Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

•Efficiency of OPVs was found to increase from 0.8% to 3.3% for ZnO film thickness ranging from 4 nm up to 100 nm.•Thickness of PLD ZnO significantly influences its workfunction.•S-shape in the IV curves eliminated beyond a threshold ZnO thickness of 25 nm.•PLD ZnO band structure evolves with increasing film thickness.•XPS shows a change in ZnO stoichiometry with increased film thickness.

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.

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