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.     

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.     

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。     

Optimization of interface structures in crystalline silicon heterojunction solar cells

In heterojunction solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si), suppression of epitaxial growth at the heterointerface is found to be crucial to achieve high solar cell efficiencies. In order to avoid the epitaxial growth, wide-gap hydrogenated amorphous silicon oxide (a-SiO:H) has been applied to the heterojunction solar cells. We have fabricated a-SiO:H/c-Si solar cells using n-type and p-type c-Si substrates and demonstrated that incorporation of the a-SiO:H i layer prevents the harmful epitaxial growth at the heterointerface completely.     

Radiation resistance of MBE-grown GaInP/GaAs cascade solar cells flown onboard Equator-S satellite

The radiation resistance of GaInP/GaAs cascade solar cells grown by molecular beam epitaxy (MBE) was tested onboard Equator-S satellite. The short-circuit current, open-circuit voltage, and power data were obtained for a period of about half a year. The remaining factors of these parameters were determined at the standard end-of-life (EOL) condition of an equivalent dose of 1×10¹⁵ cm⁻² 1 MeV electrons. Electron irradiation tests were also performed in the laboratory. Consistent results were obtained with flight and laboratory data. The remaining power at the EOL condition was 0.89–0.90 for these cells.     

Properties of tandem-junction heterophotoconverters with GaAs <Emphasis Type="Italic">p–n</Emphasis> junctions under exposure by bilaterally concentrated sunlight

Properties of GaAs/Al_xGa_1–xAs heterophotoconverters fabricated on two sides of monocrystal plates from GaP and GaAs under lighting conditions by V-shaped concentrators are described. It it found that, owing to the increased transparency of the photoconverter structure with respect to thermal photons and comparatively low GaP thermal resistance, the temperature increment of the p–n junctions and relative losses of the electrical power are notably lower than for photoconverters of the same structure on the basis of GaAs.     

Light-splitting photovoltaic system utilizing two dual-junction solar cells

There are many difficulties limiting the further development of monolithic multi-junction solar cells, such as the growth of lattice-mismatched material and the current matching constraint. As an alternative approach, the light-splitting photovoltaic system is investigated intensively in different aspects, including the energy loss mechanism and the choice of energy bandgaps of solar cells. Based on the investigation, a two-dual junction system has been implemented employing lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells grown epitaxially on GaAs and InP substrates, respectively.     

High-lifetime GaAs on Si using GeSi buffers and its potential for space photovoltaics

Record-high minority carrier lifetimes exceeding 10 ns are reported for GaAs grown on Si wafers using compositionallygraded GeSi buffers coupled with monolayer-scale control of the GaAs/Ge interface nucleation during molecular beam epitaxy. The GaAs layers are free of anti-phase domain disorder, with threading dislocation densities at or below 2×10⁶ cm⁻². Secondary ion mass spectroscopy (SIMS) reveals that crossdiffusion of Ga, As and Ge at the GaAs/Ge interface formed on the graded buffers are below detection limits in the interface region. Test diodes yielded excellent I–V characteristics, matching those of GaAs diodes on Ge and GaAs wafers, indicating that to a first order, threading dislocations do not limit device performance.     

Improvement of life time of minority carriers in GaAs epi-layer grown on Ge substrate

A buffer layer structure on Ge substrate was studied for MOCVD growth of a high-quality GaAs layer. The buffer layer structure was designed taking into consideration both lattice constants and thermal expansion coefficients of GaAs and Ge. It consisted of a preliminarily grown thin layer of Al_xGa_1−xAs and a GaAs layer. Photoluminescence (PL) decay of a GaAs layer in an Alo_0.2Ga_0.8As-GaAs-Al_0.2Ga_0.8As double-hetero (DH) structure, which was grown on the buffer layer structure, was observed by time-resolved PL method to estimate the quality of epilayers in the DH structure. The PL decay time strongly depended on Al content (x) of the Al_xGa_1−x As preliminary layer, and the highest value was obtained when the x was 0.25. A PL decay time above 20 ns was successfully obtained for the DH structure grown on the buffer layer structure, which consisted of a 0.05 μm thick Al_0.25Ga_0.75As layer and a 1 μm thick GaAs layer. Although this value was half of that for the DH structure grown on GaAs substrate, it was much longer than the value of 3 ns for the DH structure grown on Ge substrate with a conventional GaAs buffer layer 1 μm thick.     

