Injection layers, OLED

B Werner, J Posdorfer, B Webling, H Becker, S Heun, H Vestweber, and T Hassenkam, Polyaniline as hole injection layer for OLEDs and PLEDs, SID Digest Tech. Pap., 33 603-605, 2002. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

Fig. 1 Basic set-up of a layered OLED structure. Electrons and holes are injected from the respective electrodes (metal cathode, semiconducting and transparent anode). The charge carriers move from different sides into the recombination/emitter layer, where electrons and holes recombine and excite the doped emitter molecules (asterisks, e.g., or-ganometallic triplet emitters). For more details see Fig. 2. For clarity, light emission is only shown for one direction although the photons are emitted in all directions...

The fabrication of OLEDs involves the deposition of one or more layers of organic semiconductors onto the transparent anode, indium tin oxide (ITO) in most cases, often coated with a hole-injection layer. Generally, two classes of devices are distinguished, according to the materials involved and - as a result of this - the fabrication process ... 

When in the stationary state equal numbers of holes and electrons are injected per unit hme, but e.g. the hole mobility is much higher than the electron mobility, then in a single-layer OLED, the recombination occurs very near the cathode. This leads as a rule to an increase in non-radiative recombination and thus to a reduction of the Hght yield. If different numbers of holes and electrons are injected and transported per unit time, then that part of the charge carriers which is in excess, i.e. a part of the current, cannot contribute to the production of Ught This too reduces the efficiency of the OLED. 

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