Electron-only devices

Figure 9-27. Experimental (dots) and theoretical (solid line) t/V characteristics of. a Ca/PPV/Ca electron-only device with a thickness, L, of 310 nm. The theoretical curve is obtained assuming an exponential trap distribution with a trap density of Nt=5-I()17 cm 1, a trap distribution parameter Tt 1500 K, and an equilibrium electron density n = L5-I011 cm"1. The dashed line gives the hole SLC according to Eq. (9.13). Reproduced from Ref. 85J.

The same equations as for the electron-only devices can be applied to describe the 1/V characteristics for organic LEDs based on Alq3 (in single and multihetero layer devices) 82. ... 

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... 

The upper panel of Figure 11-17 shows the effect of increasing the electron mobility in the layer near to the electron contact of the iwo-polymer-laycr electron-only device. The solid line is the calculated / V characteristic when the electron mobilities ol lhe two layers are the same and given by the value used above. The dotted line... 

Fig. 3.24. J-V characteristics of the Al/LiF/Alq3/LiF/Al electron only device as a function of temperature for Alq3 with a thickness of 160 nm [54].

Fig. 3.25. Experimental and theoretical J-V characteristics of theAko electron only device at higher temperatures. The solid lines are fits of the theoretical prediction to trap model where the current obeys the power law J = V H, while the symbols are the experimental data. From these measurements characteristic temperature (7c) of 2300 K can be derived [54].

FIG. 3.33. (a) Comparison of the FDTO model (solid lines) with the experimental (symbols) of Crone et al. [59] for a Ca/MEH-PPV/Ca electron only device, (b) Comparison of the FDTO model (dashed line) and FDTO with background impurity model (solid curve) with the experimental data for A1/MEH-OPV5/ITO (curve 1) and for A1/MEH-OPV5/PEDOT/ITO (curve 2). The thickness of both samples is 110 nm. 

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... 

Figure 9-27. Experimental (dots) and theoretical (solid line) //V characteristics of a Ca/PPV/Ca electron-only device with a thickness, L, of 310 nm. The theoretical curve is obtained assuming an expo-...

A similar approach can be taken to study electron injection. Replacing ITO with a lower work function metal (e. g. Nd or Mg) yields devices in which the carriers are almost exclusively electrons. A similar analysis of the data from a range of electron-only devices indicates that electrons tunnel into the jt -band of MEH-PPV at 4.9 eV through a triangular barrier at the polymer/cathode interface. 

The carrier mobility in the mixed CuPcC o films has been obtained by modeling the current density-voltage (j-V) characteristics for hole- and electron-only devices using the space-charge-limited current (SCLC) theory (Pope and Swenberg, 1999 Rand et al., 2005), viz. ... 

Fig. 6 (a) Simulated current/voltage curve of an electron-only device with and without p-type doping (Na = cm ). (b) Band diagram of the simulated electron-only device with doping. 

Figure 6 shows the current/voltage curve (Fig. 6a) and the band diagram for an electron only device with a p-type dopant (Fig. 6b). Figure 6a shows that, relative to the undoped case, the current is greatly reduced at low voltages. This is due to the fact that part of the voltage is used to overcome the barrier that can be seen in... 

Figure 7.8 J-V characteristics of electron-only devices using Ti02 and optimized NP-Ti02 as ETLs measured with and without (w/o) UV excitation. (Reprinted from ref. 65, with permission from the Royal Society of Chemistry, http //pubs.rsc.org/en/Content/ArticleLanding/2013/EE/ c3ee42440e divAbstract.)...

Another interesting property of the metal NP-ineorporated Ti02 is that the dark current of the electron-only device reduces when the eurrent is enhanced under UV excitation when compared to that of pristine Ti02 devices, as shown in Figure 7.8. This can enhance the dynamie range of the OSCs and shows potential for photodetector applications. 

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