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The development of near-infrared (NIR) photodetection technologies is driven by emerging applications such as medical imaging or optical sensors for electronic displays and machine vision. An all-organic upconverter (OUD) is a device that converts incident NIR light directly into visible light and consists of a monolithic stack of a NIR organic photodetector (OPD) and a visible organic light-emitting diode (OLED). We present OUDs that consist of NIR heptamethine cyanine dyes/C60 fullerene OPDs and a fluorescent tris(8-hydroxyquinolinato)aluminium (Alq3) OLED. The device metrics of performance are a high external quantum efficiency (EQE) of 43% of the OPD part and an OUD luminance turn-on at low bias voltages of 2 V. The dynamic response of the photocurrent and luminance is linear over a NIR light range corresponding to 26 dB. These OUDs upconvert NIR light at 830 nm to green light with a photon-to-photon conversion efficiency of 0.61%, close to the expected maximum.

Imaging in the near-infrared (NIR) is getting increasingly important for applications such as machine vision or medical imaging. NIR-to-visible optical upconverters consist of a monolithic stack of a NIR photodetector and a visible light-emitting unit. Such devices convert NIR light directly to visible light and allow capturing a NIR image with an ordinary camera. Here, five-layer organic solution-processed upconverters (OUCs) are reported which consist of a squaraine dye NIR photodetector and a fluorescent poly( para-phenylene vinylene) copolymer (super yellow)-based organic light-emitting diode (OLED) or light-emitting electrochemical cell (LEC), respectively. Both OLED-OUCs and LEC-OUCs convert NIR light at 980 nm to yellow light at around 575 nm with comparable device metrics of performance, such as a turn-on voltage of 2.7-2.9 V and a NIR-to-visible photon conversion efficiency of around 1.6%. Because of the presence of a salt in the emitting layer, the LEC-OUC is a temporally dynamic device. The LEC-OUC turn-on and relaxation behavior is characterized in detail. It is demonstrated that a particular ionic distribution and thereby the LEC-OUC status can be frozen by storing the device in the presence of a small voltage applied. This provides a test chart for quantitative measurements.

The growing interest in near-infrared (NIR) imaging is explained by the increasing number of applications in this spectral range, which includes process monitoring and medical imaging. NIR-to-visible optical upconverters made by integrating a NIR photosensitive unit with a visible emitting unit convert incident NIR light to visible light, allowing imaging of a NIR scene directly with the naked eye. Optical upconverters made entirely from organic and hybrid materials – which include colloidal quantum dots, and metal-halide perovskites – enable low-cost and pixel-free NIR imaging. These devices have emerged as a promising addition to current NIR imagers based on inorganic semiconductor photodiode arrays interconnected with read-out integrated circuitry. Here, we review the recent progress in the field of optical upconverters made from organic and hybrid materials, explain their functionality and characterization, and identify open challenges and opportunities., GRAPHICAL ABSTRACT 10.1080/14686996.2019.1610057-UF0001

Narrowband photodetectors are useful for a myriad of applications involving color discrimination as well as biological imaging and machine vision. Prevalent devices use broadband light absorbing materials in conjunction with electric (charge collection narrowing, photo multiplication) and optical strategies (optical cavity) to narrow down the spectral response of the detector. Here we exploit the intrinsic narrow absorption width of ∼13–50 nm peculiar to J-aggregates of strongly coupled cyanine dye molecules for narrowband photodetectors. Albeit using ultrathin J-aggregate films (10 nm) the unparalleled optical aggregate properties lead to high external quantum efficiency (15%) and response speed (15 kHz) at low bias voltage (−1 V). We show that the self-assembly process and device architecture used permit the incorporation of J-aggregates of various cyanine dyes with photodetection maxima ranging from the visible to the near-infrared region up to 1000 nm. Narrow bandwidth (

Polymer light-emitting electrochemical cells (PLECs) and light-emitting diodes (PLEDs) receive interest for large-area lighting and signage applications. During operation of a PLEC, a p-i-n junction develops where electrons and holes are injected into the film and are transported along n- and p-doped regions to the intrinsic (i) region, where they recombine under light emission. Conceptually, this resembles the PLED device architecture equipped with Ohmic charge-injection and transport layers. The similarity between the i-region of the PLEC and the emissive layer of the PLED is obvious; however, implication of this has not been examined in detail so far. For example, for PLEDs it is known that electron traps hinder the electron transport, and that hole trap formation dictates the long-term durability. Here, for PLECs the electrical and optical response to electrical driving and breaks are studied, the current response to external light irradiation is probed, and degradation is followed with long-term absorption and capacitance measurements. The electron traps in PLECs are identified and it is found that hole trap formation limits the device lifetime, in the same manner as established for PLEDs. It is concluded that charge traps in semiconducting polymers present important, but so far overlooked intrinsic performance limitations for PLECs.

