Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Super high-efficiency multi-junction and concentrator solar cells

III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications.We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm²) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.     

Novel materials for high-efficiency III–V multi-junction solar cells

As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm². In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.     

Annual output estimation of concentrator photovoltaic systems using high-efficiency InGaP/InGaAs/Ge triple-junction solar cells based on experimental solar cell's characteristics and field-test meteorological data

The temperature dependences of the electrical characteristics of InGaP/InGaAs/Ge triple-junction solar cells under concentration were evaluated. For these solar cells, conversion efficiency (η) decreased with increasing temperature, and increased with increasing concentration ratio owing to an increase in open-circuit voltage. The decrease in η with increasing temperature decreases with increasing concentration ratio. Moreover, the annual output of a concentrator system with a high-efficiency triple-junction cell was estimated utilizing the experimental solar cell's characteristics obtained in this study and field-test meteorological data collected for 1 year at the Nara Institute of Science and Technology, and compared with that of a nonconcentration flat-plate system.     

Over 27% efficiency GaAs/InGaAs mechanically stacked solar cell

We have applied an InGaAs solar cell (band GAP = 0.75 eV) to the bottom cell of the super-high-efficiency tandem solar cell aiming an over 35% conversion efficiency. The InGaAs cell which is lattice-matched to the InP substrate showed the efficiency of 5.5% under the GaAs substrate with low carrier concentration. Combining with the GaAs cell by means of a mechanically stacking technique, we obtained an efficiency of 28.8% at air mass (AM) 1.5, 1-sun. This result suggests the possibility of the cells with the efficiency of over 35% with combining a GalnP/GaAs monolithic tandem cell and the InGaAs cell (or InGaAsP cell).     

Evaluation of temperature characteristics of high-efficiency InGaP/InGaAs/Ge triple-junction solar cells under concentration

Temperature characteristics of the open-circuit voltage (V_oc) were investigated in the temperature range from 30°C to 240°C for the InGaP/InGaAs/Ge triple-junction cells. Also, single-junction cells that had the similar structure to the subcells in the triple-junction cells were studied. In the high-temperature range (from 170°C to 240°C), the temperature coefficients of V_oc of the InGaP/InGaAs/Ge triple-junction solar cell (dV_oc/dT) were different from those in the low-temperature range (from 30°C to 100°C). This is because photo-voltage from the Ge subcell becomes almost 0 V in the high-temperature range. It was found that the open-circuit voltage of a Ge single-junction cell reduced to almost 0 V temperatures over 120°C under 1 sun condition.     

Evaluation of InGaP/InGaAs/Ge triple-junction solar cell and optimization of solar cell's structure focusing on series resistance for high-efficiency concentrator photovoltaic systems

The series resistance of an InGaP/InGaAs/Ge triple-junction solar cell was evaluated in detail. Series resistance components such as electrode resistance, tunnel junction resistance and lateral resistance between electrodes were estimated separately. The characteristics of the triple-junction solar cell under concentrated light were evaluated by equivalent circuit calculation with a simulation program with integrated circuit emphasis (SPICE). By equivalent circuit calculation, the optimization of cell designs was performed, focusing on series resistance and cell current in order to realize high-efficiency concentrator cells.     

Numerical and experimental research of the characteristics of concentration solar cells

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (I_sc) and the open-circuit voltage (V_oc) of multi-junction solar cells increases with the increasing concentration ratio, while the cell efficiency (η_c) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, I_sc increases, while V_oc and η_c decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.     

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells with a world-record efficiency of 26.9% (AM0, 28°C) has been evaluated by 1 MeV electron irradiation. Degradation in tandem cell performance has been confirmed to be mainly attributed to large degradation in the GaAs bottom cell. Similar radiation resistance with GaAs-on-Ge cells has been observed for the InGaP/GaAs tandem cell. Moreover, recovery of the tandem cell performance has been found due to minority-carrier injection under light illumination or forward bias, which causes defect annealing in InGaP top cells. The optimal design of the InGaP base layer thickness for current matching at end of life (EOL) (after irradiation with 10¹⁵ electrons cm⁻²) has been examined.     

