Cast Polycrystalline Silicon Research Articles

Influence of temperature on certain properties of n +-p and n +-p-p + structures based on secondary cast polycrystalline silicon

This paper discusses the Isc and Uxx temperature dependencies of n+-p and n+-p-p+ structures made of comparatively fine-grained secondary cast polycrystalline silicon upon their being homogenously heating and heated with a temperature gradient, as well as upon alteration of heat flux direction.

Hydrogen plasma treatment for improving bulk passivation quality of c-Si solar cells

We have investigated the effects of radio frequency hydrogen plasma (H-plasma) treatment on bulk passivation qualities of cast poly-crystalline silicon (poly c-Si) wafers. When the hydrogen treatment was performed directly to poly c-Si wafers, the plasma damage was introduced and it was difficult to improve the minority carrier lifetime by optimizing the plasma condition. Hydrogenated amorphous silicon oxide (a-SiO x :H) films deposited by RF-PECVD using a mixture of SiH 4, H 2 and CO 2 were employed as the protection and the passivation layers. We found that the lifetime of poly c-Si substrates passivated by a RF-PECVD a-SiO x :H films with additional H-plasma treatment showed an improvement of the overall effective lifetime. Annealing up to 400 °C also enhances the further diffusion of hydrogen, and the effective lifetime of the samples with H-plasma treatment increased 20–30% compared to the samples without treatment. This result clearly indicated that the H-plasma treatment improves the bulk passivation quality of poly c-Si wafers.

Boron-doped Microcrystalline Silicon Oxide Film for Use as Back Surface Field in Cast Polycrystalline Silicon Solar Cells

We have studied the properties of boron-doped hydrogenated microcrystalline silicon oxide (p++ µc-SiO:H) with the aim of applying this film as the back surface field (BSF) in silicon solar cells. We found that the p++ µc-SiO:H film showed high conductivity and excellent passivation effect. A cast polycrystalline silicon solar cell using the p++ µc-SiO:H film as BSF showed an efficiency of 15.5% (2 ×2 cm2). The high open circuit voltage and quantum efficiency at a wavelength of approximately 1000 nm clearly indicated the excellent BSF properties of the p++ µc-SiO:H film.

EELS analysis of {1 1 1} Σ3 and {1 1 2} Σ3 twin boundaries and their junctions in phosphor-doped cast polycrystalline silicon

{1 1 1} Σ3 and {1 1 2} Σ3 twin boundaries and their junctions in a phosphor-doped cast polycrystalline silicon were investigated by EELS using HREM to study the local electrical properties and impurity effects at these boundaries and junctions. FWHMs of the silicon plasmon-loss peaks are wider at the grain boundary junctions and {1 1 2} Σ3 twin boundary as compared with other area partly because of the overlapped effect of plasmon loss of SiO 2 and carbon. In the inner-shell edge part of EELS spectrum, the grain boundary junction having distorted structure shows slightly strong intensity at around 110 eV, suggesting the formation of nano-size SiO 2, even though the presence of the SiO 2 could not be observed by HREM. The effects of carbon K-edge can be recognized in the EELS spectra acquired at {1 1 1} Σ3 and {1 1 2} Σ3 boundaries and their junctions, suggesting the grain boundary segregation of carbon atoms.

Effect of thermal annealing on the properties of a-SiCN:H films by hot wire chemical vapor deposition using hexamethyldisilazane

We investigated the effect of thermal annealing on the properties of a-SiCN:H films prepared by HWCVD using hexamethyldisilazane focusing on the change in the passivation quality. We found that annealing a-SiCN:H films at the temperature around 600 °C led to an effective hydrogen diffusion, resulting in the enhancement of the passivation effect. The performance of cast polycrystalline silicon solar cells using a-SiCN:H films showed a strong dependence on the contact firing temperature. The best efficiency of 13.75% was achieved at the firing temperature of 750 °C.

