Manipulating spin and electrostatic effects in organic semiconductors

Spin is expected to be a good quantum number in organic semiconductors where spin mixing effects associated e.g. to spin-orbit coupling are weak. In that respect, the formation of triplet excitations by electron-hole recombination in electroluminescent or photovoltaic devices ususally leads to light, current or voltage losses. In the first part of the talk, we will present a very recent combined experimental-theoretical work demonstrating that recombination of charge-transfer triplet excitations into localized triplets, acting as a sink for charge photogeneration, can be avoided by properly designing donor and acceptor molecules in order to favor hybridization or configurational mixing in the excited state. We will further demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles through spin-exchange and charge-transfer interactions that turn the dark triplets into bright states.
In the second part of the talk, we will review our recent work on the role played by electrostatics on the energetics of charge carriers in organics. We will namely report on the suprizingly high charge separation efficiency measured in a single component α-sexithiophene (α-6T) solar cell, which is driven by an electrostatically-driven energy mismatch at the interface between lying and standing α-6T molecules.
This work reshapes the common understanding in the role of ‘donor’ and ‘acceptor’ in organic semiconductors where this character is not only fixed by the primary chemical structure of the molecules but can also be modulated by interfacial packing and electrostatics.