Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), have been grown by organic molecular beam deposition. The individual layer thicknesses in the multilayer samples were varied from 10 to 200 Å. X-ray diffraction and birefringence data show that there is a strong structural ordering in all layers, as well as across large spatial distances along the sample surface. Thus, the growth is ‘‘quasi-epitaxial’’ even though the PTCDA and NTCDA crystal structures are incommensurate. From the optical absorption spectra, it was found that the lowest energy PTCDA singlet exciton line shifts to higher energy with decreasing layer thickness. Comparison of these results with a quantum mechanical model based on exciton confinement in the PTCDA layers is proposed to describe the energy shift.
We have studied the electrical and optical characteristics of isotype p-P organic heterojunctions (HJ) consisting of CuPc (copper phthalocyanine) and PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride). It is found that the charge-transport properties of the heterojunction are limited by thermionic emission of holes over the energy barrier at the CuPc/PTCDA heterojunction at low forward and reverse bias, and by series resistance at high voltage. The heterojunction energy barrier at the CuPc/PTCDA valence-band edge was measured using both current-voltage and capacitance-voltage analysis and was found to be ΔEvC,P=0.48±0.05 eV. Similar measurements made for HJs consisting of CuPc and PTCDA in combination with another perylene-based material, 3, 4, 9, 10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), suggest that the band offsets for these three materials follow a transitive relationship. That is, ΔEvC,P=ΔEvC,B−ΔEvB,P, where subscripts C, P, and B refer to CuPc, PTCDA, and PTCBI, respectively. The results are discussed in terms of energy-band and molecular energy-level models.
Organic-on-inorganic semiconductor heterojunctions (OI-HJs) exhibit rectification whereby the current-voltage characteristics are limited by the properties of the inorganic semiconductor substrate and the magnitude of the energy barrier at the heterointerface. In this paper we calculate the potential distribution and the quasi-Fermi level energy (or imref) across the OI diode bulk. Both ohmic as well as space-charge-limited conduction regimes of the organic thin film are considered. Previous work considered the OI-HJ to be similar to a Schottky, metal-semiconductor contact. While this can give a good approximation to OI-HJ transport processes under some bias regimes, it results in a misleading picture of the position of the imrefs under reverse bias, as well as errors in measurements of the band discontinuity energy at the OI-HJ. Unlike Schottky contacts, the imref in the OI diode is flat throughout the substrate under both low forward and reverse biases. These results are used to calculate carrier velocities within the organic film. The hole velocity is in the range of 100–2000 cm/s under reverse bias and is as high as 105 cm/s under forward bias. Experimental measurements of the energy-band discontinuities are presented that are in agreement with the predictions of the current-voltage model.
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