Understanding the role of interface properties still remains a crucial aspect in the search for alternative design strategies to optimize the efficiency and performance of organic semiconductor devices. Indeed, with this aim, the modulation and the control of electronic properties of surface and interface are of fundamental importance. The use of self-assembled monolayers (SAM) [1] constitutes a highly efficient strategy for achieving that [2,3]. Indeed, many studies have shown the possibility of tuning the energy level of the interface, and thus controlling the charge transfer process, by an appropriate selection of SAM incorporating dipolar molecules. However, these electronics properties of interface are strongly affected by the SAM morphology such as molecular orientation, nature of the molecule-surface interactions, molecular packing in the film and so on. All these SAM parameters directly depend strongly on the deposition conditions and the molecular structure. Therefore, mastering the formation of SAMs of molecules anchored perpendicularly to the surface, thus allowing to orient the dipoles of dipolar compounds, represents a key challenge to optimize the interface properties. Therefore, we propose in this subject to design a new interface based on self-assembled monolayers of multichromophore molecules [4] and to optimize their organization in the active layer to achieve the best electronic properties.

During the last years, chemists have paid deep attention to “push-pull” organic compounds bearing electron donor (D) and acceptor (A) groups linked by π–conjugated bridges [5] owing to their dipole induced by a charge transfer from D to A, and their appealing non−linear optical response. The non−linear optical properties of such D–π–A compounds can be finely tuned by selecting appropriate D & A units, and π–bridges at suitable positions. However, very few studies have focused on their adsorption on the surface [6], perhaps due to their uncontrolled orientation. 

In this subject, the peculiar molecular structure of the chromophore compounds that will be grafted onto the surface is highly innovative. Indeed, novel bichromophore molecules composed by two complementary push-pull chromophores borne by the same grafting head will be synthetized, and then deposited by self-assembly onto the surface thus ensuring ordering and equipartition of each chromophore, which has not been studied yet. Such an approach could then be extended to multichromophores. This work will therefore be focused on the uncovering of relationships between structure and electronic properties of these new SAMs. This unique system will lead to novel electrical interfaces with exclusive properties rendering them interesting for improving light harvesting, charge separation, and facilitating charge injections. Success of this work will have tremendous impact in the development of miniaturized organic electronic devices.

Self-assembled monolayers will be obtained by transfer of the molecules from a solution to the surface. First, the deposition parameters (solvent nature, concentration, temperature, duration,…) will be optimized.  The quality of the layers produced will then be evaluated by various surface analysis techniques available in the laboratory or at partner’s (ellipsometry, contact angles, quartz crystal microbalance, UV-visible, infrared, XPS, UPS, IPES,… spectroscopies), and in particular at the local level by atomic force and electric force microscopy (AFM, EFM) and scanning tunneling microscopy (STM). Then, the electric properties will be studied via capacitance-voltage (C-V) and current-voltage (I-V) analyzes locally by STM/EFM microscopy, and more generally by means of InGa droplet at the eutectic.

The following goals should be reached :

• Synthesis of a new bi- or multichromophoric molecule series with various donor and acceptor groups allowing to modulate the dipole moment (achieved by CINaM in Marseille).
• Ab initio calculations to identify the level position of HOMO (highest occupied molecular orbital) & LUMO (lowest unoccupied molecular orbital) of multichromophores (collab. with LPS at Orsay)
• Optimization of the deposition parameters such as temperature, solvent, concentration, to make a dense, compact, and well controlled SAM with oriented dipoles.
• Highlighting the electrical properties versus the structure of the interface. Particularly, we will focus on novel SAMs as ultra-thin weakly resistive interfacial electron & hole transport layers (ETL & HTL) based on new π-conjugated bichromophoric compounds tethered on a unique anchoring unit for organic solar cells. These bichromophoric structures will have added values compared to the simple ones in terms of dipole, charge transport, photoinduced charge transfer, charge recombination hindrance. Their implementation in model organic solar cells (regular-based devices) will be done in order to demonstrate their efficiencies. In addition, studies on the stability and degradation mechanisms of the novel SAMs under operation and damp/heat/light conditions will be performed in order to identify and get deeper insights about structure-property relationships.

Funding: Doctoral contract from the ministry (MESRI)

Keywords: Self-assembled molecular monolayers, push-pull bichromophores, dipole

References:

1. A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press Ed., Boston (1991); R.K. Vijayaraghavan et al., J. Phys. Chem. C 117 (2013) 16820

2. J.J. Cartus and al, ACS Omega 6 (2021) 32270-32276

3. A. Asyuda et al., J. Phys. Chem. C 124 (2020), 8775-8785 

4. A. Aster et al, Chem. Sci. 10 (2019) 10629; J.L. Weber et al. Chem. Sci. (2021) doi : 10.1039/d0sc03381b  

5. V.Malytskyi, J.J.Simon, L.Patrone, J.M.Raimundo, RSC Adv. 5 (2015) 354

6. V.Malytskyi, J.J.Simon, L.Patrone, J.M.Raimundo,  RSC Adv. 5 (2015) 26308, and Tetrahedron 73 (2017) 57381

Funding category: Contrat doctoral



PHD title: PhD in Physics - Condensed Matter & Nanosciences

PHD Country: France