(418ai) Determining the Effects of Unit Cell Parameters and Solvent Size on the Energy Profile of Organic Semiconducting Crystals

Determining the Effects of Unit Cell Parameters and Solvent Size on the Energy Profile of Organic Semiconducting Crystals

Kristina Lenn, Paulette Clancy Cornell University

Gaurav Giri, Ying Diao and Zhenan Bao Stanford University

Organic semiconducting materials are gaining popularity as solar cell materials, in large part because of their ability to be processed onto flexible substrates and to be manufactured more cheaply. However, due to their “charge-hopping”-like mechanism, organic materials are not nearly as efficient as silicon in regards to charge transport. To circumvent this, the Bao research group at Stanford has initiated a new process called solution shearing on 6,13-bis(triisopropylsilylethynyl) pentacene (6,13-TIPS pentacene) that significantly reduces the pi-orbital overlap, thereby enhancing the charge transport by an 80% increase.

To fully appreciate what is happening at an atomic level and how shearing the unit cell to conform to a configuration that is both energetically favorable and conducive to charge transport, an atomically explicit model of molecules of 6,13-TIPS pentacene were analyzed via ab initio and Molecular Dynamics (MD) techniques. In vacuo, the parameters of the unit cell [a,b,γ] were varied and subjected to minimization calculations via the MD package TINKER. From these minimizations, an energy profile was generated which allowed us to identify the five lowest energy configurations which we found were well aligned with five polymorphs determined experimentally by the Bao group.

Closer examination of the molecular configurations suggested that the fluctuations in the energy profile were due to the bending and twisting of the acene backbone, the wagging of the silylethynyl groups, and the rotating of the methyl groups. Compared to an equilibrium structure, there is a 4.5° deviation in the acene backbone and a nearly 10° difference in the silylethynyl angle across this energy landscape. However, when comparing the changes in these different angles against the energy profile, the minima for both are closely aligned.

6,13-TIPS pentacene was also studied in ten different solvents, ranging in size from tetrahydrofuran (THF) at 3.18 Å to decalin at 3.94 Å. A Lennard-Jones atom was used to represent these solvents, varying only in size. The atom was placed within the unit cell in an energetically favorable location and submitted to energy minimization calculations. From these energies, it can be seen that as the solvent size increases, the stability of the unit cell decreases.

Based on the data gathered, the energy profile of 6,13-TIPS pentacene is significantly affected by the rotating, wagging, and bending of the various side groups and the acene backbone. Whether these deviations occur in sync or are a result of a “domino effect” (i.e., the rotation of the methyl groups leads to the wagging of the silylethynyl groups which in turn leads to the bend in the backbone) is not yet clear. The size of the solvent also impacts the energy profile; as the solvent radius increases, the molecules in the unit cell can be seen to almost push at the solvent in an effort to expel it from the unit cell.