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Publications/Abstracts TPV

Abstracts of TPV Publications

Abstract: The solar thermophotovoltaic system is a kind of thermophotovoltaic (TPV) system where the emitter is powered by the concentrated sunlight. Narrow bandgap PV cells based on GaSb can be used in such a system as the efficient TPV converters operating with an infrared energy source of photons emitted from heated up to 1200-1800 0C solar powered emitters. High efficient thermophotovoltaic cells based on GaSb and InGaAsSb/GaSb have been developed in this work with the use of the liquid phase epitaxy, MOCVD growth and zinc diffusion from the gas phase. Results of manufacture and development of TPV cells and modules of two types: flat and cylindrical, also results of their testing in outdoor and indoor conditions are presented. TPV arrays of the cylindrical type were tested under irradiation of the different length emitters produced from W and Ta. Emitters used in the measurements were heated by the 4600x concentrated sunlight obtained with the help of a 60x60 cm2 Fresnel lens. PV cells connected in series intended for the conical system were fabricated and tested under a flash tester. The cells were fixed on a plate of BeO ceramics soldered on a copper base. The TPV array in the conical system of four 1 x 1 cm2 GaSb cells connected in series under a flash tester have showed the open circuit voltage Voc = 1.77 V and the fill factor FF = 66.4 % with the short circuit photocurrent Isc = 1 A. The array load characteristics of the cylindrical system obtained with the use of different length emitters has been investigated. The TPV array in the cylindrical system of three 1 x 1 cm2 GaSb cells connected in parallel had ISC = 3.5 A, VOC = 0.41 V, FF = 62.4 % and Pmax= 0.88 W with tungsten emitter temperature T = 1400 °C. For effective conversion of low-temperature emitter radiation, photocells based on InGaAsSb/GaSb has been developed and manufactured. Band gap energy of photoactive InGaAsSb layers was about 0.6 eV. Voc = 0.43 V and FF = 60 % at Jsc ≈ 7 A/cm2 were obtained in these cells. The effect of different diffusion profiles of a p-n junction obtained by etching the anodic oxidation of a diffused layer on the behaviour of basic characteristics of GaSb TPV cells: the load characteristic fill factor, the spectral sensitivity, the open circuit voltage was investigated in detail at different densities of photocell illumination. High efficient photocells based on GaSb with reduced internal losses intended for conversion of radiation from the emitters heated by the concentrated sunlight have been developed and fabricated by means of the Zn diffusion technique. The optimum depth of the initial p-n junction etching has been found allowing to obtain maximal efficiencies for the cell photocurrent densities of up to 5 A/cm2. The possibility to obtain GaSb TPV cells by means of a non-traditional diffusion – from a solid state film diffusant – was studied as well. Realization of this procedure does not require complicated equipment, gives a possibility to use sources containing several doping elements and to obtain low surface charge carrier concentration. The quality of such diffusion p-n junctions was being estimated by 3.5x3.5 mm2 GaSb cell characteristics. The open circuit voltage reached 0.5 V and the fill factor – 66 % at the current density of 10 A/cm2 with external quantum yield of 0.82-0.87 at the wavelength range of 800-1600 nm. Quite high parameters of the manufactured cells indicate a good quality of the diffusion p-n junction obtained by diffusion from a polymer film.

