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 (TA) 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-C61-butyric acid methyl ester with TA as low as 80 °C exhibit power-conversion efficiencies of 3.6%, which is equal to those of devices with higher TA. 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 TA = 60 to 80 °C is due to the improvement of energy-level alignment arising from the increases in the relative amount and the crystallinity of ZnO.

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► We fabricate inverted polymer solar cells with ZnO layers annealed at temperatures as low as 80 °C.
► The origins of the effects of annealing temperature on device performance are identified.
► The aqueous solution-processed ZnO is well suited for organic solar cells on flexible substrates.