Polyoxoplatinates as covalently dynamic electron sponges and molecular electronics materials

In organic systems, dynamic covalent chemistry provides an adaptive approach (i.e., "covalent dynamics") where thermodynamic equilibria are used to tailor structural and electronic changes in molecular assemblies. The covalent dynamics finds utility in the design of novel self-healing materials, sensors, and actuators. Herein, using density functional theory (DFT) we explore the structural, electronic and transport properties of the Pt-based polyoxometalate (POM) [Pt^III ₁₂O₈(SO₄)₁₂]^4- and its derivatives. The latter POM has six redox responsive {O-Pt-Pt-O} moieties and prospects for storage of up to twelve electrons, thus exemplifying how dynamic covalent chemistry may manifest itself in fully inorganic systems. Simulations of the Au/POM/Au junction show that the electron conduction strongly depends on the redox of the POM but more weakly on its rotations with respect to the Au surface. Moreover, the POM shows promising spin-polarized current behaviour, which can be modulated using bias and gate voltages.

Insights from Adsorption and Electron Modification Studies of Polyoxometalates on Surfaces for Molecular Memory Applications

This Account highlights recent experimental and theoretical work focusing on the development of polyoxometalates (POMs) as possible active switching units in what may be called "molecule-based memory cells". Herein, we critically discuss how multiply charged vanadium-containing POMs, which exhibit stable metal-oxo bonds and are characterized by the excellent ability to change their redox states without significant structural distortions of the central polyoxoanion core, can be immobilized best and how they may work optimally at appropriate surfaces. Furthermore, we critically discuss important issues and challenges on the long way toward POM-based nanoelectronics. This Account is divided into four sections shedding light on POM interplay in solution and on surfaces, ion soft-landing of mass-selected POMs on surfaces, electronic modification of POMs on surfaces, and computational modeling of POMs on surfaces. The sections showcase the complex nature of far-reaching POM interactions with the chemical surroundings in solution and the properties of POMs in the macroscopic environment of electrode surfaces. Section 2 describes complex relationships of POMs with their counter-cations, solvent molecules, and water impurities, which have been shown to exhibit a direct impact on the resulting surface morphology, where a concentration-dependent formation of micellar structures can be potentially observed. Section 3 gives insights into the ion soft-landing deposition of mass-selected POMs on electrode surfaces, which emerges as an appealing method because the simultaneous deposition of agglomeration-stimulating counter-cations can be avoided. Section 4 provides details of electronic properties of POMs and their modification by external electronic stimuli toward the development of multiple-state resistive (memristive) switches. Section 5 sheds light on issues of the determination of the electronic structure properties of POMs across their interfaces, which is difficult to address by experiment. The studies summarized in these four sections have employed various X-ray-scattering, microscopy, spectroscopy, and computational techniques for imaging of POM interfaces in solution and on surfaces to determine the adsorption type, agglomeration tendency, distribution, and oxidation state of deposited molecules. The presented research findings and conceptual ideas may assist experimentalists and theoreticians to advance the exploration of POM electrical conductivity as a function of metal redox and spin states and to pave the way for a realization of ("brain-inspired") POM-based memory devices, memristive POM-surface device engineering, and energy efficient nonvolatile data storage and processing technologies.

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage, or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO₂ substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS, and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UV-visible absorption spectra, and theoretical calculations. The electronic properties of these POM-based molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Inorganic Molecular Clusters with Facile Preparation and Neutral pH for Efficient Hole Extraction in Organic Solar Cells

The development of electrode interlayers for hole extraction is a great challenge in the field of organic solar cells (OSCs). At present, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the only solution-processed anode interlayer (AIL) that can be used to achieve power conversion efficiencies (PCEs) over 15% in OSC devices, even though there are several well-known drawbacks in practical applications of PEDOT:PSS. Herein, we use an inorganic molecular cluster (IMC) as the AIL for making highly efficient and large-area OSCs. The IMC possesses several advantages in serving as the AIL, such as neutral pH, excellent optical transmittance, high work function, good film-forming properties, and low cost. OSCs using the IMC can achieve a high PCE of 13.38%, which is superior to the PCE of the PEDOT:PSS device. This is among the few examples of OSC devices with solution-processed and pH neutral AILs showing higher PCE than PEDOT:PSS devices. Ultraviolet photoelectron spectroscopy and electron spin resonance results indicate the formation of inorganic-organic heterojunction, which is crucial for efficient hole extraction. More importantly, the IMC is compatible with printing processing. Using a blade-coated IMC film, we fabricated a large-area OSC of 1 cm² and a high PCE of 9.5% was achieved.

