OLED

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


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



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. Thedotted 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 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


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

Engineering

aadish choprauce

emitting oled

oled display

aadish chopradr

plastic substratesoleds

glass substrate

lcd vs plasma vs oled

emitting area