26.1% thin-film GaAs solar cell using epitaxial lift-off

The epitaxial lift-off technique can be used to separate a III-V solar cell structure from its underlying GaAs substrate. Processing a thin-film cell is somewhat different from a regular cell on substrate. In this work a number of critical issues, e.g., a low-temperature anneal front contact and the metal mirror on backside of the thin-film are optimized. Together with an improved active layer material quality, grid mask and anti-reflection coating this leads to thin-film cells as good as cells on a substrate, with record efficiencies for single junction GaAs solar cells of 26.1% for both cell types.     

Optimal growth procedure of GaInP/GaAs heterostructure for high-efficiency solar cells

We have developed an optimal growth procedure for gas-source MBE production of a GaInP/GaAs heterointerface. The interface quality is crucial to obtaining high-performance GaAs solar cells with a GaInP barrier layer because minority carrier lifetime depends strongly on the interface structure. In situ Reflective High-Energy Electron Diffraction (RHEED) observation during the growth across the GaInP/GaAs heterointerface revealed that the phosphorus atoms are replaced by arsenic atoms in the near-interface region of the GaInP layer, and a transient layer acting as a carrier trap is formed. Introduction of a GaP layer into the interface was found to be effective in suppressing carrier loss. From Composition Analysis by Thickness Fringe-Transmission Electron Microscopy (CAT-TEM) images, it was also found that the optimum thickness of inserted GaP to avoid the generation of misfit dislocations is 1 nm.     

Process damage free thin-film GaAs solar cells by epitaxial liftoff with GaInP window layer

We report on process damage free thin-film GaAs cells detached from the GaAs substrates. GaAs cells grown by gas-source MBE were thinned by the epitaxial liftoff (ELO) technique. Photoluminescence spectroscopy showed a peak splitting in the band emission, indicating that a strain was induced in the thin-film cell fixed on the quartz glass substrate. The strain, however, was found not to affect the quality of the thin-film cells, based on the fact that the peak intensity was almost twice that before ELO. The thin-film GaAs cells showed no evidence of degradation in diode characteristics and spectral responses. The keys to avoiding damage on the active region of the solar cell during the thinning process are the introducing a GaInP window layer and improving the thin film process including metallization on thin film cells. These results demonstrates that the thinning and transfer processes dol-not affect the quality of the active region of the cells.     

用于GaInP/GaAs双结叠层太阳电池的MOVPE外延材料

采用金属有机化合物气相淀积(MOVPE)技术生长用于GaInP/GaAs双结叠层太阳电池的外延材料。针对材料生长中的隧穿结、p-AlInP层等关键问题,通过对掺杂剂、生长技术及条件的调整与改进,极大地提高了GaInP/GaAs双结叠层太阳电池性能,最高转换效率达到23.8%(AMO,25℃)。     

Energy requirements of thin-film solar cell modules-a review

In this paper a number of energy analysis studies for thin-film solar cell modules are compared and reviewed. We start with a short introduction into methodological issues related to energy analysis (of PV systems) such as system boundary definition, treatment of different (secondary) energy types and the choice of functional unit.Subsequently we review results from 6 studies on a-Si modules and 3 studies on CdTe modules. The aim is to present results in a unified format, compare them and try to clarify observed differences. Although significant differences were found, many of these differences could be explained by the choice of materials for the module encapsulation. For categories with large observed differences, like indirect process energy and capital equipment energy, we performed additional analyses in order to gain a better understanding of these aspects.Finally we present best estimates of the energy requirement for present-day a-Si and CdTe thin film modules which are between 600 and 1500 MJ (primary energy) per m^fn3 module area, depending on cell and encapsulation type. This means that the energy pay-back time is below two years for a grid-connected module under 1700 kWhm²yr irradiation. In the near future an energy pay-back time below one year seems feasible.     

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).     

Thin film GaAs solar cells with increased quantum efficiency due to light reflection

Thin film GaAs solar cells, separated from their substrate using the weight-induced epitaxial lift-off technique, were compared with conventional cells on a substrate. The thin film cells can be illuminated from both sides using a mirror. The thickness of the p-type GaAs layer, which is the base layer for front illumination and the emitter layer for rear illumination, was varied between 0.25 and 2.5 μm. For both front and rear illumination, the cell efficiency shows a maximum at a thickness of 1.5 μm. The rear illuminated cell current is only 10% lower than for front illumination. Light reflection in the thin film cell enhances the external quantum efficiency and the collection efficiency in the higher wavelength region from 0.84 to 0.90 and from 0.82 to 0.95, respectively.     

