Theoretical Investigations On Structures And Charge Carrier Mobilities Of Fused Thiophene And Nitrogen Heterocycles As Organic Semiconductors

Recent years, organic semiconductors have attracted much attention due to their low cost, light weight, ease of processing, versatility of chemical synthesis and flexibility. Among the organic semiconductors investigated, some organic p-type semiconductors have achieved a mobility beyond 10 cm2·V-1·s-1. However, the development of the n-type semiconductors significantly lags behind the p-type analogs, which is mainly ascribed to the intrinsic instability of n-type organic materials under air conditions. As a consequence, the search for the high-performance and ambient-stable n-type and/or ambipolar organic materials is a crucial challenge. In this paper, the crystal structures of several fused thiophene and nitrogen-containing heteraromatic compounds were predicted by the Monte Carlo-simulated annealing method with the embedded electrostatic potential charges. The crystal structures were further optimized by the density functional theory with dispersion force being included (DFT-D), and charge carrier transport properties were investigated by the incoherent charge-hopping model. We hope that our work could be useful to understand how the interplaying role of various factors affects the charge transport property and provide some important information for designing high performance semiconductors to promote the development of organic electronics. Our work mainly includes the following seven sections:1. A novel crystal structure of octaseleno[8]circulene (C]6Se8, we named it selflower) was predicted on the basis of sym-tetraselenatetrathio[8]circulene crystal (C]6S4Se4, selenosulflower). The charge transport properties of selenosulflower and its selenium analogue of selflowe as potential ambipolar materials were investigated by DFT coupled with the incoherent charge-hopping model. Insights into their geometric and electronic structures, frontier molecular orbitals, reorganization energies and transfer integrals, anisotropic mobilities as well as band structures of the two novel materials were provided in detail. The gap of the frontier molecular orbitals decreases when all sulfur atoms of C16S4Se4 are substituted by selenium, which improves the charge transfer efficiency. The predicated hole and electron mobilities of C16Se8 are 1.03 and 1.26 cm2·V-1·s-1, respectively. C16S4Se4 has hole mobility of 0.49 cm2·V-1·s-1 and electron mobility of 0.74 cm2·V-1·s-1. Both circulenes exhibit electron-dominated ambipolar performance. The small reorganization energy and larger transfer integral originated from the face to face Ï€-Ï€ stacking lead to large charge mobility for the novel compound C16Se8. The newly designed "selflower" C16Se8 is a novel organic semiconductor and worthy to synthesize.2. The charge transport properties of the six fused thiophene derivatives 2,6-diphenylbisthieno[3,2-b:2’,3’-d]thiophene (DP-DTT) 6,6’-diphenyl-2,2’-bibisthieno[3,2-b:2’,3’-d]thiophene (DP-BDTT), 2-(pentafluorophenyl)-6-phenylbisthieno[3,2-b:2’,3’-d]thiophene (FPP-DTT), 6,6’-bis(pentafluorophenyl)-2,2’-bibisthieno[3,2-b:2’,3’-d]thiophene (FPP-BDTT), 2,6-dipentafluorophenyl-bisthieno[3,2-b:2’,3’-d]thiophene (DFP-DTT) and 6,6’-dipentafluorophenyl-2,2’-bibisthieno[3,2-b:2’,3’-d]thiophene (DFP-BDTT) were explored by DFT coupled with the incoherent charge-hopping model at the molecular and crystal levels. The crystal structures of the titled compounds are either predicted by the dispersion-corrected density functional method (DFD-D) or retrieved from the Cambridge Crystallographic Database. Introducing the electron-withdrawing fluorine atoms to the end phenyl of the DTT and BDTT molecules can decrease the HOMO-LUMO gap, which is beneficial to the conductivity. The FPP-BDTT has the largest electron mobility among the six compounds because it has small electron reorganization energy and large transfer integral. The efficient overlaps of Ï€-orbital and smaller Ï€-Ï€ stacking distance are proved to be the main reason for its good hole transport property for DFP-DTT. FPP-BDTT and DFP-BDTT have shown remarkably anisotropic behaviors and the maximal charge mobilities are along a special crystal axis direction with strong Ï€-Ï€ interactions, which further confirms our finding that the fluorination effect may be an effective way to improve the charge mobilities.3. The crystal structures of known anthra-tetrathiophene (ATT) and its three fluorinated derivatives (ATT1, ATT2 and ATT3) were predicted by the Monte Carlo-simulated annealing method with the embedded electrostatic potential (ESP) charges and the most stable crystal structures were further optimized DFT-D method. The calculated results show that the introduction of fluorine atoms does not affect the molecular planarity but decreases the HOMO-LUMO gap, which is beneficial to electron injection and provides more charge carrier stabilization. The improved electron mobility from ATT to ATT3 is attributed to the