Significant increase in current density of inverted polymer solar cells by induced-crystallization of sol-gel ZnO embedded with ZnO-NP

Performance of the inverted polymer solar cells using sol-gel ZnO as cathode buffer layer is significantly improved by addition of 10 wt% crystalline ZnO nano particles (ZnO-NP) to the sol-gel ZnO. Power conversion efficiency (eta) of the inverted polymer solar cells using P3HT/PC60BM as a model active layer increases over 30% from 3.73% with the pristine sol-gel ZnO buffer layer to 4.91% (eta(max) = 5.0%) with the modified sol-gel ZnO buffer layer. Crystalline ZnO-NP acts as a nucleation agent inducing crystallization of sol-gel ZnO at 200 degrees C that increases crystallinity and carrier mobility of ZnO buffer layer. Crystal size of ZnO increases from amorphous of the pristine sol-gel ZnO to 11 nm of the modified sol-gel ZnO. The increase in crystallinity of ZnO leads to a decrease in resistivity of the ZnO buffer layer and a decrease in the series resistance of the fabricated device, which leads to performance improvement of the inverted polymer solar cells. By using this concept, it is possible to prepare inverted flexible polymer solar cells with improved device performance at low processing temperature.