A chemical approach to the orbitals of organic polymers

Abstract

The electronic structure of the prototypical unsaturated and saturated organic polymers polyacetylene and polyethylene is elucidated by a building-up process starting from a linear carbon chain, which is then kinked and finally has one or two hydrogen atoms attached to it. In this process the common features as well as the differences between polyacetylene and polyethylene (semiconductor vs large band gap insulator, unsaturation vs saturation) emerge in a natural way. The emphasis is on bringing together the essential concepts of solid-state physics and simple ideas of chemical bonding. Some topics of special interest, such as bond alternation and solitons in polyacetylene, are also discussed, in as simple and chemical manner as possible.

Impact of molecular weight on charge carrier dissociation in solar cells from a polyfluorene derivative

“…For these relatively thin active layers, using a constant generation rate G max throughout the active layer does not change the simulation results compared to the use of the calculated optical profile as input [34]. The slightly higher V oc of the low MW cell originates from the inversely proportional relationship between the polymer band gap and the average number of repeating units in each chain [35]. At short-circuit conditions, the average dissociation probability of bound electron-hole pairs in the high MW cell is 19% higher (relative) than in the low MW device, due to the reduction in k f .…”

Design of low band gap polymers employing density functional theory?hybrid functionals ameliorate band gap problem

“…Although previous studies agree that the band gap problem is due to the derivative discontinuity in the exchange correlation functional, they come to different conclusions about whether the error is caused by too-high LUMO energies or by problems with HOMO and LUMO energies. Levy et al 2 found that the center of the DFT band gap is given correctly. Thus HOMO and LUMO have to be too high or too low by the same amount.…”

Polaron hopping mediated by nuclear tunnelling in semiconducting polymers at high carrier density

“…The semiconducting properties in conjugated polymers stem from the covalent interactions between the monomers on the backbone of the p-conjugated polymer 1 . Although polymer chains provide a conjugated path for charge transport, real devices offer only limited charge mobility due to the presence of disorder, which disrupts the p-conjugation, and due to polarization of the surrounding medium, which augments the effective mass of the charge carriers.…”