Controlled solvent vapour annealing for polymer electronics† Sven H€ uttner, ab Michael Sommer, a Arnaud Chiche, c Georg Krausch, d Ullrich Steiner * b and Mukundan Thelakkat * a Received 14th April 2009, Accepted 20th July 2009 First published as an Advance Article on the web 21st August 2009 DOI: 10.1039/b907147d Solvent vapour annealing (SVA) is demonstrated as an attractive method to anneal polymer blend and block copolymer thin films at low temperatures. It is especially suitable for organic electronics, where sensitive materials with strong intermolecular interactions are used. We demonstrate the effect of solvent vapour exposure on the film properties of a perylene bisimide acrylate (PPerAcr) side-chain polymer with strong crystallinity at the perylene bisimide moieties. We record the film thickness, light absorption and fluorescence as a function of the relative solvent vapour pressure. At a certain threshold of relative solvent vapour pressure, we observe a disruption of the p–p stacking, which is responsible for perylene bisimide crystallisation. This leads to an increase in the polymer-chain mobility and therefore to changes in the film morphology. The results are applied to a film of a donor–acceptor block copolymer carrying PPerAcr segments, and the influence of solvent annealing on the nanoscale morphology is demonstrated. Introduction The field of organic electronics has enjoyed increasing interest over the past decade. Organic light emitting diodes, field-effect transistors and solar cells have a promising future, with some products already commercially available. Both, low-molecular- weight systems and polymeric materials are used in organic electronic devices. Polymers typically offer an easy processability from solution, potentially paving the way for low cost and large- area applications. Bulk heterojunction solar cells, for example, consist of two components, an acceptor and a donor material. A distinct phase morphology of the two components is required in order to guarantee efficient charge separation combined with sufficient charge percolation to the electrodes. This is important because of the limited diffusion length of the photogenerated excitons of only several nanometres. Charge separation only takes place at the donor–acceptor interface because of the low permittivity of organic materials. A general approach to achieve an inter- penetrating network of the donor and the acceptor material is blending the two materials. The most common method to produce such films is spin coating. Depending on the solubility and the boiling point of the solvent, film formation takes place within several seconds, freezing-in a non-equilibrium morphology of a polymer blend. 1 The detailed demixing process during film processing therefore determines the internal structure of binary polymeric thin films. The morphology always plays the decisive role in such devices since it has a large effect on several essential properties such as charge transport, charge separation and recombination. Post-annealing steps are therefore often applied in order to alter or induce the desired phase separation. This can be done either by temperature annealing, where the polymer film is heated above its glass transition or melting temperature, 2,3 or by solvent vapour annealing (SVA). 4,5 Alternatively, co-solvent spin coating can be used, where a high-boiling-point solvent is mixed with a low-boiling-point solvent. 6 All of these methods increase the polymer-chain mobility, giving the system sufficient time to modify its morphology towards its thermodynamic equilibrium. This provides a path for the polymer blend to evolve towards an advantageous morphology. Once a desired morphology is obtained, the system is ‘frozen’ to prevent further changes. Solvent vapour annealing involves the exposure of the film to a solvent atmosphere under controlled conditions. The film swells by solvent take-up, causing an increase in polymer-chain mobility similar to temperature annealing above the melting or glass transition temperature. SVA has the advantage that it can be done at room temperature, significantly reducing the risk of thermal degradation of the material. Perylene bisimides (PBIs) are well-known stable electron transporting materials. Extensive research has been performed on low-molecular-weight perylene bisimide derivatives, and their application in organic field-effect transistors, photovoltaic cells, and photodetectors has been demonstrated. 7,8,9 Various deriva- tives of PBIs have been synthesized, in which the molecular packing is altered by different substituents. This leads to a vari- ation in the physical properties such as melting temperature, solubility in organic solvents, liquid crystal formation, and charge carrier transport. For example, high electron mobilities of up to 0.1 cm 2 V 1 s 1 were reported for low-molecular-weight PBIs. 10 The p–p interactions between the perylene bisimide cores strongly influence the optical properties of the PBI moieties. This a Angewandte Funktionspolymere, Makromolekulare Chemie I, Universit € at Bayreuth, 95440 Bayreuth, Germany. E-mail: mukundan.thelakkat@ uni-bayreuth.de; Fax: +49 921 553206; Tel: +49 921 553018 b Cavendish Laboratory, University of Cambridge, CB3 0HE Cambridge, United Kingdom. E-mail: u.steiner@phy.cam.ac.uk; Tel: +44 1223 337073 c DSM Material Science Centre, P.O. Box 18, 6160 MD, Geleen, The Netherlands d Johannes Gutenberg Universit € at Mainz, 55128 Mainz, Germany † Electronic supplementary information (ESI) available: Optical properties of the PvTPA-b-PPerAcr block copolymer. See DOI: 10.1039/b907147d 4206 | Soft Matter , 2009, 5, 4206–4211 This journal is ª The Royal Society of Chemistry 2009 PAPER www.rsc.org/softmatter | Soft Matter