Molecular parameters responsible for thermally activated transport in doped organic semiconductors

Abstract

Doped organic semiconductors typically exhibit a thermal activation of their electrical conductivity, whose physical origin is still under scientific debate. In this study, we disclose relationships between molecular parameters and the thermal activation energy (EA) of the conductivity, revealing that charge transport is controlled by the properties of host–dopant integer charge transfer complexes (ICTCs) in efficiently doped organic semiconductors. At low doping concentrations, charge transport is limited by the Coulomb binding energy of ICTCs, which can be minimized by systematic modification of the charge distribution on the individual ions. The investigation of a wide variety of material systems reveals that static energetic disorder induced by ICTC dipole moments sets a general lower limit for EA at large doping concentrations. The impact of disorder can be reduced by adjusting the ICTC density and the intramolecular relaxation energy of host ions, allowing an increase of conductivity by many orders of magnitude.

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Fig. 1: Chemical structures of host and dopant molecules.
Fig. 2: Temperature-activated conductivity in doped organic semiconductors.
Fig. 3: ICTCs limit EA.
Fig. 4: Impact of charge distribution on the Coulomb binding energy of ICTCs (Ecoul,ICTC).
Fig. 5: Static energetic disorder limits EA at 10 mol%.

Data availability

All the data supporting the findings of this study are available within the article, its Supplementary Information files, or from the corresponding authors upon reasonable request.

References

  1. 1.

    Bässler, H. Charge transport in disordered organic photoconductors – A Monte Carlo simulation study. Phys. Status Solidi B 175, 15–56 (1993).

    Article  Google Scholar 

  2. 2.

    Coropceanu, V. et al. Charge transport in organic semiconductors. Chem. Rev. 107, 926–952 (2007).

    CAS  Article  Google Scholar 

  3. 3.

    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  Article  Google Scholar 

  4. 4.

    Reineke, S. et al. White organic light-emitting diodes with fluorescent tube efficiency. Nature 459, 234–238 (2009).

    CAS  Article  Google Scholar 

  5. 5.

    Olthof, S., Tress, W., Meerheim, R., Lüssem, B. & Leo, K. Photoelectron spectroscopy study of systematically varied doping concentrations in an organic semiconductor layer using a molecular p-dopant. J. Appl. Phys. 106, 103711 (2009).

    Article  Google Scholar 

  6. 6.

    Mayer, T. et al. Fermi level positioning in organic semiconductor phase mixed composites: The internal interface charge transfer doping model. Org. Electron. 13, 1356–1364 (2012).

    CAS  Article  Google Scholar 

  7. 7.

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

    CAS  Article  Google Scholar 

  8. 8.

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

  9. 9.

    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  Article  Google Scholar 

  10. 10.

    Pingel, P. & Neher, D. Comprehensive picture of p-type doping of P3HT with the molecular acceptor F4TCNQ. Phys. Rev. B 87, 115209 (2013).

    Article  Google Scholar 

  11. 11.

    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).

    Article  Google Scholar 

  12. 12.

    Tietze, M. L. et al. Elementary steps in electrical doping of organic semiconductors. Nat. Commun. 9, 1182 (2018).

    Article  Google Scholar 

  13. 13.

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

    CAS  Article  Google Scholar 

  14. 14.

    Pahner, P. et al. Pentacene Schottky diodes studied by impedance spectroscopy: Doping properties and trap response. Phys. Rev. B 88, 195205 (2013).

    Article  Google Scholar 

  15. 15.

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

    Article  Google Scholar 

  16. 16.

    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  Article  Google Scholar 

  17. 17.

    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).

    Article  Google Scholar 

  18. 18.

    Arkhipov, V., Emelianova, E., Heremans, P. & Bässler, H. Analytic model of carrier mobility in doped disordered organic semiconductors. Phys. Rev. B 72, 235202 (2005).

    Article  Google Scholar 

  19. 19.

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

    CAS  Article  Google Scholar 

  20. 20.

    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).

    Article  Google Scholar 

  21. 21.

    Shen, Y. et al. Charge transport in doped organic semiconductors. Phys. Rev. B 68, 081204 (2003).

    Article  Google Scholar 

  22. 22.

    Menke, T., Ray, D., 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).

    Article  Google Scholar 

  23. 23.

    Schmechel, R. Hopping transport in doped organic semiconductors: A theoretical approach and its application to p-doped zinc-phthalocyanine. J. Appl. Phys. 93, 4653 (2003).

    CAS  Article  Google Scholar 

  24. 24.

