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The temperature-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the optical properties of O and Zn polarity of a c-plane single crystal ZnO wafer. By analyzing the XEOL and XRD, we found an unprecedented blue shift of the free exciton transition with increasing the excited carrier density as tuning the X-ray energy across the Zn K-edge, and the O-polar face possesses better crystal structure than the Zn-polar one. This spectral blue shift is attributed to the Coulomb screening of the spontaneous polarization by the excited free carriers that result in decreasing the exciton-phonon Frohlich interaction to reduce exciton binding energy.

We demonstrate a low series resistance contact of gold (anode)/zinc-phthalocyznine in a reverse organic photovoltaic device aided by oxygen plasma treatment. The power conversion efficiency appreciated from 0.2% to 2.13%, post O2 plasma treatment, predominantly due to the reduction in the series resistance of the device. A depth profile of zinc from x-ray photoelectron spectroscopy study revealed a gold layer with graded zinc composition, instead of pure gold, to be an efficient hole-collector from the organic interface. The zinc alloying of the gold layer has clearly been promoted by the oxygen plasma treatment.