Energy Level Alignment in Organic-Organic Heterojunctions: The TTF-TCNQ Interface

TL;DR

This study uses DFT calculations to analyze the energy level alignment at the TTF-TCNQ organic interface, revealing a charge transfer-driven metallic state and an induced dipole consistent with experiments, and explores bias-induced electronic transitions.

Contribution

It provides a detailed theoretical analysis of the TTF-TCNQ interface, including energy level alignment, charge transfer effects, and bias-dependent electronic properties, which advances understanding of organic heterojunctions.

Findings

The interface is metallic due to charge transfer.

An induced dipole of 0.7 eV matches experimental data.

Bias voltage can switch the interface between metallic and insulating states.

Abstract

The energy level alignment of the two organic materials forming the TTF-TCNQ interface is analyzed by means of a local orbital DFT calculation, including an appropriate correction for the transport energy gaps associated with both materials. These energy gaps are determined by a combination of some experimental data and the results of our calculations for the difference between the TTF_{HOMO} and the TCNQ_{LUMO} levels. We find that the interface is metallic, as predicted by recent experiments, due to the overlap (and charge transfer) between the Density of States corresponding to these two levels, indicating that the main mechanism controlling the TTF-TCNQ energy level alignment is the charge transfer between the two materials. We find an induced interface dipole of 0.7 eV in good agreement with the experimental evidence. We have also analyzed the electronic properties of the TTF-TCNQ interface as a function of an external bias voltage \Delta, between the TCNQ and TTF crystals, finding a transition between metallic and insulator behavior for \Delta~0.5 eV.