Publications

We report the morphological and photovoltaic evaluation of a novel fully conjugated donor/acceptor block copolymer system based on the P3HT-b-PFTBT scaffold. The incorporation of hydrophilic tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to better-defined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant; however, preliminary device characterization indicates a loss of electron mobility in the hydrophilic acceptor block.

We demonstrate the read-out of the conformational state of photoswitchable molecules, without modifying that state by the read-out process itself. The ring-opening and ring-closing reactions change not only the absorption spectrum of a molecular layer but also its index of refraction. A thin layer of a dithienylethene derivative was combined with well-designed gold nanostructures. The particle plasmon resonance of these nanostructures is extremely sensitive to the photoinduced change in the environment and can therefore be used to probe the state of the photochromic switch. The probing wavelength now interrogates the plasmonic structure, not the molecular film, and can thus be conveniently placed in a transparent spectral range, e.g., the near-infrared. We find good agreement between experiments and numerical simulations with regard to spectral signatures of the plasmon resonance.

The design, synthesis, characterization, and application of a novel series of copolymers based on the electron deficient thieno[3,4-c]pyrrole-4,6-dione building block, copolymerized with either thieno[3,2-b]thiophene (PTPDTT) or thiophene (PTPDT), are reported. High molecular weights were obtained for PTPDTT via Stille polycondensation. For the PTPDTs, different molecular weights were achieved by varying the polymerization conditions. The increase in molecular weight (PTPDT-2) favors face-on alignment and increases the charge carrier mobility. Grazing-incidence wide-angle X-ray scattering measurements reveal higher crystallinity for PTPDTT with up to 5 orders of lamellar stacking compared to PTPDTs. All polymers show ambipolar charge transport with highly balanced hole and electron mobilities in organic field effect transistors (OFETs), which improve considerably upon thermal annealing. A shift of comonomer from simple thiophene in PTPDT-2 to planar and electron-dense thienothiophene in PTPDTT drastically changes the alignment from face-on to edge-on fashion. Consequently, the charge carrier mobility increases considerably by 1 order of magnitude in PTPDTT, reaching excellent charge carrier mobilities for both holes (0.11 cm2 V–1 s–1) and electrons (0.17 cm2 V–1 s–1). PTPDTT was tested as a donor material in combination with PC71BM as well as an acceptor material along with a donor polymer. As a donor material, a power conversion efficiency of 4.3% was reached in combination with PC71BM.

We report the synthesis of meso-ethynylphenyl BODIPYs and compare their properties with the corresponding meso-phenyl derivatives. Both types of BODIPYs carry a 2-cyano-3-acrylic acid anchoring moiety and either methyl groups or 4,4′-dimethoxytriphenylamine (MeOTPA) donor groups at positions 3 and 5. All compounds were characterized by NMR, UV/vis and cyclic voltammetry. The MeOTPA-substituted BODIPYs show an excellent panchromatic absorption with high molar extinction coefficients over the whole UV/vis range up to the near-IR region. The most impressive absorption was exhibited by the MeOTPA-substituted meso-ethynylphenyl BODIPY which strongly absorbs up to 1030 nm. By cyclic voltammetry measurements, all compounds were identified to be electrochemically stable in solution. Further, it was observed that the value of the LUMO level can be tuned by the meso-substituent. The HOMO level is determined by the donor substituents (−5.41 ± 0.03 eV and −4.84 ± 0.01 eV for BODIPYs with methyl groups and MeOTPA donor groups, respectively). These findings were further supported by DFT calculations. To evaluate the potential of the BODIPYs as sensitizers, the incident photon-to-current conversion efficiencies of solid-state dye-sensitized solar cells were measured. The photoaction spectra clearly show that the BODIPYs contribute to the photocurrent generation over their entire absorption region.

We present an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a novel plasticizer and doping with the well known sensitizer C<SUB>60</SUB> we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of I<SUB>write</SUB> equals 1 W/cm<SUP>2</SUP> and 450 microsecond(s) at I<SUB>write</SUB> equals 10.8 W/cm<SUP>2</SUP>. Combined with high refractive index modulations of up to (Delta) n equals 6 X 10<SUP>-3</SUP> and sample lifetimes of over 6 months this material exhibits an excellent overall performance.

