Hello Jing,

Following are some of the factors that limit the efficiency of OLEDs:

1. Efficiency of electrons and holes recombination.

2. Quantum yield of emission of excited state.

3. Efficiency of excited state formation upon annihilation.

Hopefully this helps you. Do provide some feedback.

Hello, Abdullah, thanks for your answer. The points are very helpful. Does the molecular assembly/ordering of the active films affect these parameters?

Yes Dr. Jing. The molecular structure has a profound influence on the transport of charge carriers and therefore affects the performance of the devices.

Performance enhancement due to the improvement of the electron-hole balance by introducing multi-layer periodic (MLP) structure is discussed in:

http://www.sciencedirect.com/science/article/pii/S0141938207001412

Furthermore, the influence of molecular conformations on the absorption and photoluminescence properties are briefly presented in the attached pdf.

Let me know if you find these helpful. Best of luck.

Hello Abdullah, they are very helpful. Thank you very much!

You are welcome. I see you have an interest in nanoparticles and optoelectronics. I am currently working on nanoparticles and their applications in solar cells, maybe we can discuss and share more ideas in the future.

Dear Dr Jing

Abdullah has rightly discussed important points about the efficiency of OLEDs. Current OLED efficiency is the result of combination of chemistry and device physics. OLEDs cannot work without both of them. Efficiency of oleds depends upon many factors. Remember, even a single limiting factor can ruin the efficiency of device. From chemistry point of view, it is not only the quantum yield of the material but also its thermal, morphological as well as electrochemical stability which govern the efficiency. Apart from that we need to match the energy levels of emissive material with other layers (HTL, HBL, ETL, EBL etc) to reduce the driving voltage and control the thickness of these layers for effective recombination of electrons and holes in the emissive layer. Even the technique used for deposition of these layers (solution process or vacuum deposition) has considerable influence on the efficiency. The attached pdf will be helpful to know about many aspects of oleds like requirements, difficulties and limitations.

Thanks Gopal

 ....and hence, you need to develop strategies to improve the efficiency. The simpler the better. If you use triplet state,  inter-molecular distance can be a critical factor. Aggregation is always not good.

Article Triplet states and Energy Back Transfer of Carbazole Derivatives