Al0.3Ga0.7As/GaAs concentrator solar cells

An Al_0.3Ga_0.7As/GaAs tandem solar cell was fabricated in a commercial reactor which has been specially modified for dual operation of both atomic layer epitaxy (ALE) and metalorganic chemical vapor deposition (MOCVD) growth modes. The p-type and n-type dopants were carbon and silicon, respectively, and the required doping concentrations were achieved by optimizing growth conditions such as V/III ratio (mole ratio of group V atoms to group III atoms), exposure times to reactant gases, and growth temperatures. The current-voltage (I–V) characteristics indicate that, up to 53 Suns, the tunnel junction does not seem to result in any appreciable deterioration in the multijunction solar cell's performance. The measured efficiency increases with increasing solar concentration up to a point where a region of negative resistance starts to appear in the I–V characteristics.     

Short-circuit current enhancement in Bragg stack multi-quantum-well solar cells for multi-junction space cell applications

GaInP/GaAs tandem cells are limited by the current generated in the bottom GaAs junction. Strain-balanced multi-quantum well (MQW) solar cells offer a way of achieving a lower band gap for the lower junction, whilst retaining the lattice parameter of GaAs, and avoiding non-radiative recombination through dislocations. Further, the addition of a distributed Bragg reflector (DBR) allows the possibility of light not absorbed by the wells being reflected back into the structure, whilst allowing sub-well band-gap light through to a third Ge junction. Experimental results are presented from MQW cells grown with and without DBRs. These show a higher internal quantum efficiency in the 880 nm–1 μm region without detriment to the bulk response, when compared to MQW cells without DBRs.     

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.     

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.     

Electrochemically etched triangular pore arrays on GaP and their photoelectrochemical properties from water oxidation

Large-scale, triangular pore arrays on GaP were successfully prepared via a simple and high-efficient approach of electrochemical etching under high field. The obtained ordered porous GaP exhibited high performance in the photoelectrochemical (PEC) properties compared with bulk GaP. The photocurrent of the porous GaP exceeded one order of magnitude higher than that of bulk material under 0.1 V compared to the reversible hydrogen electrode (RHE), which indicated the porous structure could enhance photoresponse and facilitate the separation of photo-induced carrier charges and their collection. The structure of triangular pore arrays cooperated with its depth determined the PEC performance of GaP. The optimal etching depth was obtained via testing the PEC performance. The hydrogen production from bulk GaP and its porous structure material were also tested from water splitting. Upon the porous structure, significantly enhanced hydrogen production has also been observed, which indicated that the porous GaP should have important potential in photocatalytic water splitting.     

Application of real-time in situ spectroscopic ellipsometry and infrared spectroscopy for characterizing interface structure of a-Si:H layer

We have applied real-time in situ spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) to investigate a-Si:H nucleation process on substrate that affects the resulting a-Si:H/substrate interface structure significantly. The analyses of these real-time measurements show the formation of a 30 Å thick H-rich interface layer having an average hydrogen content of 20 at.% on a c-Si substrate covered with native oxide (30 Å). This interface layer formation is primarily caused by the H-rich three-dimensional island growth on the substrate. We found a weak dependence of interface layer properties on a-Si:H deposition conditions. This result suggests that the interface layer formation is controlled by the nucleation site density of a-Si:H islands on the substrate, rather than plasma conditions.     

The vertical coupling effect on the electronic states in self-assembled InAs/GaAs quantum dots

A series of self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy to investigate the dependence of transition energy on GaAs spacer layer thickness. The latter was varied of 60, 45, 30, 15, and 10 monolayers (MLs) for the five different samples. The photoluminescence (PL) measurements were carried out. The electronic states in coupled self-assembled InAs/GaAs QDs are investigated through PL properties with the aid of the theoretical calculation. First the energy levels of electrons and holes are calculated by solving the three-dimensional Schrödinger equation by considering the vertical coupling effect between a finite numbers of QDs. Based on the results the energies transitions between electrons and holes levels are calculated. Modification of PL spectra by increasing number of layers was found and attributed to an increasing vertical coupling. The PL full-width at half-maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 MLs. Moreover, the observed behavior PL lines is analyzed theoretically.     

