Schmidt-Mende, Lukas
0000-0001-6867-443X Berufsbeschreibung
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A Dibenzotetrathiafulvalene-Bridged Bis(alkenylruthenium) Complex and Its One- and Two-Electron-Oxidized Forms
We report on the synthesis of the new bis(alkenylruthenium) complex DBTTF-(ViRu)2 with a longitudinally extended, π-conjugated dibenzotetrathiafulvalene (DBTTF) bridge, characterized by multinuclear NMR, IR, and UV/vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Cyclic and square-wave voltammetry revealed that DBTTF-(ViRu)2 undergoes four consecutive oxidations. IR, UV/vis/near-IR, and electron paramagnetic resonance spectroscopy indicate that the first oxidation involves the redox-noninnocent DBTTF bridge, while the second oxidation is biased toward one of the peripheral styrylruthenium entities, thereby generating an electronically coupled mixed-valent state ({Ru}-CH═CH)•+-DBTTF•+-(CH═CH-{Ru}) [{Ru} = Ru(CO)Cl(PiPr3)2]. The latter is apparently in resonance with the ({Ru}-CH═CH)•+-DBTTF-(CH═CH-{Ru})•+ and ({Ru}-CH═CH)-DBTTF2+-(CH═CH-{Ru}) forms, which are calculated to lie within 19 kJ/mol. Higher oxidized forms proved too unstable for further characterization. The reaction of DBTTF-(ViRu)2 with the strong organic acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, tetracyano-p-benzoquinodimethane (TCNQ), and F4TCNQ resulted in formation of the DBTTF-(ViRu)2•+ radical cation, as shown by various spectroscopic techniques. Solid samples of these compounds were found to be highly amorphous and electrically insulating.
Understanding the Methylammonium Chloride-Assisted Crystallization for Improved Performance of Lead-Free Tin Perovskite Solar Cells
In the quest for perovskite materials with reduced toxicity, Sn perovskites are emerging. However, they suffer from material instability and rapid crystallization, leading to high defect densities in the films. In this work, the methylammonium chloride (MACl)-assisted crystallization as a route to improve stability and optoelectronic quality of quasi 2D/3D PEA0.08FA0.92SnI3 perovskite is demonstrated. For an optimal additive amount (10 mol%), a 37% increase in power conversion efficiency is found. Notably, MACl enhances the films' stability, evidenced by temporal PL tracking. Understanding the effect of MACl addition in this system is interesting for the pursuit of efficient and stable tin-based devices. The investigations show that MACl addition causes a shift in the optical bandgap and improves morphology, indicating effects in the bulk crystal structure. X-ray photoelectron spectroscopy confirms the presence of Cl on the surface, but no indication of MA+ is found. Intriguingly, UV photoelectron spectroscopy shows pronounced changes in the density of states. For the first time, it is shown that MACl promotes the formation of a two-dimensional layer via the surface accumulation of PEA+. The MACl additive lowers the absorber's ionization energy, possibly facilitating hole extraction. Overall, this work highlights a facile route to control the crystallization of Sn perovskites.
Heterostructured 3D-Co-MOF@CoO/Ni‖3D-C-Fe4N@NiCu/SS hybrids as high-performance electrode materials for efficient hybrid supercapacitor
Designing of 3D heterostructured hybrid materials are effective step, in the pursuit of attaining higher electrochemical performance compared to their pristine metal/metal oxide counterparts, owing to the enlarged active area of electrode-electrolyte interaction, and fast ion transportation path. In this regard, binder-free heterostructured electrodes, 3D-Co-MOF@CoO/Ni (positive) and 3D-C-Fe4N@NiCu/SS (negative) are fabricated to construct a hybrid supercapacitor (HSC). This core-shell structured positive electrode revealed better electrochemical performance (592.8mC/cm2 at 2mA/cm2) than bare CoO/Ni, which is nearly twice as good. Meanwhile, the hierarchical microporous 3D-C-Fe4N@NiCu/SS electrode achieved a broad potential of 0 to − 1 V with a maximum areal capacity of 852 mC/cm2 at 2mA/cm2 and 72 % stability over 10000 GCD cycles. The 3D heterostructured morphologies of the electrodes endowed the 3D-Co-MOF@CoO/Ni‖3D-C-Fe4N@NiCu/SS HSC with a high volumetric energy value of 2.305mWh/cm3 with a volumetric power of 325mW/cm3. The unique architecture and engineering strategies adopted in this research work can pave the way to attain desired electrochemical output in the future.
