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Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Welcome to Seminar on
OLED technology and displays
Seminar Guide : Name – Aadish ChopraDr. Jankiballabh Sharma C.R.N - 11/101
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
What is OLED ?
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Types of Oled
Passive matrix OLED ( PMOLED ) Active matrix OLED ( AMOLED )
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Types of OLED
Transparent OLED Top Emitting OLED
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Principle
OLED energy diagram
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Difference between LCD vs Plasma vs OLED
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Schematic diagram of PECVD system.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Microscopic photographs of the emitting area in OLED before and after a 500-h storage test.
Electrode arrangement and emitting and observation areas.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Shrinking of the emitting area from the anode edge.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Characteristics of OLED devices on a plastic substrate
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Flexible displays using plastic substrates
Characteristics of OLED devices on a plastic substrate
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Electrical Equivalent Design
• 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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Electrical Equivalent Design
Selected OLED equivalent electrical model.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Electrical Equivalent Design
• 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).
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Electrical Equivalent Design
OLED static characteristic plotted in semilogarithmic scale. Thedotted lines show the leakage conductivity and the OLED threshold voltage
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Electrical Equivalent Design
OLED static characteristics plotted on a linear scale
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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).
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Impedance analysis
Parameter values for different bias voltages under threshold voltage.
The results show that parameter values in this operatingmode do not depend on the applied bias voltage and canbe considered constant. As no charges are injected since thebias voltage is under the threshold voltage, the capacitancecorresponds to the geometric capacity given by
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Impedance analysis
OLED impedance and phase versus frequency for a polarization voltage of 4 V and an ac amplitude of 10 mV.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Impedance analysis
OLED equivalent capacitance versus bias voltage.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
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.
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Created by Aadish Chopra UCE,RTU,Kota4/13/2015
Application of OLED in neural activity
Created by Aadish Chopra UCE,RTU,Kota4/13/2015