) perovskite with OAI passivation, (c) spiro-\nOMeTAD with LiTFSI and 4-tert-butylpyridine, (d) spiro-OMeTAD with LiTFSI and 4-\nphenylpyridine, (e) spiro-OMeTAD with LiTFSI and 3-phenylpyridine, (f) spiro-OMeTAD with\nLiTFSI and 2-phenylpyridine. Scale bar corresponds to 5 μm.\nwere calculated with photoelectron yield spectroscopy, KPFM, and bandgap from UVVIS\nspectroscopy. Bandgap of perovskite and phenylpyridine was associated with 1.56 eV and 3.02\neV, respectively. Spiro-OMeTAD include LiTFSI and each additive same as recipe with device\nfabrication.\n25\n2-6. Lithium salt was observed to accumulate at interfaces (Perovskite solar cells based on spiro-\nOMeTAD stabilized with an alkylthiol additive Nature Photonics, 2023, 17, 96-105). Are the\nadditives proposed in this work helpful to obtain homogenous dopant distribution? Also, it is\nimportant to locate the position of additives. Is it homogenously distributed throughout the HTL?\nTherefore, time-of-flight secondary-ion mass spectrometry (TOF-SIMS) or other characterization\nmethods which can clearly demonstrate the out-of-plane component distribution is important to\nhelp readers understand the benefits of new the additives proposed in this work.\nAnswer: Thank you for your valuable comments. We investigated the effect of additives on\nelemental distribution using ToF-SIMS (Supplementary Fig. S80). The measured samples were\nprepared with the configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD (with additives)/Au,\nfollowing the same procedure as device fabrication. The Au layer was peeled off immediately\nbefore ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-phenylpyridine before\nand after thermal aging at 85 °C for 100 hours. Lithium migrated toward the ETL side and slightly\naccumulated at the surface, which is consistent with a previous report103. The lithium distribution\nwas not significantly affected by the choice of dopant, nor by the thermal test. From these results,\n2-phenylpyridine does not appear to promote a more homogeneous lithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer (Supplementary Fig. S80a). After the 85 °C test, however, 4-tert-butylpyridine\nhad migrated substantially into the perovskite layer (Supplementary Fig. S80b). This migration is\nlikely driven by its reaction with the perovskite, which is consistent with the by-product confirmed\nby XRD (Figure 4b). In contrast, 2-phenylpyridine was mainly localized in the Spiro-OMeTAD\nlayer, and its distribution showed no significant change after the 85 °C test (Supplementary Fig.\nS80c,d). This stability may contribute to its beneficial effect on thermal stability, probably because\n2-phenylpyridine does not react with the perovskite and therefore does not diffuse into the\nperovskite layer.\nFurthermore, the ToF-SIMS results may help explain the absence of voids in the 2-\nphenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Fig. S81a,b). By contrast,\nthe void-free morphology observed with 2-phenylpyridine can be attributed to the stability of its\ndistribution (Supplementary Fig. S81c,d).\nAlthough it remains difficult to determine the precise mechanism of void formation from these\nresults alone, they may provide valuable design guidelines for void-free HTMs.\nThe sentences were revised on page 21.\nFor further understanding of mechanism of the void-formation, we investigated the effect of\nadditives on elemental distribution using ToF-SIMS (Supplementary Fig. S80). The measured\nsamples were prepared with the configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD (with\nadditives)/Au, following the same procedure as device fabrication. The Au layer was peeled off\nimmediately before ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-\nphenylpyridine before and after thermal aging at 85 °C for 100 hours. Lithium migrated toward\nthe ETL side and slightly accumulated at the surface, which is consistent with a previous report103.\nThe lithium distribution was not significantly affected by the choice of dopant, nor by the thermal\ntest. From these results, 2-phenylpyridine does not appear to promote a more homogeneous\nlithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer (Supplementary Fig. S80a). After the 85 °C test, however, 4-tert-butylpyridine\nhad migrated substantially into the perovskite layer (Supplementary Fig. S80b). This migration is\nlikely driven by its reaction with the perovskite, which is consistent with the by-product confirmed\nby XRD (Figure 4b). In contrast, 2-phenylpyridine was mainly localized in the Spiro-OMeTAD\nlayer, and its distribution showed no significant change after the 85 °C test (Supplementary Fig.\nS80c,d). This stability may contribute to its beneficial effect on thermal stability, probably because\n2-phenylpyridine does not react with the perovskite and therefore does not diffuse into the\nperovskite layer. These ToF-SIMS results may explain the absence of voids in the 2-\nphenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S81a,b). By contrast,\nthe void-free morphology observed with 2-phenylpyridine can be attributed to the stability of its\ndistribution (Supplementary Figure S81c,d).\n27\n[103] Perovskite solar cells based on spiro-OMeTAD stabilized with an alkylthiol additive Nature\nPhotonics, 2023, 17, 96-105\nFigure S80. ToF-SIMS depth profiles of (a) a fresh device with 4-tert-butylpyridine, (b) the same\ndevice after the 85 ℃ test, (c) a fresh device with 2-phenylpyridine, and (d) the same device after\nthe 85 ℃ test.\nFigure S81. Schematics images of void formation and void-free by additives. (a) a fresh device\n28\nwith 4-tert-butylpyridine, (b) the same device after the 85 ℃ test, (c) a fresh device with 2-\nphenylpyridine, and (d) the same device after the 85 ℃ test.