Search

The active layer crystallinity has long been associated with favourable organic solar cells (OSCs) properties such as high mobility and Fill Factor. In particular, this applies to acceptor materials such as fullerene-derivatives and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing 19% of Power Conversion Efficiency. Despite these advantages are being commonly attributed to their 3-dimensional crystal packing motif in the single crystal, the bridge that links the acceptor crystal packing from single crystals to solar cells has not clearly been shown yet. In this work, we investigate the molecular organisation of seven NFAs (o-IDTBR, IDIC, ITIC, m-ITIC, 4TIC, 4TICO, m-4TICO), following the evolution of their packing motif in single-crystals, powder and thin films made with pure NFAs and donor:NFA blends. In general, we observed a good correlation between the NFA single crystal packing and their molecular arrangement in the bulk heterojunction. However, the NFA packing motif is not directly affecting the device parameters but it provide an impact on the material propensity to form highly crystalline domain in the blend. Although that NFA crystallinity is required to obtain high mobility, the domain purity is more important to limit the bimolecular recombination and to obtain high efficiency organic solar cells., Comment: 16 pages, 8 figures

Copper(I) thiocyanate (CuSCN) is an effective interlayer material for hole injection and transport in organic electronic devices but its solution processing has conventionally utilized undesirable di-n-alkyl sulfide solvents such as diethyl- (DES) and dipropyl-sulfide (DPS). Herein, this paper reports on the use of N,N-dimethylformamide (DMF) and 1-methyl-2-pyrrolidinone (NMP) as alternative solvents for CuSCN interlayers and performs a detailed comparison of the resulting properties relative to films processed from DES and DPS and two other recent alternatives, dimethyl sulfoxide (DMSO) and ammonium hydroxide. The surface roughness, polymorphism, and surface chemistry of the resulting CuSCN layers are reported. The interlayer surface energy and ionization potential that are key to the overlying semiconductor microstructure and interfacial energy barrier, and hence to charge transport and injection, are also discussed. Finally, systematic device tests using well-known organic semiconductors in light-emitting diode, solar cell and field-effect transistor structures demonstrate the overall suitability of DMSO and DMF as solvents for CuSCN interlayer deposition to achieve better device performance. This study broadens the applicability of CuSCN as a highly efficient hole injection/transport material for organic semiconductor devices by expanding the documented range of suitable CuSCN solvents.

In order to lead to a competitive advantage, there is the need to carefully consider the pros and cons of state-of-the-art manufacturing techniques. This is frequently carried out in a competitive manner, but can also be done in a complementary way. This complementary approach is often used for the processing of difficult-to-machine materials with particular regard to high-tech parts or components. Hybrid machining processes or, more general, advanced machining processes can be brought to the point that the results would not be possible with the individual constituent processes in isolation [Hybrid Machining Processes Perspectives on Machining and Finishing (Springer International Publishing AG, 2016)]. Hence, the controlled interaction of process mechanisms and/or energy sources is frequently applied for a significant increase of the process performance [Advanced Machining Processes of Metallic Materials: Theory, Modelling, and Applications, 2nd ed. (2016)] and will be addressed within the present paper. A via electron beam melting manufactured gamma titanium aluminide nozzle is extended and adapted. This is done via hybrid laser metal deposition. The presented approach considers critical impacts like processing temperatures, temperature gradients, and solidification conditions with particular regard to crucial material properties like the phenomena of lamellar interface cracking [Laser-Based Manufacturing of Components using Materials with High Cracking Susceptibility (Laser Institute of America-LIA), pp. 586-592; Ti-2015: The 13th World Conference on Titanium, Symposium 5]. Furthermore, selected destructive and non-destructive testing is performed in order to prove the material properties. Finally, the results will be evaluated. This will also be done in the perspective of other applications.

