Fabrication and characterization of flexible organic light-emitting diodes for smart textiles application

Abstract

This thesis presents a systematic investigation to fabricate flexible OLEDs, which exhibit high performance and flexibility, and can be applied in wearable display applications. This research was conducted based on a comprehensive literature review of related technologies. The statement of the existing problems and potential prospects of this technology in wearable smart textile applications put forth the objective of this research. Various materials were used in this study, including flexible substrates, light emitters, buffers, electrodes and packaging materials. A tailor-made thermal vapor deposition system and a sputtering system in Flexible Display Laboratory of ITC were adopted to prepare the multi-layer structured FOLEDs. The working principle of the physical vapor deposition system and the operating process were described. Details of the materials preparation, device fabrication and characterization of OLEDs were described. Fabrication conditions of Alq3 based FOLEDs were experimentally investigated. The current-voltage-luminance characteristics of the FOLEDs were characterized. It has been found that the deposition temperature of Alq3 is critical to achieve a better FOLEDs performance, which was associated with the film morphology. Performance of FOLEDs was investigated by varying materials for hole transport and hole injection. TPD was deposited as the hole transport materials at various rates. Surface morphology and grain size of the TPD thin films changed with the deposition rate of TPD, hence, the carrier mobility of the films was affected as well. Higher stability and luminous intensity were achieved with TPD grown at a low deposition rate. Nanocomposite of MWCNTs and PEDOT:PSS was used as the hole injection material of the FOLEDs. The EL intensity of the resultant FOLEDs was significantly increased and the turn-on voltage of the devices was decreased by adopting an optimal concentration of MWCNTs in the composite. The metal or semimetal properties of the MWCNTs was attributed to the improvement of the nanocomposites of PEDOT:PSS and MWCNTs. Further improvement has been obtained by optimizing the FOLED structure of ITO/PEDOT:PSS: w.t. 0.4% MWCNTs/TPD/Alq3/Al. A luminance of 6800cd/m2 was achieved. The PEDOT:PSS was further investigated for replacement of ITO by varying doping levels of solvents (DMSO and glycerol) and heating treatments. The baking temperature slightly changed the conductivity of the PEDOT:PSS films, while solvent-treatment improved the conductivity by three orders of magnitude. AFM micrograph showed that the surface morphology of the PEDOT:PSS was greatly improved by the DMSO treatment. The films became more hydrophobic. Raman spectra indicated that the conformation of PEDOT:PSS was changed after the treatment, probably due to the rearrangement of the coil chain to linear chain. Particle accumulation was found in the pristine PEDOT:PSS from particle size analysis. After DMSO treatment, the bigger particles were broken into smaller ones by interaction between the DMSO and PEDOT:PSS, which may result smooth film surface. The thicker PSS shell of PEDOT:PSS was found to decrease by XPS study as the take-off angle was changed from 45o to 25o, instead of which was the increase of the surface concentration of conductive PEDOT. Better connections were formed between the PEDOT:PSS particles. A model was proposed for the conductive improvement mechanism of the DMSO treated PEDOT:PSS films. Nanocomposites of PEDOT:PSS and SWCNTs together with the DMSO treatment were investigated to develop polymeric anode for FOLEDs. The UV-VIS transmittance and electrical conductivity of the anode were studied as a function of SWCNTs concentration. Within the experimental range, the conductivity of the PEDOT:PSS composite increased with the increase of the SWCNTs concentration, as additional conductive pathway of SWCNTs was created in the PEDOT:PSS system. At the same time, a slight decrease of the transmittance was evident. FOLEDs based on this polymeric anode demonstrated a performance very close to those on ITO anode. This flexible polymeric composite anode exhibits a very good bending property over ITO/PET anode. Only slight resistance increase was found with a bending cycle up to 1000, while the resistance of the ITO/PET anode increases about 100 times of initial resistance with a bending cycles of 20 with the same maximum bending curvature. The polymeric anode is very promising for fully flexible OLEDs or other optoelectronics. Flexible polymeric composite anode with a thickness of 100nm on PET substrate has been made with an optical transmittance of 80% and an electrical resistance of 136.2o/a.