| Because of the simple structure, low cost and high conversion efficiency, the intermediate band solar cells (IBSC) have attracted renewed attention as one of promising solutions to the existing energy crisis. Approaches to introduce intermediate band sandwiched between conduction band and valence band have been proposed and demonstrated, including impurity heavy doping, quantum dots and highly mismatched alloys (HMAs). Due to the large difference of electronegativity and ionic sizes between dopants and host materials, such diluted semiconductors exhibit unique electrical and optical properties with the formation of intermediate band and its electronic band structure can be well explained by band anticrossing (BAC) model. The IBSCs prototype devices using HMAs (ZnTe:O and GaAs:N) as an active absorption layer have been demonstrated. However, the conversion efficiency of these cells is still very low and the material growth is costly. The physical mechanisms and carrier dynamics associated with IB are still in debate. In this study, ZnTe:O highly mismatched alloys have been produced by isoelectric oxygen implantation into ZnTe and the microstructural and optical properties of ZnTe:O materials have been investigated in detail. The main achievements are summarized as follows:(1) The implantation of isoelectronic 0 into ZnTe with different doses have been demonstrated, and its influence on the structural and optical properties have been investigated in detail. It was found that low dose of oxygen ions led to larger lattice constant and weaker electron-LO phonon couple coherent length. The isoelectronic trap induced the information of intermediate band located at energy level of ～0.45eV below conduction band, which was consistent with the prediction of well-known band anticrossing (BAC) model. In contrast, high dose of oxygen ions caused ZnTe surface layer amorphous and enhanced the deep level emission related to Zn vacancies. The time-resolved photoluminescence spectra indicated that the intermediate band carrier recombination dynamic process show characteristic of expanding exponential decay due to the localized states caused by lattice distortion and alloy disorder.(2) The different post-annealing processes have been employed to recover the lattice disorder and activate the intermediate band associated with oxygen isoelectronic ions in ZnTe. In comparison, the approach of pulsed laser annealing has been proven to be a better pathway to realize high efficient intermediate band while preventing the serious out-diffusion of oxygen. The XRD and XPS results suggested that the lattice disorder induced by ion implantation is reduced effectively through laser annealing, and oxygen ions tend to transfer from interstitial sites to Te lattice sites. The photoluminescence (PL) spectra show significant radiative emission at about 1.8eV, which is in good agreement with the energy level of intermediate band in ZnTe as predicted by BAC model. The optical transitions of VB-IB and IB-CB have been observed obviously in transmittance measurement. Two-photon-excitation PL spectra also provided strong evidence of the simultaneous optical transitions between VB-IB and IB-CB. The excitation power dependent time-resolved PL spectra indicated that the recombination mechanism of IB carriers was complicated. The electronic states in IB are partially delocalized while most are still in localization due to potential fluctuation and tight binding nature of impurities. |
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