The formation of a polaron, a charged quasi-particle, may account for the high power conversion efficiency (~25 %) achieved in hybrid organic-inorganic perovskite solar cells. In according to polaron screening effect hypothesis, the electric dipole reorientations of the organic cations and the structural distortions of the inorganic frameworks are the key factors to have a long-lived polaronic excited state associated with high power conversion efficiencies. This also implies that hybrid organic-inorganic perovskites have different energy minima between the neutral ground state and polaronic excited-state potential energy surfaces along with polaron formation coordinates. However, most of the studies on the polaron are based on theoretical results, and a few experimental results have shown excited-state structural changes. Thus, it is necessary to have more experimental evidence to disclose polaron structures and formation dynamics.
Recently, we have studied coherent nuclear wave packets of Pb-I framework (oscillating at <100 cm-1) employing femtosecond transient absorption spectroscopy and first-principles density functional theory. Our results clearly present the experimental evidence on the structural displacements between the ground and excited states. In addition, we have found that these wave packets turn to be correlated to the polaron formation dynamics.