Electron-donating strength dependent symmetry breaking charge transfer dynamics of quadrupolar molecules.

The excited state symmetry breaking charge transfer (SBCT) dynamics of two diacetylide-triphenylamine (DATPA) derivatives with different electron-donating abilities are investigated by femtosecond transient absorption and fluorescence spectroscopy. By tracking the evolution of the excited states by transient absorption spectra and the kinetics of the instantaneous emission dipole moments obtained from transient fluorescence spectroscopy, it is found that, in nonpolar solvent, the relaxed S1 state is quadrupolar with little change of emission dipole moments for the two molecules within 30 ps, whereas in polar solvent, the quadrupolar state evolves to a symmetry broken S1 state, in which, the emission dipole moment exhibits a fast reduction in the first few picoseconds. The larger reduction in emission transition dipole moment for the molecule with stronger electron-donating methoxy groups indicates a larger extent of symmetry breaking compared with the one with weak electron-donating methyl groups. Consequently, we revealed that the magnitude of symmetry breaking can be tuned by changing the electron-donors in quadrupolar molecules.

Effect of extending conjugation via thiophene-based oligomers on the excited state electron transfer rates to ZnO nanocrystals.

Oligothiophene dyes with two to five thiophene units were anchored to oleate-capped, quantum-confined zinc oxide nanocrystals (ZnO NCs) through a cyanoacrylate functional group. While the fluorescence of the bithiophene derivative was too weak for meaningful quenching studies, the fluorescence of the dyes with three, four and five thiophene rings was quenched upon binding to the NCs. Ultrafast pump-probe spectroscopy was used to observe the singlet excited states of the free dyes dissolved in dichloromethane as well as attached to a ZnO NC dispersed in the same solvent. When the dyes were bound to ZnO NCs, ultrafast spectroscopic measurements revealed rapid disappearance of the singlet excited state and appearance of a new transient absorption at higher energy that was assigned to the oxidized dye based on the similar absorption observed upon oxidation of the dye using nitrosonium ion. The appearance lifetimes of the oxidized dyes were assigned to the excited state electron transfer and were 36 ± 2, 22.3 ± 3.9, 26.5 ± 1.5 and 19.4 ± 0.8 ps for bi-, ter-, quarter- and quinquethiophene dyes respectively. Two factors contributed to the similarity in the electron transfer lifetime. First the excited state energies of the dyes were similar, and second, the free energy for electron transfer reaction was sufficiently large to move the event into the energy-independent regime.

Effect of alkyl chain length on the properties of triphenylamine-based hole transport materials and their performance in perovskite solar cells.

A new series of diacetylide-triphenylamine (DATPA) derivatives with five different alkyl chains in the para position, MeO, EtO, nPrO, iPrO and BuO, were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells (PSC) studied. Their thermal, optical and electrochemical properties were investigated along with their molecular packing and charge transport properties to analyse the influence of different alkyl chains in the solar cell parameters. The shorter alkyl chain facilitates more compact packing structures which enhanced the hole mobilities and reduced recombination. This work suggests that the molecule with the methoxy substituent (MeO) exhibits the best semiconductive properties with a power conversion efficiency of up to 5.63%, an open circuit voltage (Voc) of 0.83 V, a photocurrent density (Jsc) of 10.84 mA cm-2 and a fill factor of 62.3% in perovskite solar cells. Upon replacing the methoxy group with longer alkyl chain substituents without changing the energy levels, there is a decrease in the charge mobility as well as PCE (e.g. 3.29% for BuO-DATPA). The alkyl chain length of semiconductive molecules plays an important role in achieving high performance perovskite solar cells.

Interdye Hole Transport Accelerates Recombination in Dye Sensitized Mesoporous Films.

Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modeled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron-hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.

p-Type NiO Hybrid Visible Photodetector.

A novel hybrid visible-light photodetector was created using a planar p-type inorganic NiO layer in a junction with an organic electron acceptor layer. The effect of different oxygen pressures on formation of the NiO layer by pulsed laser deposition shows that higher pressure increases the charge carrier density of the film and lowers the dark current in the device. The addition of a monolayer of small molecules containing conjugated π systems and carboxyl groups at the device interface was also investigated and with correct alignment of the energy levels improves the device performance with respect to the quantum efficiency, responsivity, and photogeneration. The thickness of the organic layer was also optimized for the device, giving a responsivity of 1.54 × 10(-2) A W(-1) in 460 nm light.

Hole-transport materials with greatly-differing redox potentials give efficient TiO2-[CH3NH3][PbX3] perovskite solar cells.

Two diacetylide-triphenylamine hole-transport materials (HTM) with varying redox potential have been applied in planar junction TiO2-[CH3NH3]PbI3-xClx solar cells leading to high power-conversion efficiencies up to 8.8%. More positive oxidation potential of the HTM gives higher VOC and lower JSC illustrating the role of matching energy levels, however both HTMs gave efficient cells despite a difference of 0.44 V in their redox potentials.

Chalcogenophene comonomer comparison in small band gap diketopyrrolopyrrole-based conjugated polymers for high-performing field-effect transistors and organic solar cells.

