Terminal alkyl chain tuning of small molecule donor enables optimized morphology and efficient all-small-molecule organic solar cells

Modulation of the interfacial molecular orientation and molecular packing behaviour is essential for fine-tuning the nanoscale morphology toward efficient non-fullerene all-small-molecule organic solar cells (NFSM-OSCs) due to the anisotropic molecular structure of electron donors (D) and acceptors (A). Currently, the design of active layer material to obtain favourable nanoscale morphology is still quite challenging. This work developed four new small molecular donors, TBCA-CX (X = 2, 4, 6, and 8), with the same conjugated skeleton but different terminal alkyl chains. The GIWAXS measurement results reveal that the predominant molecular orientation of pristine TBCA-CX films began to change from edge-on to face-on with the increased terminal alkyl chains length. Furthermore, the non-fullerene acceptor IT-4F was used to fabricate NFSM-OSCs with the new donor compounds. Finally, the TBCA-C4: IT-4F blend formed a more favourable face-to-face orientation at the D/A interface and a more compact molecular stacking as proved from GIWAXS characterization, which is beneficial for charge transfer and charge carrier transport. As a result, the TBCA-C4-based device exhibited a higher power conversion efficiency (PCE) of 9.21%, while its analogs showed PCEs less than 8%. These results provide an insight to understand the impact of molecular design of interfacial molecular orientation and molecular packing behaviour on photovoltaic performance in NFSM-OSCs.

Automated summary

This study developed new small molecular donor materials and explored their potential applications in non-fullerene all-small-molecule organic solar cells. By modulating the molecular orientation and molecular packing behavior of the donor materials, a favorable nanoscale morphology for charge transfer and charge carrier transport was achieved, resulting in improved power conversion efficiency of the devices.