V. M. Andreev

Solar photovoltaics: Trends and prospects

Key areas in the development of photovoltaic methods of solar energy conversion, which open up wide prospects for semiconductor solar energy conversion, are discussed. The article focuses mainly on photovoltaic cells based on III-V heterostructures, primarily on cascade solar cells, which provide the highest efficiency of solar energy conversion and are produced by high-tech methods such as MBE or MOCVD. It is shown that the use of intermediate sunlight concentration makes the area of solar cells smaller and, hence, lowers their cost proportionally to the sunlight concentration ratio.

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).

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.

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.

Progress In Developing HCPV Modules Of SMALFOC-Design

This work presents a module generation which has been named as “SMALFOC module design”. The abbreviation reflects the main features of the modules: Small-size concentrators; Multi-junction cells; “All-glass” structure; Lamination technology; Fresnel Optics for Concentration. Investigated modules have revealed a quite low over-heating temperature of cells in the MPP regime of operation and a real way for increasing the PV efficiency, if the rear glass base is supplied with an antireflection coating. Outdoor and high/low temperature tests have shown a good potential for long-term operation of such a type of modules.

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.

Thermal Regimes of Fresnel Lenses and Cells in “All‐Glass” HCPV Modules

We report on research carried out at the Ioffe Physical Technical Institute in the field of operational conditions of the high‐concentration photovoltaic (HCPV) module components. The subject of investigations are thermal regimes of the primary Fresnel lenses and multijunction solar cells. Two main issues governing the design of a solar concentrator module with III–V triple‐junction solar cells (SCs) are considered: the effective concentration of radiation using Fresnel lenses, and effective heat removal from SCs. By theoretical and experimental simulating these processes, the design parameters of modules’ elements have been found. A test batch of sub‐modules (HCPV modules based on individual Fresnel lenses) has been fabricated and tested. The influence of different operation temperatures on the optical efficiency of Fresnel lenses and conversion efficiency of solar cells have been estimated.

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.

Photovoltaic laser-power converter based on AlGaAs/GaAs heterostructures

Photovoltaic laser-power converters for a wavelength of λ = 809 nm are developed and fabricated on the basis of single-junction AlGaAs/GaAs structures grown by metal-organic vapor-phase epitaxy. The parameters of the photovoltaic structure constituted by an optical “window” and a cladding layer are optimized by mathematical simulation. Photovoltaic converters with areas of S = 10.2 and 12.2 mm2 and 4 cm2 are fabricated and studied. For photocells with S = 10.2 mm2, the monochromatic efficiency (η) was 60% at a current density of 5.9 A/cm2. A photovoltaic module with a working voltage of 4 V (η = 56.3% at 0.34 A/cm2) is assembled.