Birgit Kunert

Crystallisation kinetics in thin films of dihexyl-terthiophene: the appearance of polymorphic phases

The presence of surface-induced crystal structures is well known within organic thin films. However, the physical parameters responsible for their formation are still under debate. In the present work, we present the formation of polymorphic crystal structures of the molecule dihexyl-terthiophene in thin films. The films are prepared by different methods using solution-based methods like spin-coating, dip-coating and drop-casting, but also by physical vapour deposition. The thin films are characterised by various X-ray diffraction methods to investigate the crystallographic properties and by microscopy techniques (atomic force microscopy and optical microscopy) to determine the thin film morphologies. Three different polymorphic crystal structures are identified and their appearance is related to the film preparation parameters. The crystallisation speed is varied by the evaporation rate of the solvent and is identified as a key parameter for the respective polymorphs present in the films. Slow crystallisation speed induces preferential growth in the stable bulk structure, while fast crystallisation leads to the occurrence of a metastable thin-film phase. Furthermore, by combining X-ray reflectivity investigations with photoelectron spectroscopy experiments, the presence of a monolayer thick wetting layer below the crystalline film could be evidenced. This work gives an example of thin film growth where the kinetics during the crystallisation rather than the film thickness is identified as the critical parameter for the presence of a thin-film phase within organic thin films.

Investigation of primary crystallite sizes in nanocrystalline ZnS powders: comparison of microwave assisted with conventional synthesis routes.

ZnS powders with primary crystallite sizes of only a few nanometers were prepared by three different synthesis routes at temperatures below 130 degrees C. The reaction of zinc acetate dihydrate with thioacetamide (TAA) in the presence of pyridine and triphenylphosphite (TPP) was carried out using either conventional heating or microwave heating. The obtained powders exhibit sphalerite structure as determined by X-ray diffraction (XRD). The primary crystallites have diameters between 1 and 7 nm obtained by XRD. Small angle X-ray scattering (SAXS) measurements were analyzed by the model-free inverse Fourier-transformation approach, as well as by a hard sphere-model from which particle size and polydispersity were extracted. The particle sizes by SAXS are in good agreement with the primary crystallite sizes obtained by XRD. It has been found that an increasing amount of sulfur and/or using microwave heating increases crystallite sizes. The presence of TPP decreases the particle sizes but no significant influence on the TPP concentration was observed. In the alternative third preparation route, hexamethyldisilathiane (HMDST) was used as precipitation reagent at ambient temperature, which leads to the smallest crystallite sizes of only 1 nm together with low polydispersities. Scanning electron microscopy, dynamic light scattering and UV-vis spectroscopy showed that all three synthesis routes lead to ZnS powders with aggregate sizes between 650 and 1200 nm. Both of the TAA-precipitation routes lead to spherical agglomerates which consist of spherical substructures, whereas the HMDST agglomerates are assembled from elongated objects.

Bismuth sulphide–polymer nanocomposites from a highly soluble bismuth xanthate precursor

Bismuth sulphide nanocrystal–polymer hybrid layers are of interest for various optoelectronic, thermoelectric or sensing applications. In this work, we present a ligand-free in situ route for the formation of Bi2S3 nanorods directly within a polymer matrix. For this purpose, we introduce a novel bismuth xanthate (bismuth(III) O-3,3-dimethylbutan-2-yl dithiocarbonate), which is highly soluble in non-polar organic solvents. The analysis of the crystal structure revealed that the prepared bismuth xanthate crystallises in the monoclinic space group C2/c and forms dimers. The bismuth xanthate can be converted into nanocrystalline Bi2S3 with an orthorhombic crystal structure via a thermally induced solid state reaction at moderate temperatures below 200 °C. In combination with the high solubility in non-polar solvents this synthetic route for Bi2S3 is of particular interest for the preparation of Bi2S3–polymer nanocomposites as exemplarily investigated on Bi2S3–poly(methyl methacrylate) and Bi2S3–poly(3-hexylthiophene-2,5-diyl) (P3HT) nanocomposite layers. Atomic force and transmission electron microscopy revealed that Bi2S3 nanorods are dispersed in the polymer matrix. Photoluminescence experiments showed a quenching of the P3HT fluorescence with increasing Bi2S3 content in the hybrid layer.

Structural characterisation of alkyl amine-capped zinc sulphide nanoparticles.

Graphical abstract Highlights ► Combined X-ray and light scattering study was performed on ZnS nanoparticles. ► Ligands with different steric properties, dodecyl- and oleylamine, are compared. ► Nanoparticles exhibit sizes of 3–5 nm. ► Thickness of the ligand shell is about 1.9 nm.

Morphologies of Phenytoin Crystals at Silica Model Surfaces: Vapor Annealing versus Drop Casting

The controlled preparation of different crystal morphologies with varying preferential orientation with respect to the substrate is of crucial importance in many fields of applications. In this work, the controlled preparation of different phenytoin morphologies and the dependency of the preferential orientation of those crystallites is related with the preparation method (solvent annealing vs drop casting), as well as the physical–chemical interaction with the solvents in use. While solvent annealing induces the formation of particular structures that are partially dewetted, the drop casting technique from various solvent results in the formation of needle-like and elongated structures, with each having a distinct morphology. The morphologies are explained via the Hansen solubility parameters and correlated with the solvent vapor pressures. X-ray diffraction experiments reveal preferential orientations with respect to the solid substrate and indicate the surface-mediated stabilization of an unknown polymorph of phenytoin with an elongated unit cell in the b-axis.

