Renewable energy offers the prospect of meeting the current power demands of society in an environmentally friendly way, harnessing abundant and clean energy resources in favor of finite and polluting energy sources. Of the numerous forms of renewable energy resources, one is particularly attractive due to its sheer potential: solar energy.
In terms of the potentiality, the earth absorbs almost 6000 times more power in a single day than what is needed to power our societies for 365 days. Thus, even utilizing a fraction of this would result in a sustainable and clean power supply. Therefore, the mathematics is quite clear: developing solar energy technology to exploit this energy resource will be quite rewarding.
The concept of making use of sunlight has been around for more than two thousand years; ancient Romans and Greeks built and positioned homes to exploit sunlight, providing natural lighting during the day for certain rooms and providing warmth for the home during the winter. Since then, the technology has developed dramatically. In more modern times, manufacturing solar cell modules to capture sunlight and generate electricity has expanded the exploitation of the power of sunlight.
The Manufacturing Revelation of 1366 Technologies
The modern day manufacture of solar cells had led to the rise of two forces that govern the progress in solar cell roll out: the manufacturing costs, and the solar cell electrical energy conversion rate. A cost-effective solar cell, from the point-of-view of the manufacturer, is the point where the solar cell can be manufactured at a reasonable profit margin and where stages of the manufacturing process can be controlled. This control refers to both the actual steps in the process chain and the cost of manufacture. By controlling the actual step in the process chain, manufacturing costs can be controlled, indicating the causal relationship of the two.
Recent developments in the manufacture of solar cells, especially by an innovative start-up called 1366 Technologies, has given rise to the possibility of a manufacturing technique that could streamline the process chain and reduce costs. 1366 Technologies derives its name from the solar constant, which describes the amount of energy in Watts (W) that could be obtained from one square meter of the Earth’s surface irradiated by sunlight, namely, 1366 kW per square meter. This fact indicates not only the actual power of sunlight but also the potential to exploit this natural resource, weaning society of the consumption of polluting and finite energy resources.
1366 Technologies has developed two methods to directly affect the cost of the solar cell module, thus offering the opportunity to completely alter the solar energy industry. The two advances put forward by 1366 Technologies, a solar cell company based in Massachusetts (United States of America) and listed as one of the fifty most innovative companies for 2011 by the MIT Technology Review, promises to push forward solar energy’s goal of reaching grid parity. Taken together, the developments will assist in reducing the manufacturing costs and support the widespread adoption of solar energy technology. The first advancement of 1366 Technologies concerns the introduction of Self-Aligned Cell (SAC) architecture, and the second industry-changing development is introducing the Direct Wafer technique to the industry.
A Promising Technique for Solar Cell Architecture
The arrangements of the components that make up a solar cell play a significant part on the manufacture and operation of the solar cell. There are numerous architectures available, and many experimental architectures currently being explored, but the self-aligned cell technology on offer by 1366 Technologies is a present and viable technique to lower the manufacturing costs of producing a solar cell. Self-aligned cell architecture allows for two distinct advantages. The first is that the technique offers scalability, allowing manufacturers to realize savings from their operations. The second is that this technique is cost-effective. This technique offers the prospect of delivering monocrystalline quality solar cells for the cost of multicrystalline solar cells with no discernable change to the manufacturing process.
There are two ways in which 1366 Technologies manages to perform this feat. The first is through the use of a proprietary-based and inventive structured texture on their solar cells, and the second is by means of pioneering proprietary-based metallization technique.
[i] Honeycomb Structure
The structured texture is a technique that allows for a better, optimized solar cell structure. A more optimized solar cell will allow for better absorption of sunlight, thus assisting in boosting the efficiency of the solar cell. Traditionally, multicrystalline silicon solar cells (especially the low cost versions) suffered from a poor texture and a high reflectivity loss, negating any benefit such a solar cell might have. 1366 Technologies developed a patterning machine and utilized a pattern that would offer better texture and reduce the reflectivity loss, and this honeycomb structure is shown in Figure 1.
Figure 1: Electron micrograph of the structured texture of the solar cell. Credit: 1366 Technologies.
This honeycomb structure also offers other benefits, benefits that will be of interest to all solar cell manufacturers. For example, this ingenious structure allows for multiple wafer orientation, showing no discrimination between monocrystalline and multicrystalline. The innovative structure also allows for a one-percentage point boost to the electrical energy efficiency conversion rate, indicating that for a typical solar cell the efficiency rate would 18 %. This figure is higher than most thin film solar cells, but still less than industry average for monocrystalline silicon solar cells. However, when taking the lower cost of the solar cell into consideration, this 18 % efficiency rate would be an acceptable compromise.
