Semicrystalline Polymer Donors for Simultaneous Dark Current Suppression and Photocurrent Enhancement in High-Performance Photomultiplication-Type Organic Photodetectors

Photomultiplication-type organic photodetectors (PM-type OPDs) have recently attracted attention. However, the development of polymer donors specifically tailored for this architecture has rarely been reported. In this study, we synthesized benzobisoxazole-based polymer donors incorporating alkylated π-spacers that simultaneously enhance photocurrent density (J_ph) and suppress dark current density (J_d), leading to high responsivity (R) and specific detectivity (D*). The introduction of π-spacers into the polymer backbone significantly improved face-on crystallinity, leading to higher hole mobility, external quantum efficiency (EQE), and R values compared to those of the polymer without π-spacers. Notably, the polymer incorporating a 6-undecyl thieno[3,2-b]thiophen-2-yl (UTT) π-spacer exhibited a higher absorption coefficient, shallower HOMO energy level, and lower bimolecular recombination, resulting in superior EQE (27 280%) and R (117 A/W) values compared to the polymer with a typical thiophene π-spacer. More importantly, the alkylated and π-extended UTT spacer improved both crystallinity and insulating properties of the polymer donor, resulting in high R and low J_d values, and a promising D* of 3.86 × 10¹³ Jones, one to 2 orders of magnitude higher than other devices. This work offers a molecular design strategy for high-performance PM-type OPDs with improved static and dynamic characteristics.