Trapping electroluminescence devices

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

Burrows. P. E. Forrest. S. R. (1994). Electroluminescence from trap-limited current transport in vacuum deposited organic light emitting devices. Applied Physics Letters, vol. 64, no. 17,2285-7. 

LEDs built from ITO substrates with 6T which is either directly covered by Al or Mg or is prepared in a sandwich device with j8-didecyL 6T and a-bis-triiso-propylsilyl- 6T as interlayer between a6T and the metal electrode were investigated by Delannoy et al [316]. The quantum yields are summarized in Table 13.10 and show that the lower workfunction metal Mg is superior over Al as an electron-injecting electrode for the simple 6T device, whereas the structures with interlayers are independent of the contact metal. In these structures, however, it is not a6T but the silyl-compound which is the light-emitting layer. A mobility of charge carriers in 6T is estimated from time-resolved electroluminescence data to be in the range of 5 x 1cm V"" s which is much lower if compared to values obtained in field-effect or SCLC measurements (compare Section 5.1). This is discussed in terms of an emptying of traps which are hence active due to the presence of two asymmetric electrodes. 

Popovic et al. proposed the formation of charge traps that act as nonradiative recombination centers in the emissive material as one possible explanation for the relative differences in the electroluminescence and photoluminescence losses in operating OLEDs [27]. Despite the absence of direct evidence of the formation of such traps or their ability to act as nonradiative recombination centers, the proposal was quite plausible, particularly in view of a complementary notion that, in some cases, recombination on highly emissive charge traps is responsible for the predominant emission by the dopants [35,36]. The rise in voltage required to drive a constant current through a device has been cited as possible evidence of the formation of traps, but the voltage rise could be, in fact, attributed to mechanistically imrelated phenomena, such as electrode deterioration. 

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