We conducted a global survey on the effects of the COVID-19 pandemic on the research activities of materials scientists by distributing a questionnaire on 9 October 2020 with a response deadline of 23 October 2020. The questions covered issues such as access to labs, effectiveness of online conferences, and effects on doctoral students for the period covering the first lockdowns until the relaxation of restrictions in late September 2020 in many countries. The survey also included online interviews with eminent materials scientists who shared their local experiences during this period. The interviews were compiled as a series of audio conversations for The STAM Podcast that is freely available worldwide. Our findings included that the majority of institutes were not prepared for such a crisis; researchers in China, Japan, and Singapore were able to resume research much quicker – for example after approximately one month in Japan – than their counterparts in the US and Europe after the first lockdowns; researchers adapted to using virtual teleconferencing to maintain contact with colleagues; and doctoral students were the hardest hit by the pandemic with deep concerns about completing their research and career prospects. We hope that the analysis from this survey will enable the global materials science community to learn from each other’s experiences and move forward from the unprecedented circumstances created by the pandemic., Graphical Abstract

Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. In this study, we address this issue by using structurally comparatively simple, well-defined, and chiral intermetallic PdGa{111} surfaces as catalytic substrates. We demonstrate the impact of chirality transfer and ensemble effect for the thermally activated azide-alkyne Huisgen cycloaddition between 3-(4-azidophenyl)propionic acid and 9-ethynylphenanthrene on these threefold symmetric intermetallic surfaces under ultrahigh vacuum conditions. Specifically, we encounter a dominating ensemble effect for this reaction as on the Pd-3-terminated PdGa{111} surfaces no stable heterocoupled structures are created, while on the Pd-1-terminated PdGa{111} surfaces, the cycloaddition proceeds regioselectively. Moreover, we observe chirality transfer from the substrate to the reaction products, as they are formed enantioselectively on the Pd-1-terminated PdGa{111} surfaces. Our results evidence a determinant ensemble effect and the immense potential of PdGa as asymmetric heterogeneous catalyst., Mechanistic insight into enantioselective reactions at intrinsically chiral surfaces can be challenging to obtain. Here the catalytic activity of Pd-1- and Pd-3-terminated PdGa{111} surfaces is shown to differ substantially, with Pd-1-terminated surfaces promoting on-surface azide- alkyne cycloadditions enantioand regioselectively.

A sandwich-type structure composed of thin films of functional materials is the dominant architecture of organic optoelectronic devices. During device operation, injected or inside-generated electrical charges arrange in a specific spatial distribution. This distribution affects the electric field profile throughout the layers and has a fundamental impact on charge transport and recombination/separation processes. However, probing of the charge distribution is challenging because the hidden active layers are experimentally not directly accessible. Here, we study the temporal evolution of organic salt-based light-emitting electrochemical cells and demonstrate that electroabsorption spectroscopy in combination with electrical capacitance measurements enables the determination of the distribution of the injected, uncompensated electronic charge inside these devices. For constant-voltage operating conditions and over a period of hours, the Stark effect signal intensity and the capacitance increase steadily, b...

There is increasing evidence that the presence of mobile ions in perovskite solar cells can cause a current-voltage curve hysteresis. However, it is still subject of ongoing debates how exactly mobile ions influence the device operation. We use drift-diffusion simulations incorporating mobile ions to describe IV-curves of preconditioned methylammonium lead iodide perovskite solar cells and compare them with experimental results. Our simulation results show that the hysteresis depends on the extent of surface recombination and on the diffusion length of charge carriers. We provide a detailed explanation for the reduced hysteresis of perovskite solar cells with high power conversion efficiencies. We find that in high-efficiency solar cells ion migration is still present, but does not cause a hysteresis effect. In these devices charge extraction is mainly driven by diffusion of free electrons and holes.

