Enhancing quality of CsPbIBr<sub>2</sub> inorganic perovskite via cellulose acetate addition for high-performance perovskite solar cells

Abstract

<sec>CsPbIBr<sub>2</sub> perovskite has been considered as a promising candidate for the light-harvesting material of perovskite solar cells (PSCs) due to its acceptable band gap and high stability. Nevertheless, the efficiency of CsPbIBr<sub>2</sub>-based PSC still lags behind that of its homologs and is far away from the theoretical value. This can be attributed to the poor quality of CsPbIBr<sub>2</sub> perovskite film. Therefore, it is highly desirable to improve the quality of CsPbIBr<sub>2</sub> perovskite film for enhancing the photovoltaic performance of CsPbIBr<sub>2</sub> PSCs. In this work, cellulose acetate (CA) is used as a polymer additive that is introduced into CsPbIBr<sub>2</sub> precursor solution for improving the quality of CsPbIBr<sub>2</sub> perovskite film via controlling crystallization process. The interaction between the C=O group of CA and Pb<sup>2+</sup> in the precursor solution and the enhanced viscosity of precursor solution induced by CA addition reduce the crystallization rate of CsPbIBr<sub>2</sub> perovskite. As a result, a compact CsPbIBr<sub>2</sub> perovskite film with high crystallinity, large grain size, and low density of defect is prepared. The remarkably improved quality of CsPbIBr<sub>2</sub> perovskite film upon CA addition can be attributed to the relatively slow crystallization rate. The slow crystallization rate allows the perovskite film to have enough time to form perfect perovskite crystal structure with large-size crystal grain and low density of defects. Furthermore, the oxygen functional groups of CA can passivate the undercoordinated Pb<sup>2+</sup>, which effectively suppresses the defects and traps induced by Pb<sup>2+</sup> in CsPbIBr<sub>2</sub> perovskite film.</sec> <sec>The stability of CsPbIBr<sub>2</sub> perovskite film is also greatly improved by CA addition. The added CA does not participate into the CsPbIBr<sub>2</sub> perovskite crystal but distributes at the grain boundaries and, or, interfaces area and forms a moisture barrier around perovskite grains, which obviously enhances the stability of CsPbIBr<sub>2</sub> perovskite film in the ambient air.</sec> <sec>The carbon-based CsPbIBr<sub>2</sub> perovskite solar cells with a configuration of FTO/TiO<sub>2</sub>/perovskite film/ carbon are fabricated by using the carbon layer as both the hole-transport layer and the back electrode. Under the illumination of 100 mW/cm<sup>2</sup>, the PSC based on CA-CsPbIBr<sub>2</sub> perovskite film delivers a high conversion efficiency of 7.52%, which is increased by 40% compared with the efficiency of the device based on the pure CsPbIBr<sub>2</sub> perovskite film. In addition, the PSC based on CA-CsPbIBr<sub>2</sub> perovskite film shows a hysteresis index (HI) of 7%, while the device based on pure CsPbIBr<sub>2</sub> film displays a higher HI of 22%. This result demonstrates that the CA addition can effectively suppress the hysteresis effect of inorganic PSCs. The stability of the PSC based on CA-CsPbIBr<sub>2</sub> perovskite film is investigated by tracking the variation of the efficiency with time in the ambient condition. The fabricated PSCs without any encapsulation are stored in the air. The photovoltaic performance is measured once a day. The efficiency of the PSC based on CA-CsPbIBr<sub>2</sub> perovskite remains more than 90% of its initial value after being stored in the air for 800 h, showing an excellent long-term stability. Therefore, this work provides a facile and effective method of improving the quality of CsPbIBr<sub>2</sub> perovskite films, which is expected to be helpful in developing high-efficiency and stable carbon-based inorganic PSCs.</sec>