A detailed study of H2 plasma passivation effects on GaAs/Si solar cell

The hydrogen plasma passivation effects of MOCVD-grown GaAs solar cell on Si substrate have been studied in detail. To get a more reproducible increase of conversion efficiency and test the thermal stability of the plasma-exposed GaAs/Si solar cell, both the plasma exposure and post-passivation annealing conditions were optimized. Annealing the H₂ plasma passivated GaAs/Si solar cell at 450°C in AsH₃/H₂ ambient seems a very essential parameter to restore the carrier concentration, especially, without losing the beneficial effects of H incorporation into GaAs on Si. For the H₂ plasma passivated GaAs/Si solar cell, a highest conversion efficiency of 18.3% was obtained compared with that of the as-grown cell (16.6%) due to the H passivation effects on nonradiative recombination centers, which increased the minority carrier lifetime.     

Thin film silicon solar cells on glass by substrate thinning

We report on the fabrication of thin film Si solar cells on glass by substrate thinning. We use thin Si films grown on thick Si substrates by either liquid phase epitaxy or chemical vapour deposition. A novel solar cell device fabrication process is then applied to the structure, in which the Si is thinned down to 20–30 μm leaving the grown Si film as the majority of the active material of the structure. We obtain a conversion efficiency of 14.4% for such a thin film Si solar cell on glass.     

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.     

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.     

Physical models for predicting the performance of Si/Si, AlGaAs/GaAs, and Si/SiGe solar cells

Analytical and physical models for homojunction and heterojunction solar cells are developed, and the performances of solar cells made by the Si/Si homojunction and made by the increasingly important and popular AlGaAs/GaAs and Si/SiGe heterojunctions compared. The models developed, which include relevant device physics such as the effective surface recombination velocity at the high-low junction and band discontinuities associated with heterojunctions, correctly explain the solar cell characteristics experimentally observed. Our calculations suggest that the highest efficiencies attainable for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, with optimized doping concentrations but without surface passivation and geometry optimization, are 21.25%, 17.8% and 13.5%, respectively, under 1 AM1.5 sun condition. For concentrator cell applications, the efficiencies improve to about 24.5%, 22.2%, and 22.0% for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, respectively, under 100 AM1.5 suns. While the AlGaAs/GaAs cell possesses the highest efficiency among the three cells, the Si/Si and Si/SiGe cells can achieve a satisfactory conversion efficiency at high sun concentration (22% at 100 suns), making them attractive for concentrator cell applications because their processing is the same as or is compatible with existing silicon technology. Model predictions for two Si/Si and one AlGaAs/GaAs cells compare favorably with data reported in the literature.     

Heteroepitaxial technologies on Si for high-efficiency solar cells

The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs/Si tandem solar cell have been studied with varying the growth conditions. The crystal quality of the AlGaAs layer was evaluated by photoluminescence, deep level transient spectroscopy, time-resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs/Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2% and 21.4% (AMO and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configuration, respectively, by the perfect photocurrent matching between the top cell and the bottom cell.     

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).     

Improvement of the MOCVD-grown InGaP-on-Si towards high-efficiency solar cell application

The thermal cycle annealing (TCA) for GaAs layer grown on Si substrate (GaAs/Si) increased the photoluminescence (PL) intensity of InGaP epilayer which was regrown on the GaAs/Si substrate by about 100 times. The full-width at half-maximum (FWHM) of double-crystal X-ray diffraction (DXRD) was decreased from 313 to 251 arcsec. From the electron-beam-induced current (EBIC) image measurements, the defect-related dark spots density (DSD) of the regrown InGaP layer was reduced by about 30% by using TCA GaAs/Si substrate. This means that TCA treatment for GaAs layer effectively increased the crystal quality of InGaP epilayer regrown on GaAs/Si substrate (InGaP/GaAs/Si). The PL intensity of InGaP epilayer was also enhanced due to the passivation of the residual defect-related nonradiative recombination centres by post-growth phosphine (PH₃/H₂=10%) plasma exposure.     

