Publications

A novel donor acceptor dyad consisting of tetraphenylbenzidine (TPD) and perylene bisimide was synthesized by linking these moieties using a dodecyl spacer. HOMO and LUMO values of the dyad were aquired by cyclic voltammetry. Photophysical properties were studied by steady-state UV−vis and fluorescence spectroscopy. After selective excitation of the TPD moiety, a quenching of donor fluorescence and the appearance of acceptor fluorescence was observed. This proves nonradiative energy transfer from the donor to the acceptor group. The energy transfer was 4 times more efficient in DBA than in a mixture of D and A. The energy transfer efficiency in the dyad is also independent of the concentration, indicating intramolecular transfer mechanism. However, the direct excitation of the acceptor in the dyad exhibited reduced fluorescence emission of the acceptor, indicating electron transfer between the moieties. Thus, this DBA is an excellent model system for the study of energy- and electron-transfer processes in organic semiconductors.

Electron and hole conducting 10-nm-wide polymer morphologies hold great promise for organic electro-optical devices such as solar cells and light emitting diodes. The self-assembly of block-copolymers (BCPs) is often viewed as an efficient way to generate such materials. Here, a functional block copolymer that contains perylene bismide (PBI) side chains which can crystallize via π–π stacking to form an electron conducting microphase is patterned harnessing hierarchical electrohydrodynamic lithography (HEHL). HEHL film destabilization creates a hierarchical structure with three distinct length scales: (1) micrometer-sized polymer pillars, containing (2) a 10-nm BCP microphase morphology that is aligned perpendicular to the substrate surface and (3) on a molecular length scale (0.35–3 nm) PBI π–π-stacks traverse the HEHL-generated plugs in a continuous fashion. The good control over BCP and PBI alignment inside the generated vertical microstructures gives rise to liquid-crystal-like optical dichroism of the HEHL patterned films, and improves the electron conductivity across the film by 3 orders of magnitude.

We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with well-defined molecular weight (5−19 kg/mol) using DSC, small angle and wide-angle X-ray scattering, and AFM. With increasing temperature, we identify three ordered phases, namely 3D crystalline, 2D crystalline with disordered side chains, and a layered phase of smectic symmetry, followed by complete melting. Although all samples crystallize in extended chain conformation, the lower molecular weight material exhibits a lower crystallinity, most likely caused by noncrystallizable end groups. The crystallinity increases strongly with increasing molecular weight, which could be a possible explanation for the known dependence of charge transport properties on molecular weight.

We investigate the excited state dynamics and the conformations of a new molecular donor-bridge-acceptor system, a Cu(ii)-phthalocyanine (CuPc) covalently linked via a flexible aliphatic spacer to a perylenebisimide (PBI). We performed time-resolved polarization anisotropy and pump-probe measurements in combination with molecular dynamics simulations. Our data suggest the existence of three conformations of the dyad: two more extended, metastable conformations with centre-of-mass distances >1 nm between the PBI and CuPc units of the dyad, and a highly stable folded structure, in which the PBI and CuPc units are stacked on top of each other with a centre-of-mass distance of 0.4 nm. In the extended conformations the dyad shows emission predominantly from the PBI unit with a very weak contribution from the CuPc unit. In contrast, for the folded conformation the PBI emission of the dyad is strongly quenched due to fast energy transfer from the PBI to the CuPc unit (3 ps) and subsequent intersystem-crossing (300 fs) from the first excited singlet state of CuPc unit into its triplet state. Finally, the CuPc triplet state is deactivated non-radiatively with a time constant of 25 ns.

