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


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Materials, structures and electronic parameters.
Fig. 2: Bulk DOS and Fermi level of the C60:2-Cyc-DMBI system.
Fig. 3: Surface DOS and photoelectron spectra of 2-Cyc-DMBI-doped C60.
Fig. 4: Effect of doping on DOS and carrier density.
Fig. 5: Conductivity and doping.


  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  5. 5.

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

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  8. 8.

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

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  17. 17.

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

    Google Scholar 

  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).

    CAS  Google Scholar 

  19. 19.

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

    CAS  Google Scholar 

  20. 20.

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

    CAS  Google Scholar 

  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).

    Google Scholar 

  22. 22.

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

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    Google Scholar 

  25. 25.

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

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  28. 28.

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

    Google Scholar 

  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  32. 32.

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

    Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    Google Scholar 

  38. 38.

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

    CAS  Google Scholar 

  39. 39.

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

    CAS  Google Scholar 

  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).

    Google Scholar 

  41. 41.

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

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  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).

    Google Scholar 

  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).

    CAS  Google Scholar 

  47. 47.

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

    CAS  Google Scholar 

Download references


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




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.

Corresponding author

Correspondence to Frank Ortmann.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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

Supplementary information

Supplementary Information

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

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Gaul, C., Hutsch, S., Schwarze, M. et al. Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc. Nature Mater 17, 439–444 (2018).

Download citation

Further reading