Probing Underlying Processes in Organic Photovoltaics with Atomic Force Microscopies

Abstract

University of Washignton Abstract Probing Underlying Processes in Organic Photovoltaics with Atomic Force Microscopies Guozheng Shao Department of Chemistry Photovoltaic is one potential solution to the ever growing demand of energy of our society. Making plastics absorb sunlight in order to generate electricity is a surprisingly easy process. However, fabricating efficient plastic solar cells is another story. A typical organic solar cell is made by blending the electron donor (polymer) and acceptor materials (fullerene derivatives) together and spin coat the solution on top of a transparent electrode to make a thin film (a process like making crêpe). No wonder that the micro-morphology determines all the key parameters of a completed device such as open circuit voltage, short circuit current, fill factor and lifetime. Atomic Force Microscopy is one unique tool to study the optoelectronic processes in organic photovoltaics (OPV), for it allows one to study the local morphology and electronic properties, e.g. photocurrent, carrier mobilities, charging rate, at the same time. I have shown in this dissertation the properties of OPV that could hardly be learned otherwise. In Chapter 1, I summarize recent thinking the OPV field and speculate on possible research projects. In Chapter 2, with the blend of PFB:F8BT, I demonstrate that the breakdown of the most sensitive part in an OPV, percolation pathway, would lead to the degradation of the whole area. In Chapter 3, I am showing a technique that I have been developing, frequency modulated Scanning Kelvin Probe Microscopy (FM-SKPM), to study the charge carrier lifetime at the buried interface in an OPV device. The application of this technique will be demonstrated on a patterned indium tin oxide surface with alternating phosphonic acids with different work functions.