Post-assembly atomic layer deposition of ultrathin metal-oxide coatings enhances the performance of an organic dye-sensitized solar cell by suppressing dye aggregation

Far-red active unsymmetrical squaraine dyes containing N-arylated indoline donors for dye sensitized solar cells

Squaraine dyes possess sharp far-red active transition with high extinction coefficient and form aggregates on TiO2 surface. Aggregation of dyes on TiO2 has been considered as a detrimental factor for DSSC device performance, which can be controlled by appending alkyl groups to the dye structures. Hence by integrating alkylated (alkyl groups with both in-plane and out-of-plane) aryl group with indoline moiety to make it compatible with other electrolytes and for controlling the dye-aggregation, a series of squaraine acceptor-based dyes SQA4-6 have been designed and synthesized. SQA4-6 dyes showed absorption between 642 and 653 nm (λmax ), photophysical and electrochemical studies indicated that the HOMO energy levels of this sets of dyes are well aligned with the potentials of I- /I 3 - $$ {\mathrm{I}}_3^{-} $$ and [Co(bpy)3 ]2+/3+ redox shuttles for better dye regeneration process. DSSC device efficiency of 3% has been achieved for SQA5 dye with iodolyte (I- /I 3 - $$ {\mathrm{I}}_3^{-} $$ ) electrolyte in the presence of 0.3 mM of chenodeoxycholic acid (CDCA). The IPCE profile of DSSC device fabricated with SQA4-6 dyes indicated the contribution of aggregated structures for the photocurrent generation.

D-π-A Structured Porphyrin and Organic Dyes with Easily Synthesizable Donor Units for Low-Cost and Efficient Dye-Sensitized Solar Cells

This study aimed to develop low-cost D-π-A structured porphyrin and organic dyes with easily synthesizable donor units instead of the conventional complex multistep synthetic donor unit of Hexyloxy-BPFA [bis(7-(2,4-bis(hexyloxy)phenyl)-9,9-dimethyl-9H-fluoren-2-yl)amine] used in SGT-021 and SGT-149 as well-known record cosensitizers with an extremely high power conversion efficiency (PCE). The design strategy concerned the easier synthesis of low-cost donor units with inversion structures in donor groups via donor structural engineering, particularly by changing the position of the fluorene and phenylene units in the donor moiety while keeping the π-bridge and acceptor unit unchanged, leading to the synthesis of two D-π-A structured porphyrins [SGT-021(D0) and SGT-021(D)] and one D-π-A structured organic sensitizer [SGT-149(D)] for dye-sensitized solar cells (DSSCs). Specifically, porphyrin SGT-021(D0) incorporated two hexyl chains into the 9-position of each fluorene, while SGT-021(D) and SGT-149(D) substituted two hexyloxy chain units to the terminal position of each fluorene in the donor groups of porphyrin dyes. The effect of the position of the fluorene and phenylene units in the donor moiety on the photochemical and electrochemical properties, as well as the photovoltaic performance, was compared with the reference dyes of SGT-021 and SGT-149, previously reported by the research group. After optimizing the DSSC devices, SGT-021(D) and SGT-021(D0) achieved a high PCE of 11.6 and 10.5%, respectively, while SGT-149(D) exhibited a little lower PCE of 10.3% under the standard AM 1.5G light intensity. The cell performance of DSSC devices based on SGT-021(D) and SGT-149(D) was inferior to the corresponding reference dyes of SGT-021 and SGT-149 due to their lower donating ability of Hexyloxy-BPFA than Hexyloxy-BFPA.

