ZnO-based semiconductors with tunable band gap for solar sell applications

In this study, we discuss the potential advantages of a new ZnO-based semiconductor, ZnInON (ZION), for application in multi quantum-well (MQW) photovoltaics. ZION is a pseudo-binary alloy of ZnO and InN, which has direct and tunable band gaps over the entire visible spectrum. It was found from simulation results that owing to the large piezoelectric constant, the spatial overlap of the electron and hole wave functions in the QWs is significantly small on the order of 10^-2, where the strong piezoelectric field enhances the separation of photo generated carriers. As a result, ZION QWs have low carrier recombination rate of 10¹⁴-10¹⁸ cm^-3s^-1, which is much lower than that in conventional QWs such as InGaAs/GaAs QW (10¹⁹ cm^-3s^-1) and InGaN/GaN QW (10¹⁸-10¹⁸ cm^-3s^-1). The long carrier life time in ZION QWs (ə1/41μs) should enable the extraction of photo-generated carriers from well layers before the recombination, and thus increase V_oc and J_sc. These simulation results are consistent with our experimental data showing that both V_oc and J_sc of a p-i-n solar cell with strained ZION MQWs and thus the efficiency were increased by the superimposition of laser light with lower photon energy than the band gap energy of the QWs. Since the laser light contributed not to carrier generation but to the carrier extraction from the QWs, and no increase in V_oc and J_sc was observed for relaxed ZION MQWs, the improvement in the efficiency was attributed to the long carrier lifetime in the strained ZION QWs.