Stability assessment of PTB7-Th and a quinoxaline-based polymer in both organic thin film transistors and in organic photovoltaic devices

Herein, we perform a stability assessment of two conjugated polymers that are conventionally used as electron donor polymers in the active layer of organic photovoltaic (OPV). More specifically, the impact of thermal annealing, a post-treatment commonly applied in the OPV community, is evaluated in terms of device performance and stability. The two polymers are PTB7-Th and QX1, and they are respectively blended with a non-fullerene electron acceptor, herein a derivative of N-annulated perylene diimide, that is, tPDI₂N-EH. These blends are targeted for their relatively high power conversion efficiency in outdoor conditions, but also for their potential as efficient active layer in low-intensity (indoor) conditions—while these blends have been reported, no study on the impact of thermal annealing on their stability has been performed yet. The performance stability of these devices, tracked via the open circuit voltage, the short-circuit current, the fill factor, and the power conversion efficiency metrics, were evaluated each day for 2 weeks and correlated to an evaluation of the microstructure of the active layer, evaluated using atomic force microscopy and UV–visible absorbance spectroscopy. Finally, transistors were prepared using only the two electron donor polymers, PTB7-Th and QX1, to assess if some correlations could be made between the behaviour of the OPV devices and that of the electronic charge mobilities. Results contribute to identify which molecular structures and which post-treatments are ideal to promote the stability of the active layers in the context of OPV devices.