Passivation of Molecular n-Doping: Exploring the Limits of Air Stability

Molecular doping is a key technique for flexible and low-cost organic complementary semiconductor technologies that requires both efficient and stable p- and n-type doping. However, in contrast to molecular p-dopants, highly efficient n-type dopants are commonly sensitive to rapid degradation in air due to their low ionization energies (IEs) required for electron donation, e.g., IE = 2.4 eV for tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W₂(hpp)₄). Here, the air stability of various host:W₂(hpp)₄ combinations is compared by conductivity measurements and photoemission spectroscopy. A partial passivation of the n-doping against degradation is found, with this effect identified to depend on the specific energy levels of the host material. Since host-W₂(hpp)₄ electronic wavefunction hybridization is unlikely due to confinement of the dopant highest occupied molecular orbital (HOMO) to its molecular center, this finding is explained via stabilization of the dopant by single-electron transfer to a host material whose energy levels are sufficiently low for avoiding further charge transfer to oxygen–water complexes. Our results show the feasibility of temporarily handling n-doped organic thin films in air, e.g., during structuring of organic field effect transistors (OFETs) by lithography.