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Communications Materials volume 6, Article number: 72 (2025) Cite this article
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CsPbI₃, an inorganic perovskite with an excellent bandgap and thermal stability, exhibits great potential for Si-perovskite tandem solar cells although it is highly sensitive to moisture. Here we address this by fabricating CsPbI₃ solar cells under high humidity, controlling annealing via the humidity-dependent volatilization of dimethylammonium iodide. Surface passivation with 1,8-diaminooctane (DAO) reduces Pb surface defects, which enhances charge transport and forms a hydrophobic, moisture-resistant film. The DAO-passivated CsPbI₃ solar cell achieves a power conversion efficiency of 17.7% and retains 92.3% of its initial efficiency after 1500 minutes of maximum power point tracking in 30% relative humidity without encapsulation, demonstrating improved humidity stability. All solution-processed layers were fabricated in ambient air, highlighting their potential for cost-effective mass production.
Since perovskite materials were first employed as active layers, perovskite solar cells (PSCs) have delivered pioneering research results^1,2,3. Recently, single-junction hybrid halide perovskite compositions reached a record efficiency of 26.7%, whereas Si-perovskite tandem cells achieved 34.6%, thereby highlighting the potential of perovskites for high-efficiency applications. Despite these advancements, PSCs still face considerable challenges in terms of their stability and scalability. Stability issues include moisture^4,5,6,7, light⁸, heat⁹, and long-term performance. Among these, moisture remains one of the most critical factors, as it penetrates through defects or grain boundaries on the perovskite surface, thereby resulting in the structural degradation of the material.
Inorganic perovskite CsPbI₃ has been extensively investigated because of its optimal bandgap (~1.7 eV) for Si-perovskite tandem solar cells and potential for thermal stability^10,11,12,13. Recently, CsPbI₃-based inorganic PSCs have garnered considerable attention, thereby achieving power conversion efficiencies (PCEs) above 21%^14,15. However, CsPbI₃ remains highly sensitive to humidity^16,17, thereby resulting in moisture-induced degradation of the perovskite films, which compromises the surface coverage and results in poor film morphology¹⁸. Previous studies employing dimethylammonium iodide (DMAI) have demonstrated the feasibility of fabricating CsPbI₃ under relative humidity (RH) levels below 30%, thereby enabling limited device operation^19,20,21. However, these approaches fail to meet the requirements for the mass production and long-term stability of perovskite solar cells. To enable fabrication under high-humidity conditions, Lu et al. introduced mercaptopropylmethyldimethoxysilane as an additive, which formed hydrophobic polymers through humidity-induced polymerisation within the film²². Fu et al. utilised maleic anhydride additives that react with atmospheric water to form hydrolysis products, thereby creating a surface “shield” to suppress phase transitions²³. Liang et al. fabricated uniform and compact CsPbI₃ films by applying an anti-solvent at various temperatures and controlling nucleation, which is difficult to fabricate under high-humidity conditions²⁴. Their study confirmed the feasibility of fabrication at RH 60% and achieved an efficiency above 16%. However, the study did not address the operational stability under moisture, as critical evaluations, such as J-V measurements, were conducted in an N₂ atmosphere.
When exposed to prolonged moisture, CsPbI₃ undergoes phase transitions owing to the reduced energy barrier, thereby decomposing into the non-photoactive δ-phase^25,26. This phase transition poses a considerable challenge, as δ-CsPbI₃ cannot function as an active layer. Consequently, prior studies have enhanced humidity resistance by incorporating hydrophobic materials such as phenyl-based or long alkyl chain passivators^{27,28,29,30,31,32,33,34}. Wang et al. demonstrated that phenylethylammonium iodide improved the stability of CsPbI₃ films by maintaining the perovskite phase under RH 85% for 24 h³⁵. This study served as a key milestone, thereby inspiring further research into humidity-resistant passivation strategies. Subsequent studies include those by Yoon et al., who used octylammonium iodide to stabilise CsPbI₃ films at RH 65% for 2 h, and Wang et al., who achieved 250 h of phase stability at RH 35% using 1,2-di(thiophen-2-yl)ethane-1,2-dione^36,37. However, these advancements were achieved under controlled low-humidity conditions, with most passivation processes conducted in glove boxes. Maintaining strict humidity control in industrial settings considerably increases mass-production costs. Further, commercialising PSCs requires rigorous product reliability testing, wherein the moisture content remains a major challenge. Hence, both fabrication and operational stability under ambient conditions must be addressed simultaneously.