Additive manufacturing (AM) addresses various benefits as the buildup of complex shaped parts, the possibility of functional integration, reduced lead times or the use of difficult machinable materials compared to conventional manufacturing possibilities. Beside these advantages, the use of more than one material in a component would strongly increase the field of applications in typical AM branches as energy, aerospace, or medical technology. By means of multi-material buildups, cost-intensive alloys could be only used in high-loaded areas of the part, whereas the remaining part could be fabricated with cheaper compositions. The selection of combined materials strongly depends on the requested thermophysical but also mechanical properties. Within this contribution, examples (e.g., used in the turbine business) show how alloys can be arranged to fit together, e.g., in terms of a well-chosen coefficient of thermal expansion. As can be seen in nature, the multi-material usage can be characterized by sharp intersections from one material to the other (e.g., in case of a thin corrosion protection), but also by graded structures enabling a smoother material transition (e.g., in case of dissimilar materials which are joined together without defects). The latter is shown for an example from aerospace within this paper. Another possibility is the simultaneous placement of several materials, e.g., hard carbide particles placed in a more ductile matrix composition. These particles can be varied in size (e.g., TiC versus WC). Also the ratio between carbides and matrix alloy can be adjusted depending on its application. Especially, nozzle-based free form fabrication technologies, e.g., laser metal deposition, enable the utilization of more than one material. Within this contribution, possibilities to feed more than one filler material are demonstrated. In addition, results of multi-material processes are shown. Finally, this work focuses on different (potential) applications, mainly on power generation, but also for medical technology or wear resistant components.

We present a comprehensive investigation of morphological changes inside the active layer of an organic solar cell induced by substrate heating during layer deposition by thermal evaporation in ultra-high vacuum. To explore the trends observed in solar cell devices, we apply absorption and photoluminescence spectroscopy, atomic force microscopy, X-ray diffraction, and organic field effect transistor measurements. The material combination we use comprises unsubstituted dicyanovinyl end-capped quaterthiophene (DCV4T) as the donor material mixed with C60 as the acceptor. The solar cell power conversion efficiency decreases with increasing substrate temperature during film deposition due to changes in the crystalline structure of the oligothiophene phase, leading to a decrease in absorption strength. Photoluminescence measurements show that substrate heating increases the amount of phase separation between the donor and acceptor, and topology and structure investigations reveal large aggregates of polycrystalline DCV4T at the surface. However, the fill factor is increased for higher substrate temperatures due to better transport properties. The highest efficiency obtained with this material combination and stack design is 3.0% under AM1.5g illumination. © 2012 Elsevier B.V. All rights reserved.

Recently organic tandem solar cells with record efficiency had been shown comprising identical absorber materials in both subcells. Such structures pose new challenges for characterization. The standard test methods for measuring spectral response of tandem solar cells can not be applied. The standard procedures demand for different bias illumination during measuring spectral response allowing to select the subcell being current limiting. With subcells comprising identical absorber materials, thus having identical absorption spectra, such a selection is not trivial. In this paper, we show that with the help of detailed optical simulations of such tandem organic solar cells, their characterization is possible, and we apply the proposed method to a sample structure. © 2014 John Wiley and Sons, Ltd.

We compare two air-stable n-dopants for the fullerene C60: AOB and DMBI-POH. Conductivity and Seebeck coefficient measurements were performed at various doping concentrations and the thermal activation of the conductivity was determined. A superlinear increase of conductivity upon doping was found for DMBI-POH doped C60 reaching a maximum conductivity of 5.3 S/cm. In contrast to this, a linear rise of conductivity and an exponential thermal activation of mobility was observed for C60 doped by AOB. This suggests a different doping mechanism for the two compounds. © 2012 Elsevier B.V. All rights reserved.

The performance of organic solar cells strongly depends on the nanoscale structure of the used mixed absorber layer. Utilizing photoconductive and conductive atomic force microscopy (pcAFM and cAFM), as well as transmission X-ray microscopy (TXM), we investigate the influence of different substrate temperatures Tsub on the thin-film structure and local photocurrent in bulk-heterojunctions (BHJs) of vacuum deposited zinc phthalocyanine (ZnPc) and Buckminsterfullerene (C60) mixed absorber layers. In this paper, we present topography maps, photocurrent maps under short-circuit current conditions, dark-current maps, and TXM images with high lateral resolution down to 25nm. We observe a strong influence of the substrate temperatures during deposition Tsub on the nanoscopical segregation of the two components in the BHJ. This segregation leads to a spatial extension of the dark-current and a reduced short-circuit current at higher substrate deposition temperatures Tsub. © 2013 Elsevier B.V.

