Hello everyone,
I am trying to make organic light-emitting diodes (OLEDs) using different anodes. I obtained the usual (exponential) j-v curves for most of the devices. However, when I used the ITO as an anode, I observed a trend like the attached one. Can you help me interpret the cause of this phenomenon? (btw, turn-on voltage of the device is about 5.5 V)
Thank you in advance
What is your device structure? It looks like that you have a rough surface of ITO substrate.
Hi Eda,
is this kind of behavior preserved when you use the OLED a second time? Maybe it is some one-time phenomenen due to some chemical change in the ITO?
I am really not an expert on this, but this was my first thougth.
Yours
Markus
Thank you for your quick responses.
Xuhua Wang the device architecture is ITO/alpha-NPD/CBP:guest emitter/TPBI/LiF/Al. I think there is no problem with the device architecture bec. we do not observe any problem when we use different anodes instead of ITO. In the case of roughness, we apply cleaning and oxygen plasma processes.
Markus Leitgeb Actually, it belongs to the first run i-v measurement and we think there is no chemical change in ITO, bec. we use the same ITO for solar cell applications and we have observed this problem only for OLED applications.
Sincerely
Dear Eda Alemdar,
It is a typical phosphorus OLED device structure. The layer thickness also plays a role in the characteristics of JV curves besides ITO surface roughness and treatment. What is the thickness of NPD in your case?
The thickness of the layer is 40 nm, which is the same when we use other anodes.
thanks a lot
Xuhua Wang
@Eda Alemdar,
It may be worth checking the roughness of ITO. Alternatively, beta-NPB has better hole injection from ITO to replace alpha-NPD.
That is a very high voltage for the current you are showing. It suggests not having good charge injection and/or process problems.
Check your process. If you are using substrate with pre-coated ITO, good process for cleaning and handling is very important for achieving good reproducible I-V curves. Are your curves reproducible?
Try adding a better injection layer. The curve should shift to lower voltage.
The change in voltage required to produce a given current for a change in thickness of one layer (usually the ETL), is sometimes a good diagnostic for injection.
The shape of the curve seems unusual. Could heating be playing a role?
You can try running your sweep at different speeds or perhaps at a lower ambient temperature and see if the heating idea pans out.
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