Using design of experiment to optimize flexible thin film solar cell processing

Next to Si-based solar cells, devices based on chalcogenide thin films, such as Cu(In,Ga)(Se,S)2 (CIGS), are at the forefront of solar cell technology. CIGS technology, as one of the main thin-film competitors of technology leader Silicon, has exceeded record module and cell efficiencies of more than 17% and 23%, respectively. The main advantage of thin film PV is the possibility to fabricate devices on flexible substrates such as plastic or metal sheets, allowing for extremely lightweight and flexible solar cell solutions, having main applications in markets such as building-integrated photovoltaics or space applications.

Whereas the fabrication of these flexible devices on plastic or metal sheets is usually done with a co-evaporation process, this master thesis topic will consist of the fabrication, characterization and optimization of thin film CIGS absorbers on flexible metal sheet substrates using a selenization of metal precursors process. This process involves first the deposition of metal layers, followed by the selenization process in a Selenium containing atmosphere. Because this process uses relatively high Se partial pressures, it is a bit more aggressive on the metal substrate. Careful optimization of the selenization process will be necessary to simultaneously optimize the light absorbing and carrier transport properties of the CIGS, without degrading the quality of the metal substrate.

Therefore, a design of experiment approach will be used to optimize the different parameters of the selenization process, such as time, temperature, temperature ramp and selenium partial pressure. The work will consist in establishing a design of experiment, performing the selenization experiments in a rapid thermal anneal oven, followed by physical and optoelectronic characterization of the fabricated layers. The layers will also be finished into fully functional solar cells, allowing the extraction of cell parameters. Routinely, the fabricated layers will be characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Electrical measurements to be performed on the solar cells consist of current-voltage and capacitance-voltage measurements. The different measurement results will be fed back into the design of experiment in order to optimize the fabrication process of the thin film solar cell absorbers.