Stationary optical concentrator designs and wafer scale monolithic integration of semiconductor devices for next generation photovoltaic panels

Abstract

Another alternative approach for sustainable high efficiency PV system design is to develop low cost PV cells for terrestrial applications. To meet the demands of low cost and large scale production, larger and thinner (or flexible) substrates are required. We demonstrated the feasibility of fabricating monolithic interconnected PV devices at the wafer scale (2 inch wafers). In this study, GaSb PV cells grown on semi-insulating GaAs were used as the model material. Crucial device fabrication steps such as a selective etching process have been developed that is necessary for isolating individual devices on the wafer and interconnecting them with sub-micron scale accuracy. Selective etching of GaSb and GaAs has been developed for isolation of GaSb devices on semi-insulating GaAs substrates. Smooth side wall morphology and desirable depth profile of the etched structures have been accomplished using optimized etching conditions presented in this thesis. Device fabrication of series interconnected GaSb PV cells on a GaAs substrate with single-sided metal contacts has been successfully demonstrated.

A major barrier in utilizing solar energy for large scale deployment is the cost of the photovoltaic (PV) systems. Several approaches have been used for the cost reduction such as by modifying PV system designs in addition to enhancing the efficiency of solar cells. Due to the high cost of materials, minimizing the use of solar cells such as in concentrator type systems is highly attractive for reducing the cost of the PV modules by focusing the incident light onto the PV cell. However concentrator PV systems (CPV) require constant tracking of the sun and hence are complex in design and expensive to operate, except in limited situations such as large scale PV power plants. It is desirable to design new concentrator type systems that do not require continuous tracking of the sun. These systems could ultimately reduce the PV system cost to a minimum while maximizing the power conversion efficiency. In this thesis we propose a simple design for a stationary concentrator photovoltaic (SCPV) system that could significantly reduce the cost of generating electricity using PV devices. Using optical ray tracing simulations, we have been able to design SCPV systems that could reduce the PV module cost by 2-10 times without compromising on the power conversion efficiency of the system.