Synthesis of conjugated organic materials

The STELORG consortium is involved in the synthesis of i) organic conjugated small molecules for emissive, light harvesting, doping and charge transport applications and of ii) organic conjugated polymers for photovoltaics, thermoelectricity and sensor applications.

The properties of advanced organic materials for such optoelectronic applications are based on π-conjugation between (hetero)aromatic units. Consequently, the synthesis of the materials involves mainly the control and development of carbon-carbon bonding reactions (organo-metallic cross-coupling reactions, C-H activation…). But it also includes the synthesis of heteroaromatic building blocks with custom substituents providing either solubility, nanophase segregation, steric hindrance, stimuli responsivity… 

Contact: Nicolas LECLERC

Design and investigation of molecular architectures for thin film devices

Advanced materials for molecular electronic, optoelectronic and photonic applications associate several molecular segments with complementary functionalities in the same (macro)molecule. A typical molecular architecture involves one or several p-conjugated units conferring the semiconducting/photophysical properties, linkers that join and space these p-conjugated units, and peripheral units modifying the physical state and the properties of the material. Most often, linkers and peripheral units are constituted by aliphatic chains, but other nature of chains are also used: siloxane, oligo(ethylene oxide),… Such a molecular architecture combining "rigid" p-conjugated units and "soft" chains offers multiple advantages for the design of the functional materials:

Solubility and processability. Molecules consisting in the bare conjugated systems are often insoluble solids melting at high temperature, with limited possibilities of handling and immutable features. In contrast, the implementation of such conjugated systems in molecular architectures containing peripheral chains generates solubility in organic solvent and softening into soft matter states. Getting solubility is a major goal of functional material research, not least as it gives access to versatile, relatively low-cost and up-scalable film deposition techniques from solutions. The soft matter states can be used to modify the deposited films and improve the device performances through thermal treatment and mechanical alignment, in particular.

Structure. Rigid conjugated units and soft chains are intrinsically incompatible, so that identical segments from neighboring molecules self-assemble into domains by repelling the antagonistic segments into juxtaposed domains. The result of this nanosegregation process are structures in which the domains of self-assembled p-conjugated units form semiconducting pathways that are isolated from each other by the insulating chains. The charge transport along these pathways could be one-dimensional for thread-like p-conjugated domains surrounded by chains, two-dimensional for p-conjugated layers alternating with chains or three-dimensional for networks of p-conjugated domains intermingled with chains. The structure might be multisegregated if the molecular architecture involves different p-conjugated units that nanosegregate in individual domains. Within domains, the interactions of p-conjugated units, and in particular their spacing, overlapping and respective orientation are driven by the choice of the linkers and the peripheral chains. These structural features determine the efficiency of charge transport between neighboring p-conjugated units, and thus the semiconducting properties of the material. Photophysical properties are also affected: the close-stacking of p-conjugated units can for instance cause the quenching of luminescence properties.

Morphology. Thin films deposited from solutions most often exhibit a spontaneous orientation of the nanosegregated structure relatively to the film surface. This orientation is reproducible but it can be influenced by the conditions of film preparation and processing. Its impact on electrooptic device performances can be tremendous, especially for one- and two-dimensional semiconductors. Indeed, the charge transport is optimal when the semiconducting pathways join the electrodes of the device, i.e. when the pathways are vertical and the electrodes are disposed of both sides of film, or when the pathways are horizontal and the electrodes are contained in the thickness of the film. Conversely, improper orientation of pathways results in strongly degraded device performances, since the path towards electrodes is interrupted by insulating chains.

In summary, molecular architectures are characterized by their versatility and their multifaceted control over material performances. Due to the strong impact of structure and morphology on properties, their characterization is inescapable for the evaluation of new materials and device fabrication procedures. The research institutions of the STELORG consortium offer facilities and skills to study the structure and the self-assembly in the bulk state. For the investigation of thin film structure and morphology in device-close conditions, experimental projects grouping the needs of the consortium in a transversal topic are regularly submitted to a synchrotron facility and lead to one experimental session per year, in the average.

The prediction of the likely structure and morphology guides the molecular design and considerably fosters the development of new materials. Although the detailed self-assembly and the accurate structural parameters cannot be anticipated, valuable predictions can be made on the main characteristics of the self-assembly, as well as on the effect of individual molecular variations. The prediction methodology uses libraries of features of molecular segments and of self-assembly rules that were created and are continuously enriched from experimental studies on previous systems. Beyond the tailored development of molecular architectures, the collected data and methods are rationalized to deepen the knowledge on self-assembly processes and soft matter structures.

