V.M. Andreev

Some advantages of intermediate band solar cells based on type II quantum dots

Unlike Type I, Type II quantum dots do not have hole bound states. This precludes that they invade the host semiconductor bandgap and prevents the reduction of voltage in intermediate band solar cells. It is proven here that the optical transition between the hole extended states and the intermediate bound states within the host bandgap is much stronger than in Type I quantum dots, increasing the current and making this structure attractive for manufacturing these cells.

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.

Concentrator tandem solar cells based on AlGaAs/GaAs-InP/InGaAs(or GaSb) structures

We have developed high-efficiency IR-transparent AlGaAs/GaAs top cells as well as InP/InGaAs and GaSb bottom cells for mechanically stacked tandem solar cells designed for space application. AlGaAs/GaAs and InP/InGaAs structures are prepared by liquid phase epitaxy, GaSb structures-by Zn-diffusion process. The highest top cell efficiency is 23.8% (AMO, 100 suns, 25/spl deg/C). The infrared-sensitive InP/InGaAs cell efficiency measured under a GaAs IR-filter is 5.9%. The maximum GaSb cell efficiency under GaAs-filter is 5.1%. Mechanically stacked four-terminal tandem solar cells have been fabricated and tested as well. The reached efficiency of an AlGaAs/GaAs-InP/InGaAs tandem cell is 28.2% (AMO, 70 suns, 25/spl deg/C).

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/.

High-efficiency AlGaAs-GaAs solar cells with internal Bragg reflector

The work presents an investigation of solar cells (SCs) based on AlGaAs/GaAs heterostructures with internal Bragg reflectors grown by low-pressure MOCVD on n-GaAs substrates in a horizontal resistively heated reactor. The typical structure consists of: Bragg reflector (BR) having 12 periods, n-GaAs base layer with the thickness of 1500-2000 nm, 400-500 nm thick p-GaAs emitter, 70 nm thick p-AlGaAs passivating window and top p-GaAs contact layers. The BR with the reflectance maximum centered at the wavelength of 860 nm consists of twelve pairs of AlAs/GaAs layers. The resulting BR thicknesses is 71.6 nm for AlAs and 59 nm for GaAs. In this case the peak of reflectance spectrum is in the area of 830-900 nm where the reflectance is close to unit. This multi-layer quasi-dielectric stack selectively reflect weakly absorbed photons with energies near the GaAs band gap for a second pass through the photoactive region increasing the photocurrent. The employment of the BR allows to increase the external quantum efficiency in the long wavelength range of the spectrum and to fabricate simultaneously a thinner n-GaAs base layer. The use of the internal BR, Ta/sub 2/O/sub 5/ for antireflecting coating and prismatic cover allowed us to obtain efficiency of 23.4% (17.7 suns, AMO, 25/spl deg/C) and 27.2% (23.4 suns, AM 1.5).

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/.

Flat-plate Fresnel lenses with improved concentrating capabilities: Designing, manufacturing and testing

This work presents the results of designing, manufacturing and testing a flat-plate Fresnel lens with improved concentrating capability. At the designing the Fresnel lens, the optimal combination of lens size, focal distance and refracting surface profile parameters was found with applying the photometric model of the concentration process. Lens optical-power characteristics were tested with use of developed indoor equipment. Based on experimentally measured geometrical parameters of the lens profile and spectral characteristics of the testing units, the mathematical model have been corrected and a good fit of simulated and experimental results is demonstrated, which indicates a right choice of the designing procedure, manufacture process and sequence of tests.

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.

Evaluation of the PV Cell Operation Temperature in the Process of Fast Switching to Open-Circuit Mode

A procedure for measuring the overheating temperature (ΔT ) of a p-n junction area in the structure of photovoltaic (PV) cells converting laser or solar radiations relative to the ambient temperature has been proposed for the conditions of connecting to an electric load. The basis of the procedure is the measurement of the open-circuit voltage (VOC ) during the initial time period after the fast disconnection of the external resistive load. The simultaneous temperature control on an external heated part of a PV module gives the means for determining the value of VOC at ambient temperature. Comparing it with that measured after switching OFF the load makes the calculation of ΔT possible. Calibration data on the VOC = f(T ) dependences for single-junction AlGaAs/GaAs and triple-junction InGaP/GaAs/Ge PV cells are presented. The temperature dynamics in the PV cells has been determined under flash illumination and during fast commutation of the load. Temperature measurements were taken in two cases: converting continuous laser power by single-junction cells and converting solar power by triple-junction cells operating in the concentrator modules.

Space Fresnel lens concentrator modules with triple-junction solar cells

The work presented in this paper is concerned with fabrication of an improved concentrator module based on linear Fresnel lenses and photoreceivers with GalnP/GaAs/Ge triple junction (TJ) cells. The manufactured module consists of: four linear Fresnel lenses with focal distance of 30 mm and dimensions of 2.5/spl times/10 cm/sup 2/ each; four linear photoreceivers of 10 cm in length with 12 TJ solar cells each; heat sink made of thin copper metallized dielectric plate. Linear Fresnel lenses with optical efficiency of 82-84% were made of silicone formed on shockproof radiation-resistant UV-protective glass 0.2 mm thick. The solar cells intended for use in linear receivers have efficiencies of 23-24% (AM0, 25 /spl deg/C) at geometrical sunlight concentration of 6-9/spl times/. The estimated module efficiency is about 21% (AM0).