A Novel Amine-Free Dianchoring Organic Dye for Efficient Dye-Sensitized Solar Cells

The role of the donor group and electron-accepting substitutions inserted in π-linkers in tuning the optoelectronic properties of D-π-A dye-sensitized solar cells: a DFT/TDDFT study

The design of low-cost and high-efficiency sensitizers is one of the most important factors in the expansion of dye-sensitized solar cells (DSSCs). To obtain effective sensitizer dyes for applications in dye-sensitized solar cells, a series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed by using computational methodologies based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We have designed JK-POZ(1–3) and JK-PTZ(1–3) D–π–A–A organic dyes by modifying the donor and π-linker units of the JK-201 reference dye. Computational calculations of the structural, photochemical properties and electrochemical properties, as well as the key parameters related to the short-circuit current density and open-circuit voltage, including light-harvesting efficiency (LHE), singlet excited state lifetime (τ), reorganization energies (λ_total), electronic injection-free energy (ΔG^inject) and regeneration driving forces (ΔG^reg) of dyes were calculated and analyzed. Moreover, charge transfer parameters, such as the amount of charge transfer (q^CT), the charge transfer distance (D^CT), and dipole moment changes (μ^CT), were investigated. The results show that ΔG^reg, λ_max, λ_total and τ of JK-POZ-3 and JK-PTZ-3 dyes are superior to those of JK-201, indicating that novel JK-POZ-3 and JK-PTZ-3 dyes could be promising candidates for improving the efficiency of the DSSCs devices.

A series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed theoretically.

Increasing the Efficiency of Dye-Sensitized Solar Cells by Adding Nickel Oxide Nanoparticles to Titanium Dioxide Working Electrodes

In this study, nickel oxide (NiO) nanoparticles were added to a titanium dioxide (TiO2) nanoparticle paste to fabricate a dye-sensitized solar cell (DSSC) working electrode by using a screen-printing method. The effects of the NiO proportion in the TiO2 paste on the TiO2 working electrode, DSSC devices, and electron transport characteristics were comprehensively investigated. The results showed that adding NiO nanoparticles to the TiO2 working electrode both inhibited electron transport (a negative effect) and prevented electron recombination with the electrolyte (a positive effect). The electron transit time was extended following an increase in the amount of NiO nanoparticles added, confirming that NiO inhibited electron transport. Furthermore, the energy level difference between TiO2 and NiO generated a potential barrier that prevented the recombination of the electrons in the TiO2 conduction band with the I3- ions in the electrolyte. When the TiO2–NiO ratio was 99:1, the positive effects outweighed the negative effects. Therefore, this ratio was the optimal TiO2–NiO ratio in the electrode for electron transport. The DSSCs with a TiO2–NiO (99:1) working electrode exhibited an optimal power conversion efficiency of 8.39%, which was higher than the DSSCs with a TiO2 working electrode.

Prediction of the Synthesis of Spiro Derivatives by Double Intramolecular Aromatic Electrophilic Substitution Using Reactivity Indices

The synthesis of heterocyclic spirobifluorene (SBF) analogs generally requires long and complicated synthetic pathways. Despite this synthetic effort, such structural modification allows the (opto)electronic properties of this remarkable three-dimensional node to be tuned especially for molecular electronic applications. For this reason, the development of a simple, robust, and efficient synthetic methodology to introduce various heterocycles in place of classical phenyl rings in the spirofluorene motif is highly and timely desirable. In this context, we describe herein our efforts to develop a straightforward and efficient synthesis leading to replacement of 2 phenyl rings by various heterocycles in spiro compounds from 2,2'-dibromobenzophenone. As the same procedure to form fully heterocyclic compounds failed, an original theoretical approach based mainly on the uncommon Fukui dual function was developed in order to determine clearly the limitation of this strategy and provide an efficient predictive tool. Indeed, such calculation allows prediction of the thermodynamic and kinetic aspects of the synthesis of spiro derivatives using a double aromatic electrophilic substitution. If this procedure reproduces well our experimental results focused on (heterocyclic) SBF compounds, it can be certainly adapted and generalized to other intramolecular substitutions.

