M. Z. Shvarts

AlGaAs/GaAs photovoltaic cells with an array of InGaAs QDs

Specific features of the fabrication of AlGaAs/GaAs single-junction photovoltaic cells with an array of quantum dots (QDs) by molecular beam epitaxy have been studied. It was shown for the first time that, in principle, vertically coupled QDs can be incorporated, with no dislocations formed, into the structure of photovoltaic cells without any noticeable deterioration of the structural quality of the p-n junction. Owing to the additional absorption of the long-wavelength part of the solar spectrum in the QD medium and to the subsequent effective separation of photogenerated carriers, a ∼1% increase in the short-circuit current density Jsc was demonstrated for the first time in the world for photovoltaic cells with QDs. The maximum efficiency of the photovoltaic cells was 18.3% in conversion of the unconcentrated ground level solar spectrum AM1.5G.

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary GaxIn1−xP and AlxIn1−xP alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm2 on the basis of heavily doped n++-GaAs:Si and p++-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).

Solar cells based on gallium antimonide

Liquid-phase epitaxy and diffusion from the gas phase have been used to create various kinds of GaSb-based solar cell structures intended for use in cascaded solar-radiation converters. A narrow-gap (GaSb) solar cell was studied in tandem based on a combination of semiconductors GaAs-GaSb (two p-n junctions) and GaInP/GaAs-GaSb (three p-n junctions). The maximum efficiency of photovoltaic conversion in GaSb behind the wide-gap cells is η = 6.5% (at sunlight concentration ratio of 275X, AM1.5D Low AOD spectrum).

Germanium Subcells for Multijunction GaInP/GaInAs/Ge Solar Cells

Photovoltaic converters based on n-GaInP/n-p-Ge heterostructures grown by the OMVPE under different conditions of formation of the p-n junction are studied. The heterostructures are intended for use as narrow-gap subcells of the GaInP/GaInAs/Ge three-junction solar cells. It is shown that, in Ge p-tn junctions, along with the diffusion mechanism, the tunneling mechanism of the current flow exists; therefore, the two-diode electrical equivalent circuit of the Ge p-n junction is used. The diode parameters are determined for both mechanisms from the analysis of both dark and “light” current-voltage dependences. It is shown that the elimination of the component of the tunneling current allows one to increase the efficiency of the Ge subcell by ∼1% with conversion of nonconcentrated solar radiation. The influence of the tunneling current on the efficiency of the Ge-based devices can be in practice reduced to zero at photogenerated current density of ∼1.5 A/cm2 due to the use of the concentrated solar radiation.

n-Si bifacial concentrator solar cell

Various approaches have been developed for reducing the cost of the photoelectricity produced by silicon solar cells (SCs). Of highest priority among these approaches are improvement of the efficiency of the SCs, transition from p-Si to n-Si, light concentration, and use of bifacial SCs. In the present study, an SC combining all these approaches has been developed. In this SC, transparent conducting oxides serve as antireflection and passivating electrodes in an indium-tin-oxide/(p+nn+)-Si/indium-fluorine-oxide structure fabricated from Cz-Si with wire contacts (Laminated Grid Cell design). The SC has front/rear efficiencies of 16.5–16.7/15.1–15.3% X (under 1–3 suns). This result is unique because the combination of bifaciality and concentrator operation has no analogs and the SC compares well with the world standard among both bifacial and concentrator SCs.

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/.

Thermophotovoltaic generators based on gallium antimonide

Designs of thermophotovoltaic (TPV) generators with infrared emitters heated by concentrated solar radiation are developed, fabricated, and tested. Emitters made of SiC, W, or Ta of various forms and sizes are studied. To the GaSb-based thermophotovoltaic cells, the efficiency of transformation of thermal radiation of W emitters was 19%. The features of operation of two variants of TPV generators, namely, of cylindrical and conical types, are considered. In a demonstration model of the TPV generator consisting of 12 photocells, the output electric power with conversion of the concentrated solar radiation was P = 3.8 W.

AlGaAs/GaAs Photovoltaic Cells with InGaAs Quantum Dots

We studied the different carrier kinetic mechanisms involved into the interband absorption of quantum dots (QDs) by photocurrent spectroscopy. It was shown that in vertically coupled InGaAs QDs an effective carrier emission, collection and separation take place due to minizone formation. The possibility for the incorporation of vertically-coupled QDs into solar cells (SC) without any deterioration of structural quality of the p-i-n-junction has been shown. Due to the additional absorption of solar spectrum in QD media and the subsequent effective separation of photogenerated carriers, an increase (~1%) in short-circuit current density (Jsc) for the QD SC-devices has been demonstrated. However the insertion of QDs into intrinsic region reduced the open circuit voltage (Voc) of such devices. Moving the QD array in the base layer as well as including the Bragg reflector (BR) centered on 920 nm resulted in increase of the Voc. Moreover an improved absorption in the QD media for SC with BR led to further increase of Jsc (~1%). The efficiency for QD SCs at the level of 25% (30 suns AM1.5D) has been demonstrated.

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.