Transient Photoconductivity in Poly(N-vinylcarbazole)

Abstract

Transport of holes through thin films of poly(N-vinylcarbazole) was studied using transient photoconductivity techniques. The photogeneration depends on the electric field with quantum efficiency of generation approaching a value of 0.1 at an applied electric field of 108 V/m. The motion of carriers cannot be characterized by a drift mobility but can be fully represented by an “effective transit time” given by (Teff)−1 = (mτ0)−1exp(γE1/2 − ε0 / kT), where m = d / d0 (d being the thickness of the film and d0 a characteristic length), τ0 is a characteristic release time from a localized state, E is the applied electric field, ε0 is an activation energy, k is the Boltzmann constant, and T is the absolute temperature. This relationship suggests a model in which holes jump from one localized state to the next one, the “effective transit time” being the sum of release times from the localized states. Bulk trapping occurs at low fields, and the release from these traps depends on both temperature and electric field.

Charge-carrier concentration dependence of the hopping mobility in organic materials with Gaussian disorder

“…Experimental studies of the mobility in systems containing conjugated small molecules in an insulating polymer matrix, as a function of the molecule concentration, have revealed values of ␣ −1 around 0.1 nm. 10 The site densities in congated polymers such as OC 1 C 10 -PPV are of the order N t Ϸ 10 26 to 10 27 m −3 , corresponding to average nearest neighbor intersite distances around 1 nm. A representative value of the reduced site density is thus N t / ␣ 3 ϳ 10 −3 .…”

“…The mobility can then be expressed as ϰ exp͓−E A / ͑k B T͔͒, with E A an effective activation energy that is of the order of a polaron binding energy. 3,10,11 Further doubts on the applicability of this so-called Gaussian disorder model were raised by the observation that the field-effect ͑hole͒ mobilities in polymer FETs are typically at least one order of magnitude larger than the hole mobility in LEDs based on the same material. Recently, Tanase and co-workers showed that this can be explained as a result of a dependence of the mobility on the hole concentration.…”

Effect of Gaussian disorder on the voltage dependence of the current density in sandwich-type devices based on organic semiconductors

“…12͒. For organic semiconducting materials, the average intersite distance a = N t −1/3 and the wave-function localization length are typically of the order of 1 and 0.1 nm, respectively, [24][25][26] so that ␣ −1 Ϸ 0.1 ϫ a. The energy levels at neighboring sites are assumed to be uncorrelated.…”

Device physics of single layer organic light-emitting diodes

“…This form for the mobility has been seen in time of flight measurements on conjugated organic materials, and has been derived theoretically. [13][14][15][16][17][18][19] It has been directly seen in time of flight measurements of MEH-PPV. 20 The device fabrication has been described previously.…”