Effects of Nitrogen Addition on the Properties of a-SiCN:H Films Using Hexamethyldisilazane

AbstractWe deposited a-SiCN:H films by HWCVD using a gas mixture of hexamethyldisilazane, H2 and N2, and fabricated cast polycrystalline silicon solar cells with the a-SiCN:H passivation and anti-reflection layer. N2 addition led to the reduction of the refractive index of the a-SiCN:H films due to the increase in nitrogen concentration of the films. This improved performance of the antireflection layer. The advantage of adding N2 to the process was demonstrated by the improvement in short circuit current (JSC) and efficiency of cast polycrystalline silicon solar cells. At present, the efficiency of cast polycrystalline silicon solar cell using a-SiCN:H film as a passivation layer reached 14.2%.

Preparation of Hydrogenated Amorphous Silicon Carbon Nitride Films by Hot-Wire Chemical Vapor Deposition Using Hexamethyldisilazane for Silicon Solar Cell Applications

We propose hydrogenated amorphous silicon carbon nitride (a-SiCN:H) films prepared by hot-wire chemical vapor deposition (HWCVD) using hexamethyldisilazane (HMDS) as a new passivation layer of silicon solar cells. The films deposited at various deposition conditions were characterized focusing on optical properties and passivation quality. We found that the effective lifetime of the samples depended on the amount of Si–H bond and had strong correlation with the interface trap densities. After the optimization, the effective surface recombination velocity of n-type single crystalline silicon (c-Si) (1–10 Ω cm) with a-SiCN:H films has become lower than 60 cm/s and that of p-type was approximately 300 cm/s. The efficiency of a cast polycrystalline silicon solar cell using a-SiCN:H as a passivation layer reached 13.7% (2×2 cm2). Experimental results indicate that a-SiCN:H films are promising low-cost antireflection and passivation layers for silicon solar cells.

Photoluminescence analysis of intra-grain defects in cast-grown polycrystalline silicon wafers

Effects of intra-grain defects in cast polycrystalline silicon (poly-Si) wafers on the solar cell performance were investigated by photoluminiscence (PL) spectroscopy and mapping at room temperature. We confirmed that the crystallinity of the longer diffusion length region of the poly-Si is almost the same as that of the single crystalline Si. For the PL macroscopic mapping, low PL intensity regions correspond to short diffusion length regions. In short diffusion length regions, plenty of dark lines and spots were observed by PL microscopic mapping, while, in contrast, longer diffusion length regions have few of them. These findings showed clearly that dark lines and spots of the PL mapping relate to defects degrading the cell performance. We also found that structures of defects depend on the fabrication process.

TEM Studies of Grain Boundary Structure in a Cast Polycrystalline Silicon

Cast polycrystalline silicon for solar cell contains mostly straight twin boundaries which are thought to have little effect on the electrical activity. There are, however, some complicated grain boundaries in it. One of these boundaries consists of slightly curved and straight parts. The structure of this boundary was analyzed to investigate the difference of these two types of boundaries. The conventional transmission electron microscopy (TEM) found that this slightly curved boundary was the zigzag shaped boundary made by (11 _ ,2) and ( _ ,211) planes. High resolution electron microscopy (HREM) confirmed that (11 _ ,2) plane was the boundary of {112} Σ3 twin boundary which formed a straight grain boundary at the other end of the analyzed grain boundary, and also confirmed that ( _ ,2 11) plane was also the boundary of {112} Σ3 twin boundary which intersected with the former twin boundary at an angle of 120 [deg].

Electroluminescence in porous polycrystalline silicon

A LED was fabricated by first forming a porous region on a cast polycrystalline silicon substrate by anodization and then depositing a Au layer. Under forward bias, the LED was observed to have a stable yellowish white emission. Based on the measured spectra, it is proposed that the origin of the emission is due to microplasma formed in the depletion region of the heterojunction formed between the porous polysilicon layer and the polysilicon bulk. The results suggested that the cast polycrystalline substrate can be used for cheap and largearea applications in porous silicon devices.