\n29\n2-7. The poor thermal stability of doped Spiro-OMeTAD is not solely attributed to the additives\n(such as Li, tBP); it may also be related to intrinsic factors of Spiro-OMeTAD itself. For instance,\nits low Tg makes it susceptible to molecular segmental motion, and non-crystalline Spiro-\nOMeTAD can undergo local recrystallization at high temperatures. Additionally, the diffusion of\nI- from the perovskite into HTL can also lead to the failure of Spiro-OMeTAD. Given this, why\ndoes the substitution of tBP with 3-phenylpyridine and 2-phenylpyridine result in a significant\nimprovement in thermal stability? Please provide more evidence to determine whether 3-\nphenylpyridine and 2-phenylpyridine influence the factors mentioned above.\nAnswer: Thank you for insightful comments. To investigate the thermodynamic properties and to\nelucidate why 2-phenylpyridine does not induce void formation whereas 4-phenylpyridine does,\ndifferential scanning calorimetry (DSC) was performed. The objective was to determine the glass\ntransition temperature (Tg) of HTLs incorporating different additives. However, the\nmeasurements exhibited poor reproducibility, precluding a clear identification of Tg. This\ninconsistency may be attributed to residual solvents in Spiro-OMeTAD. Attempts to improve\nreproducibility through pre-heating and/or vacuum treatment were unsuccessful. Accordingly,\nthese results are provided here solely for the reviewers’ reference. (see Reviewer 1, comment 4)\nTo investigate local recrystallization at high temperatures, we performed optical microscopy\nand AFM measurements of the HTLs before and after thermal aging (85 ℃ for 100 hours,\nSupplementary Figure S73, S74). (see Reviewer 2, comments 4) After thermal aging, the HTL\nwith 4-tert-butylpyridine exhibited pin-hole formation and cracks originating from crystalline-\nlike regions. These results are consistent with previous reports96. The depths of the pin-holes and\ncracks in the crystalline-like regions were in the range of 50–150 nm. Such defects are expected\nto deteriorate the electrical properties of the HTLs as well as the interfaces between HTL/Au and\nPerovskite/HTL. In contrast, the HTLs with 4-phenylpyridine and 2-phenylpyridine showed no\nsignificant morphological changes after the 85 ℃ test. These observations suggest that 4-\nphenylpyridine and 2-phenylpyridine effectively suppress pin-hole and crack formation in\ncrystalline-like regions, which account for their higher thermal stability.\nAs for the diffusion of I-, we conducted ToF-SIMS for devices before and after 85℃ test\n30\ncomparing 4-tert-butylpyridine and 2-phenylpyridine (Supplementary Fig. S80). I- is mainly\ndistributed in the perovskite layer and there was no significant changes in distribution after the\n85 ℃ test regardless of the dopants. From this result, 2-phenylpyridine does not influence of the\nI- distribution.\nFrom these results, additive migration (see Comment 6) is most likely the underlying\nmechanism by which 2-phenylpyridine enhances thermal stability. In the case of 4-tert-\nbutylpyridine, substantial migration from the Spiro-OMeTAD layer into the perovskite may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S81a,b). By\ncontrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the\nstability of its spatial distribution (Supplementary Figure S81c,d).\n31\n2-8. What are the boiling points of 3-phenylpyridine, 2-phenylpyridine, and tBP? Could the\ndifferences in the results shown in Figure 3 be due to the different boiling points of the three\nmaterials?\nAnswer: Thank you for your valuable comments. The boiling points are 274 °C for 3-\nphenylpyridine, 270 °C for 2-phenylpyridine, and 197 °C for 4-tert-butylpyridine, respectively.\nThese values are consistent with the trends observed in Figure 3. 4-tert-butylpyridine is known to\nbe highly volatile, which may lead to void formation after thermal stress. In contrast, 2-\nphenylpyridine and 3-phenylpyridine exhibit lower volatility, which can contribute to enhanced\nthermal stability. This reduced volatility, together with the stable additive distribution described\nin Comment 6, likely accounts for the superior thermal stability of 2-phenylpyridine and 3-\nphenylpyridine.\nThe sentences were revised on page 17.\nThe boiling points are 274 °C for 3-phenylpyridine, 270 °C for 2-phenylpyridine, and 197 °C\nfor 4-tert-butylpyridine, respectively. These values are consistent with the trends observed in\nFigure 3.\n2-9. Please provide the AFM images of Spiro-OMeTAD doped with the three additives, Li+3-\nphenylpyridine, Li+2-phenylpyridine, and Li+tBP, before and after heating, in order to observe\nthe behavior of Li+ before and after heating.\nAnswer: Thank you for valuable comments. We performed AFM measurements of the HTLs\nbefore and after thermal aging (85 ℃ for 100 hours). Sample structure was FTO\nglass/SnO /Perovskite/Sprio-OMeTAD/Au. Spiro-OMeTAD layer contains LiTFSI and each\n2\nadditive same as device fabrication. (Please see Comment 4)\n32\n2-10. For stability, the thickness of Spiro-OMeTAD (120 mg/ml) has been increased in this\nmanuscript. However, thicker Spiro-OMeTAD tends to increase the resistance of the HTL layer.\nAny trade-off benefit here?\nAnswer: Thank you for your comment. We examined whether the concentration of Spiro-\nOMeTAD affects device performance by comparing 30 mg/ml and 120 mg/ml solutions. For\nseries resistance (Rs), the average Rs of the 30 mg/ml devices was 5.4 Ω cm², slightly lower than\nthat of the 120 mg/ml devices (5.8 Ω cm²), which may be attributed to the thinner Spiro-OMeTAD\nlayer. This negligible difference in resistance can be explained by the excellent resistivity of 4-\nphenylpyridine (see Reviewer 3, Comment 3). In contrast, the average shunt resistance (Rsh) of\nthe 30 mg/ml devices was 7,854 Ω cm², lower than that of the 120 mg/ml devices (10,634 Ω cm²),\nresulting in reduced FF and PCE. This reduction may result from the thinner Spiro-OMeTAD\nlayer, which could induce pinholes and reduce reproducibility. Regarding the champion devices,\nno significant differences were observed in photovoltaic properties. Overall, these results indicate\nthat a higher Spiro-OMeTAD concentration (120 mg/ml) provides improved reproducibility\nwithout substantially increasing resistance.