V. P. Khvostikov

Solar thermophotovoltaic system with high temperature tungsten emitter

In this paper, R&D of a solar thermophotovoltaic (STPV) system is reported. Two types of TPV modules have been developed and tested under concentrated sunlight. Temperatures as high as 1800/spl deg/C were obtained in a 12 mm dia and 15 mm length emitter, illuminated by a 0.45 m/sup 2/ pseudoparabolic facet mirror with a secondary quartz lens (4000/spl times/ concentration ratio). Analysis of various parameters, influencing the overall performance of the developed STPV is presented.

Current flow and potential efficiency of solar cells based on GaAs and GaSb p-n junctions

Dependence of the efficiency of single-junction and multijunction solar cells on the mechanisms of current flow in photoactive p-n junctions, specifically on the form of the dark current-voltage characteristic J-V, has been studied. The resistanceless J-Vj characteristic (with the series resistance disregarded) of a multijunction solar cell has the same shape as the characteristic of a single-junction cell: both feature a set of exponential portions. This made it possible to develop a unified analytical method for calculating the efficiency of singlejunction and multijunction solar cells. The equation relating the efficiency to the photogenerated current at each portion of the J-Vj characteristic is derived. For p-n junctions in GaAs and GaSb, the following characteristics were measured: the dark J-V characteristic, the dependence of the open-circuit voltage on the illumination intensity P-VOC, and the dependence of the luminescence intensity on the forward current L-J. Calculated dependences of potential efficiency (under idealized condition for equality to unity of external quantum yield) on the photogenerated current for single-junction GaAs and GaSb solar cells and a GaAs/GaSb tandem are plotted. The form of these dependences corresponds to the shape of J-Vj characteristics: there are the diffusion- and recombination-related portions; in some cases, the tunneling-trapping portion is also observed. At low degrees of concentration of solar radiation (C < 10), an appreciable contribution to photogenerated current is made by recombination component. It is an increase in this component in the case of irradiation with 6.78-MeV protons or 1-MeV electrons that brings about a decrease in the efficiency of conversion of unconcentrated solar radiation.

Tandem GaSb/InGaAsSb thermophotovoltaic cells

Computer modelling of a tandem thermophotovoltaic (TPV) system has been carried out. The monolithic GaSb/InGaAsSb tandem TPV devices have been designed and fabricated by LPE. The cell consists of: nGaSb (substrate); (n-p)In/sub x/Ga/sub 1-x/As/sub y/Sb/sub 1-y/ (E/sub g//spl ap/0.56 eV, bottom cell); p/sup ++/-n/sup ++/GaSb (tunnel junction); (n-p)-GaSb (top cell). External quantum yields of 80% at 800-1600 nm wavelength (top cell) and of about 75% at 1800-2100 nm (bottom cell) have been measured. V/sub OC/=0.61 V and FF=0.75 were achieved in a tandem cell at current density of 0.7 A/cm/sup 2/.

Zinc-diffused InAsSbP/InAs and Ge TPV cells

By means of LPE growth and Zn diffusion, TPV cells and mid-IR photodetectors based on p-InAsSbP/n-InAsSbP/n-InAs and p-InAs/n-InAs structures have been fabricated with the photosensitivity widened in the infrared range (2.5-3.4 /spl mu/m). Zinc - diffused p-n Ge-based TPV cells have been fabricated with external quantum yield as high as 0.9-0.95 and high short circuit current of 31.6 mA/cm/sup 2/ under sunlight with cut-off at /spl lambda/< 900 nm AMO spectrum. The Ge-based TPV cells with back-surface mirror demonstrate reflection of 85% for the sub-bandgap photons. The Ge cells with GaAs window have been developed for PV and TPV applications with using the combination of liquid-phase epitaxy and Zn-diffusion processes. Efficiency of more than 5% has been measured in p-GaAs/p-Ge/n-Ge cells under cut-off (/spl lambda/<900 nm) AM1.5 spectrum at photocurrent densities of 3+20 A/cm/sup 2/.

GaAs/Ge heterostructure photovoltaic cells fabricated by a combination of MOCVD and zinc diffusion techniques

Ge photovoltaic cells based on GaAs/Ge heterostructures have been produced by a combination of metal-organic chemical-vapor deposition and Zn diffusion from the gas phase. The cells are characterized by increased photocurrent and open-circuit voltage. The calculated efficiency of a Ge solar cell under concentrated sunlight exceeds 5.5%. The photocurrent achieved in a Ge photovoltaic cell is close to that obtained in GaAs solar cells under similar conditions of illumination with air-mass-zero (AM0) sunlight, which enables one to design high-efficiency concentrator-type cascade solar cells with a GaAs top cell and a Ge bottom cell.

