25-30 August 2019
Henry Ford Building
Europe/Berlin timezone

When stronger magnets don’t help: Methods for disentangling overlapping high-field EPR spectra illustrated in record organic solar cell blend PBDB-T:ITIC

27 Aug 2019, 16:50
25m
Lecture Hall C (Henry Ford Building)

Lecture Hall C

Henry Ford Building

Talk EPR development and applications EPR

Speaker

Ms Melissa Van Landeghem (Department of Physics, University of Antwerp)

Description

Spectral overlap, even at high field, is a problem generally encountered in many EPR studies. In the specific case of bulk-heterojunction (BHJ) organic solar cells (OSCs), the paramagnetic species of interest are light-induced radicals which are created as a pair after charge transfer at the interface between the donor polymer and molecular acceptor regions making up the BHJ blend. Hence, the similar g-values expected for the positive and negative organic radicals often lead to strong spectral overlap complicating the unambiguous assignment of the light-induced (LI) EPR spectrum.

The donor-acceptor combination studied here, PBDB-T:ITIC, was the first fullerene-free OSC to recently achieve >11% efficiency, challenging the state-of-the-art polymer-PC$_{71}$BM devices [1]. For this blend, the two-component structure of the LI-EPR spectrum could not even be resolved at W-band frequency (94 GHz). Therefore we separated the two contributions to the total EPR spectrum by exploiting two different properties of the charge-transfer radicals, namely the (small) difference in their longitudinal ($\text{T}_1$) relaxation times and the presence of a unique magnetic nucleus, $^{14}$N, in ITIC. For the $\text{T}_1$-based method, we applied an inversion-recovery filter to selectively suppress one component in the spin echo analogously to the relaxation-filtered hyperfine spectroscopy (REFINE) technique first proposed by Maly et al. [2]. Sensitive detection of the $^{14}$N hyperfine couplings at W-band frequency was achieved by means of electron-electron double resonance (ELDOR)-detected NMR (EDNMR). Here we demonstrate the application of EDNMR-induced EPR to obtain an EPR spectrum containing only contributions from the ITIC radical [3]. Both approaches are validated by LI-EPR spectra on related blends which yield better-resolved spectra of the individual PBDB-T and ITIC radicals.

[1] W. Zhao et al., Adv. Mater., 2016, 28, 4734.
[2] T. Maly, T. F. Prisner, J. Magn. Reson., 2004, 170, 88.
[3] M. Van Landeghem et al., J. Magn. Reson., 2018, 288, 1.

Primary authors

Ms Melissa Van Landeghem (Department of Physics, University of Antwerp) Prof. Wouter Maes (Institute for Materials Research, Design & Synthesis of Organic Semiconductors, Hasselt University) Prof. Etienne Goovaerts (Department of Physics, University of Antwerp) Prof. Sabine Van Doorslaer (Department of Physics, University of Antwerp)

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