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Title: Gravure printed and solution-processed polymer semiconductor devices
Author: Chung, Dae-Young
ISNI:       0000 0004 2706 3079
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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The idea of printing optoelectronic devices has been developed over the last decade by various printing techniques such as screen printing, transfer printing, and inkjet printing, attributed to the advent of soluble organic semiconducting (OSC) materials. Printing of optoelectronic devices provides economical advantages for its fast and simple processing stages which is conceptually similar to the graphical printing. The advantage is expected to overcome the relatively low performance of organic materials where its charge transport is occurred by hopping process which is limited by its hopping distance and conformation of molecular chains. Printing techniques currently available should be optimized further to attract a huge impact. For example, the inkjet printing has a drawback of its low printing speed although it offers the printing of high definition pixels with its width around 60 μm. In this Thesis, gravure printing, a high throughput printing technique, is discussed to experimentally demonstrate its feasibility as a production method of optoelectronic devices. The targeted device structures are organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Both printed OFETs and OLEDs have reached device performance similar to reference devices with the same materials and structures fabricated by spin-coating. Unlike the graphic art printing, such as is used to fabricate newspapers, magazines and posters, the printing of OSC optoelectronic devices is very sensitive to processing conditions attributed to a thickness of very thin layers, usually less than 100 nm. Therefore, the surface uniformity of the printed layers must be very planar, with a surface roughness root mean square (RMS) value typically less than 3 nm. It is found that controlling hydrodynamic forces during the thin film formation, such as the coffee stain convection flow and the surface tension driven Marangoni flow, offer a clear opportunity for achieving devices with high performance in gravure contact printed thin films. Chapter 2 and 3 discuss background theory related to experiments in this thesis. Chapter 4 is an experimental chapter explaining materials used and experimental techniques. In Chapter 5, gravure printing of OLEDs with printed poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) hole injection and LUMATION Green 1300 (LG1300) light emissive layers is developed with discussions of wetting of printing formulations and fluidic movements observed during film formation. A mixture of solvent method provides the circulation of hydrodynamic flows inside the printed formulation providing a deposition of highly uniform thin film after solvent evaporation. As a result, high performance of OLEDs with its performance of 8.8 cd/A and 5.4 lm/W with a maximum brightness of 66,000 cd/m2 is reported in OLEDs where both PEDOT: PSS and LG1300 are gravure printed. The performance is the highest up to date among the OLEDs printed by the same printing method. Chapter 6 introduces an inverted structure type OLED where its high work function anode is placed on the top of the device so that it consequently improves device stability as high work function metals such as Au or Ag are less sensitive to ambient dopants. The use of carbonate or oxide layers on the top of a high work function metal at the bottom of the device induced efficient injection of electrons to the device. A very thin layer of caesium carbonate (Cs2CO3) around 5-10 nm was gravure printed onto the ITO electrode. The printed Cs2CO3 layer showed that the surface roughness is highly improved owing to molecular ordering is affected and improved by the mechanical forces such as pressure and thermal energy engaged during the printing. The inverted OLEDs with the printed Cs2CO3 layer recorded the device performance of 10 cd/A and 3 lm/W with a maximum brightness of around 7,500 cd/m2. This is a first report showing that a very thin and inorganic layer can also be gravure printed. Chapter 7 describes charge balancing and position of recombination zone in inverted OLEDs using poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly (9,9-dioctylfluoreneco- N-(4-butylphenyl)-diphenylamine) (TFB) bilayer structure. The two layers are either hole or electron transporting materials and can form a large energy offsets between the HOMO levels and the LUMO levels of the two materials at the interface which confines a large number of injected charge carriers there. It is shown that a position of recombination zone and the charge carrier confinement effect are dependent with the thicknesses of the two polymer layers. The confined charged carriers induce the recombination zone to be positioned close to the interface where charge carrier tunnelling and Föster energy transfer occur more frequently than the bulk. The experimentally optimized thicknesses of the two layers record the highest efficiency of 36 cd/A and 23 lm/W with a bright emission of 51,200 cd/m2 at a low voltage around 4 V. The efficiency is the highest efficiency reported so far to the best of our knowledge using fluorescence materials. Chapter 8 explains gravure printing of OFETs using a thiophene polymer. Poly(3- hexylthiophene)-2,5-diyl (P3HT) OSC, two dielectric layers, and top Ag gate electrode are sequentially gravure printed. The annealing condition of P3HT, choice of dielectric layer and issues related to printing P3HT are discussed. Fully gravure printed OFETs on the pre-patterend ITO source and drain pattern report a high mobility of 3 × 10-2 cm2/Vs and an on/off current ratio of 104.62. The performance is the highest among the printed OFETs using P3HT.
Supervisor: Campbell, Alasdair Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral