The fluorescence emission properties of hybrid systems consisting of a one-dimensional gold nanowire grating and an evaporated thin film of the small molecule organic semiconductor diindenoperylene (DIP) are investigated. The optical properties of the metallic systems are dominated by their plasmonic resonances. The spectral positions of the resonances are tuned through the width of the single nanowires. Additionally, the plasmonic gratings show a strong polarization dependency due to their one-dimensionality. In contrast, the pure organic system has a polarization-independent emission between 570 and 900 nm when excited with blue light, whereas no photoluminescence can be detected from the bare plasmonic system for the same illumination conditions. For the hybrid system, the intensity, the shape of the emission spectrum, and the polarization of the emission dearly correlate with the optical properties of the plasmonic nanowires. Depending on the optical properties of the plasmonic system, different emission bands of the organic thin film can be enhanced. (C) 2019 Optical Society of America

We studied the influence of, periodic growth interruptions during codeposition of diindenoperylene (DIP) and buckminsterfullerene (C60) in an equimolar mixing ratio. DIP and C60 are known to phase-separate when codeposited, but the details and, in particular, the length scales depend on kinetic factors. Using X-ray scattering and atomic force microscopy, we demonstrate that the phase separation mechanism is in fact influenced by growth interruptions, with more pronounced effects if the deposition rates are low. For high deposition rates, growth interruptions have no appreciable effect. We discuss our proof-of-concept investigation in the context of the relevant processes and their time scales.

We investigated the growth of the two phase-separating materials diindenoperylene (DIP) and buckminsterfullerene C60 with different mixing ratio in real-time and in situ by X-ray scattering experiments. We found that at room temperature, mixtures with an excess of DIP show a growth mode which is very close to the perfect layer-by-layer limit with DIP crystallites forming over the entire film thickness. An unexpected increase in the island size is observed for these mixtures as a function of film thickness. On the other hand, equimolar and C60 dominated mixtures grow with poor crystallinity but form very smooth films. Additionally, it is observed that higher substrate temperatures lead to an increase in the length scale of phase separation with film thickness.

Planar and bulk heterojunctions of organic donor and acceptor molecules are used to understand elementary processes in photovoltaic cells. The electronic structure, interface and film morphology, excitonic behavior, device characteristics, and correlations between these properties are reviewed here using a wide range of material combinations.

We report on the growth and characterization of molecular mixed thin films of α-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-α-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.

The interaction of metallic plasmonic nanostructures and organic semiconductor thin films plays a crucial role in engineering light harvesting and energy transfer processes, e.g., for optoelectronic applications. Plasmonic resonances of the metal structures can be used to increase the light emission or absorption of organic molecules. Here small molecules are employed since they can form organic layers with a defined crystalline order and orientation of the transition dipole. Extinction measurements combined with numerical simulations of a hybrid system consisting of a gold nanowire grating and a thin film of diindenoperylene (DIP) are reported. The experimental results are compared to the simulations and indicate an enhanced absorption in the wavelength region corresponding to the transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of DIP. This enhancement is found to be related to the localized field enhancement near the individual nanostructures as well as to grating-induced effects. Notably, the hybrid system also exhibits parallel lattice resonances, which have recently been discussed for two-dimensional (2D) gold nanostructure arrays. In this study a hybrid plasmonic-organic small molecule system exhibiting these modes is investigated. The results for this model system show a way to modify the optical properties of plasmonic nanostructures by collective effects to achieve stronger light-matter interaction in a wide range of hybrid plasmonic systems.

Thin films of α-sexithiophene (6T) and C 60 mixtures deposited on nSiO substrates at 303 and 373 K were investigated in real time and in situ during the film growth using X-ray diffraction. The mixtures are observed to contain the well known 6T low-temperature crystal phase and the β phase, which usually coexist in pure 6T films. The addition of C 60 modifies the structure to almost purely β-phase-dominated films if the substrate is at 303 K. In contrast, at 373 K the low-temperature crystal phase of 6T dominates the film growth of the mixtures. Post-growth annealing experiments up to 373 K on equimolar mixtures and pure 6T films were also performed and followed in real time with X-ray diffraction. Annealing of pure 6T films results in a strong increase of film ordering, whereas annealing of equimolar 6T:C 60 mixed films does not induce any significant changes in the film structure. These results lend further support to theories about the important influence of C 60 on the growth behaviour and structure formation process of 6T in mixtures of the two materials.

