Search

In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7 × 1015 cm-3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells.

The distribution of electron donor and acceptor in the active layer is known to strongly influence the electrical performance of polymer solar cells for most of the high performance polymer:fullerene systems. The formulation of the solution from which the active layer is spincoated plays an important role in the quest for morphology control. We have studied how the choice of solvent and the use of small amounts of a low vapour pressure additive in the coating solution influence the film morphology and the solar cell performance for blends of poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM). We have investigated the lateral morphology using atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM), the vertical morphology using dynamic secondary ion mass spectrometry (d-SIMS) and variable-angle spectroscopic ellipsometry (VASE), and the surface composition using near-edge X-ray absorption fine structure (NEXAFS). The lateral phase-separated domains observed in films spincoated from single solvents, increase in size with increasing solvent vapour pressure and decreasing PC70BM solubility, but are not observed when 1-chloronaphthalene (CN) is added. A strongly TQ1-enriched surface layer is formed in all TQ1:PC70BM blend films and rationalized by surface energy differences. The photocurrent and power conversion efficiency strongly increased upon the addition of CN, while the leakage current decreased by one to two orders of magnitude. The higher photocurrent correlates with the finer lateral structure and stronger TQ1-enrichment at the interface with the electron-collecting electrode. This indicates that the charge transport and collection are not hindered by this polymer-enriched surface layer. Neither the open-circuit voltage nor the series resistance of the devices are sensitive to the differences in morphology. © 2015 The Royal Society of Chemistry.

El rendimiento de las células solares basadas en moléculas y polímeros semiconductores (fotovoltaica orgánica) ha experimentado un vertiginoso progreso desde valores inferiores al 1 % hasta sobrepasar el 10 % en poco más de una década. Esta rápida evolución ha sido motivada por la posibilidad de generar paneles solares flexibles, ligeros y de bajo coste mediante métodos de fabricación parecidos a la impresión de periódicos. En este artículo se revisarán los tres pilares fundamentales sobre los que se ha basado el desarrollo de la tecnología fotovoltaica orgánica y que suponen un cambio conceptual radical con respecto a las tecnologías convencionales. En particular, se describirá la estructura nanoscópica de la capa activa, las nuevas estrategias que persiguen aumentar la absorción de las células solares mediante dispositivos con diseños novedosos y las tendencias actuales en la síntesis de mejores materiales.

We combine inkjet printed silver grids and reverse nanoimprinting transfer to demonstrate front electrode structures embedded within the substrate for indium tin oxide (ITO) replacement in organic solar cells. The smooth interface between Ag grid and transparent semiconductor polymer PEDOT:PSS improves the topology of the successive layers. In turn, this results in overall good photovoltaic performance including high fill factor and no shunting. By modelling the shadowing and resistive losses as a function of the separation between grid lines, the optimal grid structure is identified, which we then confirm experimentally. Inkjet printed ITO-free solar cells showed slightly higher performance than the ITO based reference.

The one-step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3-hexylthiophene) (P3HT) as well as P3HT:fullerene bulk-heterojunction blends can be spin-coated from a mixture of the crystallizable solvent 1,3,5-trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square-centimeter-sized domains that are composed of one spherulite-like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite-like structures. Moreover, grazing-incidence wide-angle X-ray scattering reveals an increased relative degree of crystallinity and predominantly flat-on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip-coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi-crystalline conjugated polymer systems is established. Those include other poly(3-alkylthiophene)s, two polyfluorenes, the low band-gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends. Macroscopic spherulite-like structures of the conjugated polymer poly(3-hexylthiophene) (P3HT) grow directly during spin-coating. This is achieved by processing P3HT or P3HT:fullerene bulk heterojunction blends from a mixture of the crystallizable solvent 1,3,5-trichlorobenzene and a second carrier solvent such as chlorobenzene. Epitaxial growth

We have studied the evolution of dipole–dipole all-plasmonic Fano resonances (FRs) in symmetric multilayered nanoshells as a function of their geometrical parameters. We demonstrate that symmetry breaking is not mandatory for controlling the Fano resonance in such multilayer structures. By carefully selecting the geometrical parameters, the position of the FR can be tuned between 600 and 950 nm and its intensity can be increased up to four fold with respect to the non-optimized structures. Generation of FRs in such symmetric nanostructures presents clear advantages over their asymmetric counterparts, as they are easier to fabricate and can be used in a wider range of technological applications.

Geometric confinement and interface effects can significantly alter the thermodynamic properties of thin polymer films. Phase transition temperatures have been shown to strongly depend on film thickness below a critical thickness threshold. It has been suggested that this behavior is due to an interface-induced continuous variation in phase transition 200 temperatures throughout the depth of the films. Here we employ variable-temperature spectroscopic ellipsometry to demonstrate the existence of these depth profiles. We examine four different polymer semiconductors that are of interest for organic light-emitting diodes, solar cells, and field-effect transistors. In contrast to insulating polymers, these light-absorbing materials provide detailed information about structural changes as a function of depth due to wavelength-dependent attenuation. This concept enables us to investigate a broad range of thermodynamic processes including the glass transition, crystallization as well as crystalline and liquid-crystalline melting. In general, for the here investigated systems, higher transition temperatures are found at the free surface. Finally, the deduced profiles are used to predict the thickness dependence of the mean phase transition temperature.

