Polyfluorophenyl quinoxaline derived acceptors with long exciton diffusion length for efficient thin-film/thick-film organic solar cells

Designing small-molecule acceptors (SMAs) with long exciton diffusion length (LD) and high electron mobility is vital for boosting power conversion efficiency (PCE) of organic solar cells (OSCs). However, the limited LD of most advanced SMAs hinders their practical applications in thick-film OSCs. Herein, we develop four SMAs (named Yq-0F, Yq-2F, Yq-4F, and Yq-6F) with fluorine-free/fluorinated phenyl-substituted quinoxaline cores by manipulating the number of fluorine atom. Among them, polyfluorophenyl-substituted Yq-4F exhibits superior 3D network crystal framework and favourable intermolecular packing, leading to optimal blend morphology with polymer donor D18. Therefore, D18:Yq-4F system achieves improved carrier mobilities and suppressed charge recombination loss. Consequently, the D18:Yq-4F based binary OSCs provide a PCE of 18.30%, surpassing these ones based on D18:Yq-0F (11.27%), D18:Yq-2F (17.73%), and D18:Yq-6F (17.00%). When introducing Yq-4F as a guest into D18:L8-BO host to fabricate ternary OSCs, the PCE further increases to 20.77%, as the highest reported value among quinoxaline-derived SMAs. Moreover, ternary D18:Yq-4F:BTP-eC9 and D18:L8-BO:Yq-4F OSCs with ~500 nm active layer thickness offer impressive PCEs of >17%, ranking among the top values for thick-film devices with similar thickness. This work offers an effective SMA design strategy for opening a path towards efficient thin-film and thick-film OSCs simultaneously. Organic solar cells need acceptor molecules with long exciton diffusion lengths, but most available materials fail in thick films. The authors design new fluorinated quinoxaline acceptors that significantly improve carrier mobilities resulting in superior efficiency in both binary and ternary devices.