Microscopic Description of Elementary Growth Processes and Classification of Structural Defects in Pentacene Thin Films

Abstract

Elementary growth processes such as kink initiation, adding a molecule to a kink, and adding a molecule between two neighboring kinks and between two grains are theoretically studied in pentacene films by adding one molecule at a time to a predefined aggregate. For each molecule, the potential energy surface is calculated using the MM3 molecular mechanics force field, which allowed one to identify useful parameters like the energy barrier for diffusion and the energy to create kinks, as well as defect configurations. Depending on the properties of the potential energy surface and the resulting growth-condition-dependent probabilities of initiating defect configurations in the film, three types of pentacene defects are identified: a thermally activated defect, an intrinsic defect, and a kinetic defect. Upon film growth, most defects relax into the ideal crystal configuration. Bulk defects that resist relaxation have densities lower than 10(16) defects/cm3 at typical growth conditions. Grain boundary defects, on the other hand, are very stable. Moreover, interstitial molecules at grain boundaries are identified as a source of compressive stress.

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

“…The relaxation happens, provided that the evaporation rate is low enough and that there is enough time for relaxation. [34] In this study it is suggested that structural defects within the grains of a pentacene film that resist relaxation cannot exceed densities of 10 16 cm −3 , at typical growth conditions. A structural defect can, however, influence the electronic levels of 10 surrounding molecules even if these molecules are in the perfect crystal configuration.…”

Nucleation and growth of thin films of rod-like conjugated molecules

“…The molecules can easily diffuse along the [001] direction guided by the oxygen rows. Analysing the width of the area in the trenches which is depleted from the small crystallites, one arrives at a ratio for the anisotropy between the diffusion along [001] and [110] of 4 to 64 [145]. This is clearly a property of the TiO 2 {110}-(1×1) substrate surface since second and third layer islands show an isotropic shape.…”

Effect of impurities on pentacene island nucleation

“…Extensive studies of the initial stages of pentacene film growth 3,[10][11][12][13][14][15] have shown that it follows the classical picture of nucleation, island growth, aggregation and coalescence that was developed for the growth of inorganic films. [16][17][18][19][20] In later stages of growth, the two-dimensional domains formed from island coalescence serve as the basis for three-dimensional growth due to an Ehrlich-Schwoebel energy barrier that slows diffusion from higher to lower layers of the film 5,7,21 .…”

“…7 Several mechanisms by which impurities could be incorporated into the pentacene film and limit transport have been proposed in the literature, including changes in chemical bonds, disruption of the crystalline structure within a grain, and impurity accumulation at the Published as PRB 77, 205328 (2008) 5 grain boundaries. 7,15,[26][27][28] In the following, careful analysis of the growth mode changes due to the PnQ is used to help understand where the PnQ resides in the thin films, and thus clarify the mechanism by which PnQ reduces the mobility of the Pn.Traditionally, nucleation studies have characterized the evolving submonolayer growth in terms of the island-size distribution (ISD), which under general circumstances has a coverage-insensitive form dependent only on the ratio of the island size to its mean.Another metric monitors the distribution of capture zones (CZ). 2,17,29 These CZs are essentially the proximity (Wigner-Seitz) cells of the islands: the CZ is the number of sites (times the area associated with each) that are closer to the enclosed island than to any other island.…”

“…7 Several mechanisms by which impurities could be incorporated into the pentacene film and limit transport have been proposed in the literature, including changes in chemical bonds, disruption of the crystalline structure within a grain, and impurity accumulation at the Published as PRB 77, 205328 (2008) 5 grain boundaries. 7,15,[26][27][28] In the following, careful analysis of the growth mode changes due to the PnQ is used to help understand where the PnQ resides in the thin films, and thus clarify the mechanism by which PnQ reduces the mobility of the Pn.…”