Use of Sb spray for improved performance of InAs/GaAs quantum dots for novel photovoltaic structures

Photoluminescence output from InAs/GaAs quantum dots has been improved by a Sb treatment immediately prior to capping with GaAs. Spectra taken at 300 and 80 K show a significant increase in output intensity when the quantum dots are exposed for 15 s under a Sb flux of approximately 0.1 monolayers per second, but this improvement is lost when the Sb exposure is extended to 30 s. There is no significant shift in the emission energies between samples indicating strain relief due to the cap layer is not responsible for the improvement. Analysis of temperature dependent photoluminescence taken between 80 and 300 K show increased activation energies at lower temperatures when an Sb spray is used, suggesting passivation of deep defect levels. For the higher temperature activation energy, corresponding to carrier escape from the QD to the barrier, whilst a 15 s Sb spray gives a substantial increase, the longer 30 s Sb spray sees the activation energy decrease, a result deduced to be due to Sb segregation providing shallow defect levels. A band structure including a very thin GaAsSb layer adjacent to the quantum dots is used to explain these results, with the 30 s Sb spray leading to shallow Sb segregation related defects and a lower activation energy. Depth dependent X-ray photoelectron spectroscopy data support the band structure proposed to explain the photoluminescence results and also reveals the highest concentration of Sb at the sample surface suggesting a ‘floating layer’ of Sb during growth of the GaAs cap. Some of the implications of these results, for growth of quantum dot samples and for two novel solar cell proposals, the intermediate band and hot carrier solar cells, are discussed.     

Suppression of melt convection in a proposed Bridgman crystal growth system

Numerical simulations are performed for Bridgman crystal growth of several semiconductor materials, such as InAs, InSb, GaSe, CdTe, PbTe, and GaP. For materials with low Prandtl and low Grashof numbers, melt convection is weak and the traditional Bridgman technique is a suitable growth process. For the materials with high Prandtl numbers in their melt status and the growth system with high Grashof number, the temperature field and the growth interface are significantly influenced by melt flow, resulting in the complicated flow pattern and curved interface shape. A new Bridgman crystal growth system is proposed to suppress convection and improve solidification interface shape by cooling of the top melt. The results obtained from the proposed design demonstrate that melt convection may be controlled by adjusting the design parameters. Further, parametric studies are performed to determine the influence of the control parameters on melt flow and solidification interface.     

Hydrogen evolution reaction electrocatalysis trends of confined gallium phosphide with substitutional defects

The integration of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gives better prediction of the system properties towards the applications like water-splitting and gas storage capacity/mechanism. The pursuit of generating low-cost and effective catalyst for such purposes has motivated the material scientists and researchers to design and study the novel nanostructured materials both theoretically and experimentally. We have utilized the well-established state-of-the-art density functional theory (DFT) for envisaging the HER activity of the two-dimensionally confined Gallium Phosphide (GaP). The effect of substitutional defect caused by foreign atoms like boron and nitrogen on the structural, electronic and adsorption properties of the GaP nanowire is analyzed by incorporating the van der Waals dispersion correction. The energy differences and the contributions of the individual atomic species to the electronic energy states have been observed by computing the electronic density of states. Introduction of the defect in the system significantly modifies the electronic and adsorption properties of the system. The results suggest GaP to be highly active for hydrogen adsorption which further gets pronounced by introducing boron defect in the system. The results on adsorption energy and Gibbs free energy stipulating better adsorptive nature for hydrogen give confidence to utilize GaP as an HER catalyst by further tuning the adsorption response by means of defect engineering. In a nut-shell, we assert the dependence of material properties that are very sensitive to defects and the cause root beneath this response can serve as a blueprint for designing prominent materials for HER based applications.