CuInSe2 cells and modules

Recent CuInSe₂ photovoltaic technology advances are discussed. 14.1% active area efficient test cells and the fabrication of monolithic integrated modules with power outputs of 112 W/m² on 940 cm² and 91.4 W/m² on 3900 cm² have been achieved. Packaged modules are stable outdoors. Studies indicate a recombination controlled junction mechanism and imply a wide CIS compositional range over which high-efficiency junctions are possible. Processing improvements already demonstrated on test cells and 940 cm² modules will yield 52-W, 3900-cm² CIS modules     

Computer simulation and modeling of graded bandgap CuInSe2/CdS based solar cells

This paper proposes the graded bandgap absorber material, Cu_1-xAg_xIn_1-y-zGa_yAl_z Se/sub 2(1-u$/ -/sub w/)S_2uTe_2w (CIS*) multinary system, to improve the low open-circuit voltage (V_OC) seen in CuInSe₂/CdS solar cells, without sacrificing the short-circuit current density (J_sc). It also proposes a p-i-n model for the CuInSe₂/CdS solar cell, where the intrinsic region is the graded bandgap CIS*. Reflecting surfaces are provided at the p-i and n-i interfaces to trap the light in the narrow intrinsic region for maximum generation of electron and hole pairs (EHP's). This optical confinement results in a 25-40% increase in the number of photons absorbed. An extensive numerical simulator was developed, which provides a 1-D self-consistent solution for Poisson's equation and the two continuity equations for electrons and holes. This simulator was used to generate J-V curves to delineate the effect of different grading profiles on cell performance. The effects of a uniform bandgap, normal grading, reverse grading, and a low bandgap notch have been considered. Having established the inherent advantages to these grading profiles an optimal doubly graded structure is proposed with grading between 1.5 eV and 1.3 eV regions which has V_OC=0.86 V, η=17.9%, FF=0.79 and J_sc=26.3 mA/cm² compared to 0.84 V, 14.9%, 0.76, and 23.3 mA/cm², respectively, for the highest efficiency 1.4-eV uniform bandgap cell. Replacing the thick CdS(2.42 ev) layer assumed in our simulations with a wide gap semiconductor such as ZnO(3.35 ev) increases all current densities by about 5 mA/cm², and increases the optimal calculated efficiency from 17.9% to roughly 21% for a doubly graded structure with a thickness of 1 μm and bandgaps ranging from 1.3 eV to 1.5 eV     

High-efficiency GaAs/Ge monolithic tandem solar cells

Tandem cells of GaAs grown by metalorganic chemical vapor deposition (MOCVD) on thin Ge to address both higher efficiency and reduced weight are discussed. GaAs/Ge monolithic tandem cells of 4-cm ² area have been produced with independently verified efficiencies up to 21.7% (AM0, one sun, 25°C, total area). Under AM1.5 global conditions, efficiencies are up to 24.3%. These are believed to be the highest one-sun efficiencies reported for GaAs/Ge cells, and the highest efficiency for a two-terminal monolithic tandem cell     

Over 35-percent efficient GaAs/GaSb tandem solar cells

The efficiency of GaAs solar cells can be substantially increased by locating an infrared-sensitive booster solar cell behind a transparent GaAs cell. Infrared-sensitive GaSb cells and visible-light-sensitive GaAS cells designed for use with concentrated sunlight are described. Prismatic cover slides are used to effectively eliminate grid shading losses for both the GaAs and the GaSb cells. With prismatic cover slides, the best component cell efficiencies add up to 38.2% under concentrated sunlight (100×AM1.5D). Two-terminal tandem cell circuit cards containing nine transparent GaAS cells and nine GaSb cells are also described. Without cell prismatic covers, circuit-level energy conversion efficiencies near 30% (AM1.5D) are demonstrated     

Electrical properties of CuInSe2 single crystals

Various bulk electrical properties and device characteristics have been measured. It has been shown that the majority carrier type is dependent on crystal stoichiometry. Mobilities of 660 cm²/V sec and 30 cm²/V sec have been measured for n-and p-type samples, respectively. Rectifying contacts and p-n junctions have been investigated by small signal analysis and the associated doping levels and equilibrium band diagrams have been determined. Photovoltage measurements on rectifying contacts have shown that the band-gap has a value of 0.95 ± 0.01eV.     

