*4.5.2. Triode FEOLEDs with a dynode structure (strip electron multiplier)*

As shown in Fig, 7 (b), FEOLEDs with a dynode structure are classified as triode devices. It comprises the dynode and an organic EL light emitting layer and CNTs template. The dynode is formed between the cathode and the organic EL lighting layers.

As shown in Fig. 8 (a), there is another kind of triode FEOLED, which is a FEOLED with a strip electron multiplier. In this case a strip electron multiplier is used instead of a dynode, but it is attached directly to the organic EL light emitting layer for protecting the electron injection layer from high-energy electron bombardment and allows the electrons to disperse evenly in the EML.

## *4.5.3. Luminescence mechanisms in FEOLEDs*

To further confirm that the mechanism of operation of FEOLEDs is the same as that of OLEDs, the following experiment was conducted. A hole blocking layer (BCP) is inserted between the hole transport layer (NPB) and the emission layer (Alq3) of the organic formation, as shown in Fig. 9(a). If an OLED is applied a voltage, hole carriers injected from the ITO anode electrode to the hole transport layer would be blocked at the interface of the NPB layer and the BCP layer. The electrons (emitted from the cathode and passing through Alq3) would then recombine with the holes accumulated in the NPB layer. Refer to Fig. 3, this NPB excitation generated by the recombination, according to the active energy levels of OLEDs would give rise to blue light. The Alq3 layer in such case would generate no light. If a cathode luminescence mechanism device is applied with the BCP layer, on the other hand, electron bombardment on the organic material would generate light in the emission layer Alq3, which should have green color. As such, when a BCP layer is inserted in FEOLEDs, if blue light is observed, the luminescent mechanism of the FEOLED must be similar to that of the conventional OLED: if green light is observed, the luminescent mechanism must be similar to that of cathode luminescence. The experiment results showed that blue light was observed, as shown in Fig. 9(b), which clearly illustrates that both FEOLEDs and OLEDs operate on similar mechanism of emission.

Thus, the light emission in FEOLEDs also occurs via the following five processes as in OLEDs: 1) both electrons and holes injected from anode and cathode into organic layers, 2) these injected charge carriers are transported towards each other across the organic layer, 3) formation of singlet excitons due to the Coulomb interaction between the injected electrons and holes, 4) migration singlet excitons to organic emitting layer and 5) radiative recombination of single excitons.

**Figure 9.** (a) Structure of a FEOLED with Hole blocking layer and (b) emission of blue light.