The design, synthesis, and characterization of a series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) for use in field-effect transistors and organic photovoltaic devices are reported. The effect of the heteroatom substitution on the optical, electrochemical, and photovoltaic properties and charge carrier mobilities of these polymers is discussed. The results indicate that by increasing the size of the chalcogen atom (S < Se < Te), polymer band gaps are narrowed mainly due to LUMO energy level stabilization. In addition, the larger heteroatomic size also increases intermolecular heteroatom-heteroatom interactions facilitating the formation of polymer aggregates leading to enhanced field-effect mobilities of 1.6 cm(2)/(V s). Bulk heterojunction solar cells based on the chalcogenophene polymer series blended with fullerene derivatives show good photovoltaic properties, with power conversion efficiencies ranging from 7.1-8.8%. A high photoresponse in the near-infrared (NIR) region with excellent photocurrents above 20 mA cm(-2) was achieved for all polymers, making these highly efficient low band gap polymers promising candidates for use in tandem solar cells.

Effect of torsional twist on 2nd order non-linear optical activity of anthracene and pyrene tricyanofuran derivatives.

Tricyanofuran (TCF) derivatives attached to both anthracene and pyrene moieties were synthesised and characterised by optical, electrochemical and computational techniques. Both compounds exhibited similar absorption profile as well as electrochemical behaviour, however the pyrene derivative showed 20-fold higher non-linear optical activity measured by the EFISH technique. This huge difference has been assigned to (i) a lower molar absorption and (ii) a higher torsion angle for the anthracene derivative, confirmed by both experimental X-ray diffraction and DFT calculations. Furthermore, we note that the µß1.907 value of -1700 × 10(-48) esu recorded for the pyrene derivative in CHCl3/pyridine is remarkable for a NLO chromophore lacking a classical push-pull structure.

Oligothiophene interlayer effect on photocurrent generation for hybrid TiO(2)/P3HT solar cells.

A series of conjugated 3-hexylthiophene derivatives with a cyanoacrylic acid group has been prepared with conjugation length from one up to five thiophene units (1T-5T). The UV-vis spectra, photoluminescence spectra, electrochemical data and DFT calculations show lowering of LUMO energies and red-shift of absorption into the visible as the thiophene chain length increases. TiO2/P3HT solar cells were prepared with prior functionalization of the TiO2 surface by 1T-5T and studies include cells using undoped P3HT and using P3HT doped with H-TFSI. Without H-TFSI doping, photocurrent generation occurs from both the oligothiophene and P3HT. Doping the P3HT with H-TFSI quenches photocurrent generation from excitation of P3HT, but enables very effective charge extraction upon excitation of the oligothiophene. In this case, photocurrent generation increases with the light harvesting ability of 1T-5T leading to a highest efficiency of 2.32% using 5T. Overall, we have shown that P3HT can act in either charge generation or in charge collection, but does not effectively perform both functions simultaneously, and this illustrates a central challenge in the further development of TiO2/P3HT solar cells.

Distance dependent charge separation and recombination in semiconductor/molecular catalyst systems for water splitting.

The photoinduced reduction of three Co electrocatalysts immobilised on TiO2 is 10(4) times faster than the reverse charge recombination. Both processes show an exponential dependence on the distance between the semiconductor and the catalytic core.

Design of Os(II) -based sensitizers for dye-sensitized solar cells: influence of heterocyclic ancillaries.

A series of Os(II) sensitizers (TFOS-x, in which x=1, 2, or 3) with a single 4,4'-dicarboxy-2,2'-dipyridine (H2 dcbpy) anchor and two chelating 2-pyridyl (or 2-pyrimidyl) triazolate ancillaries was successfully prepared. Single-crystal X-ray structural analysis showed that the core geometry of the Os(II) -based sensitizers consisted of one H2 dcbpy unit and two eclipsed cis-triazolate fragments; this was notably different from the Ru(II) -based counterparts, in which the azolate (both pyrazolate and triazolate) fragments are located at the mutual trans-positions. The basic properties were extensively probed by using spectroscopic and electrochemical methods as well as time-dependent density functional theory (TD-DFT) calculations. Fabrication of dye-sensitized solar cells (DSCs) was then attempted by using the I(-) /I3 (-) -based electrolyte solution. One such DSC device, which utilized TFOS-2 as the sensitizer, showed promising performance characteristics with a short-circuit current density (JSC ) of 15.7 mA cm(-2) , an open-circuit voltage of 610 mV, a fill factor of 0.63, and a power conversion efficiency of 6.08 % under AM 1.5G simulated one-sun irradiation. Importantly, adequate incident photon-to-current conversion efficiency performances were observed for all TFOS derivatives over the wide spectral region of 450 to 950 nm, showing a panchromatic light harvesting capability that extended into the near-infrared regime. Our results underlined a feasible strategy for maximizing JSC and increasing the efficiency of DSCs.

Synthesis and optical characterisation of triphenylamine-based hole extractor materials for CdSe quantum dots.

We report the synthesis and optical characterisation of different triphenylamine-based hole capture materials able to anchor to CdSe quantum dots (QDs). Cyclic voltammetry studies indicate that these materials exhibit reversible electrochemical behaviour. Photoluminescence and transient absorption spectroscopy techniques are used to study interfacial charge transfer properties of the triphenylamine functionalized CdSe QDs. Specifically, we show that the functionalized QDs based on the most easily oxidised triphenylamine display efficient hole-extraction and long-lived charge separation. The present findings should help identify new strategies to control charge transfer QD-based optoelectronic devices.