Importance of the Donor:Fullerene intermolecular arrangement for high-efficiency organic photovoltaics

Abstract
The performance of organic photovoltaic (OPV) material systems are hypothesized to depend strongly on the intermolecular arrangements at the donor:fullerene interfaces. A review of some of the most efficient polymers utilized in polymer:fullerene PV devices, combined with an analysis of reported polymer donor materials wherein the same conjugated backbone was used with varying alkyl substituents, supports this hypothesis. Specifically, the literature shows that higher-performing donor-acceptor type polymers generally have acceptor moieties that are sterically accessible for interactions with the fullerene derivative, whereas the corresponding donor moieties tend to have branched alkyl substituents that sterically hinder interactions with the fullerene. To further explore the idea that the most beneficial polymer:fullerene arrangement involves the fullerene docking with the acceptor moiety, a family of benzo[1,2-b:4,5-b]dithiophene-thieno[3,4-c]pyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized and tested in a variety of PV device types with vastly different aggregation states of the polymer. In agreement with our hypothesis, the PBDTTPD derivative with a more sterically accessible acceptor moiety and a more sterically hindered donor moiety shows the highest performance in bulk-heterojunction, bilayer, and low-polymer concentration PV devices where fullerene derivatives serve as the electron-accepting materials. Furthermore, external quantum efficiency measurements of the charge-transfer state and solid-state two-dimensional (2D) 13C{1H} heteronuclear correlation (HETCOR) NMR analyses support that a specific polymer:fullerene arrangement is present for the highest performing PBDTTPD derivative, in which the fullerene is in closer proximity to the acceptor moiety of the polymer. This work demonstrates that the polymer:fullerene arrangement and resulting intermolecular interactions may be key factors in determining the performance of OPV material systems. © 2014 American Chemical Society.

Citation
Graham, K. R., Cabanetos, C., Jahnke, J. P., Idso, M. N., El Labban, A., Ngongang Ndjawa, G. O., … McGehee, M. D. (2014). Importance of the Donor:Fullerene Intermolecular Arrangement for High-Efficiency Organic Photovoltaics. Journal of the American Chemical Society, 136(27), 9608–9618. doi:10.1021/ja502985g

Acknowledgements
This publication was supported by the Center for Advanced Molecular Photovoltaics (Award No KUS-C1-015-21) and was made possible by King Abdullah University of Science and Technology (KAUST). K.R.G. and A.A. acknowledge SABIC for a postdoctoral fellowship. G.O.N.N., K.R.G., M.D.M., and A.A. acknowledge the Office of Competitive Research Funds for a GRP-CF award. T.H. gratefully acknowledges a "DAAD Doktorantenstipendium" and the SFB 953 "Synthetic Carbon Allotropes". Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The NMR experiments were conducted in the Central Facilities of the UCSB Materials Research Laboratory supported by the MRSEC program of the U.S. NSF under Award No. DMR-1121053. The work at UCSB was supported by the USARO through the Institute for Collaborative Biotechnologies under Contract No. W911NF-09-D-0001. The authors also thank Dr. Chad Risko and Prof. Jean-Luc Bredas for helpful discussions.

Publisher
American Chemical Society (ACS)

Journal
Journal of the American Chemical Society

DOI
10.1021/ja502985g

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