Double layer antireflection coating for high-efficiency passivatedemitter silicon solar cells

Recent high-efficiency silicon solar cells employ high-quality oxides both for surface passivation and as a rudimentary antireflection coating. This gives over 3% reflection at the cell front surface, even though the surface is microstructured. A double layer antireflection coating applied to cells with reduced SiO₂ thickness reduces the cell reflection. However, although reflection is minimized by reducing the oxide thickness to values below 100 Å, a rapid falloff in both open-circuit voltage and short-circuit current is observed experimentally once this thickness is reduced below 200 Å. The best compromise is found when oxide thickness is 250 Å which allows improved short-circuit current density without appreciable loss in open-circuit voltage     

26-percent efficient point-junction concentrator solar cells with afront metal grid

Silicon concentrator cells with point diffusion and metal contacts on both the front and backsides are discussed. The design minimizes reflection losses by forming an inverted pyramid topography on the front surface and by shaping the metal grid lines in the form of a triangular ridge. A short-circuit current density of 39.6 mA/cm² has been achieved, even though the front grid covers 16% of the cell's active area of 1.56 cm². This, together with an open-circuit voltage of 700 mV, has led to an efficiency of 22% at one sun, AM1.5 global spectrum. Under direct-spectrum 8.8-W/cm² concentrated light, the efficiency is 26%. This is the highest ever reported for a silicon cell having a front metal grid     

Evaporated polycrystalline silicon films for photovoltaic applications - grain size effects

Vacuum deposited, polycrystalline silicon films were fabricated into planar photovoltaic diodes by double diffusion techniques. Scanning electron microscopy showed that the crystallites are columnar in shape, with grain lengths several times larger than grain diameters. The dependency of average grain diameter on deposition conditions is discussed. Secondary ion mass spectrometry was used to obtain doping profiles and junction depths. Dark and illuminated I-V curves, spectral responses, and minority carrier diffusion lengths are presented for photovoltaic devices having grain sizes in the range 0.2 to 5 μm. Samples formed on sapphire and on a special alkaline-earth aluminosillcate glass processed under the same conditions had similar photovoltaic characteristics. Data on open-circuit voltage, short-circuit current, and solar cell efficiency are presented as functions of average grain diameter.     

A model of front-illuminated n+-p-p+high-efficiency silicon solar cell

A realistic model of a front-illuminated n⁺-p-p⁺ silicon solar cell is developed by solving the current continuity equations for minority carriers in the quasi-neutral regions in steady state, assuming the light in the cell is trapped as a result of multiple reflections at the front and the back of the cell. This model is used to study the effects of the front emitter thickness and doping level and the light trapping on the J-V characteristic and thereby on the open-circuit voltage, short-circuit current density, curve factor, and the efficiency of the cell. A textured cell with an emitter thickness in the range of 0.3-1.0 μm with its doping ≈5×10¹⁸ cm^-3 and the recombination velocities of minority carriers as large as 200 cm/s at the n⁺ front surface and 10 cm/s at the back of the p base can exhibit an efficiency in excess of 26% (under AM 1.5 sunlight of 100 mW/cm² intensity) at 25°C if the light reflection losses at the front surface can be made small     

Two-dimensional analysis for CuxS-CdS solar cells with nonuniform skin region

Due to rapid diffusion of externally applied copper along the disordered region of the grain boundaries, the skin region of the CuxS-CdS solar cell consists of alternate layers of thick and thin regions of p-CuxS. A two-dimensional idealized analysis leading to the short-circuit current, open-circuit voltage, conversion efficiency and spectral response of the cell is presented. For a typical cell, the variations of these parameters with grain size and width of the disordered regions are shown. It was pointed out that for such a cell, there is an optimum crystallite size for maximum conversion efficiency of the cell. The spectral response characteristics shows a relative improvement in the low-frequency side with increase in density of the CdS crystallites. This is probably due to the vertical junctions formed at the disordered regions.     

