Regulating phase separation and molecular stacking by introducing siloxane to small-molecule donors enables high efficiency all-small-molecule organic solar cells

For a long time, regulating the phase separation of all-small-molecule organic solar cells (ASM-OCSs) to achieve the ideal phase morphology has been a challenging problem in the field, in particular for the system composed of non-fullerene acceptors. In this work, we have constructed two small-molecule donors (ZR-SiO and ZR-SiO-EH) with low surface tensions by introducing siloxane. Based on the difference in the surface tension between a donor and an acceptor (Y6), an appropriate morphology with nanoscale phase separation was achieved in the blend system by regulating the intermolecular compatibility, thus effectively enhancing the photovoltaic performance of the corresponding ASM-OSCs. In particular, the ZR-SiO-EH:Y6 system exhibited a nanoscale bicontinuous interpenetrating network with a small domain size and face-on molecular stacking, which guarantees effective exciton dissociation and efficient charge transport even in the mixed region. In addition, the ordered molecular orientation, optimized morphology, and reduced energy offset between donors and acceptors reduce the non-radiative energy loss to 0.2 eV, leading to a high open circuit voltage of 0.87 V for ASM-OSCs. As a result, the ZR-SiO-EH:Y6 based device exhibited a high power conversion efficiency of 16.4%. These results demonstrate that regulating the intermolecular compatibility by siloxane to obtain ordered phase separation morphology provides an efficient method for designing high performance ASM-OSCs.