Solar Electrolyzer Coupling via Load-Matching and Doping in Cadmium Telluride Solar Cells to Overcome Voltage Limitations

Abstract

This dissertation comprises two distinct topics concerning photovoltaics: studying solar electrolyzer performance with a practical consideration of their design and operation, and extrinsic p-type doping in thin-film CdTe solar cells using Sb for improved voltage output.

Solar fuel generation via water and CO₂ reduction using photovoltaics has witnessed considerable growth since the identification of photocatalysis four decades ago. Numerous photovoltaic-driven electrochemical cells (PV-ECs) and photoelectrochemical cells (PECs) with efficiencies reaching 30% for H ₂O reduction and 10% for CO₂ reduction have been reported. We will discuss the many benefits of a PV-EC system over the PEC approach. This dissertation discusses the implementation of a high-efficiency PV-EC using silicon solar cells and a flow-cell CO₂ electrolyzer (in collaboration with Prof. Feng Jiao group from UDel’s Center for Catalytic Science and Technology). With 25 cm² electrode area, this is the largest CO₂ electrolysis device yet reported that exhibited > 6.5% efficiency at operating currents in excess of 1A. The development of a model to optimize the coupling of such devices and to simulate annual field performance will be presented. Improvement in fuel generation by > 20% is demonstrated by employing power electronic devices to continuously optimize the PV-EC operating point for maximum power coupling despite variable sunlight and temperature.

Polycrystalline thin-film CdTe/CdS heterojunction solar cells are the leading commercial competitor to c-Si solar modules. While having demonstrated good performance at low cost and large scale, the potential to exceed 25% efficiency by enhancing open circuit voltage (Voc), from present ~0.9V to the near-ideal 1.1 V, is possible with carrier concentrations exceeding 5 × 10¹⁶ cm^–3 while retaining bulk minority carrier lifetime > 10 ns. State-of-the-art intrinsic CdTe solar cells, wherein n- or p-type doping is achieved through native point defect (V _Cd) control during film growth, are limited to acceptor concentration levels of < 10¹⁵ cm^–3. Bridging the Voc gap through extrinsic doping of polycrystalline CdTe films with Sb during film growth using vapor transport deposition technique is examined. Applying device characterization and analysis techniques to cells processed with different post-growth device treatments for dopant activation shows where optimization effort is needed. Admittance and current-voltage measurements indicate that despite significant improvement in acceptor density (up to 3 × 10 ¹⁵ cm^–3), Voc is limited to < 600 mV due to increased defect density and thus reduced minority carrier lifetime.