Slower Charge Recombination Rate Research Articles

Double‐Anchored Triphenylamine‐Fluorene Functionalized Dyes for High‐Performance Photocatalytic Hydrogen Generation

Two novel metal‐free organic dyes, namely the mono‐anchoring KMT1 and di‐anchoring KMT2 dyes, have been synthesized to function as photosensitizers for utilizing in photocatalytic hydrogen production systems. They adopt the donor‐donor‐π‐acceptor and donor‐donor‐(‐π‐acceptor)2 configurations, respectively, featuring triphenylamine and fluorene moieties as electron donors, thiophene as the π‐bridges and cyanoacrylic acid as the acceptors/anchoring groups. It was found that the photocatalytic performance of the di‐anchoring dye significantly outperforms that of its mono‐anchoring counterpart. KMT2‐based photocatalytic system demonstrated remarkable performance, producing 3520 μmol (84.9 mL) of hydrogen over a span of 257 hours under blue light irradiation, with a turnover number (TON) of 56300, a turnover frequency (TOFi) of 1862.7 h‐1, an initial hydrogen production activity (activityi) of 1164180 and an apparent quantum yield (AQYi%) of 19.9. This performance is among the top‐tier when compared to other dyes used in similar photocatalytic systems. The results clearly illustrate that the di‐anchoring dye possesses several advantages, including a broad absorption spectrum, higher absorptivity, and a relatively slower charge recombination rate. These attributes collectively contribute to its superior overall performance in photocatalysis.

Tuning spontaneous polarization to alter water oxidation/reduction activities of LiNbO3

Here, we investigated the effects of spontaneous polarization on photoreactivities by using a ferroelectric material n-type congruent LiNbO3 single crystal as a model. It was found that c+ LiNbO3 was superior to c− LiNbO3 in photocatalytic water reduction, while c− LiNbO3 exhibited better performances for photoelectrochemical water oxidation than c+ LiNbO3. Using Kelvin probe force microscopy and open circuit potential methods, we observed that c− LiNbO3 generated a higher photovoltage and had a slower charge-recombination rate than c+ LiNbO3. The results of electrochemical impedance spectroscopy measurements indicated that c− LiNbO3 may favor the hole transport from the bulk to the surface compared with c+ LiNbO3, leading to the anisotropic performances of c+ and c− LiNbO3 in water oxidation/reduction. Therefore, tuning the direction of the polarization may be a strategy to dramatically prompt the photoreactivities of water oxidation or reduction.

A combined experimental and computational investigation on pyrene based D-π-A dyes.

The geometry (twist vs. planar) of a dye is one of the most pivotal factors for determining intramolecular charge transfer (ICT), light harvesting and photovoltaic properties of dye-sensitized solar cells. In order to comprehend the role of dye geometry on the above properties, we have devised the pyrene based D-π-A dyes namely 2-cyano-3-(5-pyren-1-yl-furan-2-yl)-acrylic acid (PFCC) and 2-cyano-3-(5-pyren-1-ylethynyl-furan-2-yl)-acrylic acid (PEFCC). The synthesized pyrene dyes were well characterized by NMR and EI-MS spectrometry. In both the dyes, the donor (pyrene) and acceptor (cyanoacrylic acid) segments remained the same. The varied π-spacers were furan and ethynyl furan. The influences of the ethynyl spacer on the energy levels, light absorption, dynamics of excited states and photovoltaic properties of the DSCs were systematically investigated via theoretical calculations and spectroscopic measurements. UV-visible absorption spectral measurements indicated that the introduction of the ethynyl spacer enhances the molar absorptivity of a dye (PEFCC) in the order of 2, but does not shift the absorption range, which is consistent with the results obtained from density functional theory (DFT) calculations. The theoretical analysis indicated that the charge transfer transition is mainly constituted of the HOMO to the LUMO that were found to be located on donor and acceptor segments, respectively, which is supportive for efficient charge separation and electron injection processes. TDDFT calculations highlighted that the LUMO of the PEFCC dye is more stabilized by the incorporation of the ethynyl group between the pyrene and furan moieties that aid to inject electrons efficiently into TiO2 thereby resulting in an enhanced power conversion efficiency of 2.47% when compared to the PFCC dye. Notably, the overall conversion efficiency of the PEFCC dye reached 60% with respect to that of an N719-based device (4.12%) fabricated under similar conditions. Transient absorption kinetic studies demonstrated that a slower charge recombination rate is an essential factor behind enhanced efficiencies in PEFCC based cells.

