Space charge limited current measurements

Fig. 9 Temperature dependence of the hole mobility of a polyfluorene copolymer inferred from space-charge-limited current measurements on samples of thicknesses 122 nm, 1 pm, and 10 pm. The full curve is an extrapolation to the low carrier density limit using the extended Gaussian disorder model. From [90] with permission. Copyright (2008) by the American Institute of Physics...

In fact, in their recent work, Mensfoort et al. [90] conclude that in polyfluorene copolymers hole transport is entirely dominated by disorder. This is supported by a strictly linear In p cx dependence covering a dynamic range of 15 decades with a temperature range from 150 to 315 K (Fig. 8). Based upon stationary space-charge-limited current measurement, where the charge carriers are in quasi equilibrium so that dispersion effects are absent, the authors determine a width a of the DOS for holes as large as 130 meV with negligible polaron contribution. 

Koo Y-M, Choi S-J, Chu T-Y, Song O-K, Shin W-J, Lee J-Y, Kim JC, Yoon T-H (2008) Ohmic contact probed by dark injection space-charge-limited current measurements. J Appl Phys 104 123707... 

Fig. 4.20. The density of states distribution obtained from space charge limited current measurements on undoped a-Si H. The various symbols represent different samples and deposition conditions. Field efiect (FE) and DLTS data are shown for comparison. is the substrate temperature during growth (Mackenzie, LeComber and Spear 1982).

Two main methods have been used to measure the charge carrier mobility in electroluminescent polymers time of flight (TOF) carrier transit time measurements and analysis of the current-voltage (1-V) characteristics of single carrier devices in the space charge-limited current (SCLC) regime. A summary of the results for the hole mobility of PPV and PPV-related polymers is given in Table 11-1 [24, 27-32]. For... 

Support for the applicability of this model to an explanation of the Meyer-Neldel rule comes from measurements of space-charge limited currents in anthracene where a correlation (see Fig. 20) has been found between the total density of traps H and the distribution parameter Tc (Owen et al, 1974). It has been shown that this effect is not fortuitous as suggested by some workers... 

The dark and illuminated J-V curves of the solar cells for different PCBM concentrations are plotted in Fig. 5.21. Measurements for 30% and 60% PCBM were also made but they are not shown as they tit with the general trends shown by the curves for other concentrations. The features of the J-V curves in Fig. 5.21 are mentioned below. (1) For larger voltages the dark currents are space charge limited currents. The shape of the dark currents, particularly the points of inflexion, are characteristics of the space charge limited... 

Brabec and coworkers have made extensive measurements of the dark and illuminated currents of MDMO-PPV/PCBM BHSCs [124,141], The experimental results for the BH-SCs from their paper [124] are shown in Fig. 5.22. There are points of inflexion in the dark current of the organic solar cell confirming that the dark current is the space charge limited currents. These features, particularly the points of inflexion, are absent in the illuminated characteristics. At larger voltages the forward dark current intersects the illuminated current and the illuminated current becomes larger than the dark current. 

In addition to time-of-flight techniques, other methods based on current measurements have been described. Space-charge-limited current techniques have been reported by Lampert and Mark (1970), Mort and Emerald (1974), Nespurek and Silinsh (1976), Sworakowski and Nespurek (1979), Silinsh... 

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... 

---