Controlling the morphology of nanofiber-P3HT:PCBM blends for organic bulk heterojunction solar cells Sabine Bertho a, * , Wibren D. Oosterbaan a , Veerle Vrindts b , Jan D’Haen a , Thomas J. Cleij a , Laurence Lutsen b , Jean Manca a,b , Dirk Vanderzande a,b a Hasselt University, Institute for Materials Research, Wetenschapspark 1, B-3590 Diepenbeek, Belgium b IMEC vzw, IMOMEC, B-3590 Diepenbeek, Belgium article info Article history: Received 6 May 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Available online 3 July 2009 PACS: 84.60.Jt 42.70.Jk 81.05.Tp 64.75.Jk 68.37.Lp Keywords: Organic solar cells Nanofibers Transmission electron microscopy abstract Within the field of organic bulk heterojunction solar cells, the morphology of the active layer has a key role in obtaining high power conversion efficiencies. P3HT nanofibers, obtained in highly concentrated solutions, are able to give controlled morphologies directly upon depo- sition. Since the solar cell efficiency of fiber solar cells depends on the fiber content of the casting solution, it is important to control this parameter. Here, we demonstrate an easy way to control the fiber content in the casting solution, i.e. changing the solution tempera- ture. By using solution heating, the overall molecular weight of the polymer in the blend is kept constant, fiber isolation is not needed and the use of solvent mixtures is avoided. The obtained optimal power conversion efficiency is shown to be linked to the morphology of the active layer, which is studied with Transmission Electron Microscopy (TEM). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction P3HT (poly(3-hexylthiophene)) has been a workhorse material for the development of organic solar cells. Its first breakthrough within the field came with the discovery of the beneficial effect of a heat-treatment of solar cells based on a P3HT:PCBM ([6-6]-phenyl C 61 -butyric acid methyl es- ter) blend [1–3]. After this, efficiencies have gradually in- creased towards 5–6% through a control of the nanomorphology of the polymer:PCBM blend [4,5]. Lately, P3HT nanofibers obtained in highly concentrated solutions have been shown to give optimized morphologies directly upon deposition [6–8]. This makes traditional thermal treatment superfluous. The deposition of nanofibers is therefore a more appropriate method to be used with flex- ible substrates, such as poly(ethylene terephthalate) (PET), which have a low glass transition temperature. The solar cell efficiency of these fiber solar cells depends on the fiber content of the casting solution. Berson et al. [6] controlled the fiber content by mixing isolated fibers with well-dis- solved low molecular weight (MW) P3HT; a ratio of 0.75 fi- ber to 0.25 well-dissolved polymer gave optimal efficiency. Li et al. [7] used the anti-solvent hexane to control the fiber content of P3HT in o-dichlorobenzene. Moulé et al. [8] added different amounts of nitrobenzene to a solvent mix- ture of P3HT:PCBM; this caused the formation of aggre- gated P3HT networks during the drying process of the devices. Here, we show that the fiber content can be easily controlled by solution temperature. In this way, the overall MW of the P3HT in the blend is kept constant. This is important, since it has been shown that MW influences the photovoltaic performance of P3HT:PCBM solar cells 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.06.018 * Corresponding author. Tel.: +32 (0) 11 268887; fax: +32 (0) 11 268899. E-mail address: sabine.bertho@uhasselt.be (S. Bertho). Organic Electronics 10 (2009) 1248–1251 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel