Organic solar cells face efficiency challenge due to slow current flow, researchers show

Researchers from the Chair of Optics and Photonics of Condensed Matter led by Prof. Dr. Carsten Deibel at the Chemnitz University of Technology and other partner institutions are currently working on solar cells made from novel organic semiconductors that can be produced using established printing processes. The scientists are collaborating interdisciplinarily to fundamentally understand these photovoltaic cells in order to further improve them.

“Organic solar cells can be produced very easily and cheaply using printing processes,” says Deibel. In contrast to established solar modules made of crystalline silicon, however, the current flow in organic solar cells is very slow.

“Due to the production of the solar cells from a kind of ink, the organic, light-absorbing layers are very disordered. Therefore, the current flow is very slow,” explains Deibel. A consequence of the slow transport of light-generated electrons and holes is the so-called transport resistance, which reduces the fill factor of the solar cells and thus the power.

Deeper understanding: Transport resistance limits the performance of organic solar cells

To better understand the performance characteristics of organic solar cells, Deibel and his scientific assistant Maria Saladina have produced and thoroughly investigated different types of organic solar cells and uncovered the negative influence of transport resistance. The current-voltage characteristics under illumination, which result from the interplay of charge generation by light, recombination of electrons and holes, and charge transport, were measured. They contain information on the power efficiency of the solar cells.

These measurements were compared with the so-called suns-Voc method, which allows the construction of an alternative current-voltage curve that is not limited by charge transport losses such as transport resistance. “Transport resistance is a result of the slow charge carriers in the disordered solar cells processed from organic ink. Thus, the charge carriers get in their own way and lead to a loss of fill factor and thus power,” says Saladina.

The research results were published in Reports on Progress in Physics. Although the optimization of organic solar cells must be re-evaluated due to these new results, there is no fundamental obstacle to producing highly efficient, printed organic solar cells.

In a perspective article published in the journal Advanced Energy Materials, written by Chen Wang, Deibel, and Saladina together with co-authors from various German universities, the physical origin of transport resistance and its significance for solar cells is explained in detail.

“In recent years, charge transport has been continuously improved without the research community knowing the exact relationship between fill factor losses and transport resistance,” says Deibel.

Saladina adds, “In addition to recombination, transport resistance is also determined by the shape of the density of states of organic solar cells. This shows that we are, step-by-step, understanding the physical foundations of these photovoltaic devices better and better.”

These results have been achieved within the framework of the DFG Research Unit POPULAR, which continues to work on understanding and improving printed organic solar cells.