The Division of Chemistry supports Olga Griffith of the University of Arizona as an American Competitiveness in Chemistry Fellow. Dr. Griffith will work with Prof. Neal Armstrong to investigate the donor-electrode and acceptor electrode interfaces in model materials that mimic those found in solar photovoltaics. The research will use a variety of electron spectroscopies, in addition to scanning probe microscopies to study these systems. Additional work will be carried out in collaboration with Dr. David Ginley, a scientist at the National Renewable Energy Laboratory. For her plan for broadening participation, the PI will work with students (8-12 grade) and teachers at the Tucson Wildcat School to develop curricular materials which teach science and engineering principles through energy-related projects. Many of the students served will come from groups that have been traditionally underrepresented in the sciences.
Research like that of Dr. Griffith is aimed at developing a better understanding of the intricacies of the conversion of solar energy into useful electrical power. Results from research like that supported here will lead to better, more efficient, devices for the conversion of solar energy. The efforts at broadening participation being pursued by Dr. Griffith are aimed at effectively engaging a diverse group of young people in science and engineering activities that relate to important societal problems. The hope of activities like these is to encourage talented Americans to pursue careers in science and technology.
Project Outcome: The main focus of this project was to understand organic/organic’ and organic/electrode heterojunctions, that are critical in emerging organic solar cell technologies. This work provides a very important insight into the polarization effects of organic semiconductor active layers and how they impact the electronic behavior of these materials. The strong correlation between electronic properties, morphologies of donors, and electrical characteristics of corresponding photovoltaic devices has been shown. Although, the contribution of the morphological properties of organic active layers (pentacenes) caused by the presence of the "charge-selective" interlayer (such as perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride or PTCDA), into the device performance, independent from the measured energy offsets of donors’ frontier orbitals, has been observed. The intramolecular electronic structures and intermolecular electronic interactions of organic semiconductors have been investigated by the combination of gas- and solid-phase UV-photoelectron along with inverse photoelectron spectroscopies. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. Morphological properties were measured by tapping-mode atomic force microscopy. Impact & Benefits: Establishing a correlation between molecular electronic properties and molecular structure of organic semiconductors at electrode interfaces enables understanding of phenomena which limit the efficiencies of organic photovoltaic or solar cell technologies. This research plays a key role in improving the design and performance of photovoltaic devices by showing how the electronic and charge transfer properties of organic materials used as active layers in these devices are related to their electrical properties, thus providing an integration of the microscopic energy measurements with the measurements performed on macroscopic length scales. Background & Explanation: Organic photovoltaic devices (OPV) are of great interest due to their low-cost, light weight, flexibility and ease of fabrication. In order to improve the design and performance of organic solar cells it is important to gain deeper understanding of electronic properties and charge transfer mechanism of materials these devices are made of. The main goal of this research was to characterize the offsets in frontier orbital energies, and their influence on charge transfer at organic/electrode interfaces, with special emphasis on materials of interest as contacts and as "charge-selective" interlayers in organic solar or photovoltaic cell technologies. Energy offsets between the contacting materials and the organic semiconductors play a key role in determining whether the contacts are ohmic in nature, and their "selectivity" toward capture of only holes or electrons, which has been realized to be a critical aspect of OPV device performance.