Reactive ion etching (RIE) as a method for texturing polycrystalline silicon solar cells

Reactive ion etching (RIE) has been applied and developed as a method for texturing polycrystalline silicon solar cells. Two structures (microgrooves and pyramids) were produced in this work. Reflectivity measurements between 400–1200 nm show an overall reflectance of 5.6% for pyramid and 7.9% for groove structures. These results are better than those using wet anisotropic etch techniques on single-crystal silicon. RIE texturing was performed on cast polycrystalline silicon and produced better reflection control than standard TiO 2 antireflection coatings. RIE texturing also changes the incidence angle of light into the silicon, this improves the response for long wavelengths which can be utilised in thin film, polycrystalline or amorphous silicon solar cells.

A Led Based on Porous Polycrystalline Silicon

AbstractPrevious studies on electroluminescence in porous silicon were based on crystalline wafers. In this paper, we shall report the characteristics of a LED based on porous effects in a cast polycrystalline silicon substrate. A layer of porous region was first formed on a cast polycrystalline silicon substrate by anodization, followed by the deposition of a semitransparent Au layer. Under forward bias, the LED emits stable yellowish white light (with the presence of bright spots) for currents above 20 mA/cm2. From the electroluminescence spectra measured, we suggest that the emission is due to the recombination of electron-hole pairs in a microplasma region. We propose a model where the microplasma is present in the depletion region of the heterojunction formed between the bulk polysilicon and the surface porous polysilicon. The defects and grain boundaries in a polycrystalline material facilitate the formation of such microplasma. The heterojunction model will also be used to explain the current characteristics of the LED. The effect on the LED characteristics due to indium coating on the porous substrate prior to Au deposition was studied, and the results agree with the heterojunction model. Our work shows that cast polycrystalline silicon substrates have potential for LED fabrication in cheap and large area applications.

Increasing short minority carrier diffusion lengths in solar-grade polycrystalline silicon by ultrasound treatment

We have found that ultrasound treatment (UST) has a profound effect on the recombination rate in as-grown, B-doped cast polycrystalline silicon wafers for photovoltaic applications. As determined by surface photovoltage measurements of the minority carrier diffusion length L, the UST increases the corresponding lifetime by almost an order of magnitude. The maximum enhancement takes place in the wafer regions with the shortest L values. For L≳20 μm, both positive and negative changes of L after UST are revealed at different wafer regions. The UST effect is temperature dependent and exhibits maximum influence at about 60 °C. Enhanced dissociation of Fe-B pairs by UST is identified as a mechanism which leads to a negative change of large L values, and a complex post-treatment relaxation. A positive change of L is attributed to the influence of ultrasound vibrations on crystallographic defects.

Cost‐benefit analysis of high‐eficiency cast polycrystalline silicon solar cell sequences

AbstractThe incorporation of eficiency enhancement techniques and the use of high‐eficiency process sequences have led to the fabrication of polycrystalline silicon solar cells with efficiencies nearing 18% in the laboratory by various investigators. In this paper a number of cell process sequences used to fabricate high‐efficiency polycrystalline silicon solar cells are reviewed. A cost‐benefit analysis of the efficiency enhancement techniques was performed using the Interim Price Estimation Guidelines 2 model, developed by the Jet Propulsion Laboratory, Pasadena, USA. the figure of merit used to compare the sequences was that the cost of additional power, generated by the efficiency enhancement technique, should be less than the selling price of the product. Techniques like oxide passivation and emitter etch‐back, and sequences like the buried‐contact process, were found to be cost effective.

Current–voltage characteristics of grain boundaries in cast polycrystalline silicon

Current-voltage ( I - U ) characteristics of grain boundaries (GBs) in cast polycrystalline silicon are reported. It is found experimentally that these I - U characteristics show a dependence on the polarity of the applied voltage. A model is developed to understand the I-U characteristics of GBs in semiconductors, taking into account the observed difference in doping concentration in the grains forming the GB. A single trap energy level is assumed for GB interface states. The potential barriers at GBs are estimated by calculating the increase in the density of filled interface states at the GB due to the applied bias. The I-U characteristics calculated through this model show the same general features as those of measured I-U characteristics.