Metal-free indoline-dye-sensitized TiO2 nanotube solar cells

Titanium dioxide nanotubes were directly fabricated from commercial P25 TiO₂ via alkali hydrothermal transformation. The prepared titanate nanotubes were successfully used as an electrode material for dye-sensitized solar cells (DSCs). A metal-free organic dye (indoline dye D102) was used as a sensitizer. The used indoline dye D102 is of high purity (?98%) and high absorption coefficient (67,500 L mol⁻¹ cm⁻¹ at 501 nm). The TiO₂ pastes were prepared with PEG (Mw 20,000) and as-made TiO₂ nanotubes or P25 powders. Titania thin films were grown by screen printing method. High conversion efficiencies of light to electricity of around 9.8% and 7.6% under illumination of simulated AM1.5 sunlight (100 mW/cm²) were achieved with P25 and TiO₂ nanotube cells, respectively. The fill factor of DSCs based on TiO₂ nanotubes increased in comparison with that of DSCs based on TiO₂ nanoparticles. The electron transport and dye adsorption properties in both titanate nanotube and P25 electrodes were evaluated in terms of photovoltaic characteristics of the fabricated cells. The related mechanisms were discussed. The study provides a promising method for the development of high-efficiency and low-cost DSCs.     

Enhanced efficiency in GaInP/GaAs tandem solar cells using carbon doped GaAs in tunnel junction

Carbon doping of GaAs using CBr₄ (carbon tetrabromide) in metal-organic chemical vapor deposition (MOCVD) was investigated to obtain very high and sharp doping profiles required for tunnel junction in tandem solar cells. It was found that the hole concentration increased with decreasing growth temperature and V/III ratio. Hole doping profiles versus distance from the sample surface showed that the hole concentration near the surface was very low in comparison with that far below the surface. As a post-growth treatment, CBr₄ was supplied during the cool down process and produced almost constant hole concentration of 1 × 10²⁰ cm⁻³ regardless of the depth, when CBr₄ flow rate was 9.53 μmol/min. Based on these results, solar cells were fabricated using both carbon (C) and zinc (Zn) as a p-type dopant. It was shown that C doping exhibits higher efficiency and lower series resistance than those of Zn doping in GaInP/GaAs tandem solar cells.     

25.5% efficient Ga0.35In0.65P/Ga0.83In0.17As tandem solar cells grown on GaAs substrates

Theoretical calculations predict a higher power conversion efficiency for the combination of Ga_0.35In_0.65P and Ga_0.83In_0.17As in a tandem solar cell, compared to the more commonly used Ga_0.51In_0.49P/GaAs approach. A record conversion efficiency of 21.6% (AM1.5 g) was recently achieved for a 1.18 eV Ga_0.83In_0.17As solar cell, grown lattice-mismatched to the GaAs substrate material. This paper reports on the device characteristics of first Ga_0.35In_0.65P/Ga_0.83In_0.17As tandem solar cells based on this very promising GaInAs material. A high quantum efficiency, comparable to the lattice-matched Ga_0.51In_0.49P on GaAs approach was achieved. A power conversion efficiency of 25.5% was measured under AM1.5d spectral conditions     

Large-area, 8-cm2 GaAs solar cells fabricated from MBEmaterial

A GaAs solar cell with an area of 2 cm×4 cm fabricated from a film grown by molecular beam epitaxy (MBE) on a 5.08-cm-diameter GaAs substrate is discussed. This is the largest device ever fabricated from MBE material. The cell demonstrated an efficiency of 21.7% under one-sun AM1.5 conditions at 25°C and 18.8% under one-sun AM0 conditions at 28°C     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Effects of residual copper selenide on CuInGaSe2 solar cells

Large-grain, copper-poor CuInGaSe₂ (CIGS) films are favored in the fabrication of highly efficient solar cells. However, the degradation of cell performance caused by residual copper selenide (Cu_2−xSe) remains a problem. This work studies the formation and behavior of excess Cu_xSe and further compares the cell performance of typical copper-poor with that of copper-rich solar cells. Since excess Cu_2−xSe cannot be exhausted during the growth, it fully surrounds the polycrystalline CIGS grains. Excess Cu_2−xSe in the CIGS film produces serious shunt paths and causes the pn junction to be of poor quality. A short circuit in copper-rich CIGS solar cells is attributable to the conductive Cu_2−xSe. The best way to ensure high-efficiency of the cells is to exhaust Cu_2−xSe during growth. Otherwise, a dense, chemically treated CIGS film is required to prevent the negative effects of excess Cu_2−xSe.     

