Unexpectedly Slow Yet Efficient Picosecond to Nanosecond Photoinduced Hole-Transfer Occurs in a Polymer/Nonfullerene Acceptor Organic Photovoltaic Blend

We study photoinduced charge generation in a model polymer/nonfullerene acceptor (NFA) organic photovoltaic (OPV) blend. Specifically, we focus on hole-transfer kinetics from the photoexcited NFA thiophene-thieno [3,2-b]thiophene-thiophene-3-(dicyanomethylidene)-indan-1-one (4TIC) to the conjugated polymer donor poly[(4,4'-bis(2-butyloctoxycarbonyl-[2,2'-bithiophene]-5,5-diyl)-alt-(2,2'-bithiophene-5,5'-diyl)] (PDCBT) using ultrafast transient absorption spectroscopy by selectively exciting the 4TIC electron acceptor and monitoring the bleach of the PDCBT ground-state population. In the blend, the 4TIC excitons decay with an average lifetime of 7 ps, accompanied by a concomitant rise in the ground-state bleach of the polymer with a comparable average lifetime that is 60% complete by 8 ps and 95% complete by 100 ps, occurring roughly an order of magnitude slower than that in most previously reported polymer/NFA blends. Notably, the ground-state bleach of the polymer continues to grow, not reaching its maximum until similar to 1 ns. To explain this slow ground-state bleach rise, we propose that some 4TIC charge-transfer-like excitons are generated, which undergo hole transfer to the polymer on the 100 ps-1 ns time scale. Our findings provide new insight into the kinetics of hole transfer in nonfullerene OPV blends in the regime of small driving force and also support the proposal that nonfullerene acceptors may generate long-lived charge species upon direct photoexcitation.