This study investigates the volatilisation behaviour of DMAI during CsPbI₃ film formation under different humidity and annealing conditions, particularly at high humidity levels. The results revealed that the volatilisation rate of DMAI varied considerably with ambient humidity during perovskite phase formation. Based on these findings, we propose a method for fabricating CsPbI₃ under uncontrolled humidity conditions by optimising the annealing conditions. Additionally, we present processing conditions for hole transport layers (HTLs) and passivation under high humidity, thereby enabling the fabrication of all layers in ambient environments. Furthermore, we demonstrate how the DMAI behaviour results in the formation of DMAI-rich CsPbI₃ surfaces during ambient processing, thereby elucidating the origin of the undercoordinated Pb defects. By employing 1,8-diaminooctane (DAO) as a passivation layer, we improved moisture resistance and enhanced the performance of the CsPbI₃-based PSCs. DAO, with its long alkyl chain (eight carbon atoms) and diamine groups, effectively modified the CsPbI₃ surface, thereby making it hydrophobic. Additionally, DAO bonded with surface defects, particularly undercoordinated Pb, reduced charge recombination. DAO passivation increased the PCE of the CsPbI₃ PSCs to 17.7%, with a open-circuit voltage (V_OC) of 1.089 V and fill factor (FF) of 83.99%. These findings not only demonstrate the potential for processing CsPbI₃ and its other layers under high humidity but also highlight the enhanced moisture stability, thereby indicating considerable potential for cost-effective mass production.
To investigate the effect of humidity on the CsPbI₃ perovskite film formation, fabrication was performed under varying relative humidity conditions: RH 0% (Ar-filled glove box), RH 45% and 60% in ambient air (20–22 °C) (Fig. 1). CsPbI₃ films were fabricated using a precursor containing DMAI, a key additive in various aspects (Fig. 1). This figure shows the phase formation of the CsPbI₃ films during the annealing process under different RH conditions. Herein, the light-yellow intermediate phase film transitioned to a black perovskite phase as N,N-Dimethylformamide (DMF) and DMAI evaporated during annealing (Supplementary Fig. 1). The timing of the black perovskite phase formation varied considerably based on the humidity level. For example, a comparison between RH 0% and 60% showed that it took over 9 min to achieve a nearly black film at RH 0%, whereas the film at RH 60% turned black within 3 min. This behaviour highlighted the humidity-dependent DMAI evaporation temperatures, as demonstrated using thermogravimetric analysis in a previous study³⁸.
CsPbI₃ films are annealed at 190 °C for 10 min in different humidity, RH 0% (Ar), 45% and 60% (21.5 °C) respectively.
