摘要:

Thin films of undoped and tin‐doped In2O3have been investigated for use as plasma filters in spectral control applications for thermal photovoltaic cells. These films are required to exhibit high reflectance at wavelengths longer than the plasma wavelength &lgr;p, high transmittance at wavelengths shorter than &lgr;pand low absorption throughout the spectrum. Both types of films were grown via atmospheric pressure chemical vapor deposition (APCVD) on Si (100) and fused silica substrates using trimethylindium (TMI), tetraethyltin (TET), and oxygen as the precursors. The O2/TMI partial pressure ratio and substrate temperature were systematically varied to control the filter characteristics. The plasma wavelength &lgr;pwas found to be a sensitive function of the O2partial pressure and the substrate temperature. Post‐growth annealing of the films carried out either in nitrogen or air ambient at elevated temperatures did not have any beneficial effect. Tin‐doped In2O3was grown using tetraethyltin (TET) as the dopant. The material properties and consequently the optical response were found to be strongly dependent on the growth conditions such as O2and TET partial pressures. Both undoped and tin‐doped In2O3grown on fused silica exhibited enhanced transmittance due to the close matching of refractive indices of In2O3and silica. X‐ray diffractometer measurements indicated that all these films were polycrystalline and highly textured towards the (111) direction. The best undoped and tin‐doped In2O3films had a &lgr;paround 2.7 &mgr;m, peak reflectance greater than 75% and residual absorption below 20%. These results indicate the promise of undoped and tin‐doped In2O3as a material for plasma filters. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49694

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Heavily doped Si was investigated for use as spectral control filter in thermal photovoltaic (TPV) system. These filters should reflect radiation at 4 &mgr;m and above and transmit radiation at 2 &mgr;m and below. 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 (&lgr;p) of these filters could be adjusted by controlling the diffusion conditions. The minimum plasma wavelength achieved was 4.8 &mgr;m. In addition, a significant amount of absorption was observed for the wavelength 2 &mgr;m and below. The second approach was doping by ion implantation followed by thermal annealing with a capped layer of doped glass. Implantation with high dosage 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 &mgr;m. The high temperature annealing (≳1000 °C) that was necessary to activate the dopant atoms and to heal the implantation damage, also caused significant absorption at 2 &mgr;m. 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 2×1016cm−2of P was at 2.9 &mgr;m. The absorption at 2 &mgr;m was less than 20% and the reflection at 5 &mgr;m was about 70%. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49696

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A selective filter is an important component in a high‐efficiency thermophotovoltaic (TPV) system. Compared to dielectric stack interference filters, semiconductor plasma filters have the potential for higher performance at lower cost. Conventional transparent conductive oxides (TCOs), such as ITO, SnO2, and ZnO, are inadequate for low temperature (800–1200 °C) TPV system applications, because of their low mobility (∼20 cm2 V−1 s−1) and high carrier concentration (≳5×1020cm−3). A cadmium stannate (Cd2SnO4) based selective filter has been developed in this study. We will report experimental results on Cd2SnO4deposited by r.f. magnetron sputtering. The principle variables investigated were the composition of the sputtering gas, the substrate temperature, and the conditions of post‐deposition thermal treatment. The electrical, optical, and compositional properties of the films have been characterized using Hall effect measurement, optical and infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, and atomic force microscopy. Mobilities as high as 65 cm2 V−1 s−1with a carrier concentration 2–3×1020cm−3have been obtained. The results indicate the ability to control the short‐wavelength transmittance, the long‐wavelength reflectance, and the position and abruptness of the plasma edge. The plasma edge can be controlled between 1.5 and 3.0 &mgr;m. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49715

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The back surface reflector (BSR) represents the spectral‐control technology that offers the highest spectral utilization factor,Fu, whereFuis defined as the fraction of the total absorbed radiation with energy greater than the semiconductor bandgap. In order for this technology to succeed, an integrated photovoltaic cell—spectral control thermophotovoltaic device design is required which simultaneously minimizes free carrier absorption and series resistance losses. For this study, BSR technology was developed for GaSb, InAs, and InP substrate systems. Reflection and contact resistance results will be presented for the above material systems. To date,Fu≳80% have been obtained for all three material systems, with potential forFu≳90%. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49697

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this paper we present two methods for determining the conversion efficiency of TPV devices. In the first, the conversion efficiency is calculated from measurements of the external quantum efficiency and reflection as a function of wavelength, and from the I‐V characteristics under high‐level illumination. This is an indirect method based on separate differential measurements. In the second method, a novel heat transfer technique is utilized to combine both the voltaic diode and spectral control efficiency into a single measurement. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49698

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This paper reviews the use of back surface reflectance (BSR) in a variety of PV cells. The major controlling factors are illustrated by plots of reflectance versus wavelength. Possible application to TPV cells is discussed. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49713