Controllable growth of vertically aligned Bi-doped TiO2 nanorod arrays for all-oxide solid-state DSSCs

In this study, vertically aligned Bi-doped TiO2 nanorod arrays as photoanodes were successfully grown on the fluorine-doped tin oxide by hydrothermal method. Structural analysis showed that bismuth was successfully incorporated into the TiO2 lattice at low concentration, but at higher concentration, phase segregation of Bi2O3 in the TiO2 matrix was occurred. TiO2 nanorods with 3 % bismuth concentration had minimum electrical resistivity. As the solid-state electrolyte, Mg-doped CuCrO2 nanoparticles with p-type conductivity were synthesized by sol–gel method. The fabricated all-oxide solid-state dye-sensitized solar cells with Bi-doped TiO2 nanorods displayed better photovoltaic performance due to the presence of Bi. The improved cell performance was correlated with the higher dye loading, slower charge recombination rate and the higher electrical conductivity of the photoanodes. After mechanical pressing, the all-oxide solid-state DSSC exhibited enhanced photovoltaic performance due to the formation of the large neck between adjacent nanoparticles by mechanical sintering. The open-circuit photovoltage decay measurement of the devices and electrical conductivity of the nanoparticles before and after pressing revealed that the mechanical pressing technique reduces charge recombination rate and facilitates electron transport through the interconnected nanoparticles.

The Synthesis, Characterisation, Photophysical and Thermal Properties, and Photovoltaic Performance of 7-Coumarinoxy-4-Methyltetrasubstituted Metallophthalocyanines

The synthesis, characterisation, photophysical and thermal properties of 2(3),9(10),16(17),23(24)-tetrakis(7-coumarinoxy-4-methyl)-phthalocyaninatozinc(ii) (ZnPc-Coumarin) and 2(3),9(10),16(17),23(24)-tetrakis(7-coumarinoxy-4-methyl)-phthalocyaninatocobalt(ii) (CoPc-Coumarin) are reported. The ground state absorbance of ZnPc-Coumarin shows molar extinction coefficients as high as 1.80 × 105 dm3 mol–1 cm–1. The fluorescence spectrum and fluorescence quantum yields of compounds ZnPc-Coumarin and CoPc-Coumarin are also investigated. The photoluminescence decay of the two transition-metal complexes in DMF solution, in poly(methyl methacrylate) (PMMA), and on TiO2 films has been studied with time-resolved emission. This study shows that the electron transfer from the dye to TiO2 is through space. The thermal stability studies indicate that both of the two complexes are stable up to 390°C. The ZnPc-Coumarin achieved a higher overall conversion efficiency than the reported SnPcCl2-Coumarin, InPcCl-Coumarin, and RuPcCl-Coumarin because of its slower charge recombination rate and faster electron injection from the dye to the conduction band of the conducting glass.

Influence of porphyrinic structure on electron transfer processes at the electrolyte/dye/TiO₂ interface in PSSCs: a comparison between meso push-pull and β-pyrrolic architectures.

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I(-)/I3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I(-), are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

Multiple-Anchoring Triphenylamine Dyes for Dye-Sensitized Solar Cell Application

A series of triphenylamine-based dyes (TPACR1, TPACR2, and TPACR3) with multiple corhodanine derivatives as acceptors were prepared and examined as sensitizers for dye-sensitized solar cells. The overall conversion efficiencies of DSSCs based on these dyes were in the range of 2.63 to 5.31%, in which TPACR2-based DSSC showed the best photovoltaic performance: a short-circuit current (Jsc) of 15.03 mA·cm–2, an open-circuit voltage (Voc) of 552 mV, and a fill factor (FF) of 0.64, corresponding to an overall efficiency of 5.31% under simulated AM 1.5 solar irradiation (100 mW·cm–2). Compared with monoanchoring TPACR1 dye and trianchoring TPACR3 dye, dianchoring TPACR2 dye had a relatively slower charge recombination rate between injected electrons and triphenylamine dye cations, which was indicated by transient absorption kinetics Furthermore, the suppression of the charge recombination between injected electron and the I3– in the electrolyte lead to the longer electron lifetime observed with the DSSC based ...

Effect of the length of the alkyl chains in porphyrin meso-substituents on the performance of dye-sensitized solar cells

A series of novel zinc porphyrin dyes which have a D–π–A structure have been designed and synthesized for DSSC applications. The donors containing a carbazole group in which the carbazole nitrogen is bonded with a butyl, hexyl or decyl chain, had a significant influence on the spectra of the TiO2 films and the electrochemical and photovoltaic properties of these sensitizers. The sensitizer with a hexyl chain (CZ-6) achieved a higher overall conversion efficiency than that with a butyl chain (CZ-4) because of its slower charge recombination rate and faster electron injection from the dye to the conduction band of the conducting glass. Furthermore, the dye with a decyl chain (CZ-10) performed the lowest conversion efficiency, resulting from the least amount of dye loading. This was due to the steric hindrance of the molecule. The highest light-to-electricity conversion efficiency (η) of 2.13% was realized for the dye CZ-6 based DSSC, which was higher than those achieved for the CZ-4- and CZ-6-sensitized DSSCs (1.69% and 1.30% respectively).

Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells

Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles are synthesized for the first time through the facile hydrothermal process in the presence of F−. Dye-sensitized solar cell based on hierarchical Zn2SnO4 nanosheets photoelectrode shows a remarkable enhancement in power conversion efficiency (4.82%) compared to that of nanoparticles (4.01%) because of its superior light scattering ability, faster electron transport rate and slower charge recombination rate, which were confirmed by the UV−vis diffuse reflectance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy and electrochemical impedance spectroscopy. Further TiCl4 treatment of Zn2SnO4 nanosheets photoelectrode results in an impressive photovoltaic performance of 5.44%.