Zwitterionic Self-Assembled Monolayer for Simultaneous Noise Suppression and Hole Extraction in High-Performance Near-Infrared Organic Photodetectors

Abstract

Phosphonic acid (PA)-based self-assembled monolayer (SAM) materials have emerged as promising hole transport materials (HTMs) for photodiodes such as organic/perovskite solar cells and photodetectors. In this study, we reveal for the first time that triphenylamine (TPA)-based PA-type SAMs spontaneously form zwitterionic structures, which are key in strengthening the binding strength of the anchoring group on the ITO surface and enhancing interfacial adhesion. The electrostatic interaction of the zwitterion induced a strong interfacial dipole, dramatically increasing the work function (WF) to 5.33 eV (0.28 eV higher than the control). Notably, TPA does not function merely as an electron-donating group, but rather as a strong Br & oslash;nsted base. DFT calculations confirmed that zwitterion formation is energetically favorable and crucial for WF modulation. Thus, zwitterionic SAMs in near-infrared organic photodetectors effectively improve hole extraction and suppress dark current density through their dual functionality of hole transporting and electron blocking. The optimized devices achieved remarkably low dark current density (1.41 x 10-9 A/cm2) and noise current (2.8 x 10-14 A), combined with high responsivity (0.496 A/W) and specific detectivity (3.78 x 1012 Jones) at -0.1 V. This study highlights the potential of zwitterionic structures as HTMs and offers a powerful molecular design strategy for SAM-type HTMs.