Abstract: Characteristics of conventional and inverted InGaAs/InP thermophotovoltaic (TPV) cells with a back surface reflector (BSR) fabricated on electrically active n-type InP substrates are presented. Thermophotovoltaic cells based on lattice matched InP–In0.53Ga0.47As heterostructures were fabricated with the use of LPE and Zn,P diffusion technologies. In the p-n TPV cells (conventional type, spectral range 600÷1800 nm) with a frontal p-InGaAs layer, BSR was made on a n-InP substrate. In the n-p structure (inverted type, spectral range1000-1800 nm) with a frontal bulk n-InP-window-substrate, BSR was formed on a p–InGaAs layer. Antireflection coating (ARC) on the frontal cell surface consists of ZnS/MgF2 layers. Results of investigation of sub-bangap photons reflection from InP substrates with a backside MgF2/Au mirror in the range of 1800÷2000nm are described. The reflection of BSR for InP samples with the doping level in the range of 1x1017÷6x1018cm-3 evidenced a weak dependence on their thickness and doping level. A reflection of 86÷90% has been measured for substrates 100μm thick and 80% for ones 400μm thick with ARC. Study of sub-bandgap photon reflection of p-InGaAs (Zn,P) layers with surface concentration of 1÷3x1019cm-3 has been also carried out. A reflection of 68÷77% for 2€4μm layers with “hybrid” (ohmic contact plus mirror) back-surface reflector consisted of deposited Cr/Au layers was measured. It was found, that p-n and n-p thermophotovoltaic 1x1cm2 cells with identical grid design reveal similar parameters for up to 1A/cm2 current density (VOC=465mV and FF=64%) and the 76÷80% reflection of the sub-bandgap photons for wavelengths longer than 1.86μm. The developed inverted InGaAs TPV cells have been tested under illumination of silicon carbide high temperature emitter. The photocurrent density Jsc=7A/cm2, open circuit voltage Voc=0.476V and fill factor FF=0.691 have been measured in the inverted (without BSR) InGaAs cell under SiC emitter heated to the temperature of about 1550°C. Both types of devices can successfully be used as TPV cells for conversion of radiation in the range of 1500-1900K, with 14-15% efficience.

Abstract: n-type Te doped Gallium antimonide, Te-doped-Er codoped GasSb and and Tedoped- Yb-codoped GaSb single crystals have been grown by using Liquid Encapsulated Czochralski. Carrier mobility, density and resistivity along the crystal have been obtained. The axial dopant composition has also been studied by analysing different wafers by Induced Coupled Plasma (ICP). From these measurements it is concluded that codoping with Er and Yb greatly reduces the mobility and increases the resistivity in both cases compared to simply Te doped GaSb. Secondary Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis have revealed Sb clustering in Er and Yb codoped Te-doped GaSb samples. These defects could be the responsible for the reduction of the carrier mobility and the increment in the resistivity. TPV cells, with size of 4 mm2have been developed in the Te-doped GaSb sample showing relatively high performance.

Abstract: The use of tellurium as n-type dopant for GaAs and InP has several advantages, including a high incorporation efficiency, the very high doping levels achievable and a low diffusion coefficient. However, its use to dope GaxIn1 xP is not straightforward, since it shows several problems like a remarkable memory effect and an acute inertia of the material to become Te-doped, which gives rise to gradual doping profiles. In this paper, all these phenomena are studied and quantified using secondary ion mass spectroscopy (SIMS) and electrochemical CV profiling (ECV) measurements. Concerning the gradual doping profiles, its origin is linked to the interaction of Te and In in the gas phase and on the growth surface. A phenomenological explanation is given for this effect although the exact physical processes behind remain to be defined.

Abstract: Selective emitters for thermophotovoltaics consisting of intermetallic alloy MoSi2 substrate with plasma-spray coated rare-earth oxides ytterbium oxide Yb2O3, Yb-doped garnet Yb1.5Y1.5Al5O12, and erbium oxide Er2O3 have been successfully tested till 1650 C. The emitters are fully operable in an oxygen containing atmosphere, are highly thermal shock stable, and show good selective emitting properties. Shielding the high out-of-band emittance of the MoSi2 substrate with 4 lm thick Pt intermediate layer has resulted in reduced radiation power and emittance of the rare-earth oxide film due to multiple reflections at the interfaces. The novel technique of vacuum plasma-spray coated rare-earth oxide films on MoSi2 is a promising way for the production of effective and high temperature stable selective thermophotovoltaic emitters.