Photovoltaic characteristics of silicon nanowire arrays synthesized by vapor-liquid-solid process

Silicon nanowire (SiNW) arrays were grown directly on a P type Si substrate, pre-deposited with gold catalyst, and then made into solar cell for photovoltaic characteristic measurement. Different growth conditions of SiNWs, including variation of the flow rate ratio of SiH₄ versus N₂, and the thickness of Au film, which can be sputtered into different size of nanoparticles, will be made in order to obtain an optimum photovoltaic conversion efficiency. The morphologies and crystalline structure of the nanowires are studied by SEM, TEM and XRD. The SiNW array surface is shown to have good antireflection property, and is expected to raise light absorption and short circuit current. The photovoltaic performance of the solar cells with SiNWs grown at different conditions is measured and discussed. More effort is still needed to raise the performance of SiNW solar cells.     

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.     

Photoluminescence analysis of InGaP top cells for high-efficiency multi-junction solar cells

A way of evaluate the minority-carrier lifetime by using photoluminescence (PL) measurement is proposed which includes self-absorption. The room-temperature PL intensity is analyzed theoretically for bulk crystals and a device with n⁺-p junction configuration, based on a one-dimensional model. Photoluminescence analysis of In_0.5Ga_0.5P solar cells grown on GaAs and Si substrates by MOCVD (metal organic vapor deposition) have been carried out and compared with the properties of the In_0.5Ga_0.5P solar cells. By improving minority-carrier lifetime, high-efficiency In_0.5Ga_0.5P cells on GaAs substrates with an efficiency of 18.5% have been made.     

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.     

Reduction in surface recombination through hydrogen and 1-heptene passivated silicon nanocrystals film on silicon solar cells

We demonstrate reduction in surface recombination by integrating silicon (Si) nanocrystal layer on single crystalline Si solar cell. Si nanocrystals (NCs) are grown by electrochemical etching of (1 0 0) oriented p-type Si wafer. The substructures on the substrate are extracted and passivated it with hydrogen and 1-heptene molecules. Colloidal dispersion of Si NCs was spin casted on solar cell at room temperature. Apart from the I–V curve depicting the efficiency of solar cell, diffuse reflectance, measurement of short circuit current as a function of wavelength and current–voltage characteristics of solar cell were recorded with and without NCs layer. The analysis showed 9.4% increase in Si solar cell efficiency due to the surface passivation effect offered by Si NCs. Measurements of surface recombination time confirms the improved passivation by NCs.     

Cu(In,Ga)Se2 solar cells with superstrate structure using lift-off process

Cu(In,Ga)Se₂ (CIGS) solar cells with a superstrate structure were fabricated using a lift-off process. To widen the variety of substrate choices for CIGS solar cells, a lift-off process was developed without an intentional sacrificial layer between the CIGS and Mo back-contact layers. The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to an alternative SLG substrate. The conversion efficiency of the CIGS solar cells with the superstrate structure was 5.1%, which was almost half that of the CIGS solar cells with a substrate structure prior to the lift-off process. The low conversion efficiency was caused by the high series resistance and low shunt resistance, which would be due to the junction resistance between the CIGS/back contact and cracks introduced during the lift-off process, respectively.     

Characteristics of flexible indium tin oxide electrode grown by continuous roll-to-roll sputtering process for flexible organic solar cells

The preparation and characteristics of flexible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates using a specially designed roll-to-roll sputtering system for use in flexible organic solar cells are described. It was found that both electrical and optical properties of the flexible ITO electrode were critically dependent on the Ar/O₂ flow ratio in the continuous roll-to-roll sputter process. In spite of the low substrate temperature (<50 °C), we can obtain the flexible ITO electrode with a sheet resistance of 47.4 Ω/square and an average optical transmittance of 83.46% in the green region of 500-550 nm wavelength. Both X-ray diffraction and field emission scanning electron microscopy analysis results showed that all flexible ITO electrodes grown on the PET substrate were amorphous with a very smooth and featureless surface, regardless of the Ar/O₂ flow ratio due to the low substrate temperature, which is maintained by a cooling drum. In addition, the flexible ITO electrode grown on the Ar ion-beam-treated PET substrates showed more stable mechanical properties than the flexible ITO electrode grown on the wet-cleaned PET substrates, due to an increased adhesion between the flexible ITO and the PET substrates. Furthermore, the flexible organic solar cell fabricated on the roll-to-roll sputter-grown flexible ITO electrode at an optimized condition exhibited a power conversion efficiency of 1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process in preparing flexible transparent electrodes for flexible solar cells or displays.     

