N-doping of nonfullerene bulk-heterojunction organic solar cells strengthens photogeneration and exciton dissociation. Issue 36 (30th August 2022) - Xie, Jiaqi
Abstract : N -doping of the bulk-heterojunction active layer to enlarge polymeric donor (D) and nonfullerene acceptor (A) domains, dilate D/A interspace and reduce acceptor exciton binding energy to enhance light harvesting and suppress germinate recombination. Abstract : N -type doping of the bulk-heterojunction layer in nonfullerene organic solar cells allows to effectively ameliorate inferior electron transportation by filling traps and optimizing electron pathways, leading to a better balance of charge transport in device. This mechanism, however, provides an incomplete understanding of the stronger photogeneration, long-lived excitons and simultaneously increased short-circuit current density ( J SC ) and open-circuit voltage ( V OC ) that also benefit from the n-doping. Herein we investigate how molecular n-dopant impacts the optical characteristics, intermolecular packing behavior, charge carrier dynamics and photovoltaic performance in the nonfullerene-based blend. When incorporating a prototypical n-type dopant N -DMBI into a benchmark PM6:Y6 blend, the crystallization of PM6/Y6 is facilitated and the crystal coherence length is elongated, which is correlated with the optical absorbance enhancement. N -doping is unveiled to prolong exciton lifetime by retarding germinate recombination (GR) both at donor/acceptor (D/A) interfaces and within constituent domains by dilating interspace, reducing trap states and decreasing exciton binding energy. Despite slower interfacial charge transfer across the enlarged D/A interspace due to dopant intercalation, exciton dissociation remains highly effective due to the impeded interfacial GR. Consequently, the champion inverted cell at an optimal N -DMBI content delivers a decent efficiency of 15.34%, which is among the highest of the state-of-the art analogous PM6:Y6-based binary cells. Such improvement is largely ascribed to the concurrent increase of J SC (up to 26.41 mA cm −2 ) and V OC (up to 0.86 V) in comparison to the undoped device.