陷光结构应用于太阳能电池的研究进展

在太阳能电池中引入陷光结构是提高光电转换效率的一种重要方法。本文主要从晶体硅太阳能电池、薄膜太阳能电池和其他新型太阳能电池三方面,综述了近年来国内外陷光结构用于太阳能电池的最新研究进展,分析了陷光结构对各类太阳能电池性能的影响、陷光作用的原理及工艺手段,最后指出其发展潜力及未来的研究方向。     

高效率双结钙钛矿叠层太阳能电池研究进展

以钙钛矿电池为顶电池的叠层太阳电池发展迅速,成为太阳能光伏领域的研究热点之一。随着电池结构和制备工艺的优化,叠层电池的光电转换效率快速提升,单片钙钛矿/晶硅叠层电池的效率已达到31.3%。本综述对近年来以宽带隙钙钛矿电池作为顶子电池、晶体硅电池及其他新型中窄带隙电池(钙钛矿电池、有机电池、铜铟镓硒(CIGS)电池)作为底子电池的叠层电池的研究进展进行了系统梳理,总结了叠层电池的顶电池、中间互联层和底电池的材料、结构及光电性能等方面的关键技术及难点,希望能够为进一步提升叠层电池效率提供一些思路。并对未来低成本高效叠层太阳能电池的光学和电学优化需求做出了分析与展望。     

Thermal effects and plasma damage upon encapsulation of polymer solar cells

A barrier structure consisting of silicon oxide and silicon nitride films was deposited via plasma-enhanced chemical vapor deposition (PECVD) for the encapsulation of polymer solar cells (PSCs). The total concentration of the solution and the ratio of P3HT and PCBM on the performance of polymer solar cells were studied by UV-Vis absorption spectroscopy, atomic force microscopy and photocurrent measurement. Base on these measurements, there is a compromise between light absorption and phase separation with increasing blend concentration. The PSCs were annealed at 80, 100, 120 and 140 °C for 10-60 min to investigate the thermal effects and to estimate the best deposition temperature of the barrier layers. Nevertheless, the devices with the encapsulation of barrier layers had relatively low power conversion efficiencies (PCE) of 0.98% comparing to the devices heated in the PECVD system (1.57%) at the same condition of 80 °C for 45 min due to the plasma damage during the film deposition process. After inserting a 5-nm TiO_x layer between Al/barrier structure and active layer against the plasma damage, the annealed devices presented an average PCE of 2.26% and demonstrated over 50% of their initial value after constant exposure to ambient atmosphere and sunlight for 1500 h.     

Development of n-type microcrystalline SiO<Subscript>x</Subscript>:H films and its application by innovative way to improve the performance of single junction µc-Si:H solar cell

Light trapping is one of the fundamental necessities of thin film based solar cell for its performance elevation. Back reflection of unused light of first pass is the key way to improve the light trapping phenomena. In this study we have reported the development of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiO:H) layers of different characteristics. The deposition has been done by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The detailed characterization of the films include the following: (1) electrical properties (2) optical properties like E₀₄ (3) structural studies which include crystalline fraction by Raman spectroscopy and grain size by X-ray diffraction measurement, FTIR spectroscopy, AFM and TEM studies. n-µc-SiO:H layer has been introduced as the n-layer of single junction p–i–n structure µc-Si solar cells. By various techniques the optimum use of n-µc-SiO:H layer for enhancing the performance of µc-Si:H solar cells has been done. It has been found that by using suitable bilayer of two different n-µc-SiO:H layers, it is possible to increase the solar cell performances. The maximum efficiency obtained without any back reflector is 8.44% that is about 8.9% higher than that obtained by using n-µc-Si:H layer as n-layer in the solar cells.     

Atmospheric pressure chemical vapour deposition of 3C-SiC for silicon thin-film solar cells on various substrates

