Epitaxial Growth of Highly Transparent Metal-Porphyrin Framework Thin Films for Efficient Bifacial Dye-Sensitized Solar Cells

Recent advances on surface mounted metal-organic frameworks for energy storage and conversion applications: Trends, challenges, and opportunities

Establishing green and reliable energy resources is very important to counteract the carbon footprints and negative impact of non-renewable energy resources. Metal-organic frameworks (MOFs) are a class of porous material finding numerous applications due to their exceptional qualities, such as high surface area, low density, superior structural flexibility, and stability. Recently, increased attention has been paid to surface mounted MOFs (SURMOFs), which is nothing but thin film of MOF, as a new category in nanotechnology having unique properties compared to bulk MOFs. With the advancement of material growth and synthesis technologies, the fine tunability of film thickness, consistency, size, and geometry with a wide range of MOF complexes is possible. In this review, we recapitulate various synthesis approaches of SURMOFs including epitaxial synthesis approach, direct solvothermal method, Langmuir-Blodgett LBL deposition, Inkjet printing technique and others and then correlated the synthesis-structure-property relationship in terms of energy storage and conversion applications. Further the critical assessment and current problems of SURMOFs have been briefly discussed to explore the future opportunities in SURMOFs for energy storage and conversion applications.

Metal-Organic Framework Nanosheets as Templates to Enhance Performance in Semi-Crystalline Organic Photovoltaic Cells

Optimizing the orientation, crystallinity, and domain size of components within organic photovoltaic (OPV) devices is key to maximizing their performance. Here a broadly applicable approach for enhancing the morphology of bulk heterojunction OPV devices using metal-organic nanosheets (MONs) as additives is demonstrated. It is shown that addition of porphyrin-based MONs to devices with fully amorphous donor polymers lead to small improvements in performance attributed to increased light absorption due to nanosheets. However, devices based on semi-crystalline polymers show remarkable improvements in power conversion efficiency (PCE), more than doubling in some cases compared to reference devices without nanosheets. In particular, this approach led to the development of PffBT4T2OD-MON-PCBM device with a PCE of 12.3%, which to the authors' knowledge is the highest performing fullerene based OPV device reported in literature to date. Detailed analysis of these devices shows that the presence of the nanosheets results in a higher fraction of face-on oriented polymer crystals in the films. These results therefore demonstrate the potential of this highly tunable class of two-dimensional nanomaterials as additives for enhancing the morphology, and therefore performance, of semi-crystalline organic electronic devices.

Photovoltaic performance and power conversion efficiency prediction of double fence porphyrins

To explore high efficiency dye-sensitized solar cells (DSSCs), two experimentally derived (single fence and double fence porphyrins) and two theoretically designed zinc porphyrin molecules with D-D-π-A-A configurations were studied. Density functional theory and time-dependent density functional theory were employed to simulate these two experimental dyes and dye@TiO₂ systems to understand why the double fence porphyrin molecule exhibits better photovoltaic performance than the single fence porphyrin molecule. For the short-circuit current (J_SC), the various parameters that affected the experimental magnitude of J_SC were analyzed from different aspects of absorption, charge transfer and chemical parameters as well as an electron injection process. The almost equal open-circuit voltages (V_OC) in the experiment were predicted by theoretical V_OC calculations. Our model predicted power conversion efficiencies (PCEs) of 1.993% and 10.866% for the single and double fence molecules, respectively, which are in accordance with the experimental values of 3.48% and 10.69%, respectively. In addition, one designed two new molecules based on the double fence porphyrin molecule with a 2-methyl-2H-benzo[d][1,2,3]triazole (BTA) unit bearing one fluorine and two fluorine atoms as the guest acceptor, respectively. Compared to the original molecules, the engineered molecules significantly improved the photovoltaic parameters, J_SC and V_OC, thereby causing excellent PCEs. The most outstanding designed molecule reached a PCE of 12.155%, and is considered a candidate dye for high-efficiency DSSC. This study provides insights into the photoelectric properties of single and double fence porphyrins. It also demonstrated that the strong electron-withdrawing ability of fluorine atoms would enhance the photovoltaic performance and provide a guideline for the further design of double fence porphyrins.