\nThe sentences were revised on page 23.\nWe also examined whether the concentration of Spiro-OMeTAD influences device\nperformance by comparing 30 mg/ml and 120 mg/ml solutions (Supplementary Fig. S84, S85).\nThe results indicate that the higher concentration (120 mg/ml) improves reproducibility without\nsignificantly increasing resistance.\nand 120 mg/mL of 4-phenylpyridine in Spiro-OMeTAD solution for (a) Jsc, (b) Voc, (c), FF,\n(d) PCE, (e) Rs, and (f) Rsh. Total number of samples was 16.\nOMeTAD solution containing 4-phenylpyridine and LiTFSI.\nIn this manuscript, the authors consider tBP as the key factor for the instability of Spiro-\nOMeTAD-based devices at high temperatures. They employed a large number of heteroaryl\nderivatives to verify which materials could improve thermal stability. Finally, the authors\nproposed that three types of derivatives have a positive effect on thermal stability, and based on\nthis, devices were fabricated, which showed good stability in outdoor stability tests. Overall, the\nauthors are thanked for their extensive work and experiments in screening suitable derivatives,\nand for providing reliable efficiency aging data instead of the normalized data in most literature.\nThe authors also provided some mechanistic explanations to better understand the impact of such\nadditives on the stability of Spiro-OMeTAD. However, despite the numerous experiments and\ntests conducted by the authors, the research results are not persuasive enough, and the mechanistic\nexplanations lack new ideas, failing to arouse greater interest among readers. In addition, there\nare some logical issues in this research. Therefore, I do not recommend further consideration of\nthis manuscript for publication in Nature Communications. The specific comments are as follows:\nAnswer: Thank you so much for your valuable comments. We revised manuscript according to\ncomments.\n36\n3-1. The authors believe that the thermal instability of tBP stems from its volatilization at high\ntemperatures, which causes pore problems in Spiro-OMeTAD, thereby leading to the thermal\ninstability of the devices. However, for Spiro-OMeTAD-based devices, the interaction between\ntbp and Li salts cannot be ignored. The core function of tbp doping is to promote the dissolution\nof lithium salts and prevent the agglomeration of lithium salts. Nevertheless, in this study, the\nauthors used a large number of additives without considering the dissolution effect of the materials\non lithium salts. This may be a key factor affecting device performance and thermal stability,\nrather than the effect brought about by the structure of the derivatives themselves. The authors\ndid not rule out this key factor in their own research.\nAnswer: Thank you for your insightful comments. We examined the dissolution effect of the\nadditives on lithium salts. Lithium salts were dissolved in Spiro-OMeTAD solutions containing\neach additive. Supplementary Figure S75 presents the relationship between thermal stability (PCE\nafter 1000 h) and the solubility of lithium salts. tBP and 3-phenylpyridine (A81) dissolved 62.5\nmg of lithium salts, whereas 4-phenylpyridine (A15) and 2-phenylpyridine (A82) dissolved 55.6\nmg and 11.4 mg, respectively. These results show no clear correlation between thermal stability\nand lithium salt solubility, suggesting that other mechanisms contribute to thermal stability.\nThe sentences were revised on page 18.\nWe also examined the dissolution effect of the additives on lithium salts. Since the additives\npromote the dissolution of lithium salts and prevent the agglomeration of lithium salts, this may\nbe a key factor affecting device performance and thermal stability. Lithium salts were dissolved\nin Spiro-OMeTAD solutions containing each additive. Supplementary Fig. S75 presents the\nrelationship between thermal stability (PCE after 1000 h) and the solubility of lithium salts. tBP\nand 3-phenylpyridine (A81) dissolved 62.5 mg/mL of lithium salts, whereas 4-phenylpyridine\n(A15) and 2-phenylpyridine (A82) dissolved 55.6 mg/mL and 11.4 mg/mL, respectively. These\nresults show no clear correlation between thermal stability and lithium salt solubility, suggesting\nthat other mechanisms contribute to thermal stability.\nLithium salts were dissolved in Spiro-OMeTAD solutions containing each additive.\nTo reveal the mechanism for thermal stability with novel additives, we investigated the effect\nof additives on elemental distribution using ToF-SIMS (Supplementary Fig. S80). The measured\nsamples were prepared with the configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD (with\nadditives)/Au, following the same procedure as device fabrication. The Au layer was peeled off\nimmediately before ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-\nphenylpyridine before and after thermal aging at 85 °C for 100 hours. Lithium migrated toward\nthe ETL side and slightly accumulated at the surface, which is consistent with a previous report103.