Investigation of NiOx-hole transport layers in triple cation perovskite solar cells

Perovskite solar cells with a planar p-i-n device structure offer easy processability at low temperatures, suitable for roll-to-roll fabrication on flexible substrates. Herein we investigate different hole transport layers (solution processed NiOx, sputtered NiOx, PEDOT:PSS) in planar p-i-n perovskite solar cells using the triple cation lead halide perovskite Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3 as absorber layer. Overall, reproducible solar cell performances with power conversion efficiencies up to 12.8% were obtained using solution processed NiOx as hole transport layer in the devices. Compared to that, devices with PEDOT:PSS as hole transport layer yield efficiencies of approx. 8.4%. Further improvement of the fill factor was achieved by the use of an additional zinc oxide nanoparticle layer between the PC60BM film and the Ag electrode.

Room temperature synthesis of CuInS2 nanocrystals

Herein, we investigate a synthetic approach to prepare copper indium sulfide nanocrystals at room temperature. The nanocrystals have a chalcopyrite crystal structure, a diameter of approximately 3 nm and are well soluble in organic solvents like toluene or chloroform. The synthesis is performed by dissolving metal xanthates as precursors together with oleylamine in toluene followed by stirring for several hours at room temperature leading to nanocrystals stabilized with oleylamine ligands. The nanoparticles are characterized in terms of inner structure by X-ray diffraction, transmission electron microscopy, Raman-, absorption- and photoluminescence spectroscopy. Their formation process is investigated by small angle X-ray scattering, UV-Vis absorption spectroscopy and NMR spectroscopy. The formation of the copper indium sulfide nanocrystals proceeds via a chemical reaction of the amine with the thiocarbonyl functionality of the xanthate. The presented method exemplifies a synthesis strategy, which can be easily expanded to other metal sulfide nanocrystals.

Idiosyncrasies of Physical Vapor Deposition Processes from Various Knudsen Cells for Quinacridone Thin Film Growth on Silicon Dioxide

Thin films of quinacridone deposited by physical vapor deposition on silicon dioxide were investigated by thermal desorption spectroscopy (TDS), mass spectrometry (MS), atomic force microscopy (AFM), specular and grazing incidence X-ray diffraction (XRD, GIXD), and Raman spectroscopy. Using a stainless steel Knudsen cell did not allow the preparation of a pure quinacridone film. TDS and MS unambiguously showed that in addition to quinacridone, desorbing at about 500 K (γ-peak), significant amounts of indigo desorbed at about 420 K (β-peak). The existence of these two species on the surface was verified by XRD, GIXD, and Raman spectroscopy. The latter spectroscopies revealed that additional species are contained in the films, not detected by TDS. In the film mainly composed of indigo a species was identified which we tentatively attribute to carbazole. The film consisting of mainly quinacridone contained in addition p-sexiphenyl. The reason for the various decomposition species effusing from the metal Knudsen cell is the comparably high sublimation temperature of the hydrogen bonded quinacridone. With special experimental methods and by using glass Knudsen-type cells we were able to prepare films which exclusively consist of molecules either corresponding to the β-peak or the γ-peak. These findings are of relevance for choosing the proper deposition techniques in the preparation of quinacridone films in the context of organic electronic devices.

Crystallographic textures and morphologies of solution cast Ibuprofen composite films at solid surfaces.

The preparation of thin composite layers has promising advantages in a variety of applications like transdermal, buccal, or sublingual patches. Within this model study the impact of the matrix material on the film forming properties of ibuprofen–matrix composite films is investigated. As matrix materials polystyrene, methyl cellulose, or hydroxyl-ethyl cellulose were used. The film properties were either varied by the preparation route, i.e., spin coating or drop casting, or via changes in the relative ratio of the ibuprofen and the matrix material. The resulting films were investigated via X-ray diffraction and atomic force microscope experiments. The results show that preferred (100) textures can be induced via spin coating with respect to the glass surface, while the drop casting results in a powder-like behavior. The morphologies of the films are strongly impacted by the ibuprofen amount rather than the preparation method. A comparison of the various matrix materials in terms of their impact on the dissolution properties show a two times faster zero order release from methyl cellulose matrix compared to a polystyrene matrix. The slowest rate was observed within the hydroxyl ethyl cellulose as the active pharmaceutical ingredients (APIs) release is limited by diffusion through a swollen matrix. The investigation reveals that the ibuprofen crystallization and film formation is only little effected by the selected matrix material than that compared to the dissolution. A similar experimental approach using other matrix materials may therefore allow to find an optimized composite layer useful for a defined application.

The effect of polymer molecular weight on the performance of PTB7-Th:O-IDTBR non-fullerene organic solar cells

Recent advances in the development of non-fullerene acceptors have increased the power conversion efficiency of organic solar cells to approximately 13%. Fullerene-derivatives and non-fullerene acceptors possess distinctively different structural, optical and electronic properties, which also change the requirements on the polymer donor in non-fullerene organic solar cells. Therefore, in this study, the effect of the molecular weight of the conjugated polymer on the photovoltaic performance, charge carrier mobility, crystallization properties, film morphology, and non-geminate recombination dynamics is systematically investigated in polymer:small molecule organic solar cells using the low bandgap polymer PTB7-Th as the donor and the non-fullerene indacenodithiophene-based small molecule O-IDTBR as the acceptor. Among the examined polymer samples (50–300 kDa), high molecular weights of PTB7-Th (with an optimum molecular weight of 200 kDa) are advantageous to achieve high efficiencies up to 10%, which can be correlated with an increased crystallinity, an improved field-effect hole mobility (1.05 × 10−2 cm2 V−1 s−1), lower charge carrier trapping and a reduced activation energy of charge transport (98 meV). Bias-assisted charge extraction and transient photovoltage measurements reveal higher carrier concentrations (1016 cm−3) and long lifetimes (4.5 μs) as well as lower non-geminate recombination rate constants in the corresponding devices, supporting the high photocurrents (ca. 15.2 mA cm−2) and fill factors (>60%).