ASHRAF ET AL. VOL. 8 ’ NO. 1 ’ 323 –331 ’ 2014 www.acsnano.org 323 December 20, 2013 C 2013 American Chemical Society Confinement-Induced Reduction in Phase Segregation and Interchain Disorder in Bulk Heterojunction Films Ahsan Ashraf, †,‡ D. M. Nanditha M. Dissanayake, ‡ David S. Germack, § Conan Weiland, ^ and Matthew D. Eisaman †,‡, * † Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11974, United States, ‡ Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, New York 11973, United States, § Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States, and ^ Synchrotron Research, Inc., Upton, New York 11973, United States R estriction of the film thickness below the radius of gyration in macromole- cules leads to thin-film confinement effects, in which a significant transition of the materials properties occurs compared to bulk films. 14 Even though thin-film con- finement is well studied for both amor- phous and crystalline pure polymers, 5 its effects on the morphology and the inter- facial interactions of ultrathin (<20 nm, the length scales comparable to the polymers radius of gyration) 6 polymer (electron donor): fullerene (electron acceptor) interpenetrat- ing composites, bulk heterojunction (BHJ) films, have not been well investigated. 7 BHJs are a more complex and functionally more advanced class of composite materi- als, and a fundamental understanding of the effect of thin-film confinement on BHJ films could be crucial for applications such as novel organic photovoltaics. In BHJ films, even within a bulk thickness regime of ∼200 nm, its morphology is reported to be affected by various processing conditions such as choice of solvent, drying time, ther- mal treatments, 8 and the use of solvent additives. 9,10 Furthermore, depending on the surface energy of the substrate on which such BHJs are coated, an inhomoge- neous distribution of the polymer:fullerene volume fraction, phase segregation, in the direction normal to the substrate has been reported. 11,12 These morphological and in- terfacial phenomena directly impact the optical and electronic properties of the BHJ layer critical for device applications. 8 In particular, the high- and low-bandgap polymer regions within the bulk of the BHJ layer believed to be formed by phase seg- regation are directly linked to the reduction in bimolecular recombination of BHJ based organic photovoltaics, and this is perceived * Address correspondence to meisaman@bnl.gov. Received for review August 9, 2013 and accepted December 20, 2013. Published online 10.1021/nn404172m ABSTRACT The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donoracceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk hetero- junction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (00 and 01) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics. KEYWORDS: phase segregation . confinement . spectroscopic ellipsometry . polymer:fullerene bulk heterojunction . organic photovoltaics . thin films ARTICLE