The production of crystalline silicon thin-film solar cells on cost effective ceramic substrates depends on a highly reliable diffusion barrier to separate the light absorbing layers from the substrate. Ideally this intermediate layer should be deposited with cost effective techniques, be conductive and should feature optical confinement. Furthermore the intermediate layer should withstand high temperatures and harsh chemical environments like they occur during solar cell processing. Especially stability against oxidizing solvents like HNO3 or inactivity during e.g., oxide removing steps with HF is required. Crystalline silicon carbide (c-SiC) deposited by atmospheric pressure chemical vapour deposition (APCVD) can match all those requirements and additionally fits the thermal properties of crystalline silicon. The c-SiC intermediate layer is deposited from methyltrichlorosilane (MTS) and H2 at 1100 degrees C. Under these conditions, growth of solely cubic 3C-SiC could be observed by X-ray diffraction measurements. Use of such intermediate layers during high temperature steps prevents diffusion of transition metals, originating from the substrates, into active silicon layers. Doping of these 3C-SiC layers with nitrogen results in specific resistivity of less than 100 ohms cm. The different potentially cost-effective substrates are made from graphite, crystalline silicon, sintered silicon carbide and sintered zircon (ZrSiO4). Surface properties of the coated substrates were investigated, explaining changes in surface roughness and influences on the solar cell processing.     

Large area ZnO:Al films with tailored light scattering properties for photovoltaic applications

Aluminum-doped zinc oxide films on glass are promising substrates for use in thin film solar cells based on amorphous and amorphous/microcrystalline silicon absorber material. The films can be produced by magnetron sputtering on large scale at relative low cost. Especially reactive sputtering of metallic Zn/Al compound targets is a cheap way to produce films at high deposition rate. One drawback of amorphous silicon is the low absorption in the near infrared spectral range. Wet chemical etching has been used to produce a rough TCO surface that enables light trapping in the absorber. The etching behaviour of ZnO:Al films can be tuned by changing oxygen partial pressure during deposition. The etching behaviour is compared to ZnO structure and discussed regarding the performance of solar cells deposited onto the etched films.     

Growth of TiO2 anti-reflection layer on textured Si (100) wafer substrate by metal-organic chemical vapor deposition method

Recently anti-reflective films (AR) have been intensely studied. Particularly for textured silicon solar cells, the AR films can further reduce the reflection of the incident light through trapping the incident light into the cells. In this work, TiO2 anti-reflection films have been grown on the textured Si (100) substrate which is processed in two steps, and the films are deposited using metal-organic chemical vapor deposition (MOCVD) with a precursor of titanium tetra-isopropoxide (TTIP). The effect of the substrate texture and the growth conditions of TiO2 films on the reflectance has been investigated. Pyramid size of textured silicon had approximately 2-9 microm. A well-textured silicon surface can lower the reflectance to 10%. For more reduced reflection, TiO2 anti-reflection films on the textured silicon were deposited at 600 degrees C using titanium tetra-isopropoxide (TTIP) as a precursor by metal-organic chemical vapor deposition (MOCVD), and the deposited TiO2 layers were then treated by annealing for 2 h in air at 600 and 1000 degrees C, respectively. In this process, the treated samples by annealing showed anatase and rutile phases, respectively. The thickness of TiO2 films was about 75 +/- 5 nm. The reflectance at specific wavelength can be reduced to 3% in optimum layer.     

Radial junction amorphous silicon solar cells on PECVD-grown silicon nanowires

Constructing radial junction hydrogenated amorphous silicon (a-Si:H) solar cells on top of silicon nanowires (SiNWs) represents a promising approach towards high performance and cost-effective thin film photovoltaics. We here develop an all-in?situ strategy to grow SiNWs, via a vapour-liquid-solid (VLS) mechanism on top of ZnO-coated glass substrate, in a plasma-enhanced chemical vapour deposition (PECVD) reactor. Controlling the distribution of indium catalyst drops allows us to tailor the as-grown SiNW arrays into suitable size and density, which in turn results in both a sufficient light trapping effect and a suitable arrangement allowing for conformal coverage of SiNWs by subsequent a-Si:H layers. We then demonstrate the fabrication of radial junction solar cells and carry on a parametric study designed to shed light on the absorption and quantum efficiency response, as functions of the intrinsic a-Si:H layer thickness and the density of SiNWs. These results lay a solid foundation for future structural optimization and performance ramp-up of the radial junction thin film a-Si:H photovoltaics.     

High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography

Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of 99% (upper bound) over wavelengths 400-1100 nm by a thickness of only 5 μm, is realized by our recently reported technique self-powered parallel electron lithography that has high-throughput and sub-35-nm high resolution. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. These related fabrication techniques can also be transferred to low-cost substrate solar energy harvesting device applications.     

