Thermal annealing dependent dielectric properties and energetic disorder in PffBT4T-2OD based organic solar cells

Organic photovoltaics (OPV) has emerged as a promising renewable energy generation technology in the past decades. The thermal annealing effect on organic solar cells as one of the most widely used processing technologies have been substantially investigated. However, to the best of our knowledge, no research has investigated the changes in dielectric property and energetic disorder behind the thermal annealing processes. Herein, we conducted a systematic thermal annealing study for PffBT4T-2OD: PC71BM based organic solar cell and focused on the changes in dielectric property and energetic disorder. The characterisations such as J-V, EQE, PL, C-V, SCLC, AFM and TEM have been conducted for investigation. We systematically studied the annealing temperature up to 140 degrees C. It was found that devices annealed at 80 degrees C possess the most efficient exciton dissociation and charge extraction process and the highly suppressed the charge recombination which may be ascribed to its optimized interpenetrate morphology with optimized phase separation and domain size. The 80 degrees C annealed devices showed the largest dielectric constant and lowest capture coloumbic radius (R-c) among all devices which indicates that the optimized thermal annealing can tune the dielectric property of the device and induce more efficient charge separation. Moreover, we firstly estimated the annealing temperature-dependent Urbach energy. As results, all thermal annealed devices showed a reduced Urbach energy when compared to fresh devices, which proved that the thermal annealing process owns the ability to reduce the energetic disorder in the device.