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Forschungsarbeit

Block Copolymers in Organic Solar Cells and Transistors – One Material two Functionalities

Von Sven Hüttner (19.03.2010)

Harvesting energy in a sustainable, inexpensive way is one of the great challenges of the 21st century. The sun provides us with an almost limitless amount of energy making the development of solar cells crucial to tackle our energy needs. Today’s inorganic silicon based solar cells work efficiently, however their production costs are too high for a widespread application. Polymer-based solar cells offer a way for a low-cost production of large area photovoltaic panels since they can be printed or roll-to-roll processed onto flexible substrates.Organic photovoltaic cells rely on two different materials, a donor (p-type) and an acceptor (n-type) material. In conventional approaches, both materials are blended together to form the active layer. Our approach is to use block copolymers that carry donor-acceptor functionalities instead (b). Thus, we combine the well established concept of block copolymers with organic electronics.

One of the key aspects in organic photovoltaics is the morphology of donor and acceptor material. An interpenetrating network on a nanometre scale with a large interface between both material phases is highly desirable. This is due to the short exciton diffusion length: photons are absorbed and excitons are formed, but only those generated within 10 nm of the donor-acceptor interface can contribute to charge separation, others would recombine before. When the charges are separated they need to find percolation paths to the respective electrodes, which is the other important requirement for the morphology. Figure (c) schematically shows a polymer blend of a donor and acceptor material, where the charge separation of the exciton takes place at the interface. Our intention is to improve the morphology of the donor and acceptor material.

We use a fully functionalized block copolymer, that consists of a donor and an acceptor block (b),[1] since block copolymers phase separate into highly ordered nanostructures: The immiscibility of both blocks on the one hand, and their molecular interconnectivity on the other hand  lead to a microphase separation. The result are highly ordered self-assembled morphologies such as  spheres, cylinders, gyroids or lamella, depending on the volume ratio of the two blocks (a). Cylindrical or gyroidal nanostructures are ideal morphologies for organic solar cells, since they are continuous and have domain sizes in the range of the exciton diffusion length. Figure (f) depicts an ideal block copolymer solar cell made of a cylinder-forming block copolymer. Figure (c) shows one of the fully functionalized block copolymers as it was synthesized for this purpose.[2,3] We have demonstrated photovoltaic devices based on these unique polymers with high external quantum efficiencies of up to 30%.[4]

Although the power conversion efficiencies of our solar cells are still relatively low (<1%), this concept has great potential for improvement and is the basis of further current research, We also target the fundamental understanding of photo-physical aspects via device characterization and charge recombination studies, and chemical properties via novel materials synthesis. The unique controlled morphologies and interfaces between the acceptor and donor material provide the platform for elaborate investigations of charge generation, recombination and transport. In this regard, we use organic field effect transistors for example (f), and analyse the charge transport between the source and drain electrode that can be either n-tpye, p-type or even ambipolar transport. The research on these new materials and its application in solar cells is a fascinating work, that involves the interdisciplinary work of material science, chemistry, physics and nanotechnology that all have to work together towards new ways of new light harvesting organic electronics.

Block Copolymers in Organic Solar Cells and Transistors[Bildunterschrift / Subline]: Figure: a) Schematics of block copolymer phase separated morphologies. Depending on the volume ratio of each block highly regular structures can form with distances in the range of some tens of nanometres. b) Schematic of a block copolymer showing two covalently connected polymerchains. c) Donor-acceptor block copolymer consisting of polyhexylthiophene (P3HT) (donor) and a perylene bisimide derivative (acceptor). d) Conventional blended organic solar cell. The charge separation takes place at the interface, but only photons that are absorbed right at the interface can contribute due to the short exciton diffusion length. e) Ideal block copolymer photovoltaic cell made of one material with both functionalities. f) Schematic of an organic field effect transistor showing ambipolar charge transport as it can be achieved with dual functionalized block copolymers.

References:

[1] S.M. Lindner, S. Hüttner, A. Chiche, M. Thelakkat, G. Krausch, Angew. Chem. Int. Ed. 2006, 45, 3364. 

[2] M. Sommer, A. S. Lang, M. Thelakkat, Angew. Chem. Int. Ed. 2008, 47, 7901.

[3] S. Hüttner, M. Sommer, M. Thelakkat, Appl. Phys. Lett. 2008, 92, 093302.

[4] M. Sommer, S. Hüttner, U. Steiner, M. Thelakkat, Appl. Phys. Lett. in print.

 


Sven Hüttner
Sven Hüttner
* 1980, Hof

Stationen
  • seit 2010
  • Research Associate/Post-Doc an der University of Cambridge
  • seit 2007
  • Doktorarbeit in Kooperation zwischen den Universitäten in Bayreuth und Cambridge
  • 2006-2009
  • Promotion in Bayreuth am Lehrstuhl Physikalische Chemie II und am Lehrstuhl für Angewandte Funktionspolymere
  • 2005
  • Im Rahmen des Studiums der Technischen Physik an der Universität Bayreuth, Diplomarbeit am Lehrstuhl Physikalische Chemie II
  • 2003-2004
  • Auslandsaufenthalt in Berkeley, University of California

Ausgewählte Veröffentlichungen
  • *S. Hüttner, M. Sommer, U. Steiner, M. Thelakkat: "Organic field effect transistors from triarylamine side-chain polymers". Appl. Phys. Lett. 96, 073503 (2010).
  • *S. Hüttner, M. Sommer, A. Chiche, G. Krausch, U. Steiner, M. Thelakkat: "Controlled solvent vapour annealing for polymer electronics". Soft Matter, 5, 4206-4211 (2009).
  • *M. Sommer, S. Hüttner, U. Steiner, M. Thelakkat: "Influence of molecular weight on the solar cell performance of double-crystalline donor-acceptor block copolymers". Appl. Phys. Lett. 95, 183308 (2009).
  • *S. King, M. Sommer, S. Hüttner, M. Thelakkat, S. Haque: "Charge separation and recombination in self-organizing nanostructured donor-acceptor block-copolymer films". J.Mater. Chem.19, 5436-5441 (2009).
  • *M. Nedelcu, J. Lee, E. J. W. Crossland , S. C. Warren, S. Guldin, S. Hüttner, C. Ducati, D. Eder, U. Wiesner, U. Steiner, H. J. Snait: "Block-copolymer directed synthesis of mesoporous TiO2 for dye-sensitized solar cells". Soft Matter 5, 134-139 (2009).
  • *S. Hüttner, M. Sommer, M. Thelakkat: "n-type organic field effect transistors from perylene bisimide block copolymers and homopolymers". Appl. Phys. Lett. 92, 093302 (2008).
  • *M. Sommer, S. Hüttner, S. Wunder, M. Thelakkat: "Novel electron-conducting block-copolymers: morphological, optical and electronic properties". Adv. Mater. 20, 2523-2527 (2008).
  • *S. Lindner, S. Hüttner, A. Chiche, M. Thelakkat, G. Krausch: "Charge separation at self-assembled nanostructured bulk interface in block cpolymers". Angew. Chem. Int. Ed. 45, 3364-3368 (2006).