Uncovering solvent-engineering mechanisms in Y6:PM6 solar cells
Additives, like 1-chloronaphtalene (CN), are commonly used in Y6:PM6 solar cells as they lead to an increased power conversion efficiency. In this work, we investigate the influence of CN during spin coating of Y6:PM6 dissolved in chloroform via an in situ transmission setup. We show that, in the presence of CN, the film formation of Y6:PM6 can be divided into two parts: one related to the evaporation of chloroform and one related to the evaporation of CN. This is mostly related to Y6 being dissolved in CN. We find that even for low CN concentration, the film formation is not completed for several minutes after the spin coating process. Furthermore, the removal of CN is needed to achieve a smooth film surface. We demonstrate that this fast removal can be achieved by spin coating the electron transport layer PDINN from methanol. The methanol is acting as an anti-solvent for the CN, leading to its removal from the film. Using this approach, solar cells fabricated with a high CN concentration of 5% feature a comparable performance to ones with more common concentrations between 0.5% and 1%.
Nanoscale Surface Photovoltage Spectroscopy
Abstract Understanding electron and ion dynamics is an important task for improving modern energy materials, such as photovoltaic perovskites. These materials usually have delicate nano‐ and microstructures that influence the device parameters. To resolve detailed structure–function relationships on the relevant micro‐ and nanometer length scales, the current macroscopic and microscopic measurement techniques are often not sufficient. Here, nanoscale surface photovoltage spectroscopy (nano‐SPV) and nanoscale ideality factor mapping (nano‐IFM) via time‐resolved Kelvin probe force microscopy are introduced. These methods can map nanoscale variations in charge carrier recombination, ion migration, and defects. To show the potential of nano‐SPV and nano‐IFM, these methods are applied to perovskite samples with different morphologies. The results clearly show an improved uniformity of the SPV and SPV decay distribution within the perovskite films upon passivation and increasing the grain size. Nevertheless, nano‐SPV and nano‐IFM can still detect local variations in the defect density on these optimized samples, guiding the way for further optimization.
Charge transfer in copper oxide thin films deposited at different electrodeposition potential
The deposition potential affects the structural, morphological, optical, and electrochemical impedance spectroscopy properties of cuprous oxide (Cu2O) thin films formed on copper (Cu) substrates adopting a three-electrode electrochemical deposition procedure. XRD data revealed that the deposited films have a cubic structure established with desired (111) growth orientation. Scanning electron microscopy (SEM) images reveal that Cu2O film has very well three-sided pyramid-shaped grains which are equally spread over the surface of the Cu substrates and change substantially when the plating potential is changed. The photo-current density of prepared Cu2O thin films was increased from −1.41 × 10−4 to −3.01 × 10−4 A/cm2 with increasing the deposition potential of −0.3 to −0.6 V, respectively. Further, Cu2O thin films obtained at −0.6 V have the minimum charge transfer resistance (Rct) than Cu2O thin films synthesized at −0.3 to −0.5 V, suggesting that Cu2O thin films produced at −0.6 V have the highest electron transfer efficiency.
A Triethyleneglycol C60 Mono-adduct Derivative for Efficient Electron Transport in Inverted Perovskite Solar Cells
Inverted perovskite solar cells (PSCs) have attracted increasing attention in recent years owing to their low-temperature fabrication proces s. However, they suffer from a limited number of electron transport materials available with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) to be the most widely studied based on its appropriate energy levels and high electron mobility. The low relative permittivity and aggregation tendency upon illumination of PCBM, however, compromises the solar cell efficiency whereas its modest hydrophobicity negatively impacts on the device stability. Alternative electron transport materials with desired properties and appropriate degree of hydrophobicity are thus desirable for further developments in inverted PSCs. Herein, we synthesize a triethyleneglycol C60 mono-adduct derivative (termed as EPF03) and test it as a novel electron transport material to replace PCBM in inverted PSCs based on a quadruple cation (RbCsMAFA) perovskite. We also compare this derivative with two novel fullerenes decorated with two (EPF01) or one dodecyl (EPF02) long side chains. The latter two fail to perform efficiently in inverted PSCs whereas the former enabled a power conversion efficiency of 18.43%, which represents a 9% improvement compared to the reference device using PCBM (17.21%). The enhanced performance mainly stems from improved electron extraction and reduced recombination enabled by the insertion of the large relative permittivity amongst other properties of EPF03. Furthermore, our results indicate that triethylene glycol side chains can also passivate perovskite trap states, suppress ion migration and enhance photostability and long-term stability of EPF03 based perovskite solar cells.
