Photoemission Study of Charge Transfer between ET (BEDT-TTF) and Acceptors F₆TCNNQ and F₂TCNQ

Abstract

The electronic properties of organic semiconductors can be modified by a charge transfer to electron accepting molecules. In this study, the charge transfer reactions between BEDT-TTF (ET) and the two molecular acceptors F2TCNQ and F6TCNNQ were investigated by photoemission spectroscopy. The materials were evaporated under vacuum conditions on top of each other, and changes in the core levels as well as the valence orbitals were probed after each deposition step. The core level spectra showed charge transfer for both interfaces, which was higher for the ET/F6TCNNQ interface according to different intensity ratios as well as different BE shifts of neutral and cationic spectra. The unequal contribution of the sulfur atoms to the HOMO of ET translated to an additional contribution to the S 2p core level spectra of oxidized ET. The valence spectra showed the occupation of the acceptor’s LUMO. Both interfaces did not show signs of occupied density of states at the Fermi level. The progression of the work function suggested that ET and F6TCNNQ form relatively sharp interfaces while signs for interdiffusion were observed for the ET/F2TCNQ combination.

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“…[63] The fluorine group is usually introduced into these molecules as an electron-withdrawing group to increase their EA value. [64] Thus, among them, F4TCNQ is the most widely used π-electron acceptor due to its relatively high EA of 5.2 eV that matches well with IE value of a wide range of HTMs. Huang et al used F4TCNQ to p-dope spiro-OMeTAD, increasing its p-type conductivity by four orders of magnitude from 6 × 10 À 8 to 1.3 × 10 À 4 S cm À 1 and resulting in a 160 % improvement for PCE of spiro-OMeTAD-based devices compared to the undoped one.…”

“…They have been used to p‐dope organic and inorganic semiconductors for various photoelectronic devices [63] . The fluorine group is usually introduced into these molecules as an electron‐withdrawing group to increase their EA value [64] . Thus, among them, F4TCNQ is the most widely used π‐electron acceptor due to its relatively high EA of 5.2 eV that matches well with IE value of a wide range of HTMs.…”