favorable molecular packing with strong Ï€-Ï€ interaction and the short stacking distance. The band structures reveal that all the paths with larger transfer integrals are along the directions of large dispersions in the valence band (VB) and conduction band (CB). ATT3 has the largest electron mobility (0.48 cm2·V-1·s-1) among the four compounds, indicating that fluorination is an effective approach to improve electron transport.4. In order to probe the effects of substituents (F and CN) attached to benzo[1,2-b:3,4-b’:5,6-b"]tristhianaphthene (BTTP) on their charge carrier transport properties, we investigated the characteristics of molecular structures and charge transport properties of BTTP and its derivatives (BTTP1, BTTP2, BTTP3, BTTP4 and BTTP5). The investigation reveals that even a subtle change of geometrical structures may result in a great change of the reorganization energy. With increasing numbers of substituted fluorine atoms, the reorganization energy of the BTTP derivative increases, which is disadvantageous to electron transport. In contrast, the attachment of the electron-withdrawing cyano groups to BTTP decreases the reorganization energy and raises the electron affinity, which is beneficial to electron injection and charge carrier stabilization. The introduction of cyano groups also results in an enhancement of Ï€-Ï€- interaction and leads to an increase in the transfer integrals. Among the six compounds, the novel compound BTTP4 has the largest electron mobility (1.154 cm2·V-1·s-1) on account of its larger transfer integral and smaller reorganization energy, indicating that BTTP4 is a promising high-performance n-type organic semiconductor and worth to synthesize.5. The DFT and the incoherent charge-hopping model were employed to investigate the charge transport properties of 4,4’-bis(perfluorophenyl)-2,2’-bithiazole (PFBT) and its Br substituted derivative Br-PFBT at the molecular and crystal levels. It was found that the introduction of the electron-withdrawing bromine atoms to PFBT lowers the HOMO-LUMO energy gap, which is beneficial to the efficiency of charge transport. However, the Br destroys the coplanar feature of the Br-PFBT, which is unfavorable to the charge transport due to the large reorganization energy and small transfer integral. The transfer integral among the dominant hopping pathways showed that both electron and hole transport processes take place in the parallel dimers between neighboring molecules with Ï€-Ï€ stacking interactions. The simulation for the angle dependence of the charge mobility reveals that the electron transport in both molecular crystals is remarkably anisotropic. For PFBT, the electron mobility (0.084 cm2·V-1·s-1) of PFBT is slightly larger than that of Br-PFBT (0.034 cm2·V-1·s-1), indicating that the attachment of the electron-withdrawing bromine atoms to the thiazole ring of PFBT is not beneficial to the charge transport.6. Three novel alkoxypheny] N-substituted naphthalene bisimide derivatives, N,N’-bis(4-n-butoxyphenyl)-1,8:4,5-naphthalenetetracarboxylic (NBI1), N,N’-bis(4-n-hexyloxyphenyl)-1,8:4,5-naphthalenetetracarboxylic (NBI2), and N,N’-bis(4-n-octyloxyphenyl)-1,8:4,5-naphthalenetetracarboxylic (NBI3) as potential organic semiconductors, have been investigated by DFT calculations. The calculated results demonstrate that the low-lying and delocalized LUMOs and larger adiabatic electron affinities of these compounds are beneficial to their stability when acting as n-type organic semiconductors. The electron coupling among the dominant hopping pathways indicate that the charge transport processes happen in the parallel dimer of neighboring molecules with Ï€-Ï€ interaction. The calculated absorption spectra by the time-dependent density functional theory (TDDFT) revealed that the strongest absorption peaks in the visible region are assigned toÏ€â†'Ï€* transition and these peaks are regulated by the transitions of HOMO â†' LUMO. The calculated electron mobilities of NBI1, NBI2 and NBI3 are 0.0365,0.0312 and 0.0801 cm2·V-1·s-1, respectively.7. Electron mobilities of two pentacenquinone derivatives as n-type organic semiconductors,5,7,12,14-tetraaza-6,13-pentacenquinone (TAPQ5) and 1,4,8,11-tetraaza-6,13-pentacenquinone (TAPQ7), have been investigated at the molecular and crystal levels by means of DFT calculations. It was found that the two compounds have lower LUMO energy levels, which can easily function as n-type organic semiconductors. The small reorganization energy and the large transfer integral for electron transport of TAPQ7 suggested that it has relatively high electron mobility (1.03 cm2·V-1·s-1). The transfer integral among the dominant hopping pathways for TAPQ7 showed that the electron transport processes occur in parallel dimers between two neighboring molecules with Ï€-stacking interactions. The analysis of angular-resolution anisotropic mobilities for TAPQ5 and TAPQ7 shows that it is helpful to control the orientations of the conducting channels for a better charge transport efficiency.