Deposition of CuInSe2 films by a two-stage processutilizing E-beam evaporation

The deposition technique involves first depositing Cu and In metallic layers onto a substrate using the electron-beam evaporation method and then selenizing these layers in an H₂Se atmosphere. High-quality CuInSe₂ films of chalcopyrite phase have been obtained by this technique, and the films have been characterized by scanning electron microscopy, X-ray diffraction, electron microprobe, and resistivity measurements. Solar cells have been fabricated using these CuInSe₂ films, and active-area conversion efficiency values approaching 11% have been demonstrated for these devices     

Structure and properties of high efficiencyZnO/CdZnS/CuInGaSe2 solar cells

Thin-film polycrystalline solar cells with the structure ZnO/CdZnS/CuInGaSe₂ fabricated with total area efficiencies of up to 12.5% under AM1.5 equivalent illumination and 10.5% under AM0 equivalent are discussed. These are among the highest total area efficiencies reported for polycrystalline thin-film solar cells. Current-voltage and quantum efficiency data for such a high-efficiency cell are given. Described are the deposition of the CuInGaSe₂ by physical vapor deposition in vacuum, the CdZnS by chemical deposition from solution, and the ZnO by reactive sputtering. The electrical and optical properties of the individual layers have been inferred from measurements on complete devices and on separate witness layers. Optical constants and thicknesses obtained for the device layers from these measurements are presented, and the requirements for optimizing the device efficiency are discussed     

N2 effect on GaAs etching at 150 mTorr capacitively-coupled Cl2/N2 plasma

The role of N₂ on GaAs etching at 150 mTorr capacitively-coupled Cl₂/N₂ plasma is reported. A catalytic effect of N₂ was found at 20-25% N₂ composition in the Cl₂/N₂ discharges. The peak intensities of the Cl₂/N₂ plasma were monitored with optical emission spectroscopy (OES). Both atomic Cl (725.66 nm) and atomic N (367.05 nm) were detected during the Cl₂/N₂ plasma etching. With the etch rate and OES results, we developed a simple model in order to explain the etch mechanism of GaAs in the high pressure capacitively-coupled Cl₂/N₂ plasma as a function of N₂ ratio. If the plasma chemistry condition became positive ion-deficient at low % N₂ or reactive chlorine-deficient at high % N₂ in the Cl₂/N₂ plasma, the GaAs etch rate is reduced. However, if the plasma had a more balanced ratio of Cl₂/N₂ (i.e. 20-25% N₂) in the plasma, much higher etch rates (up to 150 nm/min) than that in pure Cl₂ (50 nm/min) were produced due to synergetic effect of neutral chlorine adsorption and reaction, and positive ion bombardment. Pure Cl₂ etching produced 14 nm of RMS surface roughness of GaAs. Introduction of ?20% N₂ gas in Cl₂/N₂ discharges significantly reduced the surface roughness to 2-4 nm. SEM photos showed that the morphology of photoresist mask was strongly degraded. Etch rate of GaAs slightly increased from 10 to 40 nm/min when RIE chuck power changed from 10 to 150 W at 12 sccm Cl₂/8 sccm N₂ plasma condition. The surface roughness of GaAs etched at 12 sccm Cl₂/8 sccm N₂ plasma was 2-3 nm.     

Water-soluble poly(3,4-ethylenedioxythiophene)/nano-crystalline TiO2 heterojunction solar cells

Nanocrystalline titanium dioxide (TiO₂) thin films were prepared by the sol-gel method and were then used to fabricate an indium-tin oxide (ITO)/nano-crystalline TiO₂/poly(3,4-ethylenedioxythiophene) (PEDOT)/Au device. The junction thus obtained shows a rectifying behavior. Their current-voltage (I-V) characteristics in dark indicate that a heterojunction at the nano-crystalline TiO₂/PEDOT interface has been created. The measured open-circuit voltage (V_oc) and short-circuit current (I_sc) for the device under illumination with 50 mW/cm² light intensity under AM 1.5 conditions (device dimension was 1 cm²) are V_oc=0.39 V, I_sc=54.9 μA/cm², the filling factor (FF)=0.429 and the energy conversion efficiency (η)=0.03%.     

