Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.334
Publication date: 6th July 2018
Due to their sizable refractive index, reflectivity of visible light off the lead halide perovskite - air interface exceeds 15%. This has prompted a number of investigations into the prominence of photo-reflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI3) perovskite. The rationale is to compare pump-probe measurements samples where the relative contributions of changes in the real and imaginary components of the refractive index are very different. The absorption cross section of a nanocrystal of volume v in terms of its complex refractive index and that of its (non-absorbing) surroundings n0 is
is a local field factor which must be included since it depends on the same refractive indices. For a polycrystalline film or a crystal, reflectivity at the interface with a non-absorbing dielectric whose refractive index is n0, is given by the Fresnel relation {2}:
The first derivatives of sabs or of R (both of which reduce transmission) with respect to n or k quantify their sensitivity to variations in either constant. By analyzing how complex refractive index changes impact the two experiments we can show that reflectivity changes due to variations in n would not only differ in amplitude but even in sign in both experiments. The results presented in the figure below demonstrate virtually identical TA data for both samples proving that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments is exclusively due to, or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide perovskite film was formed on high index flint glass, and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump-probe experiments on thin perovskite films in terms of photo-induced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photo-reflective effects.
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