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

Abstract

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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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.

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Acknowledgements

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.

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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.

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Correspondence to Frank Ortmann.

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

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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). https://doi.org/10.1038/s41563-018-0030-8

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