Perspectives on Photovoltaics - Costs Decrease As Solar Cell Technology Advances

When it comes to photovoltaic (PV) cells, Wall Street is concerned primarily with established companies employing traditional silicon-based technologies. However, record high oil prices based on genuine long-term supply worries has Wall Street increasing its interest in companies developing other PV technologies and materials, because at least for the immediate future, all types of PV technologies will thrive. This is despite the fact that the price of polysilicon, a key material used in traditional silicon-based solar cells and semiconductors, is expected to come down within the next six months. Lower polysilicon prices would seem to dampen interest in alternative non-silicon PV technologies because of their lower efficiencies in harnessing solar energy.

In fact, it is easy to foresee a segmented industry with a dual focus. One segment would focus on silicon-based photovoltaics using rigid, bulky solar panels primarily in large-scale applications such as producing power for utilities. A second segment would focus on low-cost technologies based on nanomaterials and conductive polymers to provide flexible PV products for buildings with better efficiencies and aesthetics.

Silicon-based Photovoltaics

The generally higher efficiencies (12-22 percent) of rigid solar cells based on silicon technology have made silicon the photovoltaic of choice despite its relatively high manufacturing costs. One way to bring down cost is through a modified manufacturing process called silicon ribbon growth that reduces the number of processing steps to six from the nine used in conventional bulk silicon growth based on ingot technology. Evergreen Solar (www.evergreensolar.com), a recognized leader in the field, has been developing interesting manufacturing processes using ribbon silicon technology.

Regardless of whether silicon solar cells are based on ingot or ribbon growth manufacturing technologies, however, increasing energy conversion efficiency will always be an issue. One way to attain greater efficiency is to increase solar cells' spectral sensitivity by using broader or different regions of solar radiation, by better matching the solar emission and producing higher absorption coefficients, and by using a higher fraction of sunlight that eliminates losses through excessive heating of the silicon cell. For example, a 2004 patent, "High Efficient PN Junction Solar Cell" (US6696739B2) describes a solar cell showing improved energy conversion efficiency by minimizing the shading loss while reducing the manufacturing costs.

Still another way of lowering cost is through the technique of concentrated photovoltaics. Passive optical elements are used to concentrate sunlight onto photovoltaic cells resulting in more energy output while using fewer PV cells.

Thin Films and Plastic or Polymer-based Photovoltaics

The first generation PV cells, developed in the 1970s, used monocrystalline or polycrystalline silicon. These are the rigid panels most people think of whenever solar cells are mentioned. These PV cells are made of semiconductor wafers in glass and require complex manufacturing processes.

The second generation, developed in the '80s, is known as thin films. It still requires low-pressure, high-temperature film deposition and complex manufacturing processes. Cadmium telluride (CdTe) cells are the most successful technology of this generation because of their very high conversion efficiency combined with a bandgap that is very close to the theoretically calculated optimum value for solar cells under un-concentrated sunlight. This is also an ideal PV cell for use in concentrated photovoltaics.

The majority of these second-generation cells are placed on glass, so they remain rigid. However, Global Solar (www.globalsolar.com) announced in March 2008 that it has developed a proprietary process for manufacturing flexible thin-film copper indium gallium diselenide (CIGS) photovoltaic modules. While other companies produce CIGS on glass, Global Solar is thought to be the only company with CIGS on flexible materials. CIGS cells are deposited on a stainless steel backing which also makes them lightweight and durable.

Organic Solar Cells

Plastic or polymer-based photovoltaics, developed in the '90s, are considered third- generation solar cells. Also called organic solar cells, these cells use photoactive or photosensitive dyes and conducting polymers that can be manufactured at high speeds and low temperatures.

Manufacturing costs can be reduced as a result of using a low temperature process similar to printing instead of the high temperature vacuum deposition process typically used to produce the first and second generation cells. Reduced installation costs are achieved by producing flexible rolls instead of rigid crystalline panels.

