Abstract
This paper presents the results of a combined experimental and computational study of the mechanisms of blister formation, and their effects on the degradation of organic light emitting devices (OLEDs). Blister formation is attributed to the effects of thermally induced mismatch stresses associated with applied bias. These result in interfacial cracking phenomena that are affected by the solvents that are used in OLED fabrication. The OLEDs are first fabricated using an electron transport layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) deposited on an active layer made from solutions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] dissolved in different solvents (toluene, chloroform, and chlorobenzene). The formation of blisters and degradation is then studied under applied bias for devices fabricated using different solvents (toluene, chloroform, and chlorobenzene) and emissive layer thicknesses. The underlying layer mechanical properties are then incorporated into interfacial fracture mechanic models that explain the formation of blisters that degrade the performance of OLED structures.
Highlights
The development of Organic Light Emitting Devices (OLEDs)[1] has led to important applications in lighting,[2] display technologies,[3] and bio-sensing technologies.[4]
Since different solvents have been used for the preparation of organic-based photon emissive layers of the organic light emitting devices (OLEDs),[37] we started by trying to understand the effects of solvent on blister formation
A combination of in situ observations and analytical/computational modeling has been used to study the crack driving forces associated with blister formation in OLEDs with different active MEH:PPV layer thicknesses
Summary
The development of Organic Light Emitting Devices (OLEDs)[1] has led to important applications in lighting,[2] display technologies,[3] and bio-sensing technologies.[4]. The mechanisms for extrinsic degradation include diffusion of water vapor and oxygen through device cathode during light emission,[23,24] electrode deterioration,[26] morphological changes due to enhanced temperature,[25] and delamination-induced buckling of films due to thermal mismatch.[27]. The dark spots are due to carbonization (reaction without oxygen, where carbon is formed from an organic compound) of the organic emissive layer.[31] It is possible for the device’s cathode to delaminate and buckle due to a combination of residual stress and thermal mismatch. Akande and Soboyejo[27] attributed blister formation to thermal expansion mismatch between the metallic cathode layer and the organic emissive layer. This resulted in stresses that were high enough to cause interfacial fracture between the two layers of dissimilar materials.[25]. The effects of the emissive layer (MEH:PPV polymer) thicknesses will be studied using a combination of experiments and interfacial fracture mechanic models that will be used to provide insights into the mechanisms of blister formation and the degradation of OLED performance that is associated with blister formation and growth
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