  In hybrid lead halide perovskites, the coupling between photogenerated
charges and the ionic degrees of freedom plays a crucial role in defining the
intrinsic limit of carrier mobility and lifetime. However, direct investigation
of this fundamental interaction remains challenging because its relevant
dynamics occur on ultrashort spatial and ultrafast temporal scales. Here, we
unveil the coupled electron-lattice dynamics of a CH3NH3PbI3 single crystal
upon intense photoexcitation through a unique combination of ultrafast electron
diffraction, time-resolved photoelectron spectroscopy, and time-dependent ab
initio calculations. We observe the structural signature of a hot-phonon
bottleneck effect that prevents rapid carrier relaxation, and we uncover a
phonon avalanche mechanism responsible for breaking the bottleneck. The
avalanche involves a collective emission of low-energy phonons - mainly
associated with the organic sub-lattice - that proceeds in a regenerative
manner and correlates with the accumulation and confinement of photocarriers at
the crystal surface. Our results indicate that in hybrid perovskites carrier
transport and spatial confinement are key to controlling the electron-phonon
interaction and their rational engineering is relevant for future applications
in optoelectronic devices.
