Authors: Gailan Al-Dainy, Fumiya Watanabe, Alexandru S. Biris, and Shawn E. Bourdo
Publication: ACS Appl. Energy Mater. 2021, XXXX, XXX, XXX-XXX, Publication Date: March 22, 2021. https://doi.org/10.1021/acsaem.0c03059
Abstract: Cesium formadinium methylammonium triple-cation (CsFAMA) lead mixed-halide perovskites have been reported to promote unique photovoltaic properties with high efficiency and minimal hysteresis effects. Perovskite film quality is a vital parameter for improving the performance of perovskite solar cells (PSCs). Here, we report the growth of CsFAMA perovskites with microscale grains passivation grain boundaries by mixing HC(NH2)2I (FAI) and CH3NH3Br (MABr) in isopropanol (IPA) as a post-treatment step to cause controlled Ostwald ripening, leading to secondary grain growth. Compared to the conventional preparation of perovskite films with a free organic halide antisolvent, this mixed-cation mixed-halide post-treatment created higher quality perovskite films in terms of morphology, electronic properties, energy level alignment, and carrier recombination, with the band gap adjusted at the optimal concentration. Treatment with the single cation and single halide presented by FAI and/or MABr in IPA treatment was also investigated, and the results showed that multiple band gap perovskite structures were obtained, which could be beneficial for band gap engineering. This method of perovskite preparation was tested in an inverted planar configuration with a hole transport layer (HTL) based on sulfonated poly(thiophene-3-[2-(2-methoxy-ethoxy)ethoxy]-2,5-diyl) (SP3MEET). The efficiency of the PSCs was dramatically boosted from 13.80% with the organic halide-free antisolvent to 17.62% when the mixed-cation mixed-halide approach was used. In addition, the PSCs treated with the mixed-cation mixed-halide solution exhibited excellent reproducibility, with a high fill factor and eliminated hysteresis. The CsFAMA photovoltaic device based on the single organic cation and single halide treatment approach showed power conversion efficiency of 15.82 and 15.90% for the FAI and MABr-treated films, respectively. Characterization of the S-P3MEET as HTLs found that good optoelectronic and morphological properties lead to improve the performance of the inverted PSC. However, with further research to align the energy levels at the S-P3MEET/perovskite interface, greater improvements in the PSCs are expected.