(a) A series of time-resolved PL decays from a CH3NH3PbI3 film measured over time under illumination. The sample was photoexcited with pulsed excitation (507 nm, 1 MHz repetition rate, 0.3 μJ cm−2 per pulse) and the emission was detected at 780 nm. (b) The PL over time under initial illumination determined from integrating acquired PL decays (black open symbols) or monitoring the PL count rate (black solid line). The red symbols are inverse trap densities 1/NT, where NT are extracted from the fits to the data in a (grey lines). Inset: open-circuit voltage (Voc) rise of a full solar cell with an illumination intensity comparable to full sunlight (532-nm cw laser, ∼60 mW cm−2). (c) The normalized integrated PL over time under illumination at different temperatures. Inset: temperature dependence of the rate of PL rise deduced from exponential fits to the data in the main panel (Supplementary Notes 5 and 6). The solid line represents a fit to the data using the Arrhenius relation to extract an activation energy (Ea=0.19±0.05 eV).