Temperature dependence of photocurrent components on enhanced performance GaAs/AlGaAs multiple quantum well solar cells

The performance of Al_0.36Ga_0.64As p/i/n solar cells with multiple quantum wells (MQW) of GaAs/Al_0.36Ga_0.64As in the i-region has been investigated at various temperatures, ranging from −10°C to 100°C, and compared with that of conventional solar cells composed of either the quantum well material (GaAs) or the barrier material (Al_0.36Ga_0.64As) alone. The dark currents of the MQW cells were found to lie between those of the conventional cells. The increase of dark current with temperature was accompanied by a slight decrease of the diode ideality factor. A linear dependence of open-circuit voltage (V_oc) on temperature was observed for all cells when illuminated with a 100W halogen lamp. V_oc for the MQW cells was found to be independent of the number of wells, lying between the V_oc's for the two conventional cells. The MQW cells exhibited performance improvement with temperature when compared to the conventional cells and there was a significant enhancement in the short-circuit current with temperature of those MQW cells that exhibited poorer performance at lower temperatures. Theoretical calculations have quantified the contribution of the tunneling current component to the total observed photocurrent at the various temperatures examined. It was found that tunneling currents are present at all temperatures and can be the dominant component in MQW cells of thinner wells at low temperatures. These results suggest that GaAs/Al_0.36Ga_0.64As MQW structures, of good-quality material, when processed as conventional solar cells with antireflective coatings should deliver more output power under intense illumination than conventional solar cells composed of the quantum well material alone.     

AlxGa1−x)0.65In0.35As monolithic multijunction solar cells

A simple graphical method is used to establish that the (Al_xGa_1−x)_0.65In_0.35As semiconductor alloy provides the range of energy-band gaps required to both maximize power conversion efficiency and achieve current-matching for two-terminal, multijunction solar cells. Within this framework, the development needs of a three junction, monolithic solar cell with lattice-matched subcells and a strain-relieved Ga_yIn_1−yAs/GaAs pseudo-substrate are discussed. The theoretical limiting efficiency of the proposed design is approximately 47.2% at 1 sun (AM 1.5 spectrum).     

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.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.     

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.     

Importance of the diffuse sky radiation in evaluation of the performance of a solar cell

Representative results of the numerical simulation of responses (viz. photocurrent and optimum power output as well as efficiency) of the conventional and violet Si cells and of the Ga_1−xAl_xAs---GaAs and GaAs cells are presented as a function of the solar zenith angle for seven different models of the terrestrial atmosphere. The atmospheric models used vary from an aerosol-free and cloud-free model with gaseous absorption to several models with moderately thick stratus cloud layer and high concentrations of aerosols. This study, restricted to horizontally situated solar cells, illustrates the manner in which characteristics are significantly affected by position of the sun, turbidity and cloudiness of the atmosphere, as well as reflectivity of the underlying surface.     

Theoretical possibilities of InxGa1−xN tandem PV structures

We designed a model of In_xGa_1−xN tandem structure made of N successive p–n junctions going from two junctions for the less sophisticated structure to six junctions for the most sophisticated. We simulated the photocurrent density and the open-circuit voltage of each structure under AM 1.5 illumination in goal to optimize the number of successive junctions forming one structure.For each value of N, we assumed that each junction absorbs the photons that are not absorbed by the preceding one. From the repartition of photons in the solar spectrum and starting from the energy gap of GaN, we fixed the gap of each junction that gives the same amount of photocurrent density in the structure. Then we calculated the current density accurately and optimized the thicknesses of p and n layers of each junction to make it give the same output current density. The evaluation of n_i: the intrinsic concentration permitted to calculate the saturation current density and the open-circuit voltage of each junction. Assuming an overall fill factor of 80%, we divided the output peak power by the incident solar power and obtained the efficiency of each structure.The numerical values for In_xGa_1−xN were taken from the relevant literature. The calculated efficiency goes from 27.49% for the two-junction tandem structure to 40.35% for a six-junction structure. The six-junction In_xGa_1−xN tandem structure has an open-circuit voltage of about 5.34 V and a short circuit current density of 9.1 mA/cm².     

Properties of GaAs/InGaAs quantum well solar cells under low concentration operation

Multi-quantum well GaAs/In_0.19Ga_0.81As solar cells have been measured under low concentration levels (1–10 suns) of AM1.5 illumination. An efficiency of 22% has been obtained at a ratio of 4 suns as opposed to 18% under 1 sun AM1.5 conditions. We explain the improvements in conversion efficiency in terms of an enhancement in minority-carrier lifetime under concentration. Even when the concentration ratio is low, the high-injection regime can be achieved since the carrier concentration in the intrinsic layer is very low. The existence of a high concentration of defects at 0.36 eV below the conduction band in the base layer has been observed by the DLTS analysis. Enhancement of the minority-carrier lifetime under concentration is thought to be due to filling of recombination centers by the injection minority carriers.     

