The thermal effect of PLEDs by Raman spectra

Raman spectra and infrared imaging systems are used for the study of internal temperatures of PLEDs. The aim is to investigate the thermal degradation of PLEDs with different current densities. Raman intensity is proportional to the number of molecules in the next higher vibration energy level, and accurate internal temperature of PLEDs at thermal equilibrium can be calculated with the ratio of anti-stokes to stokes Raman density by Boltzmann equation. With the current density of PLED going from 0 mA/cm(2) to 169 mA/cm(2), it is found that the internal temperature of PLED increases accordingly. When the temperature comes to the glass transition temperature (T-g) of the emission layer, there is a phase change in it and the layer becomes free state as liquid, which is not stable. Local disfigurement in the emission layer results in short circuit between the cathode and the anode of a PLED, and the luminescence of PLED fails. Therefore, Raman spectrum is considered as a good method for detecting temperatures of thin-film semiconductor devices.