Stable 2D anti-ferromagnetically coupled fluorenyl radical dendrons

The first class of stable two-dimensional anti-ferromagnetically coupled dendritic polyradicaloids was synthesized, which show polyradical character and unique properties.

We report the first class of stable, two-dimensional (2D) anti-ferromagnetically coupled dendritic polyradicaloids. A kinetically blocked fluorenyl radical was used to build up the first and second generation dendrons FR-G1 and FR-G2 containing three and seven fluorenyl units, respectively. The neighboring fluorenyl radicals in FR-G1 and FR-G2 show moderate anti-ferromagnetic exchange interaction, resulting in a doublet and quartet ground state, respectively, with a small doublet–quartet energy gap. From FR-G1 to FR-G2, the energy gap decreased and the two-photon absorption was enhanced owing to more extended 2D π-conjugation. Both compounds showed multiple redox waves due to their polyradical character.

Molecular Design of Efficient Organic D-A-π -A Dye Featuring Triphenylamine as Donor Fragment for Application in Dye-Sensitized Solar Cells

A metal-free organic sensitizer, suitable for the application in dye-sensitized solar cells (DSSCs), has been designed, synthesized and characterized both experimentally and theoretically. The structure of the novel donor-acceptor-π-bridge-acceptor (D-A-π-A) dye incorporates a triphenylamine (TPA) segment and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA). The triphenylamine unit is widely used as an electron donor for photosensitizers, owing to its nonplanar molecular configuration and excellent electron-donating capability, whereas 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid is used as an electron acceptor unit. The influences of I₃^- /I^- , [Co(bpy)₃ ]^3+/2+ and [Cu(tmby)₂ ]^2+/+ (tmby=4,4',6,6'-tetramethyl-2,2'-bipyridine) as redox electrolytes on the DSSC device performance were also investigated. The maximal monochromatic incident photon-to-current conversion efficiency (IPCE) reached 81 % and the solar light to electrical energy conversion efficiency of devices with [Cu(tmby)₂ ]^2+/+ reached 7.15 %. The devices with [Co(bpy)₃ ]^3+/2+ and I₃^- /I^- electrolytes gave efficiencies of 5.22 % and 6.14 %, respectively. The lowest device performance with a [Co(bpy)₃ ]^3+/2+ -based electrolyte is attributed to increased charge recombination.

Development of Spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene]-Based A-π-D-π-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells

Three acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules (STFYT, STFRDN, and STFRCN) with spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene] (STF) as the central donor unit, terthiophene as the π-conjugated bridge, indenedione, 3-ethylrhodanine, or 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit are designed, synthesized, and characterized as electron donor materials in solution-processing organic solar cells (OSCs). The effects of the spiro STF-based central core and different acceptors on the molecular configuration, absorption properties, electronic energy levels, carrier transport properties, the morphology of active layers, and photovoltaic properties are investigated in detail. The three molecules exhibit desirable physicochemical features: wide absorption bands (300-850 nm) and high molar absorption coefficients (4.82 × 10⁴ to 7.56 × 10⁴ M^-1 cm^-1) and relatively low HOMO levels (-5.15 to -5.38 eV). Density functional theory calculations reveal that the spiro STF central core benefits to reduce the steric hindrance effect between the central donor block and terthiophene bridge and suppress excessive intermolecular aggregations. The optimized OSCs based on these molecules deliver power conversion efficiencies (PCEs) of 6.68%, 3.30%, and 4.33% for STFYT, STFRDN, and STFRCN, respectively. The higher PCE of STFYT-based OSCs should be ascribed to its better absorption ability, higher and balanced hole and electron mobilities, and superior active layer morphology as compared to the other two compounds. So far, this is the first example of developing the A-π-D-π-A type small molecules with a spiro central donor core for high-performance OSC applications. Meanwhile, these results demonstrate that using spiro central block to construct A-π-D-π-A molecule is an alternative and effective strategy for achieving high-performance small molecule donor materials.

