Dear Faten Ben Chaabane,

Charge carrier mobility can be extracted from the Current density-voltage J-V characteristics measured in the dark.

At low voltages, the J-V characteristics typically show ohmic behaviour.

At high voltages, the current becomes space-charge-limited.

When the contact between the electrode and the organic layer is ohmic and the current is transport limited instead of injection limited, the Space-Charge-Limited Current (SCLC) J is given by Mott-Gurney equation.

I hope I have answered your question.

With Best Wishes,

Samir G. Pandya

Dear Samir G. Pandya,

Thank you for your answer. Actually, I have a short circuit device which means that I have fabrication problems. I do not know for what reason i have this short circuit.

Best regards,

Faten

From the device structure " Glass/ITO/PEDOT:PSS/Red emitter/ZnO/Al", the "Red emitter" layer should be critical (like thickness, material, fabrication, etc.). It would be interesting to give the detail information of device fabrication processes, structure layers thicknesses and sizes, I-V curves (in different scales, linear or semi-log plot, etc.).

At low voltages, the J-V behavior is generally ohmic and at high voltages it can become trap limited if the trap density is high in your device or injection limited if the conduction is limited by the injection of carriers at the electrodes or SCLC if the conduction is limited by the space charge.

Other reason for seeing ohmic behavior could be due to the presence of shorts in your device. These can be eliminated by 1. Proper cleaning of Glass/ITO substrate

- use a brush to scrub the substrate

- use clean filtered water

- do the cleaning in dust free atmosphere, preferably without exposing the substrates to the ambience.

2. Spin coating PEDOT:PSS/Red emitter in clean filtered solvents

- ensure sufficient thickness of the films so that there is uniform coverage

3. Ensure that organic molecules are not degraded due to high temperatures involved in ZnO deposition. Too high a deposition rate may not be good.

4. Optimize Al deposition conditions. High deposition rates may not be good.

From your question it is clear that you have short circuit in your device. This may be due to less thickness of the emissive layer or pinholes in it. My suggestion will be to play with the thickness of your emissive layer. Initially forget about getting light from your device and the turn on voltage. Improve the thickness of the emissive layer till you get a diode characteristics curve. Once you get that you can easily modify the thickness and structure to improve device properties. Initially for simplicity you can also remove ZnO from the device. Even though it looks like a waste of time, It will help you to learn a lot. My point is even if you use a semiconductor between two electrode, you should get diode characteristics and not ohmic curve.

While answering I assumed that you are getting a ohmic curve throughout the entire voltage range. Correct me if I am wrong.

Pinholes can be controlled/avoided by maintaining proper conditions as mentioned above in my previous post. They could be because of dust or due to non-uniform coverage of different layers involved. These cause an OLED to show ohmic behavior across the entire voltage range and not just at low voltages.

Hello,

Thank you all for your answers.

I do all organic layers deposition in the ambiante (we don't have a glovebox).

And yes Dipjyoti Das I get the ohmic curve throught the entire voltage range.

I will follow your advices ( Dipjyoti Das and V. N. Ravi Kishore V. ) and i will let you know if there is any progress.

Thank you again,

Faten

Try fabricating a device with materials and architecture which are known to work. Once you have a working OLED then you could try novel materials and novel architecture.