OLEDs may undergo internal decay through various mechanisms like chemical reactions, morphological changes, and physical changes like charge accumulation. These mechanisms can impact the device’s color, luminance, current, and voltage properties, which may eventually reduce the device’s efficiency and lifetime. The luminance behavior of OLEDs over time typically assessed at a constant physical environment such as temperature, current density, voltage, or humidity, is the most critical factor in determining the device’s lifetime. The device lifetime is usually expressed as T₅₀ or T_1/2, the duration for the luminance to dim to half its initial brightness at a constant current density [31]. More complex metrics such as T₇₀ and T₉₇ measure how long it takes for the luminance to drop to 70% and 97% of its initial value, respectively. The T₉₇ metric is significant for the display industry as it measures the threshold at which humans can perceive differences in brightness between adjacent display elements. Although some researchers have assessed device lifetime using constant voltage sources, this can cause luminance drops to occur more quickly as the device’s impedance rises with time, decreasing current density.

Interestingly, the luminance of OLEDs often varies directly with the current density across a wide range. This suggests that as the current density delivered to the OLED increases, its lifetime typically decreases, according to the well-known empirical scaling law Ln0⸱T1/2=CL0n⸱T1/2=C [31,32,33]. In fact, researchers have found they can predict the lifetime of very stable OLEDs with reasonable accuracy by studying the relationship between a device’s initial luminance at time zero (L₀) and its lifetime (T_1/2), using a constant factor (C), as described in studies by Meerheim et al. and Jarikov et al. [34,35]. The rate of degradation of a device over time can be influenced by the acceleration factor (n), which is specific to the materials and design of the device under study. However, accurately predicting the lifetime of very stable organic devices can be challenging due to the potential for inaccuracies [33,34]. In some cases, it can be difficult to predict an OLED’s actual lifetime since various factors can cause degradation. Researchers may observe an initial sharp decline in luminance followed by a more gradual, long-term decline. To account for these complex patterns, researchers use a combination of different exponential decay functions, such as LtL0/=ae−αt+be−βtLtL0=ae−�t+be−�t. This equation includes several fitting parameters, including a, b, α, and β, which can be used to predict the OLED’s lifetime more precisely [33]. The alternative method for predicting the lifetime of OLEDs is through the stretched exponential decay (SED) function, which is represented by, LtL0=exp[−(tτ)β]LtL0=exp⁡[−t��] [33,34,36]. The degradation of OLEDs is linked to the formation of defects within the device, which can function as luminescent quenchers, nonradiative recombination centers, or deep charge traps based on their energy levels [37,38]. It has been found that at 50% luminance loss in a particular system, the areal density of fixed charges linked with these defects is around 6 × 10¹¹ cm⁻² (or 10⁻⁷ C/cm²) in size [39]. Giebink et al. investigated electroluminescence loss utilizing a mathematical model with various arbitrary factors, which showed that a defect density of 10¹⁸ cm⁻³ (about 0.1% of the molecular density) might cause a loss of more than 50% of the device luminance [38]. However, it is crucial to remember that these equations might underestimate or overestimate the lifetime if inadequate data were utilized for fitting [34,40]. The SED function has two fitting parameters, τ and β, representing the decay constant and stretching factor, respectively, but neither holds any specific physical significance. It has been empirically found that β varies for different material configurations and stack designs but remains the same for various current densities of identical OLED stacks.

The way lifetime data are reported and compared has changed over the last few years, in line with the criteria for device application by industrial standards. An overview of relevant OLED brightness, needed for several applications is shown in Figure 4. Initially, device lifetime was commonly reported at an expected display brightness of 100 cd/m², but, nowadays, the majority of lifetime data are published as luminance, which is 1 order of magnitude higher, i.e., 1000 cd/m², suitable for both simple lighting applications and demanding display applications. It is well-known that indoor displays usually have an overall brightness ranging from 100 to 300 cd/m² [41]. To attain this range of brightness, individual pixels on the device must emit light at levels between 200 and 600 cd/m², depending on the pixel’s size, color, and device architecture [32,42]. In 2001, Howard and Orache from IBM and Chwang et al. from UDC predicted that a display would need to last for over 10,000 h at 140 cd/m², assuming continuous operation and integration into devices such as entertainment headsets [36,43]. The incorporation of filters such as polarizers in displays also increases the device lifetime requirements. According to the same IBM study, even modest outdoor displays may require brightness as high as 3000 cd/m². This requirement extends to the industry and governmental bodies, as evidenced by the German Federal Ministry of Education and Research project to achieve a lifetime of 30,000 h for white OLEDs at 1000 cd/m².