    Li, L., Meller, G. & Kosina, H. Analytical conductivity model for doped organic semiconductors. J. Appl. Phys. 101, 033716 (2007).

    Article  Google Scholar 

  25. 25.

    Menke, T. et al. Highly efficient p-dopants in amorphous hosts. Org. Electron. 15, 365–371 (2014).

    CAS  Article  Google Scholar 

  26. 26.

    Pfeiffer, M., Beyer, A., Fritz, T. & Leo, K. Controlled doping of phthalocyanine layers by cosublimation with acceptor molecules: A systematic Seebeck and conductivity study. Appl. Phys. Lett. 73, 3202 (1998).

    CAS  Article  Google Scholar 

  27. 27.

    Menke, T. Molecular Doping of Organic Semiconductors – A Conductivity and Seebeck Study (TU Dresden, Dresden, 2013).

  28. 28.

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

    CAS  Article  Google Scholar 

  29. 29.

    Ueno, N. Tuning organic band structures with Coulomb interactions. Science 352, 1395–1396 (2016).

    CAS  Article  Google Scholar 

  30. 30.

    Schwarze, M. et al. Analyzing the n-doping mechanism of an air-stable small-molecule precursor. ACS Appl. Mater. Interfaces 10, 1340–1346 (2018).

    CAS  Article  Google Scholar 

  31. 31.

    Lin, X. et al. Impact of a low concentration of dopants on the distribution of gap states in a molecular semiconductor. Chem. Mater. 28, 2677–2684 (2016).

    CAS  Article  Google Scholar 

  32. 32.

    Marcus, R. A. Electron transfer reactions in chemistry. Theory and experiment. Rev. Mod. Phys. 65, 599–610 (1993).

    CAS  Article  Google Scholar 

  33. 33.

    Fishchuk, I. I. et al. Temperature dependence of the charge carrier mobility in disordered organic semiconductors at large carrier concentrations. Phys. Rev. B 81, 45202 (2010).

    Article  Google Scholar 

  34. 34.

    Fishchuk, I. I. et al. Origin of Meyer–Neldel type compensation behavior in organic semiconductors at large carrier concentrations: Disorder versus thermodynamic description. Phys. Rev. B 90, 245201 (2014).

    Article  Google Scholar 

  35. 35.

    Vandewal, K. et al. Absorption tails of donor:C60 blends provide insight into thermally activated charge-transfer processes and polaron relaxation. J. Am. Chem. Soc. 139, 1699–1704 (2017).

    CAS  Article  Google Scholar 

  36. 36.

    Massé, A. et al. Effects of energy correlations and superexchange on charge transport and exciton formation in amorphous molecular semiconductors: An ab initio study. Phys. Rev. B 95, 115204 (2017).

    Article  Google Scholar 

  37. 37.

    Zhang, F. & Kahn, A. Investigation of the high electron affinity molecular dopant F6-TCNNQ for hole-transport materials. Adv. Funct. Mater. 28, 1703780 (2017).

    Article  Google Scholar 

  38. 38.

    Nell, B., Ortstein, K., Boltalina, O. V. & Vandewal, K. Influence of dopant–host energy level offset on thermoelectric properties of doped organic semiconductors. J. Phys. Chem. C 122, 11730–11735 (2018).

    CAS  Article  Google Scholar 

  39. 39.

    Pollak, M. A theory for many-electron hopping rates. J. Phys. C Solid State Phys. 14, 2977–2993 (1981).

    CAS  Article  Google Scholar 

  40. 40.

    Scholz, R. et al. Quantifying charge transfer energies at donor–acceptor interfaces in small-molecule solar cells with constrained DFTB and spectroscopic methods. J. Phys. Condens. Matter. 25, 473201 (2013).

    Article  Google Scholar 

  41. 41.

    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).

    Article  Google Scholar 

  42. 42.

    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).

    Article  Google Scholar 

  43. 43.

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

    Article  Google Scholar 

  44. 44.

    Ueno, N. in Physics of Organic Semiconductors (eds. Brütting, W. & Adachi, C.) 65–89 (Wiley-VCH, Weinheim, 2013); https://doi.org/10.1002/9783527654949.ch3

  45. 45.

    Yang, J.-P., Bussolotti, F., Kera, S. & Ueno, N. Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals. J. Phys. D Appl. Phys. 50, 423002 (2017).

    Article  Google Scholar 

  46. 46.

    Zhao, Y. & Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other function. Theor. Chem. Acc. 120, 215–241 (2008).

