Investigation of device failure mechanisms in polymer light-emitting diodes

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices

“…222 Another mechanism proposed to lead to catastrophic failures is electromigration of the cathode metal during device operation resulting in the formation of thin metal filaments extending into organic layers or even reaching the opposite electrode. 223,224 Some features of this mechanism make it difficult to distinguish it from other failure modes: strong dependence on current density and temperature, role of pinhole defects in initiation of filament growth, and self-healing behavior presumably caused by periodic formation and destruction of filaments. Although no new studies have been reported in the past decade, it is possible that electromigration is still an important contributor to both catastrophic failures and to leakage currents (e.g., as observed below turn-on voltage) even in state-of-the-art OLED devices.…”

Interfacial degradation in organic optoelectronics

“…Table 1 List of commonly used time constants to define device stability t 0 initial measurement time immediately after device fabrication t 50 time for performance to decay to 50% of initial measured value (half-life) t 80 time for performance to decay to 80% of initial measured value t s second testing measurement time defined arbitrarily by user: used mainly for OPVs, generally taken as onset of linear or 2nd exponential behavior after initial rapid decay t s80 time for performance to decay to 80% of measured value at t = t s The two different stages of degradation observed in typical decay curves are important to consider independently, as they are believed to arise from different conditions: the initial decay is attributed to interfacial degradation, 8,21,[30][31][32][36][37][38][39] often driven by extrinsic factors (such as moisture) and the longer time scale decay to intrinsic or oxidation driven degradation of the bulk active layers. 8,[21][22][23]25,31,[40][41][42][43][44][45][46][47][48][49][50][51] Although two exponentials are widely used, it is also common for researchers to ignore the initial degradation, and only fit the monotonic decay.…”

“…10 The common benchmark value defined for OLEDs is the lifetime for an initial luminance of 100 cd m 22 . As t 50 generally decreases as a function of applied voltage, 36,55,56 accelerated lifetime testing can be performed for higher initial luminance, with an acceleration factor n, assuming L 0 n t 50 = const (2) For OPVs, the PCE is tracked, but it is the normalized short circuit current (J sc ) that is used to determine rate constants. The open circuit voltage (V oc ) typically does not change over time, 57 although sometimes degradation of the fill factor (FF) has been observed.…”

“…The ITO surface is highly reactive with many organic materials, especially under electrical stress. 36,73,144,170 Fig. 2 shows optically visible craters on the ITO surface after OLED device operation at high voltages.…”

“…The most significant effect of In diffusion into the active layers for OLEDs is the formation of dark spots (non-emissive regions) in the opaque low work function counter electrode. One of the earliest observed failure modes in OLEDs, the development of dark/dead regions has a number of proposed mechanisms including oxidation through pinholes in the counterelectrode 188,[205][206][207][208] (see section 4.3), organic layer crystallization, 73,195,208,209 surface roughness 73,119,188,[210][211][212] and shorting across the organic layer 21,36,213 where metal diffusion leads to the formation of leakage paths for charge carriers.…”