Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This research was funded by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile project “Organische p-i-n Bauelemente 2.2,” as well as competitive funding from the King Abdullah University of Science and Technology. In addition, this work received funding from the European Union Seventh Framework Programme under the grant agreement number 607232 (THINFACE), from the Austrian Science Fund (FWF), grant I2081-N20, and finally from the German Research Foundation (DFG) through the project MatWorldNet LE-747/44-1. We thank Professor Björn Lüssem and Dr Christian Körner for fruitful discussions. K.L. thanks the Canadian Institute for Advanced Research (CIFAR) for support.