307 publications from this institution
We report the first synthesis of regioregular poly(3-hexylthiophene) with carboxylic end groups (P3HT–COOH), show its potential to anchor onto mesoporous TiO2 and show its application as a polymer sensitizer in a solid-state dye-sensitized solar cell. The incorporation of COOH groups was done by a polymer analogous reaction on P3HT. The product is characterized in comparison with P3HT and low molecular weight model molecules of substituted thiophene. These model compounds are efficient tools to identify end groups in P3HT and P3HT–COOH. The solar cell with 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spiro-bifluorene (spiro-OMeTAD) as a hole conductor and P3HT–COOH as a polymer sensitizer on mesoporous TiO2 shows a short-circuit current of 3.7 mA cm−2, an open circuit voltage of 0.54 V and a power conversion efficiency of 0.9%.
In this article we present a setup for the combinatorial vapor deposition of thin-film multilayer devices as well as methods for the fast and efficient analytic screening of the libraries obtained. The preparation setup is based on a commercially available evaporation chamber equipped with various evaporation sources for both organic and metallic materials. The combinatorial approach is realized by the combination of a rotation stage for the substrate, a five-mask sampler, and an additional mask whose position can be deliberately varied along one axis during the evaporation process. The latter is used to evaporate linear as well as step gradients by continuous or stepwise movement of a shutter mask. The mask sampler allows to define the sectors of the library and to evaporate more complex structures, e.g., an electrode layout. Finally, the simultaneous evaporation of two or more materials enables us to produce layers of varying composition ratio in general and doped materials, in particular. For the control of the evaporation process we have developed an automation software, which is particularly helpful for complex library designs and which grants excellent repeatability of experiments. Efficient and fast characterization of the obtained libraries is realized by (i) a purely optical setup and (ii) an electro-optical setup. (i) The UV/vis reader FLASHScan 530 permits to map out the UV/vis absorbance or fluorescence of the whole library. The UV/vis absorbance is primarily used to determine layer thicknesses and to confirm thickness uniformity across larger regions. The fluorescence measurements are used to determine the composition of layers containing fluorescent dyes. (ii) For a detailed short- and long-term electro-optical analysis we have developed an automated measurement system, which allows the characterization of 8x8 optoelectronic devices and to study their degradation behavior. Both solar cells and organic light-emitting diodes can be tested. Finally, we have developed a data analysis software to extract characteristic values from the huge amount of data and with this facilitate the finding of systematic dependencies.
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We utilize the direct, monomer-activated anionic ring-opening polymerization of glycidyl propargyl ether (GPE) to obtain a clickable polyether backbone [(P(GPE)] with high molecular weights and narrow distributions. First, P(GPE) was individually clicked with three different azide-functionalized pendant groups: LiTFSI-N3, EG3-N3, and Bn-N3. LiTFSI serves as a lithium ion source with immobilized anion, tri(ethylene glycol) as an additional source of ion-conductive medium, and the benzyl substituent is used to improve the mechanical properties. To combine all these properties in one single polymer electrolyte, we sequentially co-clicked the named azides onto P(GPE). The final material composition, the [O]/[Li] ratio, as well as the glass transition temperature can be adjusted via the azide feed ratios. A self-standing electrolyte material with close-to-unity lithium transport number and very high electrochemical stability was obtained. We present an innovative clickable PEG platform to synthesize a ready-to-use anion-immobilized solid polymer electrolyte with tunable thermal properties in a single, sequential reaction step.
Two series of copolyimides, poly(quinoxaline perylene bisimide)s and poly(quinoxaline perylene bisimide ether)s, carrying quinoxaline units and varying amounts of substituted perylene bisimides in the main chain have been synthesized. The polymers were characterized by FT-IR, NMR, UV/vis, and fluorescence spectroscopy as well as GPC, DSC, and TGA measurements. They are highly soluble in usual solvents like CHCl3, THF, etc., and form optically transparent films. The incorporation of varying amounts of perylene diimides with different substitution patterns allowed the control of fluorescence wavelength and intensity in solution and in thin films. The copolyimides with 1 mol % of perylene diimide content exhibit highest solid-state fluorescence. The observed intense fluorescence and a constant maximum wavelength of absorption in the solid state for copolymers containing up to 2.82 mol % perylene content indicate the absence of chromophore aggregation. Both types of copolymers are thermally stable up to 420 °C and exhibit glass transition temperatures in the range 225−270 °C. Blends of these perylene-containing polymers with hole transport molecules show complete photoluminescence quenching due to efficient electron transfer.
Counterion exchange has been introduced as a method to modify properties of anionic conjugated polyelectrolyte (CPE) blends. Blending of two self-doped CPEs having metallic and semiconducting behavior has been achieved from two different solvents, by exchanging the counterion of the metallic component. Different blending conditions lead to films exhibiting different optical properties, depending on the aggregation states of the CPEs. Conductance responses for the blends showed the opportunity to tune threshold voltage of the films both by blending and counterion exchange. Therefore, the blends have been exploited for the fabrication of accumulation mode organic electrochemical transistors. These devices exhibit short switching times and high transconductance, up to 15.3 mS, as well as high stability upon fast pulsed cycles, retaining 88% of the drain currents after 2 × 103 cycles.
