Lei Diao Tsinghua University - Beijing, China; 1997 B.S. Materials Science and Engineering Tsinghua University - Beijing, China; 2000 Masters Materials Science Email: diao@cems.umn.edu

The performance of organic devices is strongly affected by the electrode materials, because the injection barrier for charge transport is determined by the property of metal/organic interface. When a metal is in an intimate contact with a semiconductor, a potential barrier is developed across the interface. In many cases, the description of metal/organic contact is not satisfactory because it based on the concept of work functions, where the barriers for the injection of charge carriers are simply given by the difference of the metal work function and the energy levels (HOMO and LUMO) of the organic material. My research focuses on the fabrication and characterization of the Pt, Au, Al, Ag and Ca Schottky barrier contacts to undoped pentacene. The geometry of devices consists of a metal contact on top of a pentacene film thermally deposited on a conductive indium tin oxide (ITO) substrate. I use a combination of J-V and activation energy measurements over a wide temperature range to investigate the diode characteristics of the Schottky barrier structures. The Arrhenius plots of both forward current and reverse current were used to obtain the Schottky barrier heights. Besides activation energy measurement, internal photoemission is another way to determine the Schottky barrier height. During the internal photoemission measurement, chopped monochromatic light is incident upon metal electrode through ITO and pentacene film. In this process photons are absorbed in the metal contact and induce photo current. The quantum yield Y which is defined as photo current per absorbed photon is a function of the photon energy. For Schottky contacts fabricated by different metals, the metal work function dependence of barrier height has been observed in this study. The goal of my research is to gain a better understanding of the metal/organic contacts, and then step further to manipulate the contact interfaces thus to optimize the performance of organic devices.

My research also centers on High Efficiency Organic Photovoltaic (PV) Cells. Today’s our main energy sources are oil and coal, which currently are in limited supply. The combustion of fossil fuels always produces harmful gases. For a better future of our planet, we need to find a renewable energy source with less harm to human nature. As we know, sunlight consists of tiny particles of pure energy - so called photons. Every second our sun emits billions of these photons. Essentially, the sunlight is renewable energy source which neither runs out nor has any significant harmful effects on our environment. The solar cell is the device that we use to convert the solar light into electric power. Because of simple processing at much lower temperatures, organic semiconductors have received increased interest as solar cells materials. Progress has been rapid in the last 5 years, with developments in solution-processed polymer blend cells leading to solar power conversion efficiencies of up to 2.5%, as well as in vacuum or vapor-phase deposited small molecule based cells with efficiencies up to 4.2%. The purpose of my research is to improve efficiencies of organic solar cells in order to make the technology commercially competitive with traditional energy sources in certain applications. The geometry of the solar cell employed in this research is based on a donor-acceptor (D/A) heterojunction deposited on ITO substrate. When we select the D/A materials, we consider 1) maximizing the energy band mismatch between D/A materials to achieve large open circuit voltage. 2) chose materials with large absorption coefficient and large exciton diffusion length, thus to maximize the short circuit current. Currently the electron donor materials we use include pentacene, P3HT, and the electron acceptor materials include PTCDI-C8 and C60. The fabrication of organic photovoltaic device involves vacuum deposition of organic molecular crystal and metal contact, and spin-coating of polymer on top of ITO. The characterizations include J-V measurement of photovoltaic device under dark and under light, absorption spectrum, etc.