Morphology of Nanometric Overlayers Made of Porphyrin-Type Molecules Physisorbed on Cellulose Iβ Crystals and Nanocrystals

Molecular dynamics simulations based on an atomistic empirical force field have been carried out to investigate structural, thermodynamic, and dynamical properties of adlayers made of porphyrin-type molecules physisorbed on surfaces of cellulose Iβ nanocrystals. The results show that low-index surfaces provide a thermally stable, weakly perturbing support for the deposition of non-hydrogen-bonded organic molecules. At submonolayer coverage, the discoidal porphyrin molecules lay flat on the surface, forming compact 2D clusters with clear elements of ordering. The adlayer grows layer-by-layer for the smallest porphyrin species on compact cellulose surfaces, while forming 3D clusters on a first relatively ordered adlayer (Stranski-Krastanov growth) in all other cases. The adsorption energy exceeds ∼1 eV per molecule, underlying the thermal stability of the adsorbate. Entropy plays a non-negligible role, destabilizing to some extent the adlayer. The in-plane dynamics of the smallest porphyrin species, i.e., porphine, on compact surfaces shows signs of superlubricity, due to the low energy and momentum exchange between the flat admolecule and the equally flat cellulose surface.

Room-Temperature Negative Differential Resistance in Surface-Supported Metal-Organic Framework Vertical Heterojunctions

The advances of surface-supported metal-organic framework (SURMOF) thin-film synthesis have provided a novel strategy for effectively integrating metal-organic framework (MOF) structures into electronic devices. The considerable potential of SURMOFs for electronics results from their low cost, high versatility, and good mechanical flexibility. Here, the first observation of room-temperature negative differential resistance (NDR) in SURMOF vertical heterojunctions is reported. By employing the rolled-up nanomembrane approach, highly porous sub-15 nm thick HKUST-1 films are integrated into a functional device. The NDR is tailored by precisely controlling the relative humidity (RH) around the device and the applied electric field. The peak-to-valley current ratio (PVCR) of about two is obtained for low voltages (<2 V). A transition from a metastable state to a field emission-like tunneling is responsible for the NDR effect. The results are interpreted through band diagram analysis, density functional theory (DFT) calculations, and ab initio molecular dynamics simulations for quasisaturated water conditions. Furthermore, a low-voltage ternary inverter as a multivalued logic (MVL) application is demonstrated. These findings point out new advances in employing unprecedented physical effects in SURMOF heterojunctions, projecting these hybrid structures toward the future generation of scalable functional devices.

Porphyrin and phthalocyanine-based metal organic frameworks beyond metal-carboxylates

Given the ubiquitous role of porphyrins in natural systems, these molecules and related derivatives such as phthalocyanines are fascinating building units to achieve functional porous materials. Porphyrin-based MOFs have been developed over the past three decades, yet chemically robust frameworks, necessary for applications, have been achieved much more recently and this field is expanding. This progress is partially driven by the development of porphyrins and phthalocyanines bearing alternative coordinating groups (phosphonate, azolates, phenolates…) that allowed moving the related MOFs beyond metal-carboxylates and achieving new topologies and properties. In this perspective article we first give a brief outline of the synthetic pathways towards simple porphyrins and phthalocyanines bearing these complexing groups. The related MOF compounds are then described; their structural and textural properties are discussed, as well as their stability and physical properties. An overview of the resulting nets and topologies is proposed, showing both the similarities with metal-carboxylate phases and the peculiarities related to the alternative coordinating groups. Eventually, the opportunities offered by this recent research topic, in terms of both synthesis pathways and modulation of pore size and shape, stability and physical properties, are discussed.

Polyoxometalate modified transparent metal selenide counter electrodes for high-efficiency bifacial dye-sensitized solar cells

We present a facile one-step hydrothermal approach for the growth of PW₁₁Co/Co_0.85Se on a conductive glass substrate, which could be used as transparent CE in bifacial DSSCs with enhanced front and back efficiencies of 7.56% and 5.82%, respectively.

