Effects of Ga- and Al-Codoped ZnO Buffer Layer on the Performance of Inverted Polymer Solar Cells

A Ga, Al-doped zinc oxide (GAZO) buffer layer was applied to inverted polymer solar cells (PSCs) based on P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C-61-butyric acid methyl ester] blend films. The work function of the GAZO layer on indium-tin oxide (ITO) was measured to be 4.45 eV. The insertion of the GAZO layer between the ITO electrode and the P3HT:PCBM blend film in the inverted PSC led to an improved short-circuit current (J(sc)), open-circuit voltage (V-oc) and fill factor (FF) compared to those of the reference cell without GAZO layer. The J(sc) enhancement in the inverted PSC with the GAZO layer was attributed to both the effective electron extraction and the increased crystallinity of P3HT, and the work function difference between Ag and GAZO layer induced the increase in V-oc. The improved FF value was due to the lowered series resistance and elevated shunt resistance. Thus, the power conversion efficiency of the device with the GAZO layer was improved by more than 200% relative to the reference cell.