Multifaceted Excited State Dynamics of Coumarin Dyes Anchored on Al2O3 Film

The co-facially stacked dyes on semiconductor films serve as an alternative model to elucidate the photo-driven exciton dynamics occurring in a molecular assembly. In this study, we report the unique emission properties of coumarin dye adsorbed on the surface of the semiconductor film, measured by ultrafast time-resolved fluorescence. When a rigid coumarin derivative, 7-hydroxycoumarin-3-carboxylic acid (OHCCA), is anchored on the Al₂O₃ film, the dye manifests dual emissions from the two lowest excited states. Various anchoring modes of a carboxylic acid group on the Al₂O₃ surface are invoked to account for the unusual emission process. Additionally, we identified characteristic transition dipole interactions in the well-stacked dye aggregates, which leads to discernible excitonic splitting in the electronic transitions. Femtosecond time-resolved fluorescence reveals that the excimer formation in the aggregate occurs with the time constant of 550 fs. Picosecond time-resolved emission spectra confirm the subsequent structural relaxations of the nascent excimer. The enhanced transition dipole via the electronic coupling between OHCCA and metal oxide can be responsible for the dual emission and the ultrafast excimer formation.

Embedding an Amphiphilic 4-Hydroxy Thiazole Dye in Langmuir Matrices: Studying Miscibilities with Arylic and Alkylic Matrix Amphiphiles via Langmuir Isotherms and Photo-induced Force Microscopy

We present here a fundamental study on the miscibility between a prototype amphiphilic dye and alkylic and arylic Langmuir monolayers. Embedding dyes in such matrices is crucial for utilizing dyes in any photo-energy conversion process if the involved dyes form aggregates that provide thermal deactivation channels. Because miscibility in Langmuir matrices depends on the blending ratio between the dye and matrix and on the Langmuir film density, as characterized via the surface pressure and the mean molecular area, we employ Langmuir miscibility studies to identify ideal miscibility parameters for each matrix. Atomic force microscopy (AFM) results support miscibility between the dye and both matrix materials at low surface pressures, where smooth and homogeneous films are obtained. AFM and photo-induced force microscopy (PiFM) reveal phase separation if the Langmuir monolayers are deposited at surface pressures above 8 mN/m at which reorientation of the chromophores has been reported. The nanoscale chemical fingerprint mapping enabled by PiFM enables assigning segregated spots to small stearic acid (SA)-enriched domains that have not been detected via AFM, thus demonstrating the value of the IR-spectroscopic contrast provided by PiFM. In this work, we have presented a so far unexploited matrix material (terphenylene carboxylic acid; TPCA) and found it equally suitable for embedding dyes as the standard amphiphile SA. In contrast to SA, TPCA is composed of rigid and electrically conducting π-electron systems, hence, being predestined for aligning dyes in Langmuir matrices and for application in optoelectronic systems.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Flexible to rigid: IR spectroscopic investigation of a rhenium-tricarbonyl-complex at a buried interface

This work explores the solid-liquid interface of a rhenium-tricarbonyl complex embedded in a layer of zirconium oxide deposited by atomic layer deposition (ALD). Time-resolved and steady state infrared spectroscopy were applied to reveal the correlations between the thickness of the ALD layer and the spectroscopic response of the system. We observed a transition of the molecular environment from flexible to rigid, as well as limitations to ligand exchange and excited state quenching on the embedded complexes, when the ALD layer is roughly of the same height as the molecules.

Cause, Regulation and Utilization of Dye Aggregation in Dye-Sensitized Solar Cells

As an important member of third generation solar cell, dye-sensitized solar cells (DSSCs) have the advantages of being low cost, having an easy fabrication process, utilizing rich raw materials and a high-power conversion efficiency (PCE), prompting nearly three decades as a research hotspot. Recently, increasing the photoelectric conversion efficiency of DSSCs has proven troublesome. Sensitizers, as the most important part, are no longer limited to molecular engineering, and the regulation of dye aggregation has become a widely held concern, especially in liquid DSSCs. This review first presents the operational mechanism of liquid and solid-state dye-sensitized solar cells, including the influencing factors of various parameters on device efficiency. Secondly, the mechanism of dye aggregation was explained by molecular exciton theory, and the influence of various factors on dye aggregation was summarized. We focused on a review of several methods for regulating dye aggregation in liquid and solid-state dye-sensitized solar cells, and the advantages and disadvantages of these methods were analyzed. In addition, the important application of quantum computational chemistry in the study of dye aggregation was introduced. Finally, an outlook was proposed that utilizing the advantages of dye aggregation by combining molecular engineering with dye aggregation regulation is a research direction to improve the performance of liquid DSSCs in the future. For solid-state dye-sensitized solar cells (ssDSSCs), the effects of solid electrolytes also need to be taken into account.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces

Three chromophores of the general form [Ru(bpy')₂(4,4'-(PO₃H₂)₂-2,2'-bipyridine)]²⁺, where bpy' is 4,4'-(C(CH₃)₃)₂-2,2'-bipyridine (Ru(dtb)₂P); 4,4'-(CH₃O)₂-2,2'-bipyridine (Ru(OMe)₂P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO₂ nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru^III/II intermolecular hole hopping in 0.1 M LiClO₄/CH₃CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E_1/2 (Ru^III/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant k_R extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO₂|Ru(OMe)₂P (k_et = 1.4 × 10⁶ s^-1) > TiO₂|RuP (7.1 × 10⁵ s^-1) > TiO₂|Ru(dtb)₂P (6.5 × 10⁴ s^-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO₂|RuP and TiO₂|Ru(dtb)₂P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO₂(e^-)|Ru^III → TiO₂|Ru^II charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al₂O₃ on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

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.

Competitive Binding of Organic Dyes between Cucurbiturils and Octa Acid

Employing six cationic water-soluble organic dye molecules as probes, we have attempted to qualitatively understand the factors that govern the attraction between such molecules and the anionic water-soluble host, octa acid (OA). Examination of the competitive host-guest complexation between cucurbit[8]uril (CB[8]) and OA using absorption and emission spectroscopy revealed that the dye molecules included within CB[8] could be "pulled out" by OA. However, an order of magnitude higher concentration of OA was required to shift the equilibrium toward OA, suggesting that attraction between the anionic host OA and the cationic dye molecules such as cresyl violet perchlorate and methylene blue is weaker than the hydrophobic and cation-dipolar interaction between these dye molecules and CB[8]. The importance of Coulombic attraction between OA and dye molecules is also revealed by monomer-to-dimer conversion upon addition of OA to an aqueous solution of monomeric dye molecules. Under conditions where the dye-to-OA ratio is high, freely dissolved monomeric dye molecules are attracted to the exterior of OA and aggregate as dimers on the exterior wall of OA. On the other hand, at high ratios of OA to dye molecules, the dye molecules adsorb as monomers on the exterior of OA. Thus, the monomer-to-dimer ratio in aqueous solution can be controlled by adjusting the ratio of dye to OA molecules. The results presented are of value in qualitatively understanding the relative binding properties of ionic guests with ionic hosts. Studies are qualitative in nature, and further detailed quantitative studies planned for the future are likely to provide deeper understanding of the interaction between water-soluble dye molecules, OA, and CB.

A program for automatically predicting supramolecular aggregates and its application to urea and porphin

Not only the molecular structure but also the presence or absence of aggregates determines many properties of organic materials. Theoretical investigation of such aggregates requires the prediction of a suitable set of diverse structures. Here, we present the open-source program EnergyScan for the unbiased prediction of geometrically diverse sets of small aggregates. Its bottom-up approach is complementary to existing ones by performing a detailed scan of an aggregate's potential energy surface, from which diverse local energy minima are selected. We crossvalidate this approach by predicting both literature-known and heretofore unreported geometries of the urea dimer. We also predict a diverse set of dimers of the less intensely studied case of porphin, which we investigate further using quantum chemistry. For several dimers, we find strong deviations from a reference absorption spectrum, which we explain using computed transition densities. This proof of principle clearly shows that EnergyScan successfully predicts aggregates exhibiting large structural and spectral diversity. © 2018 Wiley Periodicals, Inc.

