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Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc

Nature Materialsvolume 17pages439444 (2018) | Download Citation


Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ1 between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ1 may provide alternative strategies to optimize the electronic properties of organic semiconductors.

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  1. 1.

    Maennig, B. et al. Controlled p-type doping of polycrystalline and amorphous organic layers: self-consistent description of conductivity and field-effect mobility by a microscopic percolation model. Phys. Rev. B 64, 195208 (2001).

  2. 2.

    Blochwitz, J., Pfeiffer, M., Fritz, T. & Leo, K. Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material. Appl. Phys. Lett. 73, 729 (1998).

  3. 3.

    Yamamori, A., Adachi, C., Koyama, T. & Taniguchi, Y. Doped organic light emitting diodes having a 650-nm-thick hole transport layer. Appl. Phys. Lett. 72, 2147 (1998).

  4. 4.

    Walzer, K., Maennig, B., Pfeiffer, M. & Leo, K. Highly efficient organic devices based on electrically doped transport layers. Chem. Rev. 107, 1233–1271 (2007).

  5. 5.

    Lüssem, B., Riede, M. & Leo, K. Doping of organic semiconductors. Phys. Stat. Sol. A 210, 9–43 (2013).

  6. 6.

    Salzmann, I., Heimel, G., Oehzelt, M., Winkler, S. & Koch, N. Molecular electrical doping of organic semiconductors: fundamental mechanisms and emerging dopant design rules. Acc. Chem. Res. 49, 370–378 (2016).

  7. 7.

    Rossbauer, S., Müller, C. & Anthopoulos, T. D. Comparative study of the n-type doping efficiency in solution-processed fullerenes and fullerene derivatives. Adv. Funct. Mater. 24, 7116–7124 (2014).

  8. 8.

    Salzmann, I. et al. Intermolecular hybridization governs molecular electrical doping. Phys. Rev. Lett. 108, 035502 (2012).

  9. 9.

    Gao, W. & Kahn, A. Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: a direct and inverse photoemission study. Appl. Phys. Lett. 79, 4040–4042 (2001).

  10. 10.

    Lögdlund, M., Lazzaroni, R., Stafström, S., Salaneck, W. R. & Brédas, J.-L. Direct observation of charge-induced π-electronic structural changes in a conjugated polymer. Phys. Rev. Lett. 63, 1841–1844 (1989).

  11. 11.

    Lee, B. H., Bazan, G. C. & Heeger, A. J. Doping-induced carrier density modulation in polymer field-effect transistors. Adv. Mater. 28, 57–62 (2016).

  12. 12.

    Wang, C., Duong, D. T., Vandewal, K., Rivnay, J. & Salleo, A. Optical measurement of doping efficiency in poly(3-hexylthiophene) solutions and thin films. Phys. Rev. B 91, 085205 (2015).

  13. 13.

    Kang, K. et al. 2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion. Nat. Mater. 15, 896–902 (2016).

  14. 14.

    Yang, J.-P. et al. Quantitative Fermi level tuning in amorphous organic semiconductor by molecular doping: toward full understanding of the doping mechanism. Appl. Phys. Lett. 109, 093302 (2016).

  15. 15.

    Lin, X. et al. Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors. Nat. Mater. 16, 1209–1215 (2017).

  16. 16.

    Tietze, M. L., Burtone, L., Riede, M., Lüssem, B. & Leo, K. Fermi level shift and doping efficiency in p-doped small molecule organic semiconductors: a photoelectron spectroscopy and theoretical study. Phys. Rev. B 86, 035320 (2012).

  17. 17.

    Olthof, S. et al. Ultralow doping in organic semiconductors: evidence of trap filling. Phys. Rev. Lett. 109, 176601 (2012).

  18. 18.

    Tietze, M. L., Pahner, P., Schmidt, K., Leo, K. & Lüssem, B. Doped organic semiconductors: trap-filling, impurity saturation, and reserve regimes. Adv. Funct. Mater. 25, 2701–2707 (2015).

  19. 19.

    Mityashin, A. et al. Unraveling the mechanism of molecular doping in organic semiconductors. Adv. Mater. 24, 1535–1539 (2012).

  20. 20.

    Winkler, S. et al. Probing the energy levels in hole-doped molecular semiconductors. Mater. Horiz. 2, 427–433 (2015).

  21. 21.

    Arkhipov, V. I., Heremans, P., Emelianova, E. V. & Bässler, H. Effect of doping on the density-of-states distribution and carrier hopping in disordered organic semiconductors. Phys. Rev. B 71, 045214 (2005).

  22. 22.

    Schwarze, M. et al. Band structure engineering in organic semiconductors. Science 352, 1446–1449 (2016).

  23. 23.

    Sueyoshi, T., Fukagawa, H., Ono, M., Kera, S. & Ueno, N. Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy. Appl. Phys. Lett. 95, 183303 (2009).

  24. 24.

    Bussolotti, F., Kera, S., Kudo, K., Kahn, A. & Ueno, N. Gap states in pentacene thin film induced by inert gas exposure. Phys. Rev. Lett. 110, 267602 (2013).

  25. 25.

    Yoshida, H. Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons. Chem. Phys. Lett. 539–540, 180–185 (2012).

  26. 26.

    Naab, B. D. et al. Effective solution- and vacuum-processed n-doping by dimers of benzimidazoline radicals. Adv. Mater. 26, 4268–4272 (2014).

  27. 27.

    Huang, D.-L., Dau, P. D., Liu, H.-T. & Wang, L.-S. High-resolution photoelectron imaging of cold C60 anions and accurate determination of the electron affinity of C60. J. Chem. Phys. 140, 224315 (2014).

  28. 28.