Currently, third generation solar cells are not as efficient as the first- or second-generation cells, but their lower cost offsets this deficiency. In the long term, these materials should cost even less and, using quantum dots to decrease the bandgap of the base material, they should reach higher efficiency levels than conventional cells.

Efficiency Improvements Being Explored

University of Notre Dame researchers have shown that adding carbon nanotubes to a titanium dioxide film doubles the efficiency of converting ultraviolet light into electrons when compared with the performance of nanoparticles alone. (Titanium dioxide is a main ingredient in white paint.) Without the carbon nanotubes, electrons generated when light is absorbed by titanium dioxide particles have to jump from particle to particle to reach an electrode. Many never make it out to generate an electrical current. The carbon nanotubes provide a conduit for electrons for a more direct route to the electrode, improving solar cell efficiency.

Titanium dioxide, however, absorbs only ultraviolet light, leaving most of the visible spectrum of sunlight to be reflected rather than absorbed. In dye-sensitized solar cells, a one-molecule thick layer of light-absorbing dye is applied to the titanium dioxide nanoparticles to catch more of the spectrum. Another approach coats nanoparticles with quantum dots or nanocrystals, which act as tiny semiconductors. Unlike conventional materials in which one photon generates just one electron, quantum dots are able to convert high-energy photons into multiple electrons. Other ways of improving collection of electrons within a solar cell include forming titanium dioxide nanotubes or complex branching structures made of various semiconductors.

Emerging Leaders in Printed Photovoltaics

Konarka Technologies (www.konarka.com) recently announced the first demonstration of manufacturing solar cells by highly efficient inkjet printing. "Demonstrating the use of inkjet-printing technology as a fabrication tool for highly efficient solar cells and sensors with small area requirements is a major milestone," says Rick Hess, President and CEO at Konarka. "This essential breakthrough in the field of printed solar cells positions Konarka as an emerging leader in printed photovoltaics." Inkjet printing is commonly used for controlled applications of functional materials solutions in specific locations on a substrate (RFID tags, for example), and it can provide easy and fast deposition of polymer films over a large area. Another leader in organic or plastic solar cells (third generation PV) is Plextronics (http://www.plextronics.com), a company concentrating on printed electronics technology.

Konarka's Power Plastic technology is focused on delivering lightweight, flexible, scalable, and manufacturable products. The inkjet demonstration confirms that organic solar cells can be processed using printing technologies with little or no loss compared with clean-room semiconductor technologies, such as spin coating. Inkjet printing could become a smart tool to manufacture solar cells with multiple colors and patterns for lower-power requirement products, such as indoor or sensor applications.

According to Solar Cells Info (solarcellsinfo.com), by 2009 at the latest, Konarka plans to bring multiple forms of its product to market-everything from tiny cells for sensors to fabric-based (solar cells embedded in awnings, for example) and larger building panels. The process involves printing or coating nanoparticles such as quantum dots or nanocrystals onto other material. Hess says Konarka is currently working with U.S. Green Building Council LEED designers on custom installations.

Final Thoughts

On the environmental side, it is estimated that compared to fossil fuel electricity generation, each kW of installed solar PV power annually saves up to 25 kg (55 lbs) of nitrogen & sulfur oxides, and offsets 600 to 2300 kg (1300 to 5100 lbs) of carbon dioxide, depending on the fuel mix and solar insolation (Incident solar radiation). It is worth noting that only a few years ago, while oil prices were relatively low, the growth of interest in PV technologies was based mostly on the environmental concerns rather than the concern on exhaustion of fossil fuel reserves and the recent higher oil prices. It is now clear that the dual focus of PV technologies along with improving the efficiency and reducing costs of the various PV systems will ensure sustained growth in this industry.

