Effects of annealing temperature of aqueous solution-processed ZnO electron-selective layers on inverted polymer solar cells

We investigate the effects of ZnO annealing temperature (T-A) on the performance of inverted polymer solar cells with ZnO electron-selective layers deposited by spin coating aqueous solutions of an ammine-hydroxo zinc complex. The inverted solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C-61-butyric acid methyl ester with T-A as low as 80 degrees C exhibit power-conversion efficiencies of 3.6%, which is equal to those of devices with higher T-A. Characterizations of the ZnO films using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, grazing incidence wide-angle X-ray scattering, and optical transmittance measurements show that the abrupt improvement of device performance from T-A = 60 to 80 degrees C is due to the improvement of energy-level alignment arising from the increases in the relative amount and the crystallinity of ZnO. (C) 2012 Elsevier B.V. All rights reserved.