Novel three-layer TiO2 nanoparticle stacking architecture for efficient dye-sensitized solar cells

In this work, we report a novel three-layer TiO₂ nanoparticle photoelectrode for dye-sensitized solar cells (DSSCs). In such a DSSC, a very thin front scattering layer (∼1 μm thick) composed of small nanoparticles (∼20 nm) and larger scattering nanoparticles (>100 nm) is inserted in front of the typical small-nanoparticle absorption layer and the large-nanoparticle back-scattering/reflection layer. Such a very thin front scattering layer having mixture of small/large nanoparticles provides a larger haze (i.e., stronger scattering) and yet still retains a high integrated transmittance. With effectively scattering portion of the incident light into larger oblique angles and therefore increasing optical paths and absorption, DSSC efficiencies are enhanced by 15.2%, compared to the conventional two-layer DSSCs.     

A novel and effective PECVD SiO2/SiN antireflectioncoating for Si solar cells

It is shown that sequential plasma-enhanced chemical vapor deposition (PECVD) of SiN and SiO₂ can produce a very effective double-layer antireflection (AR) coating. This AR coating is compared with the frequently used and highly efficient MgF₂/ZnS double layer coating. The SiO₂/SiN coating improves the short-circuit current (J_SC) by 47%, open-circuit voltage (V_OC) by 3.7%, and efficiency (Eff) by 55% for silicon cells with oxide surface passivation. The counterpart MgF₂/ZnS coating gives similar but slightly smaller improvement in V_OC and Eff. However, if silicon cells do not have the oxide passivation, the PECVD SiO₂/SiN gives much greater improvement in the cell parameters, 57% in J_SC, 8% in V_OC, and 66% in efficiency, compared to the MgF₂/ZnS coating which improves J_SC by 50%, V_OC by 2%, and cell efficiency by 54%. This significant additional improvement results from the PECVD deposition-induced surface/defect passivation. The internal quantum efficiency (IQE) measurements showed that the PECVD SiO₂/SiN coating a absorbs fair amount of photons in the short-wavelength range (<500 nm); however, the improved surface/defect passivation more than compensates for the loss in J_SC and gives higher improvement in the cell efficiency compared to the MgF₂/ZnS coating     

Electrical measurements of voltage stressed Al2O3/GaAs MOSFET

Electrical characteristics of GaAs metal–oxide–semiconductor field effect transistor with atomic layer deposition deposited Al₂O₃ gate dielectric have been investigated. The IV characteristics were studied after various constant voltage stress (CVS) has been applied. A power law dependence of the gate leakage current (I_g) on the gate voltage (V_g) was found to fit the CVS data of the low positive V_g range. The percolation model well explains the degradation of I_g after a high positive V_g stress. A positive threshold voltage (V_th) shift for both +1.5 V and +2 V CVS was observed. Our data indicated that positive mobile charges may be first removed from the Al₂O₃ layer during the initial CVS, while the trapping of electrons by existing traps in the Al₂O₃ layer is responsible for the V_th shift during the subsequent CVS.     

CH4/H2/Ar ECR plasma etching forAlGaAs/InGaAs/GaAs pseudomorphic HFETs

The effect of electron cyclotron resonance (ECR) plasma etching using CH₄H/₂/Ar on Si δ-doped pseudomorphic AlGaAs/InGaAs/GaAs heterostructures and field-effect transistors has been investigated. Hall measurements were performed as a function of temperature (5-300 K) and the Hall mobility and the sheet density compared to wet chemically etched reference samples. Direct current and high-frequency measurements were performed on dry gate-recessed PMHFETs     

High-efficiency GaAs solar cells

An updated review of the state of the art in the development of GaAs solar cells is provided, with emphasis on AlGaAs-GaAs cells suitable for space applications. A set of theoretically derived characteristics is given for this type of solar cell. Comparison of measured performance with theory shows excellent agreement. Data on the effects of radiation damage (high-energy electrons, protons, and neutrons) is also integrated into a form useful for evaluation purposes. Techniques for fabricating (AlGa)As-GaAs solar cells in quantities large enough for practical applications are discussed and are shown to have been demonstrated. The possibility of extending these techniques to the fabrication of very thin low-weight cells for space applications is also considered. Finally, the results obtained to date in the development of GaAs solar cells for applications requiring concentrated sunlight are reviewed, for terrestrial as well as for space applications. As a milestone toward the practical application of AlGaAs-GaAs solar cells in space systems, a brief account is provided on the development status of small experimental AlGaAs-GaAs solar-cell panels for specific space flights.