Article | Published:

Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies

Nature Materialsvolume 17pages329334 (2018) | Download Citation

Abstract

Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

A correction to this article is available online at https://doi.org/10.1038/s41563-018-0043-3.

Change history

  • 06 March 2018

    In the html version of this Article originally published, Paul W. M. Blom and Gert-Jan A. H. Wetzelaer were incorrectly listed as Paul M. W. Blom and Gert-Jan H. A. Wetzelaer, respectively, due to a technical error. This has now been amended in all online versions of the Article.

References

  1. 1.

    Tao, Y., Yang, C. & Qin, J. Organic host materials for phosphorescent organic light-emitting diodes. Chem. Soc. Rev. 40, 2943–2970 (2011).

  2. 2.

    Wong, M. Y. & Zysman-Colman, E. Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Adv. Mater. 29, 1605444 (2017).

  3. 3.

    Koch, N. & Vollmer, A. Electrode-molecular semiconductor contacts: Work-function-dependent hole injection barriers versus Fermi-level pinning. Appl. Phys. Lett. 89, 162107 (2006).

  4. 4.

    Helander, M. G. et al. Chlorinated indium tin oxide electrodes with high work function for organic device compatibility. Science 332, 944–947 (2011).

  5. 5.

    Simmons, J. G. Richardson–Schottky effect in solids. Phys. Rev. Lett. 15, 967–968 (1965).

  6. 6.

    Méndez, H. et al. Charge-transfer crystallites as molecular electrical dopants. Nat. Commun. 6, 8560 (2015).

  7. 7.

    Tang, C. G. et al. Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts. Nature 539, 536–540 (2016).

  8. 8.

    Kröger, M. et al. Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films. Appl. Phys. Lett. 95, 123301 (2009).

  9. 9.

    Kröger, M. et al. P-type doping of organic wide band gap materials by transition metal oxides: A case-study on molybdenum trioxide. Org. Electron. 10, 932–938 (2009).

  10. 10.

    Meyer, J. et al. Transition metal oxides for organic electronics: Energetics, device physics and applications. Adv. Mater. 24, 5408–5427 (2012).

  11. 11.

    Blom, P. W. M., de Jong, M. J. M. & Vleggaar, J. J. M. Electron and hole transport in poly(p-phenylene vinylene) devices. Appl. Phys. Lett. 68, 3308–3310 (1996).

  12. 12.

    Davids, P. S., Campbell, I. H. & Smith, D. L. Device model for single carrier organic diodes. J. Appl. Phys. 82, 6319–6325 (1997).

  13. 13.

    Belisle, R. A., Jain, P., Prasanna, R., Leijtens, T. & McGehee, M. D. Minimal effect of the hole-transport material ionization potential on the open-circuit voltage of perovskite solar cells. ACS Energy Lett. 1, 556–560 (2016).

  14. 14.

    White, R. T., Thibau, E. S. & Lu, Z.-H. Interface structure of MoO3 on organic semiconductors. Sci. Rep. 6, 21109 (2016).

  15. 15.

    Mott, N. F. & Gurney, R. W. Electronic Processes in Ionic Crystals (Oxford University Press: Oxford, 1940).

  16. 16.

    Li, C., Duan, L., Li, H. & Qiu, Y. Universal trap effect in carrier transport of disordered organic semiconductors: Transition from shallow trapping to deep trapping. J. Phys. Chem. C 118, 10651–10660 (2014).

  17. 17.

    Greiner, M. T. et al. Universal energy-level alignment of molecules on metal oxides. Nat. Mater. 11, 76–81 (2012).

  18. 18.

    Blakesley, J. C. & Greenham, N. C. Charge transfer at polymer-electrode interfaces: The effect of energetic disorder and thermal injection on band bending and open-circuit voltage. J. Appl. Phys. 106, 034507 (2009).

  19. 19.

    Oehzelt, M., Koch, N. & Heimel, G. Organic semiconductor density of states controls the energy level alignment at electrode interfaces. Nat. Comm. 5, 4174 (2014).

  20. 20.

    Oehzelt, M., Akaike, K., Koch, N. & Heimel, G. Energy-level alignment at organic heterointerfaces. Sci. Adv. 1, e1501127 (2015).