Charge transport through redox active [H7P8W48O184]33- polyoxometalates self-assembled onto gold surfaces and gold nanodots

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

Multi-electron reduction of Wells-Dawson polyoxometalate films onto metallic, semiconducting and dielectric substrates

The investigation of conditions allowing multi-electron reduction and reoxidation of polyoxometalate (POM) films onto solid substrates is considered an issue of critical importance for their successful incorporation in electronic devices, different types of sensors and catalytic systems. In the present paper, the rich multi-electron redox chemistry of films of Wells-Dawson ammonium salts, namely (NH4)6P2Mo18O62 and (NH4)6P2W18O62, on top of metallic (Al), semiconducting (ITO) and dielectric (SiO2) substrates under ambient conditions is investigated. The respective Keggin heteropolyacids, H3PMo12O40 and H3PW12O40, are also investigated for comparison. On Al substrates, the Wells-Dawson ammonium salts are found to be significantly more reduced (4-6e-) compared to the respective Keggin heteropolyacids (∼2e-), in accordance with their deeper lying lowest unoccupied molecular orbital (LUMO) level. Subsequent thermal treatment in air results in reoxidation of the initially highly reduced POM films. Similar behavior is found on ITO substrates, but in initially less reduced (2-4e-) Wells-Dawson POM films. On the other hand, on SiO2 substrates, the thermal reduction of (NH4)6P2Mo18O62 film is observed and attributed to the thermal oxidation of ammonium counterions by [P2Mo18O62]6- anions. Overall, the multi-electron reduction of Wells-Dawson ammonium salts onto metallic and semiconducting substrates (Al, ITO) is determined by the relative position of the LUMO level of POMs in relation to the Fermi level of the substrate (i.e. substrate work function) and affected in a synergistic way by the presence of ammonium counterions. In contrast, on dielectric substrates (SiO2) the reduction of Wells-Dawson POMs ((NH4)6P2Mo18O62) is attributed only to the oxidation of ammonium counterions.

Fully Coated Semitransparent Organic Solar Cells with a Doctor-Blade-Coated Composite Anode Buffer Layer of Phosphomolybdic Acid and PEDOT:PSS and a Spray-Coated Silver Nanowire Top Electrode

In the aim to realize high performance semitransparent fully coated organic solar cells, printable electrode buffer layers and top electrodes are two important key technologies. An ideal ink for the preparation of the electrode buffer layer for printed top electrodes should have good wettability and negligible solvent corrosion to the underlying layer. This work reports a novel organic-inorganic composite of phosphomolybdic acid (PMA) and PEDOT:PSS that features excellent wettability with the active layer and printed top Ag nanowires and high resistibility to solvent corrosion. This composite buffer layer can be easily deposited on a polymer surface to form a smooth, homogeneous film via spin-coating or doctor-blade coating. Through the use of this composite anode buffer layer, fully coated semitransparent devices with doctor-blade-coated functional layers and spray-coated Ag nanowire top electrodes showed the highest power conversion efficiency (PCE) of 5.01% with an excellent average visible-light transmittance (AVT) of 50.3%, demonstrating superior overall characteristics with a comparable performance to and a much higher AVT than cells based on a thermally evaporated MoO₃/Ag/MoO₃ thin film electrode (with a PCE of 5.77% and AVT of 19.5%). The current work reports the fabrication of fully coated inverted organic solar cells by combining doctor-blade coating and spray coating and, more importantly, demonstrates that a nanocomposite of a polyoxometalate and conjugated polymer could be an excellent anode buffer layer for the fully coated polymer solar cells with favorable interfacial contact, hole extraction efficiency, and high comparability with full printing.

Evolutionary Metal Oxide Clusters for Novel Applications: Toward High-Density Data Storage in Nonvolatile Memories

Because of current fabrication limitations, miniaturizing nonvolatile memory devices for managing the explosive increase in big data is challenging. Molecular memories constitute a promising candidate for next-generation memories because their properties can be readily modulated through chemical synthesis. Moreover, these memories can be fabricated through mild solution processing, which can be easily scaled up. Among the various materials, polyoxometalate (POM) molecules have attracted considerable attention for use as novel data-storage nodes for nonvolatile memories. Here, an overview of recent advances in the development of POMs for nonvolatile memories is presented. The general background knowledge of the structure and property diversity of POMs is also summarized. Finally, the challenges and perspectives in the application of POMs in memories are discussed.

Non-covalent polyhedral oligomeric silsesquioxane-polyoxometalates as inorganic–organic–inorganic hybrid materials for visible-light photocatalytic splitting of water

Hydrophilic to hydrophobic conversion of polyoxometalate by integration of POSS-NH₂via non-covalent interactions for sustainable photocatalytic hydrogen production reactions.

Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells

Thin films of a heteropolytungstate, lithium silicotungstate (Li₄SiW₁₂O₄₀, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH₃NH₃PbI₃ PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with J_SC of 22.16 mA cm^-2, V_OC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO₂ layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH₃NH₃PbI₃ more efficiently and also exhibits longer electron lifetime than the TiO₂ layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.

