Single-Molecule Spectroscopic Studies of Conjugated Polymers

A semiconductor is a type of material which has electrical properties in between an insulator and a conductor. A conductor can be thought of as a group of atoms surrounded by a sea of free electrons, which are free to move throughout the material; in contrast, the electrons in an insulator are tightly localized on a specific atom or molecule. In a semiconductor, the electrons are localized as in insulators; however, with the addition of energy, the electrons in semiconductors are easily excited into a state where they are free to move throughout the material as in a conductor. Thus, with the proper combination of materials, electrons can flow from the semiconductor into a circuit when light strikes it. This is the basis behind solar energy, in which a current is generated when sunlight hits silicon-based panels known as photovoltaic cells. Materials known as conjugated organic polymers are semiconductors made up of elements like carbon, hydrogen, and nitrogen. A polymer is a chain of small repeated units known as monomers. In a conjugated polymer, the electrons of the molecule are shared across several atoms in the polymer; when these electrons are excited, they are then free to move essentially along the entire length of the polymer. Theoretically, then, conjugated polymers could be used for applications like solar energy, as well. In order to study single conjugated polymer molecules, a technique known as singlemolecule spectroscopy (SMS) was used. In SMS, light from a laser is focused to a very small spot (∼10 nm) on a sample composed of individual, isolated single molecules. This laser light excites electrons in the molecules; when the electrons relax back to their ground (lowest-energy) state, they re-release the energy as a characteristic group of wavelengths of light known as a spectrum. I studied the spectroscopy of the conjugated polymers F8BT and MEH-PPV. For F8BT, I studied the effect of polymer size and temperature on the fluorescence of the polymer. The spectrum of F8BT displays a bimodal distribution: some molecules have a peak in their emission spectrum at approximately ∼570 nm (“red”), while some have it at ∼530 nm (“blue”). This bimodal distribution collapses at low temperatures to a single emission peak. Furthermore, small F8BT molecules emit almost entirely in the blue, while many more large F8BT molecules emit in the red form. This indicates that there is some type of low-energy electron “trap” that becomes more abundant as the polymer gets larger. The most commonly-proposed mechanism for this trap is contact between different parts of the polymer chain. For MEH-PPV, I studied ∼100-molecule aggregates of MEH-PPV polymers. These aggregates contain enough molecules to display characteristics of bulk MEH-PPV with on the order of 10 molecules, but are small enough to be essentially homogeneous – that is, all of the molecules within a single aggregate have identical environments. MEHPPV shows a similar bimodal emission distribution to that of F8BT; the aggregates emit almost exclusively in the “red” form of MEH-PPV. This further supports the hypothesis that chain-chain contact makes a large contribution to the formation of low-energy traps. Solar energy production is just one of many areas where conjugated polymers such as F8BT and MEH-PPV could have a huge impact on the world. If good efficiencies can be achieved in converting sunlight into electrical current, things like photovoltaic cells could be much cheaper, easier to produce, and more environmentally friendly.