A Structurally Simple Polymer Donor Enables High-Efficiency Organic Solar Cells with Minimal Energy Losses

Energy loss (E_loss) between optical energy gap (E_g) and open-circuit voltage (eV_oc) sets efficiency upper limits for organic solar cells (OSCs). Nevertheless, further breaking the limit of E_loss in OSCs is challenging, especially via structurally simple materials in binary OSCs. Herein, a structurally simple nonhalogenated polymer donor, namely PBDCT, is developed for realizing high-efficiency OSCs with record-breaking E_loss. The critical building block 3,4-dicyanothiophene with high electron affinity results in a deep-lying highest occupied molecular orbital (HOMO), which effectively reduces radiative and nonradiative recombination energy losses in OSCs. Meanwhile, the finely tuned alkyl chains offer high crystallinity and low energetic disorder for the polymer, which enables efficient exciton dissociation at low energy loss. Moreover, bi-continuous crystalline fibrillary network structure is formed in the blend consisting of PBDCT due to the optimal aggregation property of the polymer, which is conducive to exciton diffusion and charge transport. Consequently, the OSC with a record-breaking low E_loss of 0.476 eV has been achieved, which thereby resulted in a power conversion efficiency (PCE) of 19.84%, the highest value achieved by nonhalogenated polymer donors in binary OSCs to date. This work demonstrates the prospect of breaking the limit of E_loss and realizing efficiency breakthroughs in OSCs.