Charge Transfer Dynamics and Device Performance of Environmentally Friendly Processed Nonfullerene Organic Solar Cells

In the last years, one of the pursuits has been to replace the use of halogenated solvent during the processing of organic photovoltaic (OPV) devices. Herein, we investigate the nonhalogenated solvent, o-methylanisole (o-MA) and the well-stabilized o-dichlorobenzene (o-DCB) to process the bulk heterojunction (BHJ) based on PTB7-Th donor (D) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′] dithiophene) (ITIC) acceptor (A). The formation of D–A interfaces with different (solvent-dependent) characteristics was verified by steady-state photoluminescence and morphological and electrical measurements. These measurements show a rather comparable device efficiency of the PTB7-th:ITIC BHJ processed by o-MA (compared to the device processed using o-DCB) despite the lower absorption of the films and the lower VOC. Also, the charge-transfer (CT) state formation was investigated and the reasons behind the VOC losses were correlated to the interface variations when processed by different solvents. Some experimental results are then discussed in light of the electronic structure of the molecules calculated using the density functional theory (DFT) method. The comparison between the experimental data and the theoretical calculations give some insights about the microscopic processes involved in the variation of the devices properties processed using the o-DCB and o-Ma solvents. We concluded that the D–A distance clearly affects the CT state energy and consequently the VOC. Furthermore, higher air stability is observed when the active layer is processed using o-MA instead of o-DCB. The better stability was observed in self-lifetime measurements and air-processed devices.