Low Work Function Lacunary Polyoxometalates as Electron Transport Interlayers for Inverted Polymer Solar Cells of Improved Efficiency and Stability

Effective interface engineering has been shown to play a vital role in facilitating efficient charge-carrier transport, thus boosting the performance of organic photovoltaic devices. Herein, we employ water-soluble lacunary polyoxometalates (POMs) as multifunctional interlayers between the titanium dioxide (TiO₂) electron extraction/transport layer and the organic photoactive film to simultaneously enhance the efficiency, lifetime, and photostability of polymer solar cells (PSCs). A significant reduction in the work function (W_F) of TiO₂ upon POM utilization was observed, with the magnitude being controlled by the negative charge of the anion and the selection of the addenda atom (W or Mo). By inserting a POM interlayer with ∼10 nm thickness into the device structure, a significant improvement in the power conversion efficiency was obtained; the optimized POM-modified poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- 33 ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C₇₀ butyric acid methyl ester (PTB7:PC₇₀BM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO₂ cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PC₇₀BM or indene-C₆₀ bisadduct (IC₆₀BA), which enhanced their efficiency up to 4.34 and 6.21%, respectively, when using POM interlayers; this represents a 25-33% improvement as compared to the reference cells. Moreover, increased lifetime under ambient air and improved photostability under constant illumination were observed in POM-modified devices. Detailed analysis shows that the improvements in efficiency and stability synergistically stem from the reduced work function of TiO₂ upon POM coverage, the improved nanomorphology of the photoactive blend, the reduced interfacial recombination losses, the superior electron transfer, and the more effective exciton dissociation at the photoactive layer/POM/TiO₂ interfaces.

Solution-based electrical doping of semiconducting polymer films over a limited depth

Solution-based electrical doping protocols may allow more versatility in the design of organic electronic devices; yet, controlling the diffusion of dopants in organic semiconductors and their stability has proven challenging. Here we present a solution-based approach for electrical p-doping of films of donor conjugated organic semiconductors and their blends with acceptors over a limited depth with a decay constant of 10-20 nm by post-process immersion into a polyoxometalate solution (phosphomolybdic acid, PMA) in nitromethane. PMA-doped films show increased electrical conductivity and work function, reduced solubility in the processing solvent, and improved photo-oxidative stability in air. This approach is applicable to a variety of organic semiconductors used in photovoltaics and field-effect transistors. PMA doping over a limited depth of bulk heterojunction polymeric films, in which amine-containing polymers were mixed in the solution used for film formation, enables single-layer organic photovoltaic devices, processed at room temperature, with power conversion efficiencies up to 5.9 ± 0.2% and stable performance on shelf-lifetime studies at 60 °C for at least 280 h.

Extended visible photosensitivity of carboxyethyltin functionalized polyoxometalates with common organic dyes enabling enhanced photoelectric performance

A kind of broad spectral responsive photoelectrode was assembled from carboxyethyltin–POM derivatives and organic dyes.

Tailor-made Covalent Organic-Inorganic Polyoxometalate Hybrids: Versatile Platforms for the Elaboration of Functional Molecular Architectures

Post-functionalization of organically modified polyoxometalates (POMs) is a powerful synthetic tool to devise functional building blocks for the rational elaboration of POM-based molecular materials. In this personal account we focus on iodoaryl-terminated POM platforms, describe reliable routes to the synthesis of covalent organic-inorganic POM-based hybrids and their integration into advanced molecular architectures or multi-scale assemblies as well as their immobilization onto surfaces. Valorisation of the remarkable redox properties of POMs in the fields of artificial synthesis and molecular electronic is especially considered.

A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices

We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO3 units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the postbaking atmosphere and temperature after film formation. When the PMA film was baked in N2, the Mo in the PMA was reduced to Mo(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional hole-injection materials (HIMs), poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), and evaporated MoO3, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM.

Polyoxometalate-conductive polymer composites for energy conversion, energy storage and nanostructured sensors

The exchange of electric charges between a chemical reaction centre and an external electrical circuit is critical for many real-life technologies. This perspective explores the "wiring" of highly redox-active molecular metal oxide anions, so-called polyoxometalates (POMs) to conductive organic polymers (CPs). The major synthetic approaches to these organic-inorganic hybrid materials are reviewed. Typical applications are highlighted, emphasizing the current bottlenecks in materials development. Utilization of the composites in the fields of energy conversion, electrochemical energy storage, sensors and nanoparticle "wiring" into conductive materials are discussed. The outlook section presents the authors' views on emerging fields of research where the combination of POMs and CPs can be expected to provide novel materials for groundbreaking new technologies. These include light-weight energy storage, high-sensitivity toxin sensors, artificial muscles, photoelectrochemical devices and components for fuel cells.