Polymer solar cells with long-term thermal stability and high efficiency based on photocrosslinkable donor-acceptor conjugated polymers

This work reports on the long-term thermal stability of highly efficient organic photovoltaic devices (OPVs) based on donor-acceptor copolymers. Photocrosslinkable derivatives of a thieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymer were developed and used as electron donating material in BHJ solar cells. By synthetically tuning the amount of photocrosslinkable moiety in the polymer and by employing UV-mediated photocrosslinking, itwas possible to fabricate OPV devices with high power conversion efficiency (PCE) and excellent thermal stability. In particular, after 72 h of thermal annealing at 150 °C, an average PCE of 4.6% was obtained with a short circuit current density (JSC) of 10.1 mA/cm2, an open circuit voltage (VOC) of 0.85 V, and a fill factor (FF) of 54%. The peak PCE obtained was as high as 4.7%. In addition, the choice of fullerene acceptor in the BHJ was found to be critical not only for achieving high PCE but also for stabilizing the morphology of the active layer at high temperature. Atomic force microscopy (AFM) was employed to investigate the surface morphology of the active layer in both crosslinked and non-crosslinked films before and after annealing.