Electrically detected magnetic resonance on fullerene-based organic semiconductor devices and microcrystals
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Abstract
This thesis deals with electrically detected magnetic resonance (EDMR) – a technique that probes
spin-dependent electronic transport processes by a combination of highly sensitive current detection
and electron paramagnetic resonance (EPR) – and its application to organic semiconductor
microcrystals and solar cell devices. The ability to directly observe and identify spin-dependent
processes makes EDMR especially useful for the device analysis in the field of organic solar cells,
where charge carrier recombination is one of the limiting factors and particularly hard to quantify.
The design and implementation of a compact setup for measuring EDMR, which is based on
a commercially available benchtop EPR spectrometer, is presented first. Using a silicon-based
reference sample, the setup’s performance is compared to a state-of-the-art instrument. It is
concluded that the EDMR spectra recorded with the new spectrometer are quantitatively comparable
in all spectroscopic dimensions with those obtained in the “large-scale” setup. The system
is capable of nearly simultaneous collection of EDMR and current-voltage data, opening up the
way to rapid research cycles and measurement series.
The benefits of close proximity between device fabrication and spectroscopic characterization
offered by the new setup are then used to record EDMR spectra of microcrystals made from
pure, oxidized, and N@C60-doped Buckminsterfullerene. These environment-sensitive objects
have not been studied in EDMR before. An evidence for strongly dipolar-coupled spin pairs
in the N@C60-doped material is presented. Although an unambiguous quantitative modeling of
the presented data is not possible due to the limited signal-to-noise ratio, a symmetric structure
in the obtained spectra is clearly visible. The performed experiments give a first spectroscopic
evidence for strongly dipolar-coupled spin pairs, never reported before in EDMR. This result illustrates
that using microcrystals and paramagnetic doping can significantly enhance the EDMR
resolution.
Making use of the presented benchtop EDMR system, the recombination in freshly prepared
P3HT:PC61BM solar cells is studied and the observed EDMR signals are assigned to P3HTrelated
species and processes. The spin-dependent recombination currents measured by EDMR
are quantitatively correlated to the current-voltage characteristics of the device, systematically
varying both optical and electronic operating conditions. A strong voltage dependence of the
signal shape and intensity is revealed, with a characteristic maximum at the quasi flat-band conditions
(corresponding to an inflection point in the voltage-dependent photocurrent). Two clearly
distinct bias regimes can be distinguished, in which the recombination is dominated by either
photo-generated or injected charge carriers. It is disclosed that the spin-independent recombination
in this study is a bimolecular process (Langevin type) and that the observed spin-dependent
recombination is monomolecular in nature (Shockley-Read-Hall type).
Finally, a brief EDMR study on a degraded P3HT:PC61BM solar cell device is presented. It
is concluded that the degradation does not change the species actively involved in the spindependent
processes, but that the injection-current-related EDMR signal component is suppressed,
most probably due to the oxidation of the top contact.