The DMAI additive facilitated the formation of the CsPbI₃ perovskite structure by enabling the evaporation of the DMA⁺ ions, which were subsequently replaced by Cs⁺ ions³⁹. The variation in the DMAI evaporation temperature with humidity indicated that the formation rate of CsPbI₃ perovskite was similarly affected by humidity. To confirm this, we prepared CsPbI₃ films under various conditions and analysed their properties. As presented in Supplementary Table 1, we compared the bandgaps under varying humidity levels to identify the formation point of CsPbI₃. At RH 0%, the film annealed for 10 min exhibited a band gap of 1.68 eV as shown in Fig. 2a, thereby indicating the presence of the DMA-β-CsPbI₃ phase⁴⁰. Conversely, the films annealed for 6 and 8 min exhibited band gaps lower than 1.67 eV, thereby indicating incomplete DMAI evaporation and the presence of considerable residual DMAI within the film. This suggests that the stable DMA-β-CsPbI₃ perovskite had not yet fully formed, thereby corresponding with the observed lower absorbance (Supplementary Table 1). At RH 45% and 60% (Fig. 2b and c), the band gaps ranged from 1.69–1.70 eV, thereby indicating the coexistence of DMA-β-CsPbI₃ and γ-CsPbI₃ phases. This overlap made it challenging to determine an optimal annealing time, as less annealed films transitioned from the γ-CsPbI₃ phase to the δ-phase owing to unstable perovskite stoichiometry, while over-annealed films formed γ-CsPbI₃. The over-annealed films were subjected to high humidity and temperature stress after forming γ-CsPbI₃, thereby highlighting the need for the precise optimisation of the annealing time. As shown in Supplementary Fig. 3, areas not noticeably covered by the perovskite films appeared after 20 min of heat treatment at RH 45% and 60%, respectively. As shown in the X-ray diffraction (XRD) pattern of Fig. 2e and f, CsPbI₃ films annealed for 10 min at RH 45% and for 8–10 min at RH 60% exhibited a pronounced δ-peak, thereby indicating a phase transition induced by stress from high humidity and temperature. The CsPbI₃ film annealed for 6 min at RH 60% exhibited a strong perovskite (220) peak and relatively high δ-peak, thereby highlighting the influence of humidity in triggering phase transitions to the δ-phase owing to the residual moisture on the surface. Conversely, the film annealed for 8 min at RH 45% exhibited almost no δ-peak, thereby confirming that DMAI can resist phase transitions up to approximately RH 40%³⁸.
Tauc plot of CsPbI₃ film annealed at a RH 0%, b 45%, c 60% for 6, 8, 10 min. d–f XRD spectra of CsPbI₃ films with different RH and annealing time. The symbols mark diffraction peaks related to black γ‐CsPbI₃ (▲,★), yellow δ‐CsPbI₃ (●) and DMAPbI₃ (†).
As presented in Supplementary Table 2, the peak ratio from the perovskite films provided a means to assess the film crystallinity, thereby showing that the highest perovskite ratios were achieved under the following optimised annealing conditions: 10 min for RH 0%, 8 min for RH 45% and 6 min for RH 60%. It is assumed that DMA-β-CsPbI₃ forms at RH 0%, while DMA-β-CsPbI₃ and γ-CsPbI₃ coexist at RH 45% and 60%, respectively. Additionally, a continuous peak at approximately 11.8°, which corresponds with DMAPbI₃, was observed in the 0% RH film, although it was absent in the RH 45% and 60% films (Fig. 2d)⁴¹.
The absence of a DMAPbI₃ peak suggests that higher humidity effectively enhances DMAI evaporation, thereby facilitating the formation of stable CsPbI₃-based perovskite. The SEM images (Fig. 3a, b, and d) show that the less-annealed CsPbI₃ films (RH 0% for 6 and 8 min and RH 45% for 6 min) exhibited indistinct grain boundaries and an amorphous surface. Over-annealed films (RH 45% RH for 10 min and RH 60% for 8 and 10 min) transitioned to the δ phase, with grain boundaries collapsing and elongating into the δ phase grains around visible pinholes (Fig. 3f, h, and i). Conversely, the optimised films (RH 0% for 10 min, RH 45% for 8 min and RH 60% for 6 min) exhibited well-oriented grains with uniform surfaces, thereby highlighting the importance of precise annealing conditions in achieving high-quality perovskite films (Fig. 3c, e, and g).
a–i Top-view SEM images of CsPbI₃ films with different RH and annealing time. XPS spectra of j N 1s, k Pb 4f, and l I 3d for the CsPbI₃ films in Ambient and Ar.