True-amplitude migration and demigration are two familiar processes in the world of seismic imaging. They transform data between the time and depth domains in such a way that geometrical spreading effects are accounted for. That means, the migration process eliminates the geometrical spreading factor while the demigration process reintroduces it in order to recover the original data. Applying both processes in sequence enables arbitrary seismic data transformations from an input domain (either the time or the depth domain) to an output domain which may or may not differ from the input domain. Since relevant parameters can be changed from step to step, e.g., the macro-model, the measurement configuration, or the ray code (i.e., according to ray theory, the subset of the wave field selected for imaging), a multitude of target-oriented seismic imaging problems can be addressed. For example, configuration transformations like migration to zero offset (MZO) or redatuming, ray code transformations (e.g., transformations of P-P reflections into P-S reflections), trace interpolation, remigration (RM), or even the simulation of time-lapse (4D) seismic monitoring are all suitable candidates for this approach. In this paper, we briefly review the Kirchhoff-type true-amplitude migration and demigration processes in 2.5D and 3D which are the fundamental transformations to this approach. Furthermore, we show quantitatively that true-amplitude migration and demigration are asymptotically inverse imaging processes which justifies the application of the method to arbitrary seismic imaging problems. Finally, we give a real data example focusing on the application of cascading migration and demigration operations for, e.g., trace interpolation.

The open-circuit voltage (VOC) of an organic solar cell is limited by the donor-acceptor material system. The effective gap E g eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for VOC, which is only reached for T → 0 K. This relation is confirmed for a number of small-molecule bulk heterojunction p-i-n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of VOC is identical to E g eff. Furthermore, the influence of the hole transport material in a p-doped hole transport layer and the donor-acceptor mixing ratio on this limit V0 is found to be negligible. Varying the active material system, the quantitative relation between V0 and E g eff is found to be identity. A comparison of V 0 in a series of nine different donor-acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material. The effective gap of a photovoltaic donor-acceptor system equals the open-circuit voltage extrapolated to temperature zero. The extrapolation is independent of the illumination intensity, and material variations in the doped transport layers do not affect the measurement result. This is shown for bulk-heterojunction devices with different mixing ratios and with small-molecular materials from various classes. Copyright © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60, as already employed in organic photovoltaics. By using DFT calculations and analyzing the recombination characteristics of the excited states revealed by photoinduced absorption (PIA) spectroscopy, the absorption peaks are assigned to triplet exciton, cation, and anion transitions. We determine the temperature dependent generation and recombination behavior of triplet excitons and cations in the mixed layer. At 10 K, we observe an enhanced triplet exciton generation rate compared to the pristine donor layer due to back recombination from a charge-transfer (CT) state at the donor-acceptor interface. With increasing temperature, the triplet generation rate first increases which is ascribed to an enhanced singlet exciton migration to this interface. Above 150 K, the triplet generation rate declines due to the beginning CT exciton separation, leading to the generation of free charge carriers. This temperature activated behavior is ascribed to a temperature activated increase of charge carrier mobility, facilitating CT exciton splitting. © 2012 American Chemical Society.

In this contribution the effect of changes in the activematerial system and modification of contact properties in organic solar cells is investigated at the example of ZnPc: C60 small molecule solar cells. Devices with different blend mixing ratios and a variation of hole transport layer are prepared by vacuum deposition and compared to drift-diffusion simulation data. It is shown that the open circuit voltage is mainly defined by themixing ratio,whereas the fill factor is strongly influenced by the choice of hole transport layer. Extraction barriers for photogenerated holes lead to S-shaped IV curves. The strength of the S-shape scales with the height of the extraction barrier. The slope of the IV curves at open circuit suggests that the observed increase in open circuit voltage with a higher amount of C60 in the blend might be due to a downshift of the highest occupied molecular orbital of ZnPc. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

We observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current-voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power-generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

