Exploring the energy landscape of the charge transport levels in organic semiconductors at the molecular scale.

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

Origin of the low-energy emission band in epitaxially grown para-sexiphenyl nanocrystallites.

A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl (PSP) films grown by organic molecular beam epitaxy (OMBE) and hot wall epitaxy (HWE) under comparable conditions is presented. Using different template substrates [mica(001) and KCl(001) surfaces] as well as different OMBE growth conditions has enabled us to vary greatly the morphology of the PSP crystallites while keeping their chemical structure virtually untouched. We prove that the broad redshifted emission band has a structure-related origin rather than being due to monomolecular oxidative defects. We conclude that the growth conditions and type of template substrate impacts substantially on the film morphology (measured by atomic force microscopy) and emission properties of the PSP films. The relative intensity of the defect emission band observed in the delayed spectra was found to correlate with the structural quality of PSP crystallites. In particular, the defect emission has been found to be drastically suppressed when (i) a KCl template substrate was used instead of mica in HWE-grown films, and (ii) in the OMBE-grown films dominated by growth mounds composed of upright standing molecules as opposed to the films consisting of crystallites formed by molecules lying parallel to the substrate.

Origin of the apparent delocalization of the conduction band in a high-mobility amorphous semiconductor.

In this paper, we show that the apparent delocalization of the conduction band reported from first-principles simulations for the high-mobility amorphous oxide semiconductor [Formula: see text] (a-IGZO) is an artifact induced by the periodic conditions imposed to the model. Given a sufficiently large unit-cell dimension (over 40 Å), the conduction band becomes localized. Such a model size is up to four times the size of commonly used models for the study of a-IGZO. This finding challenges the analyses done so far on the nature of the defects and on the interpretation of numerous electrical measurements. In particular, we re-interpret the meaning of the computed effective mass reported so far in literature. Our finding also applies to materials such as SiZnSnO, ZnSnO, InZnSnO, In2O3 or InAlZnO4 whose models have been reported to display a fully delocalized conduction band in the amorphous phase.

[To better integrate prevention into general medicine in deprived milieus]. / Mieux intégrer la prévention en médecine générale dans les milieux défavorisés.

Generalists occupy a strategic position in the area of prevention within the general population, and among disadvantaged areas in particular. Yet, practice shows that this position is not a given, as the socio-cultural gap separating doctor and patient renders communication particularly difficult in this context. In order to analyse this situation, a review of the literature, surveys among generalists, and a European seminar were carried out. This article deals with the definition of "disadvantaged environments" by the practitioner, practices in prevention in disadvantaged environments and their related obstacles, and tracks for improving prevention in disadvantaged environments by visits with a generalist.

Nanoimprinted semiconducting polymer films with 50 nm features and their application to organic heterojunction solar cells.

Nanoimprint lithography is used to directly pattern the conjugated polymer semiconductor poly(3-hexylthiophene) (P3HT). We obtain trenches with aspect ratios up to 2 and feature sizes as small as 50 nm in this polymer. The application to organic solar cells is shown by creating an interpenetrated donor-acceptor interface, based on P3HT and N,N'-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(13)), deposited from the vapor phase to reduce shadow effects. A planarizing layer of spin-coated zinc oxide (ZnO) nanoparticles is used to reduce the roughness of the layer stack. The response of the photovoltaic devices follows the increased interface area, up to a 2.5-fold enhancement.

Maximum detection with a two-dimensional optoelectronic p-n-p-n winner-take-all network.

An optoelectronic p-n-p-n winner-take-all network is presented that can identify the location of the maximum intensity in two-dimensional spatially distributed light. An input pattern of illumination is incident upon an array of p-n-p-n devices that are connected in parallel to a common load resistor and therefore have identical voltage biases. If the light intensity at one or several elements exceeds a threshold value, only one p-n-p-n element stays on and emits light when the input light pattern is turned off. The element that is turned on is the one that receives the maximum illumination.

Integration of refractive micro-optical elements with differential-pair optical-thyristor arrays.

We demonstrate a refractive micr-optical system by using ion-exchange microlenses and microprisms, which are combined to generate a superposition of two shifted images. The microlenses, fabricated with field-assisted Ag-Na exchange, achieve diffraction-limited imaging with a single-lens system and with a double-lens system for a field of 800 µm × 800 µm. Furthermore, we demonstrate cascading of two separate differential-pair optical-thyristor arrays by transcribing the information of a source array onto a second destination array.

Compact optical imaging system for arrays of optical thyristors.

A compact and modular optical system that employs gradient-refractive-index rod lenses to image arrays of Lambertian sources is characterized both experimentally and by ray-tracing simulations. A hybrid optical system that incorporates additional microlens arrays to reduce transmittance losses and aberrations is also modeled, and the two systems are compared.

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers.

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.