Full Spectrum Ultrahigh Efficiency Photovoltaics: System Design, Integration, and Characterization

Citation

Flowers, Cristofer Addison (2017) Full Spectrum Ultrahigh Efficiency Photovoltaics: System Design, Integration, and Characterization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9DF6P90. https://resolver.caltech.edu/CaltechTHESIS:06082017-225442178

Abstract

The sun is an immense source of power, radiating more energy than all known non-renewable reserves onto the Earth every year in the form of sunlight. In spite of this abundant availability, photovoltaic electricity conversion provides less than 1% of the of the global energy consumption. This lack of deployment is largely a consequence of the cost of photovoltaics relative to other technologies, but increased efficiency is a strong driver for cost reduction due to its ability to impact both photovoltaic module and balance of systems costs. In this thesis, we present enabling technologies for achieving increased efficiency and energy yield for photovoltaic conversion of sunlight. First, we develop finite element cell modeling and electrical contact optimization tools. These models are used to deploy unconstrained optimization techniques that expand the design space of solar cell contacts. Additionally, constrained optimization techniques are used to design solar cell electrical contacts for lateral spectrum-splitting photovoltaic submodules. The lateral spectrum-splitting submodule uses a series of filters to divide broadband sunlight into seven wavelength bands, sending each onto a solar cell with bandgap chosen to minimize thermalization and sub-bandgap transmission losses. By employing a wholistic design model covering limiting efficiency, material constraints, optical ray tracing, and electrical modeling, we generate designs capable of ultrahigh (>50%) efficiency. We then design, integrate, and prototype the first photovoltaic converter with seven unique bandgaps. Characterization of this prototype and its constituent components shows an integrated 84.5% optical efficiency and 30.2% submodule efficiency. The exemplary optical performance highlights the promise of the design with further development of the cells. Finally, we develop module circuit and power combination topologies that enable independent electrical connection to two or more subcells in a multijunction photovoltaic converter. This circuit architecture enables independent power production from each device, which reduces the module sensitivity to diurnal and seasonal spectral changes and increases panel annual energy yield. The photovoltaic technologies developed herein often break with convention and demonstrate a feasible pathway to very high (>40%) and ultrahigh (>50%) efficiency modules.

Thesis Files