Hole metal/polymer interfaces

The direct evidence for this morphological dependence of A

hole-only devices.25,37 For example, it is observed that a hole-only device consisting of Au(anode POM contact)/polymer/Au(cathode MOP contact) has different I-V curves under forward and reversed biases.37 This phenomenon is also observable with other high-workfunction metals such as Cu and Ag.25 In these devices, if the anode/polymer and the cathode/polymer interfaces have the same

significantly different since the two metals have different workfunctions (4.5 eV for Cu and 4.3 eV for Al). However, it was found by Roman et al. that the I-V curves were almost identical under forward and reverse biases.36... 

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). 

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.

Figure 1-3. In Ihis improved bilaycr device structure lor a polymer LED an extra ECHB layer has been inserted between the PPV and the cathode metal. The EC11B material enhances the How of electrons but resists oxidation. Electrons and holes then accumulate near the PPV/EC1113 layer interface. Charge recombination and photon generation occurs in the PPV layer and away from the cathode.

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. 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

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