Figure 3c, e, and g revealed a notable difference: while the film processed at RH 0% at 10 min did not exhibit large grains, RH 45% and 60% at 8 and 6 min, respectively, exhibited pronounced surface grains. Analysis of the full width at half maximum (FWHM) values (Supplementary Table 2) showed that the overall grain size, calculated using the Scherrer equation, was larger for RH 0% at 10 min. However, the surface characteristics suggested an opposite trend. This discrepancy was confirmed using the X-ray photoelectron spectroscopy (XPS) analysis, as the film processed in ambient air exhibited a peak at 399.8 eV in the N 1s spectrum, while no such peak was observed in the film processed under RH 0% conditions in an Ar-filled glove box (Fig. 3j). CsPbI₃ films fabricated under RH 0% remain unaffected by the reaction between DMAI and H₂O, displaying only a single peak at 402 eV, which corresponds to DMAI. In contrast, films prepared under ambient air exhibit an additional peak at 399.8 eV, resulting from the reaction between DMAI and H₂O. This reaction reduces the oxidation state of nitrogen from +1 to 0, leading to a corresponding decrease in binding energy, as observed in the N 1s spectra. Additionally, the film processed in ambient air exhibited relatively lower peak intensities in the Pb 4f and I 3d spectra than that processed under RH 0% conditions (Fig. 3k and l). These findings indicate that in the films processed under ambient air conditions (RH 45% and 60%), DMAI volatilized rapidly, thereby leaving little to no DMAI in the bulk and concentrating it near the surface⁴². Conversely, films processed at RH 0% showed a uniform distribution of DMAI across the surface and bulk⁴⁰. Consequently, the large grains observed in the films processed under ambient air conditions are likely DMAI-rich phase grains, with the surface exhibiting a pronounced DMAI-rich composition. These results can be explained by incorporating the moisture-induced effects on DMAI volatilization into the reaction mechanism, as expressed in Equation 1 as follows:
This equation shows that during the annealing process, H₂O acts as a catalyst, thereby decomposing DMAI into C₂H₇N and HI. While DMAI possesses a melting point of 153.85 °C, C₂H₇N and HI have lower boiling points of 7 °C and −35.1 °C, respectively. This explains the increased volatilisation of DMAI under high-humidity conditions.
Supplementary Fig. 4 shows a graph of the area wherein the PCE exceeds 10% at different RH values (0% and 40–60%). By optimising the annealing times at RH 0%, RH 45%, and 60%, a higher humidity results in shorter annealing durations, thereby allowing the perovskite solar cell to function effectively.
Because [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz), which is used as an HTL, must also be fabricated under varying humidity conditions, XPS analysis was conducted^{43,44,45,46,47}. The binding energy of the P 2p peak remained at 133.4 eV across all humidity conditions, thereby indicating that the anchor portion of MeO-2PACz effectively formed a covalent bond with the FTO substrate (Supplementary Fig. 5)⁴⁸. The ultraviolet photoelectron spectroscopy (UPS) measurements revealed work functions of 5.17, 5.16, and 5.11 eV for RH 0%, RH 45%, and 60%, respectively, thereby suggesting minimal impact on the device performance (Supplementary Fig. 6). To investigate whether the terminal group of the self-assembled monolayer (SAM) influences the crystallinity of the perovskite, a CsPbI₃ perovskite was fabricated on top of MeO-2PACz, followed by absorbance, bandgap, XRD, and scanning electron microscope (SEM) analyses (Supplementary Figs. 7, 8, and 9). CsPbI₃ was fabricated under optimised conditions for each humidity level, and no considerable differences were observed, thereby confirming that the SAM processed under different humidity conditions did not alter the perovskite characteristics.
Although the conditions for fabricating CsPbI₃ under various high-humidity conditions have been identified, passivation is crucial for the long-term stability of the perovskite solar cell, thereby enabling them to withstand the stress induced by moisture exposure.
CsPbI₃ is a perovskite material that degrades rapidly in the presence of moisture, thereby resulting in extensive studies aimed at improving its humidity resistance via passivation³⁵. In this study, DAO, a passivation material with a diamine and long alkyl chain, was selected. Although experiments have been conducted using 1,12-Diaminododecane (DAD), which features a longer alkyl chain than DAO, its low polarity results in poor solubility in solvents, thereby limiting the process to concentrations below 4 mg/mL. At a low concentration of 1 mg/mL, although DAD exhibited optimal efficiency, it deviated from the intended hydrophobicity, thereby resulting in an efficiency of only 13%, which was lower than expected. This result suggests that excessively long alkyl chains are unsuitable as passivation layers (Supplementary Fig. 10).