Light-emitting electrochemical cells (LECs) can be fabricated as a single emissive organic/salt layer sandwiched between two electrodes, offering cost-effective next generation signage and lighting applications. Cyanine dyes are especially attractive to exploit the low cost potential of LECs. Cyanines denote a large class of fluorescent organic salts with tuneable emission wavelength, inherent conductivity for ionic and electronic charges, and many cyanines are commercially available at low cost. We systematically tested a set of cyanine dyes for visible emitting LECs. To circumvent non-radiative quenching processes in pure cyanine films (monomer fluorescence quantum yields, PLQE

Tinted and colour-neutral semitransparent organic photovoltaic elements are of interest for building-integrated applications in windows, on glass roofs or on facades. We demonstrate a semitransparent organic photovoltaic cell with a dry-laminated top electrode that achieves a uniform average visible transmittance of 51% and a power conversion efficiency of 3%. The photo-active material is based on a majority blend composed of a visibly absorbing donor polymer and a fullerene acceptor, to which a selective near-infrared absorbing cyanine dye is added as a minority component. Our results show that organic ternary blends are attractive for the fabrication of semitransparent solar cells in general, because a guest component with a complementary absorption can compensate for the inevitably reduced current generation capability of a high-performing binary blend when applied as a thin, semitransparent film. ISSN:1468-6996 ISSN:1878-5514

In light‐emitting electrochemical cells (LECs), the position of the emission zone (EZ) is not predefined via a multilayer architecture design, but governed by a complex motion of electrical and ionic charges. As a result of the evolution of doped charge transport layers that enclose a dynamic intrinsic region until steady state is reached, the EZ is often dynamic during turn‐on. For thick sandwich polymer LECs, a continuous change of the emission color provides a direct visual indication of a moving EZ. Results from an optical and electrical analysis indicate that the intrinsic zone is narrow at early times, but starts to widen during operation, notably well before the electrical device optimum is reached. Results from numerical simulations demonstrate that the only precondition for this event to occur is that the mobilities of anions (µa) and cations (µc) are not equal, and the direction of the EZ shift dictates µc > µa. Quantitative ion profiles reveal that the displacement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. Finally, simulations indicate that the experimental current peak for constant‐voltage operation is intrinsic and the subsequent decay does not result from degradation, as commonly stated.

Made available in DSpace on 2018-12-11T16:43:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-01-01 Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung A simple lamination process of the top electrode for perovskite solar cells is demonstrated. The laminate electrode consists of a transparent and conductive plastic/metal mesh substrate, coated with an adhesive mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, and sorbitol. The laminate electrode showed a high degree of transparency of 85%. Best cell performance was achieved for laminate electrodes prepared with a sorbitol concentration of ~30 wt% per milliliter PEDOT:PSS dispersion, and using a pre-annealing temperature of 120°C for 10 min before lamination. Thereby, perovskite solar cells with stabilized power conversion efficiencies of (7.6 ± 1.0)% were obtained which corresponds to 80% of the reference devices with reflective opaque gold electrodes. Empa Swiss Federal Institute for Materials Science and Technology Laboratory for Functional Polymers UNESP–Universidade Estadual Paulista–Instituto de Química POSMAT CSEM Centre Suisse d’Electronique et Microtechnique SA Nanotechnology Group ETH Zurich UNESP–Universidade Estadual Paulista–Instituto de Química POSMAT Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung: 200021-144120/1 Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung: NRP70 PV2050 407040-153976/1

Cyanine dyes are fluorescent organic salts with intrinsic conductivity for ionic and electronic charges. Recently (J. Am. Chem. Soc. 2013, 135, 18008-18011), these features have been exploited in cyanine light-emitting electrochemical cells (LECs). Here, we demonstrate that stacked, constant-voltage driven trimethine cyanine LECs with various counteranions develop a p-i-n junction that is composed of p- and n-doped zones and an intrinsic region where light-emission occurs. We introduce a method that combines spectral photocurrent response measurements with optical modeling and find that at maximum current the intrinsic region is centered at similar to 37% away from the anode. Transient capacitance, photoluminescence and attenuance experiments indicate a device situation with a narrow p-doped region, an undoped region that occupies similar to 72% of the dye layer thickness and an n-doped region with a maximum doping concentration of 0.08 dopant/cyanine molecule. Finally, we observe that during device relaxation the parent cyanines are not reformed. We ascribe this to irreversible reactions between doped cyanine radicals. For sterically conservative cyanine dyes, this suggests that undesired radical decomposition pathways limit the LEC song-term stability in general.

