Hi. There can be various explanations, but two of them are the most probable (and only you can decide which suits well your experimental conditions):

(1) generation of the interfacial dipole on the metal-organic interface. This is very common approach, but it also means that the layer does not really enhance the injection, because you needs first to compensate the field of the dipole (i.e. to obtain the flat-band conditions) and afterwards only the charges are injected to the device;

(2) creation of interfacial traps. In other words, even though the HIL improve the hole injection, for some reason it also creates trap states on the interface. As a result, the trapped charges partially compensate the applied external field and shift the threshold voltage to more negative.

Unfortunately, both cases are related to the potential drop on the interface (and very lose region), hence there is perhaps no direct experimental way how to distinguish these two possibilities.

Have a nice day.

Thank you very much for your reply.

I am agree with your reason in addition I would like to share that

I studied somewhereby inserting HIL layer the drain current is highly increased and the interface barrier between the metal electrode and the semiconductor is reduced which produce a large injection described by the Fowler Nordheim Equation.

please check the attached file

Yup, you are right. These approaches are used for the injection problem (i.e. potential drop as well as the contact resistance) and the threshold voltage.

Regarding the FN equation I can mention this paper:

http://dx.doi.org/10.1016/j.tsf.2009.07.088

and the paper talking about the relation between the electric field, the semiconducotr/insulator thicknesses, and the threshold voltage:

http://link.aip.org/link/doi/10.1063/1.3449078

Hence, the reason you proposed is, of course, right. It is because of the electric field (i.e. change of potential profile) and my previous answer was more about the origin of this electric field, whereas your discussion was about the impact of this field on the threshold.

Maybe I can mention here also another one paper which talks about the relation between the contact resistace, channel resistance and the threshold voltage:

http://dx.doi.org/10.1143/JJAP.51.100205

In summary, you are right that these topics are coupled and it is impossible to study the separately.

What is the reason for increase in threshold voltage (Vt) with increase in thickness of thin film in memory type electrical switching.

Usually threshold voltage increases by increases thickness of semiconductor thickness.its mainly because of electric field as discussed above.check this reference

@Martin Weis Thank you very much for your useful comments

Thanks frnd Mir

Your welcome dear....