GaAs/Ge solar cell AC parameters under illumination

The ac parameters of GaAs/Ge solar cell were measured under illumination at different cell temperatures using impedance spectroscopy technique. They are compared with the dark measurements. It is found that the cell capacitance is higher and cell resistance is lower under illumination than in dark for all cell terminal voltages. The cell capacitances at the corresponding maximum power point voltage (terminal) do not vary with temperature where as the cell resistance decreases. The cell capacitance under illumination is estimated from the dark cell capacitance and it is in good agreement with the measured illumination data.     

RF sputtered indium-tin-oxide as antireflective coating for GaAs solar cells

Conductive and antireflective indium-tin-oxide (ITO) has been prepared by RF sputtering in Ar atmosphere, without introducing oxygen into the plasma and on room temperature substrates in order to be used as antireflective coating on GaAs solar cells. The electrical resistivity of the n-type, degenerate ITO films exhibited a reduction with deposition rate and an increase with total pressure, while it was independent of the film thickness in the range of 20 nm to 130 run. Further reduction of resistivity, up to 4 × 10⁻⁴ Ωcm, was obtained by annealing at 400°C. This is the lowest resistivity that has been reported for TTO films prepared under similar conditions. The transmittance of 90 nm thick ITO film was 85% and the reflectance of p/n GaAs solar cell was reduced from 35% to 2% after the ITO layer application.     

Characterization of crystalline silicon grown by plasma-enhanced CVD for thin-film solar cells

High growth-rate Si epitaxy by plasma-enhanced chemical vapor deposition (PECVD) has been investigated for a thin-film solar cell application. A high growth rate of 50 μm/h was obtained at 1050°C with plasma which is 50% larger than that by the conventional CVD without plasma. The electrical properties are almost the same for epitaxial layers with and without plasma. For undoped n-type layers, the Hall mobility and carrier density were about 600 cm²/V s and low 10¹⁵ cm⁻³, respectively. The electron diffusion length in doped p-type layers was about 20 μm. These electrical properties for the layer with plasma, in spite of higher growth rate, are comparable or better than those without plasma.     

GaAs/Ge solar cell AC parameters at different temperatures

The AC parameters of Gallium Arsenide (GaAs/Ge) solar cell were measured at different cell temperatures (198–348 K) by varying the cell bias voltage (forward and reverse) under dark condition using impedance spectroscopy technique. It was found that the cell capacitance increases with the cell temperature where as the cell resistance decreases, at any bias voltage. The measured cell parameters were used to calculate the intrinsic concentration of electron–hole pair, cell material relative permittivity and its band gap energy. The diode factor and the cell dynamic resistance at the corresponding maximum power point decrease with the cell temperature.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.     

Effects of grain boundaries in polycrystalline silicon thin-film solar cells based on the two-dimensional model

The two-dimensional calculation for polycrystalline Si thin-film solar cells was performed. Two models, “stripe structure” and “columnar structure”, were applied for the solar cells composed of grains. For the stripe structure of 20 μm active layer, to keep the efficiency distribution within 5% for individual unit cells, the stripe width requires more than 500 μm for a minority-carrier lifetime of 1×10⁻⁵ s and recombination velocity at the grain boundary of 1×10⁴ cm/s. For the columnar structure of 10 μm active layer, to keep the efficiency independent of grain size, the recombination velocity should be kept less than 1×10³ cm/s. If imperfect passivation of a grain boundary is given, the way of decreasing carrier concentration to 10¹⁴ cm⁻³ in an active layer may realize insusceptible output. An appropriate device modeling is needed in the two-dimensional calculation for polycrystalline Si thin films with an electron diffusion length close to or more than grain size and with a poorly passivated grain boundary. The calculated efficiency using bad model will include an error of about 1% as overestimation.     

Progress in chalcopyrite compound semiconductor research for photovoltaic applications and transfer of results into actual solar cell production

This paper gives an overview of the main research directions in chalcopyrite material research and the application of results for the improvement and fabrication of solar cells. So far the copper indium gallium sulphur selenide material family is the base for the highest efficiency thin-film solar cells and the most advanced in terms of actual commercialisation. The transfer of research results into actual production from its early stage and the development of the chalcopyrite thin-film solar cell industry are sketched. The last part of the review shortly describes a number of current industrial players involved in the manufacturing of chalcopyrite solar cells.     

