Tuning the charge transfer in Fx-TCNQ/rubrene single-crystal interfaces

TL;DR

This study systematically investigates charge transfer mechanisms at rubrene and Fx-TCNQ organic semiconductor interfaces, revealing how electron affinity influences charge transfer and highlighting the limitations of single-particle models in describing interfacial energetics.

Contribution

It provides the first detailed analysis linking electron affinity to charge transfer in Fx-TCNQ/rubrene interfaces and discusses the limitations of band diagram approaches.

Findings

Higher electron affinity in Fx-TCNQ leads to increased charge transfer.

Transport properties fit a mobility-edge model, but energy level alignment shows a ~100 meV discrepancy.

Discrepancies are attributed to molecular relaxation and screening effects not captured by single-particle models.

Abstract

Interfaces formed by two different organic semiconductors often exhibit a large conductivity, originating from transfer of charge between the constituent materials. The precise mechanisms driving charge transfer and determining its magnitude remain vastly unexplored, and are not understood microscopically. To start addressing this issue, we have performed a systematic study of highly reproducible single-crystal interfaces based on rubrene and Fx-TCNQ, a family of molecules whose electron affinity can be tuned by increasing the fluorine content. The combined analysis of transport and scanning Kelvin probe measurements reveals that the interfacial charge carrier density, resistivity, and activation energy correlate with the electron affinity of Fx-TCNQ crystals, with a higher affinity resulting in larger charge transfer. Although the transport properties can be described consistently and quantitatively using a mobility-edge model, we find that a quantitative analysis of charge transfer in terms of single-particle band diagrams reveals a discrepancy ~ 100 meV in the interfacial energy level alignment. We attribute the discrepancy to phenomena known to affect the energetics of organic semiconductors, which are neglected by a single-particle description, such as molecular relaxation and band-gap renormalization due to screening. The systematic behavior of the Fx-TCNQ/rubrene interfaces opens the possibility to investigate these phenomena experimentally, under controlled conditions.