Mobility measurement in organic charge transport materials by charge-retraction time-of-flight (2008)

In both organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), the mobility of charge carriers in the various organic layers is a key parameter. A number of methods have been developed to measure this mobility, each with their own advantages and disadvantages. In the transient space-charge-limited current (SCLC) method, the sample is subjected to a rectangular voltage pulse, and the resulting current exhibits a characteristic peak "cusp" at a space-charge perturbed time-of-flight (TOF) before settling down to a steady-state level. Both the TOF and the steady-state level can be used to determine the bulk mobility. The SCLC technique, however, requires that the injecting contact be ohmic, a requirement that is rarely met. Transient electroluminescence is another technique, and one that lessens the demand on these injecting contacts somewhat. In transient electroluminescence measurements, a voltage pulse is applied to a suitably structured OLED, and the onset of light emission is evaluated for the TOF; however, analysis is complicated by the kinetics of supplying carriers of the opposite sign. In these three methods, it is difficult to reliably extract any information beyond the effective mobility of the faster charge carriers. Charge extraction in a linearly increasing voltage is principally applicable to samples with appreciable conductivity in the dark, but has begun to be combined with photoexcitation. While all these methods remain in use, the most popular technique for studying charge carrier mobility in organic materials is photocurrent time-of-flight. Here a thin layer of a sample is photo-excited by a flash of light, often a pulsed laser, and the resultant charges are swept across the sample by an applied electric field. Due to the penetration depth of the light, charges are generated well within the sample layer, often necessitating samples of several microns in thickness. Fabricating such thick samples can be materials intensive, and it is challenging to keep the thickness and morphology uniform. In addition, such thick films are potentially unrepresentative of the layers used in working devices, which are typically one-tenth to one-hundredth as thick. In this presentation, a relatively new method for mobility measurement will be discussed. Charge-retraction time-of-flight (CR-TOF) involves the injection, blocking, accumulation, and retraction of charge carriers to realize a transient functionally equivalent to those obtained by a traditional photocurrent TOF experiment, giving information about both charge carriers. It has been validated by measurement of the hole mobility of two well-known compounds, 4,4′,4″-tris-[N-(3-methylphenyl)-N-phenylamino]triphenylamine and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl, as well as preliminary measurement of the electron mobility of 4,7-biphenyl-phenanthroline, with appropriate blocking layers. With this technique, a sample layer thickness of less than 300 nm can be used because of the thinness of the charge sheet accumulated at the blocking layer present. The availability of this blocking layer is the primary constraint for the method; an excellent one has been found for blocking holes, while a feasible one has been found for electrons. Two additional points of interest have emerged from developing this method. First, the arbitrariness associated with evaluating the transition voltage at organic-organic interfaces was addressed by examining the excess charges collected as a function of charging voltage. Second, by changing the charging time, charges could be retracted while in transit to the blocking layer, thus providing opportunities to study their distribution and possibly trapping or leakage at the blocking interface.