Charge carrier dynamics of methylammonium lead iodide: from PbI2-rich to low-dimensional broadly emitting perovskites

Abstract

We provide an investigation of the charge carrier dynamics of the (MAI)_x(PbI₂)_1−x system in the range x = 0.32–0.90 following the recently published “pseudobinary phase-composition processing diagram” of Song et al. (Chem. Mater., 2015, 27, 4612). The dynamics were studied using ultrafast pump-supercontinuum probe spectroscopy over the pump fluence range 2–50 μJ cm⁻², allowing for a wide variation of the initial carrier density. At high MAI excess (x = 0.90), low-dimensional perovskites (LDPs) are formed, and their luminescence spectra are significantly blue-shifted by ca. 50 nm and broadened compared to the 3D perovskite. The shift is due to quantum confinement effects, and the inhomogeneous broadening arises from different low-dimensional structures (predominantly 2D, but presumably also 1D and 0D). Accurate transient carrier temperatures are extracted from the transient absorption spectra. The regimes of carrier–carrier, carrier–optical phonon and acoustic phonon scattering are clearly distinguished. Perovskites with mole fractions x ≤ 0.71 exhibit extremely fast carrier cooling (ca. 300 fs) at low fluence of 2 μJ cm⁻², however cooling slows down significantly at high fluence of 50 μJ cm⁻² due to the “hot phonon effect” (ca. 2.8 ps). A kinetic analysis of the electron–hole recombination dynamics provides second-order recombination rate constants k₂ which decrease from 5.3 to 1.5 × 10⁻⁹ cm³ s⁻¹ in the range x = 0.32–0.71. In contrast, recombination in the LDPs (x = 0.90) is more than one order of magnitude faster, 6.4 × 10⁻⁸ cm³ s⁻¹, which is related to the confined perovskite structure. Recombination in these LDPs should be however still slow enough for their potential application as efficient broadband emitters or solar light-harvesting materials.