Abstract: Selective emitters for thermophotovoltaics have been produced by vacuum plasma-spray coating of erbium doped garnet Er1.5Y1.5Al5O12 and Er2O3 on the intermetallic alloy MoSi2. The emitters are fully operable in an oxygen-containing10 atmosphere at a temperature of 1600C, are highly thermal-shock stable, and show good selective-emitting properties. The film thickness of the rare-earth oxide was varied between 200 and 600 lm and an optimal thickness for maximum selectivity was found. Measurements with Si and GaSb photocells have been performed in order to evaluate the optimal combination emitter photocell for real thermophotovoltaic systems.

Abstract: A solar TPV generator development and characterization is presented. A cost-effective Fresnel lens based double stage sunlight concentrator ensuring 4600x concentration ratio is used for outdoor measurements. For the indoor characterization of the TPV part of the generator a solar simulator based on a 5kW Xe lamp with an ellipsoidal mirror is used, and its performance is compared with the real conditions. TPV modules based on tungsten emitters and GaSb cells were designed, fabricated and tested at indoor and outdoor conditions. The performance of tungsten and MoSi2 emitters under concentrated solar radiation was investigated. Emitter temperatures in the range of 1400-2000 K, depending on the emitter size, were measured under the concentrated sun light. The light distribution in the module has been characterized with both concentrated solar radiation and solar simulator. The differences in the light distribution on the system input appear to bring minor influence on the TPV module performance. A continuous progress takes place in improvement of gallium antimonide PV cells. 1x1 cm GaSb TPV cells were fabricated with the use of the Zn-diffusion and LPE technologies. Efficiencies of 18-18.8% were obtained at tungsten emitter (1750-2000 K) radiation. The developed GaSb PV cells appear to be the most relevant for the use in STPV systems. The series connection of PV cells was ensured by the use of various ceramics. The performance of the cells mounted on BeO2 and Al2O3 ceramics was compared under continuous thermal radiation of a graphite heater simulating the TPV module conditions. A temperature increase of a PV cell mounted on a water-cooled base with BeO2 or Al2O3 ceramics by 5-15 degrees correspondingly was estimated from the I-V curve measurements. This estimation is based on both the I-V measurements under flash-lamp and dark I-V measurements. This leads to a decrease of the Voc by 5-20 mV and few percent in FF value, which is affordable. The whole solar TPV system was examined outdoors. The tungsten emitters, sealed in quartz bulb, displayed themselves stable under concentrated sunlight. The power output of 5.5 W was measured with a full-size 24 cell module and 25x12 mm tungsten emitter under the solar simulator, providing the same power density, as the Fresnel lens concentrator. The ways for the system improvement are discussed

Abstract: In a first and completely new approach, a vacuum plasma-spray coating technique was used to deposit selective emitting rare-earth oxide films of ytterbia Yb2O3 on porous silicon-infiltrated silicon carbide foams (Si–SiC). The plasma-spray coating technique offers a new and promising way to produce selective emitting coatings on different refractory substrates with complex geometries. The adhesion and thermal shock stability were tested until a film thickness of 130 m was achieved; the selective emittance of the oxide coating has been found to be dependent on the film thickness. The material combination Si–SiC and Yb2O3, however, needs some major improvement regarding hightemperature stability and high thermal cycling loads. In a different approach, the advantage of low emitting Al2O3 fibers and good thermal matching was combined with Yb2O3 slurry coating of flexible alumina Al2O3 fiber bundles, formed into a cylindrical shape. The thin fiber structure tried to imitate the famous incandescent mantle emitters of Auer von Welsbach, but with a more rugged structure. Even though the fibers of the woven emitter were thin, the low thermal conductivity of Al2O3 led to a distinct reduction of the surface temperature and emittance, and a shielding effect of the radiation emanating from the hot inner walls by the cooler outer grid structure was inevitable. Optical filters consisting of a water film and of transparent conducting oxides (TCO) have been developed and tested to protect the photocells against overheating and to reflect nonconvertible off-band radiation back to the emitter. The water film led to a significant reduction of the cell temperature and increased cell performance, whereas with the TCO filters only a reduction of the cell temperature was observed.

