Prof. Jan Behrends "Spins in Solar Cells: Charge Separation from an EPR Perspective"

Photocurrent generation in solar cells based on organic semiconductors involves localised states which, if occupied by single electrons or holes, are paramagnetic and can be detected by electron paramagnetic resonance (EPR) spectroscopy. Spin-sensitive techniques are capable of probing separated polarons, weakly coupled charge transfer states as well as triplet excitons. Further, EPR spectroscopy allows studying the conversion between these different types of excited states because they exhibit clearly distinguishable signals.
The first part of this presentation will give an overview about how EPR-based techniques in combination with optical excitation can usefully complement all-optical experiments in studying charge separation in materials for organic solar cells. As one example, the generation of triplet excitons via a back electron transfer process in solar cell absorber layers based on low-bandgap polymers will be discussed. The second part of the talk will address the role of electron spins in charge transport and recombination processes. In particular, I will report on the development of a setup that allows for a simultaneous detection of transient EPR as well as transient electrically detected magnetic resonance signals from fully processed solar cells. Moreover, I will discuss the EPR properties of intermediate paramagnetic states generated upon fission of one singlet exciton into two separated triplet excitons. This singlet fission process provides the possibility of building solar cells with quantum efficiencies exceeding 100%.

invited by Till Biskup