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78 || AWSAR Awarded Popular Science Stories - 2019
Conductivity is a property of a material that allows the electric current to pass through it. The structure of an OLED device is shown in Fig. 3. Alq3 organic layer as an OLED emitter, cobalt (Co) as the cathode,
iron (Fe) as the anode and
silicon (Si) as a substrate
were used during my research
work for the fabrication of an
OLED. The chemical name of
Alq3 is [8-(hydroxyquinoline)
aluminium]. Alq3 (Fig. 3)
consists of atoms with a low
atomic number, such as carbon
(C), hydrogen (H), nitrogen
(N), oxygen (O), aluminium
(Al), etc. Due to a low atomic
number, Alq3 possesses long
spin relaxation time. Long
spin relaxation time means the
spin remains in the same state
(either in up- or downstate) for a longer time. Long spin relaxation time makes Alq3organic semiconductor an ideal platform for the fabrication of OLEDs.
From Fig. 3, it is clear that the OLED structure is made up of multiple layers. The boundary region between any two layers is called the interface. It can be understood that the number of electrons passing through the interface is directly proportional to the light emitted by the OLED structure. Generally, the Co/ Alq3 interface gives rise to high voltages and low efficiency. Hence, there is a requirement to further modify this interface.
Fig. 4. (a) Coin flipping and (b) spin flipping.
SP current through Co/ Alq3/Fe device is mainly affected by the spin-flip phenomenon at Co/ Alq3 interface. This can be understood by an example of tossing (flipping) of a coin,
as shown in Fig. 4(a). Just as the head becomes tail or tail becomes head as the coin flips in the air, the spin-flip phenomenon is the change of spin state of electrons from up to down or down to up when the electrons flow from cathode to anode via organic semiconductor as indicated in Fig. 4(b). The following spin-flip phenomenon are the two important factors that can cause this spin flipping at Co/ Alq3 interface:
1. (Interfacial diffusion) Mixing of atoms of Co into Alq3. We have addressed this problem in our manuscript published in the Journal of Physics
D: Applied Physics in May 2018. [link]
2. Conductivity mismatch between Co
and Alq3.
Problem: The conductivity mismatch
arises due to different conductivities of Co and Alq3. A large difference in the conductivity of these two materials induces spin flipping at Co/ Alq3 interface. This results in the blocking of SP current from metallic Co into Alq3layer, as a result of which there is no light emitted by Alq3.
Objective: The objective of my research work was to minimize this conductivity mismatch problem at Co/ Alq3 interface for the low-power consumption OLED display.
Solution: To circumvent the conductivity mismatch problem, a transition metal oxide TiO2 was used as a barrier layer at the Co/Alq3 interface in my research work. Conductivity mismatch between Co and Alq3 was determined with a parameter called work function using
Organic spintronics now supports a rapidly growing industry, including organic light- emitting diode (OLED) displays used in cell phone displays and smart TVs. The OLED has several attractive features such as low driving voltage, high efficiency, wide colour variation and fast response speed.