Quantitative relationships between film morphology, charge carrier dynamics, and photovoltaic performance in bulk-heterojunction binary vs. ternary acceptor blends

Addressing pertinent and perplexing questions regarding why nonfullerene acceptors (NFAs) promote higher power conversion efficiencies (PCEs) than traditional fullerenes and how photoactive bulk heterojunction (BHJ) film morphology, charge photogeneration, and recombination dynamics dictate solar cell performance have stimulated many studies of polymer solar cells (PSCs), yet quantitative relationships remain limited. Better understanding in these areas offers the potential to advance materials design and device engineering, afford higher PCEs, and ultimate commercialization. Here we probe quantitative relationships between BHJ film morphology, charge carrier dynamics, and photovoltaic performance in model binary and ternary blend systems having a wide bandgap donor polymer, a fullerene, and a promising NFA. We show that optimal PC71BM incorporation in a PBDB-TF:ITIC-Th binary system matrix retains the original pi-face-on orientation, ITIC-Th crystallinity and BHJ film crystallite dimensions, and reduces film upper surface ITIC-Th segregation. Such morphology changes together simultaneously increase hole (mu(h)) and electron (mu(e)) mobilities, facilitate light-activated ITIC-Th to PC71BM domain electron delocalization, reduce free charge carrier (FC) bimolecular recombination (BR) within PBDB-TF:ITIC-Th mixed regions, and increase FC extraction pathways viaPBDB-TF:PC71BM pairs. The interplay of these effects yields significantly enhanced inverted cell short-circuit current density (J(SC)), fill factor (FF), and PCE. Unexpectedly, we also find that excessive PC71BM in the PBDB-TF:ITIC-Th binary system alters the PBDB-TF orientation to pi-edge-on, increases large scale PC71BM-rich aggregations and BHJ upper surface PC71BM composition. These morphology changes increase parasitic decay processes such as intersystem crossing from photoexcited PC71BM, compromising the J(SC), FF, and PCE metrics. ITIC-Th X-ray diffraction reveals a unique sidechain-dominated molecular network with previously unknown sidechain-end group stacking, rationalizing the STEM and GIWAXS results, photophysics, and the high mu(e). DFT computation reveals charge transfer networks within ITIC-Th crystallites, supporting excited-state electron delocalization from ITIC-Th to PC71BM. This structure-property understanding leads to a newly reported NFA blend with PCE near 17%.

Automated summary

Recent studies have shown that nonfullerene acceptors (NFAs) can achieve higher power conversion efficiencies (PCEs) compared to traditional fullerenes in polymer solar cells (PSCs). Understanding the relationship between film morphology, charge carrier dynamics, and photovoltaic performance is crucial for improving materials design and device engineering. Optimization of the incorporation of a fullerene, PC71BM, in a binary blend system can enhance PCE by increasing the charge carrier mobilities and reducing recombination. However, excessive PC71BM can lead to undesirable morphology changes and decreased performance.