Enhanced open circuit voltage of small molecule acceptors containing angular-shaped indacenodithiophene units for P3HT-based organic solar cells

Angular indaceno[2,1-b:6,5-b′]dithiophene (a-IDT) as an analogue of linear-IDT (l-IDT), in which the central phenyl ring was linked to the β-position of the thiophene ring but fused on its α-position, was designed and exploited via intramolecular annulation. Two A-D-A type small molecule acceptors (SMAs) l-IDTBTRh and a-IDTBTRh, with l-IDT and a-IDT as the central cores, benzothiadiazole (BT) as the π-bridge acceptor segments and 3-ethylrhodanine as the end-capping groups, were synthesized and employed for P3HT-based fullerene free organic solar cells. The geometric shape of the l-IDT and a-IDT subunits plays a pivotal role in governing the optoelectronic properties, the charge mobility, the morphology and the photovoltaic performance of the resulting acceptors. Preliminarily, the device based on P3HT/l-IDTBTRh (1 : 08, w/w) displayed a circuit current density (J_sc) of 8.81 mA cm⁻², an open-circuit voltage (V_oc) of 0.86 V and a fill factor (FF) of 71.0%, delivering a decent PCE of 5.38%, which is associated with the lower bandgap, the better charge mobility and the morphology. Despite the inferior photovoltaic performance (2.53%), solar cells based on BHJ blends of a-IDTBTRh and P3HT offer a remarkably higher V_oc of 0.92 V as a result of the high-lying LUMO energy level (−3.59 eV), representing one of the highest values among the reported IDT-based A-D-A type SMAs for solar cells containing P3HT. The results suggested that introducing an a-IDT segment as the central core into A-D-A type SMAs can effectively increase the LUMO energy level of SMAs, offering an efficient strategy to design non-fullerene small molecule acceptors with high V_oc based-P3HT organic solar cells.