Abstract: The efficiency of Ge TPV cells under high power densities has been optimized by an adaptation of the cell structure and processing technology. An adequate front- and back-contact structure was developed. A fill factor of 71 % has been achieved assuming a micro-structured tungsten emitter spectrum at 1100 °C with an incident radiation density of 2.5 W/cm2. A new back-side of the solar cell was developed which combines an electrical passivation layer for the Ge cell with a back-side mirror yielding in a high reflection in the infrared range. This new structure increases the cell spectral response in the long wavelength range between 1.6 and 2 μm if the diffusion length is comparable or larger than the thickness of the base layer. Cells with a doping level in the range of p=1016 cm-3 show an increase of 10 % in voltage using this structure. An external reflection between 70 % and 80 % was achieved for wavelengths below the band gap of Ge, allowing to recycle these photons by heating the TPV emitter.

Abstract: Characteristics of inverted In0.53Ga0.47As /InP thermophotovoltaic (TPV) cells with the uniformly doped base grown on electrically active n-type InP substrates are presented. Thermophotovoltaic cells based on lattice matched InP–In0.53Ga0.47As heterostructures were fabricated with the use of LPE and Zn,P diffusion from a local source in an open system in the hydrogen atmosphere. The TPV cells (spectral range of 1000÷1800nm) with different base and emitter thicknesses have been investigated. These TPV cells had doping concentrations up to 2÷4x1019cm-3 for the base layers and up to 2÷5.1017cm-3 for the emitter layers. From obtained dark I-V characteristics and photovoltaic characteristics of the InGaAs/InP TPV cell p-n junctions, the dark resistanceless I-V characteristic has been determined. Note, that the diffusion component value determining the potential efficiency of the InGaAs/InP photocell corresponds to the present-day quality of the best InGaAs TPV cells fabricated by the MOCVD technique. Electrical measurements using a flash tester reveal the open circuit voltage of 0.44÷0.49V, the fill factor of 75÷76% at short circuit current density of 1÷10A/cm2. The calculated efficiencies for a TPV cell with uniformly doped base are 16÷17% for the tungsten emitter temperature of 1800÷2000K. Thus, the developed technique of Zn diffusion from a local source is a highly efficient low cost method for manufacture of TPV cells and can be successfully used in industrial production.

Abstract: In this work, high efficient photovoltaic (PV) cells based on gallium antimonide have been developed with the use of the liquid phase epitaxy (LPE), MOCVD and diffusion from the gas phase techniques. There are intended for conversion of the thermal radiation of emitters heated by the sunlight. On the ground of investigation of the LPE temperature regimes and the tellurium doping during the process, growth epilayers from various melts, three types of GaSb PV cells with different structures have been fabricated. The maximum photoluminescence (PL) intensity corresponding to the interband charge carrier transitions was observed for epitaxial layers grown by LPE and by MOCVD.

