Technical: This project studies the interfaces in small-molecule organic photovoltaic materials using surface analytical probes. The objective of the experimental research is to understand the fundamental aspects of the interface electronic structures and the exciton dissociation at the donor-acceptor interfaces. The main experimental techniques include x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and inverse photoemission spectroscopy. Exciton dissociation at the donor-acceptor interfaces is measured by observing the quenching of the photoluminescence spectroscopy intensity. The charge transfer dynamics across the donor-acceptor interfaces is investigated with time-resolved two-photon photoemission spectroscopy. X-ray photoemission spectroscopy is used to study the interface chemistry. The morphology of the organic layers is characterized with scanning tunneling microscopy, atomic force microscopy, and near-field scanning optical microscopy. The project will also test the concepts developed through the surface/interface analysis by fabricating and characterize simple organic photovoltaic devices fabricated based on the findings of surface analytical investigations.
The project addresses basic research issues in a topical area of materials science with high technological relevance. It addresses fundamental understanding of organic photovoltaics, a promising candidate important for renewable and environmentally friendly energy source. A major part of the project is educating and training graduate and undergraduate students through interdisciplinary research in advanced organic materials and devices and in materials surface/interface characterization. A strong focus of the program is providing early exposure of research to middle and high school students through programs including University of Rochester Pre-College Experience in Physics for women.
Photovoltaic devices can convert light (or photon) into electricity (or mobile charges such as electrons), providing a clean and renewable energy source that is of very high priority given the continually rising oil prices and degrading global environment from fossil fuel burning. Organic photovoltaic (OPV) cells offer an attractive avenue in terms of lower production costs and versatility of applications, such as light weight, large area, and flexible solar panels. In this project, the interfaces in small molecule organic photovoltaic devices have been studied using surface analytical probes. The objective of the experimental program is to understand the fundamental aspects of the interface electronic structures and the exiton dissociation at the donor-acceptor (DA) interface. The interfaces were investigated using surface analytical techniques including x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). The investigation on the melecular orientation dependence of the donor/acceptor interface formation is significant as it provides a direction of improving the efficiency of organic solar cells by controlling the interface molecular packing geometry. The study directly probes the interface electronic structure in organic photovoltaic devices. The investigaions on inserting a metal oxide between the anode and the organic donor is significant as it provides a microscopic mechanism that is useful for the application of organic solar cells as a clean and renewable energy source. Another important outcome is on the oxygen and air exposure that resolves the controversy in the electronic structure of MoO3, and provides an upper limit on the exposure in OPV device fabrication and operation. The work on ITO and the finding of high work function after a proper treatment broaden and strengthen the utility of this important material as anode in OPV. The impact of the research on surface doping by metal oxide is the first direct verification of the electronic structure in p-type doping of C60. Furthermore, as no dopant is mixed with C60, the intrinsic conductivity of C60 is not compromised. The work on CdTe/CdS thin film solar cells provides the optimum parameter for acid treatment of the CdTe surface. The impact of the research also comes from the training of the next generation of scientists in a field that is aiming at solving the energy issue facing the humankind.
