Tuning charge transport in solution-sheared organic semiconductors using lattice strain

Abstract

Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications1,2,3,4,5. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice6. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility7. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33 Å to 3.08 Å. We believe that 3.08 Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04 Å has been achieved through intramolecular bonding8,9,10). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from 0.8 cm2 V−1 s−1 for unstrained films to a high mobility of 4.6 cm2 V−1 s−1 for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.

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Figure 1: Solution-shearing schematic and cross-polarized optical microscope images of solution-sheared films.
Figure 2: Change in GIXD pattern of TIPS-pentacene as function of shearing speed.
Figure 3: Molecular packing structure of TIPS-pentacene thin films prepared under different conditions.
Figure 4: Lattice strain and charge transport properties as a function of shearing speed.

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Acknowledgements

We thank A. Salleo, J. E. Anthony, H. Li, A. Sokolov and J. Rivnay for discussions. We thank J. E. Anthony and M. M. Nelson of 3M Corp. for providing high-purity TIPS-pentacene. This publication was partially supported by the National Science Foundation DMR-Solid State Chemistry (DMR-0705687-002), the Samsung Advanced Institute of Technology, the Global Climate and Energy Project at Stanford University (SPO 25591130-45282-A) and the Air Force Office of Scientific Research (award number FA9550-09--0256). E.V. thanks the Eastman Kodak Corporation Kodak Fellows Program for support. Z.B. acknowledges support from the David Filo and Jerry Yang Faculty Fellowship from Stanford University. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of Stanford University, the Sponsors of the Global Climate and Energy Project, or others involved with the Global Climate and Energy Project.

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Contributions

G.G. and H.A.B. built the current version of the solution-shearing set-up. G.G. and E.V. performed X-ray and transistor measurements. S.C.B.M. performed unit cell and molecular packing calculations. G.G., E.V., S.C.B.M., D.H.K., M.F.T. and Z.B. analysed the X-ray data. S.A.-E. and A.A.-G. performed transfer integral calculations. G.G., E.V. and Z.B. wrote the manuscript, and all other authors had input. Z.B. and S.Y.L. directed the project.

Corresponding author

Correspondence to Zhenan Bao.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Methods and a Discussion, Supplementary Figures 1-13 with legends, Supplementary Tables 1-4, legends for Supplementary Movies 1-2 and additional references. (PDF 1309 kb)

Supplementary Movie 1

This movie shows in-situ strain relief of GIXD peaks during toluene vapor annealing. (MPG 3408 kb)

Supplementary Movie 2

This movie shows in-situ heating TIPS-pentacene thin films, showing no strain relief of the (010) GIXD peak. (MPG 3074 kb)

Supplementary Data 1

This file shows the structure of thin film unstrained TIPS-pentacene. (TXT 3 kb)

Supplementary Data 2

This file shows the structure of thin film strained TIPS-pentacene solution sheared at 8mm/s (TXT 3 kb)

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Giri, G., Verploegen, E., Mannsfeld, S. et al. Tuning charge transport in solution-sheared organic semiconductors using lattice strain. Nature 480, 504–508 (2011). https://doi.org/10.1038/nature10683

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