    CAS  Article  Google Scholar 

  47. 47.

    Dunning, T. H. Jr Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 90, 1007 (1989).

    CAS  Article  Google Scholar 

  48. 48.

    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  Article  Google Scholar 

  49. 49.

    Frisch, M. J. et al. Gaussian 09 (Revision A.02) (Gaussian Inc, 2016).

  50. 50.

    Schünemann, C. et al. Zinc phthalocyanine - Influence of substrate temperature, film thickness, and kind of substrate on the morphology. Thin Solid Films 519, 3939–3945 (2011).

    Article  Google Scholar 

  51. 51.

    Brendel, M. et al. The effect of gradual fluorination on the properties of FnZnPc thin films and FnZnPc/C60 bilayer photovoltaic cells. Adv. Funct. Mater. 25, 1565–1573 (2015).

    CAS  Article  Google Scholar 

  52. 52.

    Efros, A. L., Van Lien, N. & Shklovskii, B. I. Impurity band structure in lightly doped semiconductors. J. Phys. C Solid State Phys. 12, 1869–1881 (1979).

    CAS  Article  Google Scholar 

  53. 53.

    Baranovskii, S. D., Hensel, F., Ruckes, K., Thomas, P. & Adriaenssens, G. J. Potential fluctuations in amorphous silicon. J. Non. Cryst. Solids 190, 117–122 (1995).

    CAS  Article  Google Scholar 

  54. 54.

    Shimakawa, K. & Ganjoo, A. ac photoconductivity of hydrogenated amorphous silicon: Influence of long-range potential fluctuations. Phys. Rev. B 65, 165213 (2002).

    Article  Google Scholar 

  55. 55.

    Efros, A. L. et al. Coulomb gap and low temperature conductivity of disordered systems. J. Phys. C Solid State Phys. 8, L49–L51 (1975).

    CAS  Article  Google Scholar 

  56. 56.

    Baranovskii, S. D., Efros, A. L., Gelmont, B. L. & Shklovskii, B. I. Coulomb gap in disordered systems: Computer simulation. Solid State Commun. 27, 1–3 (1978).

    CAS  Article  Google Scholar 

  57. 57.

    Crispin, A. et al. Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors. J. Chem. Phys. 116, 8159–8167 (2002).

    CAS  Article  Google Scholar 

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Acknowledgements

We thank O. Kaveh and D. Schütze for performing conductivity measurements, D. Wöhrle for supplying F8ZnPc, and M. L. Tietze for insightful discussions. M.S. acknowledges financial support by the German Research Foundation (DFG) through the project MatWorldNet LE-747/44-1, the German Academic Exchange Service within the frame of the IPID4all Program and the Graduate Academy of TU Dresden. A.H. acknowledges financial support from the project UNVEiL of the German Federal Ministry of Education and Research (BMBF). S.K. thanks JSPS for financial support (KAKENHI 26248062). N.U. acknowledges support of the Global-COE Program of MEXT (G03) and 21st Century-COE Program of MEXT(G-4) for developing an ultrahigh-sensitivity UPS system. B.N. received funding from the European Union Seventh Framework Programme under grant agreement no. 607232 (THINFACE). F.O. would like to thank the DFG for financial support (project OR-349/1). Grants for computing time from the Zentrum für Informationsdienste und Hochleistungsrechnen Dresden (ZIH) are gratefully acknowledged.

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Contributions

M.S. designed the study and acquired the UPS data in Dresden. C.G., R.S., K.S.S. and F.O. performed DFT simulations. A.H. performed transport simulations. B.N. performed part of the conductivity measurements. F.B. acquired UPS and LEIPS data in Okazaki. B.D.N. and Z.B. provided the highly efficient dopant (2-Cyc-DMBI)2. K.L., F.O., S.K., N.U., J.W. and K.V. supervised different parts of the study. D.S. contributed valuably to the physical understanding of charge transport. M.S. and F.O. wrote the manuscript and all authors contributed to discussions and finalizing the manuscript.

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Correspondence to Martin Schwarze or Frank Ortmann or Karl Leo.

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Supplementary Information

Supplementary Materials and Methods, Supplementary Tables 1–3, Supplementary Figures 1–15, Supplementary References 1–22

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Schwarze, M., Gaul, C., Scholz, R. et al. Molecular parameters responsible for thermally activated transport in doped organic semiconductors. Nature Mater 18, 242–248 (2019). https://doi.org/10.1038/s41563-018-0277-0

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