We present a simple and versatile technique to introduce plasmonic silver nanoparticles into organic thin film devices by in situ vacuum deposition. Silver particles with 80 nm diameter at the back of small molecule organic solar cells increase the power conversion efficiency (PCE). Doped organic transport layers allow one to separate electrical and optical effects. By a systematic variation of the position of the silver particles within the solar cell stack, we can thus clearly distinguish a near-field photocurrent gain in the IR that decays to one-half on length scales of around 4 nm, and a less distance-dependent selective mirror effect for short wavelength, which allows one to optimize devices for different wavelengths simultaneously. Device optimization reveals that plasmonic increased absorption can be used to significantly reduce the thickness of the absorber layers and gain efficiency through improved transport properties. A plasmonic zinc phthalocyanine fullerene-C60 solar cell that yields improved photocurrent, fill factor, and PCE of 2.6% includes one-half of the absorber material of an optimized reference device with PCE of 2.4%. The design priciples for plasmonic solar cells are general and were confirmed in thin devices containing zinc 1,8,15,22-tetrafluoro-phthalocyanine, improving the PCE from 2.7% to 3.4%. © 2014 American Chemical Society.

The maximum open-circuit voltage VOC of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene C60 with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C60 from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C60) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:C60 as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C60 blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C60 phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested. © 2013 American Physical Society.

Meanwhile the laser build-up welding technology is a well industrially established technique for highly precise coating tasks and additive manufacturing. By means of new precise powder nozzle systems, modified process equipment as well as suited manufacturing strategies, the generation of customized components can be realized up to a detail resolution within the double-digit micrometer range. This opens a variety of new applications, e. g. in aviation, tooling industry or medical technology.

In contrast to p-dopants, highly efficient molecular n-dopants are prone to degradation in air due to their low ionization potentials, limiting the processing conditions of doped functional organic devices. In this contribution, we investigate the air-stability of pure films of the n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4) and of C60 layers doped by W2(hpp)4. We find that 1/3 of the initial conductivity of the doped C60 thin films can be restored by thermal annealing in vacuum after a drop by 5 orders of magnitude upon air exposure. Furthermore, we show by ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements that the Fermi level shift toward the lowest unoccupied molecular orbital (LUMO) of C60 remains after air exposure, clearly indicating a conservation of n-doping. We explain these findings by a down-shift of the W2(hpp)4 energy levels upon charge-transfer to a host material with deeper lying energy-levels, facilitating a protection against oxidation in air. Consequently, the observed recovery of the conductivity can be understood in terms of a self-passivation of the molecular n-doping. Hence, an application of highly efficient n-doped thin films in functional organic devices handled even under ambient conditions during fabrication is feasible. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Enhancement of the efficiency of organic solar cell devices requires knowledge about the structure of the organic layers involved. Films of the donor material dicyanovinyl-sexithiophene bearing four ethyl side-chains at thiophenes two and five DCV6T-Et(2,2,5,5) (DCV6T-Et) are prepared by thermal evaporation in high vacuum at various thicknesses and substrate temperatures. Infrared spectroscopic ellipsometry is used for determination of the molecular orientation in the thin films grown on room temperature (RT) substrate. From simulation of the IR ellipsometric data, the film thickness and the anisotropic optical constants of the DCV6T-Et films are determined. It is found that the optical constants strongly depend on the film thickness. Different average molecular orientations are determined for a few molecules thin (4 nm) and somewhat thicker (20 nm) films. Furthermore, the evolution of the surface morphology of films deposited at elevated substrate temperatures (80°C, 100°C) is studied in comparison to the thick RT-film. Atomic force microscopy images indicate that the growth on heated substrate is accompanied by an increase in grain size and surface roughness of the films. Simultaneously, the measured optical absorption spectra display structured and increased absorption in the red spectral region for the DCV6T-Et films deposited at higher substrate temperatures. The changes in surface topography and optical response relate to improved molecular arrangement induced by the substrate heating. To demonstrate the morphological influence on solar cell performance, we finally discuss DCV6T-Et/C60 planar heterojunction solar cells composed of DCV6T-Et films deposited at different substrate temperatures. © 2012 Elsevier B.V.

