Evolution of the Dynamics of As-Deposited and Annealed Lead Halide Perovskites

The rapid rise of organolead trihalide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices and lasers due to their unique and promising optical properties. Evolution of the photophysical properties in as-deposited or annealed CH3NH3PbBr3 and CH3NH3PbI3 perovskite films processed through the interdiffusion method has been investigated. Absorption spectra showed broad band edge saturation in the as-deposited films in contrast to sharp excitonic absorption in the annealed films. Fluorescence emission of the perovskite films showed strong dependence on the halogen type with a very high quantum yield of ∼90% for the annealed CH3NH3PbBr3 film. An explanation for this was provided based on its crystallinity and quantum confinement of the excitons. The emission showed weakly Stokes shifted bands. Time-resolved spectroscopic measurements were carried out to probe the ultrafast dynamics for the perovskites for the as-deposited or annealed films. We classified the evolution in the absorption features in the excited state of CH3NH3PbBr3 perovskite films for the first time and compared them to CH3NH3PbI3. We suggest a bleach feature below 400 nm as the charge transfer band, which results in the photoinduced absorption in the CH3NH3PbBr3 perovskite film, a charge-separated band gap state, and the existence of intermediate excited-state species that regenerate the ground state.