[ii] Better Metallization
Bonus features to this technique developed by 1366 Technologies include the metallization step, as well as the polishing of the back contact on a solar cell, which opens up the possibility of adding advanced structures to the solar cell that might boost the efficiency rate even further. This proprietary-based texturing technique allows for a shorter process chain, as there would be fewer steps, and this will translate into reduced costs for solar cell manufacturers.
Two issues blight the metallization of a solar cell: the width of the lines on the front side, and the use of silver paste. One of the highest costs for the manufacture of a solar cell is the cost associated with the use of the silver paste, so research into alternatives to the use silver paste is under way. However, while that research is being carried out, creative ways to improve the line width would bring more practical results. And with the solar cell manufacturing technique devised by 1366 Technologies, better line widths have been achieved.
1366 Technologies devised a method to deposit finger grooves on the flat surface of the solar cell, instead of finger lines as is common. Such an arrangement brings forth two benefits. The first is that the shading caused by the deposited lines is reduced. The second is that the finger grooves offer better adhesion strengths. By utilizing grooves, the distance between each line was reduced to 30 microns, a significant reduction over the standard 100 – 120 microns. These narrower fingers resulted in a reduction in resistivity losses, further improving the efficiency rate. Figure 2 shows the narrower fingers on a flat surface.
Figure 2: Electron micrograph of the cross-section of a finger groove, showing the width of less than 30 microns. Credit: 1366 Technologies.
Coupled to this metallization process is the addition of the ‘Light Capturing Busbar’ from 1366 Technologies, a front contact development that has allowed for further reductions to light shading. Emanuel Sachs, Professor of Mechanical Engineering at the Massachusetts Institute of Technology (MIT), co-founder and chief technology office at 1366 Technologies, redesigned the front-end of the solar cell so that the busbars would reflect back almost 80% of the incident sunlight back onto the solar cell module. This redesign allowed for an increase in the efficiency rate at very little cost. The narrow fingers, with the redesigned busbars, allowed for shading to be reduced from 9 % to 2 %, allowing for an efficiency gain of 1 % to be made.
Simplification of the Wafer Production Chain
As mentioned, the cost of the silver paste is one of the highest costs associated with the manufacture of a solar cell. In fact, this silver paste cost is second only to the cost of producing the wafer. Using the direct wafer technique developed by 1366 Technologies, this cost has now been reduced by an astonishing 65 %. In addition, the multi-step manufacturing process was reduced using this proprietary methodology.
The conventional manufacturing process involves raw silicon particles being melted and recast. This step ensures foreign particles / impurities are removed. The result is a casting of monocrystalline silicon, commonly referred to as an ingot. These ingots are then sliced (or sawed) into blocks. Each block is then sliced to produce silicon wafers. After this fourth step, the p-n junctions are deposited, the electrodes are attached, the anti-reflective coating added, and the cell encapsulated, producing a photovoltaic cell. However, the first four main steps have compelled this recent advancement.
The four-step manufacturing process results in a high degree of wastage, especially at the slicing and sawing stage. The manufacturing technique created by 1366 Technologies has made a quantum leap in bringing the process to a single step, literally extracting the fully formed wafer direct from the molten silicon. While the exact details are not available due to commercial considerations, the result is that this technique devised by professor Sachs reduces the amount of silicon lost during the slicing step and the sawing step. This will help manufacturers realize savings from their use of the silicon raw material, with figures from the U.S. Department of Energy suggesting wafer capital costs could be reduced by 90 %. This reduction has given rise to the possibility of a fast, low-cost manufacture of a solar cell, pushing the solar industry in reaching grid parity.
Ripple Effects of 1366 Technologies
The technological improvements have all been proprietary-based, but the advancements do give an indication of the areas of the solar cell that can be improved. Using 1366 Technologies as a source of inspiration, solar cell manufacturers worldwide, from low cost manufacturers such as Trina Solar to high-end solar cell manufacturers such as Silex Solar, and even thin film manufacturers such as First Solar, can all direct research towards producing solar cells that incorporate multicrystalline cost structures yet maintain monocrystalline efficiency rates.
With the developments listed in this article from 1366 Technologies, developments that have led to better designed solar cells and ones with comparable efficiency rates, the day the solar industry reaches grid parity has been brought forward. Therefore, the solar-powered future does indeed look bright.