If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq 3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N ,N-Di(1-naphthyl)-N ,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB) as nonpolar host and Alq 3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.
Microcrystalline strontium citrate monohydrate Sr3(C6H5O7)2·H2O was prepared by a microwave‐assisted hydrothermal synthesis. Single‐crystal X‐ray structure determination (P1, a = 10.0572(4), b = 10.1506(5), c = 10.8531(5) Å, α = 89.642(2)°, β = 67.156(2)°, γ = 62.367(2)°, 3040 independent reflections, 353 refined parameters, wR2 = 0.066) revealed three different Sr sites coordinated eight‐, nine and ten‐fold by two crystallographically different citrate molecules with one comprising oriental disorder, but not by the crystal water molecule. These findings are supported by energy dispersive X‐ray spectroscopy, powder XRD, infrared spectroscopy and thermal analysis. Further, the latter three methods are applied to the hitherto only strontium citrate hydrate with known crystal structure Sr3(C6H5O7)2·5H2O and both compounds are compared, especially with respect to their possible application as precursors.
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