The impact of axisymmetric and centrosymmetric molecular architectures in non-fused ring electron acceptors on photovoltaic performance

Abstract

The geometric configuration of electron acceptors significantly governs molecular dipole moments and stacking behavior, thereby critically influencing power conversion efficiencies (PCEs) in organic solar cells (OSCs). In this study, we designed and synthesized two non-fused ring electron acceptors (NFREAs), TTCIC (axisymmetric) and TCIC (centrosymmetric), by incorporating 3,6-bis(octan-3-yloxy)thieno[3,2-b]thiophene and 3,4-bis(octan-3-yloxy)thiophene units, respectively. Compared to TTCIC, TCIC exhibits a higher LUMO (−3.89 eV vs. −3.98 eV), a lower HOMO (−5.40 eV vs. −5.35 eV), a large dipole moment change (0.217 D vs. 0 D) between the ground state and excited state dipoles, and weaker intermolecular interactions. Interestingly, both acceptors showed an edge-on molecular orientation in the films; however, after blending with a polymer donor, PBDB-T, TTCIC blend films exhibited preferential edge-on molecular alignment, whereas TCIC blend films adopted a face-on orientation. This morphological contrast induced stronger charge carrier recombination in PBDB-T:TTCIC blends. Consequently, PBDB-T:TTCIC-based OSCs achieved an exceptionally low PCE of 0.60%, while PBDB-T:TCIC devices delivered a moderate PCE of 8.66%. These results demonstrate that fine-tuning of NFREA geometric configurations is essential for optimizing the molecular stacking orientation and enhancing the OSC performance.