With the growing utilization of solar power for electricity and heat generation, photovoltaic-thermal (PVT) systems possess tremendous potential as sustainable energy solutions. This review covers recent advances in concentrated photovoltaic-thermal and photovoltaic-thermal technologies, providing insights into improving system performance. Our review concludes that recent innovations in materials, operating configurations, and integration with other technologies have largely optimized PVT designs. Moving forward, PVT systems present a promising pathway for clean, renewable energy generation through effective utilization of the full spectrum of solar energy. Further advancements in cost-competitiveness may facilitate widespread adoption.

Pin fin heat sinks have garnered considerable attention within the realm of thermal management for high-heat-flux electronics systems. This study advances the understanding of perforated pin fin systems, offering novel insights into heat transfer enhancement. The research explores circular and square perforation geometries located at varying positions along the pin fin length, aiming to enhance heat dissipation and extend the lifespan of electronic components. A parametric analysis examines the impact of perforation quantity on thermal performance with a rigorous comparison against non-perforated pin fins. The investigation incorporates machine learning techniques, including artificial neural networks, to predict cooling system thermal efficiency. Using computational fluid dynamics simulations and Artificial Neural Network modeling, the obtained results indicate the optimal design mode is the use of 3 square holes on the pin fin, which leads to an increase in thermal efficiency by 16.63% compared to the case without pin fins. Additionally, the mean absolute error value calculated across all data extracted from the computational fluid dynamics simulation for predicting thermal efficiency was 2.25%. This low error value further demonstrates the accuracy of the simulation data and neural network model in predicting the heat transfer performance of the pin fin designs.

Analysis of the composition, strain-relaxation, layer-tilt, and the crystalline quality of InyGa1-yAs/ InP1-xAsx thermophotovoltaic (TPV) diodes grown by metal-organic vapor phase epitaxy (MOVPE) is demonstrated using triple-axis X-ray reciprocal space mapping techniques. [n(0.53)Ga(0.47)s (E-gap = 0.74 eV) n/p junction diodes are grown lattice matched (LM) to InP substrates and lattice-mismatched (LMM) In0.67Ga0.33As (E-gap = 0.6 eV) TP%/ diodes are grown on three-step InP1-xAsx (0 <x<0.32) buffer layers on InP substrates. X-ray reciprocal space maps about the symmetric (400) and asymmetric (5 3 3) reciprocal lattice points (RELPs) determine the in-plane and out-of-plane lattice parameters and strain of the InyGa1-yAs TPV active layer and underlying InP1-xAsx. buffers. Triple-axis X-ray rocking Curves about the LMM In0.67Ga0.33As RELPshow an order of magnitude increase of its full-width at half-maximum (FWHM) compared to that from the LM In0.53Ga0.47As (250 vs. 30 arcsec). Despite the significant RELP broadening, the photovoltaic figure of merits show that the electronic quality of the LMM In0.67Ga0.33As approaches that of the LM diode material. This indicates that misfit-related crystalline imperfections are not dominating the photovoltaic response of the optimized LMM In0.67Ga0.33As material compared with the intrinsic recombination processes and/or recombination through native point defects, which would be present in both LMM and LM diode material. However, additional RELP broadening in non-optimized LMM In0.67Ga0.33As n/p junction diodes does correspond to significant degradation of TPV diode open-circuit voltage and minority carrier lifetime demonstrating that there is correlation between X-ray FWHM and the electronic performance of the LMM TPV diodes. (C) 2008 Elsevier B.V. All rights reserved.

The results of study of minority carrier lifetime in p-type and n-type InGaAs double heterostructures (DH) and thermophotovoltaic (TPV) cells with InPAs step-graded buffer layers grown on InP substrates are summarized. The active layer carrier concentration was varied in the range from 1015 to 1018 cm-3. The carrier lifetime constants were determined from photoluminescence (PL) transient and frequency responses under low-injection conditions. The rapid decrease of electron lifetime with decrease of excitation was observed in p-type InGaAs DHs and was attributed to capture of electrons on positively-charged deep-donor recombination centers. It was found that Te-doping of the InPAs buffer layers improves the low-injection electron lifetime. Temperature dependences of radiative efficiency and minority carrier lifetime were studied in sets of DHs with different doping level in order to separate the radiative and non-radiative recombination processes