\nThe lithium distribution was not significantly affected by the choice of dopant, nor by the thermal\ntest. From these results, 2-phenylpyridine does not appear to promote a more homogeneous\nlithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer (Supplementary Fig. S80a). After the 85 °C test, however, 4-tert-butylpyridine\nhad migrated substantially into the perovskite layer (Supplementary Fig. S80b). This migration is\nlikely driven by its reaction with the perovskite, which is consistent with the by-product confirmed\nby XRD (Figure 4b). In contrast, 2-phenylpyridine was mainly localized in the Spiro-OMeTAD\nlayer, and its distribution showed no significant change after the 85 °C test (Supplementary Fig.\nS80c,d). This stability may contribute to its beneficial effect on thermal stability, probably because\n38\n2-phenylpyridine does not react with the perovskite and therefore does not diffuse into the\nperovskite layer.\nFurthermore, the ToF-SIMS results may help explain the absence of voids in the 2-\nphenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S81a,b). By\ncontrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the\nstability of its distribution (Supplementary Figure S81c,d).\nAlthough it remains difficult to determine the precise mechanism of void formation from these\nresults alone, they may provide valuable design guidelines for void-free HTMs.\nThe sentences were revised on page 23.\nWe investigated the effect of additives on elemental distribution using ToF-SIMS. The\nmeasured samples were prepared with the configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD\n(with additives)/Au, following the same procedure as device fabrication. The Au layer was peeled\noff immediately before ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-\nphenylpyridine before and after thermal aging at 85 °C for 100 hours. Lithium migrated toward\nthe ETL side and slightly accumulated at the surface, which is consistent with a previous report103.\nThe lithium distribution was not significantly affected by the choice of dopant, nor by the thermal\ntest. From these results, 2-phenylpyridine does not appear to promote a more homogeneous\nlithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer. After the 85 °C test, however, 4-tert-butylpyridine had migrated substantially\ninto the perovskite layer. This migration is likely driven by its reaction with the perovskite, which\nis consistent with the by-product confirmed by XRD (Figure 4b). In contrast, 2-phenylpyridine\nwas mainly localized in the Spiro-OMeTAD layer, and its distribution showed no significant\nchange after the 85 °C test. This stability may contribute to its beneficial effect on thermal stability,\nprobably because 2-phenylpyridine does not react with the perovskite and therefore does not\ndiffuse into the perovskite layer. These ToF-SIMS results may explain the absence of voids in the\n39\n2-phenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S80a,b). By\ncontrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the\nstability of its distribution (Supplementary Figure S80c,d).\nFigure S80. ToF-SIMS depth profiles of (a) a fresh device with 4-tert-butylpyridine, (b) the same\ndevice after the 85 ℃ test, (c) a fresh device with 2-phenylpyridine, and (d) the same device after\nthe 85 ℃ test.\nwith 4-tert-butylpyridine, (b) the same device after the 85 ℃ test, (c) a fresh device with 2-\nphenylpyridine, and (d) the same device after the 85 ℃ test.\n41\n3-2. Although the author provided a lot of device aging data for different additives in the\nsupporting information, the device performance indicated by these data is really too low. Five\nyears ago, it was very easy for devices using TBP to achieve an efficiency of over 20%, and it\nwas not difficult to reach an efficiency of over 23% after passivation. However, the author only\nobtained an efficiency of around 15%, which indicates that there are significant problems with\nthe perovskite thin film itself. The initial efficiency of the additives A80 and A81 proposed by the\nauthor is only above 14%, and using such data to measure the thermal stability of the devices\nlacks sufficient persuasiveness. In addition, after the modification of the electron transport layer\n(ETL) and passivation, the efficiency can be increased to 25%, but the final efficiency of the\ndevice modified with TBP is only 20.5%, which is hardly convincing.\nAnswer: Thank you for valuable comments. We tried to improve the efficiency of perovskite solar\ncells by optimization of fabrication recipe. To improve the photovoltaic performance of the\nperovskite solar cells with tBP, we optimized device perovskite recipe by increasing MACl\nconcentration from 10% to 20%. By this optimization, devices with tBP achieved 0.771 of FF and\n22.4% of PCE. These results were added to the manuscript.\nFig. 5 a I-V scans of the perovskite solar cells with 4-tert-butylpyridine (Li+tBP). Note that MACl\nconcentration increased from 10% to 20% to improve efficiency for devices with tBP.\nMoreover, we optimized the efficiency of the 2-phenylpyridine (A82) by increasing the MACl\nconcentration from 10% to 20%. By this optimization, devices with 2-phenylpyridine obtained\n42\n20.1% as an initial PCE and achieved improved thermal stability. Also, we conducted the 85 ℃\ntest, showing excellent thermal stability with improved PCE. These results were included in the\nsupplementary information. Although this work focuses on the thermal stability of the perovskite\nsolar cells rather than their photoconversion efficiency, we think that these improved\nphotoconversion efficiency and high thermal stability result are sufficient for stability study.\nFigure S71. Stability comparison of perovskite solar cells. (a) dispersed-solution-SnO\n2\n(SnO₂_DS) vs chemical bath deposition (SnO₂_CBD). (b) CsFAPbI vs RbCsFAPbI .\n3 3\n43\n3-3. It is insufficient to merely use TRPL to characterize the performance of the Spiro-OMeTAD\nthin films after the addition of different additives. If the modified Spiro-OMeTAD has sufficient\nperformance advantages, the author should provide more characteristics of the optimized Spiro-\nOMeTAD, including the energy band, electrical conductivity, and so on.