    D’Avino, G. et al. Electrostatic phenomena in organic semiconductors: fundamentals and implications for photovoltaics. J. Phys. Condens. Matter 28, 433002 (2016).

  29. 29.

    de Vries, J. et al. Single-photon ionization of C60- and C70-fullerene with synchrotron radiation: determination of the ionization potential of C60. Chem. Phys. Lett. 188, 159–162 (1992).

  30. 30.

    Shirley, E. L. & Louie, S. G. Electron excitations in solid C60: energy gap, band dispersions, and effects of orientational disorder. Phys. Rev. Lett. 71, 133–136 (1993).

  31. 31.

    Blase, X., Attaccalite, C. & Olevano, V. First-principles GW calculations for fullerenes, porphyrins, phtalocyanine, and other molecules of interest for organic photovoltaic applications. Phys. Rev. B 83, 115103 (2011).

  32. 32.

    Refaely-Abramson, S. et al. Gap renormalization of molecular crystals from density-functional theory. Phys. Rev. B 88, 081204 (2013).

  33. 33.

    Hebard, A. F., Haddon, R. C., Fleming, R. M. & Kortan, A. R. Deposition and characterization of fullerene films. Appl. Phys. Lett. 59, 2109–2111 (1991).

  34. 34.

    Takahashi, T., Morikawa, T., Katayama-Yoshida, H., Hasegawa, S. & Inokuchi, H. Photoemission and inverse photoemission of alkali-doped C60. J. Phys. Chem. Solids 53, 1699–1705 (1992).

  35. 35.

    Hill, I. G., Kahn, A., Soos, Z. G. & Pascal, R. A. Jr Charge-separation energy in films of π-conjugated organic molecules. Chem. Phys. Lett. 327, 181–188 (2000).

  36. 36.

    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).

  37. 37.

    Ortmann, F., Bechstedt, F. & Hannewald, K. Theory of charge transport in organic crystals: beyond Holstein’s small-polaron model. Phys. Rev. B 79, 235206 (2009).

  38. 38.

    Cotton, F. A. et al. Closed-shell molecules that ionize more readily than cesium. Science 298, 1971–1974 (2002).

  39. 39.

    Guo, S. et al. n-Doping of organic electronic materials using air-stable organometallics. Adv. Mater. 24, 699–703 (2012).

  40. 40.

    Menke, T., Debdutta, R., Meiss, J., Leo, K. & Riede, M. In-situ conductivity and Seebeck measurements of highly efficient n-dopants in fullerene C60. Appl. Phys. Lett. 100, 093304 (2012).

  41. 41.

    Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652 (1993).

  42. 42.

    Lee, C., Yang, W. & Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B. 37, 785–789 (1988).

  43. 43.

    Vosko, S. H., Wilk, L. & Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis. Can. J. Phys. 58, 1200–1211 (1980).

  44. 44.

    Dobbs, K. D. & Hehre, W. J. Molecular-orbital theory of the properties of inorganic and organometallic compounds. 6. Extended basis-sets for 2nd-row transition-metals. J. Comp. Chem. 8, 880–893 (1987).

  45. 45.

    Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parameterization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132, 154104 (2010).

  46. 46.

    Valiev, M. et al. NWChem: a comprehensive and scalable open-source solution for large scale molecular simulations. Comput. Phys. Commun. 181, 1477–1489 (2010).

  47. 47.

    Poelking, C. et al. Impact of mesoscale order on open-circuit voltage in organic solar cells. Nat. Mater. 14, 434–439 (2015).

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We would like to thank the Deutsche Forschungsgemeinschaft for financial support (OR 349/1-1 and MatWorldNet LE-747/44-1). This work was partly supported by the excellence cluster ‘Center for Advancing Electronics Dresden’. Financial support was provided by the German Academic Exchange Service within the IPID4all program and the Graduate Academy of TU Dresden. This work was partly supported by JSPS KAKENHI (2624806). Grants for HPC computer time from the Zentrum für Informationsdienste und Hochleichstungsrechnen of TU Dresden (ZIH), the Partnership for Advanced Computing in Europe (PRACE), and the Supercomputer Center in Garching (SuperMUC) are gratefully acknowledged. We thank B. Naab and Z. Bao from Stanford University for providing the 2-Cyc-DMBI dopant and O. Kaveh and D. Schütze for conductivity measurements.

Author information

Author notes

    • Fabio Bussolotti

    Present address: Institute of Materials Research and Engineering, Agency of Science, Technology and Research (A*STAR), Singapore, Singapore

  1. These authors contributed equally: Christopher Gaul, Sebastian Hutsch, Martin Schwarze and Karl Sebastian Schellhammer.


  1. Center for Advancing Electronics Dresden and Dresden Center for Computational Materials Science, Technische Universität Dresden, Dresden, Germany

    • Christopher Gaul
    • , Sebastian Hutsch
    • , Karl Sebastian Schellhammer
    • , Gianaurelio Cuniberti
    •  & Frank Ortmann
  2. Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, Germany

    • Martin Schwarze
    •  & Karl Leo
  3. Institute for Materials Science and Max Bergmann Center for Biomaterials, Technische Universität Dresden, Dresden, Germany

    • Karl Sebastian Schellhammer
    •  & Gianaurelio Cuniberti
  4. Institute for Molecular Science, Department of Photo-Molecular Science, Myodaiji, Okazaki, Japan

    • Fabio Bussolotti
    •  & Satoshi Kera


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C.G., K.S.S., S.H. and F.O. performed the calculations. M.S. and F.B. carried out the experiments and data analysis. F.O. wrote the paper. F.O., S.K., G.C. and K.L. supervised different parts of the work. All authors commented on the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Frank Ortmann.

Supplementary information

  1. Supplementary Information

    Simulation results; Tables 1–4, Figures 1–7, References 1–30

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