The impact of controlled solvent exposure on the morphology, structure and function of bulk heterojunction solar cells Raghavendra Hegde a , Nathan Henry a , Ben Whittle a , Huidong Zang c , Bin Hu c , Jihua Chen d , Kai Xiao d , Mark Dadmun a,b,n a Department of Chemistry, The University of Tennessee, Knoxville, TN, USA b Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA c Department of Material Science and Engineering, The University of Tennessee, Knoxville, TN, USA d Center for Nanophase Materials Sciences Oak Ridge National Laboratory, Oak Ridge, TN, USA article info Article history: Received 13 April 2012 Received in revised form 12 July 2012 Accepted 17 July 2012 Available online 7 September 2012 Keywords: Morphology Organic solar cell Solvent annealing Crystallinity Depth profile abstract Films of poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) were controllably exposed to CS 2 vapor in a column with a linear solvent vapor pressure gradient. Changes in the morphology of the P3HT:PCBM thin film were monitored and correlated to the ability of this thin film to act as the active layer in an organic solar cell. The results show that the crystallinity and crystal size of the P3HT increase initially with solvent vapor pressure and annealing time, but longer exposure to solvent decreases P3HT crystallinity and photovoltaic efficiency. Neutron reflectivity indicates that the PCBM segregates to the Si substrate in the as-cast thin film, but distributes throughout the film with solvent annealing. The changes in crystallinity and the depth profile of the P3HT:PCBM mixture differ from those induced by thermal annealing. The structural variation with solvent exposure is correlated to photovoltaic function, demonstrating that the solvent annealing provides a window of optimum efficiency, which depends on solvent exposure. Moreover, the control of depth profile and structure should be generally applicable to a broad range of polymer-nanoparticle mixtures and thus these results provide fundamental information that can be used to control the depth profile, morphology and function of thin film nanocomposites. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Conjugated polymers are intriguing materials due to their poten- tial use in organic electronics such as low cost, flexible solar cells, low-voltage field-effect transistors and organic optoelectronic devices such as modulators, optical filters, and polarization rotators. Due to their high absorption coefficient, they have become primary compo- nents in the active layer of organic solar cells (OSC), as thin layers can generate sufficiently high photocurrent to be practical. Organic solar cells are promising materials to convert sunlight into electricity, however in order to be economically competitive with inorganic photovoltaics, their power conversion efficiency must be improved. This improvement requires a more complete understanding of the correlation between active layer morphology and performance. This is because in the function of an OSC, the absorption of a photon forms a Frankel exciton, which has a large binding energy due to the low dielectric constant of the organic matrix. The exciton must, therefore, migrate to a donor–acceptor interface to dissociate, and then each phase must have a pathway to the electrodes to realize a photo-generated current [1]. Thus, the morphology of the OSC is critical in realizing an efficient photovoltaic cell. The bulk heterojunction consisting of regioregular poly(3-hexy- lthiophene) (P3HT) and [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) has become the archetypal OSC active layer in recent years, as power conversion efficiencies approaching 5% have been reported for the P3HT:PCBM active layer. In this system, the P3HT provides adequate hole mobility, the ability to self organize into a crystalline structure and a reasonable spectral overlap of its absorption spectrum with the solar emission spectrum. The improved solubility of PCBM appears to aid in the formation of desired morphology [2]. However, as-cast devices often exhibit poor photovoltaic efficiencies, and subsequent processing, such as casting from a mixed solvent, micro- wave annealing, thermal annealing, [3] and solvent vapor annealing (SVA) processes [4–11] are required to improve the photovoltaic efficiency. Previous studies ascribe the increased efficiency in PCBM:P3HT with thermal annealing to an increase in P3HT crystallinity [12], which improves hole transport, as well as segregation of the PCBM to the device cathode [13]. It is reasoned that the increase in charge Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.07.014 n Corresponding author at: Department of Chemistry, The University of Tennessee, Knoxville TN, USA. Tel.: þ1 865 974 6582. E-mail address: dad@utk.edu (M. Dadmun). Solar Energy Materials & Solar Cells 107 (2012) 112–124