Tuning Exciton Dynamics in Asymmetric Molecular Acceptors by End-Group Modulation Enables Efficient All-Small Molecule Organic Solar Cells

Asymmetric small-molecule acceptors have been recognized as a pivotal component in the development of organic solar cells (OSCs). Here, we report the design and synthesis of two cost-effective asymmetric molecular acceptors, SM-11 and SM-12, featuring a simple coumarin-thiophene scaffold linked to different electron-acceptor end groups, such as dicyanoindanone and 3-ethylrhodanine units, respectively. This strategic variation precisely controls the absorption properties, molecular dipole moments, energy levels, and binding energies, thus effectively mitigating intramolecular charge transfer effects beneficial for OSCs. In binary OSCs with CA-CO as the donor, the achieved power conversion efficiencies (PCEs) were 11.73% (SM-11) and 6.78% (SM-12), respectively. The broader absorption profile, higher exciton generation/dissociation efficiency, along with lower binding energy of SM-11 account for the performance enhancement. Furthermore, integrating SM-11 into a ternary blend with CA-CO:Y7 further enhanced the PCE to 15.58%, outpacing the binary reference CA-CO:Y7 device (12.17%). The enhanced PCEs stem from the alteration of end-group functionalization in the small molecules, which accelerates exciton dissociation and charge transfer in the ternary system, promoting favorable blend morphology and efficient guest-to-host energy transfer.