Plasmabeschichtungsverfahren für die Solarzellenherstellung

Photovoltaics, i.e. the conversion of light into electric energy, is an important link of the chain of regenerative energies, at which the further growing of the photovoltaic industry depends on the reducing of the costs from the manufacturing of the solar modules. Potential to reach the aim are cheaper basic material for the solar cells, as well as the reducing of the cost to manufacture cells and modules and the increasing efficiency of the solar cells. An essential process step during production of crystalline silicon solar cells is their coating with an antireflective and passivation layer. Amorphous hydrogenated silicon nitride (SiN) layers, deposited under vacuum by PECVD (Plasma Enhanced Chemical Vapour Deposition), have been demonstrated to be – besides good anti‐reflection layers – excellent means for surface and bulk passivation of silicon solar cells. Therefore they are especially effecting a significant increase of the conversion efficiency of multi‐crystalline silicon solar cells. With the common concept “SiNA” a series of effective, industrial scale systems for the SiN‐coating has been developed and qualified, caused by its excellent layer properties, high throughput, high up‐time and a maintenance‐friendly design. Consecutively performance parameters of the systems and relevant data of their use under production conditions are introduced.     

Crystalline Silicon Thin‐Film Solar Cells on Silicon Nitride Ceramic Substrates

Photovoltaics—the generation of electricity from sunlight—is a technically challenging but environmentally benign technology to generate electricity with a large economical potential. However, the major hurdle for its widespread usage is its present high cost. Various thin‐film solar cell technologies are investigated to bring down the total cost to an economic value. One of them, the crystalline silicon thin‐film (CSiTF) solar cell combines the advantages of conventional wafer‐based silicon solar cells such as high efficiency and non‐toxicity with the benefits of thin‐film technologies such as serial interconnection and large area deposition. This paper reports for the first time the preparation of CSiTF solar cells on specially developed Si₃N₄ ceramic substrates. Three different types of Si₃N₄ ceramic wafers were single‐sided coated with 10μm of microcrystalline silicon, which was recrystallized by a zone melting step and subsequently thickened to approx. 30 μm. Optical analysis of the layer surface and cross sections was done to determine the crystallographic properties of the silicon layers, as well as mass spectroscopy to measure the concentration of transition metal impurities. A one‐side contacted solar cell process was applied on non‐conducting Si₃N₄ substrates. The best 1 cm² cells achieved an efficiency of 8.0 % with an excellent fill factor of 74 % and an open circuit voltage of 554 mV. The solar cell characterization was complemented by measurements of dark current–voltage characteristics, spectrally resolved light beam induced current mapping, and external quantum efficiency.     

Bismuth-catalyzed and doped silicon nanowires for one-pump-down fabrication of radial junction solar cells

Silicon nanowires (SiNWs) are becoming a popular choice to develop a new generation of radial junction solar cells. We here explore a bismuth- (Bi-) catalyzed growth and doping of SiNWs, via vapor-liquid-solid (VLS) mode, to fabricate amorphous Si radial n-i-p junction solar cells in a one-pump-down and low-temperature process in a single chamber plasma deposition system. We provide the first evidence that catalyst doping in the SiNW cores, caused by incorporating Bi catalyst atoms as n-type dopant, can be utilized to fabricate radial junction solar cells, with a record open circuit voltage of V(oc) = 0.76 V and an enhanced light trapping effect that boosts the short circuit current to J(sc) = 11.23 mA/cm(2). More importantly, this bi-catalyzed SiNW growth and doping strategy exempts the use of extremely toxic phosphine gas, leading to significant procedure simplification and cost reduction for building radial junction thin film solar cells.     

Amorphous/crystalline silicon heterojunction solar cells with varying i-layer thickness

We study the effect on various properties of varying the intrinsic layer (i-layer) thickness of amorphous/crystalline silicon heterojunction (SHJ) solar cells. Double-side monocrystalline silicon (c-Si) heterojunction solar cells are made using hot-wire chemical vapor deposition on high-lifetime n-type Czochralski wafers. We fabricate a series of SHJ solar cells with the amorphous silicon (a-Si:H) i-layer thickness at the front emitter varying from 3.2 nm (0.8xi) to ~ 96 nm (24xi). Our optimized i-layer thickness is about 4 nm (1xi). Our reference cell (1xi) performance has an efficiency of 17.1% with open-circuit voltage (V_oc) of 684 mV, fill factor (FF) of 76%, and short-circuit current density (J_sc) of 33.1 mA/cm². With an increase of i-layer thickness, V_oc changes little, whereas the FF falls significantly after 12 nm (3xi) of i-layer. Transient capacitance measurements are used to probe the effect of the potential barrier at the n-type c-Si/a-Si interface on minority-carrier collection. We show that hole transport through the i-layer is field-driven transport rather than tunneling.     