Small-molecule organic photovoltaic cells often employ a planar heterojunction (PHJ) geometry where the electron donor and acceptor materials are stacked one on top of the other. The thin-film growth scenario of such PHJs can be very different from the one of a single compound on a bare substrate. We have investigated the growth of PHJs, consisting of two different donor−acceptor pairs, namely, α-sexithiophene (6T)/C60 and 6T/diindenoperylene (DIP) using real-time in situ X-ray scattering. For both donor− acceptor material combinations, we observe that the coherent in-plane crystalline size of the second material strongly correlates with the one of the bottom one, and hence a strong templating effect of the 6T on the material deposited subsequently, indicating a strong interaction between the two materials in the PHJ. Furthermore, a change in the structure of the 6T film during the deposition of the second material was observed, which shows that the deposition of an additional material on top of a templating layer can partially change the crystal structure of the templating film itself.

We present an extensive structural analysis of hybrid architectures prepared by the "soft" incorporation of gold nanoparticles (AuNPs) within an organic semiconductor matrix of diindenoperylene (DIP). Such "soft" or noninvasive deposition of nanoparticles within organic semiconducting host matrices not only minimizes the influence of the deposition process on the order and properties of the organic host molecules, but also offers additional control in the process of incorporation. The hybrid structures were characterized by X-ray scattering techniques including grazing incidence small angle X-ray scattering (GISAXS), grazing incidence X-ray diffraction (GIXD), X-ray reflectivity (XRR), and complemented by atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM) measurements. We show that different strategies of incorporating the nanoparticles in the host matrix lead to drastically different structure and morphologies. Particularly remarkable is the morphological change observed in the matrix of DIP as well as the AuNPs due to the influence of organic solvents, as evidenced by TEM tomography measurements, which revealed the exact location of the AuNPs within the organic host. It is also demonstrated that AuNPs can be successfully used as tunable templates for the growth of the organic semiconductors with desired island sizes and distances.

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (JSC) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in JSC, elevating the power conversion efficiency by a factor of 1.2 to more than 4%.

Small-molecule organic photovoltaic cells often employ a planar heterojunction (PHJ) geometry where the electron donor and acceptor materials are stacked one on top of the other. The thin-film growth scenario of such PHJs can be very different from the one of a single compound on a bare substrate. We have investigated the growth of PHJs, consisting of two different donor acceptor pairs, namely, alpha-serithiophene (6T)/C-60 and 6T/diindenoperylene (DIP) using real-time in situ X-ray scattering. For both donor acceptor material combinations, we observe that the coherent in-plane crystalline size of the second material strongly correlates with the one of the bottom one, and hence a strong templating effect of the 6T on the material deposited subsequently, indicating a strong interaction between the two materials in the PHJ. Furthermore, a change in the structure of the 6T film during the deposition of the second material was observed, which shows that the deposition of an additional material on top of a templating layer can partially change the crystal structure of the templating film itself.

We report on a real-time in situ study of the growth of α-sexithiophene on silicon oxide substrates. Synchrotron-based X-ray diffraction experiments were performed during and directly after the growth in order to monitor the growth process. We observed a coexistence of two different crystal phases for different substrate temperatures. For films prepared at 233 and 308 K a disordered phase (β-phase) seems to be dominant compared to films prepared at 373 K where the so-called low-temperature bulk crystal phase (LT-phase) is dominant. From real-time measurements during growth we observed a temperature and film thickness dependent effect on the fraction of both phases in one sample. At 373 K the film growth begins primarily in the β-phase, and above a certain thickness the film growth proceeds mainly in the LT-phase. However, at 308 K the film growth is dominated by the β-phase for the entire thickness. We show that for kinetically limited growth conditions (high deposition rate and/or low substrate temperature) substrate induced growth effects are dominant.

