We thoroughly investigated the carrier-phonon relaxation process in 2D halide perovskites with the general formula of (BA)₂(MA)_n-1Pb_nI_3n+1, where n = 2, 3, and 4, by femtosecond transient absorption spectroscopy. A significant enhancement of the hot phonon bottleneck effect is observed in the natural multiple quantum well with n = 2 and 3. Specifically, the sample with n = 3 shows a 1000 ps hot carrier relaxation time to reach room temperature, which is 10 times longer compared with its three-dimensional counterpart. We believe that both the organic cation and quantum confinement effect are responsible for this phenomenon. The acoustic phonon cannot propagate due to the decrease in group velocity caused by the confinement effect in such a quantum well structure. In addition, the confined acoustic phonons can up-convert to optical phonons due to the presence of a “hybrid phonon” induced by the organic cation. This result suggests a promising way to obtain long-live hot carrier materials.

Multiple quantum wells have been shown significantly reduced hot carrier cooling rates compared to bulk material and are thus a promising candidate for hot carrier solar cell absorbers. However, the mechanism(s) by which hot carrier cooling is restricted is not clear. A systematic study of carrier cooling rates in GaAs/AlAs multiple quantum wells (MQWs) with either varying barrier thickness or varying well thickness is presented in this paper. These allow an investigation as to whether the mechanisms of either a modification in hot carrier diffusion or a localisation of phonons emitted by hot carriers are primarily responsible for reduced carrier cooling rates. With the conclusion that for the structures investigated the situation is rather more complex with both carrier mobility to modify hot carrier diffusion, different diffusion rates for electrons and holes and reflection and localisation of phonons to enhance phonon bottleneck all playing their parts in modulating phonon reabsorption and hot carrier behaviour.