Comparing the electron and hole mobilities in the α and β phases of perylene: role of π-stacking

Abstract

Computational studies are carried out on a and b polymorphs of perylene molecular crystals to estimate their hole and electron mobilities (m hole and m electron ). For both the polymorphs, it is found that the LUMO bandwidhs are larger than the HOMO bandwidth resulting in larger electron conductance, as observed experimentally and which has remained unexplained until now. For a microscopic understanding, we perform quantum chemical calculations for the hole and electron transfer matrix elements (H mn hole and H mn electron ) on some selected unique nearest neighbour pairs of molecules in the crystal geometries. This along with calculations for the hole and electron reorganization energies (l hole and l electron ) within the embedded cluster method reveal that for both the crystals, m electron exceeds m hole by two orders of magnitude. The electron mobility for the a-phase (m electron = 67.2 cm 2 V 21 s 21 ) is found to be three times that for the b-phase. The major driving force for preferential electron conductance in a-perylene is the slipped parallel p-stacking arrangement of the molecules at short intermolecular distances (d = 3.9 A ˚) in the crystal. We suggest that experimental strategies that further enhance the percentage of such specific p-stacking dimers in molecular assemblies have the potential to further increase m electron . The present theoretical calculations provide a unified understanding of the parameters that optimize an organic crystal for enhanced electron and hole mobilities.

Theoretical study on the effects of nitrogen and methyl substitution on tris-(8-hydroxyquinoline) aluminum: An efficient exciton blocking layer for organic photovoltaic cells

“…[21][22][23][24][25][26] For each calculation, the Becke-style 3-parameter exchange and Lee-Yang-Parr correlation (B3LYP) hybrid functional was used in the calculation of single molecular properties including geometry optimization, vibrational frequency and λ calculation 27, 28 since it has described molecular properties quite well. We chose meridianal form of Alq 3 and its derivatives because meridianal Alq 3 is energetically more stable than facial form 29,30 and thus real Alq 3 system mostly consists of meridianal form.…”

Single-crystal field-effect transistors of new Cl2-NDI polymorph processed by sublimation in air

“…Although at any given temperature and pressure, only one polymorph corresponds to the minimum in free energy and can be regarded as the thermodynamically stable phase, it is very challenging to clarify the phase diagram because of the high activation barriers encountered in the structural transformation of molecular crystals 15,19,[22][23][24][25] . On the other hand, besides temperature, the solvent also plays an explicit role in the crystal nucleation and growth dynamics in solution and therefore many crystals formed in solution correspond to metastable polymorphs [9][10][11][12][13][14][15][18][19][20][21][22][23][24][25][26][27][28][29] . For Cl 2 -NDI, regardless of the utilization of pure or mixed solvents (CHCl 3 , CH 2 Cl 2 , chlorobenzene, CH 3 OH, THF, hexane), the crystals grown from solution are always colourless ribbon-shaped crystals, whose packing arrangement was confirmed as a-phase by XRD ( Supplementary Fig.…”

“…As the morphology of organic semiconducting thin films on a dielectric layer has a crucial impact on the charge transport properties and determines the device performance, for the demand of high device performance, many studies have been devoted to manufacture highly ordered crystalline films, which has been effectively solved by interface engineering and the improvement of fabrication processes, for examples, by introduction of self-assembled monolayers, annealing and inkjet printing technology [4][5][6][7][8] . However, despite the well-known fact that polymorphism in thin films is not easy to avoid and different phases have a great impact on the charge carrier transport properties, only for few organic semiconductors, detailed studies on crystalline polymorphs investigated in organic thin-film transistors (OTFTs) are available [9][10][11][12][13][14][15] . The reason might be that individual pure single crystalline phases are difficult to prepare in thin films and challenging analytics are needed to distinguish coexisting polymorphs in various domain structures, not to mention the impact of grain boundaries between domains of different phases on the charge carrier transport.…”

Electronic Polarization in Organic Crystals: A Comparative Study of Induced Dipoles and Intramolecular Charge Redistribution Schemes

“…Our results show a decreasing trend of PEs with the number of aromatic rings (exception made for ME 0 results reaching a maximum for A5), with larger values for holes than for electrons. Figure 10 and Table 4 also report available experimental data, 19,79 from which we subtracted the specific intramolecular reorganization energies, 65,66 in order to compare the electronic contribution to PE.…”