Intramolecular and interfacial dynamics of triarylamine-based hole transport materials.

We present steady-state and time-resolved spectroscopic investigations combined with detailed kinetic modelling for the triarylamine derivatives X60 and PTAA which serve as hole transport materials (HTMs) in perovskite solar cells and represent model compounds for organic electronic materials. Photoexcitation of the spiro-fluorene-xanthene X60 populates its S1 state which decays via intersystem crossing (ISC) as well as fluorescence and internal conversion on a nanosecond time scale. Photoexcitation of the PTAA polymer leads to the formation of singlet excitons which relax and migrate in the time regime from a few to about a hundred picoseconds and decay to the ground state with a lifetime of ca. 900 ps. Both X60 and PTAA exhibit efficient photoinduced electron injection into mesoporous TiO2 thin films forming radical cations with characteristic spectral bands, as confirmed by spectroelectrochemistry in solution. Photoluminescence (PL) experiments performed for the HTMs on methylammonium lead iodide perovskite deposited on mesoporous TiO2 show that small molecular HTMs, such as X60, quench the PL much better than the PTAA polymer. Ultrafast transient absorption experiments on the other hand suggest that the hole transfer at the interface between the perovskite and these HTMs is very fast, regardless of the type of HTM. It is therefore concluded that small molecular HTMs infiltrate much better into mesoporous structures and therefore more efficiently accept holes from the perovskite on such thin film architectures due to the better interfacial contact compared with their polymer-based counterparts.

Photoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin films.

We present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterise its photophysics in different organic solvents and on mesoporous Al2O3 and TiO2 thin films. Photoexcitation of H101 at 400 nm in organic solvents populates the S1 state which shows intramolecular relaxation on a picosecond time scale. Branching from the relaxed S1 state leads to population of the T1 triplet state and the ground electronic state S0. Triplet formation is induced by the internal heavy-atom effect of the sulphur atom, and the triplet yield decreases substantially with solvent polarity. On mesoporous Al2O3, intermolecular exciton-splitting is observed leading to the formation of a radical cation - radical anion pair (H101(S1) + H101(S0) → H101˙+ + H101˙-) followed by exciton recombination. On mesoporous TiO2, efficient electron injection is observed in addition to exciton-splitting. Complementary spectroelectrochemistry experiments enable a full spectral characterisation of the cation species H101˙+ and H1012+. Extensive DFT/TDDFT calculations successfully assign the spectral features of all experimentally observed species. Implications for the function of H101 in photovoltaic devices are discussed.

Quantifying ultrafast charge carrier injection from methylammonium lead iodide into the hole-transport material H101 and mesoporous TiO2 using Vis-NIR transient absorption.

Organic-inorganic hybrid lead halide perovskites already reach very high power conversion efficiencies above 22% on architectures employing mesoporous TiO2, but the carrier injection processes across the different interfaces are still not fully understood. Here we use ultrafast broadband transient absorption spectroscopy to determine time constants and yields for hole and electron injection. We show that hole transfer from the perovskite valence band (VB) to the hole-transport material (HTM) H101 at the perovskite/HTM interface occurs in less than 500 fs, but is limited by imperfections of the contact layer and poor infiltration of the HTM into the mesoporous structure. Electron injection from the perovskite conduction band (CB) into the CB of mesoporous TiO2 is only a small channel (25%). Electron transport inside mesoporous MAPI/TiO2 architectures therefore mainly occurs via the perovskite. We also show that electron injection from H101 into the perovskite is feasible for excitation at 400 nm resulting in light-harvesting of high-energy photons by the HTM. Accurate absolute NIR absorption coefficients for CB electrons in mesoporous TiO2 are provided.

Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films.

We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous Al2O3 and TiO2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramolecular charge transfer character ("S1/ICT"). On Al2O3 and in the three organic solvents, the dynamics are fully explained in terms of S1/ICT stabilisation (by reorientation of adjacent solvent or D49 molecules and collisional cooling), intramolecular vibrational redistribution and S1/ICT → S0 electronic decay. A substantial decrease of the S1/ICT lifetime is observed with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addition of the nonpolar co-adsorbent deoxycholic acid (DCA) to the Al2O3 surface leads to a substantial increase of the S1/ICT lifetime. DCA spacers reduce the local polarity around the dye molecules, thus interrupting D49 "self-solvation". These results are in contrast to a recent experimental study for the indoline dye D131 on Al2O3, where charge transfer from electronically excited D131 to adjacent dye molecules was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 molecules and also not for electron injection from D49 into Al2O3 trap states. Charge separation is only observed for D49 bound to TiO2 thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49˙+e-] complex and a transient Stark effect signalling the formation of mobile electrons upon dissociation of the complex.

Charge carrier dynamics of methylammonium lead iodide: from PbI2-rich to low-dimensional broadly emitting perovskites.

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.

Ultrafast photoinduced dynamics of the organolead trihalide perovskite CH3NH3PbI3 on mesoporous TiO2 scaffolds in the 320-920 nm range.

We present femtosecond broadband transient absorption experiments for the investigation of the carrier dynamics in the organolead trihalide perovskite CH3NH3PbI3. The perovskite was prepared on a mesoporous TiO2 scaffold either by 1-step deposition from solution or by 2-step methods employing deposition of lead iodide followed by an on-surface reaction with methylammonium iodide. The thin films were characterized by XRD and FTIR chemical mapping. After pumping with an ultrashort laser pulse at 400 or 500 nm, the dynamics were monitored by a broadband supercontinuum reaching from the near IR (920 nm) into the UV. Specifically, the usage of quartz substrates and thin perovskite/TiO2 films enabled us to cover the spectral development down to 320 nm. The charge carrier dynamics were largely independent from the specific route of perovskite preparation: initial ultrafast carrier relaxation steps with time constants τCC and τCOP of <0.08, 0.2 and 2.6 ps were assigned to carrier-carrier and carrier-optical phonon scattering. Pronounced sub-band-gap absorption was found in the near IR at early times. Transient carrier temperatures were extracted from a Boltzmann fit to the blue wing of the photobleach band in the time range 0.2-700 ps, allowing us to distinguish between the decay of acoustic phonons (τAP = 50 and >1000 ps) and Auger recombination (τAR = 9, 75 and 450 ps). Carrier relaxation was accompanied by formation of an absorption band around 550 nm, with a characteristic structure assignable to a transient Stark effect, i.e. a red-shift of the perovskite spectrum due to the appearance of a directed electric field in the material and possibly additional influence of lattice heating. We observed a substantial Stokes shift between the relaxed photobleach and photoluminescence bands. Contributions of unreacted PbI2 to the transient absorption features appear to be negligible.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile.

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 (<90 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 µs, 170 µs 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.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films.

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 ≤70 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.