EAPSI:Investigation of Radiation Tolerance of Novel Semiconductor Materials for Space Power

Bittner Zachary S, Rochester NY

Investigators

Abstract

Photovoltaics (PV) is an enabling technology for space exploration, and the lifecycle of the technology employed needs to be well characterized and well understood before the full system can be designed. A major concern for the longevity of satellites in Medium Earth orbit (MEO), or orbital radii from 1.8 to 2.5 times the Earth's radius(Re) is the high energy radiation effects from passing through the Van Allen belts where charge particles are trapped by the Earth's magnetic field. Since the PV system degrades from exposure to radiation, the total power consumption of the satellite or vessel is limited by what the PV system can provide at end-of-life. The underlying goal of this project is to understand and engineer materials that exhibit better electrical characteristics under high radiation fluences, a topic of great interest to the space PV community. This research will be completed at the Semiconductor Analysis & Radiation Effects group of the Japan Atomic Energy Agency with the assistance of Dr. Shin-Ichiro Sato, an expert in the field of radiation effects in semiconductor materials. This facility possesses state-of-the-art solar cell irradiation and testing facilities, enabling completion of an extensive study in a relatively short period of time. For this project, three sets of strain balanced quantum dot/quantum well solar cells (QDSC/QWSC) and quantum dot/quantum well (QD/QW) test structures will be grown. The first set will target a strain-neutral condition. The second set will be designed to have a slightly compressive strain (~1000 parts per million (ppm) verified via x-ray diffractometry) by thinning the GaP strain compensation layer. Finally, the third set will have a thicker GaP strain compensation layer, resulting in a slightly tensile (~1000 ppm) strained QD stack. A change in required displacement knock-on energy should be measurable via changes in defect density from DLTS, and if the QD stack is kept thin, relatively minor changes in residual strain should not have detrimental effects on electrical properties. Since QD solar cells will also be included, it will be possible to correlate these changes to changes in electrical properties of devices. This array of devices makes it possible to deconvolve effects of residual strain from inherent QD properties in order to enhance understanding of the effects adding nanostructures has on radiation tolerance of the material. The study will be repeated with InGaAs/GaAs QWs in order to investigate the impact of 3D quantum confinement vs 1D quantum confinement on suppressing radiation effects in solar cells. This NSF EAPSI award is funded in collaboration with the Japan Society for the Promotion of Science.