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.     

High conversion efficiency p-n+InP homojunction solar cells

p-n⁺InP homojunction solar cells have been fabricated and investigated. The best experimental cell without an antireflection coating exhibits a conversion efficiency of 13.5 percent (active area) under AM1 illumination; the corresponding open-circuit voltage, short-circuit current density, and fill factor (F.F.) are 0.817 V, 21.0 mA/cm², and 0.787, respectively.     

Influence of surface texturization on the light trapping andspectral response of silicon solar cells

A quantitative model that explains the spectral response, internal quantum efficiency, total short-circuit current, open-circuit voltage, and efficiency of high-efficiency solar cells with textured front surface and Lambertian back-surface reflectors is presented. A comparison of the textured cell characteristics is made with those of planar cells, and the separate roles of the front surface reflection coefficient and internal quantum efficiency in enhancing the short-circuit current have been investigated. It is shown that, in the case of large diffusion lengths, almost all the contribution to the increase of spectral response on texturization is due to the reduced reflection coefficient whereas, for small diffusion lengths, there is a significant increase in internal quantum efficiency on texturization, especially in the region of higher wavelengths. However, there is a small decrease in open-circuit voltage for large diffusion lengths, whereas no significant change is observed for small diffusion lengths on texturization. Nevertheless, there is a net gain in power conversion efficiency which is larger for smaller diffusion lengths     

Effect of antireflection coatings and coverglasses on silicon solar cell performance

The high reflectivity of the polished silicon surface of the newer N⁺/P silicon solar cells has emphasized the need for properly designed antireflection coatings to obtain improved solar cell performance. The problem is complicated by the facts that solar cells are generally tested in air, but are for their final application covered with a glass or quartz slide which is adhesive-bonded to the cell surface, and further, that solar cells operating in a nuclear particle radiation environment change their spectral response and are frequently optimized for performance at the end of design-life. Experiments have been performed to explore the antireflection characteristics of thin films of silicon monoxide which have been evaporated on the solar cell surface. The effect of the antireflection coating thickness on cell response as a function of wavelength has been determined and the improvement in cell short circuit current for Air Mass Zero space sunlight evaluated. Included in this study was the evaluation of the antireflection characteristics after the application of a coverglass with adhesive over the antireflection coating. For comparison, coverglasses were also applied to bare cells with no antireflection coating present. In all cases the various coating comparisons were based on the cell short-circuit current performance in Air Mass Zero sunlight.     

空间高效硅太阳电池减反射膜设计与数值分析

结合AM0太阳光谱特性对空间硅太阳电池的减反射膜进行了设计分析,得到了最小反射时的最佳膜厚.分别讨论了单、双、三层减反射膜厚度变化对反射率的影响,并对有钝化层的Si O2 (94 nm) / Ti O2 (6 0 nm)双层减反射膜进行了优化设计,优化后硅太阳电池的短路电流和效率分别提高了2 .1%和1.4 % .     

Use of tin oxide as an inexpensive antireflection coating for p on n polycrystalline silicon solar cells

The potential of tin oxide as an inexpensive antireflection (AR) coating for polycrystalline silicon solar cells has been investigated. Undoped tin oxide films of a desired thickness were deposited over p on n polycrystalline silicon solar cells by spray pyrolysis of an alcoholic solution of hydrated stannic chloride at 500°C. Evaluation of cell performance before and after this AR coating showed that the AR coating is highly compatible with the polycrystalline silicon solar cells. About 40-50 percent improvement in the short-circuit current of p on n polycrystalline cells has been measured. The coating may be highly suited to large-scale production of low-cost polycrystalline silicon solar cells for terrestrial application.     

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     

Wide-bandgap epitaxial heterojunction windows for silicon solarcells

It is shown that the efficiency of a solar cell can be improved if minority carriers are confined by use of a wide-bandgap heterojunction window. For silicon (lattice constant a=5.43 Å), nearly lattice-matched wide-bandgap materials are ZnS (a=5.41 Å) and GaP (a=5.45 Å). Isotype n-n heterojunctions of both ZnS/Si and GaP/Si were grown on silicon n-p homojunction solar cells. Successful deposition processes used were metalorganic chemical vapor deposition (MO-CVD) for GaP and ZnS, and vacuum evaporation of ZnS. Planar (100) and (111) and texture-etched ((111) faceted) surfaces were used. A decrease in minority-carrier surface recombination compared to a bare surface was seen from increased short-wavelength spectral response, increased open-circuit voltage, and reduced dark saturation current, with no degradation of the minority carrier diffusion length     

Minority-carrier lifetime measurements on silicon solar cells using Iscand Voctransient decay

Measurements of the transient decay of short-circuit current and open-circuit voltage of solar cells provide sufficient information, in principle, to determine both the effective back-surface recombination velocity S and the base minority-carrier lifetime τ. The practical use of the method is illustrated by an example, and the technique is applied to a variety of cells. Analysis of the effect of different cell thickness on the measurement is included. Finally, some limitations, both fundamental and experimental, are discussed.     