\nAnswer: Thank you for valuable comments. We investigated energy band and electrical\nconductivity to provide more characteristics. Also, UV/vis and FTIR measurement were also\nconducted. (see reviewer 2, Comment 3 and 5)\nPYS/KPFM is measured to investigate the associated working function/energy level changes.\nPhotoelectron yield spectroscopy (PYS) was used to measure the valence band instead of UPS.\nFrom these data, energy diagrams were calculated as shown supplementary Figrure S89. Valence\nband edges (E ) of Spiro-OMeTAD with 4-tert-butylpyridine was -5.58 eV. Interestingly, E of\nv v\nSpiro-OMeTAD with 4-phenylpyridine, 3-phenylpyridine, and 2-phenylpyridine were -5.76 eV, -\n5.76 eV, and -5.83 eV, which is closer to perovskite (-5.89 eV) than 4-tert-butylpyridine. This\ncould be the reason for effective charge transfer and improved photoconversion efficiency.\nThe sentences were revised on page 26.\nFurthermore, energy level diagrams of Spiro-OMeTAD with additives were constructed based\non photoelectron yield spectroscopy and Kelvin probe force microscopy (KPFM) (Supplementary\nFig. S87-89). The valence band edge (Ev) of Spiro-OMeTAD with 4-tert-butylpyridine was -5.58\neV. In contrast, the Ev values of Spiro-OMeTAD with 4-phenylpyridine, 3-phenylpyridine, and\n2-phenylpyridine were -5.76 eV, -5.76 eV, and -5.83 eV, respectively, which are closer to that of\nperovskite (–5.89 eV) than the value obtained with 4-tert-butylpyridine. This alignment is likely\nto facilitate more efficient charge transfer, thereby contributing to improved photoconversion\nefficiency.\npassivation, (c) spiro-OMeTAD with LiTFSI and 4-tert-butylpyridine, (d) spiro-OMeTAD with\nLiTFSI and 4-phenylpyridine, (e) spiro-OMeTAD with LiTFSI and 3-phenylpyridine, (f) spiro-\nOMeTAD with LiTFSI and 2-phenylpyridine.\nFigure S88. KPFM mapping of (a) perovskite, (b) perovskite with OAI passivation, (c) spiro-\nphenylpyridine, (e) spiro-OMeTAD with LiTFSI and 3-phenylpyridine, (f) spiro-OMeTAD with\nLiTFSI and 2-phenylpyridine. Scale bar corresponds to 5 μm.\nFigure S89. Energy diagram of perovskite and spiro-OMeTAD with additives. Energy diagrams\nwere calculated with photoelectron yield spectroscopy, KPFM, and bandgap from UVVIS\nspectroscopy. Bandgap of perovskite and phenylpyridine was associated with 1.56 eV and 3.02\neV, respectively. Spiro-OMeTAD include LiTFSI and each additive same as recipe with device\nfabrication.\nIn terms of conductivity, the resistivity of Spiro-OMeTAD layers containing LiTFSI and each\nadditive was measured using the van der Pauw method (Supplementary Fig. S90). The Spiro-\nOMeTAD layers were deposited on glass substrates by spin-coating. Spiro-OMeTAD solutions\nwere prepared using the same procedure as in the device fabrication. The resistivity of the 4-tert-\nbutylpyridine, 4-phenylpyridine, 3-phenylpyridine, and 2-phenylpyridine were 19,100 Ω cm,\n8,540 Ω cm, 6,050 Ω cm, and 2,680 Ω cm, respectively. From these results, 2,3,4-phenylpyridine\n46\nhave an advantage in the conductivity compared to 4-tert-butylpyridine, resulting in improved\nphotovoltaic performance.\nThe sentences were revised on page 26.\nThe resistivity of Spiro-OMeTAD layers containing LiTFSI and each additive were 19,100 Ω\ncm for 4-tert-butylpyridine, 8,540 Ω cm for 4-phenylpyridine, 6,050 Ω cm for 3-phenylpyridine,\nand 2,680 Ω cm for 2-phenylpyridine, respectively (Supplementary Figure S90). These results\nsuggest that 2,3,4-phenylpyridine have an advantage in the resistivity compared to 4-tert-\nbutylpyridine, resulting in improved FF and PCE.\nFigure S90. The resistivity of Spiro-OMeTAD layers containing LiTFSI and each additive was\nmeasured using the van der Pauw method. The Spiro-OMeTAD layers were deposited on glass\nsubstrates by spin-coating. Spiro-OMeTAD solutions were prepared using the same procedure as\nin the device fabrication.\n47\n3-4. The author spent a great deal of space in the previous part of the manuscript on using additives\nto address the thermal instability of perovskite devices. However, in the subsequent\ncharacterization of the optimized devices, the author tested the operational stability under actual\noutdoor ambient. Although this outdoor stability is crucial for perovskite devices, I fail to find\nsufficient logical coherence between the two parts. According to the temperature diagram\nprovided by the author in the SI, it seems that the stability at high temperatures is not covered.\nInstead, the author emphasizes the performance under diurnal cycles. Therefore, I believe there\nare significant logical issues in this section. The author should focus on and provide the thermal\nstability performance when the efficiency is optimized to over 25%.\nAnswer: Thank you for valuable comments. High efficiency and durability often involve a trade-\noff relationship. Given this situation, we optimized the device to achieve both high efficiency and\ndurability as much as possible. As a result, a device incorporating 2-phenylpyridine achieved an\ninitial PCE of 20.1% and retained 18.7% after 936 hours at 85 °C. These results are included in\nthe manuscript.\nIn contrast, rubidium-doped perovskite (RbCsFAPbI₃) exhibited higher efficiency (24.2%)\ncompared to CsFAPbI₃ (20.1%). However, the stability of RbCsFAPbI₃ dropped significantly after\nonly 87 hours at 85 °C, suggesting that rubidium-doped perovskites themselves may have issues\nwith thermal durability. Correa-Baena et al. reported that a dosage of only 1% Rb was sufficient\nto induce segregation, which represents a different type of problem compared with those\nassociated with additives\n(https://www.sciencedirect.com/science/article/pii/S266693582100104X?utm_source=chatgpt.c\nom). These results highlight that optimization of perovskite composition is also crucial for\nachieving high thermal stability. The corresponding data are provided in the Supplementary\nInformation.