Measurement and analysis techniques for device agnostic electrical characterization of thin film solar cells
Abstract
Numerous electrical and optical techniques such as IV characterization, admittance spectroscopy, and photoluminescence studies are commonly used to characterize solar cell materials and devices. The analysis of these techniques is often well defined for ideal, crystalline devices; however thin film solar cells often show several non-ideal behaviors that lead to complications in their analysis. Phenomena such as voltage dependent current collection, superposition failure, grain non-uniformity, Schottky contacts, and band tail effects are often present and may lead to ambiguities in experimental data that can be difficult to interpret. Our primary purpose is, therefore, to develop a procedure utilizing a relatively small number of measurements that, when integrated with numerical simulation, can give us a comprehensive understanding of the physical operation of thin film photovoltaic devices. One problem common in many thin film solar technologies is the presence of a low open circuit voltage (Voc). We show that it is possible to use a series of electrical characterization techniques coupled with numerical modeling to determine when low Voc’s and superposition failure are caused by doping issues, band offsets, band tails, or some other mechanism. This work also demonstrates how numerical modeling can be used to analyze optical measurements. A numerical simulation that self-consistently fits multiple measurements can be used to create a comprehensive model that fully describes the recombination properties of a device. Using this technique, we are able to accurately measure the lifetime and surface recombination velocities for GaAs, InP, and CdTe absorber materials. Developing measurement and analysis techniques for device agnostic electrical and optical characterization provides a useful framework for developing new solar cell materials and technologies and could provide an example of a more general model-driven technology development paradigm that is useful in areas other than solar cells. This research has several important applications. First, it promises to help increase the efficiencies for thin film solar cells, an important class of photovoltaics. Second, it extends previous work for crystalline and amorphous solar cells to encompass several new solar cell materials including CZTS, InP, and CdTe. Finally, it creates a framework for technology development that tightly couples device simulation and characterization with performance.
Degree
Ph.D.
Advisors
Bermel, Purdue University.
Subject Area
Electrical engineering|Energy
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