By Ramesh G. Raju
Trying to understand how the Theory of Inventive Problem Solving (TRIZ) can be used in applications in an individual field can be difficult. Looking at clever designs created by others within the desired field and determining what principles are embedded within is a frequently used teaching/learning tool in TRIZ. In particular, the 40 inventive principles have broad use potential.
A technology for solar cells was recently developed at the Australian National University (ANU). A team created a process that significantly increased the amount of energy that could be generated from a single silicon wafer. (Note: The inventors of the technology, to the author's knowledge, did not make use of TRIZ during the creation of their technology.)
The function of a solar cell is to convert light to electricity. The structure of a basic solar cell is shown in Figure 1.
It consists of a wafer with junctions (P and n) connected by metal wires to an external circuit. There is a depletion zone between the P and n areas, which causes an electric field to develop. When a light photon enters the wafer it has the potential to create an electron hole pair. The electric field sweeps the electron and hole to opposite sides of the P and n junctions. This causes voltage to develop and a current to flow. The metal "fingers" at the wafer's top help to collect the electrons to bring them into the external circuit. The anti-reflection coating on the top enables maximum coupling of the light into the cell. The textured surface at the top helps keep the light in the cell via total internal reflection.
One problem with the present generation of solar cells is that the metal "fingers" on the top surface block some of the light from the surface from reaching into the cell. Another problem is that the cost of silicon is very high. The technology developed by ANU takes care of these two problems. Since the metal fingers block the top surface, sunlight is instead directed to go in through the side of the wafer. This makes use of the TRIZ inventive principle of "transitioning to another dimension."
The edges of the wafer, however, are thin and do not allow much light to pass through the wafer. The TRIZ principle "segmentation" is used to separate the wafer into thin sections by etching through the width of the wafer and rotating it so that the side faces up. The sections are then reintegrated in a frame. The solar cell is evolving toward the micro-level.
Separating the wafer into thin segments is difficult after it has been processed. The anti-reflection coating and texturing has to be applied to the sides of the wafer since the light now travels through the sides. "Do it in advance" is applicable in this situation, where the silicon wafer is etched into small strips before the process of fabricating the solar cell starts.
The strips need to be held in place during processing. To help, "partial action" can be seen in use as the silicon strips are not completely etched until they are separated; the strips are held in place at the sides. A new technology for solar cells has been created.
The primary advantage of the new technology is that the metal fingers do not block the path of light because they are now at the sides instead of the top. The surface area that each silicon wafer covers is enlarged. For the same amount of silicon, a much greater amount of power can be generated.
As is generally the case for all products in the evolution of the technology, the new technology provides some benefits that conventional technology does not. The new solar cells are bi-facial, meaning that the light from both sides of the cell can be used in power generation. The cells are also much more flexible due to their thinness.
The actions of various TRIZ principles are a powerful tool in the creation of new technologies. Individuals should routinely look for opportunities in their fields for applying TRIZ.
Ramesh G. Raju is an engineer in the microelectronics industry with seven years of experience. He has one patent with two others pending. Mr. Raju got hooked on TRIZ in 2006. Contact Ramesh G. Raju at ramesh.g.raju (at) gmail.com.