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A window to trap-free charge transport in organic semiconducting thin films

Abstract

Organic semiconductors, which serve as the active component in devices, such as solar cells, light-emitting diodes and field-effect transistors1, often exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. Here, we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm, leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV, hole trapping will limit the hole transport, whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window, trap-free bipolar charge transport occurs. Based on simulations, water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However, for blue-emitting light-emitting diodes, which require an energy gap of 3 eV, removing or disabling charge traps will remain a challenge.

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Fig. 1: Energy window for trap-free charge transport.
Fig. 2: Hole transport in C60, PCBM and ICBA.
Fig. 3: Close-up view of a water-filled pocket in a simulated C60 morphology.
Fig. 4: Calculated densities of states of water clusters in four organic films.

Data availability

Experimental data are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors thank C. Bauer, M. Beuchel, Hs-J. Guttmann, F. Keller and V. Maus for technical support and Y. Ie for the synthesis of 4CzIPN. This project has received funding from the European Union Horizon 2020 research and innovation programme under grant agreement no. 646176 (EXTMOS) and no. 646259 (MOSTOPHOS). D.A. thanks the BMBF grant InterPhase (grant no. FKZ 13N13661).

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Contributions

G.A.H.W. proposed the project. N.B.K. carried out sample preparation and electrical measurements. G.A.H.W. and N.B.K. analysed the experimental data. A.M. and D.A. devised and performed molecular-dynamics simulations. G.A.H.W., P.W.M.B. and D.A. supervised the project and wrote the manuscript, with input from N.B.K. and A.M.

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Correspondence to Gert-Jan A. H. Wetzelaer.

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Supplementary materials, Supplementary Figs. 1–15, Supplementary Refs. 1–29

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Kotadiya, N.B., Mondal, A., Blom, P.W.M. et al. A window to trap-free charge transport in organic semiconducting thin films. Nat. Mater. 18, 1182–1186 (2019). https://doi.org/10.1038/s41563-019-0473-6

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