Optical and electrical properties of isotype crystalline molecular organic heterojunctions

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.