Hole injection, energy barrier

One can, nevertheless, conclude that (i) there is only a very small barrier for hole injection from ITO to PTV, if any barrier at all, (ii) a finite energy should exist for hole transport across the PTVIDASMB interface, and (iii) PBD should act as an efficient internal blockade for hole transport towards the cathode. 

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... 

When an electric field is applied between the anode and the cathode, electrons and holes are injected from the cathode and the anode, respectively, into the organic layers. With a matched energy barrier between the electron and the hole injection layers (EILs and HILs) and the cathode and the anode, electrons and holes are efficiently injected into the ETL and HTL. 

The magnitude of the injection barrier is open to conjecture. Meanwhile there is consensus that energy barriers can deviate significantly from the values estimated from vacuum values of the work-function of the electrode and from the center of the hole and electron transporting states, respectively. The reason is related to the possible formation of interfacial dipole layers that are specific for the kind of material. Photoelectron spectroscopy indicates that injection barriers can differ by more than 1 eV from values that assume vacuum level alignment [176, 177]. Photoemission studies can also delineate band bending close to the interface [178]. 

Figure 118 (a) Schematic representation (not to scale) of separated polymer channel domains individual submicrometer size LEDs in the polymer blend (PMCHT PTOPT)/PBD EL structure, (b) The energy level structure of the EL device. The data on polymer energy levels taken from Ref. 345. The hole injection barriers from ITO into PMCHT [AEh(l) = 1 eV] and PTOPT [AEh(2) 9 0.6 eV] are indicated in the figure. For the molecular structure of the polymers, see Fig. 119. 

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