Quantitative EPR Analysis of Doped Spiro-OMeTAD
Ula Yasin a b, Lukas Kegelmann c, Felix Kraffert a, Steve Albrecht c, Jan Behrends a b
a Berlin Joint EPR Lab, Freie Universität Berlin, Berlin (Germany)
b Institute for Nanospectroscopy, Helmholtz Zentrum Berlin, Berlin (Germany)
c Young Investigator Group Perovskite Tandem Solar Cells, Helmholtz-Zentrum Berlin, Berlin, Germany
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Ula Yasin, 158
Publication date: 11th February 2019

Perovskite based solar cells (PSCs) have ignited a competition for record power conversion efficiencies (PCEs) since their first successful implementation in 2009 [1]. With ongoing and extensive research, power conversion efficiencies have risen to more than 23% in less than a decade [2]. One reason for this remarkable boost is the substantial effort in improving charge selective contacts. Therefore a decisive approach in increasing efficiency is the optimization of the hole extraction at the perovskite interface. The small molecule [2,2′,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′ -spirobifluorene] (spiro-OMeTAD) is to-date the most commonly used hole transport material and has proven high PCEs above 21% [3,4]. As its pristine form is low in conductivity, p-doping of spiro-OMeTAD is required, typically by adding lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)-TFSI (FK209). It is, however, unclear to what extent each additive enhances the charge carrier density in spiro-OMeTAD, which usually is associated with an increase in conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy allows to quantitatively investigate the extent of doping independent of the charge conductivity and mobility. With quantitative EPR we determine doping efficiencies for both FK209 and Li-TFSI, allowing the evaluation of their role in the mixture. Our results indicate that FK209 is a prominent p-dopant with a doping efficiency of approximately 80% compared to Li-TFSI with an efficiency of only a few percent.

We furthermore conduct a spectral analysis of the doped spiro-OMeTAD EPR spectra obtained from solution and thin film samples. We identify the origin of the EPR spectrum as a consequence of the hole on spiro-OMeTAD. In addition, deviations in the spectral shape between the two phases are detected. With the aid of Density Functional Theory (DFT) calculations, spectroscopic parameters are obtained that enable a complete interpretation of the EPR spectra. We show that the spectral deviation between solutions and films is a direct consequence of the difference in spin-density distribution on the molecule existing in the corresponding morphological environment.

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