(340d) Solution Processing of Thin-Film Chalcogenide Solar Cell Materials

I’m interested in working on technical problems related to sustainability and renewable energy. Specifically, I’m interested in work related to the synthesis and characterization of materials for clean energy applications. My expertise lies in studying the relationships between synthesis conditions and electronic properties of solar cell materials.

Research Summary

Solution processing of photovoltaic materials is a promising approach to produce efficient solar cells in a high throughput, low-cost manufacturing paradigm. Better understanding each step of the fabrication process is key to accelerating work on solution processed solar cells and advancing this technology towards large-scale manufacturing.

The first phase of my research focused on understanding the amine-thiol chemistry used to solution process chalcogenide solar cells. By performing careful reactions and analyzing the products with NMR, ESI-MS, and X-Ray absorption, we were able to determine the reactions that form metal thiolate species in solution, identify their structure, and understand the pyrolysis that occurs when a coated film is heated to form a semiconductor layer.

The second phase of my research has taken my knowledge of amine-thiol chemistry and applied it to the synthesis of AgIn(S,Se)₂ (AISSe). This involved developing a solution processing route for the synthesis of AISSe thin films, studying the electronic properties of AISSe to identify its merits and areas for improvement, and laying a path towards successful AISSe solar cells.

Using the amine-thiol system, I was able to cast films of AgInS₂. These films were heated in an atmosphere of argon and elemental selenium, causing the sulfur to be largely replaced by selenium, giving large-grain films of chalcopyrite phase AISSe.

Hall effect measurements performed on AISSe films identified ­n-type carrier concentrations of ~10¹³cm^-3. Furthermore, it was found that the carrier mobility (~12 cm²/Vs) was substantially higher than in other solution processed inorganic thin film materials, demonstrating an advantage of AISSe.

Photoluminescence demonstrated strong, narrow emission spectra in room-temperature measurements, indicating few defects in the material and pointing towards minimal voltage losses in future solar cells. In addition, Kelvin Probe Force Microscopy suggests that the electronic properties of grain boundaries in AISSe will help prevent the recombination of charge carriers even without intentional grain boundary passivation commonly used in other materials.

To take full advantage of these exciting properties, work must be done to find an optimal device architecture for AISSe solar cells. Based on the low carrier concentrations of AISSe I propose that it should be applied in a p-i-n device architecture, with AISSe as the “intrinsic” absorber layer. Investigations into possible candidates for the p and n layers are ongoing.