Intricacies and Mechanism of p-Doping Spiro-MeOTAD Using Cu(TFSI)₂

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Hochgesang, Adrian ; Biberger, Simon ; Grüne, Jeannine ; Mohanraj, John ; Kahle, Frank-Julian ; Dyakonov, Vladimir ; Köhler, Anna ; Thelakkat, Mukundan:
Intricacies and Mechanism of p-Doping Spiro-MeOTAD Using Cu(TFSI)₂.
In: Advanced Electronic Materials. Bd. 8 (2022) Heft 10 . - 2200113.
ISSN 2199-160X
DOI: https://doi.org/10.1002/aelm.202200113

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Abstract

Abstract Copper salts are a popular choice as p-dopants for organic semiconductors, particularly in N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi9H-fluoren-2,2′,7,7′-tetramine (Spiro-MeOTAD) hole transport material for solar cells. While being exceptionally effective, no scientific consensus about their doping mechanism has been established so far. This study describes the thermodynamic equilibria of involved species in copper(II) bis(trifluoromethanesulfonyl)imide (Cu(TFSI)2) doped, co-evaporated Spiro-MeOTAD. A temperature-independent formation of charge transfer states is found, followed by an endothermic release of free charge carriers. Impedance and electron paramagnetic resonance spectroscopy unravel low activation energies for hole release and hopping transport. As a result, (52.0 ± 6.4)\% of the total Cu(TFSI)2 molecules form free, dissociated holes at 10 mol\% and room temperature. CuI species arising out of doping are stabilized by formation of a CuI(TFSI)2- cuprate, inhibiting elemental copper formation. This CuI species presents a potent hole trap reducing their mobility, which can be averted by simple addition of a bathocuproine complexing agent. A nonlinear temperature-dependent conductivity and mobility that contradicts current charge transport models is observed. This is attributed to a combination of trap- and charge transfer state freeze-out. These insights may be adapted to other metal salts, providing guidelines for designing next-generation ultra-high efficiency dopants.