Effects of 1,8-diiodooctane on ultrafast charge carrier dynamics and photovoltaic performance in organic solar cells: A comparison of PC71BM and nonfullerene acceptor IT-M.

Nonfullerene acceptors are currently dominating the research and development of organic photovoltaics (OPVs). Exploring the charge separation mechanism is helpful to understand the photovoltaic process in efficient nonfullerene-based OPVs. Herein, we have selected two typical acceptors: PC 71 BM and nonfullerene acceptor IT-M, and have studied the device function and the ultrafast charge carrier dynamics in the PBDB-TF:IT-M and PBDB-TF:PC 71 BM blends to investigate how they are affected by different blend morphologies induced by using a popular solvent additive 1,8-diiodooctane (DIO). It is found that ultrafast charge generation occurs in both of the two blend systems. For the IT-M blend film, the DIO additive strongly affects the ultrafast photophysical process. Efficient hole transfer is observed for the IT-M blend processed with DIO compared to the pristine IT-M blend. Moreover, the charge density is significantly increased in the DIO-processed IT-M blend film. In contrast to the IT-M blend, the DIO does not strongly affect the ultrafast charge carrier dynamics in PC 71 BM blend possibly due to its slight effect on the blend morphology. The reionization of spin-triplet charge transfer states can occur faster than relaxation to triplet in both of the well-ordered blend film processed with DIO resulting in an enhanced current density and fill factor. Image 1 • Ultrafast charge separation is observed in both PBDB-TF:IT-M and PBDB-TF:PC 71 BM blend films. • Efficient hole transfer occurs and the charge density is significantly increased in DIO-processed PBDB-TF:IT-M blend film. • The reionization of 3CT states can occur faster than relaxation to triplet excitons in both fullerene and nonfullerene blend films processed with DIO. [ABSTRACT FROM AUTHOR]