FeCl3-Mediated Oxidative Spirocyclization of Difluorenylidene Diarylethanes Leading to Dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s

A novel FeCl3-mediated oxidative spirocyclization for construction of a new class of di-spirolinked π-conjugated molecules, dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s (DSFIIFs), has been reported. The combination of FeCl3 with FeO(OH) triggered an unprecedented double one-electron oxidation of difluorenylidene diarylethanes to afford the corresponding dispirocycles in high yields. The highest fluorescence quantum yield was up to 0.94 in solution. This protocol is also applicable to the synthesis of the non-spirolinked dihydroindenoindenes.

Investigation of Coral-Like Cu2O Nano/Microstructures as Counter Electrodes for Dye-Sensitized Solar Cells

In this study, a chemical oxidation method was employed to fabricate coral-like Cu₂O nano/microstructures on Cu foils as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The Cu₂O nano/microstructures were prepared at various sintering temperatures (400, 500, 600 and 700 °C) to investigate the influences of the sintering temperature on the DSSC characteristics. First, the Cu foil substrates were immersed in an aqueous solution containing (NH₄)₂S₂O₈ and NaOH. After reacting at 25 °C for 30 min, the Cu substrates were converted to Cu(OH)₂ nanostructures. Subsequently, the nanostructures were subjected to nitrogen sintering, leading to Cu(OH)₂ being dehydrated into CuO, which was then deoxidized to form coral-like Cu₂O nano/microstructures. The material properties of the Cu₂O CEs were comprehensively determined using a scanning electron microscope, energy dispersive X-ray spectrometer, X-ray diffractometer, Raman spectrometer, X-ray photoelectron spectroscope, and cyclic voltameter. The Cu₂O CEs sintered at various temperatures were used in DSSC devices and analyzed according to the current density–voltage characteristics, incident photon-to-current conversion efficiency, and electrochemical impedance characteristics. The Cu₂O CEs sintered at 600 °C exhibited the optimal electrode properties and DSSC performance, yielding a power conversion efficiency of 3.62%. The Cu₂O CEs fabricated on Cu foil were generally mechanically flexible and could therefore be applied to flexible DSSCs.

Attempt to improve the performance of pyrrole-containing dyes in dye sensitized solar cells by adjusting isolation groups

Four new pyrrole-based organic sensitizers with different isolation groups were conveniently synthesized and applied to dye sensitized solar cells (DSCs). The introduction of isolation group in the side chain could both suppress the formation of dye aggregates and electron recombination. Especially, when two pieces of D-π-A chromophore moieties shared one isolation group to construct the "H" type dye, the performance was further improved. Consequently, in the corresponding solar cell of LI-57, a short-circuit photocurrent density (Jsc) was tested to be 13.85 mA cm(-2), while 0.72 V for the open-circuit photovoltage (Voc), 0.64 for the fill factor (FF), and 6.43% for the overall conversion efficiency (η), exceeding its analogue LI-55 (5.94%) with the same isolation group. The results demonstrated that both the size (bulk and shape) and the linkage mode between the D-π-A chromophores and the isolation groups, could affect the performance of sensitizers in DSCs in a large degree, providing a new approach to optimize the chemical structure of dyes to achieve high conversion efficiencies.

High-performance dye-sensitized solar cells based on phenothiazine dyes containing double anchors and thiophene spacers

A series of new push-pull phenothiazine-based dyes (HL1-HL4) featuring various π spacers (thiophene, 3-hexylthiophene, 4-hexyl-2,2'-bithiophene) and double acceptors/anchors have been synthesized, characterized, and used as sensitizers for dye-sensitized solar cells (DSSCs). Among them, the best conversion efficiency (7.31%) reaches approximately 99% of the N719-based (7.38%) DSSCs fabricated and measured under similar conditions. The dyes with two anchors have more efficient interfacial charge generation and transport compared with their congeners with only single anchor. Incorporation of hexyl chains into the π-conjugated spacer of these double-anchoring dyes can efficiently suppress dye aggregation and reduce charge recombination.