Controlling Spontaneous Orientation Polarization in Organic Semiconductors -- The Case of Phosphine Oxides

TL;DR

This study demonstrates how molecular design and growth conditions can be used to control spontaneous orientation polarization in phosphine oxide organic semiconductors, achieving high surface potentials and near-perfect molecular alignment.

Contribution

It introduces methods to tune SOP in phosphine oxides through molecular structure and deposition parameters, enabling precise control of film polarization.

Findings

High SOP with giant surface potential (>150mV/nm) in BCPO films.

SOP magnitude can be controlled from 0 to 300mV/nm by adjusting growth conditions.

Achieved over 80% upright PDM alignment through combined temperature and doping strategies.

Abstract

Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a non-zero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides. We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar phosphine oxides with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope) reaching more than 150mV/nm in a neat BCPO film grown at room temperature. Additionally, by altering the evaporation rate and the substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300mV/nm. Diluting BCPO in a non-polar host enhances the PDM alignment only marginally, but combining temperature control together with dipolar doping can result in almost perfectly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.