Figure 2. Typical normalized L−t curve, showing the experimental data (full circles), together with the fit using eq 2 up to 1100 h (dotted line), and up to 2200 h (full line). Reprinted with permission from ref 32.

Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is presented. For small-molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-efficiency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.

We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene—emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5×75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.