Manipulating crystallization kinetics and vertical phase distribution via a small molecule donor guest for organic photovoltaic cells with 20% efficiency†

Abstract

Precise control over molecular crystallization and vertical phase distribution of photovoltaic bulk-heterojunction (BHJ) films is crucial for enhancing their optoelectronic properties toward high-performance polymer solar cells (PSCs). Herein, a kinetics-controlling strategy is implemented in the PM6:L8-BO blend system by introducing a small molecule donor (SMD), namely BTR-SCl, which possesses strong crystallization properties and excellent miscibility with the host polymer donor. In situ spectroscopic characterization indicates that BTR-SCl can effectively advance the aggregation of PM6 from the blend solution and prolong its self-assembly time during the film formation process, which leads to well-defined vertical phase distribution with a more ordered polymer donor enriched at the anode, effectively facilitating charge transport and collection, alleviating trap density and energetic disorder, and reducing energy loss. Ultimately, the PM6:BTR-SCl:L8-BO ternary PSCs (T-PSCs) achieve a remarkably enhanced power conversion efficiency (PCE) of 19.4% in comparison with 18.0% for the binary device. Notably, by replacing PM6 with D18, the PCE of ternary devices is further boosted to 20.0%, which represents the highest efficiency for SMD-based T-PSCs reported to date. Our findings elucidate the great potential of crystalline SMD in optimizing the vertical phase distribution and molecular packing within the BHJ film, leading to considerable improvements in the PCE of PSCs.