High-efficiency (49%) and high-power photovoltaic cells based on gallium antimonide

High-efficiency GaSb-based photovoltaic cells designed for conversion of high-power laser radiation and infrared radiation of emitters heated by concentrated solar radiation are fabricated and studied. The maximum efficiency of conversion of the radiation with λ = 1680 nm was 49% at the photocurrent density of 50–100 A/cm2 for the fabricated photovoltaic cells. The methods for reducing the losses at ohmic contacts to p-and n-GaSb are investigated. The minimum values of the specific resistance, (1–3) × 10−6 Θ cm2, of contact to p-GaSb with the doping level of 1020 cm−3 were obtained using the Ti/Pt/Au contact system. The minimum values of the specific contact resistance were (1–3) × 10−6 Θ cm2 in the case of n-GaSb with the doping level of 2 × 1018 cm−3 if the Au(Ge)/Ni/Au and Au/Ni/Au contact systems are used.

Single-junction GaSb and tandem GaSb/InGaAsSb and AlGaAsSb/GaSb thermophotovoltaic cells

The advanced technology of the single- and dual-junction TPV cells, based on GaSb was developed. Photocurrent density as high as 54 mA/cm/sup 2/ under AM0 spectrum, efficiencies of 12.8% (AM1.5D, 120 suns) and 19.1% under cut-off AM0-spectrum (/spl lambda/=900-1800 nm) were obtained. The deviations of V/sub oc/ and FF in the GaSb 1 cm/sup 2/ cells are in the following ranges: V/sub oc/=0.42-0.45 V and FF=0.7-0.72 at photocurrent of 1 A. The cells of 2 cm/sup 2/ in area generate photocurrent up to 9 A at V/sub oc/=0.52 V. Tandem InGaAsSb (0.55 eV)/GaSb cells with V/sub oc/=0.65 V at I/sub sc/1A/cm/sup 2/ were developed. Tandem AlGaAsSb(1 eV)/GaSb cells with expected V/sub oc/=1.1 V at I/sub sc/=1A/cm/sup 2/ are under development as well.

Quantum-well AlGaAs heterostructures grown by low-temperature liquid-phase epitaxy

Abstract An investigation of the low-temperature liquid-phase epitaxy (LTLPE) process at 400–650 °C for crystallization of AlGaAs heterostructures with ultrathin (2–100 nm) layers has been carried out. In particular, low-threshold quantum-well heterolasers (threshold current density j th as low as 120 A cm −2 ) and high-efficiency solar cells (efficiency as high as 24.0–24.7% under AMO conditions at concentration ratio 20–100 ×) have been fabricated by LT LPE.

Tritium-powered betacells based on Al/sub x/Ga/sub 1-x/As

The paper describes betavoltaic cells (BVC) based on A/sup 3/B/sup 5/ semiconductors. It is shown that wide band gap semiconductors such as AlGaAs ensure the efficient betavoltaic conversion of beta radiation from tritium and photovoltaic (PV) conversion of the light from tritium powered radioluminescent sources. Betavoltaic cells based on Al/sub x/Ga/sub 1-x/As with ultrathin (0.01-0.03 /spl mu/m) wide bandgap passivating window were developed. Output current densities as high as (0.75-1)10/sup -6/ A/cm/sup 2/, open circuit voltage of 0.65-0.93 V and maximum power output of 0.55 /spl mu/W/cm/sup 2/ were achieved in the betacells in a capsule with tritium gas. Specific output power of 335 nW/Ci is ensured in the battery based on green tritium lamps and developed PV cells.

Space concentrator module based on short focus linear Fresnel lenses and GaAs/GaSb tandem stacks

The module has a dimensions of 2.5/spl times/10/spl times/10 cm/sup 3/ and consists of four Fresnel lenses with dimensions of 2.5/spl times/10 cm/sup 2/ each. Two photoreceiver strings are made of tandem GaAs/GaSb triplets and two strings are made of AlGaAs/GaAs single junction cells. The best fabricated GaAs/GaSb tandem cell efficiency of 27.5% (AM0, 10 suns, 25/spl deg/C), triplet efficiency of 25.6% (11 suns) and linear (10 cm in length) receiver efficiency of 24.3% (AM0, 6 suns, 25/spl deg/C) were received. Fresnel lens efficiencies of 0.82-0.85 were achieved. The estimated module efficiency is 21% (AM0, 25/spl deg/C). Module specific weight of 3 kg/m/sup 2/ was realized.