Organic binary charge-transfer compounds of 2,2′ : 6′,2′′ : 6′′,6-trioxotriphenylamine and a pyrene-annulated azaacene as donors
Three binary charge-transfer (CT) compounds resulting from the donor 2,2′ : 6′,2′′ : 6′′,6-trioxotriphenylamine (TOTA) and the acceptors F4TCNQ and F4BQ and of a pyrene-annulated azaacene (PAA) with the acceptor F4TCNQ are reported. The identity of these CT compounds are confirmed by single-crystal X-ray diffraction as well as by IR, UV-vis-NIR and EPR spectroscopy. X-ray diffraction analysis reveals a 1 : 1 stoichiometry for TOTA·F4TCNQ, a 2 : 1 donor : acceptor ratio in (TOTA)2·F4BQ, and a rare 4 : 1 stoichiometry in (PAA)4·F4TCNQ, respectively. Metrical parameters of the donor (D) and acceptor (A) constituents as well as IR spectra indicate full CT in TOTA·F4TCNQ, partial CT in (TOTA)2·F4BQ and only a very modest one in (PAA)4·F4TCNQ. Intricate packing motifs are present in the crystal lattice with encaged, π-stacked (F4TCNQ-)2 dimers in TOTA·F4TCNQ or mixed D/A stacks in the other two compounds. Their solid-state UV-vis-NIR spectra feature CT transitions. The CT compounds with F4TCNQ are electrical insulators, while (TOTA)2·F4BQ is weakly conducting.
Hybrid supercapacitors, formation, and new advances with different electrochemical electrodes based on layered double hydroxides (LDHs), metal–organic framework (MOF) materials, smart supercapacitors
Hybrid supercapacitors (HSCs) are made by the combination of electric double-layer capacitor (EDLC) materials, various types of pseudocapacitive, and battery-type materials. The progress made on the improvement of energy density without sacrificing the power density attracted the researchers to move toward HSC. The specific capacitance of the HSC showed a superior value than the EDLC or pseudocapacitance-based supercapacitors. Numerous advancements have been made on the HSCs with the development in preparation of electrode materials, electrolyte, formation of component, device structure, and the new mechanisms for the improvement of electrochemical characteristics. This chapter specifically emphasis the new development made on the HSCs based on the layered double hydroxides, metal–organic frameworks (MOFs), MOF-derived materials and their composites. The specific characteristics requirement of the electrode material, electrolyte, the substrate in support of electrode materials, and the progress made on them for smart supercapacitors are discussed.
Solvent-Assisted Crystallization of an α-Fe2O3 Electron Transport Layer for Efficient and Stable Perovskite Solar Cells Featuring Negligible Hysteresis
Inorganic–organic metal halide perovskite solar cells (PSCs) show power conversion efficiency values approaching those of state-of-the-art silicon solar cells. In a quest to find suitable charge transport materials in PSCs, hematite (α-Fe2O3) has emerged as a potential electron transport layer (ETL) in n–i–p planar PSCs due to its low cost, UV light stability, and nontoxicity. Yet, the performance of α-Fe2O3-based PSCs is far lower than that of state-of-the-art PSCs owing to the poor quality of the α-Fe2O3 ETL. In this work, solvent-assisted crystallization of α-Fe2O3 ETLs was carried out to examine the impact of solvents on the optoelectronic properties of α-Fe2O3 thin films. Among the various solvents used in this study (deionized water, ethanol, iso-propanol, and iso-butanol), optimized ethanol-based α-Fe2O3 ETLs lead to champion device performance with a power conversion efficiency of 13% with a reduced hysteresis index of 0.04 in an n–i–p-configured PSC. The PSC also exhibited superior long-term inert and ambient stabilities compared to a reference device made using a SnO2 ETL. Through a series of experiments spanning structural, morphological, and optoelectronic properties of the various α-Fe2O3 thin films and their devices, we provide insights into the reasons for the improved photovoltaic performance. It is noted that the formation of a pinhole-free compact morphology of ETLs facilitates crack-free surface coverage of the perovskite film atop an α-Fe2O3 ETL, reduces interfacial recombination, and enhances charge transfer efficiency. This work opens up the route toward novel ETLs for the development of efficient and photo-stable PSCs.