Non-halogenated-solvent-processed highly efficient organic solar cells with a record open circuit voltage enabled by noncovalently locked novel polymer donors

Abstract

The relatively low open circuit voltage (V_oc) of organic solar cells (OSCs) with narrow and ultra-narrow bandgap fused-ring electron acceptors limits further improvement of the OSCs. Simply down-shifting the highest occupied molecular orbital (HOMO) levels of the donors always results in the trade-off between the V_oc and short circuit current (J_sc). In this work, we reported three novel noncovalently locked polymer donors based on different side-chain-modified benzodithiophene (BDT) units alternately copolymerized with an electron-deficient 3,3′-dicarboxylate-substituted difluorotetrathiophene building block for efficient OSCs. Due to the existence of the electron-affinity moiety, deep HOMO levels are obtained for these copolymer donors, enabling the highest recorded V_oc of 0.99 V when blended with the IT-4F acceptor. Meanwhile, intramolecular noncovalent interactions in these copolymers favor a preferential face-on orientation. Efficient charge transport and exciton dissociation under a low driving force are observed in these novel polymer donors. Consequently, the device processed from a non-halogenated solvent shows a high efficiency of 12.5% with simultaneously high V_oc and J_sc, which is one of the highest performances of non-halogenated-solvent-processed OSCs to date. These results demonstrate that the synergistic effect of the energy band structure and molecular geometry can provide an effective molecular design strategy for high performance OSCs.