Conjugated Polyelectrolyte-Sensitized TiO2 Solar Cells: Effects of Chain Length and Aggregation on Efficiency

Sensitizers of Metal Complexes with Sulfur Coordination Achieving a Power Conversion Efficiency of 12.89

In the study to improve the light absorption range and intensity of dye sensitizers in the visible region and promote their photovoltaic performance, five novel polymeric metal complexes with sulfur coordination (BDTT-VBT-Ni, BDTT-VBT-Cu, BDTT-VBT-Zn, BDTT-VBT-Cd, and BDTT-VBT-Hg) to be used as D-A-π-A motif dye sensitizers were designed, synthesized, and characterized. In these polymeric metal complexes with sulfur coordination, the metal complexes with sulfur coordination of benzodithiophene derivatives are used as auxiliary electron acceptors, 8-quinolinol derivatives are used as π-bridge and electron acceptors, and thienylbenzene-[1,2-b:4,5-b'] dithiophene (BDTT) are used as electron donors. The effect of different metal complexes with sulfur coordination on the photovoltaic performance of dye sensitizers has been systematically studied. Under AM 1.5 irradiation (100 mW cm^-2), the devices of dye-sensitized solar cells (DSSCs) based on five polymeric metal complexes with sulfur coordination exhibited a short-circuit current density (J_sc) of 13.43, 15.07, 18.00, 18.99, and 20.78 mA cm^-2, respectively, and their power conversion efficiencies (PCEs) were 7.10, 8.59, 10.68, 11.23, and 12.89%, respectively, and their thermal decomposition temperatures (T_d) were 251, 257, 265, 276, and 277 °C, respectively. The result shows that the J_sc and PCE of five polymeric metal complexes increase by degrees, and the PCE of BDTT-VBT-Hg is up to 12.89%, which is because of the strength of the coordination bonds between Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) and sulfur increases in turn so that the electron-withdrawing ability and electron-transfer ability of auxiliary electron acceptors is enhanced. These results provide a new way to develop stable and efficient metal complexes with sulfur coordination dye sensitizers in the future.

Enhanced Photocatalytic Alcohol Oxidation at the Interface of RuC-Coated TiO2 Nanorod Arrays

Visible-light-driven organic oxidations carried out under mild conditions offer a sustainable approach to performing chemical transformations important to the chemical industry. This work reports an efficient photocatalytic benzyl alcohol oxidation process using one-dimensional (1D) TiO2 nanorod (NR)-based photoanodes with surface-adsorbed ruthenium polypyridyl photocatalysts at room temperature. The photocatalyst bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine)Ru(II) (RuC) was covalently anchored onto TiO2 nanorod arrays grown on fluorine-doped tin oxide (FTO) electrode surfaces (FTO|t-TiO2|RuC, t = the thickness of TiO2 NR). Under aerobic conditions, the photophysical and photocatalytic properties of FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes were investigated in a solution containing a hydrogen atom transfer mediator (4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl, ACT) as cocatalyst. Dye-sensitized photoelectrochemical cells (DSPECs) using the FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes and ACT-containing electrolyte were investigated for carrying out photocatalytic oxidation of a lignin model compound containing a benzylic alcohol functional group. The best-performing anode surface, FTO|1-TiO2|RuC (shortest NR length), oxidized the 2° alcohol of the lignin model compound to the Cα-ketone form with a > 99% yield over a 4 h photocatalytic experiment with a Faradaic efficiency of 88%. The length of TiO2 NR arrays (TiO2 NRAs) on the FTO substrate influenced the photocatalytic performance with longer NRAs underperforming compared to the shorter arrays. The influence of the NR length is hypothesized to affect the homogeneity of the RuC coating and accessibility of the ACT mediator to the RuC-coated TiO2 surface. The efficient photocatalytic alcohol oxidation with visible light at room temperature as demonstrated in this study is important to the development of sustainable approaches for lignin depolymerization and biomass conversion.

