Prolonging the exciton diffusion length by manipulating molecular stacking enables pseudo-planar heterojunction organic solar cells to achieve over 19% efficiency

The limited exciton diffusion length (LD) of organic semiconductors constraints exciton dissociation in pseudo-planar heterojunction (PPHJ) organic solar cells (OSCs), which is deemed the bottleneck hampering the further improvement in their power conversion efficiencies (PCEs). Herein, a solid-additive strategy was proposed to efficiently prolong the LD in PPHJ OSCs by developing a volatile solid additive, SA-5F, and blending it in non-fullerene acceptors. The addition of SA-5F endowed a Y6+ film with improved crystallinity and more compact molecular stacking, resulting in a higher photoluminescence quantum yield, larger overlap between its absorption and emission spectra, and shorter intermolecular distance compared with a pristine Y6 film. Thus, the Y6+ film achieved a higher exciton diffusion coefficient of 1.36 x 10-3 cm2 s-1 and, thus, similar to 24% improvement in the LD to 10.27 nm. The prolonged LD enabled D18/Y6+-based PPHJ devices to realize more efficient exciton dissociation and weaker charge recombination compared with their D18/Y6 counterparts. Therefore, D18/Y6+-based PPHJ OSCs afforded an impressive PCE of 19.11%, which is among the highest values reported to date. Moreover, this strategy exhibits excellent universality in improving the photovoltaic performance of different systems. Thus, it was demonstrated that introducing a solid additive is an efficient and universal strategy to prolong the LD in PPHJ OSCs and consequently boost their PCEs. A solid additive, SA-5F, is employed to promote molecular stacking, which leads to higher crystallinity, boosting the exciton diffusion coefficient and then exciton diffusion length. Thus, D18/Y6+-based PPHJ OSCs afford an impressive PCE of 19.11%.