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© 2020 American Chemical Society. Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to "free" molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene-carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.

A variety of charge extraction (CE) techniques have been developed to measure charge density and recombination coefficients in bulk heterojunction solar cells. Charge recombination during charge extraction as a major limitation of this method has not been systematically quantified. This study reports CE measurements using a newly designed fast switch, which enables the application of a reverse bias to the solar cells facilitating charge extraction. With applied reverse bias, more than 40% increase in the extracted charge is obtained in solar cells with thicker active layers or with fast recombination. The measured charge carrier lifetime increases by up to a factor of three at sufficiently high applied biases (up to 8 V), suggesting significant errors in CE measurements without applied bias. The increased extracted charges with increasing applied bias are attributed to a combination of three cases: (i) slightly faster charge extraction due to the larger electric field; (ii) increased charge extraction rate at high light intensities when the transients are space charge disturbed; (iii) increased charge separated lifetime during charge extraction attributed to the spatial separation of the electron and hole density due to the applied electric field.

Reduction kinetics of oxidized dyes absorbed on semiconductor surfaces and immersed in redox active electrolytes has been mainly modeled based on the free energy difference between the oxidation potential of the dye and the redox potential of the electrolyte. Only a few mechanisms have been demonstrated to enhance the kinetics by other means. In this work, the rate constant of the reduction of oxidized porphyrin dye is enhanced by attaching non-conjugated carbazole triphenylamine moiety using iodine/triiodide and tris(2,2′-bispyridinium)cobalt II/III electrolytes. These results are obtained using transient absorption spectroscopy by selectively probing the regeneration kinetics at the porphyrin radical cation and the carbazole triphenylamine radical cation absorption wavelengths. The enhancement in the reduction kinetics is not attributed to changes in the driving force, but to the more exposed dye cation radical orbitals of the dichromophoric dye. The results are important for the development of high efficiency photo-electrochemical devices with minimalized energy loss at electron transfer interfaces.

A novel Zn phthalocyanine-Zn porphyrin (ZnPc-ZnPor) dyad (1), in which a zinc porphyrin moiety is linked covalently with an ABAB-type zinc phthalocyanine moiety, has been designed and synthesized. The ZnPc-ZnPor dyad 1 can absorb over a wide spectral range of visible light. The fluorescence spectrum of dyad 1 revealed an intramolecular efficient energy transfer from Zn porphyrin to Zn phthalocyanine. When dyad 1 was used as a light-harvesting sensitizer for dye-sensitized solar cells, the cell sensitized by dyad 1 provided a power conversion efficiency of 2.7% under simulated air mass 1.5 global sunlight. The incident photon-to-current efficiency spectrum of the dyad 1 cell suggested the contribution of both components in dyad 1 for the current generation.

Porphyrins are some of the most studied chromophores employed in photo-electrochemical energy conversion devices. However, the molar extinction coefficient of most simple porphyrins is small within the 450-550 nm wavelength region, referred to here as the absorption gap, which limits the light harvesting efficiency of thin photoelectrodes. The purpose of this work is to fill the absorption gap by covalently attaching additional chromophores with complementary absorption in the 450-550 nm wavelength region. To this end, three carbazole-fused thiophene-substituted zinc porphyrin dyes were synthesized, and their photophysical properties were investigated using UV-vis absorption, photoluminescence, resonance Raman, and electrochemical methods, supported by density functional theory calculations. All three dyes showed much-improved light harvesting up to 550 nm when attached to TiO2 photoelectrodes, resulting in doubling the short circuit current of dye-sensitized solar cells using the Co2+/Co3+ electrolyte. The highest power conversion efficiency of 4.7% was achieved using dithieno[3,2-b:2′,3′-d]thiophene attached to carbazole as the additional chromophore. All three carbazole-fused thiophene dichromophoric porphyrin dyes studied have attained increased electron lifetimes contributing to their higher open circuit voltage (VOC) compared to that of a simple porphyrin. Absorbed photon to collected electron efficiency together with charge extraction studies suggests that the performance of the carbazole-fused thiophene dyes is limited by electron injection.