Ultrathin bilayer heterojunction solar cells using cyanine electron donors and electron acceptor C-60 are used to fabricate monolithically stacked tandem and triple junction devices. Sub-cell stack sequences as well as C-60 layer thicknesses are optimized by optical modeling and maximum efficiency is corroborated experimentally. The highest power conversion efficiency of 4.3% under full sun irradiation is achieved with a tandem cell where heptamethine and trimethine cyanine dyes are used in the front and back cell, respectively. The open circuit voltage matches the sum of the two respective open circuit voltages of the individual single junction solar cells within 3%. Triple junction cells using an additional sub-cell with a pentamethine cyanine suffer from electrical series resistance. At low light irradiation intensity, however, both triple and tandem solar cells reach power conversion efficiencies above 5% in agreement with the performance increase predicted from numerical simulation. (C) 2015 Elsevier B.V. All rights reserved.

Nanoporous surface morphologies often perform better than their planar or bulk counterparts in phenomena such as catalysis and charge transport. In organic photovoltaics, for example, fullerene films structured on a scale corresponding to the exciton diffusion length, typically a few tens of nanometers, could improve charge separation. Existing methods to form such films, however, typically do not reach features smaller than 100 nm. Here, we propose a simple method based on interfacial nucleation and growth to produce large and flexible 2D percolating assemblies of C60 nanoparticles with diameters below 50 nm that cover up to 60% of the surface. These films can be rendered insoluble by photopolymerization and infiltrated with other functional molecules. We illustrate the benefit of such stabilized nanoporous films on the example of organic solar cells of the architecture ITO/TiO2/C60/P3HT/MoO3/Ag. Structured C60 films with interconnected morphological features perform better than planar C60 films by 50% on average, reaching a power conversion efficiency of 1.3%. Due to the ease of film fabrication and small dimensions of the C60 morphological features, these results may find application beyond organic photovoltaics, in a variety of fields where an enhanced interface with a fullerene surface is needed.

The redistribution of ions in light-emitting electrochemical cells (LECs) plays a key role in their functionality. The direct quantitative mapping of ion density distributions in operating realistic sandwich-type devices, however, has not been experimentally achieved. Here we operate high-performing [Super Yellow/trimethylolpropane ethoxylate/lithium trifluoromethanesulfonate (Li+CF3SO3–)] LEC devices inside a time-of-flight secondary ion mass spectrometer and cool the devices after different operation times to liquid nitrogen temperatures before depth profiling is performed. The results reveal the dependence of the elemental and molecular distributions across the device layer on operation conditions. We find that the ion displacements lead to a substantial shift of the local chemical equilibria governing the free ion concentration.

Intrinsic photogeneration of charge carriers in organic semiconductors is generally attributed to high energy ionization or exciton dissociation by a strong electric field. Here, high bulk photoconductivity is reported in pristine pentamethine cyanine films with photocurrent onset at the band‐edge of the organic semiconductor. Single‐layer cyanine diodes with selective hole and electron contacts show linear dependence of photocurrent with reverse voltage and light intensity. Numerical drift‐diffusion simulations reveal that the linear resistor behavior stems from low and unbalanced carrier mobilities giving rise to negative space charge. Slow bimolecular recombination kinetics of photoinduced charges obtained by time delayed charge extraction measurements show strongly reduced Langevin recombination with long carrier lifetime of the order of a millisecond. Such reduced charge carrier recombination puts forward a materials concept to be exploited in photodiodes and more generally in optoelectronic devices.

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye/fullerene organic solar cells enables one to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers' generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

C-60 fullerene derivatives bearing aliphatic chains can self-assemble into versatile supramolecular structures. Cross-linking of such self-assembled morphologies is an attractive approach to enhance the structural stability of these self-organized structures. We describe the synthesis of a C-60 fimctionalized with a single alkyl chain bearing a diacetylene moiety. In a thin film, the molecule self-assembles into lamellar arrays. The character of the side chain attached to the fullerene is key to the observed packing ability. The stabilization proceeds through solid-state polymerization of the diacetylene moieties. By blending the fiillerene derivative with a cyanine dye, various nanostructured fiillerene morphologies are obtained that can be selectively stabilized by thermal polymerization. These films can serve as basis for nanostructured fullerene scaffolds that can find applications in optics and electronics.

