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

Energy loss (Eloss) between optical energy gap (Eg) and open‐circuit voltage (eVoc) sets efficiency upper limits for organic solar cells (OSCs). Nevertheless, further breaking the limit of Eloss in OSCs is challenging, especially via structurally simple materials in binary OSCs. Herein, a structurally simple non‐halogenated polymer donor, namely PBDCT, is developed for realizing high‐efficiency OSCs with record‐breaking Eloss. The critical building block 3,4‐dicyanothiophene with high electron affinity results in a deep‐lying highest occupied molecular orbital (HOMO), which effectively reduced 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 enable 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 Eloss of 0.476 eV has been achieved, which thereby resulted in a power conversion efficiency (PCE) of 19.84%, the highest value achieved by non‐halogenated polymer donors in binary OSCs to date. This work demonstrates the prospect in breaking the limit of Eloss and realizing efficiency breakthroughs in OSCs.