The influence of the nematic phase on the phase separation of blended organic semiconductors for photovoltaics Steven A. Myers a,n , Manea S. Al Kalifah a,1 , Chunghong Lei a , Mary O'Neill a , Stuart P. Kitney b , Stephen M. Kelly b a Department of Physics and Mathematics, University of Hull, Hull HU6 7RX, UK b Department of Chemistry, University of Hull, Hull HU6 7RX, UK article info Article history: Received 28 November 2012 Received in revised form 25 April 2013 Accepted 7 May 2013 Available online 6 June 2013 Keywords: Photovoltaics Liquid crystals Nematic Self-assembly Nano-morphology Phase transitions abstract Differential scanning calorimetry in combination with atomic force microscopy is used to examine the phase separation of a blended nematic liquid crystalline electron-donor and crystalline perylene electron- acceptor mixture. Separate domains of donor and acceptor material are mostly retained in the blend, although a small proportion of the acceptor, increasing with increasing donor concentration, is mixed in with the donor domains. Annealing in the nematic phase allows the donor and acceptor molecules to move and generate phase-separated domains of the required size, thus enhancing the performance of bulk heterojunction photovoltaic devices based on these blends. We show that the optimum annealing temperature can be controlled by manipulation of the temperature range of the nematic phase of the donor. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Columnar, smectic and, more recently, nematic liquid crystals have been shown to be very promising charge-transporting organic semiconductors due to their ability to spontaneously self-assemble into highly ordered domains in uniform, thin films over large areas [1–3]. This combination of properties allied with broad absorption spectra render them particularly suitable as active materials for organic photovoltaics, such as bulk hetero- junction devices based on blends of liquid crystalline electron donors and crystalline electron acceptors. Photogenerated excitons are dissociated at the interface between the phase-separated domains of the donor and acceptor components. The separated electrons and holes then travel along different and distinct con- tinuous pathways in opposite directions to the electrodes. Photo- voltaic cells comprising a coronene compound as a donor with columnar liquid crystalline phases blended with an electron acceptor showed power conversion efficiencies up to 1.5% under standard measurement conditions [4–6]. The performance of these organic photovoltaic (OPV) cells was optimized by effective use of thermal annealing to control the degree of molecular ordering within the individual domains of electron donors and electron acceptors. Thioenothiophene polymers, which have high- temperature liquid crystalline phases, were blended with soluble fullerene derivatives [7,8], with optimized devices having a power conversion efficiency of 2.5%. It is noted that annealing in the liquid crystal phase improved the performance of photovoltaic devices incorporating donors with either columnar or lamellar polymeric phases [6,9,10]. A nematic liquid crystal composite with a porous surface and sub-micron scaled grooves has also been used to provide a distributed interface to vertically separate electron-donating and electron-accepting films in a bilayer PV device [11,12]. However, quite surprisingly, the use in OPVs of low- molar-mass nematic liquid crystals (small molecules) has not been studied to any significant degree [13]. The nematic phase of such compounds possesses a much lower viscosity than that of either highly ordered columnar liquid crystals or high molecular weight liquid crystalline polymers, which are exceptionally viscous mate- rials. Therefore, nematic liquid crystals should offer significant advantages in controlling the morphology of the donor–acceptor composites for organic photovoltaics, especially when using thermal annealing to control domain size and morphology. This work addresses the important question of how the self- assembly properties of liquid crystals can be exploited to control the morphology of the donor–acceptor composites for organic photovoltaics. Our model system consists of a nematic donor blended with a crystalline perylene based acceptor. We propose Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.05.006 n Corresponding author. Tel.: +44 1482465396. E-mail address: S.A.Myers@hull.ac.uk (S.A. Myers). 1 Current Address: Department of Physics, Faculty of Science and Art at Alrass, Qassim University, Kingdom of Saudi Arabia. Solar Energy Materials & Solar Cells 116 (2013) 262–269