Modeling analysis for OLED degradation mechanism using impedance spectroscopy

Abstract

Organic light emitting diodes(OLED) was first discovered by W. Helfrich, M. Pole and et al. in 1963 and became gradually known by C.W Tang team at Eastman-Kodak and made green light emitting OLED that had thin film structures. Soon, OLED started to apply to display market as OLED can be fabricated in ultra-thin film structure and operated under low driving voltage. But they have a critical weak point that is chemical compound. Because chemical compound devices are generally weak at moisture, oxidation and heat, the fabrication is carefully progressed to get rid of these and after OLED products are fabricated, encapsulation progress is absolutely necessary. Other weak point is OLED degradation that appeals after electric power supply. These drawbacks shorten the display life time as well as cause what is called a blur that drops uniformity of display. These drawbacks are fatal fault when OLED competes with other display. Thus effort to slow down the degradation process is gradual issue in OLED study. This thesis, we noticed that the voltage supply shape can effect to OLED degradation and measured. The result is OLED is more stable when alternated voltage is supplied than when direct voltage is supplied. This result shows that the role of accumulated charge movement in space charge must be noticed in OLED degradation modeling. That is, recombination and luminescence occur in interface of HTL and EML thus the accumulation of charge in this interface cannot avoid causing bottleneck phenomenon and the fixed charge as cation influence OLED degradation. Therefore frequency power supply method can recover the OLEL life time. And as additional study, using impedance spectroscopy OLED modeling is tried. This modeling is added the inductor to notice the negative capacitance that occur in low frequency while the typical OLED model is composed parallel circuit of capacitance and resistance. By this model, each parameter value can be extracted before and after OLED degrades and figure each parameter changes and these result applied to OLED degradation model. As a result, the inductance component is increased and the internal capacitance component is slightly decreased. In addition, the overall resistance is increased. The increased inductance component can be monitored by the increased value of negative capacitance and this may mean that internal localized electric field that arises by the accumulated charge inside OLED is increase, so that this field impedes injection of the new carrier. In addition, the decreased capacitance means can be analyzed that the effective EML layer thickness is shrink. Thus the degradation phenomenon affects spatial variations of the internal interface. In this study, it revealed that the life time of the OLED is changed by frequency driving method and I developed the mathematical program tool that is able to implement physical meaning and to extract the accurate parameters of equivalent OLED circuit comparatively. As a result, degradation mechanism can be explained by each extracted impedance parameters using impedance spectroscopy. This study is relevant not only to present comparatively simple way that OLED performance can be evaluated but also to support the physical and chemical analysis through OLED electrical analysis in further enlarged, complicated and mass produced OLED industry.