Traps distributed

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.

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. 

Figure 9-19. Bund diagram of LPPP with hole traps and gold electrodes with Va<- vacuum level. Ec conduction band, Eva valence band. E, Fermi level. . baudgup energy. and , " trap depths. ,( ) trap distribution, X electron affmity, and All work function of the gold electrodes.

The luminance reaches 100 cd/m2 at 2.5 V with EL efficiency of 2.5 cd/ A. The corresponding external quantum efficiency is about 2% ph/el. At —10 V bias, the photosensitivity at 430 nm is around 90 mA/W, corresponding to a quantum yield of 20% el/ph [135], The carrier collection efficiency at zero bias was relatively low in the order of 10-3 ph/el. The photosensitivity showed a field dependence with activation energy of 10 2 eV [135], This value is consistent with the trap distribution measured in the PPV-based conjugated polymers [136,137],... 

The concentration of the traps distributed exponentially in energy is given by,... 

Assuming exponential trap distribution, combining continuity and Poisson equations and integrating the resultant equation, we obtain,... 

Fig. 3.20. (a) Calculated current density as a function of inverse temperature for V = 10 V for a conducting polymer sample with exponentially distributed traps. The values of the trap distribution parameter Tc are Tc = 2500 K (long dash line), Tc = 1800 K (dash-dot line), Tc = 1500 K (solid line), Tc = 1200 K (dotted line) and Tc = 1000 K (small dash line). The values of the other parameters are Hb = 3 x 1018 cm-3, Nv = 3 - 1020 cm-3, e=2 and /ip = 5 x 10 cm2 V 1 s 1. The inset shows the calculated effective activation energy, Eeb. as a function of the characteristic trap distribution energy ) = kTc. (b) J-V characteristics of a sample of MEH-PPV with a thickness of 94 nm on a log-linear scale. The symbols represent the data of Campbell et al. [46]. The lines represent calculated values using the mobility model. The figure is taken from [42],... 

Kumar et al. [39] made numerical calculations for traps at a single energy level and for traps distributed exponentially in the energy space. The effect of high field is qualitatively similar in the two cases. We show the effect of high field on the electric field in Fig. 3.32. The electric field and Poole-Frenkel effect suppress the actual electric field considerably. Near the exit end the field is suppressed by more than one order of... 

In studies of low-mobility insulators, two types of continuous trap distributions are commonly used the exponential distribution of traps [364] and the Gaussian distribution of traps [365]. 

Figure 99 The trap distribution function in evaporated polycrystalline films of thionaphtenoindole (TNI), calculated from Eq. (258), using time-dependent tangent of the experimental log i(t)—log t plots (See Ref. 450).

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