Abstract: For production of highly effective photoconverters good semiconductor materials with strictly determined parameters are required. For thermophotovoltaic (TPV) GaSb cells an homogeneous Tedopin level in the bulk semiconductor of (2-7)•1017cm-3 is required to produce high efficient PV cells by Zn diffusion process [1-2]. In this paper we propose to investigate the cell performances obtained on different GaSb:Te wafers (100) and (221) orientation. Based on classical I(V) measurements and external quantum efficiency (EQE) curves, we analyse cell performances in order to improve all fabrication stages like wafer surface preparation, p-type GaSb emitter elaboration by Zinc diffusion process, anti reflecting coating deposition and contact realisation. Most of the technological steps are described in [3]. Today good performances are obtained on both 3.5 x 3.5 mm² (221) and 10 x 10 mm² (100) GaSb cells. We obtained an EQE (Figure 1) between 70-76 % in the 800-1600 nm range for the first one and 80-88 % in the same spectrum for the second one. Electrical characterisation give respectively a fill factor (FF) between 67.8 % down to 65.8% in the 1-5 A/cm² range and 67.5 % down to 52.5% (Figure 2). The open circuit voltage Voc increase from 0.44 V (1 A/cm²) up to 0.49 V (5 A/cm²) for the small surface cell. Post-process information like Zn junction depth and profile obtained by SIMS analysis will help us to understand deeply the cell behaviour in order to fabricate large TPV arrays (100 cm²) and to test them in real Solar TPV system. It is apparent that improvement of ingot parameters, wafers and “buffer” layers is of great importance for fabrication of future cells.
This work has been partially supported by the European Commission through the funding of the project FULLSPECTRUM (Ref. N: SES6-CT-2003-502620).

Abstract: Reflection of infrared radiation from n -InP substrates with a rear MgF2 /Au mirror is investigated in the wavelength range 1000–2200 nm. It is found that the reflectance weakly depends on substrate thickness and free-carrier concentration in the (0.1–6)× 1018 cm–3range. Thermophotovoltaic cells based on the InP/In0.53Ga0.47As lattice-matched heterostructure of p–n and n–p are fabricated by liquid-phase epitaxy and Zn and P diffusion from a gas phase. The characteristics of p–n and n–p thermophotovoltaic cells with an identical configuration of the contacts of 1 cm2 area are determined. These characteristics are the open-circuit voltage U oc = 0.465 V, the filling factor FF = 64% at the current density of 1 A/cm2, and the reflectance R= 76 − 80% for wavelengths longer than 1.86μm.

Abstract: Polycrystalline silicon, pc-Si, plays a significant role in the world's photovoltaic cell and module production. The market share of pc-Si modules has increased from 51% in 2001 to about 56% at present. It is expected that pc-Si modules continue to keep this position for many more years and that their market share will also grow in the future. Cell and module efficiencies are increasing steadily and specific module prices will further decline. Therefore the economic assessment of modules needs to be updated continuously. For this purpose the annual module output is required. This in turn implies knowing the impact of climatic parameters on the efficiency. This work covers extensive outdoor testing of a pc-Si module from Kyocera (LA361K51S). All tests were performed at PSI’s Solar Test Facility. The location of PSI represents a typical site in the Swiss Midland. The module was fixed on a sun tracker and tested under clear sky conditions as well as under a cloudy sky. During testing, the global normal irradiance varied between 24 and 946 W/m2, the cell temperature between 1 and 42°C, and the relative air mass between 1.9 and 7.7. About 600 current/voltage characteristics were acquired, leading to the efficiency as a function of irradiance, cell temperature and air mass. The data were used to develop a new efficiency model to determine the efficiency under all relevant operating conditions. The model contains six parameters, determined by applying non-linear fitting techniques. Applying transformation techniques reported on earlier, measurements and the efficiency model can be compared and validated in two-dimensional representations, Figs. 1 to 3. Fig. 1 shows excellent efficiency behavior over the whole irradiance range. The STC efficiency (referred to the active cell area) was found to be 12.7%, corresponding to an STC modul power of 45.7 W. An efficiency maximum of 13.3% was found at 419 W/m2. Fig. 2 shows a linear decrease of the efficiency with temperature. Its temperature coefficient was found to be –0.0493 abs.-%/°C. Fig. 3
shows the dependence of the efficiency for varying air mass. The efficiency exhibits a maximum at an air mass of 2.55 and fairly good red light sensitivity in the late afternoon. Even though the module tested is fairly old (launched in September 1993), it shows very attractive behavior in the climate of the Swiss Midland. From recent testing, it is known that today’s advanced pc-Si modules, e.g. Kyocera’s module KC125GHT-2, behave even better, at remarkably higher efficiency levels.

