Hole Transfer Prompted by Viscous Oligomer Solid Additives in Non-Fullerene Bulk-Heterojunction Layers

Modulating the charge transfer in bulk-heterojunction (BHJ) layers is essential for achieving high power conversion efficiencies (PCEs) for organic solar cells (OSCs). Adding additives to the active layer blend films is a feasible and effective strategy. Here, two donor-acceptor (D-A)-type pi-conjugated oligomers, PCID-1 and PCID-2, based on building blocks cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and isoindigo (IID) with a long side alkyl chain were synthesized via C-H direct arylation polymerization (DArP). The oligomers obtained from the DArP are soft and viscous that can be well compatible with the donors and acceptors in the blend films. It is proposed that the binding energy of the electron-hole pairs is markedly decreased upon the introduction of viscosity additives into BHJs. As a result, an order of magnitude improvement in hole mobility (from 2.71 x 10(-5) to 2.19 x 10(-4) cm(2) V-1 s(-1)) of the BHJs is successfully achieved. The corresponding non-fullerene OSCs exhibit an enhanced PCE with higher J(SC) and FF. This finding suggests a facile strategy for highly efficient non-fullerene OSCs by modulating the charge transfer process via oligomeric solid additives.