Intricacies and Mechanism of p‐Doping Spiro‐MeOTAD Using Cu(TFSI)2. (25th June 2022)

• Record Type: Journal Article
• Authors: Hochgesang, Adrian; Biberger, Simon; Grüne, Jeannine; Mohanraj, John; Kahle, Frank‐Julian; Dyakonov, Vladimir; Köhler, Anna; Thelakkat, Mukundan
• Is Part Of: Advanced Electronic Materials. Volume 8:Number 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-25
• Subjects: copper -- doping -- electron transfer -- reaction mechanisms -- semiconductors; Materials -- Electric properties -- Periodicals; Materials science -- Periodicals; Magnetic materials -- Periodicals; Electronic apparatus and appliances -- Periodicals; 537
• DOI: 10.1002/aelm.202200113
• Languages: English
• ISSNs: 2199-160X

Abstract

Abstract: Copper salts are a popular choice as p ‐dopants for organic semiconductors, particularly in N 2, N 2, N 2′, N 2′, N 7, N 7, N 7′, N 7′ ‐octakis(4‐methoxyphenyl)‐9, 9′‐spirobi[9H‐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. Cu I species arising out of doping are stabilized by formation of a [Cu I (TFSI)2 ] ‐ cuprate, inhibiting elemental copper formation. This Cu I 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. Abstract : A systematic study of the p‐doping of Spiro‐MeOTAD using Cu(TFSI)2 as dopant reveals the doping mechanism and origins of macroscopic property changes. The stable product of doping [Cu I (TFSI)2 ] – boosts doping efficiency and functions as hole trapping species. Reversible equilibrium between charge transfer state and free charge carriers causes nonlinear effects in temperature dependent electronic properties.