(608d) Perylene-Thiophene Based Donor-Acceptor Oligomers for Molecular Scale Photovoltaics

Organic photovoltaic solar cells represent a cheap and lightweight alternative energy source that hold promise for solving the world's energy needs. The highest efficiency devices to date are heterojunction solar cells, in which donor (D) and acceptor (A) semiconductors are blended together. The efficiencies of the devices are strongly dependent on the properties of the interfaces on a scale of 5-10 nm, where excitons are created. Control of this length scale and the interface morphology has proven to be quite difficult. D-A oligomers provide an elegant route to achieve this goal by variation of several tunable parameters, such as the length of oligomer blocks, the functionality of each block, and the bridging moieties. We have prepared a series of perylene-thiophene-based oligomers to study the relationships between molecular structure, thin film morphology, and device performance.

We started with one perylene molecule linked to a thiophene dimer or trimer by a double bond. This p-n junction molecule shows high thermal stability and can be transferred to different substrates by gas phase evaporation methods. With different R groups at one terminal end of the perylene, we observed disordered, short range ordered, and highly ordered structures on an Au (111) surface by STM studies. Grazing-angle incidence X-ray diffraction (GIXD) has also been applied to understand the three dimensional structures of the films. This structural information provides significant understanding of the thin film morphology, and should help in the optimization of device performance.

In addition, a series of p-n junction oligomers have been prepared. In this type of molecular assembly, perylene-naphthalene (A) is connected by different bridging units to hexathiophene (D). These molecules are designed to help answer some fundamental questions about the structural requirements of charge separations and recombination: What is the minimum length to achieve molecular dipolar character? What is the function of various bridges? Does the configuration of the bridge affect the morphology of the thin film? The synthesis of these molecules will be discussed, along with the electrical and optical properties of the new materials, in both solution and the solid state.