We report a systematic investigation on the role of excess PbI₂ content in CH₃NH₃PbI₃ perovskite film properties, solar cell parameters and device storage stability. We used the CH₃NH₃I vapor assisted method for the preparation of PbI₂-free CH₃NH₃PbI₃ films under a N₂ atmosphere. These pristine CH₃NH₃PbI₃ films were annealed at 165 °C for different time intervals in a N₂ atmosphere to generate additional PbI₂ in these films. From XRD measurements, the excess of PbI₂ was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CH₃NH₃PbI₃ films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI₂ contents on degradation of the CH₃NH₃PbI₃ films. We find that the rate of degradation of CH₃NH₃PbI₃ is accelerated due to the amount of PbI₂ present in the film. A comparison of solar cell parameters of devices prepared using CH₃NH₃PbI₃ samples having different PbI₂ contents reveals a strong influence on the current density-voltage hysteresis as well as storage stability. We demonstrate that CH₃NH₃PbI₃ devices do not require any residual PbI₂ for a high performance. Moreover, a small amount of excess PbI₂, which improves the initial performance of the devices slightly, has undesirable effects on the CH₃NH₃PbI₃ film stability as well as on device hysteresis and stability.

Abstract In the quest for indium‐tin oxide (ITO)‐free photovoltaics and for building integrated as well as automobile roof applications, novel transparent electrodes for both front and back electrodes are required. Here we report the fabrication and integration of submicrometer transparent silver (Ag) and gold (Au) metal network electrodes, which are invisible to the naked eye, in organic photovoltaic devices. We exploit the idea of using the spontaneous cracking of a polymer layer as template to prepare the metal network. The main challenge is to apply the cracked template approach on top of soluble organic layers and to lift off the template without damaging the photoactive layer. We demonstrate that Ag or Au back electrodes can be fabricated maintaining a transmittance of 80 % for the whole visible range. These electrodes exhibit ultralow haze of approximately 5 % and an excellent figure of merit value. Moreover, the ITO‐free semitransparent polymer solar cell incorporating the Ag/Ag network electrodes exhibits 57 % transmittance above 650 nm.

Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated polymers and are thus of pivotal relevance for the mobility of charge carriers in electro-optical device applications. Currently, poly(3-akylthiophenes) are discussed as one of the most promising classes of materials for photovoltaic applications. Here, we use 13C solid-state NMR to determine the crystallinity and details on crystal−crystal phase transformations in regioregular head-to-tail poly(3-hexylthiophene) of well-defined molecular weight and demonstrate that the crystallinity was previously severely underestimated. Typical crystallinities are at least around 37% for the lowest molecular weights and as high as about 64% upon increasing MW, corresponding to a corrected maximum value for the reference melting enthalpy of ΔHm∞ ≈ 37 J/g for use in DSC experiments. Using 1D 13C spectra and 2D experiments that measure the strength of 13C−1H dipolar couplings, we observe that the crystal−crystal phase transition between a 3D- and a 2D-ordered crystalline phase at around 60 °C entails a structural disordering process of the alkyl side chains, while not affecting rigidity and conformation of the backbones. This phase transition is suppressed at higher molecular weights due to a kinetic suppression of the formation of the alkyl-ordered low-temperature phase.

Predictions made by two models describing dynamic processes in organic photorefractive materials are verified for a photorefractive polymer, consisting of a Poly-TPD, a dicyano dye and the sensitizer C₆₀. One of the models can describe our measurements under the assumption of optically active traps as dominant trap species. This is confirmed by altering the sensitizer concentration, which has an influence on the charge transport properties of the material. Also, increasing the sensitizer concentration yielded a significant increase of the grating erasure rate in our material.

Photochromic molecules that are covalently linked to a strong fluorophore combine the requirements of external control and strong fluorescence, which will become increasingly important for super-resolution microscopy techniques based on single molecules. However, given the bulky structure of such constructs, steric hindrance might affect their photoconversion efficiencies upon immobilising them for imaging purposes. In this study the efficiencies of the photochromic conversion processes of molecular triads that are embedded in a polymer have been studied as a function of temperature. The triads consist of two perylene bisimide dye molecules that are connected via a dithienylcyclopentene photochromic bridge that undergoes a cyclization/cycloreversion reaction upon appropriate illumination. It is found that photochromic switching remains active, even at 5 K, yet with reduced but finite efficiency for the cycloreversion reaction. This might even be an advantage for the achievement of high labelling densities in super-resolution microscopy.