Third generation multi-layer graded band gap solar cells for achieving high conversion efficiencies—II: Experimental results

New designs of multi-layer graded band gap solar cell structures were experimentally tested using well-understood Al_xGa_(1−x)As materials grown by the MOVPE technique. Laboratory scale devices (0.5 mm diameter) were processed and measured for their performance as solar cells. Both V_oc (1110 mV) and fill factors (83%) for the best devices have shown drastic improvements over existing cells and the short-circuit current densities measured are in the range (10–20) mA cm⁻² .     

Low cost surface passivation for p-type mc-Si based on pseudobinary alloys (Al2O3)x(TiO2)1−x

The conventional process for back side passivation with full face Al screen printing layer is not suitable for very thin multicrystalline (mc-Si) solar cells and approaches to new technological processes are searched for. More investigations have been concentrated on local aluminum contacts and passivation coatings with different layers on mc-Si wafers. The aim of this work is to prove that (Al₂O₃)_x(TiO₂)_1−x is one promising candidate to be applied as passivation layer on multicrystalline Si. Investigations were performed on dielectric films of pseudobinary alloy (PBA) (Al₂O₃)_x(TiO₂)_1−x, prepared by chemical solution deposition known initially as sol–gel method. It was determined that their optical, dielectric and electrophysical properties are suitable for applications of these layers as back side surface passivation for thin multicrystalline silicon cells.     

Fabrication of wide-gap Cu(In1−xGax)Se2 thin film solar cells: a study on the correlation of cell performance with highly resistive i-ZnO layer thickness

The correlation of the cell performance of wide-gap Cu(In_1−xGa_x)Se₂ (CIGS) solar cells with the thickness of highly resistive i-ZnO layers, which are commonly introduced between the buffer layer and the transparent conductive oxide (TCO) layer in CIGS solar cell devices, was studied. It was found that cell parameters, in particular, the fill factor (F.F.) varied with the thickness of the i-ZnO layers and the variation of the F.F. was directly related to cell efficiency. A 16%-efficiency was achieved without use of an anti-reflection coating from wide-gap (E_g1.3 eV) CIGS solar cells by adjusting the deposition conditions of the i-ZnO layers.     

Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell

This paper reports the preparation of a core-shell nanoporous electrode consisting of an inner TiO₂ porous matrix and a thin overlayer of Al₂O₃, and its application for solid-state dye-sensitized solar cell using p-CuI as hole conductor. Al₂O₃ overlayer was coated onto TiO₂ porous film by the surface sol–gel process. The role of Al₂O₃ layer thickness on the cell performance was investigated. The solar cells fabricated from Al₂O₃-coated electrodes showed superior performance to the bare TiO₂ electrode. Under illumination of AM 1.5 simulated sunlight (89 mW/cm²), a ca. 0.19 nm Al₂O₃ overlayer increased the photo-to-electric conversion efficiency from 1.94% to 2.59%.     

Numerical simulation of the effect of the Al molar fraction and thickness of an AlxGa1−xAs window on the sensitivity of a p+–n–n+ GaAs solar cell to 1 MeV electron irradiation

In this paper numerical simulation has been used to predict the effect of the thickness and aluminium (Al) mole fraction of an AlGaAs layer, used as a window for a p⁺–n–n⁺ GaAs solar cell under AM0 illumination and exposed to 1 MeV electron irradiation. Such solar cells are used in satellites and undergo severe degradation in their performance due to induced structural defects. The irradiation-induced defects are modelled as energy levels in the energy gap of GaAs. To predict this effect, the spectral response is evaluated for different electron irradiation fluences for two types of cells. In the first a narrow Al_0.31Ga_0.69As window is a small part of the p⁺ layer while in the second type the whole window is an Al_xGa_1?xAs layer with a gradual Al mole fraction. The obtained results show that the Al_xGa_1?xAs window with a gradual Al mole fraction improves the resistance of the solar cell to electron irradiation especially in the short wavelengths range.     

A thin-film solar cell of high-quality -FeSi2/Si heterojunction prepared by sputtering

High-quality (1 1 0)/(1 0 1)-oriented epitaxial β-FeSi₂ films were fabricated on Si (1 1 1) substrate by the sputtering method. The critical feature was the formation of a high-quality thin β-FeSi₂ template buffer layer on Si (1 1 1) substrate at low temperature. It was demonstrated that the template is very important for the epitaxial growth of thick β-FeSi₂ films and for the blocking of Fe diffusion into the Si at the β-FeSi₂/Si interface. Hall effect measurements for β-FeSi₂ films showed n-type conductivity, with residual electron concentration around 2.0 × 10¹⁷ cm⁻³ and mobility of 50–400 cm²/V s. A prototype thin-film solar cell was fabricated by depositing n-β-FeSi₂ on p-Si (1 1 1). Under 100 mW/cm² sunlight, an energy conversion efficiency of 3.7%, with an open-circuit voltage of 0.45 V, a short-circuit current density of 14.8 mA/cm² and a fill factor of 0.55, was obtained.     

