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

Abstract

Perylene monomers (PDI) and twisted dimers (TPDI) are investigated here as non- fullerene acceptors and are paired with the polymer donor PTB7-Th in organic photo- voltaic blends. A combination of device characteristics, morphology studies and tran- sient absorption techniques reveal a complete picture of the photophysics in these two systems. TPDI-based devices are shown to far outperform PDI-based devices, and intensity-dependent JSC and VOC measurements reveal that they do not suffer from non-geminate recombination to the same extent as PDI-based devices. Transient ab- sorption (TA) spectroscopy also demonstrates dual-direction energy harvesting from excitation of both PTB7-Th and (T)PDI domains. TA reveals that charges live signif- icantly longer in TPDI blends and are less susceptible to bimolecular recombination. The modelling 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 favourable morphology in the TPDI:PTB7-Th blend, demonstrating the critical impor- tance of photoactive layer morphology considerations in the design of OPV acceptor materials.