Nerac Inc. is a global research and advisory firm for companies developing innovative products and technologies. Nerac Analysts deliver custom assessments of product and technology development opportunities, competitor intelligence, intellectual property strategies, and compliance requirements through a proven blended approach to custom analysis: review of technical knowledge, investigation of intellectual property, and appraisal of business impacts. Nerac deploys analysts in diverse disciplines to help clients discover new applications, serving as a catalyst for new thinking and creative approaches to business problems or identifying strategic growth opportunities. On the web at http://www.nerac.com

Thin Film Solar Panels - An Exciting Breakthrough in Solar Technology

The thin film solar panels are one of the newest breakthroughs in the booming solar industry.  Compared to their predecessor, they are much thinner and affordable and may well lead to a much wider use of solar energy in near future.

The working mechanism behind the thin film solar panels is the same as their "thick" counterparts.  Both use photovoltaic cells to collect sunlight and convert it into electric current through the interaction between the sunlight and the semiconductor material contained in the PV cells.    The  electricity thus generated can be put into use right away at your home or office.  You can also store it with batteries to back up the power at nights or on cloudy days.

What exactly has enabled the thin film PV cells to work with the same efficiency but at a much reduced cost?  The answer is in the semiconductor material.  The first generation of solar panels, which are the thick ones that we are all used to seeing, use crystalline silicon as the semiconductor material.  Each solar cell is produced on a different silicon wafer, one by one.  This is an extremely labor-intensive process, which makes the solar panels unaffordable by mass people.

The semiconductor material used in thin film PV cells, as a contrast, is much thinner and cheaper.  What's better, it can be mass produced with an automated system and thereby cuts down the labor work by 3 times. You can imagine that, with this reduced cost, more and more businesses will be encouraged to enter the manufacturing of solar panels.  If this happens, the prices of solar panels will become even more affordable.

And, there are more exciting applications.  With the solar cells being smaller, they are also more light-weighted and can be flexibly placed onto various smaller and light-weighted objects.  For instance, the solar roof shingles are produced by covering the traditional asphalt roof with a layer of thin file solar cells.  Now,  instead of holding your solar panel with the large and heavy steel arrays, your solar roof can look almost the same as that of your neighbors.

Portable solar panels are another product of the thin film technology.  They are being manufactured to power up just any type of electric appliances, from cell phones, GPS devices, MP3 players, to televisions and laptops.   How handy it will be if you can install a solar panel on your backpack or in your purse?  You can carry it wherever you go and charge your portable electric devices whenever needed.

If the thin film solar panels still sound to you like a complex technical innovation rather than an easily accessible daily product, don't worry.  Think about the digital watches.  They cost dearly a few decades ago, but can be  purchased today at a very friendly price.  This will be the future of our solar panels!

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Solar Technology and Photovoltaic Cells

Concentrating photovoltaic's (CPV) uses lenses and mirrors to focus the sun's energy. This technology includes both a low-concentration approach, which increases the sun's magnification by between 2 and 100 times, and a high concentration approach, which can increase the magnification by hundreds of suns when the PV efficiency exceeds 40%. CPV uses less photovoltaic material and increases performance, hopefully enough to offset any additional costs.

Concentrating Photovoltaics and Thermal (CPVT) is another technology; this produces both electricity and thermal heat in the same module. Thermal energy itself is a benefit from the sun, and other plants have a design of a solar power tower in which the mirrors focus sunlight on a heat receiver at the top that collects the heat and transfers it to piping inside the tower where is it circulated and used to make electricity. The design minimizes the field of piping to the vertical tower height to a few hundred meters and can reach temperatures in excess of 1000 degrees.While currently there are very few commercially operating tower installations, based on announcements, this technology may grow rapidly.

The Solar Two tower in California is an example of this technology and has the capability to produce 10 megawatts of power. Because of its success, Solar Tres is being built in Spain; this will be three times larger than the Solar Two plant and have a capacity of 17 megawatts. As it is, Solar Two's tower has been removed in 2009 to make way for a larger solar project. Another solar thermal technology is the parabolic trough. The SEGS plants in California utilize this technology and have a capacity of 33 megawatts each. Nevada Solar One is another very large CSP project with a capacity of 64 megawatts, using Flabeg AG troughs made in Germany.

