Synthesis and Environmental Assessment of Arsenic-Containing Copper Chalcogenides for Photovoltaic Applications

2020-07-15T19:41:36Z (GMT) by Joseph Andler
As the demand for energy increases, competition for a sustainable alternative to non-renewable energy resources has resulted in the growth of the photovoltaic industry. Although most photovoltaic technologies are based on crystalline silicon, thin film technologies have been developed with the expectation of generating a comparably high-performing product with lower processing costs. These materials have demonstrated sufficiently high optoelectronic performance to enable commercialization but concerns such as material scarcity limit terawatt level power production.

In the continuous pursuit of earth abundant solar absorber materials appropriate for thin film technologies, enargite Cu3AsS4 has been identified as a promising material due to its ideal direct band gap, stability, and high absorption. Recent efforts have demonstrated this class of copper chalcogenides exhibits band gap tunability and has solution processing capabilities for potentially scalable manufacturing. Furthermore, recent first-principles calculations of enargite Cu3AsS4 have hypothesized this material may have high carrier mobility and defect-tolerant optoelectronic properties, which further support investigation into this material.

In this dissertation, a novel reactive deposition processing route has been developed which has produced dense, single-phase enargite thin films. A champion device efficiency of 0.54% was achieved following a post deposition etching procedure on these films, which demonstrates the density and observable secondary phases were not limiting to initial nanoparticle-based device performances. Together with recent modeling efforts, the non-ideal band alignment with both the back contact and diode junction is concluded to be the primary limiting factor for high efficiency devices.

As this technology contains arsenic, concerns have been raised about its potential carcinogenicity and toxicity. Similar concerns were raised during the development of cadmium telluride technology, but these concerns have been mitigated through careful life cycle analyses and identifying strategies for responsible life cycle management. Therefore, a life cycle analysis and two risk assessments have been completed on Cu3AsS4 systems. Although emissions of arsenic and its contributions to life cycle impacts are expected to be low due to the small quantity required, hot spots have been identified to reduce waste and emissions. Reduction strategies for this material system are found to be applicable to other PV systems and include minimizing molybdenum sputter waste, reusing and recycling balance of system components, and investigating low-energy processing routes on thin substrates. This work serves to establish a basis on which the potential environmental implications of this thin film technology are understood.

This dissertation will serve as a guide toward the technical and environmental development of Cu3AsS4 thin films. Having a life cycle perspective during the systematic development of a technology will enable sustainable engineering. Furthermore, the processing and characterization methods detailed herein are expected to be generally applicable to other copper chalcogenide systems.