Homogeneity Holds the Key to CIGS Cell Efficiency

Solar cells are one of the primary environmentally friendly non-fossil energy sources, together with wind power and hydroelectric power. As oil supplies dwindle, these sources become more and more important.

Thin film solar cells are a very important branch of photovoltaics as they are only a few microns thick and offer savings on material and manufacturing costs over conventional crystalline silicon cells.

CIGS thin-film solar cells absorb the sunlight through a thin layer made of copper, indium, gallium, selenium, and sulphur. With a chemical formula of Cu(In,Ga)(Se,S)2),CIGS have been claimed to offer the highest efficiency of all thin film solar cells. The efficiency is especially important in regions where sun illuminated areas are limited and you want to get the most out of say, the roof area of your house.

The research team at Mainz University headed by Claudia Felser, is using computer simulations to investigate the characteristics of CIGS. Working on the ‘comCIGS’ project alongside IBM Mainz, Schott AG, the Helmholtz Center Berlin for Materials and Energy and Jena University, the government-funded project is targeted at finding ways of optimizing CIGS solar cells.

Thomas Gruhn, head of the theory group in Felser’s team, points out that, “CIGS cells are very interesting here because they combine low production costs with good efficiency but the efficiencies reached so far, about 20%, are far below the theoretical limits - above 30%. Besides big panels, thin films solar cells are relevant as light-weight and mechanically flexible photovoltaics that can be put on cloths or tents. Here, again, it is extremely important to get as much power as possible out of the devices, which in these cases are rather small.”

Previous calculations by Huang and Wei, Zhang, and Zunger, have predicted that the optimum band gap for the CIGS cells should be about 1.5eV, which corresponds to a CIGS material incorporating about 30% indium and 70% gallium. However, in practice, the cells with the best performance have an In:Ga ratio of 70:30, the opposite way around.

With the support of IBM Mainz, Christian Ludwig of 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. He was able to use a mainframe for his investigations which was recently donated to Mainz University by IBM as part of a Shared University Research (SUR) science award.

"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", explained Thomas Gruhn.

With the aid of the simulations, it was discovered that the indium and gallium atoms are not distributed 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 separate. If the material is heated to above this demixing temperature, differently sized clusters of indium and gallium atoms form.

The higher the temperature, the more homogeneous the material becomes. It has now become apparent that gallium-rich CIGS is always less homogeneous than indium-rich CIGS. Because of this lack of homogeneity, the optoelectronic properties of the gallium-rich material are poorer, resulting in the low efficiency levels of gallium-rich CIGS cells - an effect that has now been explained for the first time.

The calculations also provide a concrete indication of the best way to manufacture CIGS solar cells. If it is produced at higher temperatures, the material is significantly more homogeneous. To retain the desired homogeneity, the material then needs to be cooled down sufficiently rapidly.

Gruhn explained that the theoretical predictions were based on the assumption of a perfectly homogeneous distribution of indium and gallium. He pointed out that the researchers doubted that this would really be the case and that at that point they had no idea how strong the inhomogeneity might be. He commented, “In practice, the demixing of In and Ga at about room temperature does not occur, because as the systems cools down from the production process, the diffusion of In and Ga decreases and the system structure "freezes" in.”

When asked if they had explored the simulations on a practical level, Gruhn commented that, “One confirmation is that our colleagues from the company Schott Mainz and scientists from the Universität Jena and the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH have shown that, indeed, the efficiency of the cells increases if the CIGS layer is created at a higher temperature. This has become possible due to the development of new glass materials at the Schott AG.”

He added that these new glass materials can be used as substrates for CIGS cells and they “allow production temperatures above 600°C - distinctly higher than conventionally used glasses.” Gruhn also remarked, “the higher production temperatures lead, indeed, to higher efficiencies. The cells that result from this process are considerably more homogeneous, meaning that the production of cells with a much greater efficiency level has become possible. “

He continued, “The investigations are ongoing. The observed phenomena of inhomogenous In-Ga distributions can be compared directly with measurements by Gütay and Bauer. They have previously measured the spatial In-Ga distribution with photoluminescence measurements. Currently, Levent Gütay is working on new experiments for testing the relation between homogeneity, production temperature and cooling rate."

These findings provide new insights into the structures of CIGS absorber materials and suggest how one can improve the efficiency of CIGS cells (by manufacturing the CIGS layer at higher temperature and then cool it down reasonably fast.)

Further details on this research is available in the latest edition of Physical Review Letters (PRL 105, 025702 (2010)).

The comCIGS project scientists are continuing their research in a number of ways. They are already working on large-format solar cells which should outperform conventional cells in terms of efficiency. Regarding their findings so far, Gruhn said, "The prospects look promising."

Other lines of investigation include finding a substitute for cadmium in the CIGS buffer layer and trying to work out why the presence of cadmium in the buffer layer increases the efficiency.

At the moment, commercial CIGS cells use cadmium sulfide (CdS) as a buffer layer on top of the CIGS absorber layer. The CdS layer increases the efficiency but cadmium, as a heavy metal, could potentially cause environmental problems. With electron structure calculations the physicists, together with researchers from the Helmholtz-Zentrum Berlin, have studied so called ‘half-Heusler’ compounds in order to find suitable Cd-free substitute materials for the buffer layer. One article has already been published (see PHYSICAL REVIEW B 81, 075208, (2010)) andtwo others have been submitted.

The researchers have previously found that close to the CIGS buffer layer, the material has a copper deficiency. They are now studying long-range spatial structures in this region as well as the influence of cadmium on the material. It is currently unclear why cadmium (currently coming from the CdS layer) has such a beneficial influence on the cell efficiency. The scientists hope to shed light on this by finding substitute doping materials that work as efficiently as cadmium does.