Dynamics of Photocarrier Separation in MAPbI3 Perovskite Multigrain Films under a Quasistatic Electric Field

Applying time-resolved electroabsorption spectroscopy for the first time to methylammonium lead triiodide perovskite (MAPbI(3)) thin films under reverse bias, we monitored optically the ultrafast evolution of the local counter-electric field produced by the drift of photogenerated electrons and holes in opposite directions. Under an externally applied electric field of vertical bar E vertical bar < 10(5) V cm(-1), the carriers were found to reach a separation of 40 nm within similar to 1 ps. This distance corresponds to the average dimensions of crystalline grains in the active film, at the boundaries of which charges were trapped. An intragrain average carrier drift mobility of mu(+/-) = 23 cm(2) V-1 s(-1) was inferred. Subsequent charge detrapping, migration through the entire film, and accumulation at its insulated surfaces caused a blue shift of the perovskite absorption edge that arose within tens of piceseconds, owing to a trap-limited electron drift mobility mu(n) = 6 cm(2) V-1 s(-1). Charge recombination was entirely suppressed between,field-separated photocarriers generated at initial densities of n(0) <= 2 x 10(16) cm(-3). Accnmulation of electrons at the interface between a mesoporous TiO2 electron-transport layer and a multigrain MAPbI(3) film was also observed, which was indicative of delayed charge injection through a poor contact junction.