Photo-Induced ChArge Separation in Self-assembled donor-acceptor block co-Oligomers for photovoltaic applications – PICASSO

The PICASSO project aims at optimizing the active organic layer via the control of its morphology and the control of the photogeneration of free charges.
The approach used to control the active layer morphology does not consist anymore in using D and A as mixture of compounds, but in assembling them together into a unique compound (a block-copolymer). The designed molecular structure is such that a spontaneous organization of the D and A blocks is expected, leading to an ideal morphology of D/A alternated lamellae.
The approach used to improve the photogenerated charges ratio and their lifetime consist in finely tuning the chemical composition and the sequence units of the molecule.

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The PICASSO project explores the synthesis and characterization of novel organic pi-conjugated block co-oligomers and their utilization in prototype photovoltaic devices. These new materials are designed i) to self-assemble into a nanostructure composed of alternating electron-donor (D) and electron-acceptor (A) lamellae and ii) to promote dissociation of charge transfer states into free charges. More specifically, by building the donor block from a sequence of chemical moieties with gradually changing electron affinity (so-called “electronic density gradient”), we expect to obtain reduced Coulomb binding of the inter-block charge transfer states and thus negligible charge recombination. In this respect, different partly electron-deficient donor head groups and acceptor molecules will be screened for best performances, The lamellar nanostructure will in turn lead to an efficient intra-lamellar charge transport and thereby facilitate charge collection within a photovoltaic device.

Advanced synthesis methods, molecular modelling, femtosecond time-resolved absorption spectroscopy, transmission electron microscopy and photovoltaic device elaboration and characterization are the core of the present project. The expected project outcomes are:
i). An enhanced control of the charge photogeneration yield at a molecular D/A interface,
ii). Ability to engineer, via molecular design, the charge-transfer-state geminate recombination rate,
iii). D/A heterojunctions with a well-defined lamellar morphology,
iv). Rationalization of the ambipolar charge transport properties,
v). Elaboration of photovoltaic devices based on a self-assembled, mono-constituent, active layer.

The project is predominantly oriented towards fundamental material investigations but includes analysis of application-oriented properties. It is therefore highly multidisciplinary and based on the complementary expertises brought in by four academic partners, all located on the same CNRS campus:
1. the departments of organic materials (DOM) and of ultrafast optics and nanophotonics (DON) of the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS, UMR 7504);
2. the Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies (LIPHT, EA 4379);
3. the Institut Charles Sadron (ICS, UPR 22) and
4. the Institute d’Electronique du Solide et des Systèmes (InESS, UMR 7163).