DISCLC Measurement

DISCLC is a well-known technique for measuring carrier mobility in organic materials [47-49]. Moreover, DISCLC is also used to examine the condition of the injection interface [50]. The DISCLC experimental setup is depicted in Table 3.2. The structure of the DISCLC sample is the same as TOF. An organic layer is sandwiched by two electrodes metal as the cathode and transparent ITO as the anode. The thickness of the organic layer can vary between a few hundred nanometers to several microns. However, at lower thicknesses, the RC decay can dominate the current transient (C°=l/d), making the determination of the transit time more difficult. 

Organic Electronics Materials, Processing, Devices and Applications 

An ideal DISCLC transient current, is the arrival time of the leading front carrier. and JscL are the instantaneous current, the current when f=Xdi and the space-charge-limited current, respectively. (From Tse, S.C. et al., /. Appl. Phys., 100,063708,2006. With permission.) 

Second, the ratio /di//o should be 2.72. However, in the real case, the transient current consists of a dark-injection current and a RC response of the circuit. The latter comes from the inevitable fact that the sample itself acts as a capacitor and its needs a finite time to charge up. Thus Jg is overestimated due to the effect of RC response, and therefore the ratio Joi/Jo is always smaller than the ideal value. Third, another ratio /di//scl should be 1.21 [41,51]. Departure from the ideal value of 1.21 may indicate that the injecting contact deviates from ohmic injection. Alternatively, if the charge injection remains ohmic, deviation of this ratio could also be a consequence of nonnegligible charge diffusion or field-dependent mobilities. A critical examination of this ratio under the condition of ohmic injection has been reported elsewhere [52]. 

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Several experimental techniques are used to investigate the carrier injection and transport in organic solids. TOP technique, current-voltage (JV) measurement, AS measurement, and DISCLC measurement are used and cross-compared. Table 3.2 shows a summary of fhese fechniques. 

MTDATA, NPB, and TPD samples were 0.76, 4.11, and 5.13 pm, respectively. After coating, the samples were immediately loaded inside a vacuum cryostat with a pressure less than 10 torr for V, DISCLC, and AS measurements. 

Besides V and DISCLC, AS measurement can also be used to probe the contact conditions between PDEDOTPSS and PA compounds. Figure 3.21... 

The hole injection properties of PA-based compounds were investigated and cross-compared by V, DISCLC, and AS measurements. We demonstrate that PEDOTPSS can act as an excellent hole injection material for PA compounds... 

Applications of PEDOT PSS anode in D1 experimenfs reveal currenf fran-sients with a cusp in all PA compounds. Meanwhile, a well-defined minimum can be observed from the plot of frequency-dependenf capacifance in AS measurement. Thus, the results from DISCLC and AS are entirely consisfent with the V results, which by themselves indicate that the contact between PEDOTPSS and PA compounds is very close to ohmic. The hole mobilities of MTDATA, NPB, and TPD could also be defermined from DISCLC and AS techniques. Those mobilities are in good agreement with the TOP measurement. In addition, both injection efficiency (q) and mobilities ratio are reduced due to the injection barrier increase, and indicate that the hole current of TPD still exhibits the injection-limited behavior. 

The DISCLC method shows several disadvantages. The ratio iDi/to should he ideally 2.72 hut in fact the transient current consists not only of the dark injection current but also the RC decay. Therefore, to is overestimated and then the ratio toi/to is underestimated. Also the measured ratio of tDi/tscL deviates often from an ideal value 1.21 which may indicate that the injecting contacts are not ohmic or that mobility is field-dependent or that there is nonnegligible charge diffusion. 

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