Quantifying triplet formation in conjugated polymer/non-fullerene acceptor blends

Abstract

Triplet formation is generally regarded as an energy loss process in organic photovoltaics. Understanding charge photogeneration and triplet formation mechanisms in non-fullerene acceptor blends is essential for deepening understanding of photophysics in these important organic photovoltaic materials. Here, we present a comprehensive spectroscopy and morphology study on non-fullerene acceptors ITIC, ITIC-Th, ITIC-2F and Y6, both pristine and blended with reference polymer PffBT4T-C9C13. Atomic force microscopy and grazing-incidence X-ray diffraction provided information regarding the morphology of the films while spectroelectrochemistry combined with microsecond transient absorption spectroscopy allowed triplets and charge carriers to be investigated in detail. Crucially, we used triplet sensitisation to determine molar extinction coefficients of the non-fullerene acceptor triplets (2.7–6.5 × 10⁴ L mol⁻¹ cm⁻¹), allowing triplet populations to be quantified in the blends. Intriguingly, no consistent trends were found in the photophysics of the studied blend systems, with each presenting its own unique mechanism. PffBT4T-C9C13:Y6 showed no triplet formation, only charge carriers that decayed rapidly in a relatively crystalline environment, consistent with the observed highly segregated morphology. In contrast, all blends in the ITIC series produced evidence of considerable triplet formation in addition to charge carriers. PffBT4T-C9C13:ITIC-Th blend produced acceptor triplets irrespective of excitation wavelength, and these were formed via intersystem crossing and/or energy transfer. Conversely, both ITIC and ITIC-2F blends displayed triplet formation via non-geminate recombination of charge carriers, with both NFA and polymer triplets observed. However, PffBT4T-C9C13:ITIC-2F produced a substantially higher charge carrier population than the ITIC blend. Because its triplet formation mechanism relies on the presence of charge carriers, PffBT4T-C9C13:ITIC-2F, with the highest charge carrier population, also had the highest triplet population. These results exemplify the prevalence of triplet states across a range of NFA blend systems, despite the varying formation mechanisms. Furthermore, they showcase that triplet populations can reach very high levels, particularly in cases of concomitantly high charge populations. Since high charge carrier densities correlate with large short circuit currents, this has significant ramifications for organic photovoltaic performance.