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Mono-like Silicon Processing as a Route to a Better Solar Cell

published: 2011-08-02 9:08

Making the transition from burning carbon-based fossil fuels to provide power to one that uses cleaner sources of energy is a sensible solution for society’s current energy predicament.  For a variety of reasons, this trend is both a positive and powerful one and allows for conventional and unconventional ideas that range from the normal to the madcap to be developed, tested, and deployed.

Of the alternative energy resources available for exploitation, solar energy is one energy resource that stands out.  This abundant energy resource has the potential to meet the demands of our societies many times over and is thus ripe for utilization.

Solar Energy Systems

The process of solar utilization is achieved primarily through the use of silicon-based solar cells. A conventional silicon solar cell is the primary component of all solar energy technologies, and over the decades, researchers in academia have tried to improve the efficiency of the solar cell. In addition, the entities in the solar industry have strived to improve the efficiency rating of the solar cell with a focus also on reducing the manufacturing costs. The solar energy industry has been advanced (with the associated reductions in manufacturing costs and improvements to efficiency rates) by the introduction of generational solar cells; currently, first generation conventional silicon cells, second-generation thin film solar cells, and certain third generation solar cells are readily available. Most third-and fourth- generation solar cells still in the basic research stage but showing great promise in further advancing the solar energy industry. The development of variants to solar cell systems, such as the development of concentrated solar power (CSP) and concentrated photovoltaic (CPV) systems, has further evolved the solar energy industry.  In parallel to these developments, there is also a significant body of research on advancing the manufacturing process to produce cells that are cost effective and more efficient.

Solar Cell Manufacturing Techniques 

Manufacturing techniques for solar cells are quite similar to those found in the semiconductor industry, chiefly due to solar cells being viewed as semiconducting devices: even though the manufacture of these two products – solar cells and semiconductors – share some commonality, one difference is the intensity of the quality control process. For solar cells, the quality control procedure during the manufacturing process is less rigid than that for semiconductors.

The solar cell manufacturing process, and in particular for monocrystalline solar cells, can be simplistically viewed as follows: first, the silicon undergoes a process of purification in an electric arc furnace, and this is then followed by a process of dipping in melted polycrystalline silicon solution. This dipping process allows for a very pure single crystal of silicon (known as an ingot) to be made, and this process is known as the Czochralski process. Slicing and dicing the ingot to form wafers, which are then fitted to make a solar cell, follow the Czochralski process. The wafers are then doped and placed back into the electric furnace, followed by placing electrical contacts on the wafers. The anti-reflective coating is then applied and then the entire solar cell is encapsulated, ready for the market and for consumers.

Types of Crystalline Silicon for Solar Cells

The two main types of silicon used in solar cells are monocrystalline and multicrystalline (sometimes termed polycrystalline) silicon. Other types, such as amorphous silicon (which is a non-crystal form of silicon) and ribbon silicon, are also utilized for solar cells.  Amorphous silicon is a material that has been used for solar-powered applications for some time, with the lower efficiency being tolerated, as the applications were lower-level ones such as for use in electronic calculators. However, its advantage over crystalline silicon is that it is flexible and can be manufactured as very thin sheets, allowing for manufacturing savings.

Ribbon silicon (also known as String Ribbon), a technique developed by Evergreen Solar, is a remarkable use of multicrystalline silicon and shows great promise for use in solar cells and for the widespread adoption of solar cell technology.  From an efficiency point of view, ribbon silicon solar cells were rated at between 17.8% - 18.3% efficiency; this recent rating indicates that ribbon silicon is reaching a comparable efficiency rating of thin film solar technology and will only improve, and that due to their very high radiating ability, and being very thin, the cost of manufacture is reduced and thus makes ribbon silicon a very noteworthy material for future silicon-based solar cells.

For major industrial players such as JA Solar, Suntech, Kyocera, Gintech, Motech, Trina Solar, and Yingli Green Energy, the manufacturing process is vital to their operations as a business entity. Developments to this manufacturing process will be particularly attractive as efficiency savings could be realized, and one such development is the production of monocrystalline-like silicon for use in solar cells.

Monocrystalline Silicon Processing

The manufacturing process for solar cells is a complicated multi-step process, going from granulated silicon to very pure silicon wafers. Different methods are deployed depending on the base material. For example, if using monocrystalline silicon, one technique is utilized. If using multicrystalline silicon, a different technique is used.

For monocrystalline processing, the most common method for manufacture involves the Czochralski process but other techniques include the Float-zone technique and the Bridgeman technique. The three techniques are used for crystal growth, but the Czochralski process is the most common as this process allows for large ingots of a semiconducting material (like silicon) to be produced at acceptable purity levels. The Float-zone and Bridgeman techniques offer very high purity levels but for most applications, such high purity levels are not required.

