Irradiation Effects in Space Solar Cells Made of Multiple Absorbers
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Irradiation Effects in Space Solar Cells Made of Multiple Absorbers M.J. Romero, R.J. Walters,1 M.M. Al-Jassim, S.R. Messenger,1 and G.P. Summers1 National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401-3393 Phone: 303-384-6653, Fax: 303-384-6604, Email: [email protected] 1 Naval Research Laboratory (NRL), Code 6615, 4555 Overlook Ave., S.W., Washington DC 20375 ABSTRACT Solar cells made of multiple absorbers are a commonly used approach for improving efficiency due to their extended range of spectral sensitivity. Indeed, efficiencies nearing the theoretical maximum have been achieved with a triple-junction device made of In0.51Ga0.49P (InGaP2), GaAs, and Ge solar cells connected in series. For extraterrestrial applications, there is the added requirement of radiation tolerance. The main challenge for space power-generation is therefore the development of highly efficient and radiation-tolerant devices. We have investigated several aspects of the radiation response of solar cells made of multiple absorbers, such as multijunction devices and quantum-well solar cells. Novel possibilities such as quantumdot solar cells and ordered-disordered heterostructures are proposed. INTRODUCTION Future satellite systems are projected to fly in orbit in or near the proton radiation belts, which extend from 2,000 to 10,000 km of altitude (MEO: Medium-Earth Orbit). Radiation effects can be very severe in these orbits, and high-efficiency solar cells with minimal degradation under cosmic particle bombardment are required. Recent attempts to boost efficiencies are based on extending the spectral sensitivity by the use of multiple absorbers. Multijunction solar cells hold the promise of increasing the maximum attainable conversion efficiency well above the Shockley and Queisser (SQ) limit [1]. Indeed, the maximum practical efficiency for a solar cell (of 32.2% at 1-sun, AM1.5) has been achieved with a triple-junction device made of In0.51Ga0.49P (InGaP2), GaAs, and Ge solar cells connected in series. Quadruple-junction devices are being developed, and the search for 1-eV absorbers to add a junction to the InGaP/GaAs/Ge cell is being conducted at NREL [2]. Other efforts have been made to improve solar cell efficiencies by the use of quantum wells as an intermediate level that absorbs additional lower-energy photons [3]. One question is whether quantum-well solar cells have their efficiency restricted by the SQ limit. Quantum-well solar cells have the potential to increase the maximum attainable conversion efficiency above the limit of conventional solar cells by extracting hot carriers to produce either higher photovoltages or photocurrents. However, if phonon-assisted relaxation of hot carriers is not prevented, a quantum-well solar cell ideally behaves as a single-junction solar cell. The use of these advanced devices for space power-generation is limited by their radiation resistance. We have investigated several aspects of the radiation response of these solar cells by beam injection methods. In multijuncti
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