title: Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO₂: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J-V Hysteresis.
abstract: Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 μC/cm(2)) and another, larger charge (40 μC/cm(2)), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 μs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J-V hysteresis. Capacitance vs open circuit voltage (V(oc)) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the V(oc) hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at V(oc) suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J-V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.
authors.affiliation: †Chemistry Department, Imperial College, 1 Exhibition Road, London SW7 2AZ, U.K.
authors.forename: Brian C
authors.initials: BC
authors.lastname: O'Regan
authors.affiliation: ‡Physics Department, Imperial College, 1 Exhibition Road, London SW7 2AZ, U.K.
authors.forename: Piers R F
authors.initials: PR
authors.lastname: Barnes
authors.affiliation: †Chemistry Department, Imperial College, 1 Exhibition Road, London SW7 2AZ, U.K.
authors.forename: Xiaoe
authors.initials: X
authors.lastname: Li
authors.affiliation: †Chemistry Department, Imperial College, 1 Exhibition Road, London SW7 2AZ, U.K.
authors.forename: Chunhung
authors.initials: C
authors.lastname: Law
authors.affiliation: §Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans,16, Tarragona E-43007, Spain.|∥ICREA, Passeig Lluis Companys, 23, E-08010 Barcelona, Spain.
authors.forename: Emilio
authors.initials: E
authors.lastname: Palomares
authors.affiliation: §Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans,16, Tarragona E-43007, Spain.
authors.forename: Jose M
authors.initials: JM
authors.lastname: Marin-Beloqui