Highly mismatched alloys are a class of materials whose fundamental properties are dramatically modified through the substitution of a relatively small fraction of host atoms with an element of very much different electronegativity. In ZnTe, the incorporation of a small amount of isoelectronic O leads to the formation of a narrow, O-derived intermediate band (IB, E-) located well below the conduction band (CB, E+) edge of the ZnTe through an anticrossing interaction between localized states of O and the CB of the ZnTe matrix. Therefore, ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC). We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. Here, we review our recent progress on the development of ZnTeO based IBSCs using n-ZnS window layer and Cldoped ZnTeO. With n-ZnS window having a small conduction band offset with ZnTe, the open circuit voltage of ZnTeO IBSC was improved. Cl-doping was performed to introduce electrons into the IB of ZnTeO that is required to be halffilled with electrons for the efficient operation of an IBSC. Low temperature photoluminescence spectra indicated that the doped Cl atoms act as donors in ZnTeO. The improved photovoltaic properties were demonstrated in the IBSC using Cl-doped ZnTeO.
Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (Voc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved VOC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.
High VI/II transport rate ratio for Al-doped ZnTe homoepitaxial layers grown by metalorganic vapor phase epitaxy leads
to distinct shallow and deep donor-acceptor-pair (DAP) emissions in the photoluminescence spectrum together with
donor-related bound excitonic emission (Id), independent of the growth conditions. From the analysis of excitation power
dependence of shallow DAP emission, donor and acceptor levels are estimated to be ~19.5 and ~53.5meV for Al-doped
ZnTe layer, respectively. Thermal quenching effects of Id and shallow DAP were examined based on two step quenching
processes, and the derived donor ionization energy is of ~ 19 meV and acceptor level is of ~52.8 meV, which are in good
agreement with the result on its excitation power dependence for the latter case.