Recent progress of all-polymer solar cells - From chemical structure and device physics to photovoltaic performance

Single junction organic solar cells (OSCs) have now achieved power conversion efficiencies (PCEs) exceeding 17 %. Most of these high performance OSCs contain fullerene acceptors (FAs) and non-fullerene small-molecule acceptors (NFSMAs). In contrast, there are very limited usages of polymer acceptors. Recently, there are escalating recognition among perylene-diimide/naphthalene-diimide (PDI/NDI) and B <- N-unit n-type polymers as electron acceptors in the all-polymer solar cells. FAs like PC71BM suffer from multiple limitations. They include restricted energy level tuning, weak absorptions in visible region, narrow spectral breadth, and morphological instability. In contrast to FAs, NFSMAs offer numerous advantages. They include strong and broad absorption in the visible and even the NIR region, tunable energy levels, and simple synthesis and purification procedures. Despite these advantages, the long-term device stability and large-area roll-to-roll (R2R) fabrication remain the major issues for the commercialization for NFSMA-based OSCs. All-polymer solar cells, on the other hand, largely address the problems of device stability and large-area film processing. Many all-polymer solar cells have been demonstrated to possess long-term thermal, photo and mechanical stability. Meanwhile, the precursor solutions for all-polymer solar cells enjoy superior control in the solution viscosity, which is an important factor for the solution processing of large-scale OSCs. Before 2015, all-polymer solar cells received little attention due to their disappointing device performance. Afterwards, PCEs of all-polymer solar cells are picking up. Currently, the best cells have achieved PCEs in excess of 11 %. Here, we provide a systematic review on the evolution of n-type polymeric acceptors used in OSCs. In addition, we summarize the morphological and charge carrier transport properties of all-polymer solar cells and compare with their small molecule acceptor counterparts. The outstanding properties of all-polymer solar cells are discussed from the perspectives of morphology and electron transport in bulk heterojunctions (BHJs). The concept of electron percolation in all-polymer BHJs is introduced and correlated with the excellent device stability. This review should have a broad appeal and enable researchers in comprehending the achievements, challenges, and future directions of all-polymer solar cells.