Electron lifetimes in dye-sensitized solar cells employing a porphyrin dye, an organic dye, a 1:1 mixture of the two dyes and a di-chromophoric dye design consisting of the two dyes using a non-conjugated linker were measured, suggesting that the dispersion force of the organic dyes has a significant detrimental effect on the electron lifetime and that the di-chromophoric design can be utilized to control the effect of the dispersion force.

Dye sensitized solar cells (DSSCs) employing a dimer porphyrin, which was synthesised with two porphyrin units connected without conjugation, have shown that both porphyrin components can contribute to photocurrent generation, that is, more than 50 % internal quantum efficiency. In addition, the open-circuit voltage (Voc) of the DSSCs was higher than that of DSSCs using monomer porphyrins. In this paper, we first optimized cell structure and fabrication conditions. We obtained more than 80% incident photon to current conversion efficiency from the dimer porphyrin sensitized DSSCs and higher Voc and energy conversion efficiency than monomer porphyrin sensitized solar cells. To examine the origin of the higher Voc, we measured electron lifetime in the DSSCs with various conditions, and found that the dimer system increased the electron lifetime by improving the steric blocking effect of the dye layer, whilst the lack of a conjugated linker prevents an increase in the attractive force between conjugated sensitizers and the acceptor species in the electrolyte. The results support a hypothesis; dispersion force is one of the factors influencing the electron lifetime in DSSCs.

The origin of simultaneous improvements in the short-circuit current density (Jsc) and open-circuit voltage (Voc) of porphyrin dye-sensitized TiO2 solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO2 surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li+) ions with dimethylpropylimidazolium (DMPIm+) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the light exposure effect was initiated by formation of dye cation molecules following photoexcitation. The substituents and linker group on the porphyrin chromophore were both varied, with light exposure producing increased electron lifetime and Voc for all dyes; however, increased Jsc values were only measured for dyes containing binding moieties with multiple carboxylic acids. It was proposed that the initial injection limitation and/or fast recombination process in these dyes arises from the presence of lithium at the surface, and the improved injection and/or retardation of fast recombination after light exposure is caused by the Li+ removal by cation exchange under illumination.

Porphyrin molecules offer immense potential as the light harvesting component of dye-sensitised nanocrystalline TiO2 solar cells. Synthetic porphyrin dyes were amongst the first dyes trialled for sensitisation of inorganic semiconducting oxides. Today, they exhibit the best performance reported for dye-sensitised solar cells. Accompanying the significant performance improvement over the last two decades is a much improved understanding of efficiency-determining fundamental electron transfer steps, from charge photogeneration to recombination. In this feature article we highlight our recent discoveries of the influence of porphyrin molecule structure on efficiency determining electron transfer kinetics and device performance by systematically changing the molecular structure and observing electron injection and recombination kinetics using time-resolved optical and electrical probes. Despite our observation of ultrafast charge injection for all porphyrin dyes studied by transient absorption spectroscopy, the injection yield estimated using an internal standard remains below 100% and depends strongly on the molecular structure. The observed discrepancy between kinetic competition and the injection yield is attributed to non-injecting dyes, probably arising due to inhomogeneity. A very interesting sub-ns (0.5 ns to 100 ns) charge recombination channel between photo-injected electrons and porphyrin cations is observed, which is found to be more prominent in free-base porphyrin dyes with a conjugated linker. Charge recombination between the acceptor species in the redox containing electrolyte and injected electrons is shown to be an important limitation of most porphyrin-sensitised solar cells, accelerated by the presence of porphyrin molecules at the TiO2-electrolyte interface. This recombination reaction is strongly dependent on the porphyrin molecular structure. Bulky substituents, using a porphyrin dimer instead of a porphyrin monomer, a light soaking treatment of fr

A remarkable 300% efficiency enhancement driven by a matching increase in the short circuit current was observed in a mixed porphyrin dye-sensitized solar cell constructed from two dyes in a 3:1 ratio. Absorbed photon-to-current conversion efficiency measurements indicate an improved charge injection yield for both dyes in the mixture. Several possible origins for the observed performance enhancement are discussed.