\nAlthough the main focus of this work is the thermal stability of perovskite solar cells rather\nthan their power conversion efficiency, we think that the observed improvements in both\nefficiency and thermal stability are sufficiently relevant to the stability study.\n2\n(SnO₂_DS) vs chemical bath deposition (SnO₂_CBD). (b) CsFAPbI vs RbCsFAPbI .\n3 3\nThe sentences were added to supplementary information.\nWe also provided stability comparison data of dispersed-solution-SnO (SnO₂_DS) vs chemical\n2\nbath deposition (SnO₂_CBD) as well as CsFAPbI vs RbCsFAPbI (Supplementary Fig. S71). To\n3 3\nprovide high efficiency PSCs, we optimized the recipe for perovskite fabrication for each\ncondition, which is explained at experimental part. In terms of dispersed-solution SnO₂\n(SnO₂_DS) vs. CBD SnO₂ (SnO₂_CBD), the initial efficiency of SnO₂_CBD with 2-\nphenylpyridine was 20.1% and remained at 18.7% after 936 hours of testing at 85 ℃. In contrast,\nSnO₂_DS with 2-phenylpyridine showed an initial efficiency of 14.9% and retained 16.2% after\n936 hours at 85 ℃. In both cases, 2-phenylpyridine provided higher stability compared with 4-\ntert-butylpyridine. As for CsFAPbI vs RbCsFAPbI , rubidium doped perovskite (RbCsFAPbI )\n3 3 3\nachieved higher efficiency (24.2%) than CsFAPbI (20.1%), however, stability with RbCsFAPbI\n3 3\nshowed significantly dropped after 87 hours of 85 ℃ test. From this result, It is possible that\nrubidium doped perovskite itself has problems with thermal durability. Correa-Baena et al.\n49\nindicated that a dosage of 1% Rb was enough for causing segregation, which is a different kind\nof issue from those associated with additives100. These results suggest that optimization of\nperovskite composition is also important to achieve high thermal stability.\n3-5. Table 1 shows the code of different additives, which is not necessary to present it in main\ntext.\nAnswer: Thank you for pointing out. We moved the code of different additives to supplementary\ninformation.\n3-6. What is the author's numbering criterion for the additives? It seems rather chaotic in the\narticle, as there is no specific order and some of the numbers are missing.\nAnswer: Thank you for your valuable comments. We've cleaned up the additive code to make it\neasier to read.\n50\n3-7. In fig.3, the SEM images of perovskite film based on different spiro-OMeTAD show different\nthickness, the authors should confirm this.\nAnswer: Thank you for your insightful comments. We examined the reproducibility of the Spiro-\nOMeTAD layer and confirmed that its thickness remained the same regardless of the additives.\nVariations in thickness can arise from the spin-coating process, which is influenced by the\nmorphology of the underlying layers, such as SnO₂ and perovskite. By carefully preparing the\nSnO₂ and perovskite layers, we consistently obtained the same thickness for the Spiro-OMeTAD\nlayer.\nFigure 3. SEM images of perovskite solar cells\n51\n3-8. From the EQE curves and integrated Jsc in Fig.S69, the PCE of devices in Fig.5a-c should\nnot obtain such high Jsc. The performance of these devices is overestimated.\nAnswer: Thank you for your suggestions. We have provided the IPCE data and the integrated\ncurrent density of the highly efficient PCSs in Fig. 5c. The integrated current density was 25.14\nmA/cm² for 4-tert-butylpyridine, 25.33 mA/cm² for 4-phenylpyridine, and 25.39 mA/cm² for 2-\nphenylpyridine. The difference between Jsc obtained from the IV curves and the integrated current\ndensity was within 4%, which is acceptable considering recent publications in Nature\nCommunications, Nature, etc. These results have been included in the supplementary information.\nFigure S72. IPCE of perovskite solar cells with 4-tert-butylpyridine (tBP), 4-phenylpyridine\n(A15), and 2-phenylpyridine (A82).\n52\n3-9. It is rarely that 9.8 mg/mL OAI was used for passivating the perovskite surface because it is\nhydrophobic to influence the deposition of Spiro-OMeTAD, the authors should confirm this.\nAnswer: Thank you for your valuable comments. OAI passivation has been introduced on some\npaper[a-c], and we also confirmed that OAI passivation and its hydrophobic properties do not\nsignificantly influence the photovoltaic properties and stability of perovskite solar cells\ncomparing to BAI passivation[d] and PAI passivation[e], which are less hydrophobic due to their\nshorter alkyl chains.\n[a] Zhao, C., Zhou, Z., Almalki, M. et al. Stabilization of highly efficient perovskite solar cells\nwith a tailored supramolecular interface. Nat Commun 15, 7139 (2024).\n[b] Kim, H., Yoo, SM., Ding, B. et al. Shallow-level defect passivation by 6H perovskite polytype\nfor highly efficient and stable perovskite solar cells. Nat Commun 15, 5632 (2024).\n[c] S. Zouhair et al. Employing 2D-Perovskite as an Electron Blocking Layer in Highly Efficient\n(18.5%) Perovskite Solar Cells with Printable Low Temperature Carbon Electrode. Adv. Ener.\nMater., 12, 2200837, (2022)\n[d] S. Mondal et al. Mixed 2D-cation passivation towards improved durability of perovskite solar\ncells and dynamics of 2D-perovskites under light irradiation and at high temperature, Sustain.\nEner. & Fuels, 9, 247, 2025.\n[e] M. Aduhelaiqa et al. Mixed cation 2D perovskite: a novel approach for enhanced perovskite\nsolar cell stability, Sustain. Ener. & Fuels, 6, 2471 (2025).