Benzothieno[1,2-b]furan: an atomically symmetric-breaking volatile solid additive for high-performance thick-film planar heterojunction organic solar cells

The fabrication of thick active layers in organic solar cells (OSCs) is essential for roll-to-roll commercial production. However, thicker layers often lead to a notable decline in device performance, mainly due to the limited exciton diffusion length (LD). Herein, we fabricated efficient thick-film planar heterojunction (PHJ) OSCs via the assistance of a new volatile solid additive (VSA) benzothieno[1,2-b]furan (TBF), featuring with atomically symmetric breaking skeleton. The asymmetric VSA treatment not only constructed a long-range ordered yet moderately loose molecular packing, but also induced an increased dielectric screening effect in the L8-BO-X layer through enhancing dipole interactions and mitigating exciton binding energy, simultaneously yielding a substantially higher photoluminescence quantum yield (PLQY) and effectively suppressing exciton-vibration coupling than the symmetric VSA counterparts benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[1,2-b:4,5-b]difuran (BDF). Consequently, the TBF-treated film exhibits an extended LD ​>40 nm, favoring exciton dissociation, and charge generation/transport, as well as reducing energy disorder. Remarkably, TBF-based empowered devices achieve high PCEs of 20.4%, 18.5%, and 16.5% at 110-, 300-, and 500-nm thicknesses, respectively, demonstrating exceptional thickness tolerance. This atomically symmetric breaking VSA strategy provides a promising strategy for extending the LD to achieve thick-film OSCs with prominent performance.