Ultrafast photoinduced dynamics of the organolead trihalide perovskite CH 3 NH 3 PbI 3 on mesoporous TiO 2 scaffolds in the 320-920 nm range

Abstract

We present femtosecond broadband transient absorption experiments for the investigation of the carrier dynamics in the organolead trihalide perovskite CH 3 NH 3 PbI 3. The perovskite was prepared on a mesoporous TiO 2 scaffold either by 1-step deposition from solution or by 2-step methods employing deposition of lead iodide followed by an on-surface reaction with methylammonium iodide. The thin films were characterized by XRD and FTIR chemical mapping. After pumping with an ultrashort laser pulse at 400 or 500 nm, the dynamics were monitored by a broadband supercontinuum reaching from the near IR (920 nm) into the UV. Specifically, the usage of quartz substrates and thin perovskite/TiO 2 films enabled us to cover the spectral development down to 320 nm. The charge carrier dynamics were largely independent from the specific route of perovskite preparation: initial ultrafast carrier relaxation steps with time constants τ CC and τ COP of <0.08, 0.2 and 2.6 ps were assigned to carrier-carrier and carrier-optical phonon scattering. Pronounced sub-band-gap absorption was found in the near IR at early times. Transient carrier temperatures were extracted from a Boltzmann fit to the blue wing of the photobleach band in the time range 0.2-700 ps, allowing us to distinguish between the decay of acoustic phonons (τ AP = 50 and >1000 ps) and Auger recombination (τ AR = 9, 75 and 450 ps). Carrier relaxation was accompanied by formation of an absorption band around 550 nm, with a characteristic structure assignable to a transient Stark effect, i.e. a red-shift of the perovskite spectrum due to the appearance of a directed electric field in the material and possibly additional influence of lattice heating. We observed a substantial Stokes shift between the relaxed photobleach and photoluminescence bands. Contributions of unreacted PbI 2 to the transient absorption features appear to be negligible.