When we look into photovoltaic cell technology and the materials used, throughout the world crystalline silicon has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies. There are two types of crystalline silicon are used in the industry. The first is mono crystalline, produced by slicing wafers from a high-purity single crystal. The second is multi crystalline silicon, made by sawing a cast block of silicon first into bars and then wafers. Most efficient production cells use mono crystalline c-Si with laser grooved, buried grid contacts for maximum light absorption and current collection. The main trend in crystalline silicon cell manufacture is toward multicrystalline technology. And for both mono- and multicrystalline Si, a semiconductor homo junction is formed by diffusing phosphorus into the top surface of the boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact pattern specially designed to allow maximum light exposure of the Si material with minimum electrical (resistive) losses in the cell. Crystalline silicon cell technology forms about 90% of solar cell demand. The balance comes from thin film technologies. Approximately 45% of the cost of a silicon cell solar module is driven by the cost of the silicon wafer, a further 35% is driven by the materials required to assemble the solar module.

Deevan Solar Panel Hot Water Heating.

Concentrated Solar Power California.

Epitaxy on Silicon-On-Insulator Technology

Introduction:

As at current, we see that CMOS technology is the driving technology of the microelectronics industry, and the conventional way of fabricating integrated circuits on bulk silicon substrates has illustrated problems such as unwanted parasitic effects, latchup, and the difficulty of making shallow junctions. In the recent years, the advent of Silicon-on-Insulator has proven superior in many aspects to their bulk counterparts, and the benefits include the absence of latch-up, the reduced parasitic source and drain capacitances, the ease of making shallow junctions, radiation hardness, ability to operate at high temperature, improved transconductance and sharper subthreshold slope. There are several approaches available to create SOI wafers, and we discuss two particular techniques over here. First, we seek to illustrate a heteroepitaxy technique through the Ultra-Thin Silicon (UTSi) process where high quality Silicon-on-Sapphire (SOS) material is formed. Next, we look at a homoepitaxy technique called Epitaxial Lateral Overgrowth (ELO) technique which seeks to grow a homogenous crystal laterally on an insulator.

Ultra-Thin Silicon (UTSi) Process

Silicon-on-Sapphire (SOS) material was first introduced in 1964. SOS was recognized for its high speed and low power potential. The usage of Czochralski growth of sapphire crystals and the subsequent deposition of a silicon film in an epitaxial reactor had proved inefficient as there was high defect density due to lattice mismatch with defect densities near the Si-Sapphire interface reaching up to planar faults /cm and line defects/cm. This resulted in low resistivity, mobility, and lifetime near the interface. The silicon film deposited is also under compressive stress at room temperature due to different thermal expansion coefficients which may possibly result in relaxation in the film through crystallographic defects such as microtwins, stacking faults, and dislocations. Such consequences are undesired.

Hence, these reasons advocate the need for better heteroepitaxy technique, and in which the UTSi process is one such potential candidate. The steps involved in a UTSi process are as follows: See Figure 1.

Step 1: Grow a relatively thick film of silicon on sapphire. Silane (SiH4) is commonly used as the source of silicon for SOS growth. Its pyrolysis reaction in a carrier hydrogen gas, SiH4 --> Si + 2H2, results in the deposition of a silicon layer over the sapphire substrate. The deposition temperature is usually kept below 1050 deg C in order to prevent the autodeposition of aluminum from the sapphire substrate to the silicon layer. The desired silicon orientation is , which has been achieved on various sapphire orientations, i.e., , , .

Step 2: Implantation of Si into the silicon film is carried out to amorphize the bottom 2/3 of the silicon film, with the exception of a thin superficial layer, where the original defect density is the lowest.

Step 3: A low temperature thermal annealing step is then used to induce solid-phase regrowth of the amorphized silicon, using the top silicon layer as a seed.

Step 4: The silicon film is then thinned to the desired thickness by thermal oxidation, and the subsequent HF strip of the SiO. What remains is the final product of Silicon-on-Sapphire (SOS).

