Manufacturing tolerances of terrestrial concentrator p‐on‐n GaAs solar cells

The increasing interest in the manufacturing of terrestrial concentrator GaAs solar cells calls for the need to establish the guidelines, assessments and tolerances of the production process. Accordingly, this paper analyses, for the first time, the tolerances of this process. The influence of the entire solar cell (antireflecting coatings, semiconductor structure and series resistance) is considered. More specifically, the parameters analysed are: the thickness of the double antireflecting coating, window thickness, emitter thickness, base thickness, emitter doping level, base doping level, shadowing factor and specific front contact resistance. The results are presented in the form of iso‐efficiency plots as a function of the aforementioned parameters. Therefore, besides the conclusions extracted in this paper, numerous others may be formed according to particular necessities. Copyright © 2001 John Wiley & Sons, Ltd.     

Investigation of alternative window materials for GaAs solar cells

The optimum window material for surface passivation of GaAs solar cells is investigated using theoretical analysis of optical losses due to window bandgap energy and thickness. A simplified expression is developed to calculate the effective surface recombination velocity in terms of lattice mismatch between the window layer and GaAs, which suggests using a window material with and indirect bandgap energy greater than 2.0 eV, a thickness of less than 0.05 μm, and a lattice mismatch of less than 0.05%. Experimental GaAs solar cells were fabricated and quantum efficiency measurements were made using no window (bare GaAs), Al_0.7Ga_0.3As, Na₂S, and ZnSe/Na₂S windows. The Al_0.7Ga_0.3As and Na₂S windows are shown to passivate the GaAs surface and reduce the surface recombination velocity to less than 10⁵ cm/s, while the ZnSe encapsulating layer was used to permanently maintain the temporary surface passivation effects from Na₂S     

High-efficiency AlGaAs/GaAs concentrator (2500 suns) solar cells

A study is made AlGaAs/GaAs heterostructure solar cells that have been optimized for ultrahigh concentrations of sunlight (1000–2500 suns). The maximum efficiencies were 25.1% at 500 suns, 25% at 1000 suns, and 22.8% at 2000 suns for sunlight passing through an air mass of AM1.5. Cells of this type open up the possibility of reducing the surface area of solar cells by more than three orders of magnitude and, as a result, substantially lowering the cost of electrical energy in power equipment with sunlight concentrators. Fiz. Tekh. Poluprovodn. 33, 1070–1072 (September 1999)     

High‐irradiance degradation tests on concentrator GaAs solar cells

The present work summarises the results of an experiment of light‐soaking high‐concentrator MOVPE‐grown GaAs solar cells under monochromatic light (808 nm). The irradiance level was set so that the short‐circuit current obtained was 1100 times that produced with the AM1ċ5D spectrum at 1 kW/m². This test caused no morphological changes in the devices. The main phenomenon discovered has been a slight increase with time of the reverse current I₀₂. This increase is analogous to that observed in similar degradation experiments based on high forward currents. In general, the results of these tests show that the drop in performance is very limited, supporting the idea that concentrator GaAs solar cells are rugged devices, capable of achieving long lifetimes in field operation. Copyright © 2003 John Wiley & Sons, Ltd.     

Flash technique for GaAs concentrator solar cell measurement

A flash-lamp testing technique for the measurement of GaAs concentrator solar cells is described. This method allows a simple and rapid evaluation of efficiency, series resistance and fill factor of the cell at high concentrations. A comparison with outdoor measurements is also reported.     

Silicon point contact concentrator solar cells

Experimental results are presented for thin high resistivity concentrator silicon solar cells which use a back-side point-contact geometry. Cells of 130 and 233 µm thickness were fabricated and characterized. The thin cells were found to have efficiencies greater than 22 percent for incident solar intensities of 3 to 30 W/cm²(30-300 "suns"). Efficiency peaked at 23 percent at 11 W/cm²measured at 22-25°C. Strategies for obtaining higher efficiencies with this solar cell design are discussed.     

