Perovskite solar cells have emerged as a promising alternative to traditional silicon-based photovoltaics due to their low cost, tunable bandgaps, and high defect tolerance. Among them, inorganic perovskite solar cells, particularly those based on CsPbI3 and its derivatives like CsPbI2.85Br0.15, offer superior thermal and photostability compared to their organic-inorganic hybrid counterparts. However, the high defect density at the film surface, acting as non-radiative recombination centers, leads to significant open-circuit voltage losses (Vloss), limiting the power conversion efficiency (PCE). Additionally, the phase instability of inorganic perovskites in ambient conditions poses a challenge for large-scale production. In this study, we address these issues by employing 1,8-octylenediamine hydroiodate (ODADI) as a surface modifier for CsPbI2.85Br0.15 films processed in air. The ODADI treatment passivates iodide vacancies (VI–) through hydrogen bonding and iodide ion supplementation, effectively suppressing ion migration and non-radiative recombination. As a result, we achieved a champion PCE of 19.46% for N-I-P structured devices fabricated entirely in air, along with enhanced stability under humidity and thermal stress. This work demonstrates a straightforward strategy for advancing inorganic perovskite solar cells towards practical applications by improving efficiency and processability in ambient environments.
The development of high-performance perovskite solar cells has been a focal point in renewable energy research, driven by the urgent need for sustainable power sources. Traditional silicon solar cells, while efficient, face limitations in cost and scalability. In contrast, perovskite materials, with their ABX3 structure (where A is a cation like Cs+, B is Pb2+, and X is a halide like I– or Br–), exhibit excellent light absorption and charge transport properties. Inorganic perovskite solar cells, specifically those using CsPbI3, have a bandgap of approximately 1.70 eV, making them suitable for tandem applications and offering better stability under thermal stress. However, the small radius of Cs+ ions can lead to lattice distortions and phase transitions from the photoactive black phase (β) to a non-photoactive yellow phase (δ) in the presence of moisture. This instability is exacerbated by defects such as iodide vacancies (VI–) and lead vacancies (VPb2+), which act as traps for charge carriers, increasing non-radiative recombination and reducing VOC. To mitigate these issues, various passivation strategies have been explored, including solvent engineering, interface modification, and dimensional control. Most of these methods, however, rely on inert atmospheres like nitrogen gloveboxes, which hinder industrial scalability. Our approach focuses on air-processable inorganic perovskite solar cells using ODADI, a long alkyl chain halide, to enhance surface passivation and device performance. By fabricating devices in air with controlled humidity (RH ≤20%), we demonstrate that ODADI not only reduces defect densities but also improves hydrophobicity, leading to robust perovskite solar cells with high efficiency and stability.
In our experimental setup, we fabricated N-I-P structured perovskite solar cells with the configuration FTO/SnO2/CsPbI2.85Br0.15/ODADI/Spiro-OMeTAD/Au. The FTO substrates were cleaned sequentially with detergent, deionized water, and isopropanol, followed by UV-ozone treatment. A SnO2 electron transport layer was deposited by spin-coating a diluted colloidal solution and annealing at 150°C in air. The perovskite precursor was prepared by dissolving CsI, HPbI3, and PbBr2 in a DMF:DMSO (1:1) mixture, which was then spin-coated onto the SnO2 layer and annealed at 180°C in air. For the ODADI treatment, a solution of 0.3 mg/mL in isopropanol was spin-coated on the perovskite film, followed by annealing at 80°C. The hole transport layer, Spiro-OMeTAD, was applied with additives like 4-tert-butylpyridine and Li-TFSI, and finally, an 80 nm gold electrode was evaporated. This process ensures that all steps, except for the ODADI treatment and electrode deposition, are performed in air, making it suitable for large-scale production. The use of ODADI as a surface modifier leverages its bifunctional nature: the amine groups act as Lewis bases to coordinate with undercoordinated Pb2+ sites, while the iodide ions fill VI– vacancies, thereby reducing defect states. We characterized the films using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy, while device performance was evaluated through current density-voltage (J-V) measurements, external quantum efficiency (EQE), time-resolved photoluminescence (TRPL), space-charge-limited current (SCLC), and electrochemical impedance spectroscopy (EIS).
