LMU shows self-organization gives rise to more efficient organic solar cells

LMU shows self-organization gives rise to more efficient organic solar cells

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By eeNews Europe

“The mechanisms and the timescale of charge separation have been the subject of controversial scientific debate for many years,” explained LMU physics professor Eberhard Riedle. In cooperation with investigators at the Technical University in Munich and at Bayreuth University, Riedle and his group at LMU have been able to dissect the process in detail.

The researchers used a novel hybrid PV cell type containing both organic and inorganic constituents, in which silicon serves as the electron acceptor. Based on the insights obtained with this system, they developed a processing strategy to improve the structural order of the polymer – and found that this enhances the efficiency of charge separation in organic semiconductors by up to twofold.

The key to this breakthrough lies in a laser-based experimental setup, which combines high temporal resolution of 40 femtoseconds with a broadband detection. This allowed the team to follow the ultrafast processes induced by photon absorption in real time as they occur. Instead of the fullerenes used in typical organic cells, the researchers used silicon as the electron acceptor, a choice that has two major advantages.

“First, with these novel hybrid solar cells, we were able to probe the photophysical processes taking place in the polymer with greater precision than ever before, and secondly through the use of silicon, a much larger segment of the solar spectrum can be harnessed for electricity,” said Riedle. It turns out that free charge carriers – so called polarons – are not generated immediately upon photoexcitation, but with a delay of about 140 fs. Primary photoexcitation of a polymer molecule first leads to the formation of an excited state, called an exciton. This then dissociates, releasing an electron, which is then transferred to the electron acceptor.

The loss of electrons leaves behind positively charged “holes” in the polymer and, as oppositely charged entities are attracted to one another by the Coulomb force, the two have a tendency to recombine. “In order to obtain free charge carriers, electron and hole must both be sufficiently mobile to overcome the Coulomb force,” explained Daniel Herrmann, the first author of the new study. The team was able to show, for the first time, that this is much easier to achieve in polymers with an ordered, regular structure than with polymers that are chaotically arranged. In other words, a high degree of self-organization of the polymer significantly increases the efficiency of charge separation.

“The polymer that we used is one of the few known to have a tendency to self-organize. This tendency can be inhibited, but one can also increase the polymer’s intrinsic propensity for self-organization by choosing appropriate processing parameters,” Herrmann explained. By optimizing the processing of the polymer P3HT, the researchers succeeded in doubling the yield of free charge carriers – and thereby significantly enhancing the efficiency of their experimental solar cells.

Reference: Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics probed in Real Time by Near-Infrared Broadband Transient Absorption,” D. Herrmann, S. Niesar, C. Scharsich, A. Köhler, M. Stutzmann, E. Riedle, J. Am.Chem. Soc. online, 21. September 2011,

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