The influence of GaSb substrate surface defects such as native oxides on the crystalline quality of epitaxial layers was investigated using transmission electron microscopy (TEM). Cross-sectional TEM imaging showed that there are discrete defects at the GaSb-substrate/epilayer interface. Secondary ion mass spectroscopy (SIMS) results revealed high oxygen concentration at the interface, indicating that the defects are likely oxides and presumed to be native oxides since other impurities were not detected. High-resolution TEM micrographs showed that the subsequent growth of the epilayer continues beyond the defects without any additional defect generation or propagation. Tellurium-doped AlGaAsSb epitaxial layers were grown lattice-matched on GaSb substrates and lattice-mismatched on semi-insulating GaAs substrates by organometallic vapor phase epitaxy. SIMS and Hall data showed that the ratio of carrier concentration to Te concentration decreases significantly when the carrier concentration increases from 2.5×1017 to 6.5×1017cm−3. TEM imaging showed that the material with heavily doped Te generates a high density (about 108cm2) of planar defects (stacking fault) located on (111) planes. Most of the Te-related defects originate at the GaSb buffer layer/AlGaAsSb epilayer interface. In addition, discrete precipitates were observed in the heavily doped AlGaAsSb layer.

Thermophotovoltaic (TPV) diodes fabricated from InGaAsSb alloys lattice-matched to GaSb substrates are grown by Metal Organic Vapor Phase Epitaxy (MOVPE). 0.53eV InGaAsSb TPV diodes utilizing front-surface spectral control filters have been tested in a vacuum cavity and a TPV thermal-to-electric conversion efficiency ({eta}{sub TPV}) and a power density (PD) of {eta}{sub TPV} = 19% and PD=0.58 W/cm{sup 2} were measured for T{sub radiator} = 950 C and T{sub diode} = 27 C. Recombination coefficients deduced from minority carrier measurements and the theory reviewed in this article predict a practical limit to the maximum achievable conversion efficiency and power density for 0.53eV InGaAsSb TPV. The limits for the above operating temperatures are projected to be {eta}{sub TPV} = 26% and PD = 0.75 W/cm{sup 2}. These limits are extended to {eta}{sub TPV} = 30% and PD = 0.85W/cm{sup 2} if the diode active region is bounded by a reflective back surface to enable photon recycling and a two-pass optical path length. The internal quantum efficiency of the InGaAsSb TPV diode is close to the theoretically predicted limits, with the exception of short wavelength absorption in GaSb contact layers. Experiments show that the open circuit voltage of the 0.53eV InGaAsSb TPV diodes is not strongly dependent on the device architectures studied in this work where both N/P and P/N double heterostructure diodes have been grown with various acceptor and donor doping levels, having GaSb and AlGaAsSb confinement, and also partial back surface reflectors. Lattice matched InGaAsSb TPV diodes were fabricated with bandgaps ranging from 0.6 to 0.5eV without significant degradation of the open circuit voltage factor, quantum efficiency, or fill factor as the composition approached the miscibility gap. The key diode performance parameter which is limiting efficiency and power density below the theoretical limits in InGaAsSb TPV devices is the open circuit voltage. The open circuit voltages of state-of-the-art 0.53eV InGaAsSb TPV diode are {approx}10% lower than the predicted semi-empirical limit to open circuit voltage for a device having absorbing substrate; the voltages are {approx}17% below that for an Auger-limited device having back surface reflector and two-pass optical design.

Yield data from a pilot-production run of thermophotovoltaic (TPV) devices are presented. A single lattice-mismatched 0.6 eV InGaAs epilayer device design was grown on 166 3-inch InP wafers by metalorganic vapor phase epitaxy (MOVPE) in a commercial reactor using standard chemical precursors. Epiwafers were processed in batch-style as 30-junction monolithically interconnected modules (MIMs) using standard proximity photolithography, wet chemical etching and plasma-enhanced chemical vapor deposition (PECVD). It is understood that yield is the product of several loss parameters. This work has shown that process consistency can be maintained and a reduction of losses at crystal growth can be achieved with the implementation of appropriate characterization techniques such as surfscan, photoreflectance, triple-axis X-ray diffraction (TAXRD) and in-situ temperature monitoring. In addition, InP as a substrate material has often been associated with an undesirably high incidence of wafer breakage. However, this work has shown that with some care in wafer handling, mechanical yield issues are not necessarily worse than in a standard GaAs fabrication line

Thermophotovoltaic (TPV) diodes fabricated from 0.52eV lattice-matched InGaAsSb alloys are grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on GaSb substrates. 4cm(2) multi chip diode modules with front-surface spectral filters were tested in a vacuum cavity and attained measured efficiency and power density of 19% and 0.58 W/cm(2) respectively at operating at temperatures of T-radiator = 950 degreesC and T-diode = 27 degreesC. Device modeling and minority carrier lifetime measurements of double heterostructure lifetime specimens indicate that diode conversion efficiency is limited predominantly by interface recombination and photon energy loss to the GaSb substrate and back ohmic contact. Recent improvements to the diode include lattice-matched p-type AlGaAsSb passivating layers with interface recombination velocities less than 100 cm/s and new processing techniques enabling thinned substrates and back surface reflectors. Modeling predictions of these improvements to the diode architecture indicate that conversion efficiencies from 27-30% and similar to0.85 W/cm(2) could be attained under the above operating temperatures.