Abstract: Developing of a solar thermophotovoltaic system is being reported. Theoretical calculations for system parameters (emitter aperture absorptance, emitter efficiency, PV cell band gap etc.) optimization and overall efficiency estimation are presented. The calculations are mainly oriented to make all the parts of the system matched to each other: the choice of tungsten emitter dimensions, PV cell material etc. 20% STPV module efficiency is reachable for GaSb based receiver with a possible increase to 29% for a tandem PV cell and advanced technology of STPV module. A cost-effective two-stage concentrator module based on a primary Fresnel lens and secondary quartz concave-convex lens have been fabricated. Concentration ratio of ~4000x, necessary for obtaining high efficiency of the concentratoremitter system, is ensured. Two types of TPV receivers are tested under outdoor conditions (850 W/m2 average direct sun intensity) and with the solar simulator set up.
Emitter temperatures in the range of 1400-2000K are registered and GaSb PV cell short circuit current density up to 5 A/cm2 is observed. BeO ceramics is used for mounting the PV cells allowing high thermal conductivity and series connection of the cells. High PV efficiency (19% under tungsten emitter irradiation) is obtained in GaSb TPV cells.

Abstract: NO ABSTRACT

Abstract: Developed solar TPV system consists of sunlight tracker, sunlight concentrator, absorber of concentrated sunlight, selective emitter of radiation, internal reflectors of radiation from the emitter, and PV cells cooled by water or forced air. The concentration ratio exceeding 8000 suns is ensured by the developed 300W dish mirror with secondary compound parabolic concentrator. The emitter is made of tungsten evacuated in a vacuum bulb. To decrease the losses of the photons emitted back to outside of TPV system, the area of the emitter surface exceeds up to 10 times the absorber aperture area. The developed PV cells based on Ge and GaSb have a back-surface mirror, which reflects the sub-bandgap photons to the emitter increasing its temperature and overall system efficiency.
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Abstract: Solar thermophotovoltaic efficiency is theoretically estimated using the following optimisation parameters: sunlight concentration ratio, absorber/emitter temperature/efficiency, photon recirculation efficiency and TPV cell parameters. It has been found that emitter temperature exceeding 2000 K, absorber/emitter efficiency of 90% and TPV systems efficiency exceeding 30% can be obtained at sunlight concentration ratio exceeding 8·103 suns with using GaSb cells with back surface reflector and grey-body emitter in vacuum. Utilization of the selective emitter allows to increase the efficiency: calculated efficiency of TPV system with tungsten emitter increases from 30% to 36%.
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Abstract: High efficiency TPV GaSb and Ge based cells fabricated by a non-toxic and inexpensive Zn-diffusion technique have been developed. GaSb based cells optimised for operation with solar powered photon emitter allowed increasing the efficiencies up to 27-28% at black body temperature > 2000 K assuming 90% reflection of sub-bandgap photons. Combination of the MOCVD technique or LPE growth and Zn diffusion from the gas phase allows fabricating Ge photocells on the base of the GaAs/Ge heterostructures, which are characterized by high photocurrent and open circuit voltage values. Efficiencies of 13% were obtained in GaAs/Ge TPV cells under the black-body (1700-2100 K) irradiation assuming the achieved 90% reflection of sub-bandgap photons.
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Abstract: Solar powered TPV systems allow using the high-temperature (> 2000ºC) vacuum emitters that insures promises for their efficiency increase. Theoretical estimations show that efficiencies exceeding 30% can be achieved in solar TPV systems characterized by the high efficiency of sub-bandgap photon recirculation. GaSb based cells with Zn-diffused emitter were developed. GaSb TPV cell efficiencies of 27-28% can be achieved at black body temperature > 2000 K assuming 90% reflection of sub-bandgap photons from the cell to emitter. TPV cells based on the p-GaAs/p-Ge/n-Ge heterostructure have been fabricated by the MOCVD, LPE and Zn-diffusion process for TPV and PV solar applications. InAsSbP/InAs cells were fabricated with the widened photosensitivity in the infrared range up to 3.5 µm.
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Abstract: Concentrator technology is considered as a lower cost alternative to the ''flat'' solar arrays. For high concentration ratios (500x and higher) the use of high-efficiency III-V solar cells is very promising and can provide reduced cell cost contribution to the cost of an installation. The efficiencies of 25-27% were achieved for the single-junction AlGaAs/GaAs solar cells under 500-1500 suns, AM1.5d. These cells are applied in the concentrator modules based on a concept of the small-aperture refractive concentrators. A modified structure of the high concentration ''all-glass'' PV modules with III-V solar cells is proposed. In the ''all-glass'' module design, the secondary small-aperture smooth surface lenses arranged in an intermediate composite (glass-silicone) panel be inserted between a panel of the primary composite Fresnel lens concentrators and a panel of the solar cells. Such a design allows fabricating the modules of large total area (up to 0.5x1m2) and improving environmental protection of the cells. The cells as small as 1.2mm in designated area diameter operating at very high concentration ratio (more than 1000x) can be used in the developed solar PV modules. Also, solar thermophotovoltaics (TPV) is discussed and results of the narrow-gap GaSb and GaAs/Ge cell development for TPV receivers are presented.

