Activity and current status of R&D on space solar cells in Japan

Japan's Research and Development (R&D) activities on high‐performance III–V compound space solar cells are presented. Studies of new CuInGaSe₂ thin‐film terrestrial solar cells for space applications are also discussed. Performance and radiation characteristics of a newly developed InGaP/GaAs/Ge triple‐junction space solar cell, including radiation response, results of a flight demonstration test of InGaP/GaAs dual‐junction solar cells and CuInGaSe₂ thin‐film solar cells, and radiation response of three component sub‐cells are explained. This study confirms superior radiation tolerance of InGaP/GaAs dual‐junction cells and CuInGaSe₂ thin‐film cells by space flight experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

The potential of III‐V semiconductors as terrestrial photovoltaic devices

III‐V semiconductors, GaAs and in particular InGaP, are used in many different electronic applications, such as high power and high frequency devices, laser diodes and high brightness LED. Their direct bandgap and high reliability make them ideal candidates for the realisation of high efficiency solar cells: in the past years they have been successfully used as power sources for satellites in space, where they are able to produce electricity from sunlight with an overall efficiency of around 30%. Nowadays, the use of arsenides and phosphides as photovoltaic (PV) devices is confined only to space applications since their price is much higher than conventional Si flat panel modules, the leading PV market technology. But with the introduction of multijunction solar cells capable of operating in high concentration solar light, the area and, therefore, the cost of these cells can be reduced and will eventually find an application and market also on Earth. This article will review the situation of semiconductor solar cell materials, focusing on Si, GaAs, InGaP and multijunction solar cells and will discuss future trends and possibilities of bringing III‐V technology from space to Earth. Copyright © 2006 John Wiley & Sons, Ltd.     

GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

GaInP/GaAs/Ge三结太阳电池辐照损伤效应及加固技术研究进展

文章首先重点介绍了国内外开展GaInP/GaAs/Ge三结太阳电池的电子、质子及其他辐射粒子或射线辐照实验的研究进展,然后从辐照损伤效应的仿真模拟研究、抗辐射加固技术、损伤预估方法等方面综述了GaInP/GaAs/Ge三结太阳电池辐照损伤效应及加固技术的研究进展,最后梳理了当前GaInP/GaAs/Ge三结太阳电池辐照损伤效应研究中亟待解决的关键技术问题,为深入开展GaInP/GaAs/Ge三结太阳电池辐照损伤效应实验方法标准制定、损伤机理分析、在轨寿命预估及抗辐射加固技术研究提供了理论指导和实验技术支持。     

Heterostructure solar cells

Heterostructure solar cells based on III–V compounds are studied. Record-high efficiencies are obtained for solar cells based on AlGaAs/GaAs heterostructures: 24.6% for 100-fold concentration of sunlight in outer space (AM0) and 27.5% for 100-fold concentration of the light on the ground (AM1.5). A substantial increase in radiation resistance is obtained for solar cells with a built-in Bragg mirror. Cascaded solar cells with efficiencies of up to 32% for 100 suns (AM1.5) are created and studied; in these cells the upper wide-gap materials are infrared transparent elements based on GaAs, while the lower narrow-band elements are made of GaSb or an InGaAs solid solution. Fiz. Tekh. Poluprovodn. 33, 1035–1038 (September 1999)     

Modelling of GaAs solar cells under wide angle cones of homogeneous light

A new theoretical model is presented for the performance of solar cells when light in the shape of a cone impinges on them. This is the case for concentration applications. The model is applied to 1000 sun concentrator p/n heteroface GaAs solar cells for the general purpose of a cone angle of between 0° and 90°. A multidimensional optimization of the whole GaAs solar cell structure (antireflecting coatings + ohmic contacts + semiconductor structure) is carried out. It is the first time that the whole structure has been simultaneously optimized under a maximum efficiency criterion. This is thanks to the dual consideration of the opto-electrical characteristics of the window layer. An assessment of GaAs solar cells working inside optical concentrators is also derived. Copyright © 1999 John Wiley & Sons, Ltd.     

