Currently, I am studying the working Principle of the OLED screen panels. Why do we use the emissive layer in between the Electron & Hole transfer layer in OLED devices? Why do we did not directly connect ETL to HTL? Just like in normal LED we connect N-type semiconductors to P-type semiconductors. Why do we need an Emissive layer?
Dear Prerna Gupta,
You raised an interesting question!
Actually the early OLED was based on hole transport layer directly contacted electron emissive layer such as ITO/NPB/Alq3/cathode. The device structure is very similar to the device of LED inorganic semiconductors based as your question is.
However after decades research &developments, it was found that the lifetime and efficiency of OLEDs with multi layers reached the commercial stage. The double layer OLEDs are not suitable for the applications due to the short lifetime and low efficiencies.
Now there are many products with OLED applications such as mobile phones and TVs typically. All OLEDs are based on multi-layers as you know.
You could find a lot of difference between OLED and LED applications.
Dear Prerna Gupta ,
The devices are built to achieve the best performance from them. This is the concept of building any device. The LED is a converter of electrical energy into light energy. It is required to achieve the highest conversion efficiency.
The experience showed that that the best diode structure to work as LED is the double heterojunction diode structure composed of an active i-region sandwiched between an electron injector and a hole injector. The electron injector is called ETL and the hole injector is called the HTL in the terminology of the organic LED.
The middle layer is an intrinsic layer i-layer with direct bandgap semiconductor to promote the radiative emission versus the nonradiative emitter.
One forward biasing such diode the ETL and the HTL injects electrons and holes in the i-region where they radiatively recombine and emits photons of light that will emanate from the material.
The ideal operation will be achieved when the recombination is confined only in the i-region and the recombination will only radiative. This will be achieved when the hole injector reflect the electrons to the active region and prevent them to be back injected in the HTL. The same happens with the ETL, where it must reflect the holes from back injection to itself.
This why one uses heterojunctions not homojunction. The homojunctions work well as injectors but fail tp block the back injection.
Best wishes
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