Organic single crystal-based light-emittingdevices by template stripping technique
2015
Organic single crystals have attracted extensive interests in the
field of optoelectronic and electronic devices for their high chemical
purity, high charge-carrier mobility and high luminescent quantum
efficiency. The motivation for research on organic single crystals
is their highly arranged packing structure, which provides an ideal
model to investigate the basic interactions between the molecules
and stacking modes and optoelectronic performance. Because of advantages
of these crystals, they have been employed in much functional optoelectronic
devices such as organic field-effect transistors (OFETs), light-emitting
organic field-effect transistors (LE-OFETs), optically pumped lasers,
photovoltaic cells and so on. However, the reports on organic single
crystal based organic light emitting devices (OLEDs) are scarce. There
is a big difficulty in the fabrication of organic single crystal-based
OLEDs because of their intrinsic properties like soft, fragile and
sensitive to organic solvent. In this work, a simple and non-destructive
method template stripping has been applied into the single crystal-based
OLEDs to improve the contact problem and realize its bright emission.
Based on this method, the physical vapour transport grown crystals
are firstly transferred to the modified silicon substrate. Then an
anode of 100 nm gold film is deposited on the top of the crystal surface
by thermal evaporation with metal mask. After dropping a microliter
of photoresist polymer on the device, a glass is pressed on top by
its own weight. And the polymer will spread to the edge of the glass
and totally cover the device. Then it will be cured after exposed
to a UV-light source with a power of 500 W. The device will be transferred
to the glass with the help of a knife for separation. Finally the
cathode of Ca/Ag will be evaporated on the other side of crystal.
The sandwiched structure devices can be achieved by this method. Both
anodes and cathodes of the crystal-based OLEDs can be deposited onto
the opposite surface of the organic single crystals by thermal evaporation
on both sides, so that a much improved contact between the crystals
and the electrodes can be realized, which enhance the carrier injection
into the active layer. Homogenous light emission can be observed from
these crystal-based OLEDs. And the molecules of BP2T are arranged
layer by layer which are nearly perpendicular to the crystal plane.
As we known, the directions of the transition dipole moments lie along
the long axis of the molecule. The light in the crystal will transmit
to the edge of the crystal which recognized as self-waveguide effect.
In the same way, edge-emitting can be seen from the crystal-based
OLEDs which attributed to this self-waveguide. In order to clarify
the template stripping method can work well on the fabrication of
single crystal based OLEDs. We compare the performance of both devices
between the lamination method and template stripping method. Based
on lamination method, the single crystals are simply laminated onto
the top of the anode by van der Waals force which results in an incomplete
contact. Compared to the lamination method, the devices show higher
carrier injection and low voltage threshold by this compacted contact
between crystals and electrodes without any breakages. It is expectable
that this technique would support broad applications of the organic
single crystals in the crystal-based optoelectronic devices.
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