Search

The stability of dye cations against recombination with conduction band electrons in mesoporous TiO2 electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO2 conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO2 surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a “non-injecting” Al2O3 surface generating a radical cation–radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S1/ICT. [ABSTRACT FROM AUTHOR]

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 [D49Ṗ+…e−] complex and a transient Stark effect signalling the formation of mobile electrons upon dissociation of the complex. [ABSTRACT FROM AUTHOR]

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 cm3 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 cm3 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. [ABSTRACT FROM AUTHOR]

Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a 'non-injecting' mesoporous thin film with large band-gap, consisting of Al2O3 nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250-350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0 → S1 absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+ [N(CN)2] - show that D149 is ideally suited for solar cell applications because of its long S1 lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim]+[B(CN)4]− using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6Ṗ+ radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S0→ S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (＜80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6Ṗ+ radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6Ṗ+ absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron–cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S1 quickly decays to S0 (time constants 1 and 35 ps). [ABSTRACT FROM AUTHOR]

The stability of dye cations against recombination with conduction band electrons in mesoporous TiO 2 electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO 2 conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO 2 surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a "non-injecting" Al 2 O 3 surface generating a radical cation-radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S 1 /ICT.

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 Al 2 O 3 and TiO 2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramolecular charge transfer character ("S 1 /ICT"). On Al 2 O 3 and in the three organic solvents, the dynamics are fully explained in terms of S 1 /ICT stabilisation (by reorientation of adjacent solvent or D49 molecules and collisional cooling), intramolecular vibrational redistribution and S 1 /ICT → S 0 electronic decay. A substantial decrease of the S 1 /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 Al 2 O 3 surface leads to a substantial increase of the S 1 /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 Al 2 O 3 , 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 Al 2 O 3 trap states. Charge separation is only observed for D49 bound to TiO 2 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.