Morphological characterization of fullerene and fullerene-free organic photovoltaics by combined real and reciprocal spa
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Donglin Zhao, Qinghe Wu, and Luping Yu Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA
Harald Ade Department of Physics and Organic and Carbon Electronics Laboratory, North Carolina State University, Raleigh, North Carolina 27695, USA
Dean M. DeLongchampb) and Lee J. Richtera) Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA (Received 30 December 2016; accepted 23 March 2017)
Morphology can play a critical role in determining function in organic photovoltaic (OPV) systems. Recently molecular acceptors have showed promise to replace fullerene derivatives as acceptor materials in bulk heterojunction solar cells and have achieved .10% efficiencies in single junction devices. The nearly identical mass/electron densities between the donor (polymer) and acceptor (molecule) materials results in poor material contrast compared to fullerene-based OPVs and therefore morphology characterization using techniques that rely on mass/electron density variations poses a challenge. This inhibits a fundamental understanding of the structure–property relationships for non-fullerene acceptor materials. We demonstrate that low angle annular dark field scanning transmission electron microscopy and resonant soft X-ray scattering form a set of complementary tools that can provide quantitative characterization of fullerene as well as non-fullerene based organic photovoltaic systems.
I. INTRODUCTION
Alongside chemical composition and energy level structure, morphology plays a critical role in determining function in organic semiconducting systems.1–10 However, the complexity of material architecture, chemistry, and interactions among constituent materials used in organic electronics make it a challenging task to elucidate structure–property relationships. The fundamental understanding, improvement and utilization of these complex materials requires probing the physical and chemical characteristics at the molecular scale.11,12 Among systems where morphology often plays a determining role, solution-processed bulk heterojunction (BHJ) organic photovoltaics (OSs) have attracted much attention and Contributing Editor: Moritz Riede a) Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/editor-manuscripts/. DOI: 10.1557/jmr.2017.131
have exhibited great potential for making low-cost, large area, flexible solar panels through solution coating techniques.13,14 Small area cells with power conversion efficiency (PCE) above 11%15,16 have been demonstrated and state-of-the-art printed large area semitransparent modules producing 4.5% PCE have been manufactured.17 The BHJ active layer in an organic photovoltaic device is typically composed of two organic compounds. For simplicity we will refer to them as an ele
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