A high‐efficiency LPE GaAs solar cell at concentrations ranging from 2000 to 4000 suns

III–V solar cells for terrestrial concentration applications are currently becoming of greater and greater interest. From our experience, concentrations higher than 1000 suns are required with these cells to reduce PV electricity cost to such an extent that this alternative could become cost competitive. In this paper, a single‐junction p/n GaAs solar cell, with efficiencies of 23ċ8 and 22ċ5% at concentration ratios of 2700 and 3600 suns respectively, is presented. This GaAs solar cell is well suited for use with non‐imaging optical concentrators, which possess a large aperture angle. Low‐temperature liquid phase epitaxy (LTLPE) has been the growing technique for the semiconductor structure as an attempt to use a simplified, cheap and clean technique, within a renewable energy perspective. The GaAs solar cell presented is compared with the highest efficiency tandem solar cells at concentration levels exceeding 1000 suns. The GaAs solar cell performance maintains high efficiencies up to 4000 suns, while tandem cells seem to drop very quickly after reaching their maximum. Therefore, single‐junction GaAs solar cells are a good candidate for operating at very high concentrations, and LPE is able to supply these high‐quality solar cells to work within terrestrial concentration systems, the main objective of which is the reduction of PV electricity costs. Copyright © 2003 John Wiley & Sons, Ltd.     

GaAs基GaInNAs太阳电池研究进展

在研究新型高效GaAs基三结和四结太阳电池过程中,研究者努力寻找一种既满足能隙约为1eV,同时又与GaAs衬底晶格匹配的半导体材料。通过调节组分,GaInNAs可同时满足上述两个特性,因此GaInNAs被认为是制备新型高效多结GaAs基太阳电池的理想材料。但实际上,制备高晶体质量GaInNAs材料十分困难,造成所制备的器件性能低下,未能达到实际要求。探讨了导致GaInNAs材料生长困难的机理,并对当前GaAs基GaInNAs太阳电池材料的研究历程和技术现状进行了概述。在此基础上,展望了GaInNAs技术的未来走向。     

Equivalent electron fluence for space qualification of shallow junction heteroface GaAs solar cells

It is desirable to perform qualification tests prior to deployment of solar cells in space power applications. Such test procedures are complicated by the complex mixture of differing radiation components in space which are difficult to simulate in ground test facilities. Although it has been shown that an equivalent electron fluence ratio cannot be uniquely defined for monoenergetic proton exposure of GaAs shallow junction cells, an equivalent electron fluence test can be defined for common spectral components of protons found in space. Equivalent electron fluence levels for the geosynchronous environment are presented.     

Influence of perimeter recombination on high-efficiency GaAs p/nheteroface solar cells

Perimeter recombination currents have been characterized for 0.5-cm-square and 2-cm-square p/n GaAs solar cells. Measurements show that perimeter recombination dominates the n=2 dark current component of these high-efficiency solar cells. The results also suggest that perimeter recombination will be substantial even in much-larger-area solar cells. Although little influence on open-circuit voltage is expected, perimeter recombination may adversely affect the cell's one-sun fill factor. Because of its importance to one-sun applications, recombination at the junction perimeter must be suppressed before GaAs solar cells approach their limiting conversion efficiencies     

Nanostructured Absorbers for Multiple Transition Solar Cells

This paper identifies absorbers for multiple transition solar cells that are implemented with nanostructured heterojunctions [e.g., quantum well solar cells with quasi-Fermi-level variations and quantum dot (QD) intermediate-band solar cells]. In the radiative limit, the solar cells implemented with these absorbers are capable of achieving a conversion efficiency ges50% with a geometric solar concentration of at least 1000times. The technical approach enumerates a set of quantitative design rules and applies the rules to the technologically important III-V semiconductors and their ternary alloys. A novel design rule mandates a negligible valence band discontinuity between the barrier material and confined materials. Another key design rule stipulates that the substrate have a lattice constant in between that of the barrier material and that of the quantum-confined material, which permits strain compensation. Strain compensation, in turn, allows a large number of QD layers to be incorporated into the solar cell because each layer is free of defects. Four candidate materials systems (confined/barrier/substrate) are identified: InP_0.85Sb_0.15/GaAs/InP, InAs_0.40P_0.60/GaAs/InP, InAs/GaAs_0.88Sb_0.12/InP, and InP/GaAs_0.70P_0.30/GaAs. Resulting from the design features, the candidate systems may also find use in other optoelectronic applications.     

