Quantitatively Characterized Crystallization Effect on Recombination Energy Loss in Non-Fullerene Organic Solar Cells

Recombination energy loss is the main impediment on improving the power conversion efficiency of organic solar cells (OSCs). The pernicious effect is usually induced by two dynamics, that is, the geminate recombination of nascent charge pairs soon after the exciton dissociation and nongeminate recombination of separated charges during their transportation. Both hinder achieving high open circuit voltage (V-OC). Here, we comprehensively investigated the relationship between crystallization and molecular recombination in a non-fullerene system of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c']dithiophene-4,8-dione))] (PBDB-T):((5Z,50Z)-5,50-(((4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b0]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadia-zole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothia-zolidin-4-one)) (O-IDTBR). Based on a quantitative characterization of crystallinity, it was found that the crystallization intensity ratio between components is the key factor to suppress recombination energy losses. The nongeminate recombination showed increased probability with enlarged variance of crystallinity between the donor and acceptor. The geminate recombination was proven to be restricted by the energetic disorder of the highest occupied molecular orbital and lowest unoccupied molecular orbital, as well as the phase separation induced by crystallization. The rational crystallization intensity ratio between donor/acceptor (D/A) components is vital in achieving minimum energy loss as well as best device performance. The results are favorable for comprehending the effects of crystallinity in charge transfer and charge transport dynamics and provide guidance for morphology and crystallinity optimization in non-fullerene OSCs.