\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nAnswer: We thank the reviewers, including the Early Career Researcher who co-reviewed this\nmanuscript, for their valuable comments and careful evaluation. Their suggestions have been very\nhelpful in improving the clarity and quality of our work."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have added substantial new data that more convincingly support the earlier conclusions, and they have addressed the majority of my previous questions satisfactorily. The manuscript has improved considerably.\n1, The attempt to determine the thin-film Tg by DSC was unsuccessful. Beyond DSC, there are several film-compatible methods to measure the glass transition temperature, such as variable-temperature spectroscopic ellipsometry (identifying the slope change in refractive index/thickness vs temperature) and dynamic mechanical analysis (DMA). Please consider adopting at least one of these approaches and report key parameters (heating rate, substrate, film thickness, etc.).\n2, On page 21, the manuscript states: “Interestingly, 3-phenylpyridine exhibited a slightly more compact structure than 4-phenylpyridine (A15) due to geometric distortion induced by its molecular orientation.” At present this “more compact/crowded” description is based on visual inspection, which is somewhat qualitative. I recommend quantifying the interaction/coordination strength (e.g., binding energy or dissociation barrier from calculations or experiments) to better substantiate the claimed tendency toward dissociation, and include methods and uncertainties in the main text.\n3, The compound codes have been revised, but a few instances of the old codes remain. Please perform a thorough consistency check across the entire manuscript, including figure captions, tables, Supporting Information, and cross-references.", "reviewer_2": "The authors have made detailed response to the reviewers' comments. The revision is greatly improved in the current form. I think this work is suitable for publication in NC.", "reviewer_3": "it can be accepted.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewer\nWe appreciate so much for the effort and dedicated work of the editor and the reviewers. We are\ngrateful for offering the opportunity to improve our work as major revision. We have judiciously\ntaken all comments into account following the comments raised.\nComments on the reviewers: in red color\nOur response: in blue color\nRevised manuscript: in yellow highlighted\nReviewer #1 (Remarks to the Author):\nThe authors have added substantial new data that more convincingly support the earlier\nconclusions, and they have addressed the majority of my previous questions satisfactorily. The\nmanuscript has improved considerably.\nAnswer: We sincerely appreciate your valuable comments. Your insightful suggestions have\ngreatly enhanced the quality of our manuscript.\n1\n1-1. The attempt to determine the thin-film Tg by DSC was unsuccessful. Beyond DSC, there are\nseveral film-compatible methods to measure the glass transition temperature, such as variable-\ntemperature spectroscopic ellipsometry (identifying the slope change in refractive index/thickness\nvs temperature) and dynamic mechanical analysis (DMA). Please consider adopting at least one\nof these approaches and report key parameters (heating rate, substrate, film thickness, etc.).\nAnswer: Thank you for your valuable comments. We are sorry but we do not have the equipment\nfor variable-temperature spectroscopic ellipsometry or dynamic mechanical analysis (DMA). We\nspent a lot of time measuring Tg using DSC, but we found that measurements and analysis of\nmixed materials are quite difficult due to the complex interactions, which require too much time\nto make a conclusion. We would like to research Tg our next research topic.\nInstead, to reveal the mechanism for thermal stability with novel additives, we investigated\nthe effect of additives on elemental distribution using ToF-SIMS (Supplementary Fig. S80). The\nmeasured samples were prepared with the configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD\n(with additives)/Au, following the same procedure as device fabrication. The Au layer was peeled\noff immediately before ToF-SIMS measurement. We compared 4-tert-butylpyridine and 2-\nphenylpyridine before and after thermal aging at 85 °C for 100 hours. Lithium migrated toward\nthe ETL side and slightly accumulated at the surface, which is consistent with a previous report103.\nThe lithium distribution was not significantly affected by the choice of dopant, nor by the thermal\ntest. From these results, 2-phenylpyridine does not appear to promote a more homogeneous\nlithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer (Supplementary Fig. S80a). After the 85 °C test, however, 4-tert-butylpyridine\nhad migrated substantially into the perovskite layer (Supplementary Fig. S80b). This migration is\nlikely driven by its reaction with the perovskite, which is consistent with the by-product confirmed\nby XRD (Figure 4b). In contrast, 2-phenylpyridine was mainly localized in the Spiro-OMeTAD\nlayer, and its distribution showed no significant change after the 85 °C test (Supplementary Fig.\nS80c,d). This stability may contribute to its beneficial effect on thermal stability, probably because\n2-phenylpyridine does not react with the perovskite and therefore does not diffuse into the\nperovskite layer.\nFurthermore, the ToF-SIMS results may help explain the absence of voids in the 2-\n2\nphenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S81a,b). By\ncontrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the\nstability of its distribution (Supplementary Figure S81c,d).\nAlthough it remains difficult to determine the precise mechanism of void formation from these\nresults alone, they may provide valuable design guidelines for void-free HTMs.