硅太阳电池稳步走向薄膜化

考察了硅太阳电池在光伏产业中所处的地位,分析了薄膜硅太阳电池的发展趋势。指出硅太阳电池在未来15a仍将保持优势地位,并继续沿着晶硅电池和薄膜硅电池两个方向发展。在此发展过程中,两个发展方向的主流很可能会汇合到一起,共同促使低成本、高效率、高可靠薄膜晶硅电池的诞生和产业化,从而继续保持硅太阳电池的优势地位。     

硅太阳电池材料的研究进展

目前各种太阳电池材料中，硅是最主要的材料。文章简要介绍单晶硅、多晶硅、带状硅、非晶硅以及多晶硅薄膜材料的研究状况，并对有关问题和太阳电池材料的发展趋势进行了讨论。     

Penetration of molten silicon into a bed of fines

During the casting process of silicon the mould must be buffered from the high temperatures of the molten silicon to prevent the mould melting and this is typically done by the operators laying down a layer of crushed silicon particles (fines) prior to pouring of the molten silicon. It is useful for operators to know how deep they should make the layer the fines so as to adequately separate the molten silicon from the mould. In this paper, we consider a model for the penetration of molten silicon into the pre-laid layer of silicon fines, which provides a predictive tool for estimating the necessary depth of fines in order to prevent the molten silicon touching the mould. The mathematical model developed here considers the flow of molten silicon as a Darcy flow and solidification due to heat flow as a one-phase Stefan problem. We are able to find a numerical solutions to this model, and from this we are able to extract data regarding the penetration depth of the molten silicon into the fines before solidification occurs. Our model and numerical solution can been seen as a first step toward understanding this important part of the casting process for silicon.     

Silicon and solar-grade silicon production by solar dissociation of Si3N4

The temperature required for carbothermal reduction of silica—in the range 2100–2300 °C—is past the upper limit for combustion process heat. It is therefore an interesting candidate for solar–thermal processing. The production of solar-grade silicon from carbothermally produced silicon requires an energy-intensive long-duree high-temperature purification process. We propose here a two-step solar process for the production of silicon from silica: first, a carbothermal reduction in the presence of nitrogen to yield silicon nitride and second, the solar dissociation of the nitride to yield silicon. This last step could be combined with purification of the silicon if solar-grade silicon is the desired end-product. In this paper, we report on experimental results that indicate that silicon nitride can be dissociated to yield silicon with no detectable nitride content.     

铸造单晶硅性能和应用分析

以G6型多晶硅定向凝固铸锭炉生长的铸造单晶硅为研究对象,对其性能特点及应用进行分析.铸造单晶硅中含有位错、亚晶粒、多晶晶粒、间隙态元素和硬质点等缺陷.在铸造单晶硅制备过程中,因长晶界面不平及杂质存在的原因,硅锭生长时存在较大的应力,致使硅原子排列出现错排,从而导致位错、亚晶粒和多晶晶粒的产生,其中多晶晶粒的晶向主要有(...     

Porous silicon in solar cell structures: a review of achievements and modern directions of further use

Porous silicon, which is being obtained by electrochemical etching of silicon wafers in electrolytes on the base of hydrofluoric acid, recently attracted the attention of specialists in photovoltaics even more due to a number of its unique properties. However, at present, acceptable results are obtained for the use of porous silicon as antireflecting coating for silicon solar cells only. In the present paper, previous experience of the use of por-Si in the silicon solar cells has been reviewed. On the base of examination of the porous silicon properties, a number of new directions of improvement of photoconversion efficiency of structures with optimized layers of porous silicon are proposed. The results of numerical calculations carried confirm perspectiveness of use of porous silicon for efficiency improvement for different types of silicon solar cells. These can be increased of their internal quantum efficiency, expansions of operating spectral range toward ultra-violet and infrared spectrum range, decrease of losses of photogenerated power due to the influence of bulk and surface recombination.     

Silicon feedstock for the multi-crystalline photovoltaic industry

During the last 5 years the PV industry continues to experience a strong economic growth between 15% and 30% per year. Multi-crystalline silicon became the preferred material for PV production with a share of more than 50% of the shipped PV modules world-wide. For the first time, the available quantity of the classical silicon feedstock sources for the PV industry—electronic grade silicon rejects from the silicon and microelectronics industry—is close to be not sufficient to satisfy the requirements of the PV industry. From this situation arises the need to develop short- and long-term solutions to guarantee a sustainable supply of the PV industry with suitable silicon feedstock at acceptable costs.This paper presents a possible route for short- and long-term solutions to provide solar grade (SoG) silicon feedstock for the PV industry. On a short-term basis a twofold solution is proposed: (i) reduction of silicon consumption by reducing the wafer thickness and the introduction of recycling scenarios for silicon waste produced by the PV industry, (ii) introduction of very low-resistivity silicon (0.1 Ω cm).On long term, a route towards the establishment of a SoG silicon production based on widely available metallurgical grade silicon is proposed. This route includes the development of suitable purification techniques. First results that allowed to lower the impurity tolerances for SoG silicon are presented. The introduction of silicon feedstock with higher impurity concentrations which show a tendency to interact with crystal defects and lead to a degradation of the material performance also requires passivation concepts to achieve highly performing solar cells.     