Synthesis and Characterization of a 2,3-Dialkoxynaphthalene-Based Conjugated Copolymer via Direct Arylation Polymerization (DAP) for Organic Electronics

Poly[(5,5'-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(thiophene-2,2'-diyl))-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PEHONDTBT) was synthesized for the first time and through direct arylation polymerization (DAP) for use as p-donor material in organic solar cells. Optimized reaction protocol leads to a donor-acceptor conjugated polymer in good yield, with less structural defects than its analog obtained from Suzuki polycondensation, and with similar or even higher molecular weight than other previously reported polymers based on the 2,3-dialkoxynaphthalene monomer. The batch-to-batch repeatability of the optimized DAP conditions for the synthesis of PEHONDTBT was proved, showing the robustness of the synthetic strategy. The structure of PEHONDTBT was corroborated by NMR, exhibiting good solubility in common organic solvents, good film-forming ability, and thermal stability. PEHONDTBT film presented an absorption band centered at 498 nm, a band gap of 2.15 eV, and HOMO and LUMO energy levels of -5.31 eV and -3.17 eV, respectively. Theoretical calculations were performed to understand the regioselectivity in the synthesis of PEHONDTBT and to rationalize its optoelectronic properties. Bilayer heterojunction organic photovoltaic devices with PEHONDTBT as the donor layer were fabricated to test their photovoltaic performance, affording low power-conversion efficiency in the preliminary studies.

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

Recent decades have witnessed the rapid development of semiconducting polymers in terms of high charge mobilities and applications in transistors. Significant efforts have been made to develop various conjugated frameworks and linkers. However, studies are increasingly demonstrating that the side chains of semiconducting polymers can significantly affect interchain packing, thin film crystallinity, and thus semiconducting performance. Ways to modify the side alkyl chains to improve the interchain packing order and charge mobilities for conjugated polymers are first discussed. It is shown that modifying the branching chains by moving the branching points away from the backbones can boost the charge mobilities, which can also be improved through partially replacing branching chains with linear ones. Second, the effects of side chains with heteroatoms and functional groups are discussed. The siloxane-terminated side chains are utilized to enhance the semiconducting properties. The fluorinated alkyl chains are beneficial for improving both charge mobility and air stability. Incorporating H bonding group side chains can improve thin film crystallinities and boost charge mobilities. Notably, incorporating functional groups (e.g., glycol, tetrathiafulvalene, and thymine) into side chains can improve the selectivity of field-effect transistor (FET)-based sensors, while photochromic group containing side chains in conjugated polymers result in photoresponsive semiconductors and optically tunable FETs.

Electrochemical Polymerization-Fabricated Several Triphenylamine-Carbazolyl-Based Polymers with Improved Short-Circuit Current and High Adsorption Stability in Dye-Sensitized Solar Cells

Polymer dyes have many potential advantages, such as high molecular weight, better light capture ability, thermal stability, film-forming ability, light resistance, and electrochemical corrosion resistance. They are expected to provide opportunities for the development of high-stability dye-sensitized solar cells (DSCs). However, polymer DSCs (PDSCs) have poor short-circuit current and filling factor (FF) due to polymer aggregation and chain-winding effect. Therefore, the energy conversion efficiency is low. In this work, we are trying to find a way to solve this problem. Herein, three polymers, polyPAC-01, polyPAC-02, and polyPAC-03 with different π-bridge chains were prepared on a titanium dioxide electrode using an "adsorption first, then electropolymerization (EP)" process. Meanwhile, as a comparison, three oligomers, PAC-01, PAC-02, and PAC-03 with the same skeleton were synthesized by the Suzuki coupling reaction and fabricated on a titanium dioxide electrode with a "first polymerization, then adsorption" process. Then, the photoanode adsorbed by those polymers or oligomers were applied to DSCs. The results show that polymers prepared by the EP method obtained a higher short-circuit (J _sc) increase, exceeding 30% and a FF increase of about 10%, and finally, the photo-to-electric conversion efficiency (PCE) increased exceeding 40%, compared to the oligomers. In addition, desorption experiments in a harsh environment show that the EP method-synthesized polymers (polyPAC-03 as a representative) have better solvent resistance and adsorption stability than the corresponding oligomers (PAC-03). The results show that the process of "adsorption first, then EP" may be an effective way to solve the bottlenecks of low energy conversion efficiency on PDSCs and provide a new way to develop stable and efficient DSCs.

