Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally, in situ photoluminescence characterization and efficiency measurements reveal that SOP-induced exciton-polaron quenching can be reduced through an appropriate choice of processing conditions, leading to enhanced OLED efficiency.
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.
PDMs), has been observed in many common OLED materials. [11,12] Intuitively, PDM and TDM orientation are expected to be influenced by similar factors. However, molecules with large PDMs may require additional considerations due to energetically favorable anti-parallel dipole alignment. [11,13,14] Preferential orientation of molecular PDMs, also known as spontaneous orientation polarization (SOP), was first observed in thin films of tris(8-hydroxyquinoline)aluminum (Alq 3 ). [15] Since then, the presence of SOP has been demonstrated for numerous common organic semiconductors. [11,16,17] This effect manifests itself via the presence of a surface potential across the film, which varies linearly with thickness. The internal electric field induced by the polarization charge is ≈50 mV nm −1 , [18] which is comparable to the electric field within an OLED under operating conditions. The mechanism for preferential molecular orientation in vapor-processed thin films in the context of TDM alignment has previously been understood in terms of "surface equilibration". [19] In this paradigm, the enhanced surface mobility of a molecule during vapor deposition allows it to partially equilibrate before being "locked" by the subsequently deposited molecular layer. The molecular equilibration process can be controlled by either deposition rate or substrate temperature. [20][21][22] Factors that have been shown to influence SOP include processing conditions, [19,[23][24][25] molecular shape, [16,20] dipole-dipole interactions, [11,13,14,25] and other electrostatic interactions between the molecules. [11,25,26] For example, the differences in molecular geometry have been used to explain the negative surface potential of Al(7-Prq) 3 , [16] while weakening of dipole-dipole interactions has been proposed to be responsible for enhanced orientation in dipolar mixtures consisting of the polar guest molecule in a nonpolar matrix. [13,14,27,28] Additionally, dipole moment direction plays a crucial role in determining the sign of the surface potential, which can be manipulated by introducing either electronaccepting or electron-donating functional groups into the polar molecule. [25] In the context of OLED performance, prior work has shown that SOP in one of the active layers (most frequently the electron transport layer (ETL)) can significantly impact charge injection, [29][30][31] serve as a sensitive probe of device degradation, [32,33] influence the position of the recombination zone, [31,33] Many electron transport layer (ETL) materials employed in organic light-emitting devices (OLEDs) show a preferred orientation of the molecular permanent dipole moments. This phenomenon is known as spontaneous orientation polarization (SOP) and results in the formation of bound polarization charge. In an OLED, this leads to the accumulation of polarons (typically holes) at the ETL/emissive layer interface to balance this charge. Previous work on phosphorescent OLEDs has found that exciton-polaron quenching due to SOP-induced hole accumulation can reduc...
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