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

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Efficient organic solar cells (OSCs) require effective exciton dissociation and suppressed recombination.
• Bulk heterojunctions (BHJ) utilize donor-acceptor (D/A) intermixing for charge separation.
• Layer-by-layer (LbL) processing offers alternative D/A morphology control, often considered pseudo-bilayer or quasi-planar heterojunction (PHJ).

Purpose of the Study:

• To quantitatively benchmark the recombination consequences of LbL-processed active layers against BHJ and low-intermixing PHJ references.
• To elucidate the effective D/A interfacial area and its impact on recombination in LbL OSCs.
• To challenge the assumption that LbL architectures represent intermediate recombination behavior.

Main Methods:

• Fabrication of BHJ, LbL, and PHJ model devices using a benchmark PM6/L8-BO system.
• Analysis of non-radiative recombination loss using device-architecture-dependent J-V measurements.
• Investigation of recombination pathways via transient-absorption spectroscopy.

Main Results:

• LbL-processed active layers exhibit BHJ-like interfacial contact and recombination characteristics, contrary to expectations.
• Non-radiative recombination loss in LbL (0.249 eV) and BHJ (0.248 eV) is substantially higher than in PHJ (0.187 eV).
• Transient-absorption spectroscopy reveals pronounced lowest-energy triplet state formation in BHJ and LbL films, indicating early-stage T1-mediated recombination.

Conclusions:

• LbL OSCs behave distinctly from PHJ and similarly to BHJ regarding recombination, challenging the notion of intermediate behavior.
• Minimizing D/A interfacial area is not universally practical for LbL OSCs, as extreme reduction can penalize exciton harvesting.
• Understanding specific D/A interactions and morphology is crucial for optimizing LbL OSC performance.