A simple process to remove boron from metallurgical grade silicon

It is necessary to develop solar grade (SoG) silicon for the photovoltaic industry. A desirable approach is to upgrade metallurgical grade (MG) silicon. The most problematic impurities to remove from MG silicon are B and P. A simple process to remove B from MG silicon has been developed by refining MG silicon in the molten state followed by directional solidification. With this approach, B has been reduced to 0.3 ppma, P to <10 ppma and all other impurities to <0.1 ppma using commercially available, as-received MG silicon. It remains to develop a similar P reduction process so that SoG silicon production from MG silicon can be commercialized. The B-removal process was applied to B overdoped electronic grade silicon, and the resulting material was used for crystal growth. Test solar cells of 12.5–13.4% (1 cm²) efficiency were produced.     

Increased cycling efficiency and rate capability of copper-coated silicon anodes in lithium-ion batteries

Cycling efficiency and rate capability of porous copper-coated, amorphous silicon thin-film negative electrodes are compared to equivalent silicon thin-film electrodes in lithium-ion batteries. The presence of a copper layer coated on the active material plays a beneficial role in increasing the cycling efficiency and the rate capability of silicon thin-film electrodes. Between 3C and C/8 discharge rates, the available cell energy decreased by 8% and 18% for 40 nm copper-coated silicon and equivalent silicon thin-film electrodes, respectively. Copper-coated silicon thin-film electrodes also show higher cycling efficiency, resulting in lower capacity fade, than equivalent silicon thin-film electrodes. We believe that copper appears to act as a glue that binds the electrode together and prevents the electronic isolation of silicon particles, thereby decreasing capacity loss. Rate capability decreases significantly at higher copper coating thicknesses as the silicon active material is not accessed, suggesting that the thickness and porosity of the copper coating need to be optimized for enhanced capacity retention and rate capability in this system.     

Characteristics of structural defects in the 240 kg silicon ingot grown by directional solidification process

Multi-crystalline silicon for solar cell over single crystalline silicon is its capability of using cheaper raw material. Since cheaper material contains harmful metal impurities, gettering technology has to be applied to silicon wafers to reduce the metal content in the crystal. Low dislocation density in the 240 kg multi-crystalline silicon crystal provides the strong possibility of gettering for the low cost silicon solar cell. Saw damage induced during the slicing process of multi-crystalline silicon ingot was confirmed to generate dislocation loops which can be employed for extrinsic gettering.     

非晶硅太阳能光伏/光热空气集热器性能对比实验研究

文章设计了新型非晶硅太阳能PV/T空气集热器,该空气集热器能够解决传统太阳能PV/T热水器在高温波动情况下,晶硅电池热应力大的问题,同时避免了冬季管道发生霜冻的现象。文章通过实验对比,分析了非晶硅太阳能PV/T空气集热器、单独非晶硅光伏电池和传统太阳能空气集热器的能量效率和[火用]效率的差异。分析结果表明:非晶硅太阳能PV/T空气集热器的平均热效率为45.70%,比传统太阳能空气集热器的平均热效率降低了约25.88%;当空气质量流量增大至0.048 kg/s时,非晶硅太阳能PV/T空气集热器中的非晶硅光伏电池的平均电效率高于单独非晶硅光伏电池,它们的平均电效率分别为4.70%,4.54%;非晶硅太阳能PV/T空气集热器的总[火用]效率高于传统太阳能空气集热器的热[火用]效率和单独非晶硅光伏电池的电[火用]效率,非晶硅太阳能PV/T空气集热器总[火用]效率最大值为7.14%。文章的分析结果为非晶硅太阳能PV/T空气集热器的推广提供了参考。     

Enhancement of on-chip combustion via nanoporous silicon microchannels

Due to its high energy density and MEMS compatible fabrication methods, on-chip porous silicon shows considerable promise as an energetic material. Rapid combustion events have been demonstrated with flame propagation speeds eclipsing 3 km/s, but much is still unknown about the controlling parameters of porous silicon combustion. Recent studies show that implementation of microstructure within a nanoporous silicon film greatly increases reaction rate of a relatively slow burning system. The present work utilizes porous silicon microchannels to enhance an already rapidly-reacting system. Reactions in channeled porous silicon regions of this system propagated at speeds up to 1.2 km/s faster than similar neat porous silicon films. The fastest propagation speed was 3660 m/s, the highest reported flame speed for comparable nanoenergetic systems to date. We provide evidence that the enhancement of flame propagation rates by channeled porous silicon is mechanistically different from the convectively controlled burning of neat porous silicon. This evidence suggests the presence of acoustically aided reactions for porous silicon channel combustion where the channels more readily ignite compared to neat porous silicon. We predict this allows for propagation of the reaction by intense sound waves within the porous medium.     