When passivating in ambient air with high humidity, surface moisture can interfere with the Lewis acid-base interactions between DAO and CsPbI₃ or infiltrate the perovskite structure owing to the dynamic coating damage, thereby resulting in decomposition into the δ-phase⁴⁹. To address this, widely used solvents such as isopropanol (IPA) and toluene were compared. In a glovebox environment, IPA and toluene facilitated stable passivation without affecting the absorbance (Supplementary Fig. 11). However, in ambient air, particularly under high humidity, IPA damaged the film, thereby reducing the absorbance, which was sometimes visible (Supplementary Fig. 12). Conversely, toluene caused less damage under similar conditions. To minimise this damage, a highly concentrated DAO solution (12 mg/mL) in toluene was used to coat the sample with the smallest volume necessary.
As shown in the cross-sectional SEM images, passivation was achieved without considerably affecting the thickness of the perovskite films (Fig. 4a). Unlike conventional passivation materials with smaller molecular sizes, DAO (with its long alkyl chain) adhered to the surface, as illustrated in the SEM images. Small particles preferentially formed in areas with lower surface roughness, while the grain boundaries became less distinct after DAO passivation. AFM results show that the root mean square roughness (R_q) of the perovskite film decreased from 16.04 nm to 9.35 nm after DAO passivation, forming a more uniform surface, as observed in the SEM cross-sectional images (Supplementary Fig. 13). The enhancement of the moisture owing to DAO was quantified using contact angle measurements (Fig. 4b). The control sample had a contact angle of 41°, whereas that of the DAO-passivated film showed a significant increase to 75.5°, thereby indicating that the surface became hydrophobic owing to the long alkyl chains. This reduces the likelihood of water penetrating the grain boundaries or defects, thereby protecting the perovskite structure from damage. To evaluate the improved humidity resistance of the DAO-passivated CsPbI₃ film, the control (without DAO) and DAO-passivated films were exposed to RH 70% for 30 min (Supplementary Fig. 16). UV-vis spectroscopy revealed that the absorbance of the DAO-passivated films was slightly lower than that of the fresh control films, as shown in Fig. 4c. However, after exposure to humidity, the absorbance of the control film decreased to 0.3938 at 700 nm, which was significantly lower than that of the DAO-passivated film (0.5676). This suggests that the CsPbI₃ film in the control sample partially transitioned to the δ-phase. XRD analysis showed that the DAO-passivated film maintained the CsPbI₃ peaks at (110) and (220), which is similar to the fresh samples, whereas the control sample exposed to humidity exhibited a δ-phase peak at approximately 10° (Fig. 4d). To conclude, DAO effectively bonded to the CsPbI₃ surface, thereby providing effective passivation. The long, hydrophobic alkyl chains of DAO considerably improved the moisture resistance of the CsPbI₃ film, thereby protecting its structural integrity from humidity-induced degradation.
a SEM images of CsPbI₃ films and Cross-sectional SEM images of CsPbI₃ films on FTO glass. b The contact angles of water on CsPbI₃ films with (DAO) and without DAO passivation (Control). c UV-vis absorption spectra of CsPbI₃ films with (DAO) and without DAO passivation (Control) when fresh and exposed to moisture. d XRD spectra of CsPbI₃ according to with (DAO) and without DAO passivation (Control) when fresh and exposed to moisture.
DAO passivation is not only crucial for improving moisture resistance but also serves as a multifunctional passivation material to modify the CsPbI₃-electron transport layer (ETL) interface in inverted CsPbI₃ PSCs. Recent studies on passivating inverted CsPbI₃ PSCs^{50,51,52,53,54,55,56} highlighted the importance of removing defective undercoordinated Pb from the CsPbI₃ surface, a concept first introduced in the studies conducted by Fu et al.⁵⁷. This corresponds with the XPS analysis shown in Fig. 3j, wherein the surface formed DMAI-rich CsPbI₃, thereby disrupting the ABX₃ stoichiometry. Consequently, defects arising from undercoordinated Pb dominant, thereby highlighting the necessity and effectiveness of passivation in inverted CsPbI₃ PSCs.
