Zoran D. Popovic

Degradation mechanisms in organic light-emitting devices: Metal migration model versus unstable tris(8-hydroxyquinoline) aluminum cationic model

To resolve the issue of which of the “indium migration” model and the “unstable AlQ3 cationic” model plays a more important role in luminescence degradation of organic light-emitting devices, we investigated the effect of the device structures on device operational stability. The results show that alterations in device layer structures can significantly affect the device operational stability, although they do not appear to noticeably change the magnitude of indium migrations. This suggests that the indium migration model may not play a dominant role in device degradation. On the other hand, the change in device stability with the alteration in the device structures can be plausibly explained by the unstable AlQ3 cationic model.

Improving the stability of organic light-emitting devices by using a hole-injection-tunable-anode-buffer-layer

Introducing a hole-injection-tunable-anode-buffer-layer (HITABL) at the indium tin oxide anode contact of an organic light-emitting device can finely tune hole injection to establish proper charge balance, thus remarkably improves its operational stability. The HITABL consists of two sublayers: (i) an ∼2.5nm thick metal (e.g., Ca, Mg, or Ag) sublayer and (ii) an ∼10nm thick tetrafluorotetracyanoquinodimethane (F4TCNQ) doped N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine sublayer. Hole injection can be tuned by changing (i) the metal in the first sublayer and/or (ii) the concentration of the F4TCNQ dopant in the second sublayer. The choice of the metal used in the first sublayer and/or the concentration of F4TCNQ in the second sublayer affect the hole-injection efficiency. Therefore, by using the HITABL, one can make the necessary diminutive adjustments to the hole injection of a device and achieve proper charge balance, resulting in a significant improvement in operational stability.

Charge injection at interfaces between molecularly doped polymer thin films

Charge injection between the active layers in organic semiconducting devices is a key determinant of device function. Accordingly, understanding the effect of intermixing between the layers at these interfaces is of fundamental importance. In this letter, via the use of the time-of-flight method, a comparison is made between the charge injection across discrete versus intermixed interfaces of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine and 1,1-bis((di-4-tolylamino)phenyl)-cyclohexane doped polycarbonate, semiconducting thin-film layers. No perturbation to the overall charge transport was observed with the discrete interface; however, in contrast the rate of charge transport was clearly reduced through the intermixed interface.