\nThe sentences were revised on page 23.\nFor further understanding of mechanism of the void-formation, we investigated the effect of\nadditives on elemental distribution using ToF-SIMS. The measured samples were prepared with\nthe configuration FTO/SnO₂/Perovskite/Spiro-OMeTAD (with additives)/Au, following the same\nprocedure as device fabrication. The Au layer was peeled off immediately before ToF-SIMS\nmeasurement. We compared 4-tert-butylpyridine and 2-phenylpyridine before and after thermal\naging at 85 °C for 100 hours. Lithium migrated toward the ETL side and slightly accumulated at\nthe surface, which is consistent with a previous report103. The lithium distribution was not\nsignificantly affected by the choice of dopant, nor by the thermal test. From these results, 2-\nphenylpyridine does not appear to promote a more homogeneous lithium distribution.\nFor the fresh device with 4-tert-butylpyridine, the additive was mainly distributed in the Spiro-\nOMeTAD layer. After the 85 °C test, however, 4-tert-butylpyridine had migrated substantially\ninto the perovskite layer. This migration is likely driven by its reaction with the perovskite, which\nis consistent with the by-product confirmed by XRD (Figure 4b). In contrast, 2-phenylpyridine\nwas mainly localized in the Spiro-OMeTAD layer, and its distribution showed no significant\nchange after the 85 °C test. This stability may contribute to its beneficial effect on thermal stability,\nprobably because 2-phenylpyridine does not react with the perovskite and therefore does not\ndiffuse into the perovskite layer. These ToF-SIMS results may explain the absence of voids in the\n2-phenylpyridine-containing Spiro-OMeTAD layer after the 85 °C test. In the case of 4-tert-\nbutylpyridine, its significant migration from Spiro-OMeTAD into the perovskite layer may have\ncontributed to void formation within Spiro-OMeTAD (Supplementary Figure S80a,b). By\ncontrast, the void-free morphology observed with 2-phenylpyridine can be attributed to the\n3\nstability of its distribution (Supplementary Figure S80c,d).\nFigure S80. ToF-SIMS depth profiles of (a) a fresh device with 4-tert-butylpyridine, (b) the same\ndevice after the 85 ℃ test, (c) a fresh device with 2-phenylpyridine, and (d) the same device after\nthe 85 ℃ test.\nFigure S81. Schematics images of void formation and void-free by additives. (a) a fresh device\n4\nwith 4-tert-butylpyridine, (b) the same device after the 85 ℃ test, (c) a fresh device with 2-\nphenylpyridine, and (d) the same device after the 85 ℃ test.\n5\n1-2. On page 21, the manuscript states: “Interestingly, 3-phenylpyridine exhibited a slightly more\ncompact structure than 4-phenylpyridine (A15) due to geometric distortion induced by its\nmolecular orientation.” At present this “more compact/crowded” description is based on visual\ninspection, which is somewhat qualitative. I recommend quantifying the interaction/coordination\nstrength (e.g., binding energy or dissociation barrier from calculations or experiments) to better\nsubstantiate the claimed tendency toward dissociation, and include methods and uncertainties in\nthe main text.\nAnswer: Thank you for your pointing. To provide a more quantitative assessment rather than a\nqualitative description such as “more compact,” we calculated and compared the minimum edge\nlengths of the smallest boxes that could enclose each molecule. The minimum edge lengths were\ndetermined to be 10.93 Å for 4-tert-butylpyridine, 12.16 Å for 4-phenylpyridine, 8.01 Å for 3-\nphenylpyridine, and 9.06 Å for 2-phenylpyridine, respectively. Also, the desorption energy\nrequired to remove a coordinated molecule from lithium was calculated to be 10.93 Å for 4-tert-\nbutylpyridine (tBP), 12.16 Å for 4-phenylpyridine (K27), 8.01 Å for 3-phenylpyridine (K28), and\n9.06 Å for 2-phenylpyridine (K29), respectively, indicating that there is no relationship between\nthermal stability and ease of molecular desorption energy (page 22). These results led to a more\nquantitative evaluation of the manuscript.\nThe sentence was added on page 22.\nThe minimum edge lengths of the smallest boxes that could enclose each molecule were\ndetermined to be 10.93 Å for 4-tert-butylpyridine (tBP), 12.16 Å for 4-phenylpyridine (K27), 8.01\nÅ for 3-phenylpyridine (K28), and 9.06 Å for 2-phenylpyridine (K29), respectively.\n1-3. The compound codes have been revised, but a few instances of the old codes remain. Please\nperform a thorough consistency check across the entire manuscript, including figure captions,\ntables, Supporting Information, and cross-references.\nAnswer: Thank you for your valuable comments. We have carefully checked the entire manuscript\n6\nto ensure accuracy.\nReviewer #2 (Remarks to the Author):\nRecommendation: Accept\nThe authors have made detailed response to the reviewers' comments. The revision is greatly\nimproved in the current form. I think this work is suitable for publication in NC.\nAnswer: Thank you very much for your valuable comments and for recommending acceptance.\nYour thoughtful feedback has significantly improved our manuscript.\nReviewer #3 (Remarks to the Author):\nRecommendation: Accept\nit can be accepted.\nAnswer: We sincerely appreciate your valuable comments and for recommending acceptance.\nYour insightful suggestions have greatly enhanced the quality of our manuscript.\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nAnswer: We thank the reviewers, including the Early Career Researcher who co-reviewed this\nmanuscript, for their valuable comments and careful evaluation. Their suggestions have been very\nhelpful in improving the clarity and quality of our work.\n7"}, {"round": 3, "reviewer_comments": {"reviewer_1": "I would suggest to accept the ms as is.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter