Proceedings of Online nanoGe Fall Meeting 20 (OnlineNFM20)
Publication date: 4th October 2020
Organic solar cells (OSCs) based on so-called bulk heterojunction (BHJ) represent an environmentally friendly technology that shows a great promise in solar energy conversion. New approaches in exploiting non-fullerene acceptors (NFAs), recently pushed the boundaries for power conversion efficiencies over 18% for a single junction device [1,2].
Yet, stability of OSCs remains a key obstacle for establishing its dominance on the OPV market. The photoactive layer of OSCs is highly sensitivity to photo-oxidation and, as a result, it deteriorates rapidly under real-life operating conditions [3]. Thus, investigation of oxidation mechanisms is of high importance.
We present an investigation of photodegradation in blend films of the donor polymer poly(3-hexylthiophene) (P3HT) and the rhodanine-flanked small molecule acceptor 5,5’-[(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(2,1,3-benzothiadiazole-7,4-diylmethylidyne)]bis[3-ethyl-2-thioxo-4-thiazolidinone] (FBR) in ambient atmosphere. The photobleaching kinetics of the pure materials and their blends was correlated with the generation of radicals and triplet excitons by using advanced EPR (pulsed EPR, ODMR), photoluminescence (PL) and absorption spectroscopy techniques. In addition, spin-trapping methods were employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, we found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor-acceptor charge transfer, which competes with triplet exciton formation.
[1] Q. Liu et al., Sci. Bull. 2020, 65, 272.
[2] Y. Cui et al., Adv. Mater. 2020, 32,1908205.
[3] P. Chen et al., Chem. Soc. Rev, 2016, 45, 2544
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