Trap-free SCLC

Figure 5.11 summarizes the temperature dependent transport behavior of unipolar and ambipolar diodes based on MDMO-PPV. Below 190 K, the hole-controlled device (ITO/PEDOT and Au contact) and the ambipolar device (ITO/PEDOT and LiF-Al contact) behave identically. Trap-free SCLC transport is observed and the mobility at this temperature is estimated to be around 10-8 cm2/Vs. For the ambipolar device, a diode-like turn-on is... 

Results for a 20 pm thick sample of polycarbonate containing 50 mass% TPD for a field of 1.5 x 107Vm-1 at 296 K with charges injected from an indium tin oxide (ITO) electrode coated with a 0.1 pm thick layer of PPV are shown in Fig. 8.30(b). The limiting current is close to the trap-free SCLC, indicating that the PPV-coated ITO acts as an efficient hole-injecting electrode. The lower curve is the TOF transient recorded under identical conditions. The arrow on the lower curve indicates the transit time and that on the upper curve is 0.8 of this value. The step-voltage response is therefore close to the theoretical prediction. 

The charge-carrier density thus decreases on going away from the contact into the bulk (Fig. 8.29). To be sure, the charge-carrier density diverges in this expression at the injecting contact. This problem does not occur if one takes diffusion currents into account. The case treated above, without shallow traps, will in the following be referred to as TF-SCLC (trap-free SCLC). 

Fig. 8.61 The relative depth dependence of the electric fieid strength in the trap-dominated (TCLC) and in the filled-trap/trap-free (SCLC) voltage ranges, as well as for injection limiting. t = 0.2 eV, T = 300 K, f = Et/kT7.8. After [38].

Current-voltage characteristics of ITO/HIL/NPB/Ag devices using different HIL materials. The HIL are ITO, Au, CuPc, and PEDOTPSS. The solid line is the theoretical trap-free SCLC of NPB. (From Tse, S.C. et al., /. Appl. Phys., 100,063708,2006. With permission.)... 

Estimated from the trap-free space charge limited current (SCLC) method... 

If the traps are deep, an analytical expression for J cannot be obtained. However numerical solutions can be obtained easily [38], In this case most of the injected carriers remain trapped and the current by injection remains small until all the traps are filled. As the traps are nearly filled the SCLC begins to flow. It increases rapidly and as the trap-filled limit is reached, it follows the trap-free V2 law. In this case trap-filled limit voltage Vtel and Vq are equal. Recent work [41] on the approach to trap-filled limit will be discussed later. 

Unambiguous steady-state measurements are carried out only in the trap-free space-charge-limited-current (SCLC) regime (23), when the average transit time (f j of any excess injected carrier across the sample bulk is shorter than the time required for the bulk to locally neutralize the carrier (22). The transit time ( r) has been defined in equation 1. The time to neutralize any excess injected carrier is the bulk dielectric relaxation time, Tr ( r = in which p is the bulk resistivity and e is the bulk dielectric... 

The observation of space charge limited currents (SCLC) in nanoscaled pure and chromium-doped titania was reported [310] and both the free-charge carrier density and the trapped-charge carrier density were given. 

Dark conductivities ap allow a number of conclusions [199, 200] and are thus measured first. Since Iq = a U a linear dependence (s 1) of the dark current Id on the voltage U indicates ohmic behavior, a superlinear dependence (s 2) the presence of space charge limited currents (SCLC). The constant a is directly proportional to the proportion of free to trapped charge carriers, the dielectrical constant of the sample, the electrical field constant and the mobihty of the charge carriers. Finally a is also determined by the cell geometry. As a is proportional to the mobility p of the charge carriers it is in certain cases possible to calculate p from the slope of the Id/U curve. The dark conductivity of an ohmic conductor varies with the temperature in an Arrhenius type fashion. The... 

The effect of traps on the SCLC has been widely studied, both theoretically and experimentally. In the case of a single shallow trap level of density lyii g at an energy E below the conduction band (or above the valence band), the current is simply multiplied by a factor 9 - nf/( f -f- t)> where and t are the density of free and trapped carriers, respectively. In the case where tif < n, and assuming that the carrier distribution follows a simple Boltzmann statistic, 9 is given by... 

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