Characterization of carrier recombination in lattice-mismatched InGaAs solar cells on GaAs substrates

Effects of thermal annealing on carrier recombination in lattice-mismatched InGaAs solar cells on GaAs substrates were investigated. Thermal annealing to the graded buffer layer was effective to increase the minority carrier lifetime in the solar cell layer. Electron beam-induced current (EBIC) measurements revealed that the density of dark line defects decreased after the thermal annealing, but dark spot defects were newly generated. We conclude that dark line defects were primary responsible for the high recombination in the lattice-mismatched InGaAs solar cells. The origin of dark spot defects was discussed and it was found that they were associated with the lattice mismatch between the InGaP back surface field (BSF) layer and the InGaAs cell layer.     

Radiation-resistant solar cells for space use

This paper reviews the present status of radiation-resistant solar cells made with Si, GaAs, InP and InGaP/GaAs for space use. At first, properties of radiation-induced defects in semiconductor materials and solar cells are described based on an anomalous degradation of Si space solar cells under high-energy, high-fluence electron and proton irradiations. Advantages of direct bandgap materials as radiation-resistant space cells are presented. Unique properties of InP as radiation-resistant cells have also been found. A world-record efficiency of 26.9% (AM0) has been obtained for an InGaP/GaAs tandem solar cell. Radiation-resistance of the InGaP/GaAs tandem cells is described.     

Uniqueness verification of direct solar spectral index for estimating outdoor performance of concentrator photovoltaic systems

To analyze the impact of a direct spectral distribution of the solar spectrum on the outdoor performance of concentrator photovoltaic (CPV) systems, an index for the direct spectral distribution is needed. Average photon energy (APE), the average energy of a photon in the direct solar spectrum, is one of these indexes. In this contribution, the uniqueness of APE to the direct solar spectral distribution is statistically analyzed to assure that an APE value uniquely yields the shape of a direct solar radiation spectrum. The results have exhibited the uniqueness of the direct normal solar spectrum with each APE value, in which the standard deviations are quite small. Short-circuit current density of the InGaP/InGaAs/Ge triple-junction solar cell in the CPV system is additionally calculated using the direct spectral irradiance with different APE values. It is revealed that APE is a useful index to describe the direct spectral distribution to evaluate the outdoor performance of the CPV systems.     

Characteristics of GaAs solar cells on Ge substrate with a preliminary grown thin layer of AlGaAs

Characteristics of GaAs solar cell on Ge substrate with a new buffer layer structure is reported. The buffer layer structure, which consisted of a preliminarily grown thin layer of A1_xGa_1−xAs and a 1 μm thick GaAs layer, was designed to obtain a high quality GaAs layer on Ge substrate by metalorganic chemical vapor deposition (MOCVD). Performance of a GaAs solar cell fabricated on Ge substrate with the buffer layer structure was compared with that fabricated on Ge substrate with a conventional GaAs buffer layer and also that fabricated on GaAs substrate. A conversion efficiency of 23.18% (AM1.5G) was successfully obtained for the cell fabricated on Ge substrate with the new buffer layer structure, while it was 20.92% for the cell fabricated on Ge substrate with the conventional GaAs buffer layer. Values of V_oc and J_sc, for the cell fabricated on Ge substrate with the new buffer layer structure were approximately comparable to those of a 25.39% efficiency GaAs solar cell fabricated on GaAs substrate.     

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.     

