Optimization of the zinc oxide electron transport layer in P3HT:PC61BM based organic solar cells by annealing and yttrium doping

Zinc-oxide and yttrium-doped ZnO films were fabricated by a sol-gel processing technique and were incorporated as an electron transport layer in inverted organic solar cells (with an active layer comprising a blend of P3HT and PC61BM). First, the annealing conditions for the pure sol-gel ZnO layers were optimized. An interesting observation was that the annealing temperature of the ZnO layer significantly influenced the overall organic solar cells performance. Annealing the ZnO film at temperatures of similar to 150 degrees C provided the highest device performance. The physical and surface properties of these ZnO films were examined by X-ray diffraction, atomic force microscopy and UV-vis transmittance measurements. Utilizing the optimized annealing conditions, we further fabricated high-efficiency organic solar cells by doping yttrium in the zinc-oxide (YZO) electron transport layers. The efficiency of YZO based devices was improved by 30% when compared to that of the undoped zinc oxide based devices.