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.