Hole Transport in Diketopyrrolopyrrole (DPP) Small Molecules: A Joint Theoretical and Experimental Study

Abstract

Charge transport in 3,6-bis[5-(benzofuran-2-yl)-thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo [3,4-c]pyrrole-1,4-dione [DPP(TBFu) 2 ] small molecules, a model donor material for organic photovoltaics, is studied by combined first-principles simulations and experiments. The dependence of the hole mobility on electric field, temperature, and molecular packing is examined in detail. Phononassisted hole transition rates and energetic disorder are identified as key intrinsic factors that control the hole mobility in DPP small molecules, with the latter playing a more important role than the former. In order to explain the substantial mobility differences between the as-cast and annealed films, we examine the effect of energetic traps on charge transport. For a trapping energy of 0.4 eV and a trap density of 1.5 × 10 17 /cm 3 , we find that 98% of the carriers are trapped and the mobility drops by 2 orders of magnitude. With trap densities of 4.5 × 10 18 /cm 3 and 1.5 × 10 17 /cm 3 and trapping energy of 0.4 eV, we can reproduce the experimental mobilities for the as-cast and annealed films, respectively. Finally, we speculate on possible molecular defects that could act as traps for the holes.

Trends in molecular design strategies for ambient stable n-channel organic field effect transistors

“…This goal has not been achieved yet because apart from computing the intrinsic electronic properties of the material this requires predicting the crystal structures, 89,90 accounting for anisotropy, 91 including molecular vibrations, 92 and correctly assessing the amount of structural and dynamic disorder. Research on disorder is picking up momentum [93][94][95][96][97] but the majority of studies are concerned with the intrinsic electronic properties [98][99][100][101][102] of idealized materials. Coherent transport in highly ordered materials can be calculated by applying the Landauer formula 103 or various other flavors of scattering theory.…”

A comparative analysis of symmetric diketopyrrolopyrrole‐cored small conjugated molecules with aromatic flanks: From geometry to charge transport

“…Here, we can mention only a few theoretical works devoted to structural, electronic and CT properties of DPP with thiophene or furan flanks . As an auxiliary method, the modeling usually supports the experiment with a “standard analysis” of conjugated backbones such as ground state geometries, frontier molecular orbitals, optical and electronic band gaps, torsion potentials, and less often excited states .…”

“…For instance, Wang et al have investigated a series of conjugated dyes based on DPP with different aromatic flanks using density functional theory (DFT) and time‐dependent‐DFT to evaluate the properties and to improve the solar cell performance of the materials under study. Li et al have carried out a combined theoretical and experimental research to clarify the role of the traps in DPP based materials. Gao et al have analyzed the intrachain carrier motion in DPP‐Th based oligomers using nonadiabatic molecular dynamics coupled with density functional tight binding theory.…”

Computational study of structure, electronic, and microscopic charge transport properties of small conjugated diketopyrrolopyrrole‐thiophene molecules

“…Since such factors as electronic structure, molecular or polymer chain's packing, presence of defects, etc. strongly influence the charge transport in conjugated systems and consequently the performance of electronic devices, understanding of structure–property relationships is of most importance for further development of highly efficient materials.…”