Alan Teran

AlGaAs Photovoltaics for Indoor Energy Harvesting in mm-Scale Wireless Sensor Nodes

Indoor photovoltaic energy harvesting is a promising candidate to power millimeter (mm)-scale systems. The theoretical efficiency and electrical performance of photovoltaics under typical indoor lighting conditions are analyzed. Commercial crystalline 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 open-circuit voltages >0.65 V under low lighting conditions. The GaAs and Al0.2Ga0.8As cells each provide 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.

21.4 A >78%-efficient light harvester over 100-to-100klux with reconfigurable PV-cell network and MPPT circuit

Energy harvesting is an attractive solution to extend system lifetime for internet of everything (IoE) nodes. Ambient light is a common energy source that can be harvested by photovoltaic (PV) cells. However, light intensity varies widely depending on location, ranging from ~10s of lux in dim indoor conditions to ~100klux under direct sunlight. Designing a fully integrated light harvester that spans such a wide range of light intensity with high efficiency is challenging, especially since typically low PV cell voltage requires a high upconversion ratio and PV-cell voltage/current characteristics change significantly with light intensity. Boost DC-DC converters are a typical energy-harvesting solution with high conversion efficiency, but they require a large off-chip inductor and hence cannot be fully integrated, increasing system size [1-3]. Recently, switched-capacitor (SC) DC-DC converters have been actively researched to enable fully-integrated energy harvesting using on-chip capacitors [4-6]. However, their efficiency has typically been limited to the 40-to-55% range at low input power levels (≤1μW) due to conduction/switching losses.

Energy Harvesting for GaAs Photovoltaics Under Low-Flux Indoor Lighting Conditions

GaAs photovoltaics are promising candidates for indoor energy harvesting to power small-scale (

Intermediate band to conduction band optical absorption in ZnTe:O

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Three-Bandgap Absolute Quantum Efficiency in GaSb/GaAs Quantum Dot Intermediate Band Solar Cells

mm2) electronics. This application has stringent requirements on dark current, recombination, and shunt leakage paths due to low-light conditions and small device dimensions. The power conversion efficiency and the limiting mechanisms in GaAs photovoltaic cells under indoor lighting conditions are studied experimentally. Voltage is limited by generation–recombination dark current attributed to perimeter sidewall surface recombination based on the measurements of variable cell area. Bulk and perimeter recombination coefficients of 1.464 pA/mm2 and 0.2816 pA/mm, respectively, were extracted from dark current measurements. Resulting power conversion efficiency is strongly dependent on cell area, where current GaAs of 1-mm2 indoor photovoltaic cells demonstrates power conversion efficiency of approximately 19% at 580 lx of white LED illumination. Reductions in both bulk and perimeter sidewall recombination are required to increase maximum efficiency (while maintaining small cell area near 1 mm2) to approach the theoretical power conversion efficiency of 40% for GaAs cells under typical indoor lighting conditions.

Heterojunction Band Offset Limitations on Open-Circuit Voltage in p -Z n T e/n -Z n S e Solar Cells

ZnTe doped with high concentrations of oxygen has been proposed in previous works as intermediate band (IB) material for photovoltaic applications. The existence of extra optical transitions related to the presence of an IB has already been demonstrated in this material and it has been possible to measure the absorption coefficient of the transitions from the valence band (VB) to the IB. In this work we present the first measurement of the absorption coefficient associated to transitions from the IB to the conduction band (CB) in ZnTe:O.

Indoor photovoltaic energy harvesting for mm-scale systems

In this work, we study type-II GaSb/GaAs quantum-dot intermediate band solar cells (IBSCs) by means of quantum efficiency (QE) measurements. We are able, for the first time, to measure an absolute QE which clearly reveals the three characteristic bandgaps of an IBSC; <italic>E_G</italic>, <italic>E_H</italic>, and <italic> E_L</italic>, for which we found the values 1.52, 1.02, and 0.49 eV, respectively, at 9 K. Under monochromatic illumination, QE at the energies <italic>E_H</italic> and <italic>E_L</italic> is 10 ^–4 and 10^–8, respectively. These low values are explained by the lack of efficient mechanisms of completing the second sub-bandgap transition when only monochromatic illumination is used. The addition of a secondary light source (<italic>E</italic> = 1.32 eV) during the measurements produces an increase in the measured QE at <italic>E_L</italic> of almost three orders of magnitude.

Intermediate band solar energy conversion in ZnTeO

Limitations on the open-circuit voltage of p-ZnTe/n-ZnSe heterojunction solar cells are studied via current-voltage (I-V) 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.

Multiphoton Sub-Band-Gap Photoconductivity and Critical Transition Temperature in Type-II GaSb Quantum-Dot Intermediate-Band Solar Cells

Low power electronic circuitry, including wirelessly interconnected sensor nodes, is a transformational technology that can be applied to a broad range of applications. These low power systems still require electrical power, ideally from ambient energy sources. Ambient sources of light can provide sufficient energy for these applications. Stray sunlight is more than adequate, though it is not available in all locations. Indoor lighting may also provide a sufficient energy source, though the characteristics of the spectrum are significantly different than the solar spectrum, where irradiance is confined to a narrower window in the visible spectrum. Energy-autonomous operation in mm-scale sensors have been achieved using photovoltaics based on silicon CMOS [1,2]. Improvements in energy harvesting are necessary to increase the duty cycle of the microsystem and to facilitate wireless transceivers. Photovoltaic cells consisting of materials with larger bandgap energy, such as GaAs, provide a better match to the indoor light spectrum, reducing thermalization losses and increasing power generation. The larger voltage provided by higher bandgap materials such as GaAs can also improve the efficiency of the overall system, where higher voltages are beneficial for the battery storage system and DC-DC converter. While the cost of GaAs photovoltaics is significantly higher than for silicon, and is currently prohibitive for large area solar energy production, the small power requirements and associated size requirements for photovoltaic cells makes GaAs an affordable option. Requirements for active and standby power are 10μW and 0.5nW, respectively[1,2], where perpetual operation may be achieved using a photovoltaic cell with area on the order of 1 mm2.

Three-bandgap absolute quantum efficiency in intermediate band solar cells

Energy conversion in solar cells incorporating ZnTeO base layers is presented. The ZnTeO base layers incorporate intermediate electronic states located approximately 0.4eV below the conduction band edge as a result of the substitution of O in Te sites in the ZnTe lattice. Cells with ZnTeO base layers demonstrate optical response at energies lower than the ZnTe bandedge, a feature that is absent in reference cells with ZnTe base layers. Quantum efficiency is significantly improved with the incorporation of ZnSe emitter/window layers and transition from growth on GaAs substrates to GaSb substrates with a near lattice match to ZnTe.