Molecular Diodes With Tunable Threshold Voltage Based on π‐Extended Tetrathiafulvalene

It is a challenge to control the threshold voltage (defined as the voltage at which the current density increases sharply in the direction of forward bias), one of the basic parameters that define a molecular diode. The problem is that strong Fermi‐level pinning of molecular frontier orbitals and the changes in the electronic structure are often overshadowed by the changes in the supramolecular structure. A chemical control approach is proposed to shift the effective threshold voltage of a series of molecular diodes composed of self‐assembled monolayers (SAMs) of π‐extended tetrathiafulvalene (exTTF) derivatives. The highest occupied molecular orbital of the molecules can be tuned by ≈0.6 eV (maximum) with the minimum changes in the supramolecular structure of the SAMs by introducing chlorine atoms or amino substitutional groups into the exTTF moieties. To the best of the authors’ knowledge, this is the first paper that reports that the threshold voltage of the exTTF‐based diode can be modulated from 0.65 to 1.70 V. In addition, it is shown that these four molecular diodes can operate as half‐wave rectifiers with the threshold voltage at positive direct current output ranging from 1.63 to 2.40 V, with the input voltage being 3.0 V at 500 Hz. Avoiding strong Fermi‐level pinning of molecular frontier orbitals is achieved by using bowl‐shaped π‐extended tetrathiafulvalene (exTTF) to decouple the direct‐contact of exTTF and the gallium/indium top‐electrode. Therefore, it allows demonstration of the tunability of the effective threshold voltage of a molecular diode over 1.0 V by chemical substitutions at exTTF and the applications in building efficient half‐wave rectifiers.