Hot carrier cooling and direct observation of electron–phonon coupling in two-dimensional butylammonium lead iodide perovskites

Abstract

Understanding the charge carrier dynamics in emerging two-dimensional (2D) lead halide perovskites (LHPs) is crucial to revealing the factors responsible for their high photovoltaic efficiency. We report a series of femtosecond transient absorption (TA) measurements of two-dimensional lead halide perovskite thin films upon photoexcitation above their band gap. Results from the TA measurements of the 2D (BA)₂(MA)_n−1Pb_nI_3n+1 (BA = n-butylammonium cation, MA = methylammonium cation) perovskite films of varying inorganic layers, n = 1, 2, and 3 have been analyzed to estimate the hot carrier cooling time constants and achieve direct monitoring of electron–phonon coupling. Global analysis of the TA data of the 2D perovskite films suggests that the hot carrier cooling to the band edge takes place with a time constant of ∼0.4 ps. The transient absorption spectral evolutions during the carrier cooling stage are unique for n = 1 films; however, for n > 1 the spectral changes are similar to that of the 3D perovskite films. Furthermore, analysis of the TA data further revealed weak periodic modulation of the transient absorption kinetic traces. These periodic modulations were isolated and Fourier transform analysis of the residual modulation provides frequency of the optic phonons in the range: 35–120 cm⁻¹, assigned to various modes of Pb–I inorganic framework. The high signal-to-noise (S/N) transient absorption spectroscopy used here enabled us to directly detect the electron–phonon couplings in the 2D perovskite thin films at room temperature.