Short lived charge transfer excitons in organic photovoltaic cells studied by high field magneto-photocurrent

TYPESpecial Seminar - Solid State Institute, Technion
Speaker:Mrs. Ayeleth Devir-Wolfman
Affiliation:Physics Department and Solid State Institute, Technion
Location:Solid State Auditorium(Entrance)
Remark:Ph.D. Student of Professor Eitan Ehrenfreund
Abstract:The main route of charge photo-generation in efficient organic photovoltaic cells based on bulk heterojunction (BHJ) donor-acceptor blends involves short-lived charge transfer excitons (CTE) at the donor-acceptor interfaces. The cell efficiency is critically affected by the CTE recombination and dissociation processes. By measuring the magneto-photocurrent (MPC) under ambient conditions at room temperature, we show that magnetic field induced spin-mixing among coupled pairs spin sublevels occurs in fields up to ~8.5T. We observe three different processes affecting the MPC, differing from each other in their dependence on the magnetic field, and their saturation field. We identify the observed spin-mixing mechanisms to be hyperfine interaction (HFI) at the low field regime (<0.04T), polaron pairs (PP) Δg-mechanism at the intermediate field regime (0.04T-0.7T), and a CTE Δgmechanism at the high field regime (>0.7T). We show that the difference between the intermediate field and the high field responses is due to the shorter life-time of the CTE compared to that of PP. We also attribute the observed non-Lorentzian high field response by a dispersive decay mechanism due to a broad distribution of CTE lifetimes.