Self-passivation of molecular n-type doping during air exposure using a highly efficient air-instable dopant

Journal article

In contrast to p-dopants, highly efficient molecular n-dopants are prone to degradation in air due to their low ionization potentials, limiting the processing conditions of doped functional organic devices. In this contribution, we investigate the air-stability of pure films of the n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W₂(hpp)₄) and of C₆₀ layers doped by W₂(hpp)₄. We find that 1/3 of the initial conductivity of the doped C₆₀ thin films can be restored by thermal annealing in vacuum after a drop by 5 orders of magnitude upon air exposure. Furthermore, we show by ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements that the Fermi level shift toward the lowest unoccupied molecular orbital (LUMO) of C₆₀ remains after air exposure, clearly indicating a conservation of n-doping. We explain these findings by a down-shift of the W₂(hpp)₄ energy levels upon charge-transfer to a host material with deeper lying energy-levels, facilitating a protection against oxidation in air. Consequently, the observed recovery of the conductivity can be understood in terms of a self-passivation of the molecular n-doping. Hence, an application of highly efficient n-doped thin films in functional organic devices handled even under ambient conditions during fabrication is feasible. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Authors: Tietze, M; Wölzl, F; Menke, T; Fischer, A; Riede, M

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Physica Status Solidi (A) Applications and Materials Science
• Volume: 210
• Issue: 10
• Pages: 2188–2198
• Publication date: 2013-01-01
• DOI: 10.1002/pssa.201330049
• EISSN: 1862-6319
• ISSN: 1862-6300
• Language: English
• Keywords: degradation; organic semiconductors; molecular n-doping; C60