Spiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport via mesoscale ordering

Abstract
We report the crystal structure and hole-transport mechanism in spiro-OMeTAD [2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene], the dominant hole-transporting material in perovskite and solid-state dye-sensitized solar cells. Despite spiro-OMeTAD’s paramount role in such devices, its crystal structure was unknown because of highly disordered solution-processed films; the hole-transport pathways remained ill-defined and the charge carrier mobilities were low, posing a major bottleneck for advancing cell efficiencies. We devised an antisolvent crystallization strategy to grow single crystals of spiro-OMeTAD, which allowed us to experimentally elucidate its molecular packing and transport properties. Electronic structure calculations enabled us to map spiro-OMeTAD’s intermolecular charge-hopping pathways. Promisingly, single-crystal mobilities were found to exceed their thin-film counterparts by three orders of magnitude. Our findings underscore mesoscale ordering as a key strategy to achieving breakthroughs in hole-transport material engineering of solar cells.

Citation
Spiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport via mesoscale ordering 2016, 2 (4):e1501491 Science Advances

Acknowledgements
O.M.B. and J.-L.B acknowledges the financial support of King Abdullah University of Science and Technology Grant URF/1/2268-01-01. J.-L.B. also acknowledges support from ONR Global through Grant N62909-15-1-2003. H.D. thanks the National Natural Science Foundation of China (91433115). Author contributions: D.S. conceived the idea. O.M.B. crafted the overall experimental plan and directed the research. D.S. optimized the crystallization. D.S. and W.X. performed the confocal optical microscope imaging. D.S. and Y.H. performed single-crystal XRD and data analysis. D.S., X.Q., H.D., T.L., and W.H. planned and performed the mobility measurements and analyzed the data. Y.L., C.Z., and J.-L.B. planned and performed the theoretical calculations. Y.L., C.Z., and J.-L.B. analyzed the data of the theoretical part. J.P. assisted D.S. in the experiments. D.S., Y.L., J.-L.B., and O.M.B. wrote the manuscript. All authors discussed and commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Publisher
American Association for the Advancement of Science (AAAS)

Journal
Science Advances

DOI
10.1126/sciadv.1501491

PubMed ID
27152342

Additional Links
http://advances.sciencemag.org/cgi/doi/10.1126/sciadv.1501491

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