Effects of Post-Assembly Molecular and Atomic Passivation of Sensitized Titania Surface: Dynamics of Electron Transfer Measured from Femtoseconds to Seconds

The dynamics of electron transfer at the dye-titania and titania-electrolyte interfaces is investigated in two post-sensitization processes: (i) atomic layer deposition of blocking alumina coating and (ii) hierarchical molecular multicapping. To measure the electron transfer dynamics, time-resolved spectroscopic methods (femtosecond transient absorption on the time scale from femtoseconds to nanoseconds and electrochemical impedance spectroscopy on the time scale from milliseconds to seconds) are applied to the complete dye-sensitized solar cells with cobalt-based electrolyte and champion ADEKA-1 dye (with silyl-anchor unit) or its popular carboxyl-anchor analogue, MK-2 dye. Both molecular capping and alumina blocking layers slow down the electron injection process (the average rate constant decreases from 1.1 ps^-1 to 0.4 ps^-1) and partial sub-nanosecond back electron transfer from titania to the dye (from ca. 10 ns^-1 to 5 ns^-1). Very small alumina layers (of 0.1 nm thickness) have the highest impact on reducing the rate constants of these electron transfer processes, and for the thicknesses greater than 0.3 nm the rate constants hardly change. In contrast, the electron recombination between titania and electrolyte, occurring on the millisecond time scale, starts to be significantly suppressed for the blocking layers of 0.3 nm or more in thickness (up to ca. 20 times for 0.5 nm thickness with respect to that for untreated sample), improving open circuit voltage and fill factor of the cells. The amplitude of the relative photocurrent (short circuit current per number of absorbed photons) is found to depend almost exclusively on the ultrafast and fast processes taking place in the first nanoseconds after dye excitation. The positive impact of coadsorbents on the solar cells performance for both ADEKA-1 and MK-2 is also studied.

Organic–inorganic hybrid photocatalyst for carbon dioxide reduction

Efficient hybrid photocatalysts for carbon dioxide reduction were developed from dye-sensitized TiO₂ nanoparticles and their catalytic performance was optimized by ternary organic/inorganic components. Thus, the hybrid system consists of (E)-2-cyano-3-(5′-(5′′-(p-(diphenylamino)phenyl)thiophen-2′′-yl)thiophen-2′-yl)-acrylic acid as a sensitizer and fac-[Re(4,4′-bis(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)₃Cl] as a reduction catalyst (ReP), both of which have been fixed onto TiO₂ semiconductors (s-TiO₂, h-TiO₂, d-TiO₂). Mott–Schottky analysis on flat-band potential (E_fb) of TiO₂ mesoporous films has verified that E_fb can be finely modulated by volume variation of water (0 to 20 vol%). The increase of added water resulted in substantial positive shifts of E_fb from −1.93 V at 0 vol% H₂O, to −1.74 V (3 vol% H₂O), to −1.56 V (10 vol% H₂O), and to −1.47 V (20 vol% H₂O). As a result, with addition of 3–10 vol% water in the photocatalytic reaction, conversion efficiency of CO₂ to CO increased significantly reaching a TON value of ∼350 for 30 h. Catalytic activity enhancement is mainly attributed to (1) the optimum alignment of E_fb by 3–10 vol% water with respect to the of the dye and E_red of ReP for smooth electron transfer from photo-excited dye to RePvia the TiO₂ semiconductor and (2) the water-induced acceleration of chemical processes on the fixed ReP. In addition, the energy level was further tuned by variation of the dye and ReP amounts. We also found that the intrinsic properties of TiO₂ sources (morphology, size, agglomeration) exert a great influence on the overall photocatalytic activity of this hybrid system. Implications of the present observations and reaction mechanisms are discussed in detail.

Engendering Long-Term Air and Light Stability of a TiO2-Supported Porphyrinic Dye via Atomic Layer Deposition

Organic and porphyrin-based chromophores are prevalent in liquid-junction photovoltaic and photocatalytic solar-cell chemistry; however, their long-term air and light instability may limit their practicality in real world technologies. Here, we describe the protection of a zinc porphyrin dye, adsorbed on nanoparticulate TiO₂, from air and light degradation by a protective coating of alumina grown with a previously developed post-treatment atomic layer deposition (ALD) technique. The protective Al₂O₃ ALD layer is deposited using dimethylaluminum isopropoxide as an Al source; in contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum isopropoxide does not degrade the zinc porphyrin dye, as confirmed by UV-vis measurements. The growth of this protective ALD layer around the dye can be monitored by an in-reactor quartz crystal microbalance (QCM). Furthermore, greater than 80% of porphyrin light absorption is retained over ∼1 month of exposure to air and light when the protective coating is present, whereas almost complete loss of porphyrin absorption is observed in less than 2 days in the absence of the ALD protective layer. Applying the Al₂O₃ post-treatment technique to the TiO₂-adsorbed dye allows the dye to remain in electronic contact with both the semiconductor surface and a surrounding electrolyte solution, the combination of which makes this technique promising for numerous other electrochemical photovoltaic and photocatalytic applications, especially those involving the dye-sensitized evolution of oxygen.

