Ultrafast transient absorption (TA) technique allows unique investigation of perovskite material close to the interfaces with electron and hole transporting layers (ETL and HTL, respectively). It can be realized when the excitation pulse has short enough wavelength with respect to the perovskite absorption onset (yielding penetration depth < 50 nm) and when the probe pulse delay is short enough (<100 ps) that photoexcited electrons and holes are not yet redistributed by the diffusion over the whole perovskite thickness. As we have shown earlier, different charge cooling, charge recombination dynamics and different charge extraction rates to ETL and HTL can be observed [1,2].

However, such selective probing can also give valuable and insightful information about the structural differences at both interfaces, as well as photoinduced changes occurring close to ETL and HTL. It is possible due to intense and narrow TA bleach signal observed in perovskites [3] which are sensitive to the bandgaps of the bulk phase or low-dimensional phases. We will show our new findings related to photoinduced ion segregation in triple cation mixed halide perovskite as well low-dimensional phase distribution in quasi-2D perovskites, recently reported [4,5] or currently developed.

Regarding the ion segregation, we observed the strongest photoinduced redistribution of the halides near ETL, especially when mesoporous TiO₂ is used as an ETL. A surprising dependency of the ion segregation rate and extent on the excitation conditions (pulsed or continuous), pump fluence and temperature was revealed. For example, the segregation time can increase from 30 minutes at single μJ/cm² fluence to 100 minutes at 0.1 mJ/cm² fluence, and it can be ~3 times slower when the sample temperature is increases from room temperature by ~30K [4]. The results can be explained by the model including the local electric field and local heating effects.

As for 2D perovskites, an asymmetry of the quasi-2D phases distribution close to ETL and HTL was found (typically more low-dimensional phases are near the bottom layer [6]). The asymmetry depends on the preparation conditions of perovskite layer and can be correlated with the efficiency of the cell. Moreover, adding passivation MXene layer to HTL decreases the amount of quasi-2D phases in perovskite close to this layer. For both triple cation and 2D perovskites, the photostability and damage thresholds were also found.

Finally, novel findings related to the observation of coherent acoustic phonons will be also shown [4]. Upon the ultrashort pulse photoexcitation close to ETL or HTL, at certain conditions (e.g high pump fluences of single mJ/cm²) an acoustic wave propagation in perovskite [3] can be observed and the sound velocity in perovskite material can be determined from the oscillation pattern in TA signal. The differences in the TA oscillation mechanism between the bulk and 2D phases was also revealed.

In conclusion, our results show the great potential of using ultrafast TA to obtain many information in perovskite solar cells beyond the typical time-resolved findings (e.g. charge cooling, recombination and extraction times). Important phenomena related to the structure and its photoinduced modifications of perovskite near ETL and HTL interfaces can be exclusively investigated using ultrafast TA technique. Many of them are hard or impossible to be studied by other commonly used structural techniques that only probe the average response of the whole perovskite layer.