Organic light-emitting diodes with FeCl3-FLG intercalated few-layer graphene electrodes

Abstract

Every day we interact with dozens of screens and touchscreens, as almost every consumer electronic device now has one. At the same time photovoltaic (PV) cells are becoming an important energy generation technology. All types of screens, such as those based on organic light-emitting diodes (OLEDs), and PVs, require a transparent conductive electrode (TCE) to enable charge carriers to enter/exit the device, while allowing light out of the screen or into the PV cell. Currently the material of choice for all these devices in commercial applications is indium tin oxide (ITO). However, there are some detrimental issues which face ITO. One of the most significant presently is its lack of flexibility. ITO is extremely brittle, making it unsuitable for use in bendable devices now being seen commercially for the first time. As wearables become more mainstream this will significantly limit its use in flexible applications, like clothing. Another major problem for ITO is the global indium stock. Reports vary, but it is estimated that there is as little as 30 years of indium left on the planet, of which its primary use is TCEs. Finally, there are other undesirable properties, such as the high financial cost, high environmental cost, and sub-optimal work function. Graphene materials for the past decade and a half have been lauded as a major breakthrough in materials science with many possible applications, including as TCE. One form of graphene which has proven to have electrical and transmittance properties required to replace ITO in OLED technology is FeCl3 intercalated FLG (FeCl3-FLG). First reported in 2012, it was remarkable for its low sheet resistance, high optical transmittance, and notable unforeseen stability to humidity and heat. It has since shown potential scalability, increasing from the micron to wafer scale, and its work function matching with organic materials used in OLEDs and PVs indicates its potential suitability as TCE for high efficiency devices. The work presented in this thesis investigates the potential of FeCl3-FLG as a possible replacement of ITO in OLEDs. This includes a systematic investigation of its relevant properties for OLEDs, how and if the material can be integrated into such devices, and what is the resulting performance compared to ITO for a range of substrates, both rigid and flexible, while always ensuring low temperature, and solution processed fabrication. The initial part of the thesis focuses on FeCl3-FLG material optimisation and characterisation, as well as OLED materials and device fabrication optimisation. A FeCl3-FLG material specific characterisation technique is developed using Raman spectroscopy to non-destructively assess the large-scale doping of few-layer graphene (FLG) induced by the FeCl3 intercalation. Specifically, a metric is developed to represent the quality of intercalation, allowing for the direct comparison of the levels of intercalation in different samples, giving a more representative figure of the sample as a whole. A wafer-scale transfer technique is developed to enable the material to be transferred from its fabrication substrate (silicon wafer) to transparent or flexible substrates and used in large-area displays. A bending apparatus is developed that enables the measurement of the material’s flexibility in a relevant way. This apparatus has shown that the resistance of ITO increases by a factor of 50 after just 100 bending cycles, while the resistance of FeCl3-FLG increases by a factor of just 5 after 2000 bending cycles, demonstrating the superiority of FeCl3-FLG a as flexible conductor. Fabrication of the OLEDs was developed and optimised for the combination of the equipment available, low temperature processing, and solution processed fabrication. Techniques were developed in-house to allow characterisation of OLED device performance such as luminance, emission profile, current efficiency, power efficiency, and external quantum efficiency. The second part of the work describes the integration of FeCl3-FLG into OLED devices. When comparing FeCl3-FLG based devices to ITO based devices, the turn on voltage indicates good work function matching, however there was a reduction in performance, indicating material issues primarily surrounding roughness and lifetime. I investigated multiple ways to overcome the deficits of the material, such as encapsulation using a novel wax-based method. I then investigated several other ways of improving device performance including roughness and a new transfer technique. Finally, I propose and investigate a FeCl3-FLG-ITO hybrid material which shows improved performance over FeCl3-FLG in OLEDs, and a significant improvement in flexibility. I show in this work that FeCl3-FLG can work as a TCE in OLEDs, and that there are signs that with more refined fabrication this could be achieved. I propose several ways of achieving this with refined methods. Significant further work is required to bring this material to commercial viability for use as a TCE in OLEDs.