Spectrum-Dependent Photovoltaic Energy Harvesting.

Abstract

Heterojunctions based on II-VI materials can be used to increase the efficiency of multi-junction solar cells, and may also offer the opportunity to realize next generation approaches such as the intermediate-band solar cell based on highly-mismatched alloys. One material of particular interest is ZnTe:O where IB solar energy conversion has been demonstrated. Efficient doping in many II-VI materials, however, is often a major obstacle to achieving high-quality junction diodes. In this work, studies of n-ZnSe/p-ZnTe heterojunction solar cells grown by molecular beam epitaxy will be reported. Limitations on the open-circuit voltage of p-ZnTe/n-ZnSe heterojunction solar cells are studied via current-voltage measurements under solar concentration and at variable temperature. The open-circuit voltage reaches a maximum value of 1.95 V at 77 K and 199 suns. The open-circuit voltage shows good agreement with the calculated built-in potential of 2.00 V at 77 K. These results suggest that the open-circuit voltage is limited by heterojunction band offsets associated with the type-II heterojunction band lineup, rather than the bandgap energy of the ZnTe absorber material. Low-power photovoltaic energy harvesting allows for the deployment of fully autonomous small-scale sensors in environments not previously possible. Indoor lighting is one of those environments where the illumination intensity is typically below 1,000 lx and the spectrum is narrowly centered in the visible region. The theoretical efficiency and electrical performance of photovoltaics under typical indoor lighting conditions are analyzed. Commercial crystalline Si, amorphous Si, and fabricated GaAs and Al0.2Ga0.8As photovoltaic cells were experimentally measured under simulated AM 1.5 solar irradiation and indoor illumination conditions using a white phosphor light-emitting diode to study the effects of input spectra and illuminance on performance. The Al0.2Ga0.8As cells demonstrated the highest performance with a power conversion efficiency of 21%, with open0circuit voltages >0.65 V under low lighting conditions. The GaAs and Al0.2Ga0.8As cells each provided a power density of ~100 nW/mm2 or more at 250 lx, sufficient for the perpetual operation of present-day low-power mm-scale wireless sensor nodes. The path for achieving large light harvesting efficiencies will be discussed as well as the implementation of these photovoltaic cells for mm-scale sensing applications.