Tunable Electron Donating and Accepting Properties Achieved by Modulating the Steric Hindrance of Side Chains in A-D-A Small-Molecule Photovoltaic Materials

Modulation of the electron donating and accepting properties of organic semiconductors is an important topic in the field of organic photovoltaics. Although the small-molecule semiconductors (SMs) with an A-D-A structure can act as either electron donor or acceptor in organic photovoltaic (OPV) devices, the reason why molecules with similar conjugated structures play different roles remains unclear. In this work, we designed and synthesized two A-D-A SMs named BTCN-O and BTCN-M, which have an identical backbone and differ in the alkyl substitution position. BTCN-O and BTCN-M demonstrate similar optical absorption spectra in solution and molecular energy levels in a solid film. BTCN-O forms an ordered lamellar packed structure with compact π–π stacking, whereas BTCN-M demonstrates only a weak π–π stacking effect in solid film. We also investigated their photovoltaic properties by blending each with a polymer donor, PBDB-T, and a fullerene acceptor, PC71BM, and found that the electron donating and accepting abilities of BTCN-O and BTCN-M are exactly opposite. According to the results obtained from a variety of analytical methods, we can infer that for the planar A-D-A SMs, the steric hindrance caused by the nonconjugated alkyls in their central units plays a critical role that affects their electron donating and accepting properties. More specifically, the A-D-A molecules that have low steric hindrance in their central units, which allows ordered lamellar packing and compact π–π stacking in the solid film, can act as an electron donor in OPV device, and the molecules that have high steric hindrance for intermolecular π–π interactions in their central units tend to act as electron acceptors. Overall, this work provides a new perspective in the molecular design of organic photovoltaic materials.