Anthracene-decorated TiO2 thin films with the enhanced photoelectrochemical performance

New insight of introducing new organic compounds for the efficient photogenerated charge separation is vitally important for the current solar energy conversion. Herein, (2Z,2'Z)-4,4'-(anthracene-2,6-diylbis(azanediyl))bis(4-oxobut-2-enoic acid) (ADA)/TiO₂ composite thin film is fabricated through the wet-impregnation strategy, which exhibits excellent photoelectrochemical performance (PEC). A combined study of ultraviolet-visible absorption spectra, scanning Kelvin probe maps, electrochemical and photoelectrochemical measurements reveals that the ADA/TiO₂ composite with narrow bandgap of 2.42 eV extends the photo response to the visible light region. The photocurrent generated by the optimal ADA/TiO₂ is 2.5 times higher than that of the pristine TiO₂. The result is attributed to the broader light absorption range and the separation of photoelectrons and holes prompted by ADA. Moreover, the high stability of the ADA/TiO₂ composite favors the practical application. The present work may offer a promising strategy for the low-cost PEC cell in the clean solar hydrogen production.

Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation

A polystyrene-based chromophore-catalyst assembly (poly-2) has been synthesized and assembled at a mesoporous metal oxide photoanode. The assembly contains water oxidation catalyst centers based on [Ru(trpy) (phenq)]²⁺ (Ru-Cat) and [Ru(bpy)₃]²⁺ derivatives (Ru-C) as chromophores (trpy= 2,2';6,2″- terpyridine, phenq = 2-(quinol-8'-yl)-1,10-phenanthroline and bpy = 2,2'-bipyridine). The photophysical and electrochemical properties of the polychromophore-oxidation catalyst assembly were investigated in solution and at the surface of mesoporous metal oxide films. The layer-by-layer (LbL) method was utilized to construct multilayer films with cationic poly-2 and anionic poly(acrylic acid) (PAA) for light-driven photochemical oxidations. Photocurrent measurements of (PAA/poly-2)₁₀ LbL films on mesoporous TiO₂ demonstrate light-driven oxidation of phenol and benzyl alcohol in aqueous solution. Interestingly, illumination of (PAA/poly-2)₅ LbL films on a fluorine doped SnO₂/TiO₂ core/shell photoanode in aqueous solution gives rise to an initial photocurrent (∼18.5 μA·cm^-2) that is in part ascribed to light driven water oxidation.

Polymer-Based Ruthenium(II) Polypyridyl Chromophores on TiO2 for Solar Energy Conversion

A polychromophoric light-harvesting assembly featuring a polystyrene (PS) backbone with ionic carboxylate-functionalized Ru(II) polypyridyl complexes as pendant groups (PS-Ru-A) was synthesized and successfully anchored onto mesoporous structured TiO2 films (TiO2 //PS-Ru-A). Studies of the resulting TiO2 //PS-Ru-A films carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) confirmed that the ionic carboxylated Ru(II) complexes from PS-Ru-A led to the surface immobilization on the TiO2 film. Monochromatic light photocurrent spectroscopy (IPCE) and white light (AM1.5G) current-voltage studies of dye-sensitized solar cells using the TiO2 //PS-Ru-A photoanode give rise to modest photocurrent and white light efficiency (24 % peak IPCE and 0.33 % PCE, respectively). The photostability of surface-bound TiO2 //PS-Ru-A films was tested and compared to a monomeric Ru(II) complex (TiO2 //Ru-A), showing an enhancement of ∼14 % in the photostability of PS-Ru-A. Transient absorption measurements reveal that electron injection from surface-bound pendants occurs on the picosecond time scale, similar to TiO2 //Ru-A, while time-resolved emission measurements reveal delayed electron injection occurring in TiO2 //PS-Ru-A on the nanosecond time scale, underscoring energy transport from unbound to surface-bound complexes. Additionally, charge recombination is delayed in PS-Ru-A, pointing towards intra-assembly hole transport to complexes away from the surface. Molecular dynamics simulations of PS-Ru-A in fluid solution indicate that a majority of the pendant Ru(II) complexes lie within 10-20 Å of each other, facilitating efficient energy- and charge transport among the pendant complexes.