Photoluminescence (PL) spectroscopy (Fig. 5a) revealed that the DAO-passivated film exhibited a significantly enhanced emission peak intensity at 722 nm, thereby indicating a considerable decrease in non-radiative recombination⁵⁸. To further investigate the carrier dynamics within the CsPbI₃ films, time-resolved photoluminescence (TRPL) decay measurements were conducted (Fig. 5b), and the corresponding fitted data are presented in the supplementary Information. The carrier lifetime of the DAO-passivated CsPbI₃ films was notably longer than that of the control film, thereby confirming the improved film crystallinity and reduced surface defect density. To investigate the changes in the electronic structure induced by the alkylamine treatment, UPS measurements were performed. The binding energy cutoff and onset values for these films (Fig. 5c) facilitated the calculation of the valence band maximum (VBM). The conduction band minimum (CBM) was determined by adding a bandgap (1.68 eV) to the VBM. For the control film, the CBM and VBM were determined to be −3.71 eV and −5.39 eV, respectively, whereas the DAO-passivated film exhibited shifts to −3.67 eV and −5.35 eV. The energy levels of the perovskite and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) used as ETL are illustrated in Fig. 5d. Despite a slight misalignment in the energy levels, the Fermi level of the perovskite shifted upward to −4.39 eV, thereby indicating improved alignment with the Fermi level of PCBM. This finding corresponds with the n-type modification trend observed in previous studies on alkylamine passivation.
a PL and b TRPL curves of with (DAO) and without DAO passivation (Control). c UPS spectra of the cut-off edge (left) and Valence band (right) of control and DAO passivation samples. d Energy level alignment of CsPbI₃ with (DAO) and without DAO passivation (Control).
To investigate the effect of DAO passivation on photovoltaic performance, inverted CsPbI₃ PSCs with the structure FTO/MeO-2PACz/CsPbI₃/DAO/PCBM/C₆₀/BCP/Ag were fabricated, as illustrated in Fig. 6a. The champion cell with DAO passivation achieved an improved PCE of 17.68%, with an open-circuit voltage (V_OC) of 1.089 V and fill factor (FF) of 83.99%. In comparison, the best control device exhibited a lower PCE of 10.88 %, V_OC of 0.9690 V and FF of 64.86%. Figure 6b shows the improvements in V_OC and FF, including the enhanced reproducibility. These performance gains are attributed to effective defect passivation, particularly of the undercoordinated Pb sites. DAO passivation appears to facilitate carrier transport while reducing charge recombination at the CsPbI₃-PCBM interface. This is further supported by XPS measurements of the perovskite film, which show that after DAO passivation, the Pb⁰/ (Pb⁰ + Pb²⁺) ratio decreases from 11.7% in the control sample to 4.87%. This reduction confirms that mitigating Pb⁰ through DAO passivation plays a key role in defect passivation (Supplementary Fig. 17). DAO passivation appears to facilitate carrier transport and reduce charge recombination at the CsPbI₃-PCBM interface. External quantum efficiency (EQE) measurements (Fig. 6c) validated the short-circuit current density (J_SC) values, with integrated J_SC values of 16.32 mA cm^-2 for the control device and 17.92 mA cm^-2 for the DAO-passivated device, which corresponds with the J-V characteristics. In both measurements, the increase in J_SC of the DAO passivation film is due to the better humidity stability of the DAO passivated device, which prevents the formation of a degraded phase (δ-phase).This trend corresponds with the absorbance data shown in Fig. 4c. Electron-only devices with structure FTO/SnO₂/CsPbI₃/PCBM/C₆₀/BCP/Ag were fabricated to evaluate the trap density(N_traps) of CsPbI₃ films using space-charge limited current (SCLC) measurement, with the equation N_traps = 2V_TFLε_rε₀q^-2L^-2_, Here, ε_r, ε₀, q and L represent the relative dielectric constant(ε_r of CsPbI₃ = 6.32)⁵⁹, vacuum permittivity, elementary charge and thickness of the perovskite films(~500 nm)⁶⁰. Notably, the DAO-passivated device exhibited a smaller V_TFL of 0.622 V than that of the control CsPbI₃ films (0.770 V) (Fig. 6d). Consequently, the trap densities for the control CsPbI₃ and DAO passivated-device films were calculated to be 2.15 × 10¹⁵ and 1.74 × 10¹⁵cm⁻³, respectively, thereby confirming a considerable decrease in the trap density for DAO passivated-CsPbI₃. These results indicate that DAO passivation suppresses nonradiative recombination, thereby enhancing V_OC and FF. To evaluate the stability of the PSC devices, maximum power point (MPP) tracking was conducted under continuous illumination in ambient air conditions (20–22 °C, RH 30%) using unencapsulated devices. After 1500 min of MPP tracking, the target device retained 92.38% of its initial, whereas the control device exhibited a considerable decrease, thereby maintaining only 61.30% of its initial efficiency after 1200 min.
