Recent Advance in Solution‐Processed Hole Transporting Materials for Organic Solar Cells

Abstract Solution‐processed hole transporting layers (HTLs) not only play a crucial role in realizing high performance of organic solar cells (OSCs), but also possess excellent compatibility with low‐cost and large‐area processing methods of industrialized productions. However, the number and species of HTL materials are obviously fewer than that of electron‐transporting materials, which limits the development and application of OSCs. In particular, the large energy level difference between anode and organic active layer leads to the serious energy barrier for hole collection, bringing much difficulty in developing efficient HTL materials. In this review, it is focused on the recent advances in solution‐processed HTLs in OSCs. Initially, the working mechanism, property requirement, and existing issues of solution‐processed HTLs are systematically analyzed. Afterward, the main classes of solution‐processed HTL materials are discussed, including PEDOT:PSS, conjugated polyelectrolytes (CPEs), TMOs, and others. The structure‐property relationships of solution‐processed HTL materials are analyzed, and some important design rules for such materials toward efficient and stable OSCs are presented. Finally, a brief summary is presented along with some perspectives to help researchers understanding the challenges and opportunities in this field.