Organic and Perovskite Light Emitting Devices: Effects of Interfacial and Layer Cracking on Device Performance and Failure Mechanisms

Cromwell, Jaya

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Organic and Perovskite Light Emitting Devices: Effects of Interfacial and Layer Cracking on Device Performance and Failure Mechanisms

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LEDs can be fabricated on rigid or flexible substrates. Cracking plays a role in the failure of OLEDs on rigid substrates, on flexible substrates and in the interfacial adhesion of Perovskite LEDs. This thesis aims to shed light on this role and consequently improve the performance and lifetimes of LEDs. It looks at the effects of interfacial fracture and blister formation in organic LEDs, channel cracking in flexible organic LEDs and interfacial cracking in Perovskite LEDs. Degradation reduces the lifetime of Organic Light Emitting Devices, so it is important to understand the mechanisms behind it. The lifetimes of organic LEDs are affected by defects, particularly at the interfaces. These can be voids or particles that serve as initiation centers for crack growth which compromises the lifetimes of LEDs. Here we investigate the effect of blister formation on degradation and optimize fabrication parameters in order to mitigate it. The growth of these blisters was also associated with a decrease in light emission. The effects of organic solvents (used in the processing of OLEDs) and crack driving forces on the underlying mechanisms of blister formation and degradation in OLEDs has been investigated by a combination of models and experiments. The effects of the emissive layer (MEH:PPV) thicknesses has been studied using a combination of experiments and interfacial fracture mechanic models. There is a need to understand the potential effects of bending on the performance and degradation mechanisms in flexible OLEDs. Repeated bending can be a cause of failure in flexible OLEDs by inducing crack formation. Here we investigated failure mechanisms in monotonically and cyclically deformed flexible OLEDs. Deformation was carried out by bending around rollers of varying radii. The OLEDs were tested for varying number of bend cycles for different bend radii. Interfacial failure can affect the reliability and lifetime of Perovskite LEDs, so it is important to know the robustness of each of the interfaces present in such devices. Brazil disk specimens were prepared for each pair of interfaces to be studied and the angle of the interface was varied to study mode mixities between pure mode I (opening) and pure mode II (sliding) fracture. These experimental studies shed light on the effects of interfacial and layer cracking on device performance and failure mechanisms.

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