||Rapid, accurate and contactless measurement of the recombination lifetime is a very critical activity in photovoltaics. The excess carrier lifetime (Δn(t)) is the most critical and variable parameter in the development of photovoltaic materials, Device performance can be accurately predicted from the lifetime measurement of the starting material. However, there is no single measurement that directly measures the bulk lifetime as all measurements are based on a device model. A primary issue is that the lifetime is a function of excess carrier lifetime and measurements must be linked to an injection level. The most common measurements are based on either photoconductive (PCD) or photoluminescence (TRPL) decay. PC decay senses the product of excess carrier (Δn) concentration and mobility (μ (Δn). This mobility variation must be included in order to extract the true excess carrier lifetime. TRPL works best for direct band gap materials and therefore is not applicable to silicon. For polycrystalline materials, shallow traps distort the measurement and must be included in the analysis of the data. Finally, surface and interface recombination have profound influence on most measurements and must be minimized for accurate measurement of the true bulk lifetime. I will discuss both the techniques and the analysis methods. I will show some typical sample measurements. One often uses a combination of these measurements in order to adequately extract the important parameters. The representative samples include thin film and wafer materials that are currently popular in the photovoltaic community.
Richard K. Ahrenkiel is a Research Professor of Metallurgical and Materials Engineering at the Colorado School of Mines in Golden, Colorado. He is also a Consultant and Research Fellow Emeritus at the National Renewable Energy Laboratory, where he has worked from 1981 to 2005. Prior to NREL, he works at Eacstman Kodak Resaearch laboratories, where he did some of the first work on electronic photography, and at Los Alamos National Laboratory where he first worked on the laser fusion project. His area of specialization is the measurement and characterization of photovoltaic cells and materials. He is one of the world experts in the area of carrier recombination and carrier lifetime and has invented a unique technique for measuring the excess carrier lifetime in materials. The latter has been named Resonance-coupled photoconductive decay (RCPCD), and has been extensively applied to silicon and non-silicon materials. His B.S. degree is in Engineering Physics and the M.S. and Ph.D degrees in Physics, all at the University of Illinois, Urbana