Nano‐cones on micro‐pyramids: modulated surface textures for maximal spectral response and high‐efficiency solar cells

The front‐side reflection represents a significant optical loss in solar cells. One way to minimize this optical loss is to nano‐texture the front surface. Although nano‐textured surfaces have shown a broad‐band anti‐reflective effect, their light scattering and surface passivation properties are found to be generally worse than those of standard micro‐textured surfaces. To overcome these setbacks in crystalline silicon solar cells, advanced texturing and passivation approaches are here presented. In the first approach, we propose a modulated surface texture by superimposing nano‐cones on micro‐pyramidal surface texture. This advanced texture applied at the front side of crystalline silicon wafers completely suppresses the reflection in a broad wavelength range from 300 nm up to 1000 nm and efficiently scatters light up to 1200 nm. In the second approach, we show a method to minimize recombination at nano‐textured surfaces by using defect‐removal etching followed by dry thermal oxidation. These two approaches are applied here in an interdigitated back‐contacted crystalline silicon solar cell and result in decoupling of the interplay between the mechanisms behind short‐circuit current density and open‐circuit voltage. The device exhibits a conversion efficiency equal to 19.8%, record external quantum efficiency (78%) at short wavelengths (300 nm), and electrical performance equal to the performance of the reference interdigitated back‐contacted device based on front‐side micro‐pyramids. Copyright © 2015 John Wiley & Sons, Ltd.     

Potential of using the Cd<Subscript>0.8</Subscript>Hg<Subscript>0.2</Subscript>Te alloy in solar cells

Surface-barrier diodes based on the Cd_xHg_1?x Te alloy (x ～ 0.8) sensitive in the wavelength range 0.3–1.1 μm, which were obtained by etching (bombardment) of the surface of the p-type crystal with argon ions, are studied. Using the measured spectral absorption and reflection curves, as well as the parameters of the diode structure, which were found from electrical characteristics, the spectra of photoelectric quantum efficiency of diodes are calculated. The results of the calculation of photoelectric parameters of the Gd_0.8Hg_0.2Te-based diodes are given in comparison with the CdTe-based and Si-based solar cells. For the AM1.5 solar irradiation conditions, the open-circuit voltage and short-circuit current, as well as values of limiting efficiency, are determined.     

n型单晶硅片的电阻率和扩散方阻对IBC太阳电池电性能的影响

叉指背接触式(IBC)太阳电池因正面没有金属栅线遮挡,具有较高的短路电流,且组件外观更加美观。但由于IBC太阳电池正负电极在背面交叉式分布,在制备过程中需要采用光刻掩模技术进行隔离,难以实现大规模生产。采用Quokka软件仿真模拟了电阻率和扩散方阻对n型IBC太阳电池效率的影响,并对不同电阻率和扩散方阻的电池片进行了实验验证,从n型单晶硅片电阻率的选择和扩散工艺优化方面为IBC太阳电池的规模化生产提供了理论基础。实验结果表明,电阻率为3~5Ω·cm、扩散方阻为70Ω/时,小批量生产的IBC太阳电池平均光电转换效率可达23.73%,开路电压为693 mV,短路电流密度为42.44 mA/cm²,填充因子为80.69%。     

The importance of surface recombination and energy-bandgap arrowing in p-n-junction silicon solar cells

Experimental data demonstrating the sensitivity of open-circuit voltage to front-surface conditions are presented for a variety of p-n-junction silicon solar cells. Analytical models accounting for the data are defined and supported by additional experiments. The models and the data imply that a) surface recombination significantly limits the open-circuit voltage (and the short-circuit current) of typical silicon cells, and b) energy-bandgap narrowing is important in the manifestation of these limitations. The models suggest modifications in both the structural design and the fabrication processing of the cells that would result in substantial improvements in cell performance. The benefits of one such modification-the addition of a thin thermal silicon-dioxide layer on the front surface-are indicated experimentally.     

A novel high open-circuit voltage p-n InP solar cell design

The performance of InP solar cells has been limited by low open-circuit voltages. While the reported short-circuit current densities are approaching the theoretical limit, the open-circuit voltages have yet to obtain what is expected from a semiconductor with a direct band gap of 1.35 eV. This work investigates the factors that determine the open-circuit voltage and presents the design and fabrication of a novel high open-circuit voltage p-n InP solar cell. the key aspect of the novel design is a complete analysis of the top contact metallization effects on the reverse saturation current density and the open-circuit voltage. the features of the design are not specific to InP solar cells but are applicable to other advanced material solar cells that require a thin emitter for an optimal design (those materials with a high absorption coefficient). By minimizing the reverse saturation current density, a high open-circuit voltage and high efficiency may be obtained. In addition, a complete analysis of the solar cell modelling is provided, with comparisons to other published InP solar cell models and device results to juxtapose the key material and design parameter effects.     

