Effect of positional isomerism on the excited state charge transfer dynamics of anthracene-based D-π-A systems

Understanding the dynamics of the back electron transfer (BET) rate of ion pairs from the electronically excited state of donor-acceptor systems is crucial for developing materials for organic electronics. The structure-property relationships in the organic molecular architectures play a key role in controlling the BET rate and have been utilized as a criterion to design systems with a reduced BET rate. Here, we examine the influence of isomerism on the BET rate in anthracene based systems, viz., (E)-2-(2-(anthracen-9-yl)vinyl)benzonitrile (ortho-CN) and (E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile (meta-CN) with N,N-diethylaniline (DEA) in methylcyclohexane using time-resolved spectroscopy. The radical cation (DEA˙⁺) and the radical anion (ortho-CN˙^- or meta-CN˙^-) generated after photoexcitation show synchronous decay kinetics, and the rate constant of back electron transfer (k_BET) for the DEA/ortho-CN pair was 6.6 × 10⁴ s^-1, which is ca. 2 orders of magnitude lower compared with the DEA/meta-CN pair. The role of isomerism in providing resonance stabilization for the organic radicals is expected to have implications for strategies that retard charge recombination in photovoltaics. The role of the molecular structural features that dictate the kinetics for charge recombination has been further identified using quantum calculations.

Isomeric Pyrene-Porphyrins for Efficient Dye-Sensitized Solar Cells: An Unexpected Enhancement of the Photovoltaic Performance upon Structural Modification

Four pyrene-porphyrins were synthesized to study the isomer effect on the photovoltaic performance of dye-sensitized solar cells. One of these porphyrins is conjugated with a terminal pyrene, whereas the other three are each attached with a pyrene bearing an extra donor group. According to the positions of the extra donor and porphyrin core on pyrene, the 1,6-, 1,8-, and 2,7-isomers were compared for their fundamental and photovoltaic properties. For fundamental properties, UV-visible absorption, fluorescence emission, electrochemistry, and DFT calculations were carried out. For photovoltaic measurements, the seemingly inferior 1,8-isomer outperforms others with an overall efficiency of 10.30% under one-sun irradiation. Superior photovoltaic performance of the 1,8-isomeric dye may be related to the so-called umbrella effect. The findings of this work may provide insight into isomeric dye design for future applications.

Pyridyl/hydroxyphenyl versus carboxyphenyl anchoring moieties in Zn - Thienyl porphyrins for dye sensitized solar cells

For developing efficient DSCs, anchoring moieties need to be understood in detail with respect to a particular dye. Herein, we have synthesized new Zn-thienyl porphyrin dyes based on pyridyl and hydroxyphenyl anchors and compared with traditional carboxyphenyl anchoring group. The effect of such variation in anchoring groups was investigated with UV-vis and fluorescence spectroscopy as well as electrochemical studies. The hydroxyphenyl appended dye showed higher molar extinction coefficient and red shifted emission bands. Both pyridyl and hydroxyphenyl dyes showed adsorption over TiO₂ semiconductor. The dye loading amount was found to be greater for carboxyphenyl dye due to stronger bidendate coordination of the carboxy group, as revealed by FT-IR studies. The electrochemical potentials were found to be strongly influenced by the anchoring functionality and all the dyes showed positive driving force for electron injection to the conduction band of TiO₂ and regeneration of oxidized dye by the redox electrolyte. DSC devices were fabricated for each of the dyes and characterized well. Hydroxyphenyl dye showed higher V_oc as a result of lower back reactions. Transient photovoltage decay measurments indicated lower recombination rate in hydroxyphenyl and pyridyl than the carboxyphenyl anchored thienyl porphyrins offering potential for further dye optimization.

Role of Electronic Relaxation in the Injection Process of Organic Push-Pull Dyes in Complete Dye-Sensitized Solar Cells

There is a growing consensus that the charge separation taking place in dye-sensitized solar cells is a multiscale process occurring on a times scale from a few to hundreds of picoseconds. We studied the excited-state dynamics of the robust and efficient push-pull dye RK1 in solution, on mesoporous films and in complete photovoltaics cells by femtosecond fluorescence upconversion and transient absorption. In a polar environment and cells, the dynamics at early times are dominated by an intramolecular electronic relaxation, while electron injection is predominant on thin films only. In cells, the electron injection process becomes visible at a later stage, from tens to hundreds of picoseconds. Our study shows that it is crucial to record and analyze full time-resolved fluorescence spectra in order to obtain wavelength-independent dynamics and get a correct description of the nature and the population of the excited state.

Synthesis and electronic properties of A3B-thienyl porphyrins: experimental and computational investigations

A facile synthesis ofmeso-thienyl porphyrins containing a phenyl substituent allows fine tuning of the frontier orbitals to suit applications in DSSC and photomedicine.

Push-Pull Zinc Porphyrins as Light-Harvesters for Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) has been attractive to scientific community due to its eco-friendliness, ease of fabrication, and vivid colorful property etc. Among various kinds of sensitizers, such as metal-free organic molecules, metal-complex, natural dyes etc., porphyrin is one of the most promising sensitizers for DSSC. The first application of porphyrin for sensitization of nanocrystaline TiO2 can be traced back to 1993 by using [tetrakis(4-carboxyphenyl) porphyrinato] zinc(II) with an overall conversion efficiency of 2.6%. After 10 years efforts, Officer and Grätzel improved this value to 7.1%. Later in 2009, by constructing porphyrin sensitizer with an arylamine as donor and a benzoic acid as acceptor, Diau and Yeh demonstrated that this donor-acceptor framwork porphyrins could attain remarkable photovoltaic performance. Now the highest efficiencies of DSSC are dominated by donor-acceptor porphyrins, reaching remarkable values around 13.0% with cobalt-based electrolytes. This achievement is largely contributed by the structural development of donor and acceptor groups within push-pull framwork. In this review, we summarized and discussed the developement of donor-acceptor porphyrin sensitizers and their applications in DSSC. A dicussion of the correlation between molecular structure and the spectral and photovoltaic properties is the major target of this review. Deeply dicussion of the substitution group, especially on porphyrin's meso-position were presented. Furthermore, the limitations of DSSC for commercialization, such as the long-term stability, sophisticated synthesis procedures for high efficiency dye etc., have also been discussed.

Significant enhancement in the performance of porphyrin for dye-sensitized solar cells: aggregation control using chenodeoxycholic acid

CDCA co-sensitizers prevent aggregation of the dyes on the TiO₂ surface.

Two new bulky substituted Zn porphyrins bearing carboxylate anchoring groups as promising dyes for DSSCs

Two novel zinc-metallated porphyrins, bearing three and six long alkoxy chains at the periphery, have been synthesized as sensitizers in DSSCs.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Accelerated inner charge transfer in porphyrins promotes a broad light-harvesting ability up to 840 nm and a conversion efficiency of 9.2%.