It has been demonstrated that UTSi process is capable of delivering relatively defect-free and stress free SOS material in which devices with a high effective mobility can be made.

One application of the UTSi process is seen in UTSi CMOS transistors. As seen from Figure 2, the fabrication process is much simpler since the deep implants and guard regions are unnecessary thanks to the insulating sapphire substrate, and undesired effects such as leakage currents, latchup, and the RF parasitics are eliminated since the devices now sit on an insulating layer. The performance of the CMOS process is enhanced by as much as two generations of process geometry reduction. The advantages of forming CMOS transistors in the ultra thin silicon layer over insulating sapphire include the following:

* Elimination of substrate capacitance, which allows higher speed at lower power and avoids voltage dependent capacitance distortions

* Fully depleted operation, improving linearity, speed, and low voltage performance

* Excellent isolation which allows integration of multiple RF functions without crosstalk

UTSi circuits are produced that compete in the rapidly expanding wireless and fiber optic markets at higher frequencies and data rates with lower power consumption than standard bulk CMOS, SiGe and GaAs circuits, while still using standard CMOS equipment and processing.

Epitaxial Lateral Overgrowth (ELO) Technique

This technique allows the homoepitaxial growth of silicon on silicon, with the focus placed on growing the crystal laterally on the insulator. In ELO, we can perform this in an atmospheric or in a reduced-pressure epitaxial reactor. The technique consists of the epitaxial growth of silicon from seeding windows over SiO islands or devices capped with an insulator.

The steps involved in a ELO technique are as follows: See Figure 3.

Step 1: An oxide layer is grown on the (100) silicon wafer. Next, patterning is carried out on the oxide to demarcate the windows. The edges of the windows are oriented along the direction.

Step 2: Cleaning of the wafer is carried out

Step 3: Wafer is loaded into an epitaxial reactor and submitted to a high-temperature hydrogen bake to remove the native oxide from the seeding windows.

Step 4: Epitaxial growth is performed next, using e.g: SiHCl +H+ HCL gas mixture.

Step 5: Apply an in-situ HCl etch step to remove any crystallites that may be formed on the oxide due to nucleation of small silicon crystals with random orientation during the epitaxial growth.

Step 6: Once the small nuclei are removed, a new epitaxial growth step is performed, followed by an etch step, and this repeats until the oxide is covered by epitaxial silicon.

Some points we should note is that the epitaxial growth proceeds from the seeding windows both vertically and laterally, and the silicon crystal is limited by the and facets. When two growth fronts, seeded from opposite sides of the oxide, join together, a continuous silicon-on-insulator film is formed, which contains a low-angle subgrain boundary where the two growth fronts meet. A groove is observed over the centre of the SOI area. When more growth is done, this groove disappears.

As much as this is a simple technique to have homoepitaxial growth, a major disadvantage is the nearly 1:1 lateral-to-vertical growth ratio. On the other hand, the thick ELO film allows the design engineer to obtain SOI films of different thickness easily simply by polishing the wafers to required depths as needed. Also, the low defect density and low thermal budget needed to implement a ELO-SOI is considered superior to other technologies such as SIMOX (Separation by Implanted Oxygen) or other SOI processes for submicron devices.

Applications for this technique have been seen in three-dimensional and double-gate devices.

Variations in ELO technique has been witnessed in "tunnel epitaxy", "confined lateral selective epitaxy" (CLSEG) or "pattern-constrained epitaxy" (PACE) whereby a "tunnel" of SiO is created, forcing the epitaxial silicon to propagate laterally instead of vertically. In effect, a 7:1 lateral-to-vertical growth ratio has been obtained, which is more efficient than the original approach.

Jean-Pierre Colinge, Silicon-On-Insulator, Kluwer Academic Publishers, London, 2004.

Originally Written Article here.

The author Jimmy Lee is involved in article writing, publishing, and website design on a freelance basis amid a daytime job as an electrical engineer. His favourite works can be found at http://flashgor.blogspot.com/ and [http://www.diypc.wordpress.com/]