Addressing Unresolved Scientific Challenges for CdTe-based Solar Cells

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Solar energy is the only renewable resource with the capacity to deliver carbon neutral energy in the quantities required to supplant the power currently generated by fossil fuels. Cadmium telluride (CdTe)-based photovoltaics have the lowest cost of the commercially relevant technologies, with champion cell and module efficiencies of 22.1% and 18.6%, respectively. Improvements in efficiency and stability are required to accelerate further deployment in the marketplace. The project addresses fundamental research questions required to further elevate performance towards theoretical limits (~33%). The project will look at improved material compositions to increase the power conversion efficiency of the device while retaining durability and stability for long operational life. This program will educate a PhD graduate student and several undergraduates in advanced technology. Their research experiences will be broadened and enriched by participation in collaborative programs offered by the National Renewable Energy Laboratory (NREL) and the Molecular Foundry at Lawrence Berkeley National Laboratory (LBNL). The team will also use the solar research to engage in outreach to K-12 students in the Denver public school system to inspire the next generation of scientists and engineers.The key innovation of this project is to develop ternary alloys with novel attributes that cannot be obtained using conventional binary layers. These alloys will be integrated throughout the device structure to improve both the efficiency and the stability of these devices. This project addresses three scientific challenges facing CdTe: (i) replacement of CdS to improve current collection; (ii) extrinsic doping of CdTe to improve open circuit voltage; and (iii) development of copper-free back contacts to improve stability. To improve current collection, the PI will systematically explore magnesium zinc oxide (MZO) alloys as buffer layers, focusing on tailoring its conduction band edge and conductivity for this application. Further improvements in open circuit voltage require increasing carrier concentration. Extrinsic doping of polycrystalline CdTe will involve generation of doped source materials and the use of dilute ternary alloys to facilitate dopant incorporation. Finally, the PI will leverage the research group's understanding of current ZnTe-based back contacts to design and implement effective contacts that do not employ copper for enhanced stability. All of these studies will be informed by analysis techniques including in situ XRD, spatially- and spectrally-resolved measurements of carrier lifetime, and nanoscale characterization of structure and composition using scanning TEM and atom probe tomography. These studies will help establish the process-structure-property relationships for these under-explored II-VI ternary alloys.