Abstract: Zinc diffusion is an established process for doping III-V semiconductors. The experimentally observed kink-and-tail Zn doping profile in GaSb can be explained by two different diffusion mechanisms. However, the reproducibility of the doping profile strongly depends on the treatment of the wafers before the diffusion process. Significant changes in the doping profile have been observed even under small variation in surface preparation. It is found that surface oxides strongly influence Zn profiles. Oxides change the solubility of Zn in GaSb and appear to influence the diffusion mechanism. Different cleaning procedures also alter the profile. This can be explained by defects resulting from cleaning residues. Various surface preparation methods and their resulting doping profiles are discussed.

Abstract: The potential of the thermophotovoltaic conversion of thermal and solar energy to electricity using narrow-gap semiconductor photoconverters is shown. Liquid-phase epitaxy, metal-organic chemical vapor deposition, and Zn diffusion from the vapor phase are used to fabricate thermophotovoltaic converters based on GaSb and GaAs/Ge structures and characterized by increased values of both photocurrent and open-circuit voltage. This circumstance made it possible to obtain thermophotovoltaic cells that were based on the aforementioned structures and had efficiencies of 25% (GaSb) and 16% (GaAs/Ge) at a blackbody-radiation temperature of T = 1473 K under the condition of 100% return of low-energy photons to the emitter.

Abstract: GaSb photovoltaic cells are the most common choice for receivers in thermophotovoltaic (TPV) systems. Although nowadays their manufacturing technology is well established, a theoretical simulation frame for their modelling under real TPV operating conditions is still not fully developed. This is basically due to the lack of a reliable and accurate set of GaSb material parameters as input for the semiconductor simulation tools. Thorough GaSb TPV cell models are needed to understand the electro-optical behaviour of the cells and eventually are essential in improving their design. This work will try to go beyond this key issue, carefully analysing and reviewing some of the key parameters for GaSb. A complete set of material parameters, including revised values for the intrinsic concentration, the electron and hole mobilities and the absorption coefficient, is given based on extended reviews of previously published data. For the first time, estimations for their temperature dependences are introduced. Finally, GaSb TPV cells are manufactured and characterized inside a real TPV system prototype. The comparisons between the electrical measurements and the model theoretical predictions confirm the validity of the proposed set of GaSb material parameters and their temperature dependences.