Abstract Mechanically stable and flexible composite TE thin films based on PEDOT:PSS and AgSb 0.94 Cd 0.06 Te 2 blend by hot pressing on porous PVDF substrate are fabricated. A considerable enhancement of S and σ is observed in the regime of 60–97.5 wt.% of AgSb 0.94 Cd 0.06 Te 2 . High Seebeck coefficients of up to 204.9 µV K −1 and a high‐power factor of 53.45 µW m −1 K −2 are obtained for 97.5 wt.% of AgSb 0.94 Cd 0.06 Te 2 content. However, there is a decrease in electrical conductivity on bending or storage under air. No loss of mechanical integrity can be detected in the SEM of bent films. Ultraviolet photoelectron spectroscopy reveals that overall energy levels of composite systems are either similar to PEDOT:PSS (φ = 5.00 ± 0.16 eV, IP = 5.12 ± 0.02 eV) or AgSb 0.94 Cd 0.06 Te 2 (φ = 4.43 ± 0.11 eV, IP = 4.57 ± 0.01 eV), depending on the blend ratio. Transport mechanisms are studied using Mott–Schottky measurements, and by calculating the weighted charge carrier mobility, which increases continuously up to 5.5 cm 2 V −1 s −1 for the highest inorganic content. It is demonstrated that a high inorganic content TE composite results in a flexible TE film achieving 80% of the Seebeck value of the pure inorganic component.

Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.

Lead halide perovskite solar cells have shown a tremendous rise in power conversion efficiency with reported record efficiencies of over 20% making this material very promising as a low cost alternative to conventional inorganic solar cells. However, due to a differently severe “hysteretic” behaviour during current density-voltage measurements, which strongly depends on scan rate, device and measurement history, preparation method, device architecture, etc., commonly used solar cell measurements do not give reliable or even reproducible results. For the aspect of commercialization and the possibility to compare results of different devices among different laboratories, it is necessary to establish a measurement protocol which gives reproducible results. Therefore, we compare device characteristics derived from standard current density-voltage measurements with stabilized values obtained from an adaptive tracking of the maximum power point and the open circuit voltage as well as characteristics extracted from time resolved current density-voltage measurements. Our results provide insight into the challenges of a correct determination of device performance and propose a measurement protocol for a reliable characterisation which is easy to implement and has been tested on varying perovskite solar cells fabricated in different laboratories.

The erasing dynamics of holographic gratings in a low molecular weight photorefractive glass depending on the sample temperature were investigated. Changes in the overall speed of the material by three orders of magnitude over a temperature range of 13 K were observed. We identified two distinct processes below the glass transition temperature T<SUB>g</SUB>, a fast one on time scales of seconds and a slower one with lifetimes around 10<SUP>3</SUP> s. We attribute the fast process to the electro-optic effect and the slower one to orientational diffusion processes of the glass molecules. Above T<SUB>g</SUB>, the fast process vanishes, whereas the diffusional processes accelerate up to time constants in the range of seconds. This study shows, that an accurate temperature control is indispensable when measuring photorefractive dynamics, especially in the temperature range around T<SUB>g</SUB>.

Light-induced W-band electron nuclear double resonance and density functional theory of a spin density distribution study of poly-perylene bisimide acrylate (PPerAcr) anion radicals in double-crystalline donor–bridge–acceptor block copolymers (BCPs) are under consideration. Evidence of spin density sharing in the electron acceptor block, mainly distributed on the perylenediimide frame of PPerAcr without delocalization along the π-stack direction of the PPerAcr block in BCPs, was found at the low (50 K) temperature in frozen o-1,2-dichlorobenzene solution. The electron-hopping frequency between the anion and the nearby neutral PPerAcr of assembled π-stacks in P3HT-b-PPerAcr BCPs is limited by a frequency of around 10⁸ Hz at temperatures below 50 K in the steady-state regime of continuous-wave light illumination.