Although these techniques offer advantages, the downside is that the process becomes very expensive. This brings an issue into the solar energy industry, that of the cost of manufacturing solar cells. Therefore, major manufacturers in the solar energy industry have looked into reducing the costs associated with the production of the silicon ingots. One such development is for the production of monocrystalline-like silicon, sometimes called “mono-like” silicon.

An Alternative to Crystalline Silicon

Mono-like silicon is an attractive development purely because this technique merges two manufacturing processes and produces a single, better, hybrid version. In 2006, BP Solar announced the production of their proprietary-led nucleation-growth casting process that produces silicon solar ingots ready for dicing and slicing. The resultant solar cell, termed Mono2 TM, illustrated its potential with an efficiency rating of 18% and with a reduced manufacturing cost. This low-cost high-efficiency route is what BP Solar believes is key for solar cell technology to become mainstream.

Silicon Processing Methodology

BP Solar managed to take the best features of monocrystalline manufacturing and the best features of multicrystalline manufacturing and created a process that produced solar wafers that are efficient and low cost. Through working with IMEC, a research center that focuses on cutting-edge nano-electronics and nanotechnology, BP Solar were able to bring to the market this revolutionary technique.

Multicrystalline manufacturing involves placing the base material into a ceramic crucible and melting the material at 1,500 degrees Centigrade, allowing the melted material to then cool for a day. The resultant product is a multicrystalline piece of silicon, replete with a variegated internal arrangement. Unfortunately, such flecks actually reduce the flow of electrons so this disorderly internal structure can be seen as being a barrier for electron flow. This method is cheaper than the method for monocrystalline processing, so this feature will appeal to some in industry. However, the drawback with this technique is that the product is of low quality due to the high degree of disorder, resulting in lower efficiency rates.

For monocrystalline silicon manufacturing, the process utilizes the Czochralski process; this process is very expensive but produces solar ingots with a high level of purity. Currently, this process is utilized to manufacture the monocrystalline silicon used in the electronics industry and for applications such as computers, televisions, and cellular phones.

Mono-like Silicon Processing

By combining the high purity levels associated with monocrystalline silicon processing with the lower costs prevalent with multicrystalline silicon processing, BP Solar created a monocrystalline-like silicon technique that included the following major attributes: low defects, high efficiency rating (more than 6% higher than multicrystalline silicon solar cells), and low cost. This trinity of attributes would be what all manufacturers strive for.

This low-cost methodology involves an extra step in the manufacturing process affecting the nucleation growth step. At this stage, and using proprietary technology, BP Solar managed to control the growth of the crystal and the internal arrangement. This allowed for conformity within the crystal, producing a silicon crystal that had a high degree of purity and order and was inexpensive to manufacture. This allowed for silicon bricks to be produced and eventually, silicon wafers to be sliced off.

Another benefit of this technique was the reduction in waste. Current techniques result in a significant amount of waste (generally found after the slicing and dicing of the silicon ingots), and this new technique from BP Solar resulted in a reduction in silicon waste; traditionally, the silicon waste would be recycled into polysilicon and used in other applications.

Current Industrial Status of Mono-like Silicon

In partnership with IMEC, BP Solar were able to produce Mono2 solar cells of with a thickness of 130 micrometers (130 μm; 1 μm = 1/1000 of a millimetre). Using advanced technological processes, such as dielectric passivation and the use of a localized back surface field (a complete aluminium back surface), BP Solar was able to improve the efficiency of the entire solar cell.

In 2010, AMG (Advanced Metallurgical Group) acquired the manufacturing assets and the proprietary knowledge/technique from BP Solar. AMG is a technology provider to the solar wafer industry, and with the purchase of the intellectual property and manufacturing techniques, and by integrating this inexpensive process into AMG’s own manufacturing capabilities, AMG will now be able to provide bespoke and innovative solar wafers with the appropriate crystallographic structure at a lower price.

Future Outlook for Mono-like Silicon

The development of a silicon solar cell using a multicrystalline processing technique, but displaying monocrystalline features, is one that all solar cell manufacturers – from JA Solar to Motech – should be excited about.  This has the potential to change the solar energy industry and facilitate the widespread rollout of solar technology, especially in countries that receive a great deal of insolation. With the features of reduced manufacturing costs, reduced wastage of silicon dust from the sawing off process, lower defects / impurities, and higher electrical energy conversion efficiency and cost-per-watt efficiency rates, mono-like silicon processing could be the catalyst to transform the solar energy industry for the benefit of the industry and the consumer.          

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