Bulk Heterojunction-Like Triplet Recombination Pathways in Layer-by-Layer Organic Solar Cells

Abstract

Efficient exciton dissociation and the suppression of its recombination are key to improving the performance of organic solar cells (OSCs). While bulk heterojunctions (BHJ) promote charge separation through extensive donor−acceptor (D/A) intermixing, layer-by-layer (LbL) processing offers an alternative to control D/A morphology. Although LbL active layers are frequently described as pseudo-bilayer, pseudo-BHJ, or quasi-planar heterojunction (PHJ) structures, the recombination consequences of the effective D/A interfacial area have rarely been benchmarked quantitatively against both BHJ and a low-intermixing PHJ-like reference. Using a benchmark PM6/L8-BO system, we establish BHJ/LbL/PHJ model devices and deduce recombination characteristics of LbL-processed active layers against BHJ and low-intermixing PHJ-like references. Non-radiative recombination loss analysis shows that PHJ achieves substantially lower loss (0.187 eV) than BHJ (0.248 eV) and LbL (0.249 eV), consistent with a reduced D/A interfacial area. Device-architecture-dependent J−V measurements are consistent with substantial acceptor penetration into the donor in LbL stacks, suggesting BHJ-like interfacial contact. Transient-absorption spectroscopy discloses the pronounced formation of the lowest-energy triplet state in BHJ and LbL films, strongly suppressed in the low-intermixing PHJ-like reference. This trend is consistent with an early-stage T1-mediated recombination pathway in the BHJ and LbL films, likely involving non-geminate recombination at D/A interfaces. The results show that LbL devices behave distinctly from PHJ yet BHJ-like, contrary to the general assumption that the LbL architecture represents intermediate recombination behavior. Our findings suggest that simply minimizing the D/A interfacial area is not universally practical for LbL OSCs, as achieving PHJ-like voltage-loss characteristics would require an extremely reduced interface, penalizing exciton harvesting when the exciton diffusion length is limited.