Optoelectronic Probing of Emission Layer Charge Distributions to Enable Optimization of Organic Light‐Emitting Diodes

ABSTRACT Spontaneous orientational polarization (SOP) in organic electron transport layers (ETLs) introduces internal electrostatic fields that influence charge injection and interfacial charge accumulation in organic light‐emitting diodes (OLEDs), adversely affecting device performance. Here we combine displacement current measurements (DCM) and charge modulation spectroscopy (CMS) to resolve the formation and magnitude of SOP‐driven charge populations. Using SOP‐active ETLs with positive giant surface potentials (GSPs) in OLEDs employing archetypal host–guest emissive layers (EMLs), we show via DCM that, prior to turn‐on, charge accumulates to counter the SOP‐derived GSP. In most cases, DCM further reveals charge extraction exceeding that required for local SOP compensation. CMS then provides the spatial resolution required to interpret these electrical measurements, resolving two distinct hole populations located at the electron blocking layer/emission layer (EBL|EML) and EML|ETL interfaces that spatially coincide with the primary and secondary recombination zones, respectively. ETL dilution with lithium quinolate (Liq) systematically suppresses SOP and shifts the hole‐injection onset voltage. Conversely, accumulation at the EBL|EML interface does not scale with SOP magnitude, indicating a distinct origin. Together, these results define a unified picture in which the intrinsic electrostatic vulnerability of host–guest EML OLED architectures must be considered alongside ETL SOP.