Increasing the Open-Circuit Voltage of Dye-Sensitized Solar Cells via Metal-Ion Coordination

Molecular Orientation and Energy Transfer Dynamics of a Metal Oxide Bound Self-Assembled Trilayer

Self-assembly of molecular multilayers via metal ion linkages has become an important strategy for interfacial engineering of metalloid and metal oxide (MOx) substrates, with applications in numerous areas, including energy harvesting, catalysis, and chemical sensing. An important aspect for the rational design of these multilayers is knowledge of the molecular structure-function relationships. For example, in a multilayer composed of different chromophores in each layer, the molecular orientation of each layer, both relative to the adjacent layers and the substrate, influences the efficiency of vectorial energy and electron transfer. Here, we describe an approach using UV-vis attenuated total reflection (ATR) spectroscopy to determine the mean dipole tilt angle of chromophores in each layer in a metal ion-linked trilayer self-assembled on indium-tin oxide. To our knowledge, this is the first report demonstrating the measurement of the orientation of three different chromophores in a single assembly. The ATR approach allows the adsorption of each layer to be monitored in real-time, and any changes in the orientation of an underlying layer arising from the adsorption of an overlying layer can be detected. We also performed transient absorption spectroscopy to monitor interlayer energy transfer dynamics in order to relate structure to function. We found that near unity efficiency, sub-nanosecond energy transfer between the third and second layer was primarily dictated by the distance between the chromophores. Thus, in this case, the orientation had minimal impact at such proximity.

Holistically Optimizing Charge Carrier Dynamics Enables High-Performance Dye-Sensitized Solar Cells and Photodetectors

Charge carrier events across organic electronics are ubiquitous, and the derived optimization plays a crucial effect on improving the performance of organic electronics. Herein, a two-dimensional material (Ti₃C₂T_x) is incorporated into titanium dioxide (TiO₂) to impart the Ti₃C₂T_x/TiO₂ hybrid film enriched hydroxy group distribution, defect-negligible surface, upshifted work function, and enhanced conductivity yet electron mobility versus the pristine TiO₂ film. Therefore, intensified photon-harvesting ability, reduced charge carrier recombination, and efficient charge carrier collection are realized for dye-sensitized solar cells (DSSCs) based on the Ti₃C₂T_x/TiO₂ hybrid photoanode relative to control ones. Consequently, the modified DSSCs based on Z907 deliver superior efficiencies of 10.39 and 29.68% under 100 mW/cm² illumination and ∼1.9 mW/cm² dim light, respectively, being the highest values of Z907-based DSSCs. However, control devices only obtain lower efficiencies of 8.06 and 23.91% when undergoing the abovementioned illumination. On the other hand, the self-powered homologous photodetectors with the hybrid film as an electron-transporting layer present enhanced detectivity (1.69 × 10¹¹ Jones) and a shortened responsivity of 0.26 s versus that of control ones (1.39 × 10¹¹ Jones and 0.35 s). Our work implies that the Ti₃C₂T_x/TiO₂ hybrid film features high potential for improving the performance of organic electronics for its effect of holistically optimizing charge carrier dynamics.

Photocatalyst-Mediator Interface Modification by Surface-Metal Cations of a Dye-Sensitized H2 Evolution Photocatalyst

To develop highly active H₂ evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X'-RuCP⁶-Zr-RuP⁶@Pt-TiO₂ (X'-DSP) with different surface-bound metal cations (X' = Fe²⁺, Y³⁺, Zr⁴⁺, Hf⁴⁺, and Bi³⁺). In 0.5 M KI aqueous solution, the photocatalytic H₂ evolution activity under blue light irradiation (λ = 460 ± 15 nm) increased in the following order: nonmetal-modified DSP, H⁺-DSP (turn over number for 6 h irradiation = 35.2) < Fe²⁺-DSP (54.9) ≈ Bi³⁺-DSP (55.2) < Hf⁴⁺-DSP (65.5) ≈ Zr⁴⁺-DSP (68.3) ≈ Y³⁺-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf⁴⁺-DSP (18.4) and Bi³⁺-DSP (16.6), exhibited better activity under green light irradiation (λ = 530 ± 15 nm) than Zr⁴⁺-DSP (15.7) and H⁺-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.

