Heat evolution and dissipation in organic light-emitting diodes on flexible polymer substrates

We investigated the effect of the heat generated in organic light-emitting diodes (OLEDs) on their performance with a focus on the thermo-physical properties of the substrates. OLEDs were fabricated on polyethylene terephthalate (PET), polyimide (PI), and glass substrates, and the instantaneous and time-resolved performances of the OLEDs were compared. The device operation temperature (T) was predicted via heat transfer analysis using the finite element method (FEM). The value of T during operation was experimentally measured using an infrared (IR) camera, and the results were compared with the numerically calculated values. The effects of T on the time-resolved electroluminescence (EL) spectra of the OLEDs on the different substrates were also investigated. The experimental results of this study agreed well with the numerical predictions that the T of the OLEDs on the polymers are higher than the T of the OLEDs on glass during operation due to the relatively low thermal diffusivity (α) of the polymer substrates used in this study. The results also showed that the performance and reliability of the flexible OLEDs (f-OLED) are highly dependent on the heat extraction capabilities of the substrates; thus, the current density (J), luminescence (L), voltage (V) characteristics, and efficiencies (η) over time of the OLEDs on PET and PI are inferior to the OLEDs on the glass substrate.