Vertically aligned ZnO nanorods doped with lithium for polymer solar cells: defect related photovoltaic properties

Abstract

The nanorod arrays of ZnO incorporated with lithium atoms show specific crystallinity, photoluminescence and absorption properties, which are promising for the improvement of photovoltaic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). Li ions can be incorporated into ZnO crystals during the hydrothermal growth of the nanorods. The presence of Li in ZnO crystal was confirmed through X-ray diffraction analysis and by the photoluminescence spectra obtained. The difference in photovoltaic properties brought about by Li doping was determined from concentrations of the precursor solution. It was determined that appropriate Li doping improves both the short circuit current density (J_[sc]) and open circuit voltage (V_[oc]). The quenching of photoluminescence of Li-doped ZnO nanorods/P3HT films indicates effective charge transfer at the interface due to oxygen-enrichment of the surface, corresponding to the enhancement of J_[sc]. The improvement of V_[oc] was due to the suppression of the charge injection from the electrode brought about by the increase in barrier height at the ITO/ZnO interface as the work function of the ZnO nanorods was reduced after Li ion doping. However, further substitution of Li to Zn (Li_[Zn]) leads to increased reverse current densities of minority carriers decreasing V_[oc] after the maximum value at 5 atom% incorporation. The maximum power conversion efficiency of 0.37% was obtained at 5 atom% doping, although improvements in photovoltaic performances through Li doping were still seen up to 20 atom% doping.