Increased efficiency of low band gap polymer solar cells at elevated temperature and its origins

Photovoltaic characteristics of a low bandgap polymer, poly[(4,4′-bis(2-lethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], based bulk hetero-junction organic photovoltaic were investigated from room temperature (RT) to 145 °C to evaluate its applications in harsh environments. The power conversion efficiency was found to increase from 4.1% at RT to 4.5% at 105 °C with increased short circuit current density (Jsc) and fill factor (FF) despite the decreased open circuit voltage (Voc). Detailed investigation revealed that Jsc and FF improvements were caused by the increased and balanced carrier mobilities at higher temperatures. The Voc of the low bandgap polymer solar cell is determined not only by the energy levels and dark currents, but also by the binding energy of charge transfer excitons (CTEs). A slower reduction of Voc is observed at high temperatures due to the decreased binding energy of CTEs.