We have measured the temperatures of the organic layers in operating organic light-emitting diodes (OLEDs) by Raman spectroscopy. The wavenumbers of the Raman bands due to N, N′-di-naphthaley1-1, 1′-bipheny1-4, 4′-diamine (NPD) and copper phthalocyanine (CuPc) have been measured as a function of temperature in the range of 25-191 °C. The observed positions of strong bands around 1607 cm-1 (NPD) and 1531 cm-1 (CuPc) shifted downward linearly with increasing temperature in the ranges lower than 92 and 191 °C, respectively. We have determined the temperatures of the NPD and CuPc layers in an operating OLED from the wavenumber-temperature relations of these bands.
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