Quantifying Dual-Direction Energy Harvesting in Twisted Perylene Nonfullerene Acceptor Organic Photovoltaics

Perylene monomers (PDIs) and twisted dimers (TPDIs) are investigated here as nonfullerene acceptors and are paired with the polymer donor PTB7-Th in organic photovoltaic blends. A combination of device characteristics, morphology studies, and transient absorption techniques reveals a complete picture of the photophysics in these two systems. TPDI-based devices are shown to far outperform PDI-based devices, and intensity-dependent J(SC) and V-OC measurements reveal that they do not suffer from nongeminate recombination to the same extent as PDI-based devices. Transient absorption (TA) spectroscopy also demonstrates dual-direction energy harvesting from the excitation of both PTB7-Th and (T)PDI domains. TA reveals that charges live significantly longer in TPDI blends and are less susceptible to bimolecular recombination. The modeling of these processes allows for quantification of the relative contributions of each charge generation pathway. We find that TPDI-based devices benefit from a more balanced dual-direction charge generation process, which can be linked to the favorable morphology in the TPDI:PTB7-Th blend, demonstrating the critical importance of photoactive layer morphology considerations in the design of OPV acceptor materials.