Epitaxial lift-off CdTe/MgCdTe double heterostructures for thin-film and flexible solar cells applications

This paper reports an improved epitaxial lift-off (ELO) technology for monocrystalline CdTe/MgCdTe double-heterostructure (DH) thin films using water-soluble and nearly lattice-matched MgTe as a sacrificial layer. Employing hard-baked photoresist as the superstrates with appropriate surface tension, the lift-off thin films show smooth and flat surfaces, confirmed by atomic-force microscopy profiles. Photoluminescence (PL) measurements reveal further enhancement of the light extraction from the ELO thin films with a coated Ag back reflective mirror. The increased PL intensity also confirms that the CdTe/MgCdTe DHs maintain high optical quality after ELO. External luminescence quantum efficiency (ηext) is quantitatively measured and used to calculate the implied open-circuit voltage (iVOC). A 0.5-μm-thick lift-off CdTe/MgCdTe DH with a back mirror demonstrates an ηext value of 5.35% and an iVOC value of 1.152 V. The devices based on this structure are also expected to have an improved fill factor and a short-circuit current density (JSC) of 24.7 mA/cm² according to the simulation results, promising to achieve CdTe solar cells with a record power conversion efficiency.