Understanding and engineering spontaneous orientation polarization in organic light-emitting devices

Abstract

Organic light-emitting devices (OLEDs) are a ubiquitous technology for displays with growing application in a variety of other spaces. The future success of this technology depends on further improvements in device efficiency and stability. One pathway for improvement relies on engineering molecular orientation in the organic thin films comprising an OLED. This review is focused on the subsequent spontaneous alignment of molecular electric dipole moments, known as spontaneous orientation polarization (SOP), a phenomenon observed for many common OLED materials. The magnitude of polarization fields associated with SOP rival what is experienced in an OLED under high injection and can significantly impact electronic and excitonic behavior. Here, we first review current work describing the mechanism for the formation of SOP, reflecting an interplay between several factors, such as molecular shape, intermolecular interactions, and processing conditions. We also consider several strategies to tune the polarization sign and magnitude, with emphasis on connecting observations to quantitative models of SOP formation. Building on this discussion of SOP in organic thin films, we review how polarization in OLED active layers impacts key aspects of device performance, including charge injection, luminescence efficiency, and stability. Finally, this review concludes with an outlook on areas of future development needed to realize broad control over SOP for a variety of applications, highlighting gaps in our current understanding of this phenomenon.

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

“…Please note that in this measurement at room temperature, the BCPO film showed an m GSP of 145 mV nm –1 , which is lower compared to the value derived from the IS measurement and a previously reported value . However, the samples for KP and IS measurements were prepared in different deposition chambers, and other organic molecules also showed differences in their respective m GSP values due to differences in the evaporation conditions . We think that specifically different evaporation rates may play an important role, as discussed below.…”

“…18 However, the samples for KP and IS measurements were prepared in different deposition chambers, and other organic molecules also showed differences in their respective m GSP values due to differences in the evaporation conditions. 10 We think that specifically different evaporation rates may play an important role, as discussed below. It is also worth noting that during warm-up, the final value of the surface potential difference, as measured with KP, did not change significantly.…”

“…4,6 Meanwhile, quite a number of dipolar molecules used in OLEDs are known to exhibit SOP, 7,8 and their implications for devices are intensively discussed. 9,10 For example, it was shown by drift-diffusion simulations 11 that the presence of negative interfacial charges between the HTL and Alq 3 in the prototypical bilayer OLED of Tang and Van Slyke�or, equivalently, a positive GSP at the surface of the Alq 3 film�is favorable for electron injection. By contrast, if an ETL with negative GSP is used, 12 the injection current is significantly lower.…”

“…This is shown in Figure for a compilation of materials studied in our lab over recent years (using either IS or KP)including three of the four materials studied here. For similar graphs with additional materials, we refer to the literature. ,,, …”

Impact of Intermittent Deposition on Spontaneous Orientation Polarization of Organic Amorphous Films Revealed by Rotary Kelvin Probe

“…When these molecules are subjected to vacuum vapor deposition, they are often slightly oriented with a head and tail distinction, resulting in film polarization with a giant surface potential (GSP). This phenomenon is referred to as spontaneous orientation polarization (SOP) and is often reported for various OLED-related materials. − As polarization induces interfacial charges that affect the charge injection and accumulation behavior in the device, ,, several attempts have been focused on controlling the polarity and magnitude of SOP in addition to the molecular orientation itself. , However, the detailed mechanism of such an anisotropic orientation is yet to be fully understood. In addition, the polarity of the SOP is not symmetric; most materials exhibit positive GSP, limiting the polarization control.…”