Unfused-ring small molecule acceptors based on A1-D-A2-D-A1 architecture with low non-radiative energy loss and excellent air stability

Abstract

Organic solar cells (OSCs) have made enormous progress in recent years. However, OSCs still suffer from the low price–quality ratio since the complex synthesis of photovoltaic materials and the poor stability of the devices. Comparing with traditional large conjugated ladder-type non-fullerene acceptors, the unfused-ring acceptors with non-covalent intramolecular interactions could notably simplify the synthetic routes while maintaining the device performance. Here, we designed and synthesized two A1-D-A2-D-A1 type non-fullerene acceptors named BDTC-F and BDTC-Cl based on 1,3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5c']dithiophene-4,8-dione (BDD) as A2 unit. With cyclopenta[2,1-b:3,4-b']dithiophene functions as the D unit, the weak O⋯S interaction between A2 and D units is formed, which reduces the torsion angle between them and increases the planarity of the molecular backbone. Furthermore, the participation of BDD could deepen the energy levels of these two acceptors, makes it possible to decrease the energy loss of the corresponding devices. When blending with polymer donor PM6, devices based on both acceptors exhibit PCEs over 10% with low energy losses of 0.59 and 0.57 eV for BDTC-F and BDTC-Cl, respectively. Moreover, the devices based on PM6:BDTC-F and PM6:BDTC-Cl demonstrate excellent air stability. After stored in the ambient atmosphere without encapsulation for 1,200 h, devices based on these two acceptors retained over 96% of their initial PCEs. These results demonstrate that designing the A1-D-A2-D-A1 type acceptor with non-covalent intramolecular interactions is an effective strategy to construct low-cost and air-stable OSCs.