A solar-based Utopia would see solar panels on the roof of every building, and solar material integrated into the façade of each building. Every conceivable space that receives direct sunlight would be layered in solar material. Using solar cells, such maximization of space would better harness the abundant sunlight the earth receives each day and offer a possible route to wean society off using polluting fossil fuels.
Solar cells, the technology used to exploit this natural energy resource, are traditionally composed of silicon, an element analogous to carbon. Through the long use as the primary material in the semiconductor industry, and by having properties that make the material attractive for solar cell use, such as a high temperature resistance and a favorable electrical environment, silicon found a natural home as the main material for solar cells.
In an ideal situation, fixing solar cells and modules to every flat surface and to every building to generate electricity should be enough to break free from the stranglehold of oil and gas, but unfortunately, several factors affect the solar cell industry that constrain the industry.
Solar Cell Innovation in Reducing Manufacturing Costs
One of the several issues constraining solar cells is actually the choice of silicon, but other issues are just as important. For example, the relatively high cost of manufacturing a solar cell is one drawback, and the electrical energy conversion efficiency rate is another. With the development and roll out of a rather unique solar cell, these constraints may be consigned to history.
First-generation solar cells consist of mainly silicon, either in monocrystalline form or polycrystalline form and utilize a p-n junction. Advancements in this field led to thin-film solar cells, mainly of cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). These second-generation thin film solar cells were initially developed as a way of lowering the manufacturing costs associated with first generation solar cells.
Further development to the second generation of solar cells resulted in third generation solar cells, ones with the aim of overcoming the theoretical electrical energy conversion rate of 31 – 41 % (the Shockley – Queisser limit) using a variety of methods, such as devising multijunction solar cells.
Copper, indium, gallium, and selenide together make up a tetragonal structure that has further pushed the area of solar cell development; while possessing the features of a third generation solar cell, CIGS solar cells are part of the second generation of solar cells. Figure 1 shows an illustration of a CIGS – based solar cell.
Figure 1: Illustration of CIGS solar cell, showing the CIGS layer on a glass substrate. Credit: National Renewable Energy Laboratory (NREL)
As a direct bandgap material, the CIGS solar cell can be tailored to produce the optimum band gap: each component plays a part in producing variable band gaps from 1.0 electron volts (eV) for a copper indium selenide configuration, to 1.7 eV for the copper gallium selenide configuration.
As CIGS is a very good absorber of sunlight, less material is required which lowers the manufacturing costs. What is more, taking on a polycrystalline thin film form, CIGS solar cells offer further savings in terms of the manufacturing costs. Therefore, with the development of CIGS the solar industry can offer thin films of solar cells with higher efficiencies, lower costs, and with the ability to be integrated into the architecture of a structure.
The Efficiency Issue Faced by Solar Cell Manufacturers
Conventional silicon-based solar cells offer efficiency rates of around 20 %, with pure monocrystalline silicon solar cells offering near 40 %; however, this very high efficiency rate is primarily used by the military for non-commercial aerospace applications due to the very high costs involved.
For thin films, the CdTe solar module has an average efficiency rate of round 12 %, and the efficiency rate for CIGS solar modules is a little higher at around 14 %. When considering just a thin film solar cell, the efficiency rates are different: for CdTe and CIGS thin films, the best efficiency rates are about 17 % and 20 % respectively.
However, with the near-20 % efficiency rate for CIGS being the best for the last six years, the focus of CIGS-based solar research changed tack and began to look at lowering the manufacturing costs by alternative manufacturing techniques. The current technique, a procedure of high temperature and under vacuum, was ripe for modification.
Advantages and Disadvantages of The Sputtering Technique
Using the sputtering technique, a metal film made up of three of the four components of CIGS (copper, indium, and gallium) is sputtered and then annealed in a selenium vapor to form the CIGS structure. There are some advantages to this technique. One such advantage is that the sputtering technique allows for a better and more controlled deposition rate, producing a smoother surface and better crystallinity in the final CIGS solar cell. This affects the efficiency rate in a positive way as a smoother surface and one with better crystallinity offers a higher efficiency rate.