The surface morphology of the CsPbI2.85Br0.15 films was significantly improved after ODADI treatment. SEM images revealed that the control films exhibited numerous pinholes and small grains, which are common sites for defect formation and non-radiative recombination. In contrast, the ODADI-treated films showed larger, more uniform grains with reduced pinhole density, indicating enhanced crystallinity and better surface coverage. AFM analysis further confirmed this, with the root-mean-square roughness decreasing from 34 nm for the control to 29 nm for the treated films. This smoother surface facilitates the formation of a uniform hole transport layer, improving charge extraction and reducing interfacial recombination. XPS measurements provided insights into the chemical interactions at the surface. The I 3d peaks shifted to lower binding energies after ODADI treatment, suggesting an increase in electron density around iodine atoms due to hydrogen bonding between ODADI and the perovskite lattice. This interaction helps suppress iodide migration, a key factor in VI– formation and phase instability. The Tauc plot derived from UV-vis absorption spectra indicated a bandgap of approximately 1.72 eV for both control and treated films, confirming that ODADI does not alter the optical properties of the perovskite layer. These findings highlight the role of ODADI in promoting a more ordered and defect-free surface, which is crucial for high-performance perovskite solar cells.
The photovoltaic performance of the devices was evaluated under AM 1.5G illumination. The champion ODADI-treated perovskite solar cell achieved a PCE of 19.46%, with a VOC of 1.24 V, JSC of 19.26 mA/cm², and fill factor (FF) of 0.81. In comparison, the control device exhibited a PCE of 18.60%, with VOC of 1.22 V, JSC of 18.98 mA/cm², and FF of 0.80. The improvement in VOC is particularly notable, as it reflects reduced non-radiative recombination, consistent with the passivation of surface defects. The steady-state power output (SPO) of the treated device maintained an average PCE of 19.14% over 750 seconds, demonstrating excellent operational stability. Statistical analysis of multiple devices showed that the average PCE increased from 17.93% to 18.38%, and the average VOC rose from 1.22 V to 1.24 V, indicating the reproducibility of the ODADI treatment. EQE measurements revealed a broad spectral response from 300 to 800 nm, with integrated JSC values of 18.98 mA/cm² and 19.26 mA/cm² for control and treated devices, respectively, matching the J-V results. The enhanced JSC can be attributed to improved charge collection efficiency due to better interface quality. To quantify the defect passivation, we performed TRPL measurements, which showed that the average carrier lifetime (τave) increased from 36.82 ns for the control to 124.43 ns for the ODADI-treated film. This prolonged lifetime indicates a significant reduction in non-radiative recombination, as described by the equation for average lifetime:
$$ \tau_{\text{ave}} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2} $$
where τ1 and τ2 are the fast and slow decay constants, and A1 and A2 are their respective amplitudes. The increase in τave directly correlates with the suppression of trap-assisted recombination, leading to higher VOC and overall efficiency in the perovskite solar cell.
Further analysis of defect density was conducted using the SCLC method. The current density-voltage characteristics in the dark showed a trap-filled limit voltage (VTFL) of 1.41 V for the control and 1.23 V for the ODADI-treated device. The defect density (Ntraps) can be calculated using the formula:
$$ V_{\text{TFL}} = \frac{q N_{\text{traps}} L^2}{2 \varepsilon_r \varepsilon_0} $$
where q is the elementary charge, L is the thickness of the perovskite layer, εr is the relative permittivity, and ε0 is the vacuum permittivity. Assuming L ≈ 500 nm and εr ≈ 30 for CsPbI2.85Br0.15, the Ntraps values were determined to be 3.01 × 1016 cm−3 for the control and 2.63 × 1016 cm−3 for the treated film. This reduction in defect density confirms the effectiveness of ODADI in passivating surface traps. Admittance spectroscopy revealed a decrease in the trap density of states (tDOS) after treatment, further supporting the SCLC results. EIS measurements under illumination showed that the series resistance (RS) decreased from 11 Ω to 7 Ω, while the recombination resistance (Rrec) increased from 697 Ω to 3,678 Ω for the ODADI-treated device. The lower RS facilitates better charge transport, and the higher Rrec indicates suppressed recombination, both contributing to the improved performance of the perovskite solar cell. Dark J-V curves also exhibited reduced leakage current in the treated devices, underscoring the enhanced diode characteristics and minimal shunt paths.
