Interface formation, hole injection layers

The interface between the quasi-metal PEDOT PSS and organic semiconductors has been investigated in numerous experiments. As illustrated in Figure 14.16, the energy barrier for hole injection (AE) is not simply determined by the difference between the PEDOT PSS work function (4>) and the ionization potential (/ ) of the semiconductor as predicted by the Schottky-Mott model dipole layer formation at the interface will lead to a vacuum level shift A [158-161]. 

Figure 14.16 Energy level alignment at the PEDOT PSS-semiconductor interface. The two layers are (a) separated and (b) in contact. The formation of an interface dipole (ID) might significantly determine the energy barrier for hole injection AE...

Fig. 12 Schematic energy level diagram of the impact of the formation of an interface dipole at an ITO/organic semiconductor (OS) interface (a) the barrier in the absence of the dipole layer (b) and (c) reduction of the hole injection and the electron injection barrier, respectively, in the presence of the interface dipole layer on the surfaces of ITO. Here, the interface dipoles with opposite sign are directed toward the ITO surfaces (b) increasing and (c) decreasing the work function of ITO.

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]. 

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