a J-V curves of the champion PSCs fabricated under with (DAO) and without DAO passivation (Control) at same relative humidity. b Photovoltaic parameters statistics of the open circuit voltage (V_OC) and fill factor (FF) based on control and DAO passivation films. A horizontal line across the box indicates the median, and a small box inside represents the mean. c EQE spectra of the champion PSCs fabricated under with (DAO) and without DAO passivation (Control) at same relative humidity. d current–voltage curve in electron-only devices. e MPP tracking stability of the unencapsulated devices under continuous illumination.
In summary, we investigated the effects of humidity on layer fabrication and successfully processed all the solution-processed layers in uncontrolled ambient air, thereby resulting in enhanced device performance. By analysing the phase transition of the CsPbI₃ films fabricated under varying humidity conditions (RH 0%, RH 45%, and 60%), we observed a strong dependence of the formation process on humidity. These results indicate that a higher humidity accelerates DMAI volatilisation, thereby allowing us to optimise the annealing time to fabricate high-quality CsPbI₃ films under different humidity conditions. By revealing the DMAI-rich surface of the ambient-air processed CsPbI₃, we provided evidence that passivation is crucial and effective for inverted CsPbI₃ PSCs.Utilising DAO as a passivation material considerably improved the moisture stability of the CsPbI₃ films, thereby enhancing their resistance to humidity-induced degradation owing to the long hydrophobic alkyl chains. Further, DAO passivation reduced non-radiative recombination, which is similar to the effect of other alkylamines. These improvements were reflected in the enhanced photovoltaic performance, with the DAO-passivated devices achieving a PCE of 17.7%, primarily owing to the enhanced V_OC and FF. Consequently, the DAO-passivated devices retained 92.3% of their initial efficiencies after 1500 min of MPP tracking in RH 30% air without encapsulation.
In conclusion, optimising the fabrication conditions for high-humidity environments is crucial for improving the CsPbI₃ perovskite film performance, while passivation strategies such as DAO are crucial for enhancing their stability and electronic properties. Further, all solution-processed layers were fabricated in uncontrolled ambient air, thereby indicating their potential for cost-effective mass production.
Fluorine-doped tin oxide (FTO) substrates were purchased from Pilkington. Cesium iodide (CsI, 99.999%, 203033), dimethylammonium iodide (DMAI, 98%, 805831), fullerene-C₆₀ (99.5%, 379646), 1,8-Diaminooctane (DAO, 98%, D22401), 1,12-Diaminododecane (DAD, 98%, D16401), Phenyl-C61-butyric acid methyl ester (PCBM, 99.5%, 684449), 2-propanol (IPA, anhydrous, 99.5%, 278475), N,N-dimethylformamide (DMF, anhydrous, 99.8%, 227056), toluene (anhydrous, 99.8%) and chlorobenzene (CB, anhydrous, 99.8%, 284513) were purchased from Sigma-Aldrich. Lead iodide (PbI₂, 99.99% L0279), [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz, 98.0%, D5798) and bathocuproine (BCP, 99.9%, B2694) were purchased from Tokyo Chemical Industry (TCI).
