Development of CdTe on Si Heteroepilayers for Controlled PV Material and Device Studies

ABSTRACTThe objective of the National Renewable Energy Laboratory’s (NREL) current three-year CdTe plan under the U.S. Department of Energy’s SunShot Initiative is to identify primary mechanisms that limit the open-circuit voltage and fill factor of polycrystalline CdTe photovoltaic (PV) devices, and develop CdTe synthesis processes and/or device designs that avoid these limitations. Part of this project relies on analysis of crystalline materials and pseudocrystalline CdTe layers where point and extended defects can be introduced sequentially without the complications of extensive impurities and grain boundaries that are typical of present polycrystalline films. The ultimate goals of the project include producing CdTe PV devices that demonstrate ≥20% conversion efficiency, while significantly improving our understanding of processes and materials capable of attaining cost goals of <$0.50 per watt. While NREL is investigating several options for the routine fabrication of high-quality CdTe layers, one pathway involves CdTe molecular beam heteroepitaxy (MBE) on Si in collaboration with the University of Illinois at Chicago. Although CdTe/Si heteroepitaxy is relatively unfamiliar to researchers in the PV community, it has been used successfully for more than 20 years to produce high-quality CdTe surfaces required for commercial production of large-area single-crystal HgCdTe infrared detectors and focal-plane arrays. The process involves chemical and thermal preparation of Si (211) wafers, followed by deposition of As-passivation and ZnTeaccommodation layers. MBE-grown CdTe layers deposited on top of this “template” have been shown to demonstrate low etch-pit density (EPD, preferably ≤ ∼5x105 cm-2) and high structural quality (full width at half maximum ∼ 60 arcs). These initial studies indicate that 10-μm-thick CdTe layers on Si are indeed epitaxial with cathodoluminescence-determined dislocation density consistent with historic EPD measurements, and that recombination rates are distinct from either as-deposited polycrystalline or crystalline materials.