If we make thin films by spin coating method, even we kept same RPM and Time for rotation, the thickness of both will be different. Then how can we compare absorption enhancement the thin film material?
In plasmonic organic solar cells, people have compared the absorption enhancement of thin film with nanoparticles (NPs) related to thin film without NPs.
Probably, experimental measurement is the best answer by using optical transmission perhaps comparing with ellipsometry. Then you should be able to extract both refractive index and thickness for both films. Then subtract the two measurements. Note that having double side polished substrates is necessary for the transmission measurements.
Take a look at the attached Swanepole paper. If your films are thick enough you can use it to construct a model to fit your thin films really very well like in the attached GaN paper. You will need to consider refractive index as a function of wavelength.
The first problem you face is that in optical thin film equations you have the product of refractive index and thickness always together. However the plasmonic particles will have a more limited wavelength range where the index change will be large, and outside that range you should be able to say that the index is very similar and then you have two equations and two unknowns and if the index is the same you can solve for thickness.
The second problem you face is that part of the reason the absorption is changing is also because of scattering from the nanoparticles resulting in a longer pathlength through the film.
It is often hard to do very very accurate photometric measurements, especially if there is light scattering involved. You can be limited by the dynamic range of the instruments. So for a student a few years ago, he compared the efficiency of the solar cells with and without plasmons using the electrical characteristics of the solar cell, rather than worrying about all the details of the optical absorption.
Some suggestions:
1) You can measure the refractive index of the matrix without particles and if the viscosity of the matrix of the solution is the same, you should be able to map out the thickness vs RPM pretty accurately. Then if you cross section the then films with nanoparticles and determine the thickness under an SEM or other method you could then spin films where you have reasonable confidence that you have adjusted the thickness to be the same in both cases. The you can make two transmission measurements and compare the absorption directly.
2) If you you have a dual beam spectrophotometer, you can compare the thin film with particles with the thin film that doesn't have nanoparticles, but you will want to make individual measurements, since if the film thicknesses are different, then you will need to correct for the interference fringes.
3) If you have an accurate way to measure the viscosity of the matrix with nanoparticles, you may be able to map out the thickness vs RPM curve and separately and then adjust the time for one spin as compared to the time of the other spin.
4) Some materials like photoresist have pretty good data about their refractive index etc. You could also us that as a reference to compare with and try mixing your particles with something well characterized like that.
The Swanepole paper is actually very good. You can learn a lot from it, and the formula's are relatively simple to implement in a program or matlab etc.
There are now fairly sophisticated fitting programs for thin films, if you have access to something like that it may be helpful too, but sometimes those types of software packages kind of break if the films have a lot of scattering.
Hope this helps it brings me back to my grad school days... It seems like a simple problem, but to really work it out in detail it is more subtle than 90 percent of people would think.
Dear colleague Shamjid Palappra,
Optical constants or optical parameters are functions of absorption. There are many ways to compare absorption enhancement the thin film material.
The simplest one, you can use Tauc relation and calculate the optical gap for each one. the attached paper may be useful.
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