Development of high-quantum-efficiency,lattice-mismatched, 1.0-eV GaInAs solar cells

High-quantum-efficiency, lattice-mismatched, 1.0-eV GaInAs solar cells grown by organometallic vapor phase epitaxy have been developed for ultimate integration into AlGaAs/GaAs/GaInAs 3-junction, 2-terminal monolithic devices. The more standard n/p junction was replaced with an n-i-p structure in the GaInAs cell in order to increase the short-circuit current by overcoming the material deficiencies which arise as a result of accommodating the lattice mismatch. This led to single junction 1.0-eV GaInAs cells with internal quantum efficiencies >90% and short-circuit-current densities that match or closely approach those needed to current match the upper AlGaAs and GaAs cells. A 4.1% (1-sun, air mass 0,25°C) power conversion efficiency was achieved with a developmental structure, indicating the potential of the lattice-mismatched n-i-p 1.0-eV GaInAs cell. An analogous device designed to allow direct monolithic integration with the upper AlGaAs and GaAs cells, with a modified grading layer of AlGaInAs in place of the usual GaInAs, achieved an efficiency of 2.2%, primarily due to a lower open-circuit voltage. The open-circuit voltage is perhaps limited by structural defects revealed in transmission electron micrographs.     

Opto‐electrical modelling and optimization study of a novel IBC c‐Si solar cell

Interdigitated back contact (IBC) crystalline silicon (c‐Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD‐based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo‐junction IBC c‐Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto‐electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front‐side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo‐junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero‐IBC cell). Simulation results show that a not‐smoothed pyramid‐textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low‐doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%_abs as compared with the reference IBC solar cell. Further, by improving both emitter and c‐Si bulk quality, a 22.84% efficient solar cell for 280‐μm thick c‐Si bulk was simulated. Copyright © 2017 John Wiley & Sons, Ltd.     

Parametric studies and optimization of the beta-voltaic cell—II open-circuit voltage and power efficiencies

Theoretical analyses on the silicon beta-violtaic cell have shown that contrary to the short-circuit current the open-circuit voltage is more sensitive to junction parameters than to bulk parameters. Optimum maximum power is obtained by a compromise between high current and high voltage. The resistivity of the silicon substrate under the optimum power condition is found to be about 0·3 Ω . cm. Optimization of overall efficiency involves maximizing the effectivness of the junction diode to collect generated carriers and minimizing the self-absorption in the active source layer; a layer of radioactive Pm₂O₃ with 1·0–1·5 Ci/cm² appears to be optimum. The corresponding overall power efficiency is 2·5 per cent for a cell irradiated from the front side. Higher efficiency is possible when beta particles are injected from both the front and the back of the silicon slice; under these conditions there is an optimum substrate thickness whcih is about 125 μm for maximum efficiency.     

Theoretical analysis on GaAs sub-cell doping concentration for triple-junction solar cells irradiated by proton based on TCAD simulation

The degradation on the GaInP/GaAs/Ge triple-junction solar cells was irradiated by proton, and the solar cells with various GaAs sub-cell doping concentrations are modeled by the technology computer aided design (TCAD) simulation. The degradation results of related electrical parameters and external quantum efficiency (EQE) are studied. The degradation mechanism irradiated by proton is discussed. The short-circuit current, maximum power and conversion efficiency decrease with the increasing of GaAs sub-cell doping concentration. When the base doping concentration of GaAs sub-cell is 1×1016 cm-3, the degradation of short-circuit current is less than that of other base doping concentrations. Furthermore, under proton irradiation, with the increase of doping concentration of GaAs sub-cell, the open-circuit voltage first increases and then decreases. Meanwhile, when the base doping concentration of GaAs sub-cell is 2×1017 cm-3, the degradation of open-circuit voltage is less than that of other base doping concentrations. The research will provide the basic theories and device simulation method for GaInP/GaAs/Ge triple-junction solar cells radiation damage evaluation study and radiation hardening, and can provide guidance for the production of triple-junction solar cells in orbit.     

