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 degrees 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 degrees C with increased short circuit current density (J(sc)) and fill factor (FF) despite the decreased open circuit voltage (V-oc). Detailed investigation revealed that J(sc) and FF improvements were caused by the increased and balanced carrier mobilities at higher temperatures. The V-oc 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 V-oc is observed at high temperatures due to the decreased binding energy of CTEs. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3643450]