Elucidating the Impact of Charge Transfer on Organic Spontaneous Orientation Polarization via Kelvin Probe

Preferential electric dipole alignment in vapor-processed organic semiconductors, known as spontaneous orientation polarization (SOP), is present in many systems and gives rise to large (~ 5 V for a 100-nm-thick film) internal electric fields. This effect has been shown to impact organic light-emitting (OLED) and photovoltaic (OPV) device operation, and can be utilized as an electret for vibrational power generators that does not require a preconditioned charging step. In many of these applications, the layer exhibiting SOP is in direct contact with a metal electrode. In this study, we find the magnitude of SOP to be strongly influenced by the type of metal underlayer. Kelvin Probe measurements are used to assess the role of metal-to-organic charge transfer as a potential origin for this phenomenon. We find the metal work function to be a critical parameter influencing charge transfer and demonstrate the methodology to engineer the underlayer work function that prevents an instantaneous surface potential decay. These findings suggest that choosing an appropriate metal underlayer is a critical step in designing devices with metal-SOP layer interfaces.