Dear SAURAV,

You need to illuminate your film with a source of light whose wavelength is greater than the energy gap of the perovskite material, then observe the emitted photons from the thin film. It is expected that the emitted light has a wavelength corresponding to the energy gap of the material.

If you can not observe the emitted light it can that it is very weak below the detection limit of your instrument provided you instrument is responsive to wavelengths of the PL.

Your light source may be weak or your material has small lifetime such that the nonradiative recombination dominates and the specimen look dark.

So, you have to investigate the causes of this result.

Best wishes

Dear Saurav,

Firstly you need to measure the absorption spectra of your films. Then the maximum absorption peak wavelength can be determined. Ideally you should use this wavelength or close to this wavelength as an excitation wavelength for your PL measurement.

Secondly, you should make sure that the excitation wavelength beam on your sample film. The excitation beam may be at the UV region. It is difficult to see the beam whether on the sample or not. If it is this case, you can set the wavelength at 500nm green light to align the sample, then set the excitation wavelength back to the maximum absorption peak.

Hope it helps.

Best wishes

Xuhua

Firstly you take the absorption spectra of methylammonium lead iodide solution. First, MA+ groups at room temperature exhibit strongly dynamic disorder that could invalidate the assumption in the first-principles calculations and lead to zero averaged polarization in a macro scale, even though a dipole moment might be formed at the unit-cell scale. Second, as a narrow-bandgap semiconductor, methylammonium lead iodide thin films investigated typically contain a high density of charge carriers with reasonable high mobility. Preferably you should utilize this wavelength or near this wavelength as an excitation wavelength for your PL estimation.

Dear Saurav Ojha ,

When you already know everything that Abdelhalim Zekry , Xuhua Wang and Rakesh Sonker advised you, there are also some instrument tips to consider when you need to measure low PL signals of thin films.

- First you need to be sure that your sample is placed correctly in your instrument. As excitation and detection optical axis are typicaly at an right angle your sample should be placed at about 45 deg to both (be careful not to direct the reflected excitation light to the detecting monochromator slit).

- Second thing that you can consider is to widen the monochromators slits excitation intensity is higher and also more photons can reach the detector. It will lower your spectral resolution but it is a good first try to find your peak.

- another thing is playing with scan rate the and integration time. Finding good parameters can enhance the signal to noise ratio.

good luck,

Adam

Dear Saurav,

I had found also several problems in perovskite thin films. First of all, I think that light intensity in spectrofluorometer is not enough to obtain good spectrum (due to glass and perovskite surface reflection – I got really bad signal to noise ratio) and also finding focal point without holder is also really demanding and annoying. I had troubles with reflected light facing detector – so exhausting. I recommend you laser or LED and spectrofluorometer like Ocean - in this case you can adjust optical fiber to "see" excitation point in perovskite film or setup with super-sensitive photodiode like avalanche photodiode in time-corelated single photon counting system. I got quite nice spectra this way (Ocean with laser and TCSPC) . Good luck!