Low temperature aqueous solution-processed Li doped ZnO buffer layers for high performance inverted organic solar cells

In this study, a lithium (Li) doped aqueous solution processed zinc oxide (ZnO) layer was employed as the electron buffer layer for inverted organic solar cells (OSCs). It was found that compared with the pure ZnO buffer layer, application of the 5% Li-doped ZnO buffer layer enhanced the open circuit voltage (V-oc) from 0.57 V to 0.62 V and the fill factor (FF) from 0.61 to 0.67, resulting in a better device performance. The increased V-oc resulted from the increased built-in field and decreased work function of the cathode, and the improved J(sc) and FF were due to the significantly increased charge carrier mobility, enhanced charge carrier collection efficiency and reduced charge carrier recombination. Based on the poly(3-hexylthiophene): phenyl-C-61- butyric acid methyl ester (P3HT:PC61BM) system, a power conversion efficiency (PCE) as high as 3.9% was obtained for the device with the 5% Li-doped ZnO buffer layer, which is much higher than that obtained with the pure ZnO buffer layer (3.2%). When using the poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b: 4,5-b'] dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl):[6,6]-phenyl-C-71-butyric acid methyl ester (PTB-7:PC71BM) system as the active layer, the PCE was enhanced to 8.4% for the Li-ZnO (5%) layer compared to pristine ZnO with a PCE of 7.5%. These results suggest that the Li-doped ZnO layer can act as a good electron buffer layer for inverted organic solar cells.