This paper presents results of experimental and theoretical research on antimonide-based thermophotovoltaic (TPV) materials and cells. The topics discussed include: growth of large diameter ternary GaInSb bulk crystals, substrate preparation, diffused junction processes, cell fabrication and characterization, and, cell modeling. Ternary GaInSb boules up to 2 inches in diameter have been grown using the vertical Bridgman technique with a novel self solute feeding technique. A single step diffusion process followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency, p-n junction GaSb and GaInSb thermophotovoltaic cells. The optimum junction depth to obtain the highest quantum efficiency and open circuit voltage has been identified based on diffusion lengths (or minority carrier lifetimes), carrier mobility and experimental diffused impurity profiles. Theoretical assessment of the performance of ternary (GaInSb) and binary (GaSb) cells fabricated by Zn diffusion in bulk substrates has been performed using PC-1D one-dimensional computer simulations. Several factors affecting the cell performances such as the effects of emitter doping profile, emitter thickness and recombination mechanisms (Auger, radiative and Shockley-Read-Hall), the advantages of surface passivation and the impact of dark current due to the metallic grid will be discussed. The conditions needed for diffused junction cells on ternary and binary substrates to achieve similar performance to the epitaxially grown lattice-matched quaternary cells are identified.

This paper assesses the performance of antimonide-based thermophotovoltaic cells fabricated by different technologies. In particular, the paper compares the performance of lattice matched quaternary (GaInAsSb) cells epitaxially grown on GaSb substrates to the performance of ternary (GaInSb) and binary (GaSb) cells fabricated by Zn diffusion on bulk substrates. The focus of the paper is to delineate the key performance advantages of the highest performance-to-date of the quaternary cells to the performance of the alternative ternary and binary antimonide-based diffusion technology. The performance characteristics of the cells considered are obtained from PC-1D simulations using appropriate material parameters.

A single-step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high-efficiency GaSb thermophotovoltaic (TPV) cells. The junction depth was controlled through monitoring of light current-voltage (I–V) curves (photovoltaic response) during the post-diffusion emitter-etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies (QEs) and the open-circuit voltages in the fabricated devices. An optimum junction depth for obtaining the highest QE and open-circuit voltage is presented based on diffusion lengths (or minority carrier lifetimes), carrier mobility, and the typical diffused impurity profile in GaSb.

This report was prepared as an account of work sponsored by the United States Government. Neither the United States, nor the United States Department of Energy, nor any of their employees, nor any of their contractors, or their employees, makes any warranty, express or implied, or assumes any legal liabiIity or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

The reaction of Au with GaSb occurs at a relatively low temperature (100 C). Upon annealing, a AuSb{sub 2} compound and several Au-Ga phases are produced. Phase transitions occur toward higher Ga concentration with increasing annealing temperatures. Furthermore, the depth of the contact also increases with increased annealing temperature. They found that the AuSb{sub 2} compound forms on the GaSb surface, with the compound crystal partially ordered with respect to the substrate. The transition of Schottky- to ohmic-contact behavior in Au/n-type GaSb occurs simultaneously with the formation of the AuGa compound at about a 250 C annealing temperature. This ohmic contact forms without the segregation of dopants at the metallic compound/GaSb interface. Therefore it is postulated that transition from Schottky- to ohmic-contact behavior is obtained through a series of tunneling transitions of electrons through defects in the depletion region in the Au/n-type GaSb contacts. Contact resistivities of 6-7 x 10{sup -6} {Omega}-cm{sup 2} were obtained with the annealing temperature between 300 and 350 C for 30 seconds. In Au/p-type GaSb contacts, the resistivity was independent of the annealing temperature. This suggested that the carrier transport in p-type contact dominated by thermionic emission.

For the first time, it has been demonstrated that thermophotovoltaic cells made of polycrystalline GaSb with small grain sizes (down to 100 x 100 {micro}m) have similar characteristics to the best Zinc diffused single crystal GaSb cells with identified device parameters. The grain boundaries in polycrystalline GaSb do not degrade TPV cell parameters, indicating that such material can be used for high-efficiency thermophotovoltaic cells.

