FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com 1. Introduction Organic photovoltaics (OPVs) based on blends of electron- donating conjugated polymers and electron-accepting fullerene derivatives have attracted tremendous attention as low-cost Understanding How Processing Additives Tune the Nanoscale Morphology of High Efficiency Organic Photovoltaic Blends: From Casting Solution to Spun-Cast Thin Film Ming Shao, Jong Kahk Keum, Rajeev Kumar, Jihua Chen, James F. Browning, Sanjib Das, Wei Chen, Jianhui Hou, Changwoo Do, Kenneth C. Littrell, Adam Rondinone, David B. Geohegan, Bobby G. Sumpter, and Kai Xiao* Adding a small amount of a processing additive to the casting solution of photoactive organic blends has been demonstrated to be an effective method for achieving improved power conversion efficiency (PCE) in organic photo- voltaics (OPVs). However, an understanding of the nano-structural evolution occurring in the transformation from casting solution to thin photoactive films is still lacking. In this report, the effects of the processing additive dii- odooctane (DIO) on the morphology of the established blend of PBDTTT-C-T polymer and the fullerene derivative PC 71 BM used for OPVs are investigated, starting in the casting solution and tracing the effects in spun-cast thin films by using neutron/X-ray scattering, neutron reflectometry, and other charac- terization techniques. The results reveal that DIO has no observable effect on the structures of PBDTTT-C-T and PC 71 BM in solution; however, in the spun-cast films, it significantly promotes their molecular ordering and phase segregation, resulting in improved PCE. Thermodynamic analysis based on Flory-Huggins theory provides a rationale for the effects of DIO on dif- ferent characteristics of phase segregation due to changes in concentration resulting from evaporation of the solvent and additive during film formation. Such information may help improve the rational design of ternary blends to more consistently achieve improved PCE for OPVs. DOI: 10.1002/adfm.201401547 Dr. M. Shao, Dr. J. K. Keum, Dr. R. Kumar, Dr. J. Chen, Dr. A. Rondinone, Dr. D. B. Geohegan, Dr. B. G. Sumpter, Dr. K. Xiao Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge, TN 37831, USA E-mail: xiaok@ornl.gov Dr. J. K. Keum, J. F. Browning, C. Do, K. C. Littrell Neutron Scattering Science Divisions Oak Ridge National Laboratory Oak Ridge, TN 37831, USA Dr. R. Kumar, B. G. Sumpter Computer Science and Mathematics Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA S. Das Department of Electrical Engineering and Computer Science University of Tennessee Knoxville, TN 37996, USA Dr. W. Chen Material Sciences Division Argonne National Laboratory Argonne, Illinois 60439, USA Dr. J. Hou Institute of Chemistry Chinese Academy of Sciences Beijing 10080, China and renewable next-generation energy sources. Remarkable progress has been made in improving the power conversion efficiency (PCE) of OPVs above 11% by a combination of new materials synthesis, morphology optimization and advanced device engineering. [1,2] For high efficiency solar cells, conjugated polymers are required to have a broad light absorption spectrum that effectively overlaps the solar spectrum for effective exciton generation, and high charge carrier mobility for effi- cient charge transport and collection. In order to meet these criteria, much effort has been expended to synthesize new con- jugated polymers, in particular low band gap polymers. [3,4] Also, the PCE of an OPV device strongly depends on the nanoscale morphology of the electron donor (ED) and acceptor (EA) in the active layer. [5] The ideal morphology has been postu- lated to be the bulk heterojunction (BHJ), where the ED and EA form a nanoscale bicontinuous network. This nanoscale network ensures short path lengths for exciton diffusion, large interfacial area between ED and EA for effective exciton separation, and suf- ficient network connectivity pathways for effective charge trans- port and collection. To achieve different BHJ morphologies, a variety of processing techniques have been explored such as Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201401547 www.afm-journal.de www.MaterialsViews.com