Chandler Downs

Progress in Infrared Photodetectors Since 2000

The first decade of the 21st-century has seen a rapid development in infrared photodetector technology. At the end of the last millennium there were two dominant IR systems, InSb- and HgCdTe-based detectors, which were well developed and available in commercial systems. While these two systems saw improvements over the last twelve years, their change has not nearly been as marked as that of the quantum-based detectors (i.e., QWIPs, QDIPs, DWELL-IPs, and SLS-based photodetectors). In this paper, we review the progress made in all of these systems over the last decade plus, compare the relative merits of the systems as they stand now, and discuss where some of the leading research groups in these fields are going to take these technologies in the years to come.

GaSb Thermophotovoltaic Cells Grown on GaAs Substrate Using the Interfacial Misfit Array Method

We present gallium antimonide (GaSb) p–i–n photodiodes for use as thermophotovoltaic (TPV) cells grown on gallium arsenide (100) substrates using the interfacial misfit array method. Devices were grown using molecular beam epitaxy and fabricated using standard microfabrication processes. X-ray diffraction was used to measure the strain, and current–voltage (I–V) tests were performed to determine the photovoltaic properties of the TPV cells. Energy generation at low efficiencies was achieved, and device performance was critically analyzed.

Thermophotovoltaics: An Alternative to and Potential Partner with Rectenna Energy Harvesters

A technology that can be used in place of, or in addition to, rectennas is thermophotovoltaics (TPVs). The ultimate function of TPVs, like that of the rectenna, is the conversion of electromagnetic radiation to DC current. Rectennas use a rectifying diode coupled with an antenna to achieve this conversion. TPVs achieve this conversion through a single diode which both receive the radiation and converts it to a current. While rectennas are superior at longer wavelengths (greater than 5 μm), TPVs are more efficient at shorter wavelengths (less than 5 μm). Although rectennas and TPVs have been investigated independently, a hybrid technology may be possible which incorporates components from both technologies.

Investigating thallium-based materials for use in multijunction photovoltaics

The optimum band placement for both multijunction photovoltaics, where layers of semiconductor materials are connected in series, and split junction photovoltaic systems, consisting of a number of physically separated solar cells, are investigated. Possible implementations of these designs are suggested, based on design constraints such as lattice matching, use of possible substrates, and ease of material growth. In order to realize these implementations, Tl-bearing semiconductor materials were required in the structures; this is a class of materials that has not been extensively studied, particularly for use in photovoltaics. Simulations of the most promising structures were performed, with efficiencies promising to outperform current high-efficiency cells.

Studying anomalous open-circuit voltage drop-out in concentrated photovoltaics using computational numerical analysis

Under ultra-high solar concentration, an anomalous open-circuit voltage drop-out has been observed experimentally, but not understood theoretically. This anomaly is often attributed to various thermal effects but is also observed in flash testing, where thermal effects do not have time to accumulate. As the optically generated carrier density increases past the equilibrium carrier density, open-circuit voltage and other important electrical properties could deteriorate. Using Newton linearizations and the finite-element library deal.II, we developed a computational model to solve the carrier continuity equations for optically generated charge carriers as a function of material depth in bulk III-V semiconductors.

Predicting Voc at Ultra-High Solar Concentration Using Computational Numerical Analysis

Under ultra-high solar concentration, drastic efficiency drops are attributed to a deteriorating fill factor and additional thermal effects. The effects of ultra-high solar concentration on other fundamental electrical properties, such as open-circuit voltage, have yet to be explored in detail. In this work, we discuss our theoretical examination of semiconductor performance under ultra-high irradiance. Using advanced numerical analysis techniques and the finite-element library deal.II, we develop a computational model to simultaneously solve the carrier continuity equations and Poisson’s equation for optically generated charge carriers and the resulting electric potential as functions of space and time. We use this model to analyze VOC in both dynamic and quasi-steady state conditions. Ultimately, we characterize the relationship between VOC and increasing solar concentration.

Cutoff wavelength optimization for high-efficiency split spectrum photovoltaics

Split spectrum photovoltaics, where incident light is divided onto multiple cells on the basis of wavelength, are an exciting recent development in the solar energy field. This technology has the potential to exceed record conversion efficiencies by utilizing a large number of p-n junctions while mitigating the constraints that plague monolithic cells: lattice matching and current matching. Each cell in a split spectrum system can have a different lattice constant (allowing for more combinations of materials) and to have different operating currents (allowing for more combinations of band spacing). In this work, we examine a split spectrum system utilizing a single spectrum splitting device (a dichroic filter) to divide the solar spectrum onto two cells. Whereas many split spectrum designs use numerous filters to direct light onto single junction cells, in this system each cell is composed of multiple active junctions. Each cell is then tailored to absorb a portion of the solar spectrum. The combination of the two cells allows for four, five, or more active junctions while maintaining lattice and current matching conditions in each cell. A number of different cutoff frequencies for the dichroic filter are examined. Each cutoff frequency corresponds to its own combination of ideal band placements for both the shorter and longer wavelength cells. Materials corresponding to those band placements are examined to determine if any combinations can satisfy lattice matching parameters; designs which do are then simulated using TCAD Sentaurus.

3+1 multijunction testing and operations platform for improved PV and TPV efficiencies

A new testing platform for semiconductor solar devices and solar concentration applications has been developed. By separating the solar radiation into two beams using a dichroic lens, simultaneous operation of a photovoltaic (PV) and a thermophotovoltaic (TPV) cell is made possible. Photons with a wavelength shorter than 1100 nm are reflected onto a PV cell; whereas the remaining solar spectrum is refracted towards a TPV cell. This testing platform takes advantage of auto-tracking and focusing capabilities using a digital mount and photoresistors. Alternative solutions to cooling problems have also been offered. Various concentration ratios, incident light intensities, acceptance angles and spot sizes can be achieved due to the versatile nature of this device. Issues with mismatching lattice constants have also been addressed through spectrum splitting. This design, when operated at its full capacity with a 3-junction PV cell and a TPV cell, allows for exciting new possibilities in high efficiency solar cell technology. These possibilities include the 3+1 hybrid multijunction cell design which could lead to higher overall efficiencies than conventional concentrators.