Abstract: This work deals with the analysis and optimization of GaSb TPV cells. First, the best configuration for the classical Zn-diffused structures is discussed, regarding both the semiconductor structure and grid designs for different illumination conditions. Then, the theoretical performance of both p/n and n/p GaSb epitaxial cell structures is calculated and compared to the diffused cell approach, weighing up the potential benefits of each option.

Abstract: For use in gas fired thermophotovoltaic systems, a selective emitter made from Yb2O3 foam ceramic has been developed. This foam ceramic is mechanically stable and FTIR spectroscopy showed that 10% of the radiation power emitted by the foam can be converted by Si photocells. The thermal and thermal-shock stability of Yb2O3 foam ceramic was analysed. The foam passed 200 heating / cooling cycles without major damage. Tubes were manufactured from this material and tested in a thermophotovoltaic demonstration system. An electrical power of 86W was achieved at a thermal power of 16 kW. Using a simulation model, a potential efficiency of a thermophotovoltaic system based on our technology applied for the conversion of concentrated solar radiation was estimated.

Abstract: The paper presents the review of solar thermophotovoltaic converters design as well as the growth, material characterization and device performance of TPV cells based on Ge, Si, InGaAs/InP and GaSb with related solid solutions. Zinc-diffused (p-n)-Ge based PV cells were fabricated with short circuit current density of 33.2 mA/cm2 obtained from the spectral curve of the internal quantum yield at active area under sunlight with ë ƒn> 900 nm AM0 spectrum. Ge cells with GaAs windows were developed by the combination of LPE or MOCVD growth of GaAs and Zn-diffusion. Efficiency higher than 13% was obtained in p-GaAs/(p-n)-Ge cells with ƒnunder cut off ë ƒnƒn> 900 nm AM0 spectrum at photocurrent densities range of 3-25 A/cm2. The higher efficiencies have been obtained in TPV cells based on GaSb and InGaAs (lattice matched to InP substrate): external quantum yield as high as 90% in IR-part of photosensitivity spectrum; Voc=0.45-0.52 V; FF=0.7-0.8 at photocurrent densities of 1-5 A/cm2. A reproducible and low-cost Zn-diffusion technology to n-GaSb wafers have been developed for producing the high efficiency TPV cells, which are being used for TPV generators. Significant results for the growth, material characterization and device performance of TPV cells based on InGaAsSb, InGaSb, AlGaAsSb, and InAsSbP fabricated by LPE, MOCVD, and diffusion methods are reviewed. For singlejunction TPV cells, epitaxial heterostructures with a ~0.53-eV bandgap InGaAsSb base layer and widebandgap AlGaAsSb or GaSb window/cladding layers (all closely lattice-matched to a GaSb substrate) represent the state of the art. As an alternative, a low-cost Zn-diffusion technology for fabrication of InGaAsSb p-n homojunction structures has been developed for producing the high efficiency TPV cells. External quantum yields as high as 90% at wavelengths (around 2000-nm wavelength), and response edges to about 2400 nm wavelength have been obtained with these TPV cells. Another approach for receiving the perspective TPV devices with bandgaps of 0.55-0.74 eV has been realized by growth of the lattice-matched and mismatched InGaAs layers on InP substrates. Monolithic interconnected modules (MIMs) on semi-insulated InP-substrates ensure a decrease of the Joule losses and an increase of subbandgap photon reflection in the structures with back-surface reflector that should contribute to TPV system efficiency owing to photon recirculation.

Abstract: Zinc diffusion profiles as a function of depth are investigated in Te dopes polycrystalline GaSb using SIMS and AFM. Two types of profile are found; a gradual-profile, which is caused by Zn deposits on the surface, and a step-profile, which is representative of the Zn gradient within the GaSb. It is found that the Zn deposits mask the step-profile to produce the gradual profile. The stepprofile within the material is different from the so-called "kink and tail" seen in other studies of the zinc profile within the GaSb. An activation energy of ~1.1eV is also found, which is lower that in other studies. These results are discusses in relation to the literature.