Mechanisms of Negative Capacitance in Organic Light-Emitting Diodes: A Review

Negative capacitance (NC) in organic light-emitting diodes (OLEDs) has emerged as a sensitive probe of non-equilibrium charge dynamics, yet its physical origin remains under active debate. This review provides a comprehensive overview of NC in OLEDs, focusing on how it arises from delayed and imbalanced charge responses under electrical modulation. We first summarize how NC manifests in capacitance–voltage, capacitance–frequency, Nyquist, and Bode representations, and outline the theoretical frameworks used to extract capacitance from impedance and transient spectroscopy measurements. We then examine the microscopic mechanisms proposed to account for NC. Under bipolar operation, NC originates from processes in which charge removal lags behind voltage modulation, including Langevin and trap-assisted recombination, asymmetric hole and electron injection, delayed release of space charge at organic–organic interfaces, and dispersive transport induced by energetic disorder. Although distinct in origin, these mechanisms share a common physical principle: internal charge redistribution cannot follow the applied voltage instantaneously, resulting in an inductive-like response with a negative incremental capacitance. NC under unipolar operation is also discussed, where self-heating has been identified as a dominant contribution arising from the mismatch between fast electronic transport and slow thermal relaxation. Finally, we highlight open questions and future research directions, emphasizing the need for carefully designed unipolar devices and combined frequency- and time-domain approaches to disentangle thermal and electronic effects. This review establishes NC as a powerful diagnostic tool for probing slow charge dynamics and interfacial processes in OLEDs and related organic semiconductor devices.