Magnetic resonance tools for solar materials – MARS

Spin processes are highly important for the efficiency of organic solar cells and light emitting diodes,
yet their role remains largely unexplored. Indeed, 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 enable 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 we will focus on two aspects: singlet exciton fission where a singlet exciton
splits into two triplet excitons of lower energy and on spin-dependent processes in few layer hybrid-
organic perovskites down to the monolayer limit where highly controlled samples can be prepared. In
this project we will explore the use of broadband 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. Recent experiments in LPS show that this
approach allows to characterise unambiguously the molecular sites occupied by bound triplets exciton
pairs. These experiments will be complemented by dielectric spectroscopy (at LPS) and pulsed
magnetic resonance experiments by the Berlin partner. The combination of these techniques will
characterise the microscopic positions of bound triplets in bi-exciton states, the strength of their
interaction, characterised by their exchange energy, as well as their fluorescence spectrum and
kinetic properties. NEEL will push the limits of the ODMR experiment to single geminate triplet-pair
detection in order to observe effects obscured in ensemble measurements. The inherently high optical
resolution of this technique will allow to measure the fine and ultimately the hyperfine structure
parameters of the excited triplet-pair. This will provie precise information on the local molecular
arrangement of the bi-exciton wavefunction. 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 developed in Bayreuth. The Bayreuth team will
also perform spectroscopic experiments in order to probe the role of bi-excitons in triplet-triplet
anhilation processes and optical up-conversion that are important for applications. The materials
relevant for solar cell and up-conversion will probably have a complex morphology which cannot be
probed in macroscopic experiments on single crystals. LPS and GEMaC will thus develop a
microfluorscence based ODMR experiment. This development will also allow to probe spin-properties
of Methylammonium lead halide (MAPI), 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. These samples, that have already been prepared at GEMaC/LPS, allow the creation of completely
new structures based on Van-der-Waals heterojunctions and their properties are more easily tunable
with gate voltages compared to bulk systems. Due to these advantages the exploration of spin
dependent optical properties in MAPI-nanosheets is a very promising research direction on which the
GEMaC team will concentrate. Therewithal, the MARS project will develop original spin sensitive
methods to probe the properties of new photo-excited states that appear in exciton fission systems
and novel materials like MAPI nanosheets with broad impact for fundamental optoelectronics and its
applications.