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

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 (&sim;0.2 &mu;C/ cm2) and another, larger charge (40 &mu;C/cm2), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of &sim;5, independent of bias light intensity. The fast decay (&sim;1 &mu;s 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&minus;V hysteresis. Capacitance vs open circuit voltage (Voc) 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 Voc hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at Voc 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&minus;V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.</div

Rutherford Backscattering Spectroscopy of Mass Transport by Transformation of PbI2 into CH3NH3PbI3 within np TiO2

Mass transport during transformation of PbI2 infiltrated in nanoporous TiO2 into CH3NH3PbI3 has been investigated by Rutherford backscattering spectroscopy RBS . Fast initial reaction kinetics were confirmed using optical ex situ and in situ measurements. Mapping with energy dispersive X ray spectroscopy of the cross section of samples revealed a homogeneous PbI2 infiltration in nanoporous TiO2 before transformation but an accumulation of Pb and I at the surface after transformation, in accordance with a depletion of Pb and I in a near surface region. Quantitative depth profiles of Pb and I were obtained from RBS analysis. An instant degradation of CH3NH3PbI3 to PbI2 and volatiles upon ion radiation was found. The concentration profiles of Pb could be simulated with a one dimensional diffusion model taking into account an effective diffusion coefficient of Pb in the nanocomposite about 1.5 amp; 8901;10 11 cm2 s as well as a parameter considering frazzling at the surface due to formation of crystallite

Hole blocking PbI2 CH3 NH3PbI3 interface

Modulated charge separation across MO CH3NH3PbI3 and MO PbI2 CH3NH3PbI3 MO TiO2, MoO3 interfaces was investigated by surface photovoltage SPV spectroscopy. Perovskite layers were deposited by solution based one step preparation and two step preparation methods. An unreacted PbI2 layer remained at the interface between the metal oxide and CH3NH3PbI3 for two step preparation. For the two step preparation on TiO2, the SPV signal related to absorption in CH3NH3PbI3 increased in comparison to the one step preparation due to electron transfer from CH3NH3PbI3 via PbI2 into TiO2 whereas the SPV signal related to defect transitions decreased. For the one step preparation on MoO3, holes photogenerated in CH3NH3PbI3 recombined with electrons in MoO3. In contrast, a hole transfer from CH3NH3PbI3 towards MoO3 was blocked by the PbI2 interlayer for the two step preparation on MoO