Recent technical advances in thin-film CdS/CdTe solar cells

CdS/CdTe solar cells have attracted attention recently for their potential as low-cost, high-efficiency solar cells of the future. It is because the CdTe layer (used for photoelectric conversion) has a bandgap energy of 1. 51 eV, which corresponds well to sunlight spectra, and the direct transition type energy band structure enables formation of thinner films.We have already industrialized CdS/CdTe solar cells in mass production stage using a printing-sintering process, as large-area modules for electric power generation(Higuchi , 1993, Omura , 1991), and as cells for indoor applications (primarily in calculators. Suyama , 1986). However, this solar cell has a conversion efficiency of approximately 6%.Recently, there has been considerable research into thin-film CdS/CdTe solar cells which have a thinner CdS film formed by CVD or CBD (Britt , 1993) process, and thus are photosensitive to light with wavelengths of 500 nm or less. At present stage of our art, in solar cells formed by the CSS with a CdTe film on CVD CdS, a conversion efficiency of 15. 05% has been obtained in cells with an area of 1 cm² (verified at JQA).     

Catalytic hydrogenation for improvement of GaAs/Ge solar cell efficiency

Hydrogen passivation on MOCVD grown p-GaAs epilayers on Ge substrate have been studied by plasma and catalytic hydrogenation and the results were compared. The conversion efficiency of the GaAs/Ge solar cells was found to increase by 10% after catalytic hydrogenation at AM1.5. This increase in efficiency is probably due to passivation of surface dangling bonds.     

Thin film solar cells of CdS/PbS chemically deposited by an ammonia-free process

A new type of solar cell with structure glass/ITO/CdS/PbS/conductive graphite was constructed and studied. Both window (CdS) and absorption (PbS) layers were deposited by means of the chemical bath deposition (CBD) technique. The maximum temperature employed during the solar cell processing was 70 °C and it did not include any post-treatment. In case of the CdS window layer, complexing agents alternative to ammonia were employed in the CBD process and their effects on the CdS films properties were studied. The solar cells are photosensitive in a large spectral range (all visible and near infrared regions); the cell with the area of 0.16 cm² without any special treatment has shown the values of open-circuit voltage V_oc of 290 mV and short circuit current J_sc of 14 mA/cm² with the efficiency η=1.63% (fill factor FF is 0.36) under illumination intensity of 900 W/m². It was found that the CBD-made PbS layer has a certain degree of porosity, which favorably affects its applicability in solar cell construction. The possible ways of device optimization, and in particular, the effect of the PbS grain size on its performance are discussed.     

Extreme radiation hardness and light-weighted thin-film indium phosphide solar cell and its computer simulation

A very light-weighted and extreme radiation hardness high-doping n⁺-i-p⁺ InP solar cell is developed. The total thickness of its epitaxial layer is only 0.22 μm. It is more radiation hardened than many other structures so that it can provide the required output power for spacecraft even after a decade at 3200 km polar orbits. Its end of life efficiency is about 10% (AM0, 1Sun); its highest power/weight ratio is about 130 W/g (only the weight of epitaxial layers is considered). The surprising fact is that all of these are obtained from a very simple structure.     

Vapor phase epitaxial liftoff of GaAs and silicon single crystal films

Among the technologies for integrating GaAs devices with Si VLSI chips, epitaxial liftoff (ELO) is conspicuous for maintaining the quality of the single crystal epitaxial GaAs films. Traditionally, ELO is implemented in aqueous HF solution. It would be cleaner and simpler if ELO could be implemented in a vapor process. In this article, we will present the potential improvements in the ELO process by using a vapor phase etch to undercut thin films.     

Diamond/CdTe: a new inverted heterojunction CdTe thin film solar cell

Recently the concept of the inverted photovoltaic cell has become more attainable as a practical cell. This thin film cell consists of a p–n heterojunction in which the window layer is p-type and the absorber layer is n-type. The feasibility of a new inverted p–n heterojunction p-diamond/n-CdTe solar cell has been demonstrated. The non-optimized solar cell structure grown on semi-transparent p-diamond yielded an open circuit voltage of 0.23 V and a short circuit current of 1.54 mA/cm² when illuminated.