Morphology manipulation for highly miscible photovoltaic blend of carboxylate-substituted polythiophene:Y6

Abstract

Polythiophenes and its derivatives (PTs) have attracted extensive research interest in the field of organic solar cells (OSCs) owing to their benefits of low synthesis cost and high scalability of synthesis. However, the efficiency of PT-based photovoltaic devices is far lower than that of the popular push-pull conjugated polymer-based devices. To elevate the photovoltaic performance, pairing PTs with low-bandgap acceptors is highly desirable. Nevertheless, when pairing the prevailing low-bandgap non-fullerene acceptor Y6 with a representative PT derivative named PDCBT-Cl, the OSC delivered an extremely low efficiency of ∼0.1%. The crucial reason for the abysmal performance is the poor morphology with insufficient phase separation induced by the high miscibility between PDCBT-Cl and Y6. How to tune the morphology and promote phase separation is challenging and important for the highly miscible blend system to achieve performance breakthrough. Given that PDCBT-Cl and Y6 are crystalline, in the present study, we promote the formation of phase separation for PDCBT-Cl:Y6 blend by control molecular crystallization, using thermal annealing (TA) at various temperatures. We observed that, phase-separated morphology appeared when TA temperature was higher than the aggregation transition temperature of PDCBT-Cl. Upon TA at 160 °C, ordered molecular packing and appropriate phase separation were obtained for PDCBT-Cl:Y6 blend film, and the device efficiency increased dramatically to ∼10%, which was ∼90 times higher than that of the as-cast counterpart. This study demonstrates that promoting crystallization is an effective means to induce phase separation for the highly miscible blend system with crystallinity.