Tailoring amphiphilic organic molecules for self assembled photovoltaic materials

• Applicants: Dr. Stefan Kraner; Professor Dr. Ullrich Scherf
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 419282603

Organic photovoltaics represent a cost-effective, renewable and therefore future-oriented energy supply. However, the power conversion efficiencies of this technology are still too low for the large-scale use of organic photovoltaics. The reason for the low efficiency can be attributed to the high voltage losses caused by the required donor-acceptor system and the low charge carrier mobility in the photoactive material.In this project, a concept is being tested which provides an ideal morphology for organic solar cells. The core of the concept consists in the self-assembly of amphiphilic organic donor acceptor molecules into a phase-separated bilayer, leading to donor and acceptor phases with a crystalline structure which improves the charge carrier mobility by order of magnitudes. In addition, the donor-acceptor interface is defined intramolecularly via a spacer molecule, thus enabling the donor-acceptor interface to be optimized at the molecular level. This ideal morphology presumably leads to power conversion efficiencies that significantly exceed the currently assumed maximum of 15%.The project investigates and tests the feasibility of the self-assembly process by amphiphilic donor-acceptor molecules into bilayers. First, the components of the amphiphilic donor-acceptor molecule, which consists of three conjugated submolecules (amphiphilic triad), are investigated. Each molecule is evaluated by molecular dynamics simulations, whereas suitable molecules are synthesized and then processed and characterized. Molecular dynamics simulations are used to optimize the amphiphilic molecules to form bilayers, reducing significantly the effort involved in the chemical synthesis. The morphology of the synthesized molecules is investigated in solutions or in thin films using small-angle X-ray scattering (SAXS), grazing incidence small-angle X-ray scattering (GISAXS), grazing incidence wide-angle X-ray scattering (GIWAXS), scanning electron microscopy and atomic force microscopy (AFM). These structural informations, together with the molecular dynamic simulations, enable a successive and efficient improvement of the amphiphilic donor-acceptor molecule. The transport properties of electrons and holes in thin films are investigated using the organic field effect transistor (OFET) measurement method, whereby optimized organic materials are ultimately applied and characterized in solar cells. The aim of the project is to evaluate the potential of the introduced molecular concept for highly efficient and cost-effective solar cells.

• DFG Programme: Research Grants