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Lattice‐matched and strained indium gallium arsenide solar cells can be used effectively and efficiently for thermophotovoltaic applications. A 0.75 eV bandgap InGaAs solar cell is well matched to a 2000 K blackbody source with a emission peak around 1.5 &mgr;m. A 0.60 eV bandgap InGaAs cell is well suited to a Ho‐YAG selective emitter and a blackbody at 1500 K which have emission peak around 2.0 &mgr;m. Modeling results predict that the cell efficiencies in excess of 30% are possible for the 1500 K Ho‐YAG selective emitter (with strained InGaAs) and for the 2000 K blackbody (with lattice‐matched InGaAs) sources. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49699

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Indium gallium arsenide (InxGa1−xAs) cell models extracted from measured data on thermophotovoltaic (TPV) cells with bandgaps of 0.75 to 0.55 eV are presented. The dark current model is based on a fit to values extracted from open‐circuit voltages at high photocurrents (where the ideality factor is close to unity) for the various bandgap cells. Quantum efficiency models of Hovel and a very simple base model are compared with measured data. A standard model for the series resistance of the cell is presented and agrees with measured data. The quantum efficiency model is used with the standard blackbody equations to predict the cell photocurrent at 800 and 1200 C over the 0.5 to 0.75 eV bandgap range. The dark current, series resistance, and photocurrent are used to numerically determine maximum output power for InxGa1−xAs cells over the above bandgap range at these two blackbody temperatures. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49700

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The growth by molecular beam epitaxy of In0.74Ga0.26As is investigated because of its importance as a PV converter for a variety low temperature TPV system configurations. In this work, a linearly graded buffer layer is used to grow high quality In0.74Ga0.26As layers on a lattice mismatched InP substrate. The thickness of the buffer layer and the substrate temperature during the growth of the buffer and active layers were varied in order to optimize the active layer material quality. The resultingp+−i−n+epitaxial layers were compared using double crystal x‐ray diffraction, spectral response, and current‐voltage measurements. A more conventional PV cell structure was also evaluated using current‐voltage measurements. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49701

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thermophotovoltaic (TPV) cells convert thermal energy to electricity. Modularity, portability, silent operation, absence of moving parts, reduced air pollution, rapid start‐up, high power densities, potentially high conversion efficiencies, choice of a wide range of heat sources employing fossil fuels, biomass, and even solar radiation are key advantages of TPV cells in comparison with fuel cells, thermionic and thermoelectric convertors, and heat engines. The potential applications of TPV systems include: remote electricity supplies, transportation, co‐generation, electric‐grid independent appliances, and space, aerospace, and military power applications. The range of bandgaps for achieving high conversion efficiencies using low temperature (1000–2000 K) black‐body or selective radiators is in the 0.5–0.75 eV range. Present high efficiency convertors are based on single crystalline materials such as In1−xGaxAs, GaSb, and Ga1−xInxSb. Several polycrystalline thin films such as Hg1−xCdxTe, Sn1−xCd2xTe2, and Pb1−xCdxTe, etc., have great potential for economic large‐scale applications. A small fraction of the high concentration of charge carriers generated at high fluences effectively saturates the large density of defects in polycrystalline thin films. Photovoltaic conversion efficiencies of polycrystalline thin films and PV solar cells are comparable to single crystalline Si solar cells, e.g., 17.1% for CuIn1−xGaxSe2and 15.8% for CdTe. The best recombination‐state densityNtis in the range of 10−15–10−16cm−3acceptable for TPV applications. Higher efficiencies may be achieved because of the higher fluences, possibility of bandgap tailoring, and use of selective emitters such as rare earth oxides (erbia, holmia, yttria) and rare earth‐yttrium aluminium garnets. As compared to higher bandgap semiconductors such as CdTe, it is easier to dope the lower bandgap semiconductors. TPV cell development can benefit from the more mature PV solar cell and opto‐electronic (infrared detectors, lasers, and optical communications) technologies. Low bandgaps and larger fluences employed in TPV cells result in very high current densities which make it difficult to collect the current effectively. Techniques for laser and mechanical scribing, integral interconnection, and multi‐junction tandem structures which have been fairly well developed for thin‐film PV solar cells could be further refined for enhancing the voltages from TPV modules. Thin‐film TPV cells may be deposited on metals or back‐surface reflectors. Spectral control elements such as indium‐tin oxide or tin oxide may be deposited directly on the TPV convertor. It would be possible to reduce the cost of TPV technologies based on single‐crystal materials being developed at present to the range of US$ 2–5 per watt so as to be competitive in small to medium size commercial applications. However, a further cost reduction to the range of US ¢ 35–$ 1 per watt to reach the more competitive large‐scale residential, consumer, and hybrid‐electric car markets would be possible only with the polycrystalline‐thin film TPV cells. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.49702

出版商:AIP    

年代:1996

数据来源: AIP