Institut für Physik, Universität Augsburg, Germany. E-mail: stefan.grob@physik.uni-a uni-augsburg.de Institut für Physik, Humboldt-Universität Berlin, Germany Department of Chemical Engineering, D Southern California, Los Angeles, California Institut für Angewandte Physik, Universität Tübingen, Germany Department of Engineering and Physics, Sweden † Electronic supplementary informa 10.1039/c5ta02806j Cite this: J. Mater. Chem. A, 2015, 3, 15700

The length-scale of phase separation in organic semiconductor donor-acceptor mixtures, while being crucially important for applications, is a non-trivial parameter to control in non-equilibrium thin film growth. We present a comprehensive study of all the important parameters that can be used to tailor the length-scale of phase separation in organic semiconductor mixtures. We employed different substrate temperatures, different growth rates, time-dependent deposition rates, and surface functionalization layers. We found not only that the substrate temperature is most prominent in influencing the length-scale of phase separation in the studied parameter range, but also that other routes can be used to tailor this length-scale.

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbital and threshold ionization potential (IPT), is not significantly altered. On graphite, on the other hand, we found a transition from a 2D (layer) to a 3D (island) growth mode with a variation of the molecular orientation from flat-lying to tilted one. The IPT changes significantly between the 2D and 3D growth regime in contrast to the SiO2 system. The origin of the different In of these picene thin films is discussed. The present results are compared with other planar Jr-conjugated compounds, in particular pentacene which is a structural isomer of picene and shows electronic properties strongly different from picene thin films.

The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (Voc): In the case of 6T/C60 heterojunctions, Voc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.

Graphical abstractDisplay Omitted Gold nanowire gratings for organic photovoltaic cells were fabricated on, within and below ITO electrodes.The nanogratings have been fabricated by e-beam lithography and nano imprint lithography.The gratings were analyzed by SEM, AFM and GISAXS measurements.The tuneability of the plasmon resonance has been shown by white light extinction spectroscopy. Organic photovoltaic devices are interesting alternatives to conventional silicon based photovoltaic cells, due to potentially lower material costs and energy consumption during the fabrication process. However, the energy conversion efficiency of organic photovoltaic cells may still be improved. One possible approach is a combination with metallic nanostructures to improve light absorption properties in the active layer. We report on the fabrication and characterization of metallic nanogratings integrated with an indium tin oxide (ITO) electrode to be combined with small-molecule organic photovoltaic cells. With respect to geometry and extinction properties gratings fabricated by two different fabrication methods, conventional electron beam lithography and nanoimprint lithography, are presented. Furthermore, gratings fabricated on top of ITO are compared to gratings integrated both below and within ITO electrodes. Fabricating structures below or within electrodes is advantageous for organic thin film techniques to avoid compromising the crystallinity of the organic thin film and short-circuiting across the thin layer.

We discuss the result of the competing effects of favourable intermolecular interactions and steric incompatibilities due to the size mismatch of perfluoropentacene (PFP) and diindenoperylene (DIP) on the structure formation and associated optical properties in mixed films. Using real-time grazing incidence X-ray diffraction we investigate the size of coherently scattering islands l(s) as a function of film thickness and mixing ratio. We find that for PFP:DIP 1:2 blends l(s) is by a factor of ~4 smaller than in pure DIP films, while l(s) of the PFP:DIP 2:1 blends is not significantly reduced compared with pure PFP. Yet, we observe an increase in l(s) with film thickness for all of the samples, independent on the mixing ratio. In parallel with the structural characterization we investigate the evolution of the absorption spectra in the visible spectral range and its dependence on l(s) in situ during film growth using differential reflectance spectroscopy. We observe a surprisingly strong effect of changes in the structural order on the shape of ε(2, xy)(E), evident by a pronounced evolution of characteristic peaks in the thickness range from 1.6 nm to 9.6 nm. The combined results of the real-time experiments allow to identify the thickness dependent crystal grain size as the origin of the observed transient effects in the absorption spectra.

We present depth-resolved grazing incidence x-ray diffraction, grazing incidence small angle scattering and x-ray reflectivity studies on the structure of mixed C(60) and diindinoperylene (DIP) films as a function of the mixing ratio. We observe enhanced out-of-plane order and smoothing of the mixed films compared to pure films upon coevaporation of DIP:C(60) thin films (in different mixing ratio) which otherwise phase separate. The mixing ratio of molecules can be tuned to alter the in-plane crystallite size as well as the interisland distances of the mixing molecules. Real-time in situ grazing incidence x-ray diffraction measurements show the kinetics and thickness dependence of phase separation, which appears to proceed only after a certain thickness. The crystallite grain size of the individual phase separated components is significantly larger at the top of the film than at the bottom with implications for the understanding of devices.