The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO 2 and ZrO 2 thin films, all in contact with air, using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from r70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149 + radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149 + in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149 + -electron complex to isolated D149 + and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149 + by interactions with ZnO. TiO 2 exhibits similar dynamics to ZnO. In the case of ZrO 2 , electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149 + in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D 0 -D 1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149 + spectrum employing the MPW1K functional.

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO 2 and ZrO 2 thin films. In acetonitrile, upon photoexcitation of the dye via the S 0 -S 1 transition, we observed ultrafast solvation dynamics with subpicosecond time constants.Subsequent decay of the S 1 excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S 0 . On TiO 2 , we observed fast (o90 fs) electron injection from the S 1 state of D35 into the TiO 2 conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S 0 -S 1 absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35 + radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO 2 , the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO 2 and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 ms, 170 ms and 1.4 ms. For D35 adsorbed on ZrO 2 , there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons.Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S 1 which decay by IC to S 0 .Differential steady-state absorption spectra of D35 + in acetonitrile and dichloromethane provide access to the complete D 0 -D 1 band. The absorption spectra of D35 and D35 + are well described by TDDFT calculations employing the MPW1K functional.

We provide an investigation of the charge carrier dynamics of the (MAI)(x)(PbI2)(1-x) system in the range x = 0.32-0.90 following the recently published "pseudobinary phase-composition processing diagram" of Song et al. (Chem. Mater., 2015, 27, 4612). The dynamics were studied using ultrafast pump-supercontinuum probe spectroscopy over the pump fluence range 2-50 μJ cm(-2), allowing for a wide variation of the initial carrier density. At high MAI excess (x = 0.90), low-dimensional perovskites (LDPs) are formed, and their luminescence spectra are significantly blue-shifted by ca. 50 nm and broadened compared to the 3D perovskite. The shift is due to quantum confinement effects, and the inhomogeneous broadening arises from different low-dimensional structures (predominantly 2D, but presumably also 1D and 0D). Accurate transient carrier temperatures are extracted from the transient absorption spectra. The regimes of carrier-carrier, carrier-optical phonon and acoustic phonon scattering are clearly distinguished. Perovskites with mole fractions x ≤ 0.71 exhibit extremely fast carrier cooling (ca. 300 fs) at low fluence of 2 μJ cm(-2), however cooling slows down significantly at high fluence of 50 μJ cm(-2) due to the "hot phonon effect" (ca. 2.8 ps). A kinetic analysis of the electron-hole recombination dynamics provides second-order recombination rate constants k2 which decrease from 5.3 to 1.5 × 10(-9) cm(3) s(-1) in the range x = 0.32-0.71. In contrast, recombination in the LDPs (x = 0.90) is more than one order of magnitude faster, 6.4 × 10(-8) cm(3) s(-1), which is related to the confined perovskite structure. Recombination in these LDPs should be however still slow enough for their potential application as efficient broadband emitters or solar light-harvesting materials.