Minimizing lifetime degradation associated with thermal oxidation of upright randomly textured silicon surfaces

Thermally grown silicon dioxide is commonly used in high-efficiency mono-crystalline silicon solar cell designs as a diffusion mask, electroless plating mask, passivation layer or as a rudimentary anti-reflection coating. Some of these device designs also use upright random texturing, to minimize front surface reflection, in combination with thermally grown silicon dioxide layers. The volume expansion associated with oxidation, generates large stresses on the silicon substrate when oxidizing sharp surface features. If this stress exceeds the fracture stress of silicon, defects are generated within the bulk region. This paper reviews oxidation-induced stress, and highlights two techniques for reducing lifetime degradation during the growth of silicon dioxide on textured wafers: (i) reducing the generation of stress, and (ii) decrease the relaxation time of the silicon dioxide layer. This paper experimentally investigates, and evaluates, several practical methods for implementing these techniques on upright random textured p-type float zoned silicon using photoconductance lifetime.     

离心铸造太阳电池硅片液态成形机制的研究

为了将离心铸造技术成功地移植到低成本超薄多晶太阳电池硅片的成形工艺上，提出了ＥＬＣＣ技术的硅片液态成形方法，即将铸模型腔预热至硅熔点以上温度，过热的硅液被浇注到型腔后，在离心力的作用下始终保持液态充型。这种成形机制易于实现厚度小于１ｍｍ的硅片的完整成形，而且对模具转速、硅液过热度等要求较低。采用该方法，硅片的成形与结晶不会同时发生，可以在硅片液态成形后，采用定向凝固的方法获得粗大的定向柱晶组织，提高硅片的光伏性能。采用理论分析、计算机模拟与工艺实验相结合的方法，对ＥＬＣＣ技术硅片液态成形机制进行了研究，为进一步对硅片凝固过程组织控制的研究奠定基础。     

Induction melting process using segmented graphite crucible for silicon melting

Induction melting process using segmented graphite crucible was investigated to melt silicon feedstock for solar cells. Induction melting is the key technology used in silicon melting process, such as ingot-growing, metallurgical refining and ribbon production. However, contamination of silicon from the crucible in induction melting is the main source of degrading the silicon. In this paper, new structure of crucible was suggested to minimize the contamination from the crucible wall. It was the segmentation of graphite crucible wall that introduced the non-contact between melt and inner wall of crucible. Numerical and experimental studies of induction melting process of silicon using the suggested crucible were conducted. For numerical analysis, 3D models of crucible, induction coil and silicon were constructed and electromagnetic force and temperature distribution in silicon and crucible were calculated. To evaluate a process with the suggested crucible, induction melting system was built up based on the simulated results and experiments of silicon melting were carried out.     

Photovoltaic materials, past, present, future

This paper traces briefly the history of this photovoltaic materials and it tries to look at possible future scenarios. A large part of the paper is concerned with silicon although from solid-state physics we know that silicon is not the ideal material for photovoltaic conversion. From the first solar cell developed at Bell Laboratories in 1954 photovoltaics was dominated by silicon. The reasons for this dominating position are investigated. Crystalline silicon today has a market share of 86% which is almost equally distributed between single crystal and cast silicon. Amorphous silicon has another 13%. The main endeavor is to reduce cost. Present trends in the crystalline field are reviewed. The conventional technology still has significant potential for cost reduction but this comes only with increasing volume. A problem to be solved is the supply of solar-grade silicon material. Other future possibilities include thin film crystalline silicon on different substrates. Because of the low absorption coefficient of silicon light trapping is required. True thin film materials need only 1–2 μm of material. Amorphous silicon, copper indium diselenide (CIS) and CdTe are hopeful approaches for very cost-effective solar cells. Some other, more speculative materials and concepts are described at the end of this paper     

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.     

Crystalline and thin-film silicon solar cells: state of the art and future potential

Bulk crystalline silicon solar cells have been the workhorse of the photovoltaic industry over the past decades. Recent major investments in new manufacturing facilities for monocrystalline and multicrystalline wafer-based cells, as well as for closely related silicon ribbon and sheet approaches, ensure this role will continue well into the future. Such investments suggest that the silicon wafer-based approach has successfully withstood the challenge mounted by thin-film chalcogenide-based cells, in the form of polycrystalline films of CdTe and CuInSe₂, as well as that mounted by thin-film cells based on amorphous silicon and its alloys with germanium. The encumbent now faces a fresh challenge by a new wave of thin-film technologies developed in the 1990s, more closely related to the bulk approach and with some advantages over the earlier contenders. One new approach is based on a stack of two silicon thin-film cells, one cell using amorphous silicon and the other mixed-phase microcrystalline silicon. The second uses silicon thin-films in polycrystalline form deposited onto glass, even more directly capturing the strengths of the wafer-based approach.     

Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions

We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm². From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.     

Recent advances of high-efficiency single crystalline silicon solar cells in processing technologies and substrate materials

Single crystalline silicon solar cells have demonstrated high-energy conversion efficiencies up to 24.7% in a laboratory environment. One of the recent trends in high-efficiency silicon solar cells is to fabricate these cells on different silicon substrates. Some silicon wafer suppliers are also involved in such development. Another recent trend is the increased production of high-efficiency silicon cells, some of them with low-cost structures. This paper will discuss the progress at the University of New South Wales, and these trends in other organisations.