Disorder in Charge Transport in doped polymers

Abstract

Charge transport in molecular doped polymers has important applications in xerography, as well as being a theoretical challenge. Its investigation by determination of transient photocurrents is reviewed, with experiments and pertinent theories, such as hopping in a Gaussian density of states and the small polaron model, being outlined. The temporal features of the photocurrents, the field and temperature dependence of the charge carrier mobilities, and the influence of molecular properties—and what can be inferred from these—are discussed.

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…The origin of the field-dependent mobility in many organic materials has been attributed to the electric field reducing the activation energy for charges hopping to neighboring sites in the field direction. [26][27][28] Figures 2 and 3 therefore provide an indication that the low MW might be more disordered and have broader density of states leading to stronger field dependence of mobility for low MW polymers. 29 In principle, one might expect an increase in mobility with reducing channel length as the channel length approaches the length scale of the polymer microcrystals ͑see below͒, i.e., charges need to cross fewer boundaries between microcrystals on their way from source to drain electrodes.…”

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

“…14,15 Here we show that the presence of large molecular dipole moments can be beneficial rather than disadvantageous for promoting charge hopping. Electrostatic attraction between the large fixed internal dipoles in these molecules leads to formation of centrosymmetric dimers with cofacial π-π antiparallel alignment along the long molecular axis.…”

Hopping conduction in strong electric fields: Negative differential conductivity

“…This regime succeeds the very strong increase of the mobility with rising field, 15,16 and does not precede it at lower fields as claimed on the basis of the drift equations. 3,4,5,6,7,8,9,10 The decreasing conductivity with increasing electric field at high field strengths has been observed experimentally for hopping transport in lightly doped and weakly compensated crystalline silicon. 17,18,19 The hopping transport mode in such systems at low electric fields had been described theoretically in all detail, 20 which made these systems particularly attractive for studying the new non-Ohmic effects.…”

“…The discussion was, to much extent, provoked by the reports on the apparent decrease of the drift mobility with rising electric field at relatively high temperatures and low field strengths in disordered organic materials. 3,4,5,6,7,8,9,10 This apparent decrease of the mobility with increasing electric field was reported to be succeeded by the increase of the mobility at higher field strengths. However, the self-consistent effective-medium theory for drift and diffusion at low electric fields 11 does not show any decrease of the mobility with increasing field.…”