Study of CdTe/MgCdTe Double-Heterostructure Solar Cells and an Epitaxial Lift-Off Technology for Thin-Film and Tandem Applications

CdTe/MgCdTe double heterostructures (DHs) integrated with a heavily-doped a-Si:H layer as the hole contact was demonstrated a record open-circuit voltage (VOC) of 1.11 V and an active-area efficiency of 20% in 2016. Despite this significant progress, some of the underlying device physics has not been fully understood. The first part of this dissertation reports a systematic study of the CdTe/MgCdTe DH devices. The CdTe/MgCdTe DHs are grown on InSb(001) substrates. The vertical transport mechanisms across the CdTe and InSb heterovalent interface are investigated with N-CdTe/n-InSb and N-CdTe/p-InSb heterostructures. A transport model including tunneling through CdTe barrier and InSb interband transition is developed to explain the different temperature dependent current-voltage characteristics of these two heterostructures. Different p-type layers are integrated with the CdTe/MgCdTe DHs to form solar cells with different VOC values and efficiencies. The low VOC of devices with ZnTe:Cu and ZnTe:As hole contacts is attributed to the low built-in voltage and reduced minority carrier lifetime in the CdTe absorber, respectively. The critical requirements for reaching high VOC values are analyzed. A novel epitaxial lift-off technology for monocrystalline CdTe is developed using a water-soluble and nearly lattice-matched MgTe sacrificial layer grown on InSb substrate. The freestanding CdTe/MgCdTe DH thin films obtained from the lift-off process show improved optical performance due to enhanced light extraction efficiency and photo-recycling effect. This technology enables the possible development of monocrystalline CdTe thin-film solar cells and 1.7/1.1-eV MgCdTe/Si or MgCdTe/Cu(InGa)Se2 tandem solar cells. The monocrystalline CdTe thin-film solar cells and 1.7-eV MgCdTe DH solar cells have been demonstrated with a power conversion efficiency of 9.8% and an active-area efficiency as high as 15.2%, respectively. Additionally, a study of the radiation effects on CdTe DHs under 68-MeV proton irradiation is performed and showed their superior radiation tolerance. All these findings indicate that the monocrystalline CdTe thin-film solar cells are reasonably expected to have low weight, high-efficiency and high power density, ideal for space applications.