Optimum antireflection coatings for heteroface AlGaAs/GaAs solar cells—Part I: The influence of window layer oxidation

This paper details both the theoretical and experimental results of a modified model for designing MgF₂/ZnS double-layer antireflection coatings for AlGaAs/GaAs heteroface solar cells. The main contribution of the model presented is that it takes into account the possible existence of an oxide layer in the AlGaAs window layer. In a first step the optical behavior of the oxide is modeled and that model is used to recalculate optimal double-layer antireflection coating when a thin AlGaAs oxide layer is present. Significant differences with classical double-layer antireflection coating design are found, such as, the antireflection properties of the oxide layer when formed onto originally thick windows or its equivalent role to that of the ZnS layer (as a high refractive index media) in the coating. Finally, an experimental analysis is carried out to assess empirically the conclusions of the model. These experiments have yielded an excellent agreement with the proposed theory.     

Degradation mechanism of concentrator solar receivers without protection layer

GaInP/GaInAs/Ge triple junction concentrator solar receivers without protection layer were conducted sequentially at 90, 110, 130 and 150 °C for 25, 30, 30 and 50 h, respectively. After the step stress accelerated degradation tests (SSADT), gradual degradation in the dark and light I–V characteristics were observed. Degradation of devices results in the decrease of open-circuit voltage (V_oc), fill factor (FF) and efficiency. The degradation mechanism of these triple junction concentrator solar receivers is attributed to the recombination current in the depletion region at the chip perimeter of solar cells. The mean time to failure (MTTF) of 380 h at 65 °C and active energy (E_a) of 0.16 eV were obtained by the statistical analysis with cumulative damage and exponential mode. Both of the obtained values of MTTF and E_a are low, and this results might be possibly due to our devices being without encapsulated protection layer.     

FTO导电玻璃对太阳电池窗口层材料的影响

P型微晶碳化硅薄膜(P-μc-SiC∶H)作为非晶硅薄膜太阳电池的窗口层材料,能够提高P层的光学带隙,增加窗口层可见光部分的透光率,从而提高太阳电池的填充因子和转换效率.本文研究射频功率、反应温度、SiH4和CH4气体流量比对FTO导电玻璃上P型μc-SiC∶H薄膜的光学带隙、透光率的影响.并在相同工艺条件下对比三种主流导电玻璃(ITO、 AZO 、FTO)上的薄膜光学特性,找到最适合做太阳能电池P-μc-SiC∶H薄膜的基板.     

27.5-percent silicon concentrator solar cells

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm²(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm²are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm²) range.     

窗口层对三结电池双层减反膜的影响

窗口层的厚度对高效三层太阳能电池具有重要影响。本文针对太阳能电池的地面应用,优化了三节电池（Ga0.5In0.5P/In0.02Ga0.98As/Ge）的双层反射膜（SiO2/TiO2, SiO2/ZnS）。同时讨论了在双层减反膜优化结构下,变化Al0.5In0.5P窗口层厚度引起反射率的变化。     

Influence of window layer thickness on double layer antireflection coating for triple junction solar cells

The optimization of a SiO₂/TiO₂,SiO₂/ZnS double layer antireflection coating（ARC）on Ga_0.5In_0.5P/In_0.02Ga_0.98As/Ge solar cells for terrestrial application is discussed.The Al_0.5In_0.5P window layer thickness is also taken into consideration.It is shown that the optimal parameters of double layer ARC vary with the thickness of the window layer.     