Effects of ion concentration and active layer thickness play a critical role on the performance of light-emitting electrochemical cells. Expanding on a pioneering materials system comprising the super yellow (SY) polymer and the electrolyte trimethylolpropane ethoxylate (TMPE)/Li+CF3SO3-, it is reported that a slightly lowered salt concentration and layer thickness result in a substantial efficiency increase, and that this increase is confined to a narrow concentration and thickness range. For a film thickness of 70 nm, a blend ratio SY:TMPE:Li+CF3SO3- = 1:0.075:0.0225, and a current of 7.7 mA cm(-2) the current efficacy is 11.6 cd A(-1), on a par with SY light-emitting diodes. The optimized salt content can be explained by increased exciton quenching at higher concentrations and hindered carrier injection and conduction at lower concentrations, while the optical dependence on the layer thickness is due to weak microcavity effects. A comprehensive optical modeling study is presented, which includes the doping-induced changes of the refractive indices and self-absorption losses due the emission-absorption overlap of intrinsic and doped SY. The analysis indicates either a thickness-independent emitter position (EP) close to the anode or a thickness-dependent EP, shifted to the cathode for increased thicknesses.

A selective near-infrared absorbing heptamethine cyanine dye (Cy7-P) electron donor was used for the fabrication of bilayer solar cells with the acceptor C60. Using a reflective metal top electrode, solar cell optimization resulted in an external power conversion efficiency (η) of 1.5% for layer thicknesses of ~20 nm for Cy7-P and of ~40 nm for C60, with using either PEDOT:PSS or MoO3 as anode and Alq3 as cathode buffer layers. Highly transparent devices were then fabricated by using silver/Alq3 cathodes. Average visible transmittance (450–670 nm) values of, for example, 67.2% (η=0.7%), 62.1% (η=0.9%) or 50% (η=1%) were obtained for different thickness combinations of silver and Alq3. Optimized solar cells had a maximum transparency of 79.8% at 568 nm. The operational stability under 1 sun illumination was T80=30 h, after which 80% of the initial cell performance was reached.

Studying and understanding the conditions under which organic semiconductors can be engineered to form aligned single crystals in thin films is of primary importance owing to their unique orientation-dependent optoelectronic properties. Efforts to reach this goal by self-assembly from solution-processed films have been rewarded only with limited success. In this article we present a new method to grow single crystalline thin films via solvent annealing. We identify solvate crystal growth in combination with a specific film dewetting morphology as key to successful fabrication of single crystals. Furthermore, these 2D single crystals can align on chemically patterned substrates to minimize their interfacial energy. We explore in situ the conditions for crystal formation and alignment.

The long-term (up to 100 days) stabilities of thin films of a trimethine cyanine dye (Cy3-P) and of Cy3-P/C-60 bilayer solar cells with initial power conversion efficiencies of up to 3.6% in the regular and inverted device architecture were studied. For storage conditions under nitrogen at room temperature in the dark, Cy3-P films were stable and no decrease in performance was measured for inverted ITO/TiO2/C-60/Cy3-P/ MoO3/Ag solar cells. ITO/PEDOT:PSS and aluminum were found to be responsible for a slight inherent degradation of regular solar cells under inert storage conditions. Solar cells stressed at accelerated (80 degrees C) full sun simulation showed a burn-in period of similar to 5 days with a performance loss of 75%. This initial performance decrease might be linked to the photopolymerization reaction of C-60. For longer times, performances stabilized and arrived at 15% of the initial values after a test period of 17 days. (C) 2013 Elsevier B.V. All rights reserved.

