Mobility Measurements-Hole-Only

Beside the excellent optical properties and suitable HOMO-LUMO energy levels, the PFs possess great charge-transport properties. Time-of-flight (TOF) measurements of PFO showed nondispersive hole transport with a room temperature mobility of holes of fi+ = 4 x 10-4 cm2/(V s) at a field of li 5 x 105 V/cm that is about one order of magnitude higher than that in PPV [259]. The polymer revealed only a weak-field dependence of the mobility, from /r+ = 3 x 1(U4 cm2/(V s) at E= 4 x 104 V/cm to /r+ = 4.2 x 1(V4 cm2/(V s) at E= 8 x 105 V/cm. 

Several experimental J-V curves taken from the recent literature have been compared with the PFE model [39], We quote a few typical examples here. Extensive measurements of J(V) curves have been made by Crone et al. [59], They used both hole only Au/MEH-PPV/Al and electron only Ca/MEH-PPV/Ca devices. Crone et al. [59] used the mobility model to interpret their results. Comparison of their results for the electron devices with the Field Dependent Trap Occupancy (FDTO) model is shown in Fig. 3.33(a). The thicknesses of the samples are 25 nm (circles), 60 nm (diamonds) and 100 nm (plus symbols). The parameters used in calculations are Tc = 1420 K, //h = 7 x 1018 cm-3, ]Vt = 5x 1019 cm-3, JV, = 1 x 1020 cm-3, p, = 6.5 x 10-6 cm2 V-1 s 1 and e = 3. The agreement of the experimental results is satisfactory. The model has another advantage. The calculations have been made with the same parameters for all the three samples. With the mobility model, Crone et al. [59] had to use different values of the parameters for different samples. 

Borsenberger et al. (1995a) measured hole mobilities of TTA doped polymers with polymers with different dipole moments poly(styrene) (PS-1), poly(4-f-butylstyrene) (PS-2), poly(4-chlorostyrene) (PS-3), and bisphenol-A polycaibonate (PC). The dipole moments of PS-1 and PS-2 are near zero. For PC and PS-3, the values are 1.0 and 1.7 Debye. The dipole moment of TTA is 0.8 Debye. Figure 55 shows a series of photocurrent transients at different temperatures for 30% TTA doped PS-1 at 6.4 x 105 V/cm. W increases with decreasing temperature. For fields between 104 and a few multiples of 1CP v/cm, W increases with increasing field. Values of W were between 0.25 and 0.62. In all cases, W increases with decreasing TTA concentratioa Low-temperature transitions from nondispersive to dispersive transport, where the transients no longer show plateaus, were observed only at low concentrations. 

Crisa (1983) measured hole. mobilities of a mixture of 2,5-bis(4-diethylaminophenyl)-l,3,4-oxadiazole and a polyester. The time, thickness, and field dependencies of the photocurrent transients agreed with predictions of the Scher-Montroll model (1975). According to the model, the transit time scales with thickness and field as (L/E)l/a. The experimental value of a was 0.80. The study of Crisa, and later work by Bos and Burland (1987), are the only literature references to polymers where the scaling relationships predicted by Scher and Montroll have been observed over a range of thicknesses and fields. 

Transference numbers have been introduced because they can be determined experimentally. On the other hand, the individual mobilities cannot be determined independently as in the case of electrons and holes in a semiconductor conductivity measurements yield only the sum of the cation and anion mobilities (see Eq. 3.1). Accordingly, the mobilities can be evaluated from measurements of the conductivity and the corresponding transference numbers. There are various methods for measuring transference numbers which are not described here (for details, see for example ref. [2]). Since both A and t-, depend on the concentration of the ions, data are usually evaluated from measurements in very dilute solutions because of interactions between ions. According to many investigations, the transference numbers of most ions are not far from = 0.5 i.e. the mobilities of cations and anions of dissolved salts are about the same. Large t-, val-... 

In conclusion, not only the observed larger scale of phase separation but also the difference in the material s phase percolation and thus charge transport properties influence the photovoltaic performance. As such, it becomes evident that the charge carrier mobility measured in these devices must be a function of the blend morphology [139-143]. Furthermore, the electron and hole carrier mobilities depend strongly on the polymer-fullerene blending ratio. Interestingly, the hole mobility of the donor polymer is increased considerably in blends with fullerenes (see Fig. 27) [142,144-147]. 

In contrast to the case of Se, the hole transit pulse in As2Se3 shows a long tail and there is often no discontinuity in the pulse discernable. Scharfe (1970) observed a break in the transient pulse only when he used Au contacts with evaporated As Seg. Al contacts in contrast yielded a transit pulse decaying without structure. Using Au contacts and taking the breakpoint as a measure of the transit time T Scharfe obtained a drift mobility for holes which increases linearly with field. A hole drift mobility of 4 X... 

It should be noted that previous studies have shown that when using gold source and drain electrodes, hole injection and transport can take place in [60]PCBM [79]. However, in this case the mobility is significantly lower than that of holes in P3HT [82], for example, and hence one can assume that hole field-effect mobility measurements made on a 1 1 (wt%) P3HT [60]PCBM blend should be representative of the P3HT network only, with a negligible modification due to the [60]PCBM network. The same can also be said of most other p-type polymers. 

Once values for R , Rp, and AEg are calculated at a given strain, the np product is extracted and individual values for n and p are determined from Eq. (4.19). The conductivity can then be calculated from eq. (4.18) after the mobilities are calculated. The hole mobility is the principal uncertainty since it has only been measured at small strains. In order to fit data obtained from elastic shock-loading experiments, a hole-mobility cutoff ratio is used as a parameter along with an unknown shear deformation potential. A best fit is then determined from the data for the cutoff ratio and the deformation potential. 

It has been established from conductivity measurements that thermally activated and field-assisted hole hopping is responsible for the charge transport in solid polysilanes [48,49]. The mobility of the hole is as high as 10 m /V sec, while the mobility of the electron is a few orders of magnitude lower. In this section, we will show the reason why only the hole is mobile in polysilanes and how we can construct electron-conductive polysilanes. 

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