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nanotimes News in Brief

10-07/08 :: July/August 2010

Solar Cells // Breakthrough For More Efficient Thin-Film Solar Cells

R

esearch Cooperation Predicts Significant Improvement to the Efficiency of CIGS Thin-film

Solar Cells Scientists at Johannes Gutenberg Univer- sity Mainz (JGU) have made a major breakthrough in their search for more efficient thin-film solar cells.

Computer simulations designed to investigate the so-called indium/gallium puzzle have highlighted a new way of increasing the efficiency of CIGS thin- film solar cells. It has only proved possible to date to achieve an around 20% efficiency with CIGS cells although efficiency levels of 30% are theoretically possible.

The research team at Mainz University headed by Professor Dr Claudia Felser is using computer simulations to investigate the characteristics of CIGS, whose chemical formula is Cu(In,Ga)(Se,S)2

. This

research forms part of the comCIGS project funded by the Federal German Ministry for the Environment, Nature Conservation, and Nuclear Safety (BMU).

IBM Mainz and Schott AG are collaborating with the Johannes Gutenberg University Mainz, the Helm- holtz Center Berlin for Materials and Energy and Jena University in the project that is targeted at finding ways of optimizing CIGS solar cells. The researchers focused in particular on the indium/gallium puzzle that has been baffling scientists for years: Although it has been postulated on the basis of calculations that

the optimal indium:gallium ratio should be 30:70, in practice, the maximum efficiency level has been achieved with the exactly inverse ratio of 70:30.

With the support of IBM Mainz, Christian Ludwig of Professor Felser‘s team undertook new calculations with the help of a hybrid method in which he used a combination of density functional calculations and Monte Carlo simulations.

“Density functional calculations make it possible to assess the energies of local structures from the quantum mechanical point of view. The results can be used to determine temperature effects over wide length scale ranges with the help of Monte-Carlo simulations,“ Dr Thomas Gruhn, head of the theory group in Professor Felser‘s team, explains the me- thods used. Christian Ludwig is able to use a main- frame for his investigations that was recently dona- ted to Mainz University by IBM as part of a Shared University Research (SUR) science award.

With the aid of the simulations, it was discovered that the indium and gallium atoms are not distri- buted evenly in the CIGS material. There is a phase that occurs at just below normal room temperature in which the indium and gallium are completely sepa- rate. If the material is heated to above this demixing temperature, differently sized clusters of indium and gallium atoms do form. The higher the temperature,