[Show abstract][Hide abstract] ABSTRACT: We measured the rate of hole hopping between dye molecules on titanium dioxide nanocrystals using cyclic voltammetry. Dyes commonly used in the field of dye sensitized solar cells exhibited efficient intermolecular charge transport, showing apparent diffusion coefficient values between 10−8 up to over 10−7 cm2 s−1 at room temperature. From temperature dependent measurements, we observed that hole transport across dye monolayers is a thermally activated process with Arrhenius activation energies between about 170 and 370 meV depending on the dye. Analysis of the data in terms of non-adiabatic Marcus theory of charge transfer enabled the estimation of the reorganization energy (740-1540 meV) and of an effective electronic coupling for the different systems. The measured reorganization energies show reasonable agreement with values obtained from density functional theory based calculations, validating our computational approach. Finally, we interpret the experimental and calculated data with reference to the chemical structure of the dyes and to the packing of the dyes on the surface of the TiO2 and suggest that delocalization of the HOMO and rigidity of the conjugated molecular structure result respectively in lower outer and inner sphere reorganization energies.
Chemical Science 01/2014; 5(1):281. DOI:10.1039/c3sc52359d · 9.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rigid rod polyisocyanopeptides bearing phthalocyanines as pendant groups have been synthesised through CuAAC of polyisocyanopeptides containing acetylene groups with zinc(II) phthalocyanine azide. As confirmed by UV/Vis, fluorescence, and circular dichroism spectroscopies, the phthalocyanines are arranged in a helical fashion along the polymer backbone, forming the longest reported well-defined phthalocyanine assembly described to date.
[Show abstract][Hide abstract] ABSTRACT: Anchors away! Two zinc phthalocyanine photosensitizers with different phosphinic acid anchor groups were synthesized. Solar cells sensitized with these compounds have a short-circuit photocurrent density of 7.6±0.2 mA cm(-2) , an open-circuit voltage of 559±30 mV, and a fill factor of 0.76±0.03, which corresponds to an overall conversion efficiency of 3.24 % under 1 sun.
[Show abstract][Hide abstract] ABSTRACT: The synthesis of stopcocks based on zinc phthalocyanine for selective adsorption at the channel entrances of zeolite L is reported. The introduction of either an inert SiMe₃ moiety, an imidazolium cation or a reactive isothiocyanate (NCS) group allows attachment to the channel entrances of zeolite L through van der Waals interactions, electrostatic interactions, or covalent binding, respectively. Stopcocks that rely on van-der-Waals-driven adsorption require careful selection of the solvent used for the deposition onto the zeolite surface to avoid a nonspecific distribution of the molecules. Regarding the design of photonic antenna systems, a stopcock with a cationic tail was found to be the most convenient, based on the observation that efficient energy transfer from molecules located in the zeolite nanochannels is more readily obtained than in the other cases.
Chemistry - A European Journal 02/2011; 17(6):1855-62. DOI:10.1002/chem.201002210 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Six supramolecular electron donor–acceptor hybrids, based on a ruthenium(II) phthalocyanine [RuPc] coordinating different dendritic oligothiophene (DOT) ligands [Py-nT] (n = 3, 9, 21) in either one [RuPcCO(Py-nT)] or two [RuPc(Py-nT)2] axial positions, have been characterized by standard spectroscopic methods and their photophysical behavior has been established by using ultrafast and fast time-resolved techniques. Based on the spectrochemical and radiolytically generated [Py-nT] (i.e., one-electron reduction of [Py-nT]) and [RuPcCO(Py) or RuPc(Py)2] (i.e., one-electron oxidation of [RuPcCO(Py) or RuPc(Py)2]) features, the deactivation processes were assigned to a solvent independent energy transfer in RuPcCO(Py-3T) and RuPc(Py-3T)2 and a strongly solvent dependent charge transfer mechanism, which competes with the energy transfer and the intersystem crossing for RuPcCO(Py-9T), RuPc(Py-9T)2, RuPcCO(Py-21T) and RuPc(Py-21T)2.
[Show abstract][Hide abstract] ABSTRACT: In this study, the injection efficiencies and photovoltaic device performances for two different phthalocyanine sensitisers—a Zn(II)Pc (TT-1) and a Ru(II)Pc (TT-35) in dye sensitized photoelectrochemical solar cells were compared. These dyes have similar structures and energetics, but differ significantly in their photophysics, with TT-1 exhibiting a reasonably long lived singlet state, whilst TT-35 exhibits rapid intersystem crossing to a long lived triplet state. Time correlated single photon counting (TCSPC) approach and incident photon conversion efficiency (IPCE) measurements were applied to study the injection efficiency of these two Pc dyes. A comparison of the injection efficiency determined by the two independent techniques, TCSPC and IPCE analysis, shows a good agreement. TT-35 shows higher injection efficiency in comparison to TT-1. This result is consistent with the relative energy and lifetime of the TT-35 triplet state compared to the TT-1 singlet excited state. The high injection efficiency and the long electron diffusion length shown by TT-35 make this dye an interesting red absorbing sensitizer for dye solar cells.