Polycrystalline silicon wafer engineering for photovoltaic applications

A mechanical dicing saw in combination with bevelled blades has been used to create a variety of textured structures in cast polycrystalline silicon, ribbon, and also single crystalline material for photovoltaic applications. It is shown that the mechanical grooving step may be applied consecutively to both the front and the back surfaces of a silicon blank. The independence of this method with regard to crystal orientation together with a variable blade tip angle represents a hitherto unknown freedom of poly-Si solar cell design. flexible textured foils with a minimum silicon thickness of 20 mu m are demonstrated. This is an important prerequisite for the preparation of highly efficient solar cell structures in finely grained polycrystalline sheets with diffusion lengths smaller than 50 mu m.

Overview of Cast Multicrystalline Silicon Solar Cells

Worldwide environmental problems such as the greenhouse effect and acid rain have been caused by the human race's continuous reliance on the combustion of petroleum for fuel.Solar energy, which is clean and practically unlimited, is expected to be a desirable alternate energy source to conventional power supplies, and demand for the photovoltaic system has increased throughout the world, especially in Europe and the United States.Photovoltaic cells are probably the most effective method for capturing solar energy, since they are easy to use and are the most effective means of directly generating electricity.Many kinds of solar cells have been developed in past years, especially since the first oil crisis in 1973. Among them, solar cells from cast multicrystalline silicon (also refereed to as (cast) polycrystalline silicon or semicrystalline silicon) are considered to be one of the most promising types, capable of achieving both high efficiency and low cost.In 1975, Wacker proposed a new manufacturing method for silicon substrates, using the casting method. Since then, many organizations have been involved in the research and development of multicrystalline ingots and solar cells using multicrystalline silicon substrates.Multicrystalline silicon substrates contain many kinds of defects compared to single-crystal silicon substrates, so the efficiency of multicrystalline silicon solar cells has been inferior to that of single-crystal cells. Recent research on multicrystalline silicon solar cells has resulted in substantial improvements and in the demonstration of high-efficiency cells.

Annealing‐Induced Changes of Recombination in Cast Polycrystalline Silicon

The effect of annealing on recombination properties of polycrystalline solar‐cell silicon was investigated by EBIC, using minority‐carrier diffusion length and defect contrast as measure of the occurring changes. Rapid thermal annealing was found to reduce the activity at dislocations in the grains and improved the diffusion length markedly, whereas conventional annealing in ampuls led to a degradation of the material. Transmission electron microscopy was applied to study the structure of the different recombination‐active defects.

Development of a new heterojunction structure (ACJ ‐HIT) and its application to polycrystalline silicon solar cells

AbstractA new solar cell structure named HIT (Heterojunction with Intrinsic Thin layer) has been developed based on new artificially constructed junction (ACJ) technology. In this structure a non‐doped a‐Si thin layer was inserted between the p(a‐Si)/n(c‐Si) heterojunction, improving the output characteristics and achieving a conversion efficiency of 18.1%. This structure was applied to cast polycrystalline silicon solar cells of a practical size. A high conversion efficeincy of 13.6% was obtained with a cell size of 10 cm × 10 cm using various technologies, including hydrogen plasma passivation.

Evaluation of the minority carrier lifetime and diffusion coefficient of cast polycrystalline silicon wafers by the dual mercury probe method

Minority carrier properties, i.e., the lifetime, τ, and the diffusion coefficient, D, of p-type electromagnetic cast polycrystalline silicon wafers, have successfully been evaluated from the measurement of the frequency-domain photocurrent by a novel dual mercury probe method. The difference of more than one order in τ has been observed at different portions on the same cast polycrystalline Si wafer, corresponding to the existence of grain boundaries. In addition, the deviation in D has been experimentally established to be small in spite of the large difference in τ.