Squaraine (SQ) dye-based organic semiconductor hybrids 6T-SQ and 18T-SQ functionalized with oligothiophene dendrons were synthesized via Suzuki–Miyaura coupling. The electronic coupling between the oligothiophene dendrons and the squaraine core was rather weak, as suggested from UV–vis spectra, cyclic voltammetry measurements and molecular modeling. Thin films of pure SQ were characterized by a pronounced solvent- and heat-induced crystallization tendency. The dendrons substantially hindered the squaraine core crystallization, and 18T-SQ films remained amorphous after annealing or storage for several weeks. PCBM disrupted dye crystallization in blends, and smooth and stable films could be coated. Heat treatment of blended films induced dewetting for SQ:PCBM and 6T-SQ:PCBM, but 18T-SQ:PCBM remained again stable. These morphological film features could consistently explain the performance of dye-fullerene solar cells. The best performance (η ∼ 1.5%) was obtained for simple bilayer 6T-SQ:C60 or 18T-SQ:C60 cells without annealing. Our results demonstrate that the attachment of decorating moieties to a central light-absorbing core unit in molecular push–pull systems can be used to adjust the optoelectronic and morphological film properties of small molecular semiconductors with a strong tendency towards crystallization.

We describe the self-organization of PCBM and a cyanine dye on chemically patterned surfaces during spin coating from solution. On homogeneous surfaces, a transient bilayer forms, which in a later stage decomposes into PCBM droplets in a matrix of the cyanine dye. On the patterned surface also a PCBM droplet phase develops, but the final film structure is greatly determined by contact line pinning of the PCBM domains to the substrate pattern. Three characteristic morphology regimes separated by wetting transitions were observed for different ratios between the natural domain dimensions and the underlying pattern periodicity. We demonstrate that contact line pinning can be an important means to control the film morphology in systems where films are coated from solution. This process can be exploited as a general and versatile method for patterning small semiconducting molecules into 1D and 2D photonic crystals.

We report the status and strategies to improve the performance of cyanine dye/C60 bilayer heterojunction solar cells. Cyanines are characterized by convenience of synthesis and tunable light absorption. They have high light extinction coefficients, which offer the possibility to go around the constraints of small exciton diffusion length, and ultrathin films can be applied to fabricate multi layer organic solar cells. We optimized the cell performance by oxidative doping (with O2/water/light, or NOBF4) of the cyanine layer. Doping increased the film conductivity and induced a steep rise in the short-circuit current and the open-circuit voltage. The external quantum efficiency approached 80% at maximum, meaning that the full cyanine layer of 30 nm thickness was active in charge generation. We argue that cyanine aggregates may be responsible for efficient charge transport over larger distances to the heterointerface. We also describe simple processes to match the energy to the low-lying hole-transporting cyanine level by using a thin organic salt layer or doped polyaniline as anode buffer layer. Thereby, hole injection was facilitated and good-performing devices (power conversion efficiency η = 3%) with a high fill factor (FF ∼60%) were fabricated. Cyanine dyes are accompanied by mobile counter ions. These ions can diffuse into adjacent layers and build up important space charge that alters the energy levels at heterojunctions. The control of the ionic space charge allows to control the flow of electronic charge carriers and to implement new functionalities unique to organic opto-electronic devices. Copyright © 2010 John Wiley & Sons, Ltd.

Current-voltage (J-V ) curves of photovoltaic devices can reveal important microscopic phenomena when parameterization is properly related to physical processes. Here, we identify a pronounced effect of thermal annealing on the organic-cathode metal interface and show that this interface is related to the origin of the kink often observed in J-V curves close to the open circuit. We propose that isolated metal nanoclusters that form upon cathode evaporation lead to defect states close to the interface and change the electric field distribution in the device. We express this scenario with a modified equivalent circuit and consistently fit J- V curves as a function of the annealing process. The developed model is general in the sense that any physical process that leads to the change in electric potential as described in this paper will possibly lead to a kink in the J- V curves. Knowing the origin of the kink allowed us to largely increase the device efficiency of the archetypal material combination Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) -C. We fabricated solar cells with an efficiency of 1.85% under 100 mW/cm AM1.5 illumination by using a deliberately designed interpenetrating bilayer film morphology, aluminium as cathode and thermal annealing. This is so far the highest reported efficiency for this particular combination of materials.

