Measure of carrier lifetime in nanocrystalline silicon thin films using transmission modulated photoconductive decay
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Measure of carrier lifetime in nanocrystalline silicon thin films using transmission modulated photoconductive decay Brian J. Simonds1,2, Baojie Yan4, Guozhen Yue4, Donald J. Dunlavy3, Richard K. Ahrenkiel2,3, P. Craig Taylor1,2 1 Department of Physics, Colorado School of Mines, Golden CO 80401 2 Renewable Energy Materials Research Science and Engineering Center, Colorado School of Mines 3 Department of Metallurgical and Materials Engineering, Colorado School of Mines 4 United Solar Ovonic, LLC, Troy, Michigan 48084 ABSTRACT We present results of extremely short carrier lifetime measurements on a series of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by a novel, non-destructive, noncontact method. Transmission modulated photoconductive decay (TMPCD) is a newly developed technique which appears to have high enough sensitivity and time resolution to measure the extremely short carrier lifetimes on the order of a nanosecond. As a proof of this, we measure various nc-Si:H samples of varying crystalline volume fraction as well as a fully amorphous sample. To ascribe an effective lifetime to the materials, we use a simple model which assumes a single exponential decay. By using this model, effective lifetimes can be deconvoluted from our pump beam giving nanosecond lifetimes. Lifetimes of between 1.9 and 0.9 nanoseconds are reported and trend to decreasing lifetimes as crystalline volume fraction is increased. INTRODUCTION Minority carrier lifetime has long been an important measure in discerning the quality of solar cell absorber materials. As these materials have shifted from bulk silicon which has a high mobility and long carrier decay lifetimes to thin film materials, which enjoy neither of these traits, it has become more difficult to measure carrier lifetime. Typical methods of measuring photoconductive carrier kinetics in the past include time-of-flight experiments [1-3] as well as several other transient photocurrent measurements. Both of these experiments require that contact is made to the material through Schottky contacts and that the sample is biased. Other techniques have been developed which allow for transient photoconductivity to be measured without contacts, such as resonant coupled photoconductive decay (RCPCD)[4] and microwave photoconductive decay (μ-PCD) [5]. Both of these measurements are available in our lab and we have seen that transmission modulated photoconductive decay (TMPCD) appears to combine the faster time response of μ-PCD with the sensitivity of RCPCD. Nanocrystalline silicon is a thin film material of interest to the solar cell community as a possible absorber layer in a thin film solar cell. It is deposited using plasma enhanced chemical vapor deposition, a technique long familiar to the amorphous thin film community. It is a promising material as it appears to combine strengths of both amorphous and crystalline silicon and is less sensitive to the light induced degradation seen in amorphous silicon. Recently United Solar Ovonics, Inc. has reported a current
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