Incorporation of functional polymers into metal halide perovskite thin-films: from interactions in solution to crystallization

In the last decade, metal halide perovskite materials (MHPs) have presented several innovative opportunities for optoelectronic technologies, particularly for solar cells. However, challenges related to reliable material processability, combined with inherent instability, pose major roadblocks in the path towards the industrialization of MHP-based technologies. One of the strategies, developed to overcome these issues, is the incorporation of polymeric additives to control the crystallization process of the material, which represents a key aspect for reproducible manufacturing of robust films. Owing to the vast range of properties presented by polymeric materials, a judicious selection of polymers and perovskite compositions allows tailoring the physical-chemical-mechanical properties of the final composite to the desired device application. In this review, the recent advances on polymer inclusion into perovskite photoactive layers for solar cells are discussed. Following a brief introduction to perovskite fundamental properties for photovoltaic applications, the focus is on the self-assembly process of MHPs and on the use of polymers with different functionalities as templating agents for the growth of polycrystalline films with optimal optoelectronic quality. The influence of different polymers on the stability, mechanical behavior and processability of MHPs is then rationalized. Finally, a summary of the most significant and accessible characterization techniques used to investigate the polymer-perovskite-based composite is given, providing guidelines for the further development of innovative formulations.