The stability of the perovskite solar cells was assessed under various environmental conditions. In humidity tests, the control films degraded rapidly, transitioning from the black β-phase to the yellow δ-phase within 30 minutes at RH 30–50%, whereas the ODADI-treated films remained stable for extended periods. Water contact angle measurements showed an increase from 48.39° for the control to 80.33° for the treated films, indicating enhanced hydrophobicity that mitigates moisture ingress. Long-term stability tests in air (RH 20–30%) demonstrated that the ODADI-treated devices retained 76.69% of their initial PCE after 800 hours, compared to only 30.71% for the control. Under harsher conditions (RH 40–50%), the treated devices maintained 56.41% of initial PCE after 300 hours, while the control degraded to 6.21%. Thermal stability tests in a nitrogen atmosphere at 65°C showed that the ODADI-treated devices retained 65.14% of initial PCE after 650 hours, versus 19.21% for the control. At 85°C, the treated devices preserved 47.46% of PCE after 300 hours, compared to 16.54% for the control. These results highlight the role of ODADI in improving both humidity and thermal stability by reducing defect-mediated degradation and enhancing interfacial cohesion. The table below summarizes the key performance parameters and stability metrics for the control and ODADI-treated perovskite solar cells.
| Parameter | Control Device | ODADI-Treated Device |
|---|---|---|
| Champion PCE (%) | 18.60 | 19.46 |
| Average PCE (%) | 17.93 | 18.38 |
| VOC (V) | 1.22 | 1.24 |
| JSC (mA/cm²) | 18.98 | 19.26 |
| Fill Factor | 0.80 | 0.81 |
| Defect Density, Ntraps (cm−3) | 3.01 × 1016 | 2.63 × 1016 |
| Carrier Lifetime, τave (ns) | 36.82 | 124.43 |
| Humidity Stability (PCE retention after 800 h, RH 20–30%) | 30.71% | 76.69% |
| Thermal Stability (PCE retention after 650 h at 65°C) | 19.21% | 65.14% |
In conclusion, we have demonstrated that surface modification with ODADI significantly enhances the performance and stability of air-processed inorganic perovskite solar cells. The bifunctional passivation mechanism, involving hydrogen bonding and iodide vacancy filling, reduces surface defect density and suppresses non-radiative recombination. This leads to a champion PCE of 19.46% with improved VOC and JSC, along with robust stability under humidity and thermal stress. The ability to fabricate high-efficiency perovskite solar cells in air using a simple, scalable method represents a significant step towards commercial viability. Future work could explore the application of similar long alkyl chain halides in other perovskite compositions or device architectures to further advance the field of perovskite photovoltaics. Overall, this study underscores the potential of interface engineering in overcoming the limitations of inorganic perovskite solar cells, paving the way for their integration into next-generation energy systems.
The success of this approach can be attributed to the synergistic effects of ODADI on the perovskite surface. The long alkyl chain provides hydrophobicity, while the amine and iodide groups address specific defects. This dual action is particularly beneficial for air-processed devices, where environmental factors like moisture can exacerbate degradation. Moreover, the use of ODADI does not require complex processing steps, making it economically feasible for large-scale production. As the demand for efficient and stable perovskite solar cells grows, strategies like this will be crucial in bridging the gap between laboratory research and industrial application. We believe that our findings will inspire further investigations into surface passivation techniques for perovskite solar cells, ultimately contributing to the global transition towards renewable energy sources.