GaInP single junction and GaInP/GaAs two junction thin-film solar cell structures by epitaxial lift-off

The epitaxial lift-off (ELO) technique was used in forming a thin-film GaInP/GaAs two-junction monolithic tandem solar cell structure. First, the GaInP single junction solar cell to be used in the tandem cell structure as a top cell was thinned by the ELO process. Although the ELO process and the transfer to the quartz substrate caused a strain in the thin-film cell after separation from the GaAs substrate, the photoluminescence peak intensity was not decreased. This shows that defects, such as those causing carrier loss, were not introduced on the thin-film cell during the thinning process. The key issue for thin-film cell fabrication is to avoid damaging the AlInP window layer during the selective etching (HF etchant), by which the thin-film cell is released from the GaAs substrate. A GaInP/GaAs monolithic tandem structure was also thinned by the same process with a GaInP single junction cell. Characteristics of the single-junction GaInP cell and individual cells in the GaInP/GaAs tandem structure were examined. It was found that the spectral response remains almost the same as that for cells with a GaAs substrate, thus confirming the feasibility of using the ELO process to fabricate thin-film GaInP/GaAs cells.     

Polythiophene-GaAs p-n heterojunction solar cells

Thin layers of poly (3-methylthiophene) have been grown on n-type GaAs substrates. The p-type character of the undoped polymer leads to the formation of a p-n heterojunction. An electrochemical polymerization technique is used, which allows the deposition of thin (25 nm) films. The solar cell is completed by the evaporation of a gold overlayer that gives an ohmic contact to the polymer. The reported heterojunction differs from already described organic-on-inorganic structures in that most of the incident light is absorbed within the GaAs substrate and not in the polymer front layer. Comparison with an Au/GaAs junction shows that the slight decrease of the short-circuit current, due to the absorption of light by the polymer, is overcompensated by a 57% increase of the open-circuit voltage. The net power efficiency under a 100 mW/cm² tungsten lamp illumination is 3.5%. After correction for the light-absorption by the gold overlayer, an internal efficiency of 17.5% is estimated. The improvement is attributed mainly to the reduction of the dark direct current.     

Simple and fast fabrication of a-Si:H/c-Si hetero-junction solar cells by dual-chamber hot wire chemical vapor deposition

One of the fabrication issues in hetero-junction crystalline Si solar cells is the overhead time between the deposition steps of the top and bottom surfaces, because flipping of the progressing wafer is necessary to process the both sides of the wafer. To reduce the overall processing time by reducing the overhead time, we propose a dual-chamber deposition system, where thin films on the top and bottom surfaces of the Si wafer are simultaneously deposited. We have evaluated the proposed deposition system by demonstrating fabricated hetero-junction crystalline Si solar cells, which were compared with solar cells fabricated by a conventional plasma-enhanced chemical deposition system. We have obtained the power conversion efficiency of 15.5% from solar cells fabricated by our dual-chamber system; and additional analyses confirmed that the proposed dual-chamber system is, in principle, competitive with conventional systems in terms of the fabricated solar cell performance. This novel concept for the fabrication of a hetero-junction crystalline Si solar cell is expected to lay an important foundation in the future thin film crystalline Si based photovoltaic industry.     

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl₃ flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.     

Characterisation of single crystalline silicon grown by Czochralski method

Abstract

The conversion efficiency of solar cells made from single crystalline Si is generally about 3–4% higher than those made from Si multicrystal by the casting method. In this paper, the single crystalline Si obtained by the Czochralski method using metallurgical grade silicon raw materials was characterised by minority carrier lifetime, optical microscopy, the concentration of impurities and the conversion efficiency. The influence of crystalline defects and impurity elements on the electrical property has been investigated. We can conclude that both defects and impurity elements play important roles in the deterioration of the minority carrier lifetime. The conversion efficiency of solar cell using the middle wafer can reach 11·39%.