Thin‐film GaAs epitaxial lift‐off solar cells for space applications

In the present work the space compatibility of thin‐film GaAs solar cells is studied. These cells are separated from their GaAs substrate by the epitaxial lift‐off (ELO) technique and mounted behind a CMG cover glass which at the same time serves as a stable carrier for the thin film cells. In the present initial stage of development these cells have an average efficiency of about 15·4% under AM0 illumination due to not yet optimized grid contacts and anti‐reflection coatings. Inspection after irradiation by 1 MeV electrons, thermal vacuum and thermal cycling experiments reveal that degradation of the cells is largely due to delamination and micro‐cracking. Based on these results, glass dehydration and adhesive degassing procedures are implemented in the ELO cell processing. As a consequence, even in this premature phase, newly produced cells show a radiation hardness comparable to or better than that of commercially available GaAs cells on Ge substrates and are virtually unaffected by severe thermal cycling. Copyright © 2005 John Wiley & Sons, Ltd.     

The potentialities of silicon and gallium arsenide solar batteries

The theory of the spectral response of a p-n junction solar battery unit is given, and detailed comparisons are made of the expected performance of silicon and gallium arsenide units. Data now available on absorption in GaAs show that it is a particularly favourable material because it has a very steep absorption edge. The low effective masses and correspondingly low densities of states in GaAs are also advantageous.

The efficiency of practical Si units is unlikely to exceed 15 per cent even with intensive development, whereas GaAs units with efficiencies of about 20 per cent should be achievable in the near future and the performance may ultimately approach 25 per cent, even if (as assumed) the carrier lifetimes stay well below those for Si.     

Effects of low temperatures and intensities on GaAs and GaAs/Gesolar cells

GaAs and GaAs/Ge solar cells grown by metalorganic chemical vapor deposition (MOCVD) were characterized at very low temperature (-185°C) and solar intensity (0.25 suns) to simulate the cell behavior in a severe interplanetary environment. A comparison is also made with GaAs cells grown with the liquid-phase-epitaxy (LPE) technique. The analysis carried out from -185 to +50°C shows, in particular, different behaviors for GaAs/Ge cells with active and passive Ge substrates; the GaAs/Ge passive cell behaves as a GaAs on GaAs cell, indicating that from the thermal and optical point of view, Ge acts only as a mechanical support. The GaAs cell with an active Ga substrate is affected by a notch in the I-V curves, which is more evident at low temperatures but reduces at low intensities. The GaAs/GaAs MOCVD cell shows the best performance at low temperature and intensity with a conversion efficiency of 27.2%     

Performance of antireflecting coating-AlGaAs window layer couplingfor terrestrial concentrator GaAs solar cells

In this paper, we present the performance of optical coating systems coupled with AlGaAs window layers over GaAs solar cells. Single, double, and triple antireflecting coatings and window layers with constant and graded aluminum content are considered. Comparison between constant and graded window layers is established. To better represent reality, practical factors such as absorption of materials even for antireflecting coatings and the oxidation at window layer surface due to its high aluminum content are also included in the calculations. The design criteria to determine the optimum thickness of each layer is the achievement of maximum photogenerated current density. For this purpose and to account for terrestrial concentrator GaAs solar cells, the inclusion of direct terrestrial solar spectrum together with the internal spectral response of the device are taken into account. Finally, the best antireflecting coating/AlGaAs window layer couplings for different cases are presented     

