I'm curious why do you think when increasing the thickness would increase the band gap, is't because the film result the different composition ? or?

Optical band gap depends on the grain size of a thin film. Usually you will observe a blue shift with decreased grain size, i.e. band gap is increased with smaller grain sizes.

I am sure, that there are exceptions, but this is a general rule (of a thumb) .

I agree with above, but the grain size is not depend on the thickness of the film, right?

@Ling-Hsuan Tsai - grain size depend on thickness of the film , as you increase the thickness of the film it shows decrease in grain size(i am not sure about all sample)

@Sergiusz Patela- please mention the papers related to "Optical band gap depends on the grain size of a thin film" (theory based)

Dear Muhammed, just go to Google scholar and google "grain size thin film bandgap"

Dear Ling-Hsuan Tsai

In thin film technology the grain size may depend on film thickness, but it is difficult to predict direction. Several factors will influence the final grain size: deposition technology, substrate temperature, quality of vacuum, substrate material and crystallographic orientation, deposition speed, the film postprocessing - just to mention a few.

the optical band gap is depend on the grain size of the thin film. As you increase the thickness of thin film , the grain size also found to increase. The optical band bad gap decreases as you increase the thickness and grain size of thin films.

Sir, Sergiusz Patela, i agree with you that In thin film technology the grain size may depend on film thickness, but it is difficult to predict direction. In the case of Muhammed ALI A.V., let me explain. firstly, the grain size has no direct relation with the THIN FILM thickness. secondly the grain size mainly depends on the film thickness and IN ADDITION OR in other words, chemical composition of the thin film is responsible for the grain size distribution. so, finally if your film composition is not varying with the film thickness, you will get a linear relation between the grain size and the film thickness. it may increases or decreases, but the band gap never changes in this case. If the composition changed with thickness, it will directly affect the material (thin film) band gap. Lastly, The grain size of the thin film affect the average transmission of the film, due to the scattering, but not affecting the energy band gap until the composition is not varying.

that's all . . .

Sergiusz Patela sir, will u please attach the soft copy of the book.

If grains in thin films are fully crystalline, the dependence of the bandgap on the grain size, in principle, no. This relationship holds for nanocrystals as a result of quantum size effect. However, if the grains is not completely crystalline (amorphous present on the surface), then such a relationship can be. I recommend you read a work that is attached.