The obstructed diffusion of the I3 ion in mesoscopic TiO2 membranes

The diffusional permeability of I⁻₃ ion in acetonitrile in free standing TiO₂ membrane with a porosity of 55% was examined. The apparent diffusion coefficient, D_app at 25°C of the ion was found to be 3.4×10⁻⁶ cm^{2 −1}, an order of magnitude smaller than the free diffusion at the same temperature. The temperature dependency of D_app was measured in the range 0–30°C and analysed in terms of the Walden product. The diffusional activation energy was found to be 13.5 kJ/mol. The parameters of interest for the efficiency of mesoscopic wet solar cells are discussed. A back of an envelope calculation shows that although the obstructed diffusion coefficient of the I⁻₃ ion was an order of magnitude smaller than the free diffusion the diffusional flux is still sufficient to meet a current density of 50 mA cm⁻². At incident photon flux of 1 kW m⁻² and at a photopotential of 0.6 V this would correspond to a solar energy efficiency of approximately 30%.     

Semiconductor-sensitized solar cells based on nanocrystalline In2S3/In2O3 thin film electrodes

A possibility of semiconductor-sensitized thin film solar cells have been proposed. Nanocrystalline In₂S₃-modified In₂O₃ electrodes were prepared with sulfidation of In₂O₃ thin film electrodes under H₂S atmosphere. The band gap (E_g) of In₂S₃ estimated from the onset of the absorption spectrum was approximately 2.0 eV. The photovoltaic properties of a photoelectrochemical solar cell based on In₂S₃/In₂O₃ thin film electrodes and I⁻/I₃⁻ redox electrolytes were investigated. This photoelectrochemical cell could convert visible light of 400–700 nm to electron. A highly efficient incident photon-to-electron conversion efficiency (IPCE) of 33% was obtained at 410 nm. The solar energy conversion efficiency, η, under AM 1.5 (100 mW cm⁻²) was 0.31% with a short-circuit photocurrent density (J_sc) of 3.10 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.26 V, and a fill factor ( ff ) of 0.38.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Room temperature transport measurements on Bridgman-grown p-type CuIn1−xGaxSe2

Room temperature measurements were made of electrical conductivity (σ), Hall coefficient (R_H) and Seebeck coefficient (α) on filamentary samples of p-type CuInSe₂ and CuIn_1−xGa_xSe₂ with x0.3, cut from vertically grown Bridgman ingots. Analysis of the results was done on a two-carrier basis, due to the higher ratio of electron to hole mobility (b) in these materials compared to elemental semiconductors. This treatment yielded a preferred b-value of 5 and to lower calculated hole concentrations than (R_He)⁻¹ and higher hole mobilities than R_Hσ, based on a one-carrier interpretation. This effect was particularly marked in p-type samples with a hole concentration below 10¹⁷ cm⁻³, where even a few percent of minority electrons can play an important role.     

Triple-axis X-ray reciprocal space mapping of InyGa1−yAs thermophotovoltaic diodes grown on (1 0 0) InP substrates

Analysis of the composition, strain-relaxation, layer-tilt, and the crystalline quality of In_yGa_1−yAs/InP_1−xAs_x thermophotovoltaic (TPV) diodes grown by metal-organic vapor phase epitaxy (MOVPE) is demonstrated using triple-axis X-ray reciprocal space mapping techniques. In_0.53Ga_0.47As (E_gap=0.74 eV) n/p junction diodes are grown lattice matched (LM) to InP substrates and lattice-mismatched (LMM) In_0.67Ga_0.33As (E_gap=0.6 eV) TPV diodes are grown on three-step InP_1−xAs_x (0<x<0.32) buffer layers on InP substrates. X-ray reciprocal space maps about the symmetric (4 0 0) and asymmetric (5 3 3) reciprocal lattice points (RELPs) determine the in-plane and out-of-plane lattice parameters and strain of the In_yGa_1−yAs TPV active layer and underlying InP_1−xAs_x buffers. Triple-axis X-ray rocking curves about the LMM In_0.67Ga_0.33As RELP show an order of magnitude increase of its full-width at half-maximum (FWHM) compared to that from the LM In_0.53Ga_0.47As (250 vs. 30 arcsec). Despite the significant RELP broadening, the photovoltaic figure of merits show that the electronic quality of the LMM In_0.67Ga_0.33As approaches that of the LM diode material. This indicates that misfit-related crystalline imperfections are not dominating the photovoltaic response of the optimized LMM In_0.67Ga_0.33As material compared with the intrinsic recombination processes and/or recombination through native point defects, which would be present in both LMM and LM diode material. However, additional RELP broadening in non-optimized LMM In_0.67Ga_0.33As n/p junction diodes does correspond to significant degradation of TPV diode open-circuit voltage and minority carrier lifetime demonstrating that there is correlation between X-ray FWHM and the electronic performance of the LMM TPV diodes.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.