Hole-only devices

Figure 9-26. Expcrimenial and calculated (solid lines) UV characteristics of ITO/PPV/Au hole-only devices of various thicknesses, L. The current flow in all devices is described by SLC with a hole mobility //, = 1(r cm2 V"1 s 1 and with a dielectric constant of 3. The inset shows the chemical structure of the PPV investigated in that study (R =CHi, R2=CiUH2i). Reproduced from Ref. 185).

Figure 12-10. j(V) characteristics of ITOIPPVIAu hole only devices with thicknesses T=0.13 pm (squarcs)=3 pm (triangles), and 0.70 pm (dots). Full lines represent the prediction of Child s law (Eq. (12.5) for a hole mobility of 5xl0 7cnr V"1 s and a dielectric constant c=3. The inset shows the PPV used with R,=CH3 and ft2=Cu)ll21 (Ref. [411). 

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

The structure of the low bandgap polymeric semiconductor and the dopant dye is plotted in Fig. 5.19. The average thickness of the active layers, determined by AFM measurements, is between 80 and 110 nm. In order to obtain a better understanding of the transport behavior of polymer blends, low temperature studies of cells with pristine MDMO-PPV and MDMO-PPV/PTPTB 1 1 (wt. %) with Au electrodes were carried out. Au has a high work function and should therefore be a good hole injection contact and provide a high barrier for electron injection. The device will therefore be a hole-only device, as described earlier in this chapter [14]. 

In order to understand the performance of the tandem device, low temperature transport studies are a valuable tool. Diodes made from pristine MDMO-PPV and in composites with PTPTB are compared. ITO/PEDOT and Au electrodes are chosen to guarantee hole-only devices. This special choice of the electrodes is a successful technique for improving our understanding of transport failures. The proper choice of contacts allows us to produce p-type or n-type diodes from the same semiconductor, depending on the selectivity of the contact. For instance, Au is a hole-injection contact for most of the polymeric semiconductors, while Ca is an electron-injection... 

The measured electron current in PCBM injected through Au electrode is shown in Fig. 3.35. Calculated SCL hole current in OC1C10-PPV is shown by circles in Fig. 3.35 for a thickness of L = 170 nm. Even with the high Schottky barrier of 1.4 eV (the barrier determined from the measured current is 0.76 eV) with A1 cathode for PCBM is not sufficient to suppress the electron current in PCBM below the hole current in OCiCio-PPV. With the large injection barrier the electron current is injection limited. This work shows that it is not possible to make hole only devices using OCiCio-PPV PCBM bulk heterostructures. 

In a typical polymer LED device, the anode is the electrode with a high work-function used for hole injection. Usually, the polymer thin film is spun on top of the anode. On the other hand, the cathode of the device is a metal electrode with a low workfunction used for electron injection. It is usually produced by the thermal evaporation of the metal on top of the polymer film. For the hole-only devices that will be discussed here, both electrodes consist of the same metal. Therefore, there is no logical cathode and anode. However, for the sake of consistency, we still... 

The direct evidence for this morphological dependence of A

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... 

FIGURE 6.11. The l-V curves under forward and reversed biases for a hole-only device using Cu electrodes. Solvent used for spin-coating was THF. 

Fig. 1 a Schematic diagram showing the Ip and Ea values of PFO relative to the work functions of common electrode materials used in PLEDs. The figures in brackets are the respective energies in eV. The optical gap energy g (2.95 eV) is also shown (taken from [12]). b Current density-voltage characteristics of a PEDOT/PFO/Au (hole-only device) and Ag/PFO/Ca (electron-only device). Bipolar device of ITO/PFO/Ca and PEDOT/PFO/Ca are also shown (taken from [13]). Note that PEDOT is the abbreviation of PEDOT PSS here... 

The I-Vdata from a hole-only device fabricated from MEH-PPV with Au as the cathode and ITO as the anode are plotted as I/F2 vs 1/F in Fig. 4.12. As predicted, the plot is close to linear particularly at high fields. The literal assumption of tunneling through a triangular barrier appears to be an excellent approximation. The deviation from linearity at lower fields probably indicates an additional contribution to the current from thermionic emission. 

Fig. 4.12 Fowler-Nordheim plot of ln[l/F2] vs 1 /F for a hole-only device fabricated using MEH-PPVwith ITO as the anode and Au as the cathode.

Based on the comparisons of (1) photoluminescence quenching in bipolar and holes-only devices and (2) photo- and electroluminescence loss in bipolar devices, the plausible hypo esis was put forward that the instability of Alq cation-radicals plays a key role in bipolar devices as well as in holes-only devices [62]. However, considering that the concentrations of Alq cation-radicals could be substantially less in operating bipolar devices relative to holes-only devices, the alternative degradation mechanisms, involving, for example, unimolecular reactions of excited states or bimolecular reactions of charge carriers and excited states, cannot be rigorously ruled out. 

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