The patterned FTO glass substrates were cleaned and ultrasonicated in acetone, ethanol, and 2-propanol for 10 min each. After sonication, the FTO substrates were exposed to ultraviolet ozone for 15 min. The FTO substrates were then spin-coated (3000 rpm for 30 s) with a 1.5 mg/mL of MeO-2PACz IPA solution, followed by annealing at 100 °C for 10 min. The 0.7 M CsPbI₃ perovskite precursor was prepared by dissolving stoichiometric amounts of CsI, PbI₂ and DMAI in DMF in a 1:1:1.2 molar ratios. The CsPbI₃ active layer was spin coated onto the pre-warmed MeO-2PACz/FTO substrate (70 °C) at 3,000 rpm for 30 s, followed by a 190 °C annealing treatment under varying RH conditions (RH 0% and 30~60%, at 20~22 °C). The annealed perovskite films were post-treated using 4, 8, 12, 16 mg/mL DAO toluene solutions via spin coating (3000 rpm for 30 s), followed by annealing at 70 °C for 5 min. Then, the post-treated perovskite film was spin-coated (2000 rpm for 40 s) with a 10 mg/mL PCBM CB solution. All the spin-coated layers were fabricated in ambient air (RH 30~60%, 20~22 °C). Finally, C₆₀, BCP and Ag metal electrodes were deposited using thermal evaporation in the following sequence: 30 nm, 8 nm, and 100 nm, respectively.
Ultraviolet–visible (UV–vis) spectroscopy was performed using a JASCO V-670 instrument. The crystallinity of the films was analysed using X-ray diffractometry (XRD, Rigaku SmartLab) with a Cu Kα source (1.54 nm). The morphologies of the films were examined using field-emission scanning electron microscopy (FE-SEM (SU5000) and energy-dispersive X-ray spectroscopy (EDS, Ultim Max40). The surface states and binding energy spectra were obtained using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) (Nexsa). Kelvin probe force microscopy (KPFM) was used to measure the surface potential of MeO-2PACz using an XE-100 instrument. Hydrophobicity was measured with a contact angle meter (DSA 25, Kruss) using pure DI water droplets. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses were performed using a confocal microscope (FluoTime 300, PicoQuant). External quantum efficiency (EQE) measurements were conducted using QEX10, with a single-mode pulsed diode laser (405 nm, pulse width of approximately 40 ps, and repetition rate of 1 MHz) as the excitation source. The space-charge limited current (SCLC) was measured using a CompactStat (Ivium). Photovoltaic parameters were recorded using a solar simulator (WACOM WXS-155S-10 class AAA) with a xenon lamp at 100 mW/cm² and source meter (Keithley 2400) under RH conditions of 30–60% in ambient air. During the photovoltaic performance measurements, an aperture mask with an active area of 0.075 cm² was applied. The light intensity was calibrated using a modulated reference Si solar cell. The voltage scan range was −0.2 to 1.2 V, with a scan rate of 40 mV/s.
The data supporting the findings of this study are available in the supplementary Information and from the corresponding authors upon request.
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This work was funded by the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and supported by grants (No. 2023-00236664) from the Ministry of Trade, Industry, and Energy of the Republic of Korea. This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government(MOTIE) (RS-2024-00451343).
These authors contributed equally: Sangwon Lee, Sujin Cho.
Department of Energy System Engineering, Graduate School of Energy and Environment (KU-KIST Green School), Korea University, Seoul, Korea
Sangwon Lee, Youngmin Kim, Jihyun Jang, Yoonmook Kang & Hae-Seok Lee
Department of Materials science and Engineering, Korea University, Seoul, Korea
Sujin Cho, Seok-Hyun Jeong, Wonkyu Lee, Dowon Pyun, Jiyeon Nam, Ji-Seong Hwang & Donghwan Kim
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Conceptualization and co-wrote the manuscript, S.L. and S.C.; methodology, S.-H.J.; validation. W.L; formal analysis, D.P.; investigation, J.N.; resources, J.-S.H.; data curation, Y.K.; writing—review and editing, S.L. and S.C.; visualization, J.J.; supervision, Y.K.; project administration, D.K.; funding acquisition, H.-S.L. All authors have read and agreed to the published version of the manuscript.
Correspondence to Hae-Seok Lee.
The authors declare no competing interests.
Communications Materials thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: Wei Zhang and Jet-Sing Lee. [A peer review file is available].
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Lee, S., Cho, S., Jeong, SH. et al. Inverted CsPbI₃ perovskite solar cells with all solution processed layers fabricated in high humidity. Commun Mater 6, 72 (2025). https://doi.org/10.1038/s43246-025-00796-1
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Received: 22 December 2024
Accepted: 31 March 2025
Published: 12 April 2025
Version of record: 12 April 2025
DOI: https://doi.org/10.1038/s43246-025-00796-1
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