Host-Guest Thin Films by Confining Ultrafine Pt/C QDs into Metal-Organic Frameworks for Highly Efficient Hydrogen Evolution

Combining the features of host templates and guest species is an efficient strategy to optimize the photo/electrocatalytic performance. Herein, novel host-guest thin-film electrocatalysts are designed and developed with Pt doped carbon (Pt/C) confined into porphyrin-based metal-organic frameworks (MOFs). Porous MOF PCN-222 and PCN-221 thin films are used as the host templates and fabricated using vapor-assisted deposition method, and then the guest Pt/C quantum dots are encapsulated into the MOFs by loading the glucose mixed H₂ PtCl₆ and heating at 200 °C. Thanks to the confinement effect of MOF pores, the homogenous and ultrafine Pt/C nanowires (Pt/CNWs) and nanodots (Pt/CNDs) are confined in nanochannels of PCN-222 and nanocages of PCN-221 (Pt/CNW@PCN-222 and Pt/CND@PCN-221), respectively. The electrocatalytic study shows that the host-guest thin films have highly-efficient electrocatalytic hydrogen evolution performance under light irradiation. Furthermore, the time-resolved photoluminescent results reveal that Pt/CNW@PCN-222 has a faster charge transfer (441 ps) from PCN-222 to Pt/CNWs comparing to that (557 ps) of Pt/CND@PCN-221, indicating the guests with different shapes play an important role in the electrocatalytic performance. This work serves to present both the outstanding level of control in the precise synthesis and high potential for nanocomposite thin films in photo-electrocatalytic application.

Application of Metal-Organic Frameworks and Covalent Organic Frameworks as (Photo)Active Material in Hybrid Photovoltaic Technologies

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are two innovative classes of porous coordination polymers. MOFs are three-dimensional materials made up of secondary building blocks comprised of metal ions/clusters and organic ligands whereas COFs are 2D or 3D highly porous organic solids made up by light elements (i.e., H, B, C, N, O). Both MOFs and COFs, being highly conjugated scaffolds, are very promising as photoactive materials for applications in photocatalysis and artificial photosynthesis because of their tunable electronic properties, high surface area, remarkable light and thermal stability, easy and relative low-cost synthesis, and structural versatility. These properties make them perfectly suitable for photovoltaic application: throughout this review, we summarize recent advances in the employment of both MOFs and COFs in emerging photovoltaics, namely dye-sensitized solar cells (DSSCs) organic photovoltaic (OPV) and perovskite solar cells (PSCs). MOFs are successfully implemented in DSSCs as photoanodic material or solid-state sensitizers and in PSCs mainly as hole or electron transporting materials. An innovative paradigm, in which the porous conductive polymer acts as standing-alone sensitized photoanode, is exploited too. Conversely, COFs are mostly implemented as photoactive material or as hole transporting material in PSCs.

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.

Strong metal-support interactions enable highly transparent Pt-Mo2C counter electrodes of bifacial dye-sensitized solar cells

Highly transparent and active Pt-Mo₂C counter electrodes were successfully fabricated by the strong metal-support interaction, with high dispersity of Pt nanoclusters on Mo₂C support, which endowed bifacial dye-sensitized solar cells with a rear-to-front efficiency ratio as high as 0.75.

Progress of MOF-Derived Functional Materials Toward Industrialization in Solar Cells and Metal-Air Batteries

The cutting-edge photovoltaic cells are an indispensable part of the ongoing progress of earth-friendly plans for daily life energy consumption. However, the continuous electrical demand that extends to the nighttime requires a prior deployment of efficient real-time storage systems. In this regard, metal-air batteries have presented themselves as the most suitable candidates for solar energy storage, combining extra lightweight with higher power outputs and promises of longer life cycles. Scientific research over non-precious functional catalysts has always been the milestone and still contributing significantly to exploring new advanced materials and moderating the cost of both complementary technologies. Metal-organic frameworks (MOFs)-derived functional materials have found their way to the application as storage and conversion materials, owing to their structural variety, porous advantages, as well as the tunability and high reactivity. In this review, we provide a detailed overview of the latest progress of MOF-based materials operating in metal-air batteries and photovoltaic cells.