Solution structure: defining polymer film morphology and optoelectronic device performance

Film structure plays a critical role in defining the performance of all organic optoelectronic devices, with the importance clearly illustrated in the development of organic acceptor-donor bulk heterojunction (BHJ) photovoltaic (OPV) devices where solvent and/or thermal annealing of the deposited active layer affect solar cell output. Herein we report that the polymer-polymer interactions in solution, which are dependent on the thermal history of the solution, are a first order parameter in controlling the properties of the final active layer and hence device performance. We illustrate the key role played by organic semiconductor interactions in solution with the high efficiency donor-acceptor co-polymer, poly[N-9 ''-heptadecanyl- 2,7-carbazole-alt-5,5-(4 ',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), and its blends with [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM). Differences in the cooling rate of the casting solution after dissolution can lead to a 20% variation in the ultimate efficiency of cells with identical active layer thicknesses with slow-cooled solutions giving rise to poorer devices. The oft-ignored intermolecular (polymer-polymer) interactions that occur in solution are manifest by dramatic differences in viscosity and are a function of concentration and molecular weight. Hence solution thermal history represents a critical new dimension in the processing landscape for organic polymer semiconductors

Quantum Efficiency of Organic Solar Cells: Electro-Optical Cavity Considerations

Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such, they are low finesse, multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the "solar cell stack". Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE), the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication, we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness, blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However, when correctly determined, the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predominantly originate from optical interference and parasitic absorptions rather than charge generation or transport phenomena. We also demonstrate similar results for a second model system (PCPDTBT/PC60BM) in which an energy-dependent "IQE-like" response has recently been used to justify the existence of hot excitons. Once again, we show the origin of these spectral phenomena to be optical, not electronic. These cases highlight the fact that thin film organic solar cells (even single junction) must be properly considered as low finesse electro-optical cavities, a point that is not universally appreciated

Photo-oriented trisazobenzene layers for patterned liquid-crystal alignment

Photosensitive trisazobenzene-based layers, after irradiation with polarized light, are employed to align the common liquid-crystal (LC) compound 4-cyano-4-heptylbiphenyl (7CB) into predetermined patterns. Surface analysis of the trisazobenzene films indicate that ordering of the LC material is not caused by illumination-induced surface relief structures, but through molecular mesoepitaxial phenomena. This finding can be exploited to produce in one-step in-plane patterns of randomly oriented and ordered macroscopic LC domains by irradiation of the substrates

Identifying the optimum composition in organic solar cells comprising non-fullerene electron acceptors

We explore the inter-relationship between the phase behavior and photovoltaic performance for two blend systems comprising poly(3-n-hexylthiophene-2,5-diyl) (P3HT) as the electron donating moiety and two newly developed small molecule non-fullerene electron acceptors. Binary non-equilibrium temperature/composition phase diagrams of the two systems are prepared from differential scanning calorimetry (DSC) thermograms of blends of different compositions. The phase behavior is correlated with the optoelectronic performance of corresponding binaries in bulk heterojunction (BHJ) solar cells. The thermal and optoelectronic blend characterization is supported with optical microscopy and specular X-ray diffraction (sXRD) experiments. For both electron-accepting compounds the composition yielding the maximum photocurrent generation in devices was found to be in the hypoeutectic regime, i.e. at compositions that are shifted from the eutectic towards the small molecule rich region in the phase diagrams. We demonstrate that measuring the thermal properties of the blends is useful for rapid component ratio optimization and the evaluation of unexplored materials combinations

Influence of molecular architecture and processing on properties of semiconducting arylacetylene: Insulating poly(vinylidene fluoride) blends

Blends of chemically readily accessible, small-molecular arylacetylene derivatives with poly(vinylidene fluoride) (PVDF) are presented that allow reliable solution processing of field-effect transistor (FET) architectures with electronic characteristics comparable to those of the neat semiconductors. We demonstrate that having the chemical means and corresponding processing protocols to control solid-state microstructures by either adjusting the chemical nature of the organic semiconductor, blend composition or deposition temperature, permit straight-forward comparison between materials and allow probing if electronic characteristics are affected by the chemical structure of the organic semiconductor and/or selected processing protocols. (C) 2011 Elsevier B.V. All rights reserved

Spostati e piantati: volti del "miracolo economico"

The narrow optical gap conjugated polymer poly­[<i>N</i>-9″-hepta­decanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo­thiadiazole)] (PCDTBT) has been used as the electron donor material in efficient solution-processed bulk-heterojunction (BHJ) photovoltaic and photodetector devices when blended with fullerene derivatives. It was found that the solution viscosity used to form the active films could be controlled by the cooling rate of a hot solution of the materials and that fast-cooled solutions afforded more efficient bulk-heterojunction solar cells than their slow-cooled counterparts. Viscometry measurements showed that the rheological behavior of the solutions is modeled by the Martin equation for different PCDTBT molecular weights and temperatures. The Martin constant <i>K</i>_M that describes the interpolymer interactions in solution was found to increase with polymer molecular weight and decrease with increasing temperature in an analogous manner to the Flory–Huggins interaction parameter χ. Small-angle neutron scattering (SANS) was used to show that when hot solutions of the polymer were cooled, phase separation into polymer-rich clusters and solvent-rich domains occurred. Similar phase separation was observed in the case of blend solutions. In addition, the fast-cooled solutions trapped more 70-PCBM in the polymer-rich phase, which in turn made the structure of the polymer more rodlike in the clusters. The results provide an explanation as to why fast-cooled solutions lead to devices with greater efficiency

A flexible n-type organic semiconductor for optoelectronics

n-Type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes, and field effect transistors (FETs). In spite of this clear motivation there has been significantly less development of n-type compounds relative to p-type systems. We have developed a simple, small molecule n-type material, 2-[(7-{9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12), that can be processed either by spin-coating from solution or evaporation in vacuum. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation). The electron mobility measured in a FET configuration was of the order of 10(-3) cm(2) V-1 s(-1) for films prepared via either processing method whilst Photoinduced Charge Extraction in Linearly Increasing Voltage (PhotoCELIV) gave a mobility of order 10(-4) cm(2) V-1 s(-1)

Semiconducting Arylacetylene:Insulating Polymer Blends for Organic-Based Electronic Devices

In this contribution we have reported about bi-component blends of readily accessible semiconducting molecular arylacetylenes with insulating high-density polyethylene (HDPE) and poly(vinylidene fluoride) (PVDF) that may exhibit electronic characteristics comparable to those of the neat semiconductors, as measured in field-effect transistors (FETs)