Low-energy proton-induced defects in n/p InGaP solar cells

The effects of low-energy proton-induced degradation of photovoltaic properties and generation of deep-level defects in n⁺/p InGaP solar cells have been investigated. Energy-dependent effects included decreased solar cell efficiency and increase the carrier removal rate with decreasing proton energy. The spectral response depicts that the degradation is more at longer wavelengths with the increase of proton fluence. A new majority (hole) trap HP1 has been observed in low-energy proton irradiated p-InGaP at 0.90±0.05 eV above the valence band for the first time. The carrier removal rates were found to be 61433 and 8640 cm⁻¹ for 100 and 380-keV proton irradiation, respectively.     

用于高效级联太阳电池的ZnSe材料研究

研究了用于高效Znse／GaAs/Ge(硒化锌绅化镓／锗)级联太阳电池顶电池的ZnSe材料。用MBE技术制备了ZnSe p-n结样品，测量了其外量子效率；提出了改进ZnSe顶电池性能的方法；分析了ZnSe／GaAs／Ge结构比GaInP／GaAs／Ge结构的优越之处。     

Recent developments in high-efficiency Ga0.5In0.5P/GaAs/Ge dual- and triple-junction solar cells: steps to next-generation PV cells

Dual-junction Ga_0.5In_0.5P/GaAs solar cells on Ge substrates have rapidly gone from small, high-efficiency laboratory cells, to large-area, high-efficiency cells manufactured at Spectrolab in high volume. Over 500,000 of these dual-junction (DJ) cells with 27-cm² area have been produced, with average AM0 load point efficiency of 21.4%. The next step in the evolution of this type of multijunction solar cell has been taken, with the development of triple-junction (TJ) Ga_0.5In_0.5P/GaAs/Ge cells. The addition of the germanium third junction, plus several significant improvements in the device structure, have led to a measured efficiency of 27.0% (AM0, 28°C) at Spectrolab on large-area (>30 cm²) TJ cells. The TJ cell is now in production at Spectrolab. Ga_0.5In_0.5P/GaAs/Ge cells are viable not only for non-concentrating space applications, but also for terrestrial and space concentrator systems. Efficiencies up to 32.3% at 47 suns under the terrestrial AM1.5D spectrum have been achieved.     

Numerical prediction of InGaAs/GaAs MQW solar cell characteristics under concentrated sunlight

The characteristics of InGaAs/GaAs multi-quantum well (MQW) solar cells under concentrated sunlight were analyzed by self-consistent numerical calculation and were compared with GaAs p-i-n cells that have the same structure except for the QWs. Prior to the calculations, the absorption coefficients of the InGaAs MQW cell were determined by fitting the calculated quantum efficiencies to experimental ones. In the simulation, mainly the influences of concentrated sunlight and temperature were investigated. Results of the simulation showed that the rate of the conversion efficiency and open-circuit voltage (V_oc) of MQW cells increased faster than that of p-i-n cells as the light intensity increased, while the rate of efficiency of the MQW cell decreased a little slower than that of the p-i-n cell with the temperature increase. MQW cells compensate for the disadvantage of lower efficiency and V_oc than p-i-n cells in a concentrated sunlight system.     

Optimum design and its experimental approach of a-Si/ /poly-Si tandem solar cell

A series of technical data on four-terminal a-Si/ /poly-Si stacked solar cells has been reported. The developed device has some unique significances such as high achievable efficiency, and low cost with almost no light induced degradation. It has been shown on a poly-Si bottom cell that an efficiency of 17.2% has been obtained by employing high conductivity with wide optical band-gap p-type μc-SiC as a window material and n-type μc-SiC as a back ohmic contact with BSF effects. On the optically transparent a-Si top cell, an optimum design has been experimentally made with the device structure of p μc-SiC/p a-SiC/i a-Si/n μc-Si/ITO, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer, while the best efficiency is 12.3% for p-i-n single-junction solar cell with 500 nm i-layer thickness deviced by Ag back-electrode. With the 100 nm thick ultrathin top cell, a total conversion efficiency as high as 21.0% has been achieved on a-Si/ /poly-Si four-terminal tandem solar cells.     

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.