Advanced optical characterization of disordered semiconductors by Fourier transform photocurrent spectroscopy A. Poruba a,b, * , J. Holovsky a , A. Purkrt a , M. Vanecek a a Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Cukrovarnicka ´ 10, 162 53 Prague 6, Czech Republic b Solartec s.r.o., Televiznı ´ 2618, 756 61 Roz ˇnov pod Radhos ˇte ˇm, Czech Republic Available online 15 February 2008 Abstract The paper summarizes progress in the Fourier transform photocurrent spectroscopy (FTPS) since 2001 when it was introduced for the first time for evaluation of the spectral dependence of the optical absorption coefficient in microcrystalline silicon thin films. We concen- trate on the appropriate measuring conditions and evaluation procedures for a correct data interpretation of various thin films and solar cell structures. We show how the measured and finally evaluated absorption spectra differ in case of single junction amorphous and microcrystalline silicon solar cells as well as for a micromorph tandem. The key issue of the tandem structure diagnostics is a separation of FTPS signals from amorphous and microcrystalline parts of a stacked structure and their correct interpretation. For an appropriate simulation of the light propagation within the structure and confirmation of our measuring approach the computer model cell has been used. Ó 2008 Published by Elsevier B.V. PACS: 73.20.At; 78.20.e; 78.20.Bh; 78.20.Ci; 78.90.+t Keywords: Silicon; Solar cells; Photovoltaics; Band structure; Plasma deposition; Defects; Monte Carlo simulations; Absorption; Optical spectroscopy; FTIR measurements; Photoconductivity; Structure 1. Introduction Optical, optoelectronic and electrical properties of all layers within the solar cell structure determine the resulting device performance. Fourier transform photocurrent spec- troscopy (FTPS) has recently been introduced as a fast and very sensitive method for the optical and optoelectronic characterization of photoconductive layers, i.e., ‘active absorbers’ of solar cells. The first measuring experiments were done for microcrystalline silicon thin films grown on low-alkaline glass substrates and these results were verified by constant photocurrent method [1,2]. Since that time we tested FTPS method for various photoconductive thin films and solar cells including organic materials and CuIn- Se 2 type (CIS) polycrystalline semiconductors. This method also has been used for fast quantum efficiency mea- surement of different solar cells [3,4]. Here, we will present some data of previously mentioned experiments; but we will mainly highlight some details regarding the measuring technique and evaluation procedure. It is well known that properties of thin films deposited by different methods depend on the substrate parameters such as its conductivity and morphology. Therefore, it is necessary to study optical, electrical and structural behav- ior of (at least) active absorbers when grown on the same substrate as incorporated in the whole cell structure. But in this case the measured photocurrent spectra are influ- enced by light scattering, light trapping and absorption in other layers of the multilayer structure, therefore for the correct interpretation modeling approach has to be used. We will show how the measured and finally evaluated 0022-3093/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.09.108 * Corresponding author. Address: Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Cukrovarnicka ´ 10, 162 53 Prague 6, Czech Republic. Tel.: +420 220 318 540; fax: +420 233 343 184. E-mail address: poruba@fzu.cz (A. Poruba). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2421–2425