Dependence of dark spot growth on cathode/organic interfacial adhesion in Organic Light Emitting Devices

Abstract

Organic Light Emitting Devices(OLEDs) have several advantages over traditional semiconductor devices such as the possibility of being printable, the potential for low cost fabrication, and the potential ease of integration onto a flexible substrate. However, mass commercialization for OLEDs faces several hurdles including that of ambient stability. Due to issues such as low work function of the cathode in the OLEDs, exposure to oxygen or humidity leads to dark spot formation which detrimentally affects device performance. Protection of OLEDs from ambient conditions places high importance on strict encapsulation techniques. Rigid encapsulation techniques which provide high impermeability to ambient conditions are not easily applicable for flexible OLEDs. Flex-integrable thin film encapsulations do not possess the high integrity of rigid encapsulation structures. If the demand for strict thin film encapsulation could be eased via stronger inherent resistance to dark spot growth, it can be most beneficial for the future of OLEDs, especially in the flexible arena. To ease the need for stringent encapsulation, alternate non-encapsulation related approaches for dark spot suppression are investigated. These include heating the substrate to an elevated temperature during cathode deposition and the use of cathode-organic interfacial layers in the OLED. Observations from microscopy, electrical measurements, surface morphology measurements, and adhesion tests are analyzed. Conclusions on the suppression of dark spot growth are discussed. The role of interfacial adhesion in dark spot suppression is investigated. Ambient stability of OLEDs is shown to be related to stronger interfacial adhesion between the cathode and the organic layers. A few thin-film encapsulation structures for OLEDs using materials like titanium (Ti) and MoO3 are also discussed along with the results. Though dark spot eradication via encapsulation may still prove challenging, dark spot suppression via cathode-organic interfacial adhesion strengthening can be shown to be a promising tentative solution to improve OLED lifetime yield.