Effect of UV light irradiation on photovoltaic characteristics of inverted polymer solar cells containing sol–gel zinc oxide electron collection layer

Abstract

An inverted organic bulk-heterojunction solar cell containing a zinc oxide (ZnO) based electron collection layer with a structure of ITO/ZnO/[6,6]-phenyl C61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid)/Au (ZnO cell) was fabricated. We examined the relationship between the heating temperature of the ZnO layer and the device performance under irradiation by simulated sunlight while cutting the UV light. The effects of the UV light contained in simulated sunlight were investigated by photocurrent–voltage (I–V) and alternating current impedance spectroscopy (IS) measurements. When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 250°C, 350°C, and 450°C. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250°C, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350°C and 450°C, the shapes of the I–V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1h, the PCE of the cell with ZnO prepared by heating at 350°C decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450°C fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.