A step forward in understanding electron transfer in organic solar cell materials

Organic solar cells have a promising future in renewable energy generation. However, there are a number of problems to be solved before they can break into the energy market. Characterisation of electron transfer in organic solar cell materials has taken one step forward, when a research team calculated charge carrier mobility and material structure with PRACE supercomputing resources.

Organic solar cells are envisaged as a promising alternative to inorganic cells. They are easy to produce and their production costs are lower than conventional silicon-based solar cells. Combined with the flexibility and lightness of organic materials, they are potentially lucrative for photovoltaic applications. They are easily deployed on windows, walls and roofs. But widespread use and commercialization have so far been prevented by their main disadvantages, such as poor light-to electricity conversion efficiency, low stability and limited durability. “Their low efficiency is due in part to the charge recombinations at the interface between the two semiconducting layers. One way to start addressing this problem is to study the charge carrier mobility in the two layers. We do not know very much about the microscopic mechanism of charge transport in these materials, although progress has been made in the last few years. In particular, the relationship between the molecular structure of the material and their charge conducting properties is not well understood,” tells Dr. Jochen Blumberger of University College, London.

He has been the leader for the project where the research team simulated electron transport in organic solar cell materials using PRACE resources. The project was concluded at the end of 2010. “We explored the potential of a new electronic structure method implemented in the Car-Parrinello molecular dynamics code. To use this method we treated many thousands of electrons in the calculations and we could do that only with PRACE resources. The advantage of the facility offered by PRACE is that the code we used scales very well on this highly parallel resource. It would have been very difficult to obtain a comparable resource anywhere else in Europe.”

Pictorial illustration of the charge transfer process in an organic semi-conducting material studied

Interesting results

“We obtained interesting new insights into the materials that we simulated. First of all, the validations on a material for which we know charge transfer behaviour experimentally, were quite successful. Based on these validations we carried out similar calculations for materials for which experimental charge transfer properties were not well known. We optimized their structure and computed charge transfer properties,” he explains. Charge transfer behaviour changes drastically with the structure of the material. “We saw that the electron mobility is very sensitive to the crystal structure of the material. In one structure the mobility was orders of magnitude smaller than in another structure. At the same time the calculations showed that the material is very anisotropic with respect to electron transfer. Hopefully, it will be possible to confirm some of the predictions of our calculations in experiments in the near future.” “Once we have done calculations for materials for which experimental mobilities are known, we will try to calculate structures and charge transfer properties of materials that have not been characterised before. Moreover, we will modify them to see how the modifications affect the structure and charge mobility.”

Enhancing research

Since organic semiconducting materials are a promising way for generating future energy, a lot of research is being done into them. The global market is forecast to grow from 0.5 billion dollars to about 20 billion dollars in the next 10 years. “The research focuses mainly on the materials that are used in solar cells, and much of it is done experimentally in laboratories. There are fewer theoreticians and computational scientists who are researching into charge transfer in organic semiconducting materials, a truly interdisciplinary field. We have a strong interest in electron transfer in condensed matter, and by simulating charge transfer in organic solar cell materials we wanted to apply our new simulation method to something that is important and relevant today,” Jochen Blumberger continues. All these results, obtained in the calculations performed by PRACE, are a good starting point for collaboration with experimentalists. “Supercomputing is a suitable tool for screening better materials with improved charge transfer properties that might increase the efficiency of organic solar cell materials and lead to wider commercialization.” There are still many other issues that need to be solved, such as long-term stability and stability against light and oxidation. In order to be able to sell organic solar cells they should last 20 or more years.

© Pirjo Rötkönen@PRACE

Left: Jochen Blumberger of University College, London was the project leader for the simulations

Right: Harald Oberhofer was the collaborator for the project