Losses of charge carriers, due to the interfacial charge recombination processes, in small molecule organic solar cells (SMOSCs) have been investigated under operating conditions. The devices consist of zinc phthalocyanine (ZnPc) as electron donor material and C60 as electron acceptor. The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have been compared to the measurements carried out with impedance spectroscopy (IS) and show good agreement. Significantly, much difference is observed in either the charge density distribution versus the device voltage or the charge carriers lifetime when comparing bulk heterojunction versus bilayer-type ZnPc:C60 devices. The implications of the faster charge carrier recombination with the device fill factor (FF) and the open circuit voltage (VOC) are discussed. © 2012 American Chemical Society.

To achieve efficient organic solar cells, donor and acceptor molecules are mixed in the photoactive layer to form a so-called bulk heterojunction. Due to molecular interactions, a certain degree of phase separation between donor and acceptor domains arises, which is necessary to achieve efficient charge extraction within the absorber layer. However, the mechanism that induces the phase separation is not fully understood and gaining detailed information about the molecular arrangement within these blend layers is quite challenging. We show that grazing incidence x-ray diffraction, combined with variable angle spectroscopic ellipsometry is a suitable way to investigate the molecular structure of blend layers in detail, consisting of a mixture of zinc-phthalocyanine (ZnPc) and C60. The degree of phase separation within the blend layer is influenced by substrate heating during the co-evaporation of ZnPc and C60 and by a variation of the mixing ratio. The effect of different blend layer morphologies on optical and electrical device performance is investigated by solar cell characterization and mobility measurements. We find that the molecular arrangement of C60 provides the essential driving force for efficient phase separation. Whereas spherical C60 molecules are able to form crystalline domains when deposited at elevated substrate temperatures, no ZnPc crystallites are observed, although the planar ZnPc molecules are not randomly oriented but standing upright within its domains. Comparing specular and grazing incidence x-ray diffraction, we find that only the latter method is able to detect nanocrystalline C60 in thin films due to its polycrystalline nature and small sized nanocrystallites. Solar cell measurements show an increase in fill factor and external quantum efficiency signal for blends with enhanced phase separation, induced by higher substrate temperatures. However, grazing incidence x-ray diffraction measurements reveal that ZnPc and C60 already form separate domains in unheated ZnPc:C60 blends, which provide fill factors close to 50% in the corresponding solar cells. © 2012 American Physical Society.

To achieve efficient organic solar cells, donor and acceptor molecules are mixed in the photoactive layer to form a so-called bulk heterojunction. Due to molecular interactions, a certain degree of phase separation between donor and acceptor domains arises, which is necessary to achieve efficient charge extraction within the absorber layer. However, the mechanism that induces the phase separation is not fully understood and gaining detailed information about the molecular arrangement within these blend layers is quite challenging. We show that grazing incidence x-ray diffraction, combined with variable angle spectroscopic ellipsometry is a suitable way to investigate the molecular structure of blend layers in detail, consisting of a mixture of zinc-phthalocyanine (ZnPc) and C-60. The degree of phase separation within the blend layer is influenced by substrate heating during the co-evaporation of ZnPc and C-60 and by a variation of the mixing ratio. The effect of different blend layer morphologies on optical and electrical device performance is investigated by solar cell characterization and mobility measurements. We find that the molecular arrangement of C-60 provides the essential driving force for efficient phase separation. Whereas spherical C-60 molecules are able to form crystalline domains when deposited at elevated substrate temperatures, no ZnPc crystallites are observed, although the planar ZnPc molecules are not randomly oriented but standing upright within its domains. Comparing specular and grazing incidence x-ray diffraction, we find that only the latter method is able to detect nanocrystalline C-60 in thin films due to its polycrystalline nature and small sized nanocrystallites. Solar cell measurements show an increase in fill factor and external quantum efficiency signal for blends with enhanced phase separation, induced by higher substrate temperatures. However, grazing incidence x-ray diffraction measurements reveal that ZnPc and C-60 already form separate domains in unheated ZnPc:C-60 blends, which provide fill factors close to 50% in the corresponding solar cells.