分子束外延生长的GaSb热光伏电池器件特性研究

利用分子束外延的方法在GaSb衬底上生长GaSb热光伏电池单元,制作了两种不同的1 cm×1 cm面积尺寸的热光伏电池单元,它们有着不同的电极形状。通过不断优化分子束外延的生长条件,以期得到高质量的GaSb外延层。AFM图中显示的表面形貌表明器件有着高质量的外延层,其表面形貌的RMS只有1.5 ? （1 ?=0.1 nm）。测量和比较了两种热光伏电池的器件特性,包括开路电压、短路电流密度、光电转换效率、填充因子以及暗电流密度。在一个模拟太阳光照射下,热光伏电池单元有着0.303 V的开路电压和27.1 mA/cm2的短路电流密度。和只有简单电极形状的热光伏电池单元进行对比,有栅形电极形状的热光伏电池单元在短路电流密度和填充因子上具有更优异的表现。在红外光的照射下,有栅形电极形状的热光伏电池达到了一个最优的填充因子56.8%。     

Heterostructures of metamorphic GaInAs photovoltaic converters fabricated by MOCVD on GaAs substrates

Heterostructures of metamorphic GaInAs photovoltaic converters (PVCs) are on GaAs substrates by the metal-organic chemical vapor deposition (MOCVD) method. It is shown that using a multilayer metamorphic buffer with a step of 2.5% in indium content and layer thicknesses of 120 nm provides the high quality of bulk layers subsequently grown on the buffer up to an indium content of 24%. PVCs with a long-wavelength photosensitivity edge up to 1300 nm and a quantum efficiency of ~80% in the spectral range 1050–1100 nm are fabricated. Analysis of the open-circuit voltage of the PVCs and diffusion lengths of minority carriers in the layers demonstrates that the density of misfit dislocations penetrating into the bulk layers increases at an indium content exceeding 10%.     

Effects on the open-circuit voltage of grain boundaries within the junction space-charge region of polycrystalline solar cells

A physical explanation is given for the observed dependence of open-circuit voltage on grain size in polycrystalline solar cells when no such dependence is seen for short-circuit current. This explanation identifies carrier recombination through grain-boundary surface states within the junction space-charge region as a primary mechanism underlying these dependencies. Experimental data that support this explanation are discussed, and possible ways of improving the conversion efficiency of polycrystalline solar cells are indicated.     

多晶硅太阳电池背表面刻蚀提升其性能的产线工艺研究

对比研究了产线上多晶硅太阳电池背表面刻蚀对 其光电转换性能的影响。示范性实验结果表明:多晶硅太阳电池背表面刻蚀能够改善其短路 电流, 从而相应的光电转换效 率提升了约 0．1％。依据多晶硅太阳电池背表面刻蚀前后的扫描 电镜(SEM)形貌、背表面漫 反射光谱及完整电池片外量子效率的测试结果,改进的光电转换的原因可能源于背表面刻蚀 “镜面”化有利于太阳光子在背表面内反射和改进印刷Al浆与背表面覆盖接触。背表面刻蚀 与当前晶硅电池产线工艺兼容,能够提升电池片的光电转换效率,是一种可供选择的产线升 级工艺。     

18-percent efficient terrestrial silicon solar cells

Silicon solar cells are described which operate at energy conversion efficiencies in excess of 18 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm², 28°C). These are believed to be the most efficient silicon cells reported to date. The high efficiency is a result of the combination of high open-circuit voltage due to the careful attention paid to passivation of the top surface of the cell; high fill factors due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current due to the use of shallow diffusions, a low grid coverage, and an optimized double-layer antireflection coating.     

GaAs/InGaAsN heterostructures for multi-junction solar cells

Solar-cell heterostructures based on GaAs/InGaAsN materials with an InAs/GaAsN superlattice, grown by molecular beam epitaxy, are studied. A p-GaAs/i-(InAs/GaAsN)/n-GaAs p–i–n test solar cell with a 0.9-μm-thick InGaAsN layer has an open-circuit voltage of 0.4 V (1 sun, AM1.5G) and a quantum efficiency of >0.75 at a wavelength of 940 nm (at zero reflection loss), which corresponds to a short-circuit current of 26.58 mA/cm² (AM1.5G, 100 mW/cm²). The high open-circuit voltage demonstrates that InGaAsN can be used as a material with a band gap of 1 eV in four-cascade solar cells.