Photodriven hydrogen evolution by molecular catalysts using Al2O3-protected perylene-3,4-dicarboximide on NiO electrodes

Photodriven charge transfer dynamics are described for an atomic layer deposition-stabilized, organic dye-sensitized photocathode architecture that produces hydrogen.

The design of efficient hydrogen-evolving photocathodes for dye-sensitized photoelectrochemical cells (DSPECs) requires the incorporation of molecular light absorbing chromophores that are capable of delivering reducing equivalents to molecular proton reduction catalysts at rates exceeding those of charge recombination events. Here, we report the functionalization and kinetic analysis of a nanostructured NiO electrode with a modified perylene-3,4-dicarboximide chromophore (PMI) that is stabilized against degradation by atomic layer deposition (ALD) of thick insulating Al₂O₃ layers. Following photoinduced charge injection into NiO in high yield, films with Al₂O₃ layers demonstrate longer charge separated lifetimes as characterized via femtosecond transient absorption spectroscopy and photoelectrochemical techniques. The photoelectrochemical behavior of the electrodes in the presence of Co(ii) and Ni(ii) molecular proton reduction catalysts is examined, revealing reduction of both catalysts. Under prolonged irradiation, evolved H₂ is directly observed by gas chromatography supporting the applicability of PMI embedded in Al₂O₃ as a photocathode architecture in DSPECs.

Carbonate Doping in TiO2 Microsphere: The Key Parameter Influencing Others for Efficient Dye Sensitized Solar Cell

Four key parameters namely light trapping, density of light harvesting centre, photoinduced electron injection and electron transport without self-recombination are universally important across all kinds of solar cells. In the present study, we have considered the parameters in the context of a model Dye Sensitized Solar Cell (DSSC). Our experimental studies reveal that carbonate doping of TiO2 mesoporous microspheres (doped MS) makes positive influence to all the above mentioned key parameters responsible for the enhanced solar cell efficiency. A simple method has been employed to synthesize the doped MS for the photoanode of a N719 (ruthenium dye)-based DSSC. A detail electron microscopy has been used to characterize the change in morphology of the MS upon doping. The optical absorption spectrum of the doped MS reveals significant shift of TiO2 (compared to that of the MS without doping) towards maximum solar radiance (~500 nm) and the excellent scattering in the entire absorption band of the sensitizing dye (N719). Finally, and most importantly, for the first time we have demonstrated that the solar cells with doped MS offers better efficiency (7.6%) in light harvesting compared to MS without doping (5.2%) and also reveal minimum self recombination of photoelectrons in the redox chain.

Synthesis, characterization, and water oxidation by a molecular chromophore-catalyst assembly prepared by atomic layer deposition. The "mummy" strategy

A new strategy for preparing spatially-controlled, multi-component films consisting of molecular light absorbing chromophores and water oxidation catalysts on high surface area, mesoporous metal oxide surfaces is described. Atomic layer deposition (ALD) is used to embed a surface-bound chromophore in a thin layer of inert Al₂O₃, followed by catalyst binding to the new oxide surface. In a final step, catalyst surface-binding is stabilized by a subsequent ALD overlayer of Al₂O₃. The ALD assembly procedure bypasses synthetic difficulties arising from the preparation of phosphonic acid derivatized, covalently-linked assemblies. An ALD mummy-based assembly has been used to demonstrate photoelectrochemical dehydrogenation of hydroquinone. Electrocatalytic water oxidation at pH 8.8 is observed over a 2 hour electrolysis period and light-assisted water oxidation over a 6 hour photolysis period with O₂ detected with a generator-collector electrode configuration.