Dear Minu Mohan ,

Ideal metallic films are expected to show a thickness-dependent work function at a level of 0.1 eV in a short thickness range (5 nm) . An increase of the work function with increasing film thickness level has been observed at the vacuum-metal interface according to the theoretical expectations.

A paper related to the effect of ZnO thin films on work function is attached. May be helpful for you.

Regards,

Nasar

paper related to the effect of ZnO thin films on work function is attached here

Thank you Nasar Ahmed.

Certainly the work function will change, WF is the difference between fermi level to the vacuum level of a materials. Even if we convince our self that all ZnO films were prepared in same environment, by varying the thickness, definitely vacuum level will be varied. Hence, the work function is subjected to change. But this change may not be high but in terms of 0.01-0.1, or 0.2 eV.

That is why we employ plasma or UVO treatments, where only vacuum level of a material is shifted, hence we observed a small increment in WF (for eg, ITO or FTO.....) .

I hope it may be useful.

thank you,

Dear Justin Jesuraj,

Thank you for your answer.

So as the thickness of the film increases, will the WF increase or decrease?

Dear Minu Mohan,

The WF will be vary as thickness changes, but im not sure whether it is decreasing or increasing. But as i have some experience in organic devices, i hope following points can be tried for your issue,

1) You deposited on ITO, hence the any change occurred on ZnO will be reflected in the change of ITO WF. that can be identified using UPS or XPS.

From UPS, you can directly measure the change in WF. (by monitoring secondary electron cut off region)

In XPS, you can observed a lower or higher energy shift of In or Sn 3d peaks pertaining to the WF variation occurred. From that you can suggest the WF decrements or increment.

2) when we deposited any EIL or HIL materials on ITO, the barrier for electron or hole injection/extraction is varied. and it is associated with the WF of HIL/EIL materials. (sometime instead of WF, the CB edge or VB top energy level has been suggested for carrier transport, mostly if thickness of EIL/HIL is high transport through VB/CB is considered.)

To convey the carrier transport enhancement, it is better to make some single carrier devices like hole or electron only device with your HIL/EIL with different thickness or parameters. Most of the people follow similar tactics to assure their carrier enhancement .

I hope it will be useful for you.

thank you,