Exciton formation efficiency

In general, a two-layer device structure is more efficient than single-layer architectures. There are two key reasons for this. First, each layer can be separately optimized for the injection and transport of one carrier type. Second, exciton formation and radiative decay take place close to the HTL-ETL interface away from the quenching sites at the organic-metal contacts. 

Both particles, electron and hole—coming from the different electrodes—move from opposite directions towards the recombination layer. There they can combine and form excitons. This may happen near to the layer interface, on matrix molecules within the layer, and/or at doped emitter molecules. In suitable cases, as required for OLEDs, this leads to a population of excited states of the emitter material which subsequently emits light. Obviously, this process should occur with high efficiency. Details of the mechanism of exciton formation and population processes of excited emitter states are discussed in the next section. 

Spin-orbit coupling will not strongly alter the mechanism of exciton formation in an organic matrix material, but it will have drastic effects on the efficiency of electro-luminescence in an OLED device. To illustrate this property, we will compare the efficiency which is obtainable with a purely organic molecule to the efficiency achievable with a transition metal complex, if both molecules exhibit equal photo-luminescence quantum yields. If one as-... 

The value of y is related to the injection process and depends on electrode materials. InP-LJBD, a calcium electrode is used to increase the electron injection efficiency due to its low work function(12,13). The value of 7 e.h related to not only materials but also device structure. Multi-layer structure is commonly used to increase the value of 7, generating the hole and/or electron accumulation near a light-emitting layer surface(l,14). The maximum value of 7 1 has been suggested to be 25% because of the spin statistics of singlet exciton formation (11). Toin rove g, we have to use highly efficient materials or dye-doped materials(15). 

The HTL enhances exciton formation and recombination in the emissive layer by blocking the electrons away from the ITO anode and efficiently injecting holes into the electroluminescent layer. Polysilanes which are insulators for electrons have a good hole mobility of around lO cm A s due to a-conjugation of the electrons along the polymer chain. Moreover they are usually transparent in the whole visible region. These properties make them interesting candidates for application as hole transport layers in LEDs. 

The improved order in films cast from toluene may also be induced by prolonged illumination at low temperatures, as recently discovered at the Technion [155]. Delocalized photoexcitations among adjacent chains may lead to several characteristic properties that are not common in isolated chains. These are [71] (i) reduced PL quantum efficiency, (ii) PL redshift, (iii) relatively large generation of PP excitations, rather than intrachain excitons, (iv) more substantial delayed PL due to PP recombination, (v) reduced formation efficiency of triplet excitons since the intersystem process is hampered by the interchain delocalization [156], and (vi) increased exciton dissociation efficiency in Ceo" doped polymer films. It is thus important to review the ultrafast excitation dynamics in MEH-PPV in comparison with those of DOO-PPV. 

Single-layer and heterojunction organic LEDs have been fabricated. Single-layer organic LEDs have a low efficiency, due in part to the low probability for exciton formation in the thin film. Heterojunction organic LEDs are much more efficient, since the carrier confinement provided by the heterointerface increases the probability of exciton formation. 

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