MIS solar cells: A review

The metal-thin-film insulator-semiconductor (MIS) structure is currently receiving much attention in solar-cell studies. Both theoretical and practical investigations indicate that this structure offers a means of overcoming the principal deficiency of Schottky barrier solar cells, namely low open-circuit photovoltage, while maintaining the attractive features that have led the metal-semiconductor junction to be considered as a possible alternative to the p-n junction for large-area, terrestrial, solar-cell applications. The thin insulating layer allows control over not only the magnitude of the dark current flowing through the diode, but also the dominant type (majority or minority carrier) of this current. Desirably low values of dark current have been postulated for majority carrier devices incorporating suitable charge-trapping centres, located either within the insulator or at the semiconductor-insulator interface, and for minority carrier devices employing suitable insulator thicknesses, metal work functions, and semiconductor resistivities. Theories based on these models are reviewed in this paper and their relevance to explaining photovoltage enhancement in practical Si and GaAs MIS cells is examined. The factors affecting other salient solar-cell properties (photocurrent, fill factor, conversion efficiency) are also considered and suggestions as to the parameters limiting present device performance are given.     

Analytical model for multi-junction solar cells prediction in space environment

Multi-junction solar cells, as other semiconductor devices, suffer degradation of their electrical and physical properties under particle irradiation (electrons and protons) in space environment. In this paper we present an analytical model in order to make predictions of multi-junction solar cells (GaInP/GaAs/Ge) degradation in space environment.     

0.85-/spl mu/m vertical-cavity surface-emitting laser diode arrays grown on p-type GaAs substrate

We have fabricated the first room-temperature (RT) continuous-wave (CW) 0.85-/spl mu/m 8/spl times/8 bottom-emitting vertical-cavity surface-emitting AlGaAs-GaAs DBR QW laser diode (VCSEL) arrays on a p-type GaAs substrate, which are applicable to optical interconnection. The laser characteristics are slightly inferior to those of VCSEL arrays made on n-type GaAs substrate with the same reflectivity, but exhibit for better array uniformity of threshold current density than previously reported. Such devices are applicable to N-MOS integration.     

Characterization testing of Measat GaAs/Ge solar cell assemblies

The first commercial communications satellite with gallium arsenide on germanium (GaAs/Ge) solar arrays was launched in January 1996. The spacecraft, named Measat, was built by Hughes Space and Communications Company. The solar cell assemblies consisted of large-area GaAs/Ge cells supplied by Spectrolab Inc. with infrared reflecting (IRR) coverglass supplied by Pilkington Space Technology. A comprehensive characterization program was performed on the GaAs/Ge solar cell assemblies used on the Measat array. This program served two functions: first to establish the database needed to accurately predict on-orbit performance under a variety of conditions; and second, to demonstrate the ability of the solar cell assemblies to withstand all mission environments while still providing the required power at end-of-life. Characterization testing included: measurement of electrical performance parameters as a function of radiation exposure, temperature and angle of incident light; reverse bias stability; optical and thermal properties; mechanical strength tests, panel fabrication, humidity and thermal cycling environmental tests. The results provided a complete database enabling the design of the Measat solar array, and demonstrated that the GaAs/Ge cells meet the spacecraft requirements at end-of-life.     

Study of thin-film GaAs solar cells with cylindrical Ag nanoparticles and distributed Bragg reflector

An efficient light-trapping structure,which consists of the periodic Ag nanoparticles and a distributed Bragg reflector（DBR）with high reflectivity,is presented for the thin-film gallium arsenide（GaAs）solar cells.The effects of both Ag nanoparticles and DBR on the optical absorption of GaAs solar cells are theoretically investigated by using finite-difference time-domain（FDTD）method.The optimization process of parameters for the solar cell with both structures is analyzed systematically.The great absorption enhancement in GaAs layer is demonstrated,especially in the wavelength region near the GaAs band gap.It is observed that the superposition of the two effects excited by Ag nanoparticles and DBR results in the obvious absorption enhancement.By using cylindrical Ag nanoparticles and DBR together,the maximum enhancement factor of the solar cell is obtained as 4.83 in the simulation.