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Structure-property relationships of donor-acceptor-blockcopolymers

Von Katharina Neumann (27.03.2014)

A great challenge of the 21st century is how to solve the energy consumption problem. Therefore, finding sustainable energy solutions for the future is crucial. One promising approach is the use of organic solar cells since the sun provides a great amount of energy.

Organic solar cells possess considerable advantages compared to silicon based cells like large scale production in simple roll-to-roll printing processes on flexible substrates, light weight and low cost production. In organic solar cells two different materials are needed: p-type and n-type semiconductors. These materials are blended to form the active layer of the solar cell. Since the fundamental processes in organic solar cells are not fully understood, we are interested in the design of p-type materials to investigate the relationship of the material’s structure on optical, electrochemical and electrical properties. A state of the art concept for the design of conjugated polymers for solar cell applications is the donor-acceptor strategy. Here, electron rich (donor, D) and an electron poor (acceptor, A) monomers are covalently linked. In general, D-A-copoylmers can be obtained by the introduction of the A unit as a side chain of the donor backbone or by the incorporation of D and A in the main chain of the copolymer. By whatever means, a charge-transfer (CT) state is formed between D- and A-unit which leads to the red-shifted absorption band. This allows them to harvest more photons and produce higher currents in the solar cell. The character of this CT-state depends on the position of the acceptor in the D-A-copolymer. Does the strength of this CT state control the charge separation in the solar cell? In order to answer this question, we correlate the observed CT-character with the conjugation as well as with the charge carrier mobility. Finally, we compared our results with the charge separation in fullerene bilayer devices.

First, UV-vis and fluorescence measurements showed that the copolymer P2 has a stronger CT character compared to P3. Second, we investigated the influence of the two different D-A-structures in electrochemical measurements to estimate the highest occupied molecular orbital (HOMO) and the corresponding lowest unoccupied molecular orbital (LUMO) values. Compared to the reference homopolymer P1 (without acceptor) the LUMO level is reduced in both D-A-copolymers. Surprisingly, the incorporation of the acceptor in the side chain (P2) lowers the HOMO value. Therefore, the delocalization and the electron richness of the polymer backbone are reduced. In contrast to that, the HOMO value in P3 does not shift. Consequently, the acceptor unit in the main chain does not affect the oxidation potential here. A similar trend can be seen in the hole transport properties measured in “space charge limited current” (SCLC) devices of the D-A-copolymers. The introduction of the acceptor in the side chain strongly reduces the hole transport. On the other hand a very good hole transport mobility for the main-chain copolymer was observed. Finally, the copolymer P3 shows a much better charge separation in bilayer solar cells with C60 compared to P2 which is consistent with the previous results. In conclusion, we could show that for the design of new materials a good conjugation in combination with a weak CT-character is an efficient method to reach efficient charge separation.[1]


[1] Katharina Neumann, Christian Schwarz, Anna Köhler, Mukundan Thelakkat, Journal of Physical Chemistry: C, accepted, DOI: 10.1021/jp407014q.

Scientific career
  • 2005 - 2008
  • Bachelor studies in Chemistry, University of Bayreuth
  • 2008 - 2010
  • Master studies in Polymer Science, University of Bayreuth
  • 2009
  • Accepted into Elite Graduate Program Macromolecular Science
  • since 2010
  • PhD at the Institute of Macromolecular Chemistry I with Prof.Dr. Mukundan Thelakkat (University of Bayreuth) in the PhD program „Photophysik synthetischer und biologischer multichromophorer Systeme“

  • * Katharina Neumann, Mukundan Thelakkat, Proc. SPIE 8477, Organic Photovoltaics XIII, 84771H, 2012.
  • * Katharina Neumann, Christian Schwarz, Anna Köhler, Mukundan Thelakkat , Journal of Physical Chemistry: C, accepted, 2013, DOI: 10.1021/jp407014q.