OLED Seminar Guide Aadish Chopra Explores Technology

Created by Aadish Chopra
UCE,RTU,Kota4/13/2015
Welcome to Seminar
on
OLED technology and displays
Seminar Guide : Name – Aadish Chopra
Dr. Jankiballabh Sharma C.R.N - 11/101

What is OLED ?

Types of Oled
Passive matrix OLED ( PMOLED ) Active matrix OLED ( AMOLED )

Types of OLED
Transparent OLED Top Emitting OLED

Principle
OLED energy diagram

Difference between LCD vs Plasma vs OLED

Flexible displays using plastic substrates
• OLEDs are being fabricated on a glass substrate and are then encapsulated
with metal or glass lids.
• An OLED display on a plastic substrate is thinner and lighter compared with
that on a glass substrate .
Development of a passivation film
• A high level of impermeability is needed because OLEDs are very sensitive to
moisture and oxygen.
• The deposition temperature must be low because OLEDs cannot withstand
high temperatures.
• Low film stress is required because if the passivation film stress is high, the
organic layer will peel off from the substrate.
• A silicon nitride (SiNx) film fabricated by plasma-enhanced chemical vapor
deposition (PECVD) met these requirements.

Schematic diagram of PECVD system.

Microscopic photographs of the emitting
area in OLED before and after a 500-h
storage test.
Electrode arrangement and
emitting and observation
areas.

Shrinking of the emitting area from the anode edge.

Development of plastic substrate for OLEDs
• Plastics are not impermeable enough to protect OLEDs, a barrier film must
be added on the substrate.
• The SiNx film fabricated by PECVD has a very high level of impermeability,
but it also has brownish color. When emitting light from the OLED is
viewed through the substrate, such colored films degrade their efficiency.
• We selected RF sputtering to deposit the SiON film. We used silicon nitride
Si N as a sputtering target and argon (Ar) and oxygen O as reaction gases.
• The Mocon method is a well-known method for evaluating moisture
permeability of films, but the level of permeability that is required for
OLEDs is lower than the limit of the measurable range of this method. To
solve this problem, a more sensitive method based on calcium corrosion
has been proposed.

Characteristics of OLED devices on a plastic substrate

Electrical Equivalent Design
• This substitution OLED device is based on a circuit-equivalent OLED model
and can be used to design and test OLED dedicated drivers.
• Moreover, a photodetector is not required to measure the light output
waveform. This can be simply done by measuring a voltage across a
resistance.
Simplified OLED electrical equivalent model

• From an electrical point of view, this structure can be considered as an
equivalent circuit combining both ohmic resistances and a capacitor.
• The physical origin of ohmic losses is mainly due to contact resistances
between organic layers, bulk conduction within organic layers, and
electrode resistance.
• The origin of the capacitive behavior is due to the stacked structure of the
organic layers.

Selected OLED equivalent electrical model.

• Indeed, when disconnected, the voltage V would remain constant, but
actually, in an OLED, the voltage decreases slowly with time. In order to
take into account this additional time constant, a resistance is placed in
parallel to the capacitor, which leads to the model presented before .
• In this electrical equivalent model, Rp represents the leakage resistance
due to charge injection into the structure when diode D is OFF.
• the branch containing the diode is detailed. It comprises a voltage source
Vt representing the diode threshold voltage, D (a perfect diode preventing
reverse current), and Rs (a series variable resistance expressing
theexponential link between the static OLED current and voltage).

OLED static characteristic plotted in semilogarithmic scale. The
dotted lines show the leakage conductivity and the OLED threshold
voltage

OLED static characteristics plotted on a linear scale

Model parameter identification
• Parameter identification requires only two types of measurements :
1. static regime measurement
2. Impedance analysis.
• The static V (I) curve (V and I are the voltage across the OLED and the
current flowing through it, respectively) is used to determine Rs and Vt
and also to evaluate the order of magnitude of Rp. Rp can be estimated by
measuring the V (I) slope below the threshold voltage (i.e., when diode D
is off).

Impedance analysis
OLED impedance and phase as a function of frequency for a
polarization voltage of 20 mV and an ac amplitude of 10 mV.

Impedance analysis
• It can be seen that, for low frequencies, the OLED behaveslike a pure
capacitor with a −90◦ phase.
• As frequency increases, the impedance decreases with an increasing
phase.
• When the phase crosses zero, the OLED is purely resistive,and the
electrode resistance at this point can be derived.
• For higher frequencies, the phase becomes positive, indicating a global
inductive behavior. This inductive behavior is only due to the inductance of
the wiring and is not linked to the OLED behavior itself. The equivalent
inductance value derived from measurements was typically few hundreds
of nanohenries.

Impedance analysis
Parameter values for different bias voltages under threshold
voltage.
The results show that parameter values in this operating
mode do not depend on the applied bias voltage and can
be considered constant. As no charges are injected since the
bias voltage is under the threshold voltage, the capacitance
corresponds to the geometric capacity given by

Impedance analysis
OLED impedance and phase versus frequency for a
polarization voltage of 4 V and an ac amplitude of
10 mV.

Impedance analysis
OLED equivalent capacitance versus bias voltage.

Advantages v/s Disadvantages
• It has the advantage of simplicity.
• It is a large signal model.
• Two electrical time constants are represented. When diode D is on, the
time constant to consider is determined by Re and C (the order of
magnitude is typically a few microseconds). When diode D is off, the time
constant is determined by Rp and C (the order of magnitude is typically
around a second).
• The main drawback of this model is its accuracy, particularly when it is
driven by low-frequency current pulses.
• Another issue with the model selected is the dependence of parameters
on temperature.

Application of OLED in neural activity

OLED Seminar Guide Aadish Chopra Explores Technology

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