We present a method to fabricate binary organic donor and acceptor blends exhibiting a controlled lateral gradient in morphology. Upon combining photometry, ellipsometry and Xray maps together with photoinduced absorption measurements, we show how the gradual exposure to solvent vapor results in a varying degree of polymer crystallinity for the polythiophene/soluble fullerene system along one direction. These morphologically graded samples are characterized by a spectral photoresponse that depends on the specific location in the area of the device where the light beam impinges, a property that stands as proof-of-concept for position sensitive detection. Moreover, we demonstrate that the development of graded morphologies is an effective one-step method which allows for fast performance optimization of organic solar cells. Finally, the appropriateness of eight different solvents for morphology control via vapor annealing is evaluated in a time-effective way using the advanced method, which helps to identify boiling point and solubility as the key processing parameters.

We demonstrate a solution-based method to create vertical nanoporous structures of semiconducting polymer poly(3-hexylthiophene) over large areas by taking advantage of the spontaneous phase segregation between poly(3-hexylthiophene) and polystyrene deposited from a single solution and, in a second step, removing polystyrene by selectively dissolving it. Nanoporous films with pore diameters which can be tuned down to 120 nm are produced by varying the composition. The potential of the method is further demonstrated by fabricating fully operational solar cells after deposition of [6,6]-phenyl-C61-butyric acid methyl ester from an adequate solvent. For optimized conditions, the devices based on nanostructured thin films exhibit enhanced efficiencies with respect to graded bilayers and bulk heterojunction organic photovoltaic devices. We relate the increase in fill factor observed in the nanostructured devices to changes in the orientation of poly(3-hexylthiophene) chains induced by nanoconfinement and self assembly with polystyrene resulting from this simple solution process without any use of elaborated chemistry or soft lithography., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/11443

El fotodetector comprende una capa activa continua en la que al incidir la luz genera una señal proporcional a la posición en la que incide dicha luz; se caracteriza porque la capa activa comprende un primer y un segundo semiconductor orgánico distribuidos según un gradiente longitudinal correspondiente a un gradiente de concentración relativa, un gradiente d estructura o ambos.La invención también se refiere al procedimiento de obtención del fotodetector y al procedimiento de medida de la respuesta del fotodetector.Permite obtener un sensor continuo de mayores dimensiones que los previstos en el estado de la técnica, aumentando su radio de acción sin alterar su sensibilidad.

[EN] The invention relates to a photodetector comprising a continuous active layer in which, in response to incident light, a signal is generated that is proportional to the incident position of the light. The invention is characterised in that the active layer comprises frrst (1) and second (2) organic semiconductors distributed along a longitudinal gradient corresponding to a relative concentration gradient, a structure gradient or both. The invention also relates to the method for obtaining the photodetector and to the method for measuring the response from the photodetector. The invention can be used to obtain a continuous sensor of larger dimensions than those known in the prior art, thereby increasing the operating range without altering the sensitivity thereof., [ES] El fotodetector comprende una capa activa continua en la que al incidir luz genera una señal proporcional a la posición en la que incide dicha luz; se caracteriza porque la capa activa comprende un primer (1) y un segundo semiconductor orgánico (2) distribuidos según un gradiente longitudinal correspondiente a un gradiente de concentración relativa, un gradiente de estructura o ambos. La invención también se refiere al procedimiento de obtención del fotodetector y al procedimiento de medida de la respuesta del fotodetector. Permite obtener un sensor continuo de mayores dimensiones que los previstos en el estado de la técnica, aumentando su radio de acción sin alterar su sensibilidad.

The thermal behaviour of an organic photovoltaic (OPV) binary system comprised of a liquidcrystalline fluorene-based polymer and a fullerene derivative is investigated. We employ variabletemperature ellipsometry complemented by photo- and electroluminescence spectroscopy as well as optical microscopy and scanning force nanoscopy to explore phase transitions of blend thin films. The high glass transition temperature correlates with the good thermal stability of solar cells based on these materials. Furthermore, we observe partial miscibility of the donor and acceptor together with the tendency of excess fullerene derivative to segregate into exceedingly large domains. Thus, for charge generation less adequate bulk-heterojunction nanostructures are poised to develop if this mixture is exposed to more elevated temperatures. Gratifyingly, the solubility of the fullerene derivative in the polymer phase is found to decrease if a higher molecular-weight polymer fraction is employed, which offers routes towards improving the photovoltaic performance of non-crystalline OPV blends.

In this study, it is demonstrated that a finer nanostructure produced under a rapid rate of solvent removal significantly improves charge separation in a high-performance polymer: fullerene bulk-heterojunction blend. During spin-coating, variations in solvent evaporation rate give rise to lateral phase separation gradients with the degree of coarseness decreasing away from the center of rotation. As a result, across spin-coated thin films the photocurrent at the first interference maximum varies as much as 25%, which is much larger than any optical effect. This is investigated by combining information on the surface morphology of the active layer imaged by atomic force microscopy, the 3D nanostructure imaged by electron tomography, film formation during the spin coating process imaged by optical interference and photocurrent generation distribution in devices imaged by a scanning light pulse technique. The observation that the nanostructure of organic photovoltaic blends can strongly vary across spin-coated thin films will aid the design of solvent mixtures suitable for high molecular-weight polymers and of coating techniques amenable to large area processing., Funding Agencies|Swedish Energy Agency||Spanish Ministerio de Ciencia e Innovacion

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron-sized structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by ～100 mV), which leads to an overall enhancement of power conversion efficiency., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/9197

In this contribution we show highly polarized photoluminescence (PL) from aligned polyethyleneoxide: polyphenylenevinylene derivative composite nanofibers. We demonstrate PL polarization ratios (parallel to perpendicular) greater than 13. This ratio is further increased (up to ～25) by stretching the nanofibers. Stretching also results in an increase in conjugation length, fiber density, and PL lifetime. We argue that the effect of stretching is equivalent to applying a permanent and strong pressure. Our results open up the possibility for new optoelectronic devices and fundamental science studies based on polymer nanofibers.