Solid polymer electrolytes in solvent-free lithium batteries may overcome some of the disadvantages of liquid electrolytes such as flammability and instability. 1 Thereby, the polymer electrolyte acts as both ion transport medium and electrical separator between the electrodes. Compared to rigid separators (e.g. fiber glass) a higher shape flexibility is a further advantage, as these SPEs can compensate volume changes of the electrodes by elastic and plastic deformation. 2 Poly(ethylene glycol) (PEG) possesses one of the highest ionic conductivities among solvent-free SPEs but suffers from a conductivity drop below its melting temperature about 50-60°C due to high crystallinity depending on the molecular weight. 1 In addition to linear PEG polymers, ion-conducting bottlebrush graft copolymers can be obtained by attaching PEG side chains to a polymer backbone in order to reduce the crystallinity maintaining very high molecular weight. In this work we synthesized five new bottlebrush polymers using free radical polymerization as well as ring-opening metathesis polymerization (ROMP). These brush polymers contain different lengths of PEG side chains (1 kg mol -1 and 2 kg mol -1 ) and two different backbones, poly(methacrylate) and poly(norbornene). We present the influence of the polymer architecture on mechanical stability, ionic conductivity, Li-ion transport number and electrochemical stability of a series of SPEs obtained thereof by mixing with different amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We also compare the results to the respective linear PEG counterparts. The differences in ionic conductivity were analyzed and correlated with respect to thermal properties such as T g and Δ H melt in order to understand the fundamental factors which influence the properties of solvent-free PEG-containing bottlebrush SPEs. We also examined the implications of changing a linear polymer system to a brush architecture for potential applications in batteries and correlate the occurring processes in the cell with the distribution of relaxation times (DRT). Detailed comparative measurements under similar cell configurations for diverse O/Li ratios in a temperature range of 25 to 80 °C for different SPEs were carried out to elucidate structure-property relationships. The interesting findings are that by applying a brush architecture, we suppress the crystallinity of PEG and improve the mechanical strength without losing ionic conductivity. We obtained conductivities in the range of 10 -3 to 10 -4 S cm -1 for solvent-free SPEs. Furthermore, the best ionic conductivities for any system correlate strongly with their respective T g . 3 Nevertheless, there are still several unresolved questions regarding these bottlebrushes compared to linear PEG in terms of interfacial as well as bulk processes in both blocking steel (ionic conductivity) and lithium (lithium plating/stripping, interfacial resistance) electrode setups. In addition to the polymer component, the lithium salt has a major effect on the properties of the electrolyte. LiTFSI is probably the most common Li-ion source in SPEs. 4 Besides that, lithium borate salts have gained high interest due to their high thermal stability, cost-effectiveness, favorable solid electrolyte interface (SEI) formation and ionic conductivities in the same range as LiTFSI. 5 For example, Lithium bis(oxalate)borate (LiBOB) and its asymmetric counterpart Lithium difluoro(oxalate) borate (LiDFOB) are stable in organic solvents and the electrochemical stability is higher than 4.5 V vs. Li/Li + . 6 Different salts in an electrolyte can influence the Li-ion transport as well as the processes at the interfaces or the formation of the SEI. This again requires a detailed comparative analysis. For this, we prepared promising bottlebrush polymer electrolytes (1 kg mol -1 PEG sidechain) containing LiBOB and LiDFOB and subsequently analyzed them electrochemically by impedance spectroscopy. The measurement data was finally interpreted by the extended Distribution of Relaxation Times (eDRT). Both SPEs showed similarities in the lithium-ion conducting process in possessing one major, resistive-capacitive bulk conductivity mechanism. The resulting, temperature-dependent conductivities were evaluated and are in the range of 10 -4 to 10 -5 S cm -1 . During cycling, the SPEs showed increased interface resistances over time, which are higher than respective bulk resistances. By applying eDRT, the time-dependent formation of an interphase layer in the SPEs is identified, separated from the slower charge transfer process and quantified. Thus, the electrolytes cannot be considered electrochemically stable against metallic Li, which is similar for liquid electrolytes. 7 References (1) Scrosati, B.; Energy Environ. Sci. 2011 , 4 , 3287. (2) Janek, J.; Nat. Energy 2016 , 1 , 16141. (3) Rosenbach, D.; ACS Appl. Energy Mater. 2019 , 2 , 3373–3388. (4) Etacheri, V.; Energy and Environmental Science 2011 , pp 3243–3262. (5) Xu, K. Chem. Rev. 2014 , 114 , 11503–11618. (6) Liu, Z.; Coord. Chem. Rev. 2015 , 292 , 56–73. (7) Hahn, M.; Electrochim. Acta 2020 , 344 , 136060. Figure 1