The extent of relative tilt angle, crystalline quality, and relaxation of GaxIn1−xSb layers grown on (001) GaSb substrates by organometallic vapor phase epitaxy have been investigated, using double-crystal x-ray diffraction and transmission electron microscopy. An unexpectedly large tilt is formed between Ga0.8In0.2Sb epitaxial layers and oriented (001) GaSb substrates which has not been previously reported. Double-crystal x-ray diffraction measurements revealed that the tilt angle between the Ga0.8In0.2Sb epilayers and the substrates increases as the layer thickness increases. A strong correlation has been established between the variation of the tilt angle and the residual strain in the layers. Transmission electron micrographs of Ga0.8In0.2Sb layers revealed that irregular dislocation activities occur in the layer at different distances from the interface which could be related to tilt formation and relaxation. The structural characteristics of the layers as a function of the compositional variation showed that the amount of tilt angle is small when the indium concentration was in the range from 0 to 12%, but increases at higher indium concentrations. Ga1−xInxSb layers with poor crystalline quality and small tilt angle are obtained when the indium concentration was more than 25%. The tilt angle between Ga0.8In0.2Sb epilayers and GaSb substrates grown at temperatures ranging from 560 to 620 °C was essentially identical, indicating that the tilt formation is not a kinetic effect.

P-type GaSb and Ga{sub 0.8}In{sub 0.2}Sb layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using silane as the doping precursor. Hall measurements show that the concentration and mobility of holes in GaSb and Ga{sub 0.8}In{sub 0.2}Sb are higher when the layers are grown on GaSb substrates than when grown on GaAs substrates. Secondary ion mass spectroscopy (SIMS) results show that the incorporation of Si is higher when GaSb substrates are used. The compensation of Si acceptors is negligible in GaSb, but is as high as 25% in Ga{sub 0.8}In{sub 0.2}Sb.

N-type Ga{sub 0.8}In{sub 0.2}Sb epitaxial layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using diethyltelluride (DETe) as the dopant source. The incorporation efficiency of Te in Ga{sub 0.8}In{sub 0.2}Sb and the electron mobility were found to be higher with GaSb substrates compared to using GaAs substrates. The electron concentration increased from 5 {times} 10{sup 16} cm{sup {minus}3} to 1.5 {times} 10{sup 18} cm{sup {minus}3} as the Te concentration was increased from 1 {times} 10{sup 17} cm{sup {minus}3} to 5 {times} 10{sup 18} cm{sup {minus}3}. As the Te concentration was increased further, the electron concentration decreased, with only about 1% of the Te electrically active at a Te concentration of 2 {times} 10{sup 20} cm{sup {minus}3}.

Thermophotovoltaic (TPV) devices have been fabricated using epitaxial ternary and quaternary layers grown on GaSb substrates. GaInSb ternary devices were grown by metalorganic vapor-phase epitaxy (MOVPE) with buffer layers to accommodate the lattice mismatch, and GaInAsSb lattice-matched quaternaries were grown by MOVPE and by liquid-phase epitaxy (LPE). Improved devices are obtained when optical absorption occurs in the p-layer due to the longer minority carrier diffusion length. Thick emitter p/n devices are limited by surface recombination, with highest quantum efficiency and lowest dark current being achieved with epitaxially grown surface passivation layers on lattice-matched MOVPE quaternaries. Thin emitter/thick base n/p devices are very promising, but require improved shallow high-quality n-type ohmic contacts.

A detailed study of the dependence of the plasma wavelength and free-carrier absorption on the doping concentration in silicon has been made. Two approaches have been used for introducing impurities into Si to achieve high doping concentration. One was the diffusion technique, using spin-on dopants. The plasma wavelength (λp) of these doped films could be adjusted by controlling the diffusion conditions. The minimum plasma wavelength achieved was 4.8 μm. In addition, a significant amount of absorption was observed for the wavelength 2 μm and below. The second approach was doping by ion implantation followed by thermal annealing with a capped layer of doped glass. Implantation with high dosages of B and As followed by high temperature annealing (>1000 °C) resulted in a plasma wavelength that could be controlled between 3.5 and 6 μm. The high temperature annealing (>1000 °C) that was necessary to activate the dopant atoms and to heal the implantation damage also caused significant redistribution of the dopants. For phosphorous implanted Si, a moderate temperature (800–900 °C) was sufficient to activate most of the phosphorous and to heal the implantation damage. The position of the plasma turn-on wavelength for an implantation dose of 2×1016 cm−2 of P was at 2.9 μm. The absorption at 2 μm was less than 25% and the reflection at 10 μm was about 85%.