For this study, we have synthesized two different poly(diketopyrrolopyrrole) copolymers with different chain lengths. The diketopyrrolopyrrole (DPP) core is substituted with oligoethylene glycol side chains to increase its compatibility with water and copolymerized with either fluorene or carbazole moieties. The polymers form a composite photocatalyst with anatase TiO2. Detailed characterization such as NMR spectroscopy, UV?Vis, DRIFT, and UPS is used to analyze the structural and optical properties as well as the frontier orbital energy levels of the components and the composite materials. The optical properties of the polymers are tunable with respect to the copolymer used, opening up the possibility of optimizing the photocatalytic activity. These composite materials (without the addition of a co-catalyst) provide up to an eightfold enhancement of the hydrogen evolution reaction (HER) compared to pristine TiO2. The polymers also exhibit stability in the reaction medium as shown by solid-state NMR, DRIFT, and UV?Vis spectroscopy. A significant influence of the chain length of the polymers on HER is found as well. As the chain length increases, the activity toward hydrogen evolution increases. We show a correlation between hydrogen evolution and PDPP chain length whereby the active site of the photocatalytic process remains the inorganic semiconductor.
A variety of new functional materials based on triarylamines, such as low molecular weight glasses which possess hole conducting/photoconductive properties as well as amorphous bifunctional materials which combine photoconductive and non-linear optical (NLO) properties in one compound, have been synthesized. The new hole transporting glasses belong to the class of 1,3,5-tris(triaryldiamino)benzenes (TTADB). The hyperbranched structure and the large aryl groups attached as substituents lead to high glass transition temperatures (Tg) of up to 141°C in these compounds. The TTADBs do not recrystallize upon cooling from the melt, but form stable glasses. Cyclic voltammetry studies reveal multi-oxidation stages for these compounds of which the first oxidation is reversible. The HOMO energy values determined from CV for TTADB-1 and TTADB-2 are -4.82 and -4.94 eV, respectively. Light emitting diodes with the structure ITO/TTADB-2/Alq3/Al (where ITO=indium tin oxide) show high efficiency and large current carrying capacity. Further, bifunctional compounds have been synthesized in which a photoconductive moiety such as bis(carbazolyl)triphenylamine or bis(diphenylamino)triphenylamine is covalently bound to different NLO chromophores. Some of these compounds are thermally and morphologically stable amorphous materials, possessing Tg in the range from 85 to 122°C. Cyclic voltammetry measurements reveal that the HOMO energy values are between -4.81 and -5.45 eV. In photorefractive measurements using 40 µm thick samples, a diffraction efficiency of 27%, which corresponds to a refractive index modulation (Δn) of 3.5×10-3, a maximum two beam coupling gain coefficient (Γ) of 90 cm-1 and a response time of 40 ms were obtained.
We report the synthesis, characterization, and properties of a new class of hole−transport dyes, poly(dithienylisothianaphthene phenyldiamine)s (poly-DTITNPDs). These polymers are characterized by the presence of a low band gap isothianaphthene (ITN) and triarylamine units in the main chain. A modified Ullmann polycondensation reaction using a phase-transfer catalyst was utilized to prepare these polymers. The optical, thermal, and electrochemical properties were studied and compared to those of poly(triphenyldiamine ether) without having an ITN group and poly(dithienylisothianaphthene) without having triarylamine groups in the main chain. The new polymers, reported here, exhibit improved thermal stability and higher glass-transition temperatures. The incorporation of ITN group into the main chain of a polytriarylamine causes an appreciable lowering of the band gap energy up to 1.6 eV. This results in light-harvesting hole−transport dyes having less mismatch with the solar spectrum. Moreover, these polymers exhibit reversible redox behavior and possess HOMO values of about −4.7 eV and LUMO values of about −2.9 eV.