Contact: Benoit HEINRICH

Charge transport in organic semiconductors

The inherently weak intermolecular coupling, the relatively strong electron-phonon coupling and the related large structural and energetic disorder that characterise organic semiconductors make the charge transport in these materials differ considerably from that of standard crystalline semiconductors. In particular, the mobility of charge carriers in many organic semiconductors occurs via random hopping amongst a Gaussian distribution of energy states rather than by a coherent transport mechanism. In addition, the assembly of organic molecules in thin films often leads to hopping rates that are highly anisotropic.  

These characteristics cause the mobility of charge carriers in most organic semiconductors to be highly dependent on charge carrier densities, electric field strength and transport direction. Hence, the experimental values of a charge carrier mobility will largely depend on the method used to determine it. To achieve a comprehensive description of charge transport in a given material, it is therefore necessary to use several complementary methods that explore charge transport in a wide range of parameter space. 

Our charge transport investigations generally include measurements done using organic field-effect transistors, space-charge-limited-current (SCLC) devices and time-of-flight methods. The field-effect method probes charge transport along a direction parallel to the substrate, under high charge carrier densities and electric field intensities that can be varied over a few orders of magnitude. On the other hand, the "SCLC mobility" values are representative for charge transport perpendicular to the substrate and are measured under lower electric fields and charge carrier densities. The latter can be tuned to some extent by varying the film thickness. Time-of-flight measurements are complementary to the previous methods as they explore charge transport under very low charge carrier density and along a direction perpendicular to the film substrate. 

Charge transport can also be studied by microwave conductivity methods, which allow access to the mobility at a more "local" scale. They have the advantage of not requiring the fabrication of working organic devices and to avoid trapping of charge carriers at defects. The microwave studies on charge transport in materials developed at STELORG are carried out in the framework of a collaboration with Kyoto University.

Recent work on charge transport in organic semiconductors done by STELORG can be found in the following references  J. Jiang et al. J. Mat. Chem. A 2021N. Kamatham et al Adv. Func. Mat. 2020A. Ruiz-Carretero et al. J. Mat. Chem. A 2019S.Mula et al. Chemistry-A European Journal 2019;

Contact: Thomas Heiser

Organic opto-electronic devices

Device manufacturing

Organic devices are mostly thin-film devices, i.e. their manufacturing requires the controlled deposition and structuring of a sequence of organic and inorganic (mainly metals) layers.

STELORG has expertise in depositing thin organic layers by spin-coating or blade-coating from various solutions. The deposition is mostly done under clean nitrogen atmosphere (glove-box) at a controlled temperature. If the environmental impact is to be taken into account, we rely on a reverse-engineering approach for the solvent selection developped in collaboration with the University of Toulouse​​​​. For the deposition of inorganic layers we are using physical vapor deposition. The layer structuring is done either by chemical (etching) or mechanical (shadow masks) means, as well as by more advanced photolithography tools.

Prior to device manufacturing, thin films are typically studied by optical, electro-chemical and mechanical means (e.g.: profilometer or atomic force microscopy) to assess film thickness, homogeneity, roughness, opacity, work-function and ionisation potential, each of which is critical to device performance.

Device characterization

Device characterisation can be carried out for two different purposes: (i) to assess device performance, or (ii) to extract material properties (e.g. mobility, conductivity, etc.).

Stationary electrical characterisation typically involves recording the DC current-voltage curves of devices under various conditions (temperature, light exposure) and comparing the results with appropriate device models. This allows us to determine power conversion efficiencies and quantum efficiencies of solar cells, electro-optical response of dynamic PSLM glazing, and spectral dependences of the device's response.

Time- or frequency-resolved measurements are also made to probe dynamic properties of devices or materials, such as response time, leakage currents, charge carrier lifetimes, ....

Recent work on organic opto-electronic devices done by STELORG can be found in the following references: S. Fall et al. ACS Applied Materials & Interfaces 2023A. Labiod et al. Organic Electronics 2022J. Jing et al. Molecular Systems Design & Engineering 2021; X. Ma et al. J. Mat. Chem. A 2020; O. A. Ibraikulov et al. Solar RRL 2019; O. A. Ibraikulov et al. J. Mat. Chem.A 2018;

Contact: Thomas Heiser