High-efficiency silicon solar cells: Full factor limitations and non-ideal diode behaviour due to voltage-dependent rear surface recombination velocity

Despite exceptionally high open-circuit voltages, record high-efficiency PERL (passivated emitter, rear locally diffused) silicon solar cells recently developed at the University of New South Wales demonstrate relatively low fill factors. This behaviour is shown to result from a surface recombination velocity at the rear Si-SiO₂ interface that increases with reducing voltage across the cell, leading to non-ideal I-V curves with high ideality factors (>1.3) near the maximum power point. When corrected for series resistance losses, the Air Mass 1.5 (AM1.5) fill factor of actual PERL cells is found to be limited to values below 82.9%, as opposed to the ideal theoretical limit of 85-86% for silicon cells operating in low injection conditions. Relatively large series resistance losses (R_s > 0.35 ω cm²) further reduce this value to the experimentally observed fill factors below 81.4%. Analysis of measured illuminated and dark I-V characteristics of PERL cells reveals that the AM1.5 efficiency is mainly limited by recombination losses at the rear oxidized surface. Optimum PERL cell resistivity is about 2 ω cm. Owing to increased rear surface recombination velocity, lower resistivity material shows no advantage in open-circuit voltage and suffers from short-circuit current losses, while a strong reduction in the surface recombination velocity above the maximum power point results in smaller fill factors. High-resistivity cells do show an improved short-cuircuit current but suffer from voltage and fill factor losses.     

Device operation of organic tandem solar cells

A generalized methodology is developed to obtain the current–voltage characteristic of polymer tandem solar cells by knowing the electrical performance of both sub cells. We demonstrate that the electrical characteristics of polymer tandem solar cells are correctly predicted for both the series and parallel connection of the sub cells. The agreement with experiments allows us to investigate the effect of a reduced open-circuit voltage, short-circuit current or fill factor in one of the sub cells on the performance of the tandem cell. A low fill factor in one of the sub cells leads to a stronger reduction of the efficiency in a series configuration as compared to the parallel tandem device.     

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.     

Performance enhancement of multicrystalline silicon solar cells and modules using double‐layered SiNx:H antireflection coatings

Silicon nitride coating deposited by the plasma‐enhanced chemical vapor deposition method is the most widely used antireflection coating for crystalline silicon solar cells. In this work, we employed double‐layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. An optimization procedure was presented for maximizing the photovoltaic performance of the encapsulated solar cells or modules. The dependence of their photovoltaic properties on the thickness of silicon nitride coatings was carefully analyzed. Desirable thicknesses of the individual silicon nitride layers for the double‐layered coatings were calculated. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the standard production line for both the double‐layered and single‐layered antireflection coating types. On the cell level, the double‐layered silicon nitride antireflection coating resulted in an increase of 0.21%, absolute for the average conversion efficiency, and 1.8 mV and 0.11 mA/cm² for the average open‐circuit voltage and short‐circuit current density, respectively. On the module level, the cell to module power transfer factor was analyzed, and it was demonstrated that the double‐layered silicon nitride antireflection coating provided a consistent enhancement in the photovoltaic performance for multicrystalline silicon solar cell modules than the single‐layered silicon nitride coating. Copyright © 2015 John Wiley & Sons, Ltd.     

