ORGANIC SOLAR CELLS New breakthrough changes our understanding of charge transport in organic solar cells

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Recent research at the Chemnitz University of Technology exploring the density of states has found that energetic disorder increases deeper into the bandgap, transforming our understanding of charge transport in organic solar cells.

The density of states (DOS) is fundamentally important for understanding physical processes in organic disordered semiconductors, yet this is very difficult to determine experimentally.

Now, research from scientists at the Chemnitz University of Technology in Germany has evaluated the DOS and found that energetic disorder increases deeper into the bandgap, revealing the linear dependence of the disorder on energy reveals the power-law DOS in organic solar cells and transforming our understanding of charge transport in organic solar cells.

Changing our understanding of PVs

The research team, led by Prof. Dr Carsten Deibel, is currently working on solar cells made of novel organic semiconductors, which can be manufactured using established printing or thermal evaporation methods.

In order to build an understanding of and further develop this class of photovoltaic materials, the research team is pursuing an interdisciplinary approach as part of the "Printed & Stable Organic Photovoltaics from Non-fullerene Acceptors—POPULAR” research unit which combines expertise from chemistry, materials science, physics, and mathematics with printing technology.

"With regard to the development of resource-efficient renewable energies, research on organic photovoltaics is extremely relevant, as they can be processed at high throughput at room temperature," says Deibel, who is also the head of the research group POPULAR.

"Organic semiconductors are very good light absorbers, so the light-absorbing layer in solar cells is 1,000 times thinner than in crystalline silicon solar cells,” he says. "For the transport of the electrons and holes generated in organic semiconductors by sunlight, this means they do not move on a highway, but on a bumpy road with many traps that catch electrons or holes and lead to a slower, but not lower, current flow.” One way to describe this is the density of states.

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Power law describes the density of states of organic solar cells

In a bid to understand charge transport in organic solar cells, Deibel together with his research assistant and colleagues from the University of Nuremberg-Erlangen, the Helmholtz Institute Erlangen-Nuremberg for Renewable Energy, and Heliatek GmbH in Dresden, manufactured different types of organic solar cells, analysed them and, for the first time, uncovered the electronic defect landscape.

This came about as the result of sensitive measurements of the open-circuit voltage of the solar cells. This is the voltage that’s generated when no current is flowing, and it is also a measure of how much energy the photo-generated electrons and holes have. The measurements were made under a wide range of light intensities and temperatures.

The results were surprising, with the evaluation of the data showing that the density of states of the organic solar cells has a shape that cannot be described by a Gaussian or exponential distribution. But it can be described as a power law. This means that contrary to older models, smaller open-circuit voltages in the solar cells lie in an energetic range where there are more traps. Fortunately, under working conditions of organic solar cells at room temperature under sunlight irradiation, the open-circuit voltage is higher, and the density of states contains fewer traps," says Maria Saladina, the research assistant of Professor Deibel.

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More complex than previously assumed

The results of this research were published in Physical Review Letters in early June. Although the theoretical description of organic solar cells must now be reconsidered due to the insights provided by this research, the researchers estimate that there is “no fundamental obstacle” to the production of highly efficient organic solar cells using printing technologies.

"We are convinced that the disordered nature of organic semiconductors for solar cells is directly linked to the mass-production-compatible manufacturing possibilities," says Deibel.

"We have realized that the density of states, which determines the processes of charge transport and recombination in organic solar cells, is more complex than previously assumed,” Saladina added.

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