Engineering Band-Type Alignment in CsPbBr3 Perovskite-Based Artificial Multiple Quantum Wells

Semiconductor heterostructures of multiple quantum wells (MQWs) have major applications in optoelectronics. However, for halide perovskites-the leading class of emerging semiconductors-building a variety of bandgap alignments (i.e., band-types) in MQWs is not yet realized owing to the limitations of the current set of used barrier materials. Here, artificial perovskite-based MQWs using 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), tris-(8-hydroxyquinoline)aluminum, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as quantum barrier materials are introduced. The structures of three different five-stacked perovskite-based MQWs each exhibiting a different band offset with CsPbBr₃ in the conduction and valence bands, resulting in a variety of MQW band alignments, i.e., type-I or type-II structures, are shown. Transient absorption spectroscopy reveals the disparity in charge carrier dynamics between type-I and type-II MQWs. Photodiodes of each type of perovskite artificial MQWs show entirely different carrier behaviors and photoresponse characteristics. Compared with bulk perovskite devices, type-II MQW photodiodes demonstrate a more than tenfold increase in the rectification ratio. The findings open new opportunities for producing halide-perovskite-based quantum devices by bandgap engineering using simple quantum barrier considerations.

The band bending effect of LiI/NaI treated TiO2 photoanodes on the performance of dye-sensitized solar cells

A new method has been developed for surface treatment of TiO2 photoanodes with LiI/NaI to enhance the photocurrent and, subsequently, the performance efficiency of the fabricated dye-sensitized solar cells (DSSCs). Three different concentrations (0.1, 0.25, and 0.5 mmol%) of LiI and NaI solutions were used to investigate the effect of this surface treatment on the device performance of DSSCs. A positive shift in the energy level of TiO2 has been experienced by surface treated devices, which is predominantly supported by the decrease in VOC. Furthermore, the introduction of LiI/NaI onto the TiO2 surface resulted in a reduction in the crystallite size, indicating an increase in the surface area which helps in more dye adsorption leading to higher JSC values of the devices. Besides, modification of the conduction band energy level, it also allows a fast electron injection process by shifting the density of states. Thus, this approach offers a simple but efficient route to enhance the photocurrent and efficiency of DSSCs.

Efficiency enhancement of ruthenium-based DSSCs employing A-π-D-π-A organic Co-sensitizers

A new bipyridyl Ru(ii) sensitizer incorporating triphenylamine and the 3,4-ethylenedioxythiophene (EDOT) ancillary ligand IMA5 was synthesized for dye-sensitized solar cells (DSSCs). The performance of these DSSCs has been enhanced via di-anchoring metal-free organic sensitizers, denoted IMA1-4, with structural motif A-π-D-π-A and incorporating phenyl-dibenzothiophene-phenyl (Ph-DBT-Ph) as the main building block but with different anchoring groups (A). These new organic sensitizers were well-characterized and used as efficient co-sensitizers. Their photophysical, electrochemical and photovoltaic properties were studied. Furthermore, molecular modeling studies using DFT calculations were used to investigate their suitability as effective sensitizers/co-sensitizers. The molecular orbital isodensity showed distinguishable delocalization of the intramolecular charge in the DBT moiety. The photovoltaic characterization showed that IMA3 had the best DSSC performance (η = 2.41%). In addition, IMA1-4 was co-sensitized in conjunction with the newly synthesized IMA5 complex to enhance light harvesting across expanded spectral regions and thus improve efficiency. The solar cells co-sensitized with IMA2, IMA3 and IMA4 exhibited improved efficiency (η) of 6.25, 6.19 and 5.83%, respectively, which outperformed the device employing IMA5 alone (η = 5.54%) owing to the improvement in the loading of IMA2, IMA3 and IMA4 in the presence of IMA5 on the surface of the TiO2 nanoparticles, and charge recombination was suppressed.

An in situ exchange mechanism for dye molecules and cations at the nano-semiconductor film/electrolyte interface

This communication uses EQCM in combination with the potentiostatic method to study the in situ exchange mechanism for dye molecules and cations on the nano-film surface under a constant potential.

Fabrication of a counter electrode for dye-sensitized solar cells (DSSCs) using a carbon material produced with the organic ligand 2-methyl-8-hydroxyquinolinol (Mq)

Dye sensitized solar cells (DSSCs) are low cost solar cells and their fabrication process is easy relative to silicon based solar cells. Platinum can be replaced with carbon materials as counter electrodes in DSSCs because of their good catalytic properties and low cost. A carbon material was produced by carbonization of an organic ligand (2 methyl 8-hydroxy quinolinol (Mq)) at high temperature in flowing argon gas. Polyvinylpyrrolidone (PVP) was used as a surfactant for making carbon slurry from carbon produced using Mq. For the fabrication of the counter electrode, a carbon coating was prepared by using the doctor blading technique and the carbon slurry was coated on the FTO substrate. DSSCs based on the carbon counter electrode exhibit a higher V oc of 0.75 V than that of the Pt counter electrode (0.69 V). DSSCs based on the carbon material showed a power conversion efficiency (PCE) of 4.25% and fill factor (FF) of 0.51 which are slightly lower than those of the platinum (Pt) based counter electrode which showed a PCE of 5.86% and FF of 0.68.