However, the disadvantages of using the sputtering technique is that the conditions needed almost guarantee high operating costs. As this takes place at high temperature and under vacuum (the annealing stage involving selenium vapor), the manufacturing costs are significant. Furthermore, as the components of a CIGS solar cell have different reaction rates, the reaction temperature must be set for the highest (copper) but this high temperature affects the other three components. As such, separate phases might be produced instead of CIGS; possible outcomes include a copper indium selenide (CIS) combination and a copper gallium selenide (CGS) version. Quite clearly, a multilayer deposition process is complicated and time-consuming, factors that eventually translate into higher operational costs for the manufacturers.
Boosting the Efficiency Rate Through Fabrication Techniques
A variety of alternative fabrication techniques have been developed ranging from non-vacuum techniques to electroplating. CIGS-based solar manufacturers have developed these fabrication techniques with the aim of reducing the manufacturing cost to assist the industry in reaching grid parity and thus accelerate the deployment of flexible thin films of solar cells.
One such manufacturing company is Nanosolar. Using a non-vacuum based proprietary fabrication technique, Nanosolar produces utility-scale flexible thin film solar cells and modules that are low-cost and comparably efficient; a retail price of US$0.99 per watt is coupled with a 17.1 % efficiency rating for the cell (October 2011) and around 14 % for the thin film solar module.
Nanosolar’s unique thin film fabrication technique starts with a proprietary nanoparticle ink that that does not clump during the fabrication process, allowing for a smooth surface. The nanoparticle ink is then printed onto a proprietary aluminium foil using the roll-to-roll printing technique. This technique, similar to the technique used for printing paper currency, is carried out at room temperature and does not require a clean room.
This facet greatly reduces the manufacturing cost significantly. After printing, the sheets are fitted with fingers and contacts, arranged into groups according to their characteristics and then laminated to form flexible thin film CIGS-based solar modules.
SoloPower, a company based in San Jose, California, developed a fabrication technique based on electrochemistry to produce low-cost high-volume flexible thin film solar cells. This proprietary technique involves near-100 % utilization of material, an attractive feature for manufacturers. Using the roll-to-roll printing technique, the roll is passed through an electrolyte solution (containing an anode) and then is subjected to a voltage application.
This technique allows for flexible solar cell modules to be produced with an efficiency rating of 11 %. While this efficiency rating is acceptable, the features of high utilization, minimal waste, a continuous printing process, and optimized product design make these flexible CIGS-based solar cells very promising.
A Promising Fabrication Technique
A development in 2010 by Professor H Yang from the Department of Materials Science and Engineering at the Henry Samueli School of Engineering and Applied Science at UCLA (California, USA) has offered a new fabrication technique for low-cost and high-volume solar cells. Professor Yang and his team have eliminated the vacuum step in the conventional fabrication process and constructed a three-step process.
Step One involves dissolving the primary material of copper, indium, and diselenide (CIS) in a solution of hydrazine solvent. This solution, which can also be tailored to include gallium to form a CIGS-based solar cell, undergoes Step Two where the liquid is ‘painted’ evenly onto a flat surface. Step Three involves baking the paint-applied surface.
A process-complete solar cell made using this technique offers an efficiency rating of 9.13 %, and this rating is constantly being improved upon at a rate of 1 % every two months; Professor Yang and his team believe it will be another three to four years before this technique can be commercialized, offering another roll-to-roll printing technique for manufacturers to take advantage of and produce high-volume low-cost solar cells with acceptable efficiency ratings.
Making Use of the Advantages of CIGS
By advancing the use of CIGS, and by deploying CIGS-based solar cells, manufacturers and society in general can take advantage of the numerous advantages on offer: environmentally better in terms of elemental composition and ratios; lower costs through the crystal deposition techniques; and lower manufacturing costs. There are some drawbacks to using CIGS-based solar cells; chief among them is that the efficiency rating is lower than for traditional first generation silicon-based solar cells. However, with the lower manufacturing costs and lower installation costs of CIGS-based solar cells, this technology is indeed very promising.
Entities in this rapidly developing segment of the thin film solar cell area are aggressively pursuing low-cost fabrication techniques to allow for widespread adoption of this technology. Companies such as Nanosolar, MiaSolé, HelioVolt, TSMC (through its strategic investment in Stion), Ritek Corporation, and AxunTek are examples of such entities pushing for CIGS-based solar cells to be integrated into the fabric of our society. With further developments to come from these manufacturers and others, where efficiency ratings comparable to traditional solar cells and modules are envisaged, the future of CIGS-based solar cells providing abundant and clean electricity from our sun is one that looks very promising and secure.