Record-High Surface Potential by Spontaneous Orientation Polarization in Organic Semiconductors

Molecular orientation in organic semiconductors has many different facets, such as optical anisotropy leading to birefringence, alignment of their optical transition dipole moments (TDMs) and/or their permanent electrical dipole moments (PDMs) [1]. While TDM alignment is well established and readily exploited to improve light outcoupling in organic light-emitting diodes (OLEDs) [2], PDM alignment and the resultant spontaneous orientation polarization (SOP) is by far less investigated, and it is not fully clear yet if it is beneficial or detrimental for device application [3].<br/>SOP is observed in evaporated films of polar organic molecules and leads to a so-called giant surface potential (GSP), which can be measured by Kelvin probe, impedance spectroscopy or the displacement current method [4]. In general, the GSP slope is roughly proportional to the magnitude of the molecule’s PDM. However, despite their very high PDM, many molecules exhibit only small SOP because they tend to aggregate in pairs with antiparallel alignment. Thus, the order parameter (i.e., the degree of net PDM alignment) is typically less than 10%.<br/>Here we report a new class of materials with drastically enhanced PDM alignment, exceeding 30% for films grown at room temperature [5]. These phosphine oxides exhibit a highly polar phosphor-oxygen bond, which leads to large SOP despite the presence of different structural conformers. By evaporating them on a cooled substrate in combination with dilution of the polar species in a non-polar host, record-high GSP slope of almost 300 mV/nm (with an equivalent electric field of 3x10⁸ V/m) can be achieved. This opens new perspectives for application of organic semiconductors in energy harvesting devices.<br/>[1] A.J. Hofmann, M. Schmid, W. Bruetting; Advanced Optical Materials 9, 2101004 (2021)<br/>[2] T.D. Schmidt, T. Lampe, D. Sylvinson, P.I. Djurovich, M.E. Thompson, W. Bruetting; Phys. Rev. Applied 8 (2017) 037001<br/>[3] Y. Noguchi, Y. Tanaka, H. Ishii, W. Bruetting; Synth. Metals 288, 117101 (2022)<br/>[4] Y. Noguchi, W. Bruetting, H. Ishii; Jap. J. Appl. Phys. 58, SF0801 (2019)<br/>[5] A. Cakaj, M. Schmid, A. Hofmann, W. Bruetting; ACS Appl. Mater. Interfaces 15, 54721 (2023)