Research team provides concrete approach to improve the performance of CIGS solar cells
A team of researchers used electron microscopes and computer simulations to investigate where losses occur in thin-film solar cells. The researchers from the Martin Luther University Halle-Wittenberg, the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) and the Helmholtz Zentrum Berlin (HZB) provide specific information on how the already high efficiency of CIGS solar cells can be improved. The results were published in the journal Nature Communication.
Thin-film solar cells made of copper indium gallium diselenide, or CIGS for short, exhibit record conversion efficiencies of 23.4 percent and a number of other advantages, such as the production on flexible substrate, which is not possible with conventional solar cells made of silicon wafers. The high conversion efficiency may be improved further because still losses occur when converting sunlight into electrical power. However, the manufacturers would first have to know exactly where in the solar cell these losses take place.
Grain boundaries are crucial
One answer to this question has now been provided by the team around HZB researcher Dr. Daniel Abou-Ras, confirming a suspicion existing already for some time: a considerable proportion of the losses occur at the boundaries between the CIGS crystals of a thin-film solar cell when positive and negative electrical charges neutralize each other at these "grain boundaries".
These charges are generated when sunlight hits a semiconductor material such as silicon or CIGS. The high-energy radiation knocks out negatively charged electrons from the atoms of this semiconductor, leaving behind positively charged defects, which are called "holes" in technical terminology. These mobile, electrical charges are collected at contacts and thus, contribute to the electrical power of the device. This in turn depends on two factors: The more electrons the solar radiation excites in the semiconductor, the better the current flow on the one hand. On the other hand, the electrical power also depends on the electrical voltage, which decreases when positive and negative charges come together again. This recombination of holes and electrons decreases the electrical power of a solar cell.
Tracking losses with the electron microscope and simulations
"First, we used the electron microscope to examine the structure of such CIGS thin-film solar cells and analyzed the distribution of the elements present at exactly the same place," explains Daniel Abou-Ras. This distribution gives the researcher important information about the position of the individual CIGS crystals. The team uses a special combination of other methods to clarify these microstructures very finely.
The group then transfers the resulting structure of a CIGS solar cell with very high conversion efficiency into a computer model. Using their experimental results, Daniel Abou-Ras and his team adapt this simulation until it reproduces the processes in a real CIGS solar cell as accurately as possible.
"In this computer model, we can then observe how different changes influence the electrical output of a solar cell," explains Daniel Abou-Ras. For example, the absorbing layer of a CIGS solar cell exhibits a so-called p-type doping owing to an excess of holes, which is distributed inhomogeneously. When varying the spatial distribution of these holes in the computer model, such inhomogeneities have no measurable influence on the electrical output of the solar cell. The measured efficiency losses should therefore have a different cause. Even different lifetimes of the pairs of electrons and holes change the performance of the CIGS solar cells only insignificantly.
What happens at the boundaries of the crystals?
However, the boundary areas between the individual crystals do have a significant effect on performance. "The atoms in CIGS crystals arrange themselves in certain structures," explains Daniel Abou-Ras. At the points where two such highly ordered crystals meet, these crystal lattices often do not fit together very well. These are the locations at which defects form that can easily trap electrons or holes. By their present work, the team is able to quantify the recombination of electrons and holes and correspondingly also the decreases of the voltage and power of the solar cells.
"This result gives the manufacturers an important indication of how they can further improve CIGS solar cells," says Daniel Abou-Ras. If the developers manage to enlarge the crystals considerably, there are also fewer interfaces and the previous record efficiency could probably be improved significantly.