This thesis investigates the use of the large-area, roll-to-roll compatible deposition technique of ultrasonic spray coating for the fabrication of polymeric organic light-emitting diodes (OLEDs).
Firstly, in Chapter 4 a range of different materials are investigated as potential solution processable electron-injection layers, typically this layer is deposited via thermal evaporation in OLEDs but a solution based alternative is required for deposition via ultrasonic spray coating. Solution-processed electron-injection layers: caesium carbonate (Cs2CO3), 8-hydroxy-quinolinato lithium (Liq) and polyethylenimine-ethoxylated (PEIE), are spin cast to tune thickness for optimal device performance and investigated for compatibly with ultrasonic spray coating under ambient conditions. Caesium carbonate deposited in an inert atmosphere was found to improve device performance compared to thermally evaporated lithium fluoride references but when cast in an ambient atmosphere the device performance was very poor due to the hygroscopic nature of Cs2CO3. Devices fabricated with optimal thickness Liq electron-injection layer had low mean peak luminance of 1197 cd m-2. Devices with PEIE layers of 2.7 ± 0.1 nm spin cast under ambient conditions have mean peak performance metrics of 3.18 cd A-1, 1.14 Lm W-1, and 10690 cd m-2.
In Chapter 5 the wide parameter space of ultrasonic spray coating is probed in order to determine the processing window and optimisation process for depositing uniform polymer thin films. The parameter space for ultrasonic spray coating has been investigated and an optimisation process for spraying uniform thin polymer films has been developed using hole-transporting polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’(N-(4-sec-butylphenyl))) diphenylamine] (TFB). The processing window for casting uniform films of TFB from a 4 mg ml-1 toluene solution cast onto a substrate held at 25 °C with a fluid pressure of 50 mbar have been shown to be a pass height of 40 mm and a range of pass speeds from 125 – 200 mm s-1 to fabricate films between 66 – 97 nm.
In Chapter 6 a study is undertaken to determine if there is a fundamental issues with films deposited via ultrasonic spray coating that could limit the electrical performance of devices containing these film compared to those deposited via spin coating. The influence of thin film processing technique and surface roughness on the electrical performance of unipolar polymer OLEDs are studied, and a negligible difference is found between low-roughness spray cast (Ra < 10 nm) and spin cast devices of equivalent thicknesses. However, above 10 nm roughness there is a reduction in injection efficiency, up to an 86 % loss in performance for roughnesses of the order of 40 % of the thickness of the film. As such a processing window of Ra < 10 nm for achieving comparable electrical performance between spin and spray cast devices is demonstrated.
Finally, in Chapter 7 the different layers of a White-emitting polymer OLED are deposited via ultrasonic spray coating in separate devices for optimisation, then the layers are sprayed subsequently in the same device and lastly large-area devices are fabricated to demonstrate the scalability of the process. White-emitting polymer OLEDs have been fabricated in which the hole-injection layer, emissive layer and electron-injection layer were deposited via ultrasonic spray coating.
Several different device studies have been conducted: Firstly, devices with PEDOT:PSS deposited via ultrasonic spray coating as a hole-injection layer were fabricated and show comparable device performance to those with spin-cast PEDOT:PSS.
Secondly, devices with a white-light-emitting polymer (LEP), deposited via ultrasonic spray coating, have mean peak current efficiency of 4.93 cd A-1, 90 % of the value of the spin cast references 5.45 cd A-1. The mean peak power efficiency of the spray cast devices was 2.42 Lm W-1, 82 % of the spin cast references 2.97 Lm W-1. The mean peak luminance of the spray cast devices was 8149 cd m-2, 80% of the reference spin cast value 10189 cd m-2. These results are equivalent to those in literature where devices fabricated by Gilissen et al. in which a yellow-light-emitting polymer was deposited via ultrasonic spray coating achieved 9.71 Lm W-1, 81 % of the spin cast reference.
Thirdly, attempts were made to replace the electron-injection layer with an air-stable, non-ionic and non-conjugated polymer, polyethylenimine-ethoxylated (PEIE), layer deposited via ultrasonic spray coating. Devices in which PEIE was deposited via ultrasonic spray coating as an electron-injection layer showed poor device metrics and non-uniform emission.
Finally, white-light-emitting devices were then fabricated in which the hole-injection and emissive layers were sequentially deposited via ultrasonic spray coating, a first for polymer OLEDs. The mean peak current efficiency for the spray cast devices was 2.43 cd A-1, 72 % of the value of the spin cast references 3.39 cd A-1. The mean peak power efficiency of the spray cast devices was 0.83 Lm W-1, 71 % of the spin cast references 1.17 Lm W-1. The mean peak luminance of the spray cast devices was 7409 cd m-2, 70 % of the reference spin cast value 10626 cd m-2. The same layers were then spray cast to fabricate large-area devices to demonstrate the possibility of coating over large areas and the potential compatibility of this technique with roll-to-roll processing, a yield of working pixels of 95.8 % was achieved.
