Nanoparticles-based eco-Friendly route for ORganic Conjugated polymer thin film Elaboration – FORCE

This project aims at producing semiconducting electron donor polymers and small molecule electron acceptor (SMA) nanoparticle (NP) dispersions as starting materials for the preparation of thin films active layers for optoelectronic devices. The use of NP dispersions has got several advantages over the conventional polymer solution approach as it should allow for a better control of the morphology of the films at the molecular scale (polymer or small molecule aggregation/crystallisation) as well as at the mesoscopic scale (materials domains organization and size in the case of blends of donor and acceptor materials). Indeed, the morphology of the active layer is a key factor to improve the efficiency of optoelectronic devices.
Moreover, one of the recurrent difficulties in the processing of semiconducting polymers is their poor solubility. In our approach, the semiconducting polymer will be dispersed in an environmentally friendly medium such as water.
For this project, we will focus on state-of-the-art fluorinated polymers that have been synthesized in our laboratory and that achieve very high OPV efficiencies in combination with fullerene derivative. These polymers are highly crystalline but their processibility in solution is very low and they have to be spin-casted at high temperature in o-dichlorobenzene. The enginnering of side-chains will be consider in order to enhance their solubility allowing an efficient elaboration of NP by the miniemulsion or the reprecipitation technique. Original non-fullerene small molecules electron acceptor (SMA) developed at ICPEES with very high molar extinction coefficients in the UV-visible range and showing high crystallinity responsible of too strong phase separation will also be used. In this case, their confinement into the NP should be beneficial in order to control the size of the phase separation domains.
Single organic semiconducting (OSC) materials nanoparticles and two-OSC materials composite nanoparticles will be prepared by the miniemulsion or the reprecipitation techniques. Their size, in the range of 20-40 nm, will be measured by Dynamic Light Scattering (DLS) and their morphology will be characterized by Transmission Electronic Microscopy (TEM), UV-visible spectroscopy or fluorescence. These characterization techniques give a good insight at the scale of the molecular level (presence of vibronic band in the UV-visible spectrum) and at the vicinity of the interface between the two polymers (fluorescence quenching).
In order to understand the aggregation/crystallisation and blend separation mechanisms in NP which occur during the miniemulsion and reprecipitation methods, a more fundamental study will be carried out as a function of process and materials parameters.
The NP will then be organized in thin films by deposition methods such as slow speed spin-casting, leading ideally to a bicontinuous blend morphology with narrow domains sizes (of the order of the nanoparticle size). Moderate thermal annealing will then help reducing the roughness of the film and increase contact between nanoparticles. Characterization of the film will be performed by Atomic Force Microscopy (AFM), TEM, UV-visible spectroscopy and fluorescence.
Finally, optoelectronic devices will be fabricated with the films as an active layer. The charge-carrier mobility in the films will be probed by the elaboration of an Organic Field Effect Transistor (OFET) and space charge limited current (SCLC) diodes in order to confirm the interconnectivity of the domains in the film. Finally, these polymer blend-based thin films will be used as active layer in organic photovoltaic devices.