Abstract All solid-state rechargeable lithium metal batteries (SS-LMBs) are gaining more and more importance because of their higher safety and higher energy densities in comparison to their liquid-based counterparts. In spite of this potential, their low discharge capacities and poor rate performances limit them to be used as state-of-the-art SS-LMBs. This arise due to the low intrinsic ionic and electronic transport pathways within the solid components in the cathode during the fast charge/discharge processes. Therefore, it is necessary to have a cathode with good electron conducting channels to increase the active material utilization without blocking the movement of lithium ions. Since SS-LMBs require a different morphology and composition of the cathode, we selected LiFePO 4 (LFP) as a prototype and, we have systematically studied the influence of the cathode composition by varying the contents of active material LFP, conductive additives (super C65 conductive carbon black and conductive graphite), ion conducting components (PEO and LiTFSI) in order to elucidate the best ion as well as electron conduction morphology in the cathode. In addition, a comparative study on different cathode slurry preparation methods was made, wherein ball milling was found to reduce the particle size and increase the homogeneity of LFP which further aids fast Li ion transport throughout the electrode. The SEM analysis of the resulting calendered electrode shows the formation of non-porous and crack-free structures with the presence of conductive graphite throughout the electrode. As a result, the optimum LFP cathode composition with solid polymer nanocomposite electrolyte (SPNE) delivered higher initial discharge capacities of 114 mAh g-1 at 0.2C rate at 30 ᴼC and 141 mAh g-1 at 1C rate at 70 ᴼC. When the current rate was increased to 2C, the electrode still delivered high discharge capacity of 82 mAh g-1 even after 500 cycle, which indicates that the optimum cathode formulation is one of the important parameters in building high rate and long cycle performing SS-LMBs.
Poly(ethylene oxide) (PEO) can be considered the most widely studied and applied, all solid-state (solvent free) polymeric lithium ion conductor. 1 However, the material suffers from numerous drawbacks: most importantly, PEO shows a considerable tendency towards crystallization. 2 Consequently, linear PEO shows a drastic increase in ionic conductivity above its melting temperature, typically ranging between 40 and 60 °C depending on the molecular weight. This temperature-dependence strongly impedes the commercial application of PEO as substitute for the state-of-the-art liquid, flammable carbonate electrolytes of today’s lithium ion batteries. One major benefit of solid (polymer) electrolytes lies in their high mechanical stiffness along with shape-flexibility and elasticity, which could be used to avoid spacer membranes. This could not only increase the specific capacity of the cell by weight reduction and decreased electrode distance but also impede lithium dendrite growth at the electrode interfaces. Many reports find that LiTFSI in combination with PEO delivers among the highest ionic conductivities for solid polymer electrolytes. The combination of the small Li + cation and the bulky, highly charge-delocalized TFSI - anion leads to a very weakly associated salt structure. The TFSI - anion contributes up to 80 % to the ionic conductivity in typical PEO/LiTFSI mixtures by propagating through the electrolyte. 3 However, TFSI - cannot be converted nor intercalated electrochemically at the electrodes in a battery application. During discharge, anions accumulate at the anode in opposite direction to the internal electric field, depolarizing the cell and thereby diminishing its specific capacity and power. Furthermore, the high concentration of anions at either electrode can facilitate the growth of unwanted lithium dendrites at the surface. The concept of a single - ion conducting electrolyte tackles these issues by spatially fixating the anions onto a scaffold, e.g. a polymer backbone. This restricts the anion movement and helps to increase the percentage of current transported only by lithium cations, which is referred to as the lithium transport number t Li+ . A synthetic approach to combine the lithium ion coordinating chemistry of a PEO-based polymer with a sterically frustrated chain architecture can be found in brush-like graft copolymers. 