Perovskite solar cells have been attracting the scientific world attention since 2009 due to outstanding absorption and charge transport properties. The possible ion migration, several ion types content and non-mono crystal structure make them really intriguing as well as challenging system to investigate by optical techniques. Electrons in the perovskite system, after light absorption, are promoted from the valence to the conduction band. First few hundreds of femtosecond after absorption are governed by cooling of hot carriers. When the process is finished, sharp absorption bleach due to the band filling phenomena occurs which decay correlates with photoluminescence kinetics and represents the excited carrier lifetime [1,2]. That decay proceeds by several paths such as the recombination (first-, second- and third-order) and charge injection to an electron transporting material (ETM) or a hole transporting material (HTM).        

We focused on a triple cation perovskite FA_0.76MA_0.19Cs_0.05(I_0.81Br_0.19)₃ sandwiched between a spiro-OMeTAD (HTM) and mesoporous TiO₂ (ETM) layers prepared under drybox (w/o oxygen and water) or ambient (in the presence of oxygen and ambient room humidity) conditions [3]. We performed femtosecond to nanosecond transient absorption as well as picosecond to nanosecond time-resolved emission studies of the prepared cells. We probed the cells from different sides to obtain information about the charge dynamics at ETM or HTM interfaces. The investigation was also supported by the electrochemical impedance and x-ray diffraction measurements.

The morphological studies indicate that the content of unreacted PbI₂ phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the ETM side than that at the HTM side for the cells prepared under ambient conditions, and the hole injection is faster for the solar cells with higher photocurrent in the cells prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution indicating a change in recombination mechanism from bulk to surface recombination [3].