Film formation of mixed-halide perovskites for PSCs

Abstract: Over the last few years there has been a steady increase in interest in the exploration of perovskite materials within photovoltaic research. This can to a large extent be attributed to the incredible pace at which solar cells based on perovskites have improved in efficiency during the same time period. Perovskite solar cells (SCs) are not only an alternative to today’s market leading silicon based devices, but can also be a complement and improve efficiency by being used together with silicon in a tandem configuration. For mixed halide perovskites that can be used for these types of applications, fundamental knowledge about the film formation process is still insufficient to overcome some of the challenges faced. A more detailed understanding of the mechanisms, and of important conditions for film formation can help to improve the quality of the films and the performance and stability of the solar cells, which is important in order to develop market-ready solar cell devices. This project focused on the film formation process of mixed halide perovskites and on some of the parameters affecting the film formation conditions. The MAPb(Ix, Br1-x)3 perovskite series with different Br/I ratios was studied in-situ during spin coating of the film using an optical tracking device. Together with the subsequent use of SEM imaging, the resulting film morphology could be revealed. How antisolvent treatment conditions during spin coating affect the crystallization process and resulting film was investigated as well as how this translates to device level and the performance of the solar cells. Further, the effects on film quality and device performance when adding an alkali salt as an interface modifier (IM) in between the SnO2 electron transport layer and the perovskite film were studied, as this has been shown to improve device performance when using other perovskite ionic compositions. We found that antisolvent treatment was beneficial for formation of closed films with a smooth surface and that the timing of the antisolvent drop seemed to have a slight effect on the resulting film quality as well as on device performance. Resulting film morphology can not be directly translated from tests on glass substrates to device level as we could see that films that were spin coated on a layer of SnO2 differed in morphology to those spin coated directly on a glass surface, and the improvement in film quality using antisolvent treatment became less apparent in these samples. Further, we found that devices in which KNO3 was included as an IM generally had less holes in the perovskite film at the interface and the VOC was improved. The effect of the IM seemed to depend on halide ratio of the perovskite as improvements in overall device performance could only be observed for the pure iodide perovskite.