4 We recently presented a convenient two-step pathway for the synthesis of clickable PEO-based electrolyte materials. 5 In the first step, epoxy- as well as alkyne-functionalized glycidyl propargyl ether (GPE) is directly converted to P(GPE) by monomer-activated, anionic ring-opening polymerization (AROP, fig. a ). We showed the consequent functionalization of this versatile polymer backbone in the second step by copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC, Click -chemistry) under very mild conditions and with complete removal of copper catalyst traces. We could tailor the polymer composition by the sequential addition of the corresponding, azide-equipped side-groups; in our case tri(ethylene glycol) azide (EG3-N 3 ) and benzyl azide (Bn-N 3 ) and a specially synthesized, clickable lithium bis(trifluoromethanesulfonyl)imide azide (LiTFSI-N 3 ) which integrates the concept of single-ion conduction (fig. a,b ) As an improved version of the previously presented, sequentially clicked graft copolymers, we prepared single-ion conducting P[GPE-(EG3 x - ran -LiTFSI y )] 100 electrolytes (fig. c ) by clicking EG3-N 3 as well as LiTFSI-N 3 sidegroups onto the P(GPE) backbone with different amounts of lithium salt in the structure. 6 We analyzed these materials in terms of composition (NMR, SEC), temperature behavior (TGA, DSC) as well as applicability as lithium ion conducting electrolytes (ionic conductivity, lithium transport number, cyclovoltammetry, battery cycling and extended DRT analysis). Additionally, we synthesized the fully EG3-grafted P(GPE-EG3) 100 (fig. d ) as a matrix polymer and mixed it with LiTFSI to obtain a salt-in-polymer electrolyte as comparison dual-ion system. Both the type of lithium salt as well as the ion-conducting matrix are the same in both electrolytes, but a significantly increased lithium transport number is expected for the single-ion conductor. Herein we present two solid polymer electrolyte systems based on the versatile P(GPE) backbone – one of them single-ion conducting – and compare them in a detailed and fundamental study. References (1) Armand, M. Solid State Ionics 1983 , 9-10 , 745–754. DOI: 10.1016/0167-2738(83)90083-8. (2) Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J. M.; Rigaud, P. Solid State Ionics 1983 , 11 (1), 91–95. DOI: 10.1016/0167-2738(83)90068-1. (3) Bouchet, R.; Maria, S.; Meziane, R.; Aboulaich, A.; Lienafa, L.; Bonnet, J.-P.; Phan, T. N. T.; Bertin, D.; Gigmes, D.; Devaux, D.; Denoyel, R.; Armand, M. Nature materials 2013 , 12 (5), 452–457. DOI: 10.1038/NMAT3602. (4) Nishimoto, A.; Watanabe, M.; Ikeda, Y.; Kohjiya, S. Electrochimica Acta 1998 , 43 (10-11), 1177–1184. DOI: 10.1016/S0013-4686(97)10017-2. (5) Krimalowski, A.; Thelakkat, M. Macromolecules 2019 , 52 (11), 4042–4051. DOI: 10.1021/acs.macromol.9b00206. (6) Hahn, M.; Rosenbach, D.; Krimalowski, A.; Nazarenus, T.; Moos, R.; Thelakkat, M.; Danzer, M. A. Electrochimica Acta 2020 , 136060. DOI: 10.1016/j.electacta.2020.136060. Figure 1
In hybrid photovoltaics an organic and an inorganic semiconductor are combined in the active layer to have the advantages of both material classes in a single device. In article number 1700248, Peter Müller-Buschbaum and co-workers review research related to hybrid solar cells which combine conjugated polymers with inorganic materials such as titanium dioxide, zinc oxide, silicon, germanium and quantum dots. Hybrid solar cells based on crystalline Si are discussed for comparison. Particular emphasis is put on different routes to tailor nanostructures of the organic or inorganic component. Cover Image by Christoph Hohmann, Nanosystems Initiative Munich (NIM).
Abstract We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5‐tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4‐ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between −4.97 and −5.16 eV. By using a polymeric TPD as hole transport layer and tris(8‐quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cd/m 2 were obtained under ambient conditions.
Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10 K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80 min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.
