Photovoltaics, which convert sunlight directly into electrical energy, play a key role in the alternative energy mix. According to a study conducted by the European Renewable Energy Council (EREC) and Greenpeace International, half of all electrical energy produced by the middle of the century will originate from a mix of renewable sources. But despite impressive advances, photovoltaic systems still don't achieve the cost efficiency required to supply power on the scale of the national energy market. Solar cells are not yet able to generate the amount of electricity consumed in Germany in 2007 (618 billion kilowatt-hours) and at price levels similar to those achieved with conventional power plants.
Nevertheless, the direct use of solar energy is the fuel of the future for our planet. In less than three days the sun delivers the energy equivalent to the annual global consumption of oil, coal, and gas. Even in Germany, where climate conditions are relatively unfavorable for solar energy, average annual solar radiation totals approximately 1,000 kilowatt-hours per square meter, which corresponds to the energy content of around 100 liters of oil. The key question here is: What’s the most effective way to capture solar energy?
Environmentally friendly production
Today we have various types of solar cells, each with a different efficiency rating. Crystalline silicon solar cells, which dominate the market with a share of more than 85 percent, are mainly used for producing electricity fed directly into the power grid. There are also thin-layer cells made of various types of materials, including amorphous silicon, cadmium indium selenide (CIS), and cadmium telluride. While such cells are less costly to manufacture, their efficiency is still lower as their production requires far less energy and materials than that of other types of cells. However, efficiency is not the sole criterion when it comes to selecting the right type of solar cell. Actually, the most important factor are the costs associated with producing electricity using a particular type of solar cell, and these costs depend in turn on the type of manufacturing procedure used. Other important criteria include the energy yield (the relation between the amount of energy required to produce the cell and the cell’s actual output) and the level of pollutant emissions released during production and solar cell operation.
This is exactly the area that Merck is focusing on, with the result that the company has developed an etching paste concept for efficient, environmentally friendly structuring of silicon solar cell substrate surfaces. Known as isishape®, the concept will reduce the number of process steps required for solar cell manufacturing in the future and increase cell efficiency while reducing the environmental pollution. Costs for customers are also low.
© Jochen Stuhrmann
Conventional solar cells consist of two layers of silicon. An electric field forms on their boundary surfaces, with the n-layer acting like a battery's negative terminal and the p-layer like its positive terminal. When light strikes a cell, electrons are released, creating a potential difference between the terminals. The result: electric current
“isishape® will make things possible that were previously impossible in a production environment,” says Dr. Ingo Köhler, Head of Research & Development at Merck’s Structuring Solutions unit. Today’s standard solar cell designs involve practically no structuring. “isishape® will be used for completely new cell design concepts,” says Köhler. “It will allow us to embark upon new paths with altered cell designs, and also to increase efficiency.”
The buzzword here is selective emitter (SE) cell — a term that describes a new, more efficient type of cell design with which isishape® will also be used. Industrially manufactured SE cells do in fact achieve high levels of efficiency. “We’re a pioneer here, and we are opening up a new market,” says Hans-Jürgen Lemp, Head of Merck’s Structuring Solutions unit. “We are now offering customers a better alternative to existing, cost-intensive options, and this alternative is a long-term and sustainable one, especially in light of rising cost pressures and environmental protection aspects.”