We performed a detailed and quantitative spectroscopic study of the electron injection dynamics for porphyrin as one of organic dyes at an adequate level to discuss the dye-sensitized solar cell performance. The electron injection kinetics and the electron injection yield for dye-sensitized TiO 2 electrodes in redox-containing electrolytes were measured by femtosecond transient absorption and picosecond fluorescence spectroscopy. By comparing the dynamics of two of the most studied porphyrins with those of a Ru complex (N719), we have directly elucidated that the short-circuit current for the porphyrin-sensitized solar cells is limited by the presence of excited dyes that are quenched in the subnanosecond time range without competing with the electron injection process, even though both porphyrins shows faster injection processes within the picosecond time range than N719. Therefore, it was clearly indicated the electron injection efficiency was mainly limited by the inhomogeniety, which should be carefully considered for further development of organic dye-sensitized solar cells. Copyright 2011 American Chemical Society.

The fundamental understanding on charge-transport properties of flame-synthesized TiO2 in dye-sensitized solar cells (DSSCs) is established in this work. By employing a one-step flame spray pyrolysis (FSP), predominantly anatase TiO2 nanoparticles with average nanoparticle sizes between 11 and 36 nm were achieved by varying the rate of combustion enthalpy (through varying liquid precursor feed flow rates) and using either an “openflame” or “enclosed-flame” configuration. Electron diffusion coefficient (D), electron lifetime (τ ), open circuit voltage (Voc ), and capacitance (C) measurements carried out on FSP TiO2 -based DSSCs demonstrated that interband charge trap density decreased with increase in particle size. Compared to earlier studies, interband charge trap density could be controlled more independently of particle size. Under one-sun conditions, relatively high Voc was measured with large particle size due to the lowering of interband charge trap density. This was true despite the associated shorter τ . Comparisons with commercial benchmark Nanoxide-T and Degussa P25 TiO2 were also carried out. The results from the current study have significant implications on the design of TiO2 nanoparticles by flame aerosol techniques, for DSSCs as well as other photoelectrochemical applications. Index Terms—Electron diffusion coefficient

Three donor−linker−acceptor triphenylamine-based cyanoacrylic acid organic dyes used for dye-sensitized solar cells (DSCs) have been examined with respect to their effect on the open-circuit voltage (Voc). Our previous study showed a decrease in Voc for DSCs based on dyes with increased molecular size (increased linker conjugation). In the present study, we investigate the origin of Voc with respect to (i) conduction band (ECB) positions of TiO2 and (ii) degree of recombination between electrons in TiO2 and electrolyte acceptor species at the interface. These parameters were studied as a function of dye structure, dye load, and I2 concentration. Two types of behavior were identified: the smaller polyene dyes show a surface-protecting effect preventing recombination upon increased dye loading, whereas the larger dyes enhance the recombination. How the different dye structures affect the recombination is discussed in terms of dye surface blocking and intermolecular interactions between dyes and electrolyte acceptor species.

The emulation of the extraordinary photosynthetic light harvesting apparatus has inspired researchers to investigate the assembly and chemistry of a wide variety of covalent and noncovalent porphyrin arrays,1 but surprisingly there have been very few studies of their use in solar cells.2 While the enhancement of solar-toelectrical energy conversion efficiency by porphyrin arrays has been theoretically demonstrated,3 there is little experimental data to support this. In this communication, we demonstrate for the first time that each porphyrin in an array can contribute to current generation in a solar cell.