Injection barrier

Figure 11-17. Calculated current density as a function of bias (upper panel) and electron density as a function of position at 12 V bias (lower panel) for a two-layer electron-only (0.5 cV electron injection barrier) device with the energy level diagram for the two polymer layers shown in Fig. 11-13. The mobility of the left hand polymer is increased by a factor of ten in the enhanced mobility structure (dotted line).

Figure 12-1. A comparison of theoretical and experimental injection currents into typically lOOnin thick films of conjugated polymers sandwiched between an ITO anode and an Al cathode. Open symbols refer to calculated j(F) characteristics. A is the zero field injection barrier (Ref. [21]).

Figure 11-13. Calculated current density as a limelion of bias lor two-layer hole-only structures (0.1 eV injection barrier lor holes) with a 0.0, 0.3, and 0.5 eV energy harrier for holes at the interface between the polymer layers. The upper panel is a schematic of the energy level diagram for the sttuc-turcs.

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are ,M.=0.8 eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the work functions of a Ag/AgCl and an 1TO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization cncigy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. E x values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

The shapes of experimental and theoretical j(Fj curves are in mutual agreement. By comparison one arrives at injection barriers ranging from 0.4 eV (PPV imine) to 0.7 eV (PPPV). The agreement between theory and experiment is similarly good as far as the temperature dependence is concerned. Data shown in Figure 12-7 were taken with DASMB and confirm the analytic results for A=0.4eV. 

Electrical measurements on devices with different layer thickness have shown that the diode current depends on the applied field rather than the drive voltage. This is similar to what has been observed with our alternating PPV copolymers [68]. It indicates that field-driven injection determines the electrical characteristics. From Figure 16-39 it is evident that U-OPV5 has the lowest onset for both current and emission. By means of Fowler-Nordhcini analysis of the /-V -charac-teristics and optical absorption measurements, wc estimated the injection barrier for holes and the HOMO-LUMO gap, respectively [119]. The results of... 

Tabic 16-7. Estimated HOMO-LUMO gaps and hole injection barriers. 

To optimize the performance of single-layer Oocl-OPV5 devices, Ca instead of A1 was used as the cathode, which lowers the injection barrier for electrons with approximately 1.3 eV. The change of cathode resulted in a more than twofold re-... 

Several groups introduced an oxadiazole moiety as a part of the PPV backbone (polymers 139a [169,170], 139b [171], 140 [172], 141 [169], and 142 [170]). Not unexpected, the oxadiazole moieties lowered the LUMO energy of these polymers (as demonstrated by CV measurements). The decreased electron injection barrier is manifested by lowered turn-on voltage (6 V for ITO/139b/Al) [171]. However, relatively low efficiency (0.15% for 139b [171]) was reported for these copolymers (Chart 2.28). 

Eg = 1.96 eV), thus decreasing the hole injection barrier from the ITO electrode. 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

Bis(2-(2-hydroxphenyl)benzothiazolate)zinc(II) (Zn(BTZ)2, 85) is an excellent white emitter. The HOMO and LUMO energy levels of Zn(BTZ)2 are —5.41 eV and —2.65 eV, respectively. Just as was found by Zuppiroli et al. for Znq2 derivatives, Zhu et al., found that the electron transport of Zn(BTZ)2 is better than Alq3, though the electron injection barrier is higher for Zn(BTZ)2 [136]. This has been explained by the strong intermolecular interaction of Zn(BTZ)2 molecules. This same group has examined the use of Zn(BTZ)2 as an ETM in PLEDs and the results are consistent with those with SMOLEDs [137]. 

A textbook example for the successful application of the model of Arkhipov et al. is the work of van Woudenbergh et al. [173]. More recently, Agrawal et al. [106] compared injection limited currents and space-charge-limited currents in a copper-phthalocyanine sandwich cell with TTO and Al electrodes. An analysis of experimental data yields consistent values for the width of the DOS distribution as well as for inter-site separation [174]. These studies support the model of thermally activated injection into a Gaussian DOS distribution of hopping sites and confirm the notion that disorder facilitates injection because it lowers the injection barrier, although the transport velocity decreases with increasing disorder. 

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]. 

Wang YZ, Qi DC, Chen S, Mao HY, Wee ATS, Gao XY (2010) Tuning the electron injection barrier between Co and Cgo using Alq3 buffer layer. J Appl Phys 108 103719... 

Significant improvement in the inversion channel mobility reported in [30] became possible due to a dramatic reduction in the interface state density by postoxidation annealing standard thermal oxides in an NO atmosphere. An extensive experimental and theoretical study exists showing that NO postoxidation anneal improves the dH-SiC/SiO interface quality and inversion channel mobility (e.g., [31-33]). It has also been shown for NO-annealed oxides that the electron injection barrier height increases to values close to the theoretical value at RT that will... 

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