Nanoscale large-area opto/electronics via adhesion lithography
Creators: name: Wyatt-Moon, Gwenhivir
Publisher: name: Imperial College London
Year: 2018

description: As the feature size of devices in the electronics industry has hit the nanoscale, device fabrication costs have rapidly increased. Whilst commercial technologies such as photolithography are able to produce nanoscale feature size, they are costly and unsuitable for large area printable electronics. To allow for up-scaling of devices considerable research is now focused on new manufacturing processes. Alongside this, new materials such as organics, metal oxides and 2D materials have been developed, allowing for novel device applications to be realised. The ability to deposit these materials at low cost and on large area flexible substrates has been realised with solution processing techniques such as blade coating, inkjet, gravure and screen printing used to deposit materials. To compete with traditional electronics and to allow for commercial applications, however, device performance needs to be improved with reduction in feature size seen as one avenue of interest. This thesis explores and develops the fabrication technique adhesion lithography (a-Lith). This simple process alters the adhesion forces of a metal using the unique properties of self-assembled monolayers (SAMs) to create asymmetric planar electrodes separated by sub 10nm gaps. Using this novel electrode fabrication technique in conjunction with solution processable semiconductors, highly scalable, low-cost, lateral architecture devices can be created. First the optimisation of a-Lith is explored by looking into the influence of metal deposition on the formation of the nanogap by varying the grain size and thickness of the two metal electrodes. Both factors are found to have a large effect on resultant devices with a reduce grain size causing a reduction in device variation and increased metal thickness causing an increase in gap size. The conversion of the process from ridged surfaces to flexible plastic substrates is also investigated with annealing substrates seen to improve the adhesion of the metal thin films and increasing fabrication yield. Solution processed materials were then used to fabricate photodiodes for various applications with copper thiocyanate (CuSCN) used to create deep ultraviolet photodiodes showing high responsivity (719 A/W) and photosensitivity (79). Next zinc oxide (ZnO) was utilised for ultraviolet photodiodes showing a high on/off ratio but slow response times. Finally poly[4,8-bis(5-(2-ethylhexyl)thiophen- 2-yl)benzo[1,2-b:4,5-b0]dit-hiophene-co-3-fluor-othieno[3,4-b]thio-phene-2-carboxylate] (PTB7-Th) in heterojunction structures with 6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM.) and in a Schottky configurations is explored for visible photodiodes showing responsivity of 33 A/W and a detectivity (D) of 6×10^13 Jones, with relatively fast response times (~1 ms). These devices demonstrate the viability of a-Lith for large area fabrication of photodiodes. The a-Lith electrodes were then investigated in light emitting diode (LED) applications. The asymmetric electrodes were used in conjunction with solution processable polymers of varying electroluminescence spectra to create unique nano-polymer LEDs. These devices allow for high current densities to be realised due to reduced Joule heating and showed brightness tunability when device width is varied. The response time of the devices was ~210 µs which enables the devices to be considered for application in the display industry and particularly high-definition optical displays. This work highlights the versatility of the a-Lith technique for LED applications.