Studies on the structure of the interface between a metal and an organic thin film and voltage-induced processes by using infrared reflection-absorption spectroscopy

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We have developed an infrared reflection-absorption spectroscopic method with a 45 degree incident angle in order to study organic light-emitting diodes and organic metal-insulator-semiconductor diodes. These devices have multilayer structures consisting of a metal electrode, an indium-tin-oxide (ITO) electrode, and organic compounds. In the infrared reflection-absorption spectra of organic light-emitting diodes, infrared bands are strongly observed with the p-polarization and weakly observed with the s-polarization. These results have been explained as follows. For the p-polarization, the intensity of the standing wave produced by the interference of the incident and reflected beams is strong in the direction of the surface normal at the metal surface. On the other hand, for the s-polarization, the resulting standing wave is extremely weak at the metal surface. The orientations of polymer chains have been determined by measuring the reflection-absorption and transmission-absorption spectra of polymer thin films. The voltage-induced infrared absorption spectra of polymer light-emitting diodes and polymer metal-insulator-semiconductor diodes have been measured by using the FT-IR difference spectrum method. The observed spectra have been attributed to injected carriers (polarons). The injected carrier balance of polymer light-emitting diodes have been discussed on the basis of the observed results. It has been demonstrated that infrared reflection-absorption spectroscopy is a powerful tool for studying the structures and orientation of organic compounds and injected carriers in organic electronic devices.