I am making OLED with this structure on plastic substrates: ITO/NPB/Alq3/LiF/Al, but the turn-on-voltage is high – around 9V and devices get burned often. Can anybody tell me what could be the problem - is it because of contacts? Or does it have something to do with the etching of the substrates?
I do not have much experience but had similar problems with OLEDs (with simpler structure) and found it to be caused by the poor quality of the layers. You are probably using vapour deposition, so layers should be smooth, but you should check the ITO surface quality. Any roughness (spikes) with hight of the order of microns will result in shortage between ITO and Al, causing most of the current going through this and heating the device.
Of course many other things may result in such problems. Each layer causes voltage drop, so thicknesses of all non-metallic layers are crucial.
fast google search:
http://www.sciencedirect.com/science/article/pii/S0379677904000967
Dear Sanjib,
As you have mentioned the device architecture in your question, your device have zero hole confinement, because you have deposited the LiF right after the Alq3. It is a very poor device architecture, if you will deposit another layer of TPBi or BPhen (25-30 nm) then your device performance should be good and the turn on voltage must be much lower than the now. From my experience it should not be more than 4V. Your device performance is poor because of carrier imbalance. Whilst, incorporation of extra BPhen or TPBi layer will shift the recombination zone towards NPB/Alq3 interface and simultaneously provide the excellent electron transport and hole/exciton confinement. Which will allow the device to turn on at low voltage and also improve the device performance.
Some other factors also need to understand, as you are preparing your device at flexible substrate. Most importantly, which material you are using as a substrate and what is the sheet resistance for your ITO electrode etc.
hope it will help you,
Thanks, Sudhir.
I am using substrates with sheet resistance of 60 ohm/sq and etching them with a mixture of DI water, HCl, and HNO3.
If you are talking about IV curve for the turn on voltage, either you ITO has not been cleaned properly or your Al deposition condition is very poor..ie your base pressure must be ~ 10^-5Pa.
Why are you using Nitric Acid and HCl? It isn't necessary, just use DI water, acetone, and isopropanol; in that order.
Have you done O2 Plasma treatment of ITO? What is the thickness of each layer? The normal structure of this device should be ITO (O2 treated)/NPB(70 to 100 nm)/Alq3(40 to 60 nm)/LiF(1 nm)/Al(>100 nm). Do you see any sparks when the device burn off? What is the voltage for the burn off? And how large is the device current?
I didn't do O2-plasma treatment and the structure is ITO/NPB(45nm)/Alq3(60nm)/LiF(0.8nm)/Al(100 nm).
There is a little spark, when a device gets burnt.
Now, I can drive them with less than 9V, but the brightness is not uniform. Is it because of the organic layers or Al layer?
Thanks!
O2 plasma is a must. Spark is often results from uneveness of your thin film. If you are doing your own lithography, make sure there is no dirt adhere on your ITO surface. Dirt as small as 1um has delirious effect. During vacuum deposition, make sure you rotate your substrate( if your system an do that) which will give you uniformity at +/- 2nm.
You might wanna to consider substituting NPM with PEDOT:PSS. That's what COMBOLED project was about where i was lucky to oversee what they were working. Small molecules OLED tends to get short very easy if you are not procuring smooth ITO. We have found away to smoothen the ITO and will upload it when it is being published as it is under review. I suspect Samung Galaxy AMOLED is using PEDOT:PSS as an anode....LOL.
Since you don't do oxygen plasma treatment, your ITO workfunction will be somewhere near 4.6-4.8. NPB has a LUMO of 5.2 eV. That means the offset can be as large as 0.6eV. This is one reason why your turn-on voltage is high. I don't think there is a problem in depositing LiF right after Alq3. In fact this is how it is done in many standard devices. HBL is required mostly for phosphorescent systems. ITO smoothness is very important as far as the pixel-burning is concerned. You might want to increase the NPB thickness to 90 or 100nm.
Dear Sanjib, the devices with this architecture should work. I had used the same structure and my devices were giving bright luminescence at voltages less than 4V. Only difference is that I had done ozone-treatment of ITO substrates which as mentioned by @Zhao and @Narayanan Unni is must. Also, as @Narayanan Unni mentioned, there is no problem in coating LiF next to AlQ3 as LiF reduces the work-function of Al to ~3.6 eV and helps in effectively injecting electrons into the LUMO of AlQ3. Regarding smoothening of ITO surfaces, if you are having the facility of glove box attached to your thermal evaporator, you can easily use PEDOT:PSS as suggested by @Kai Lin Woon. But if you don’t have that facility, you can try F4TCNQ, which serves the purpose as you can thermally evaporate it.
As per my experience, its not because of the shorting of ITO and Al layers, as in that condition you wouldn't have even got that faint light at 9V. Also, the thickness what you used should work.
The reason for your problem is you are not doing ozone-treatment and not using any layer for smoothening the ITO surface. Also, improper cleaning of ITO may also create lot of problems (I, myself had faced that problem). If you don't have a ozone-treatment setup, you can easily do one yourself.
You have not mentioned the details of how you are doing the measurements. Are you characterizing the devices in inert atmosphere? Or are you encapsulating the devices?
1. You could try depositing a thin layer of an HIL between the ITO and NBP, for example HATCN.
2. You need clean ITO. One of the challenges is that ITO picks up carbon from the environment, even air. Consequently, for glove box deposition systems, the results can be poor and not very reproducible.
Diagnostics that I have found revealing, are (a) deposit the cathode by itself for a set of samples and see what they look like electrically, and (b) vary the thickness of one layer at a time and graph voltage versus thickness at a given current density.
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