Polymeric solar cells based on P3HT:PCBM: Role of the casting solvent Minh Trung Dang a , Guillaume Wantz a,n , Habiba Bejbouji a , Mathieu Urien a , Olivier J. Dautel b , Laurence Vignau a , Lionel Hirsch a a Universite´ de Bordeaux, Laboratoire IMS, UMR CNRS 5218, Ecole Nationale Supe´rieure de Chimie, Biologie et Physique, 16 Avenue Pey Berland, 33607 Pessac Cedex, France b Laboratoire AM 2 N, Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Ecole Nationale Supe´rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France article info Article history: Received 20 April 2011 Received in revised form 18 July 2011 Accepted 29 July 2011 Available online 3 September 2011 Keywords: Polymer solar cells P3HT:PCBM Spin-coating solvent Tetrahydronaphthalene Trichlorobenzene abstract Polymeric photovoltaic (PV) solar cells have been fabricated using six solvents: chloroform (CHCl 3 ), toluene (T), chlorobenzene (CB), orthodichlorobenzene (ODCB), 1,2,3,4-tetrahydronaphthalene (THN) and 1,2,4-trichlorobenzene (TCB). The active layers were composed of poly(3-hexyl)thiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Special care has been taken to keep all experimental parameters constant (thickness of the active layers, donor/acceptor weight ratio, area of active surface and electrodes) in order to avoid artefacts and truly study the effect of solvents. Studies using atomic force microscopy (AFM) and optical absorption (UV–vis) showed the relationship between the photovoltaic performance and the evaporation rate of solvents. The use of solvents with high boiling point results in a higher degree of organization in the structure of P3HT. A direct comparison with devices processed with thermal treatment has also been performed. As often reported thermal annealing increases photo-conversion efficiency of devices created from common solvents, due to better separation of phase between the two materials of the blend. In the case of solvents with high boiling point such as THN and TCB, neither phase separation nor modification of P3HT crystallization induced by thermal annealing has been observed. However thermal treatment appears to enhance performance, ensuing the evaporation of remaining solvent in the active layers. An overview of the effect of solvent on the electrical properties of films containing pure P3HT and P3HT:PCBM blend reported in the literature has been completed for the discussion. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Photovoltaic (PV) solar cells are a possible answer to the need of clean and renewable energy. Despite the expanding use of inorganic semiconductors, the organic semiconductors enable the fabrication of solar modules with several potential advantages, including light-weight, flexibility, low-cost manufacturing and the possibility of creating large-area devices [1–5]. While organic semiconductors of low molecular weight are generally deposited by deposition under vacuum, the conjugated polymers are con- veniently deposited from solutions using wet processing techni- ques such as dip coating, spin coating, inkjet printing, doctor blading and compatible with high speed line processes like roll- to-roll [6–11]. These techniques represent an enormously exciting way for producing PV cells because they can be performed under ambient conditions (temperature and pressure) and scaled up to large area with minimized loss of material. Within organic semiconductors, free holes and electrons can- not be created upon light absorption due to the large exciton binding energy (0.1–1.4 eV) [12–15]. Light absorption induces the formation of Frenkel-like excitons [13]. To produce electricity, excitons must be split apart into free electrons and holes, which are then collected at the anode and cathode, respectively. Efficient exciton dissociation is achieved at the interface between two organic semiconductors with an appropriate difference in elec- tron affinities and ionization potentials [16,17]. However, the only driving force for excitons to reach such interfaces is diffusion and the typical exciton diffusion length in most polymers is confined to approximately 3–10 nm [18–26]. As excitons have to diffuse across this specific distance before undergoing recombination, a bi-layer structure is not suitable because only a small volume near the interface is effective for achieving efficient photovoltaic conversion. In order to overcome this limit, the concept of bulk heterojunction has been developed by simply mixing electron donor and electron acceptor [27,28]. The high yield of photo- current in bulk heterojunction devices is attributed to efficient exciton dissociation, resulting from the extensively dispersed interface between the domains However, if the donor and acceptor are too finely incorporated on the molecular scale, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.07.039 n Corresponding author. E-mail address: guillaume.wantz@ims-bordeaux.fr (G. Wantz). Solar Energy Materials & Solar Cells 95 (2011) 3408–3418