The performance of heterojunction organic solar cells is critically dependent on the morphology of the donor and acceptor components in the active film. We report results of photovoltaic devices consisting of bilayers and bulk heterojunctions using poly(3-hexylthiophene) (P3HT) and Buckminsterfullerene C-60. White light power efficiencies of eta similar to 2.2% (bulk heterojunction) and 2.6% (bilayer) were measured after a thermal annealing step on completed devices. Optical and structural investigations on non-annealed bilayer thin films indicated a distinct porosity of the spin-coated polymer, which allows C-60 to penetrate the P3HT layer and to touch the anode. This resulted for these bilayer solar cells in the experimental observation that electrons were collected predominantly at the cathode after photo-excitation of P3HT, but predominantly at. the anode after C-60 excitation. A morphological model to explain the ambipolar charge collection phenomenon is proposed. (c) 2007 Elsevier B.V. All rights reserved.

Two new, fully conjugated polymeric cyanine dyes based on trimethine and heptamethine moieties have been synthesized. Both polymers were characterized by gel permeation chromatography, UV-vis and IR spectroscopy, elementary analysis and cyclic voltammetry. The structure of one material could be confirmed with NMR spectroscopy. Upon head-to-tail coupling of the dye moieties distinct bathochromic shifts up to 159 nm were observed for the polymers which absorb solely in the near infrared (NIR) region with maxima up to 1002 nm and very high molar absorption coefficients. This highly efficient absorption in the NIR spectral domain combined with the strong electron accepting properties makes these dyes interesting candidates for many optical applications; investigations on photovoltaic devices based on polymeric cyanine dye/C 60 heterojunctions identify one of these possibilities.

We present atomic force microscopic images of the interphase morphology of vertically segregated thin films spin coated from two-component mixtures of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and polystyrene (PS). We investigate the mechanism leading to the formation of wetting layers and lateral structures during spin coating using different PS molecular weights, solvents and blend compositions. Spinodal decomposition competes with the formation of surface enrichment layers. The spinodal wavelength as a function of PS molecular weight follows a power-law similar to bulk-like spinodal decomposition. Our experimental results indicate that length scales of interface topographical features can be adjusted from the nanometer to micrometer range. The importance of controlled arrangement of semiconducting polymers in thin film geometries for organic optoelectronic device applications is discussed.

Growth behaviour of TiO2 particles, which has significant influence on their crystallinity and surface area, has been controlled by varying the process parameters of the flame aerosol synthesis. An existing method to determine the crystallinity of the powder has been modified which facilitates comparison of the degree of crystallinity of the nanoparticles synthesized by various methods. Resultant TiO2 nanoparticles were characterized by XRD and nitrogen physisorption to determine the degree of crystallinity quantitatively and surface area, respectively, and the decomposition rate of an industrial dye, methylene blue, has been taken as the measure of the photocatalytic activity (PCA). TiO2 nanoparticles showed improved photocatalytic activity compared to the commercial photocatalyst, Degussa P25. This is attributed to the increased degree of crystallinity and surface area of the TiO2 nanoparticles compared to Degussa P25. The results of this study elucidate the importance of the gas-phase processes and strategy to synthesize highly active TiO2 photocatalysts in a single step. Moreover, difference in the PCA of the nanoparticles synthesized by the similar gas-phase routes has been shown and the particles are tailor-made for improved PCA in the present study.

Standard chromatographic methods for the quantification of bacterial poly(3-hydroxyalkanoate) (PHA) proved to be inappropriate for the analysis of medium-chain-length PHA (mcl-PHA). Transesterification catalyzed by protic acids is not quantitative for mcl-PHA under common conditions due to slow reaction kinetics and formation of side-products in case of functionalized side-chains. To circumvent these limitations, an improved method for the quantification of mcl-PHA by GC-FID was developed. Boron trifluoride in methanol was successfully applied to quantitatively methanolyse different mcl-PHA (recovery >94%). This novel method is well-suited for the analysis of purified mcl-PHA as well as for mcl-PHA in biomass.

An improved synthesis of photosensitive homopolymers containing aryltriazene chromophores covalently incorporated into the polymer backbone is reported. Such photopolymers proved to have promising properties for novel UV-laser applications. A homologous series of new aryltriazene polymers with increasingly branched side chains (R = Me, Et, iPr, tBu) was synthesized and characterized. Homogeneous thin films with thicknesses from ≈15 to >150 nm were prepared by spin-coating. Photodecomposition was studied in solution and on thin films. Polymers with increasingly branched and bulky substituents showed decreasing photodissociation rates. NMR studies suggested an enhanced hindrance of the N(2)–N(3) bond rotation in the aryltriazene moiety with increasing steric demand of the substituents.