John Asbury

Solid-State Chemistry

Emerging technologies such as inexpensive organic photovoltaic panels for solar power generation, organic light-emitting diodes for energy efficient lighting, and flexible displays for the next generation of computers and electronic paper all rely on the development of novel materials with finely tuned properties. The scientific community is addressing these needs by realizing new organic materials that promise economical manufacture at high volume. State-of-the-art materials exhibit very promising initial behavior, yet suffer profound degradation upon continued operation. This degradation impedes advancement of the technologies because the material lifetimes are too short for practical use.

The principle objective of Professor Asbury's research program is to reveal mechanisms of chemical and structural degradation in emerging electro-active materials using ultrafast transient 2DIR vibrational spectroscopy ( t -2DIR). The practical aim is to use advanced chemical physics methods to provide nano-scale views of degradation mechanisms in real materials that will guide the development of new materials with enhanced operational lifetimes. The fundamental goal of the program is to understand the role of the rigid “solvent” matrix on the reaction coordinate in solid-state chemical reactions, and how this role differs from corresponding reactions in liquid environments.

To illustrate the sensitivity of t -2DIR spectroscopy to nano-scale structures, consider the defects displayed in Fig. 1 that have been predicted in a variety of models describing degradation in hydrogenated amorphous silicon ( a -Si:H), an amorphous inorganic photovoltaic material.

Figure 1. Structures of hydrogen related defects in a -Si:H. The Si-H stretch frequencies are: H BC , ~2000 cm -1 ; H 2 * , a) ~2060 cm -1 and b) ~1840 cm -1 ; and º SiH 2 , ~2100 cm -1 .

Figure 2. Simulated t -2DIR spectra of the Si-H stretch from microscopic models of degradation in a -Si:H. Structures that reacted to form defects are encircled with the dashed line. Other features arise from defects. Differences in the spectra demonstrate the structural sensitivity of t -2DIR.

Figure 2 depicts simulated t -2DIR spectra (green boxes) corresponding to the structures displayed in Fig. 1. The variance among features in the t -2DIR spectra (blue and red shapes) correspond to structural differences of the defects (see Fig. 1). The diagonal in the t -2DIR spectra where Wp = Wpr is represented by the dotted line. The diagonally elongated peaks displayed in Fig. 2 illustrate the ability of t -2DIR to examine discrete nano-scale structures by resolving their inhomogeneous vibrational modes into sub-components that are distributed along the diagonal of the 2D spectra. The sub-components are analyzed separately with the aid of a frequency structure correlation. Electronic structure calculations of model clusters provide fine frequency-structure correlation enabling explicit structural assignments from the data. These structural assignments are used to understand the degradation mechanism in this material.

The same level of nano-scale sensitivity is applied to studies of degradation in organic elctro-active materials used in many applications such as PPV/ BBL (organic photovoltaic), Ir(ppy) 2 P( n -Bu) 3 CN/ PVK (blue organic LED), pentacene (organic TFT) and PMMA (organic dielectric). Structures of these materials are depicted below.

A variety of vibrational modes are used to probe structures in the materials including C-H, C=O, CN, C=N, O-H and N-H stretch and bend modes with vibrational frequencies ranging from 3000 cm^ -1 to 1000 cm^ -1 .