Enhancing performance of inverted organic solar cells by nano-imprinting the active layer with a PDMS template

The potential applications of organic materials in solar cells have been widely explored for the creation of inexpensive and flexible modules compared with their inorganic counterparts. However, the power conversion efficiency (PCE) of organic solar cells (OSCs) have been severely restricted on account of the insufficient light absorption in the organic active layers. A simple method to achieve higher absorption efficiency is to increase the thickness of the active layer, but considerable electrical loss can occur during charge transport to the electrodes. Therefore, it is necessary to seek some effective ways to enhance light absorption in active layer without increasing its thickness. OSCs with inverted configuration usually present higher PCE and longer lifetime than corresponding devices with regular configuration. In this study, we demonstrate an improvement in photovoltaic properties in inverted OSCs by introducing the patterned structures in the active layer (PTB7:PC70BM) using a nano-imprinting technique with a PDMS stamp. By adjusting pressure of imprinting the active layer, the imprinted OSCs were optimized, showing the optimal optoelectronic performances. The results indicated when the imprinted cell was pressed by a 200 g weight, the absorption of the nanoimprinted cell dramatically increased compared with the control cell. Meanwhile, the fill factor (FF) also increased from 68.0% for the control to 70.0% for the optimal imprinted cell. In addition, the open voltage (Voc) was maintained in 0.73 V. Overall, the PCE of 6.95% with a 6.0% enhancement compared to the control cell (6.54%) was achieved.