Thickness dependent gate oxide quality of thin thermal oxide grownon high temperature formed SiGe

For thin oxides grown on high temperature formed Si_0.3Ge_0.7, the gate oxide quality is strongly dependent on oxide thickness and improves as thickness reduces from 50 to 30 Å. The thinner 30 Å oxide has excellent quality as evidenced by the comparable leakage current, breakdown voltage, interface-trap density and charge-to-breakdown with conventional thermal oxide grown on Si. The achieved good oxide quality is due to the high temperature formed Si_0.3Ge_0.7 that is strain relaxed and stable during oxidation. The possible reason for strong thickness dependence may be due to the lower GeO₂ content formed in thinner 30 Å oxide rather than strain relaxation related rough surface or defects     

Performance Enhancement of Crystalline Silicon Solar Cells by Coating with Luminescent Silicon Nanostructures

In this work we report a technique that is potentially capable of increasing the efficiency of crystalline silicon solar cells, which dominate the present-day market of photovoltaic devices. The simple and cost-effective method involves coating the surface of a commercially procured silicon solar cell with luminescent silicon nanocrystals. Core/shell silicon/silicon-oxide nanostructures are fabricated by an inexpensive and reproducible technique, where coarse silicon powders are repeatedly milled, oxidized, and etched until their sizes are reduced so as to exhibit room-temperature photoluminescence under ultraviolet excitation. A thin coating of these nanostructures on a standard solar cell, obtained by a simple dip-coating method, increases the open-circuit voltage and short-circuit current, which consequently increases the maximum power delivered by ~16.3% and efficiency by almost ～39%. We propose that the core/shell nanostructures act as luminescent convertors that convert higher-energy photons to lower-energy photons, thereby leading to less thermal relaxation loss of photoexcited carriers.     

17.3% efficiency metal-oxide-semiconductor (MOS) solar cells withliquid-phase-deposited silicon dioxide

Simple and high efficiency silicon metal-oxide semiconductor (MOS) solar cells, with silicon dioxide prepared by a room-temperature liquid phase deposition (LPD) method, are proposed. The thickness of LPD oxide is about 5 nm. After adding a 2~5 nm semi-transparent thin Al film between the 200 nm patterned Al cathode, all the solar cells' performance parameters are improved. For a cell exposed under 15 mW/cm ², short-circuit current density J_SC up to 10.7 mA/cm², open-circuit voltage V_OC up to 412 mV, fill factor FF up to 59, and record effective conversion efficiency η up to 17.3% are obtained for this structure. Photo-conductivity properties of LPD oxide are found and the mechanism is discussed     

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%。     

High-efficiency silicon minMIS solar cells—Design and experimental results

Grating minority-carrier Metal-Insulator-Semiconductor (minMIS) solar cells have been fabricated on a range of Czochvalski (CZ) and float-zone (FZ) substrates. The most efficient cells result when the semiconductor surface between the grating lines is inverted. This can be achieved for p-type substrates with minimal interference to light transmission using the fixed positive charge in antireflection (AR) coating materials such as SiO. To characterize the measured short-circuit current of grating cells, the concept of an effective minority-carrier collection distance Δ is introduced. Δ is shown to be proportional to the inverse square root of input light power. Knowing Δ, an appropriate grating spacing can be determined. Record open-circuit voltages of 655 mV (AM0, 25°C) for 0.1-Ω. cm FZ substrates have been achieved. AM1 active area efficiencies of 17.6 percent for polished substrates and 17.4 percent for textured substrates have been measured.     

Analysis of band-to-band tunneling leakage current intrench-capacitor DRAM cells

Evidence demonstrating that the band-to-band tunneling leakage current occurs mainly at the edge of the self-aligned isolation rather than the trench upper corners is presented. Moreover, the leakage current increases drastically with the decrease of capacitor oxide thickness. It is shown that the leakage current limits the thickness of capacitor oxide to more than 80 Å even if the operation voltage is reduced to 3.3 V from 5 V     

Fabrication of inverted pyramid structure by Cesium Chloride self-assembly lithography for silicon solar cell

Inverted pyramids were fabricated through a method combining cesium chloride (CsCl) self-assembly technology and anisotropy corrosion of silicon solar cells. Ti film with nanoporous masks was formed by lift-off the CsCl nanoislands for the inverted pyramids. The pyramids were then formed by anisotropy corrosion of alkaline solution. The average diameter and morphology of the pyramids were controlled by varying the average diameter of CsCl nanoislands from 400 nm to 1.5 µm and by varying the etching time of alkaline solution from 2 to 8 min. The inverted-pyramid texture could suppress reflection to below 10% at wavelengths from 400 to 1000 nm, which was much lower than that of planar wafer. A solar cell fabricated from the pyramids had higher short-circuit current density (J_sc) and photovoltaic conversion efficiency (PCE) compared with those of planar solar cells for the good antireflection property. The solar cell showed a PCE of 15.25%, a J_sc of 38.35 mA/cm², and an open-circuit voltage of 555.7 mV.