Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy

We
have developed a new noninvasive optical method for monitoring
charge carrier diffusion and mobility in semiconductor thin films
in the direction perpendicular to the surface which is most relevant
for devices. The method is based on standard transient absorption
measurements carried out in reflectance and transmittance modes at
wavelengths below the band gap where the transient response is mainly
determined by the change in refractive index, which in turn depends
on the distribution of photogenerated carriers across the film. This
distribution is initially inhomogeneous because of absorption at the
excitation wavelength and becomes uniform over time via diffusion.
By modeling these phenomena we can determine the diffusion constant
and respective mobility. Applying the method to a 500 nm thick triple
cation FAMACs perovskite film revealed that homogeneous carrier distribution
is established in few hundred picoseconds, which is consistent with
mobility of 66 cm2 (V s)−1.