Work hole injection

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

When the work functions of the contact electrodes are not well matched to the bands of the EL polymers, energy barriers are formed at the respective interfaces. The height of the barrier for hole injection is determined by the difference between the work function of... 

Since the ionization potential of 238 matches closely the work function of PEDOT (5.1-5.3 eV) [335], the hole injection is dramatically improved. Accordingly, the device ITO/PEDOT/237 238(7 3)/Al has a significantly improved EL efficiency, tjel= 1.5cd/A, two orders of magnitude higher than that of single-layer PLED with 237, six times higher than that of bilayer PLED with triarylamine polymer HTL, and almost twice as high as that of PF blends with low molecular triphenylamine HT materials (in device with Ca electrode) [321]. 

High work function (WF) (< >4.1 eV) to promote efficient hole injection... 

In electrochemical light-emitting cells, the semiconductive polymer can be surrounded asymmetrically with a hole-injecting material on one side and a low work function electron injecting metal (such as magnesium, calcium, or aluminum) on the other side. The emission of light may occur when a charge carrier recombines in the polymer as electrons from one side and holes from the other meet. 

The structure of the low bandgap polymeric semiconductor and the dopant dye is plotted in Fig. 5.19. The average thickness of the active layers, determined by AFM measurements, is between 80 and 110 nm. In order to obtain a better understanding of the transport behavior of polymer blends, low temperature studies of cells with pristine MDMO-PPV and MDMO-PPV/PTPTB 1 1 (wt. %) with Au electrodes were carried out. Au has a high work function and should therefore be a good hole injection contact and provide a high barrier for electron injection. The device will therefore be a hole-only device, as described earlier in this chapter [14]. 

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