Magnetic Resonance Tools for Solar Materials

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Spin processes are highly important for the efficiency of organic solar cells and light-emitting diodes, yet their role remains largely unexplored. Due to the weak spin-orbit coupling, organic materials possess strict spin-selection rules and the properties of photoexcited species can therefore vary drastically, depending on the particular spin configuration. In addition, over the last years there has been tremendous progress on the development of hybrid organic-inorganic perovskite based materials, which now enables the fabrication of optoelectronic devices with remarkable performance. It has been suggested that the spin degree of freedom is relevant for photo-physical processes in these materials as well. Thus, spin properties of excited states are actively investigated in very different contexts. In this project, which is a collaborative effort involving scientists from Paris (Université Paris-Sud), Versailles (Université Versailles Saint-Quentin), Grenoble (Institut Néel), Bayreuth (Universität Bayreuth [UBAT]) and Berlin (Freie Universität Berlin [FU Berlin]), we will focus on two aspects: singlet exciton fission where a singlet exciton splits into two triplet excitons of lower energy and spin-dependent processes in few layer hybrid-organic perovskites down to the monolayer limit. We will explore the use of broadband and time-resolved optically detected magnetic resonance (ODMR) spectroscopy as a powerful method to establish the microscopic nature of bi-exciton states with total spin S = 2 (quintets) formed through singlet fission. This will allow us to characterise the microscopic positions of bound triplets in bi-exciton states, the strength of their interaction, as well as their fluorescence spectrum and kinetic properties. The limits of the ODMR experiment will be pushed to single geminate triplet-pair detection in order to observe effects obscured in ensemble measurements. The detailed physical picture emerging from these experiments will serve as the basis for a quantitative molecular-level characterisation of the electronic structure parameters of bi-exciton states, which will be carried out within the framework of the project. The materials relevant for solar cells and up-conversion have a complex morphology which cannot be probed in macroscopic experiments on single crystals. We are thus planning to develop a microfluorescence-based ODMR experiment. This development will also allow probing spin-properties of Methylammonium lead halide, a promising solar cell material, in the almost unexplored limit of chemical vapour deposition grown monolayers and few layer single crystal flakes of micrometer sizes. All in all, the MARS project will develop original spin-sensitive methods to probe the properties of photo-excited states that appear in exciton fission systems and novel materials with broad impact on fundamental optoelectronics and its applications.