[Show abstract][Hide abstract] ABSTRACT: A supramolecular phthalocyanine-squaraine ensemble which exhibits a large coverage of the solar spectrum from 850 to 250 nm has been prepared and characterized both photophysically and in bulk heterojunction solar cells.
Chemical Communications 09/2009; DOI:10.1039/b903534f · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The development of efficient red sensitizer dyes is essential for the optimization of dye-sensitized photoelectrochemical solar cells. Ru-phthalocyanines are good candidates because they show high absorbance in the red while their axial ligands hinder the formation of aggregates, a recurrent problem among phthalocyanine dyes. In this paper, we present the photophysics and photovoltaic device performance for a series of novel Ru-phthalocyanines. We focus in particular upon the origin of the enhancement in device performance observed in the presence of two additives, Li+ and chenodeoxycholic acid. The addition of Li+ lowers the conduction band edge of the TiO2 semiconductor leading to a higher electron injection yield and a higher photocurrent in the device. The increases in injection yield and photocurrent are large for these sensitizers, compared to the widely studied ruthenium bipyridyl dye N719, due to the relatively slow injection dynamics, emphasizing the importance of injection yield in limiting device performance for this Ru-phthalocyanine dye series. Of particular interest is the effect of chenodeoxycholic acid. This coadsorbent dramatically enhances the photocurrent of the studied devices without lowering the photovoltage. Unlike previous studies, in this case the photocurrent rise can not be attributed to an increment in the electron injection yield due to the effect of the coadsorbent hindering the formation of dye aggregates. Photophysical measurements instead show that the slower recombination of dye cations with the TiO2 electrons and faster regeneration of the dye cations by the electrolyte are the reasons for the enhanced photocurrent.
[Show abstract][Hide abstract] ABSTRACT: Within the present work, two series of novel ruthenium(II) phthalocyanine (RuPc) complexes with one [RuPcCOPy-nT] or two [RuPc(Py-nT)(2)] dendritic oligothiophene (DOT) ligands in the axial positions are reported. The ability of Ru(II) for axial coordination in RuPcs allowed the attachment of the Pc through the metal site to the DOT-ligands bearing pyridine at the core position of the dendrons. These extended pyridine functionalized conjugated DOT-ligands (Py-nT) were chosen to cover the spectral window between 380 and 550 nm, where the RuPc does not exhibit a strong absorption, in order to improve the light-absorption of these complexes and hence enhance the efficiency of the corresponding solar cells. Good efficiencies of up to 1.6% have been achieved when blended together with a fullerene acceptor in solution-processed photovoltaic devices, providing by far the best phthalocyanine-based bulk heterojunction solar cells reported to-date.
Journal of the American Chemical Society 07/2009; 131(24):8669-76. DOI:10.1021/ja901537d · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the first phthalocyanine-based stopcock for selective adsorption to the channel entrances of zeolite L and realisation of a new electronic dipole moment coupling situation.
Chemical Communications 04/2008; 17(10):1187-9. DOI:10.1039/b714995f · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to increase the energy efficiency of dye-sensitized solar cells beyond 10%, an improved dye needs to be developed with greater light absorption in the red and near-infrared. Many dyes have been tested for this purpose; however, no dye with significant absorption beyond 750 nm has functioned properly. We have examined a series of ruthenium phthalocyanines, a dye class with large and tunable absorption in the red. For these dyes we observe a large reduction in the output voltage of the cells relative to the benchmark dye (N719). By examination of photovoltage transients and charge density measurements, we demonstrate that this reduction in voltage is caused by a 100-fold increase in the rate constant for recombination (iodine reduction) at the TiO2/electrolyte interface. N719, however, does not seem to catalyze this reaction. By examination of the literature, we propose that catalysis of the recombination reaction may be occurring for many other classes of potentially useful dyes including porphyrins, coumarins, perylenes, cyanines, merocyanines, and azulene. This widespread ability of the dye to catalyze recombination has not been appreciated before. This finding has important implications for future work to improve the red response of dye sensitized photovoltaics.
Journal of the American Chemical Society 04/2008; 130(10):2906-7. DOI:10.1021/ja078045o · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photoinduced electron injection in dye sensitized TiO2 is a critical step in the function of dye sensitized solar cells. High electron injection quantum yields are a requirement to obtain efficient devices. While high electron injection quantum yields are usually linked to ultrafast electron-transfer dynamics (in the fs-ps timescales), the latter are not a requirement. We present here a system, Ru-phthalocyanine sensitized TiO2, where slow electron injection (k (inj) approximate to 450 ns(-1)) and efficient electron injection are compatible owing to the long lifetime of the injecting state, the Ru-phthalocyanine triplet state. Ru-phthalocyanine dyes are attractive sensitizers because they absorb strongly in the red and their axial ligands hinder the formation of aggregates.
Journal of the American Chemical Society 08/2007; 129(30):9250-9251. DOI:10.1021/ja0722980 · 12.11 Impact Factor