  21. 21.

    Baldo, M. A. & Forrest, S. R. Interface-limited injection in amorphous organic semiconductors. Phys. Rev. B 64, 085201 (2001).

  22. 22.

    Limketkai, B. N. & Baldo, M. A. Charge injection into cathode-doped amorphous organic semiconductors. Phys. Rev. B 71, 085207 (2005).

  23. 23.

    Seino, Y., Inomata, S., Sasabe, H., Pu, Y.-J. & Kido, J. High-performance green OLEDs using thermally activated delayed fluorescence with a power efficiency of over 100 lm W−1. Adv. Mater. 28, 2638–2643 (2016).

  24. 24.

    Hung, W.-Y. et al. Employing ambipolar oligofluorene as the charge-generation layer in time-of-flight mobility measurements of organic thin films. Appl. Phys. Lett. 88, 064102 (2006).

  25. 25.

    Tse, S. C., Kwok, K. C. & So, S. K. Electron transport in naphthylamine-based organic compounds. Appl. Phys. Lett. 89, 262102 (2006).

  26. 26.

    Bach, U., De Cloedt, K., Spreitzer, H. & Grätzel, M. Characterization of hole transport in a new class of spiro-linked oligotriphenylamine compounds. Adv. Mater. 12, 1060–1063 (2000).

  27. 27.

    Noh, S., Suman, C. K., Hong, Y. & Lee, C. Carrier conduction mechanism for phosphorescent material doped organic semiconductor. J. Appl. Phys. 105, 033709 (2009).

  28. 28.

    Matsusue, N., Suzuki, Y. & Naito, H. Charge carrier transport in neat thin films of phosphorescent iridium complexes. Jpn. J. Appl. Phys. 44, 3691–3694 (2005).

  29. 29.

    Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).

  30. 30.

    Ruehle, V. et al. Microscopic simulations of charge transport in disordered organic semiconductors. J. Chem. Theory Comput. 7, 3335–3345 (2011).

  31. 31.

    Poelking, C. & Andrienko, D. Long-range embedding of molecular ions and excitations in a polarizable molecular environment. J. Chem. Theory Comput. 12, 4516–4523 (2016).

Download references

Acknowledgements

The authors thank C. Bauer, F. Keller, and H.-J. Guttmann for technical support. This project has received funding from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 646176 (EXTMOS). D.A. thanks the BMBF grant InterPhase (FKZ 13N13661) and the European Union Horizon 2020 research and innovation programme ‘Widening materials models’ under Grant Agreement No. 646259 (MOSTOPHOS).

Author information

Affiliations

  1. Max Planck Institute for Polymer Research, Mainz, Germany

    • Naresh B. Kotadiya
    • , Hao Lu
    • , Anirban Mondal
    • , Yutaka Ie
    • , Denis Andrienko
    • , Paul W. M. Blom
    •  & Gert-Jan A. H. Wetzelaer
  2. The Institute of Scientific and Industrial Research (ISIR), Osaka University, Ibaraki, Japan

    • Yutaka Ie

Authors

  1. Search for Naresh B. Kotadiya in:

  2. Search for Hao Lu in:

  3. Search for Anirban Mondal in:

  4. Search for Yutaka Ie in:

  5. Search for Denis Andrienko in:

  6. Search for Paul W. M. Blom in:

  7. Search for Gert-Jan A. H. Wetzelaer in:

Contributions

G.A.H.W. proposed and supervised the project. N.B.K. carried out sample preparation and electrical measurements. H.L. performed the UPS measurements. D.A. and A.M. performed molecular-dynamics and energy-alignment simulations. Y.I. synthesized and purified 4CzIPN. G.A.H.W., N.B.K. and P.W.M.B. analysed the experimental data. G.A.H.W. wrote the manuscript with input from D.A. and P.W.M.B.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Gert-Jan A. H. Wetzelaer.

Supplementary information

  1. Supplementary Information

    Molecular structures of materials, Experimental techniques, Supporting experimental results, Molecular dynamics simulations, Density-of-states distributions, energy-level alignment calculations.

About this article

Publication history

Received

Accepted

Published

DOI

https://doi.org/10.1038/s41563-018-0022-8

Further reading