Metal-polymer interface, hole injection

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

For devices using ITO/PEDOT anodes, because the PEDOT layer is much more conductive than the MEH-PPV film, the major energy barrier for hole injection depends on the PEDOT/MEH-PPV interface. If the organic solvent used to dissolve the MEH-PPV does not dissolve the PEDOT layer, it is expected that the resulting PEDOT/MEH-PPV interface will be similar to those obtained by spin-coating the polymer on top of a metal electrode. This is true (or nearly true) in most cases, because PEDOT has a very limited solubility in many commonly used organic solvents. In fact, it was found that the MEH-PPV film spun on top of the PEDOT layer could be easily pealed off from the PEDOT surface by a piece of Scotch tape,... 

The energy barrier for hole injection at the metal-poymer interface is determined by the vacuum work function of the metal contact and the ionization potential Ip of the polymer. For conjugated polymer films spin-coated onto hole injecting metal electrodes, it has been reported that as long as is smaller than a critical value characteristic of the polymer, no interface dipole is formed [104]. In this case, the barrier for hole injection can be estimated simply by aligning the vacuum levels of the metal and the polymer (Mott-Schottky limit) the measured work function of the metal with the polymer deposited on top increases linearly with with a slope of one (see Figure 2.3.11). 

However, when exceeds said critical valne a significant interface dipole can be formed. Positive charges are transferred from the metal to the semiconductor and the position of the Fermi level at the interface becomes pinned at an energy level interpreted as the hole polaron/bipolaron energy level in the polymer semiconductor. This simple picture suggests that, at least in the case of solntion-deposited polymers on common hole-injecting contacts, chemical interactions between the metal and... 

Figure 6. Hole injection efficiency figure of merit for substrate contacts of varying work function vs. energy step across the contact polymer interface estimated from published work function data and electrochemical redox potential data. The height of each bar reflects the variability in injection efficiency due primarily to variation in substrate surface pretreatment and for the particular case of Au, diffusion to the interface of metal atoms from underlying binder layers.

The functioning of such a device is illustrated schematically in Fig. 16-6. Here, holes injected from the ITO electrode are blocked at the interface with the electrontransporting polymer layer, which comprises a 1 1 blend of PMMA with 2-(4 biphenylyl)-5-(4-ter-butylphenyl)-l,3,4-oxadiazole)(calledbutyl-PBD).Thisblocking causes increased electron injection from the other electrode, forcing a balance in electron and hole currents. Additionally, excitons formed at the PPV/PBD-PMMA interface are kept away from the other electrode. Such tailoring allows the use of less reactive metal cathodes, such as Mg, in place of Ca, and yields quantum efficiencies as high as 0.4%. 

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