Suppressing Subnanosecond Bimolecular Charge Recombination in a High-Performance Organic Photovoltaic Material

Abstract

Nanoscale morphology and spin can have a significant impact on charge generation and short time scale recombination in organic photovoltaic materials. We reveal multiple efficient charge separation pathways and the suppression of triplet loss channels in a high-performing nematic liquid crystalline electron donor, benzodithiophene terthiophene rhodanine (BTR). BTR:PC 71 BM bulk heterojunction photovoltaic devices have been shown to exhibit charge generation quantum yields of ∼90% and power conversion efficiencies >9.5%, even in thick devices. Solvent vapor annealing increases device efficiency, delivering performance almost twice as high as that of untreated blend films, despite reduced exciton quenching. Broadband femtosecond transient absorption spectroscopy reveals both efficient hole and electron transfer on different time scales in the bulk heterojunction blends. BTR triplet excitons are formed due to subnanosecond bimolecular recombination in untreated blend films, though their formation is significantly suppressed after solvent vapor annealing. This treatment results in more crystalline BTR domains with three-dimensional percolation pathways that have an important impact on these terminal triplet loss channels formed through fast recombination of free charges. We propose that spin and nanoscale morphology have significant and interconnected roles in the prevention of loss channels that with careful control can lead to superior device performance in promising new photovoltaic materials.