Photovoltaics

Space Solar Power

Space-based solar power is an audacious concept for future baseload renewable energy generation by harvesting sunlight using a large-scale photovoltaic array in orbit which is then transmitted to Earth wirelessly through a microwave phased array transmitter, for collection at terrestrial rectenna stations and conversion back to electricity. Although the technology is still presently at a very early stage, it features several intriguing advantages: first, harvesting the energy in space is done with higher intensity sunlight; second, continuous access to sunlight 24 hours per day overcomes the intrinsic day-night power generation limitation of terrestrial solar cells; third, power can be beam to any location on Earth, mitigating the need to develop additional utility grid infrastructure on the ground, thereby addressing some of solar power's intrinsic problems. The biggest challenge for widespread implementation of space-based solar power is the payload mass and cost for the launch of conventional photovoltaic power systems, which directly affects the levelized cost of electricity. As part of the Caltech Space Solar Power project, we are designing ultralight, flexible large-scale photovoltaics suited for the space environment.

Ultralight Photovoltaics

For space solar power to be feasible, solar cells must be lightweight (<50 g/m²), efficient (>20%), radiation-resistant, and low-cost (~ terrestrial silicon). No current solar cell meets all these criteria. While commercial space III-V multijunction cells offer efficiency and radiation hardness, they are too heavy and costly. Our goal is to develop single crystal thin-film solar cells without epitaxy, retaining efficiency and radiation resistance while reducing cost and weight. To achieve this, we use diffusion doping for creating p/n junctions and controlled spalling for exfoliating thin semiconductor films from bulk crystals. Recent advances in zinc diffusion doping of GaAs have simplified the process for forming p/n junctions, yielding cells with open-circuit voltages over 950 mV. Spalling offers a straightforward method for exfoliating wafer-scale single crystal semiconductor films at micron scale thickness. We are currently exploring advanced device design and methods for passivation of defects and surfaces, as well as carrier selective contacts, to enable high photovoltaic performance.