Elektronenstrahlverdampfen von Silizium

Electron beam evaporation of silicon High rate electron beam evaporation of silicon is a versatile technique to deposit silicon layers with tailored properties on large areas in a cost effective manner. A unique feature of the process is the wide range of deposition rates that can be selected. This technique allows for the deposition of nanometer thin layers with high reproducibility on the one hand, on the other hand layers with thicknesses of several tens of micrometers and low film stress can be deposited within minutes. Due to the high quality of the deposited silicon it is possible to form ultra‐thin layers as contacts in solar cells or for TFT applications or several 10 micrometer thick films as absorbers in LPC solar cells or as “construction” material for MEMS. This article reports on the deposition process, the material properties and illustrates the applicability based on examples from our current research activities.     

Microcrystalline silicon carbide alloys prepared with HWCVD as highly transparent and conductive window layers for thin film solar cells

Crystalline silicon carbide alloys have a very high potential as transparent conductive window layers in thin-film solar cells provided they can be prepared in thin-film form and at compatible deposition temperatures. The low-temperature deposition of such material in microcrystalline form (µc-Si:C:H) was realized by use of monomethylsilane precursor gas diluted in hydrogen with the Hot-Wire Chemical Vapor Deposition process. A wide range of deposition parameters has been investigated and the structural, electronic and optical properties of the µc-SiC:H thin films have been studied. The material, which is strongly n-type from unintentional doping, has been used as window layer in n-side illuminated microcrystalline silicon solar cells. High short-circuit current densities are obtained due to the high transparency of the material resulting in a maximum solar cell conversion efficiency of 9.2%.     

A simulation study of thin film tandem solar cells with a nanoplate absorber bottom cell

A tandem thin film solar cell with a nanoplate absorber bottom cell that can solve the trade-off between light absorption and carrier transport in thin film solar cell is investigated. This structure has an n-type microcrystalline silicon nanoplate array on the substrate, and the p- and i-layers are sequentially grown along the surface of each n-type microcrystalline silicon nanoplate for bottom cell. After above bottom cell is fabricated, a similar process is used to fabricate an amorphous Si p-i-n top cell. High-aspect-ratio width/height nanoplates allow for the use of a material with sufficient thickness to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers perpendicular to light absorption. The power conversion efficiency of nanoplate solar cells with 15,000 nm plate height is around 10%, which is an approximately 40% enhancement over a planar solar cell with a similar layer stack.     

Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.     

n型层对柔性衬底微晶硅太阳电池特性的影响

在不锈钢柔性衬底上采用等离子体化学气相沉积(PECVD)方法制备了不同结构的n型硅薄膜,测试了在其上生长的微晶硅太阳电池的电学输出特性.发现太阳电池的开路电压随n型层的硅烷浓度线形变化,短路电流密度则存在一个最优值,这与n型层引起的本征层中的孵化层和结构演变有关.将优化后的n型层应用于不锈钢柔性衬底的非晶硅/微晶硅叠层...     

Incorporation of amorphous and microcrystalline silicon in n–i–p solar cells

We have investigated the material properties and n–i–p solar cell quality of hot-wire deposited amorphous and microcrystalline silicon. Although it is possible to make high quality amorphous silicon solar cells, occasionally many cells show shunting behavior. Therefore, better control over the variation in cell performance is needed. We prove that this behavior is correlated with the filament age and different methods for improving the reproducibility of the cell performance are presented. Furthermore, the influence of different deposition parameters of microcrystalline silicon layers on the material and solar cell properties was studied. Although some of these microcrystalline layers are porous and oxidize in air, an initial efficiency of 4.8% is obtained for an n–i–p cell on untextured stainless steel.     

Electrical insulation and bending properties of SiOx barrier layers prepared on flexible stainless steel foils by different preparing methods

Stainless steel foils on which flexible display devices and integrated solar modules are prepared need to be coated by barrier layers for electrical insulation. In this study, SiO_x barrier layer was prepared on steel foils (SUS 304) by ion beam assisted deposition, Sol-gel deposition and plasma enhanced chemical vapor deposition, respectively. The electrical properties of the SiO_x films, such as resistance, reactance, leakage current density, breakdown field strength and performance index were investigated, and the bending properties were evaluated by bending tests. The best electrical insulation and bending properties of barrier could be achieved with 4 μm thick SiO_x layer prepared by plasma enhanced chemical vapor deposition.