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Self‐passivation of molecular n‐type doping during air exposure using a highly efficient air‐instable dopant
Author(s) -
Tietze Max L.,
Wölzl Florian,
Menke Torben,
Fischer Axel,
Riede Moritz,
Leo Karl,
Lüssem Björn
Publication year - 2013
Publication title -
physica status solidi (a)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.532
H-Index - 104
eISSN - 1862-6319
pISSN - 1862-6300
DOI - 10.1002/pssa.201330049
Subject(s) - dopant , doping , passivation , homo/lumo , materials science , fermi level , x ray photoelectron spectroscopy , ionization energy , thin film , ultraviolet photoelectron spectroscopy , annealing (glass) , analytical chemistry (journal) , optoelectronics , ionization , chemical physics , nanotechnology , chemistry , chemical engineering , molecule , organic chemistry , ion , physics , layer (electronics) , engineering , electron , composite material , quantum mechanics
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) ( W 2( hpp ) 4 ) and ofC 60layers doped byW 2( hpp ) 4 . We find that 1/3 of the initial conductivity of the doped C 60 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 C 60 remains after air exposure, clearly indicating a conservation of n‐doping. We explain these findings by a down‐shift of the W 2 (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.