Wang Lulu ¹, Wang Lingling ², Chu Qianqian ¹, Yao Ying ¹, Bai Cunrong ¹, Han Fei ³, Feng Li ¹

(1. School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China;

2. Institute of Materials and Intelligent Manufacturing, Jiangxi Academy of Sciences, Nanchang 330096, Jiangxi, China;

3. Jiangxi Provincial Key Laboratory of Greenhouse Gas Accounting and Carbon Reduction, Institute of Energy Research, Jiangxi Academy of Sciences, Nanchang 330096, Jiangxi, China)

Extended abstract:

[Background and purposes] Perovskite solar cells (PSCs) have emerged as one of the most promising thin-film photovoltaic technologies, owing to their high power conversion efficiency, solution processability and low manufacturing cost. However, further improvements in both efficiency and stability are critically hindered by surface defects, non-radiative recombination and environment-induced phase degradation. In particular, solution-processed perovskite films inevitably contain high density of defects, such as undercoordinated Pb²⁺, iodide vacancies and local lattice distortions at their surfaces and grain boundaries. These defects act as dominant non-radiative recombination centers, severely impeding interfacial charge separation and transport, thereby deteriorating device performance. Consequently, effective regulation of perovskite surface defects is essential for achieving high-efficiency and stable devices. Interfacial engineering has proven to be a key strategy to address these challenges. By introducing functional materials between the perovskite and charge transport layers, it is possible to passivate defects, tune interfacial energy levels and optimize charge carrier dynamics. Among various approaches, organic small-molecule interfacial engineering offers unique advantages, due to their tunable molecular structures, well-defined functional groups and versatile interfacial interactions. However, most existing molecules are unable to simultaneously integrate strong chemical passivation, suppression of iodide ion migration, improved interfacial charge extraction and hydrophobic barrier. Therefore, designing a multifunctional molecule that synergistically enhances both device efficiency and stability remains a critically important research objective.

[Methods] Carbon-based PSCs with a structure of FTO/SnO₂/Perovskite/BP-4-CN/P₃HT/Carbon were fabricated. The SnO₂ electron transport layer was deposited by using spin-coating. The perovskite precursor solution was prepared using formamidinium iodide (FAI), lead iodide (PbI₂) and methylammonium chloride (MACl) in a mixed solvent of DMF and DMSO. The perovskite film was formed by using a one-step spin-coating method, followed by annealing. The BP-4-CN passivation layer was applied by spin-coating its solution (in chlorobenzene) with different concentrations onto the perovskite surface, followed by thermal treatment. Finally, a carbon paste electrode was deposited by using a doctor-blade method. Morphology and crystallinity of the films were characterized by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Chemical interaction between BP-4-CN and perovskite was examined by using Fourier transform infrared spectroscopy (FTIR). Steady-state and time-resolved photoluminescence (PL and TRPL) spectra were measured to study carrier recombination dynamics. The defect density and charge transport properties were evaluated using space-charge-limited current (SCLC) measurements. Photovoltaic performance was evaluated by using current density-voltage (J-V) measurements under AM 1.5G illumination. Electrochemical impedance spectroscopy (EIS) and dark current measurements were conducted to analyze interfacial charge transfer and recombination. Stability tests were performed by monitoring the PCE of unencapsulated devices stored in ambient air (≈30% RH, 25 ℃) and by observing the phase stability of films under thermal stress (65 ℃).

[Results] The results of SEM and XRD confirmed that the modification of BP-4-CN did not alter surface morphology or crystal structure of the perovskite films, indicating good compatibility. FTIR analysis results revealed a significant redshift of the -C≡N stretching vibration peak in the BP-4-CN modified films (from 2220 cm−1 to 2205 cm−1), providing direct evidence for coordination interaction between the cyanide group and the under-coordinated Pb2⁺ ions on the surface of the perovskite films. From the PL and TRPL test, after BP-4-CN treatment, the photoluminescence intensity significantly increased, with the average carrier lifetime increasing from 299.96 nanoseconds in the control sample to 480.69 nanoseconds, indicating effective inhibition of non-radiative recombination. SCLC measurements showed that the trap density in the modified films was significantly reduced compared with that of the control sample. The highest photoelectric conversion efficiency of the PSC modified with 3 mg·mL⁻¹ BP-4-CN reached 19.04%, exceeding that of the control device (17.18%). This improvement mainly came from the increase in Voc and FF. Statistical analysis of 20 devices confirmed the reproducibility. The results of EIS indicated that the composite resistance of the modified devices was higher and the series resistance was smaller, which was consistent with the reduction of interface recombination and the improvement of charge collection. The dark current density was also lower, indicating the suppression of trap-assisted recombination. Water contact angle measurements showed that BP-4-CN formed a hydrophobic layer on the perovskite films. The modified perovskite films stored at room temperature in air (relative humidity 30%, 25 ℃) or under heating conditions (65 ℃) exhibited significantly better phase stability than the control films. Correspondingly, the unencapsulated BP-4-CN modified photovoltaic device remained above 80% of the initial photoelectric conversion efficiency after being stored for 1000 h in air (30% relative humidity, 25 ℃), significantly superior to the control device.

[Conclusions] In conclusion, BP-4-CN successfully served as an interface modifier for the perovskite films in carbon-based photovoltaic cells. The cyano group effectively passivates the undercoordinated Pb²⁺ defects on the perovskite surface through strong coordination bonds, which suppresses non-radiative recombination. Meanwhile, the conjugated biphenyl moiety facilitates charge transport at the perovskite/carbon interface and provides a hydrophobic barrier against moisture ingress. This dual mechanism works synergistically, leading to significantly improved photovoltaic performance (particularly VOC and FF) and enhanced long-term environmental stability of the devices. This work highlights the great potential of rationally designed bifunctional molecules in interface engineering for achieving high-performance and stable perovskite photovoltaic devices and provides a valuable reference for the molecular design of new passivating agents.

Key words: perovskite solar cell; interface modification; stability; carrier behavior