Towards implementing hierarchical porous zeolitic imidazolate frameworks in dye-sensitized solar cells

A one-pot method for encapsulation of dye, which can be applied for dye-sensitized solar cells (DSSCs), and synthesis of hierarchical porous zeolitic imidazolate frameworks (ZIF-8), is reported. The size of the encapsulated dye tunes the mesoporosity and surface area of ZIF-8. The mesopore size, Langmuir surface area and pore volume are 15 nm, 960-1500 m² · g^-1 and 0.36-0.61 cm³ · g^-1, respectively. After encapsulation into ZIF-8, the dyes show longer emission lifetimes (greater than 4-8-fold) as compared to the corresponding non-encapsulated dyes, due to suppression of aggregation, and torsional motions.

Stabilization of a Cyclometalated Ruthenium Sensitizer on Nanocrystalline TiO2 by an Electrodeposited Covalent Layer

A cyclometalated ruthenium sensitizer 3 containing a triphenylamine unit was synthesized and immobilized on a nanocrystalline TiO₂ surface. By using oxidative electrochemical deposition, a covalent layer of a related cyclometalated ruthenium complex 2 was coupled to the top of dye 3. Electrochemical studies suggested that complex 2 was immobilized on the TiO₂/3 film surface by a tetraphenylbenzidine linker to form a dimer-like structure. The immobilization of 3 and 2 was further supported by absorption spectral analysis. The resulting electrodeposited TiO₂/(3+2) film displays significantly enhanced sensitizer stabilization toward basic aqueous NaOH solution with respect to the original TiO₂/3 film. The dye-sensitized solar cells with the TiO₂/(3+2) photoanode display a power conversion efficiency of 4.4%, which is slightly inferior to that with the TiO₂/3 film (5.1%) under the same measurement conditions.

Nanocrystalline Sb-doped SnO2 films modified with cyclometalated ruthenium complexes for two-step electrochromism

Sb-Doped nanocrystalline SnO2 (SnO2:Sb) thin films functionalized with cyclometalated ruthenium complexes 1 or 2 on FTO conductive glasses have been prepared and characterized. These complexes contain a redox-active amine unit separated from the ruthenium ion by a phenyl or biphenyl linker, respectively, to modify the absorption wavelengths at different redox states. Near-infrared electrochromism of both films has been examined by oxidative spectroelectrochemical measurements and double-potential-step chronoamperometry. A contrast ratio (ΔT%) of 33% at 1070 nm and 63% at 696 nm has been achieved for the SnO2:Sb/1 film in two stepwise oxidation processes, respectively. The other film with complex 2 shows two-step electrochromism at 1310 and 806 nm with ΔT% of 36% and 76%, respectively. The response time of electrochromic switching is around a few seconds. Taking advantage of the good contrast ratio, the rapid response, and the long retention time of each oxidation state, these films have been successfully used to demonstrate surface-confined flip-flop memory functions with a high ON/OFF ratio.

Impact of Ligand Substitutions on Multielectron Redox Properties of Fe Complexes Supported by Nitrogenous Chelates

Redox flow batteries (RFBs) have recently been recognized as a potentially viable technology for scalable energy storage. To take full advantage of RFBs, one possible approach for achieving high energy densities is to maximize a number of redox events by utilizing charge carriers capable of multiple one-electron transfers within the electrochemical window of solvent. However, past efforts to develop more efficient electrolytes for nonaqueous RFBs have mostly been empirical. In this manuscript, we shed light on design principles by theoretically investigating the effects of systematically substituting pyridyl moieties with imine ligands within a series of Fe complexes with some experimental validation. We found that such replacement is an effective strategy for reducing the molecular weight-to-charge ratios of these complexes. Simultaneously, calculations suggest that the reduction potentials and ligand-based redox activity of such substituted N-heterocyclic Fe compounds might be maintained within their +4 → -1 charge states. Additionally, by theoretically examining the role of coordination geometry, vis-à-vis reducing the number of redox noninnocent ligands within the first coordination sphere, we have demonstrated that Fe complexes with one such ligand were also capable of supporting multielectron reduction events and exhibited reduction potentials similar to their parent analogs supported by two or three of the same multidentate ligands. However, some differences in redox nature within the lower (+2 → -1) charge states were also noticed. Specifically, complexes containing two bidentate ligands, or one tridentate ligand, exhibited ligand-based reductions, whereas compounds with one bidentate ligand exhibited metal-centered reductions. The current results pave the way toward the design of the next-generation of Fe complexes with lower molecular weights and greater stored energy for redox flow batteries.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.