High-efficiency,thin-film InP concentrator solar cells

High-efficiency, thin-film InP solar cells grown heteroepitaxially on GaAs and Si single-crystal bulk substrates are being developed as a means of eliminating the problems associated with using single-crystal InP substrates (e.g., high cost, fragility, high mass density and low thermal conductivity). A novel device structure employing a compositionally graded Ga _x In_1−x As layer (∼8 μm thick) between the bulk substrate and the InP cell layers is used to reduce the dislocation density and improve the minority carrier properties in the InP. The structures are grown in a continuous sequence of steps using computer-controlled atmospheric-pressure metalorganic vapor-phase epitaxy (AP-MOVPE). Dislocation densities as low as 3×10⁷ cm⁻² and minority carrier lifetimes as high as 3.3 ns are achieved in the InP layers with this method using both GaAs or Si substrates. Structures prepared in this fashion are also completely free of microcracks. These results represent a substantial improvement in InP layer quality when compared to heteroepitaxial InP prepared using conventional techniques such as thermally cycled growth and post-growth annealing. The present work is concerned with the fabrication and characterization of thin-film InP solar cells designed for operation at high solar concentration (∼100 suns) which have been prepared from similar device structures grown on GaAs substrates. The cell performance is characterized as a function of the air mass zero (AM0) solar concentration ratio (1–100 suns) and operating temperature (25°–80° C). From these data, the temperature coefficients of the cell performance parameters are derived as a function of the concentration ratio. Under concentration, the cells exhibit a dramatic increase in efficiency and an improved temperature coefficient of efficiency. At 25° C, a peak conversion efficiency of 18.9% (71.8 suns, AM0 spectrum) is reported. At 80° C, the peak AM0 efficiency is 15.7% at 75.6 suns. These are the highest efficiencies yet reported for InP heteroepitaxial cells. Approaches for further improving the cell performance are discussed.     

Evaluation of the reliability of high concentrator GaAs solar cells by means of temperature accelerated aging tests

Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30‐year warranty for these 1 mm²‐sized GaAs cells (manufactured according to a light emitting diode‐like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

GaAs thin-film solar cells

Thin-film solar cells utilizing polycrystalline gallium-arsenide films have been made and investigated to determine their suitability for future solar-power systems. The gallium-arsenide films are vapor deposited onto substrates of molybdenum or aluminum foil. Of the various junctions investigated, the most successful has been one consisting of a surface barrier employing an evaporated film of platinum or semiconducting copper selenide. The efficiencies of platinum gallium-arsenide barriers on molybdenum substrates have been 3 percent for 4 cm²area, 4.5 percent for 2 cm²area and 5 percent for 0.2 cm²area. For copper selenide gallium-arsenide barriers on molybdenum an efficiency of 4.6 percent for 0.73 cm²area has been measured; using aluminum substrates this figure is 4.3 percent for the same area with a power-to-weight ratio in excess of 135 watts per pound. With an etching treatment, cells made with copper selenide barriers have shown no degradation on the shelf or under load at room ambient.     

Effect of anodic-oxide layer thickness on the performance of GaAs MOS solar cells

Native oxide layers on n-type GaAs have been grown by aqueous anodic oxidation technique using AGW electrolyte. The effect of thickness of the As2O3rich interfacial oxide layer having a resistivity ~ 10¹⁴Ω . cm on the performance of GaAs MOS solar cells has been investigated. An attempt has been made to optimize the oxide layer thickness for achieving optimum efficiency.     

Increased photogeneration in thin silicon concentrator solar cells

Recent progress in silicon concentrator solar cells has resulted in several designs capable of 25-percent efficiency with one group reporting 28 percent under 14 W/cm²of incident power at 25°C. It has been shown that further improvement is possible by restricting the sunlight acceptance angle of the cell. In this letter, a practical implementation which is equivalent in its effect is proposed which results in an increased utilization of weakly absorbed near-bandgap light. This increased absorption is obtained by placing the cells in a cavity with a small entrance aperture. An analysis is given based upon work on the acceptance angle enhancements by Campbell and Green. The design is expected to improve the efficiencies of existing solar cells to 30 percent. If used in conjunction with previously proposed cell improvements, the efficiencies will be improved towards 33 percent, very near the limit efficiency of 36 percent. This design also has the effect of decreasing the differences in performance between the leading candidate concentrator cell designs and diminishing the dependence of the efficiencies on the cell texturization and bulk carrier lifetimes.