Crosslinkable Conjugated Polymers for Organic Photovoltaics
Philipp Knauer,* Christina Saller,* Tobias Hahn,+ Anna Köhler,+ Peter Strohriegl*
*University of Bayreuth, Macromolecular Chemistry I, 95440 Bayreuth, Germany +University of Bayreuth, Experimental Physics II, 95440 Bayreuth, Germany
Low-bandgap polymers have gained large interest as efficient materials in bulk heterojunction organic solar cells (OSCs) during the last years. In our contribution we present crosslinkable conjugated polymers and a number of strategies for their application in organic solar cells.
We present crosslinking as a strategy for the stabilization of the morphology of conjugated polymers. We have functionalized the well-known low-bandgap polymers poly(2,7-(9,9-dioctylfluorene)-alt-5,5-(4’,7’-di-4-hexylthien-2-yl-2’,1†™,3’- benzothiadiazole) PFDTBT and poly(N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-dithien-2-yl-2’,1†™,3’-benzothiadiazole)) PCDTBT with polymerizable oxetane units.[1] The syntheses, characterization and two approaches for crosslinking will be presented.
One common approach for the fabrication of organic solar cells is the bulk heterojunction (BHJ) concept. Here, the active layer most often comprises a low molar mass fullerene derivative in a polymer matrix. Thermodynamically these materials are in a non-equilibrium state and problems with diffusion may occur, if BHJ solar cells are run at elevated temperatures for a prolonged time. To address the problem of C60 diffusion in conjugated polymers, we have established a new method to determine the diffusion coefficient of C60 in crosslinked and non-crosslinked polyfluorenes. The diffusion coefficient of C60 is reduced by a factor 1000, if the polymer is densely crosslinked.[2]
Besides the BHJ concept multilayer devices represent a different architecture for OSCs. Multilayer solar cells have been reported to reach efficiencies of 12%, overmatching the best BHJ devices. Such multilayer solar cells are usually made by vacuum deposition of small molecules. Crosslinkable conjugated polymers allow the fabrication of multilayers from solutions. Crosslinking of a cast polymer film renders it insoluble and another layer can be applied from solution without redissolving the first layer. We present multilayer OSCs with an exciton blocking layer formed by a crosslinked polymer.[3]
Furthermore, crosslinking opens up the possibility to stabilize small patterns in films of conjugated polymers. Hereby, OSCs with well-defined interfaces between the donor and acceptor material can be achieved. Using techniques such as nanoimprint lithography, patterns with a large surface area, for instancecomb-like structures (< 100 nm), can be realized. The patterns are subsequently stabilized by crosslinking of the low-bandgap polymer. In the next step the acceptor layer can be applied by thermal evaporation and from solution,respectively.
[1] P. Strohriegl, P. Knauer, C. Saller, E. Scheler, Proc. SPIE 8830, Organic Photovoltaics XIV, 8830P (October 2013); doi: 10.1117/12.2023899. [2] F. Fischer, T. Hahn, H. Bässler, I. Bauer, P. Strohriegl, A. Köhler, Adv. Funct. Mater. 39 (2014). 6172 [3] T. Hahn, C. Saller, M. Weigl, I. Bauer, T. Unger, A. Köhler, P. Strohriegl, Multilayer solar cells with crosslinked polymeric exciton blocking layer; submitted.