Efficiency of Thin-Film CdS/CdTe Solar Cells

Abstract

Over the last two decades, polycrystalline thin-film CdS/CdTe solar cells fabricated on glass substrates have been considered as one of the most promising candidates for large-scale applications in the field of photovoltaic energy conversion (Surek, 2005; Goetzberger et al., 2003; Romeo et al., 2004). CdTe-based modules have already made the transition from pilot scale development to large manufacturing facilities. This success is attributable to the unique physical properties of CdTe which make it ideal for converting solar energy into useful electricity at an efficiency level comparable to traditional Si technologies, but with the use of only about 1% of the semiconductor material required by Si solar cells. To date, the record efficiencies of laboratory samples of CdS/CdTe solar cells and large-area modules are ~ 16.5 % and less than 10 %, respectively (Britt & Ferekides, 1993; Hanafusa et al., 1997; Meyers & Albright, 2000; Wu et al., 2001; Hanafusa et al., 2001; Bonnet, 2003). Thus, even the record efficiency of such type solar cells is considerable lower than the theoretical limit of 28-30% (Sze, 1981). Next challenge is to improve the performance of the modules through new advances in fundamental material science and engineering, and device processing. Further studies are required to reveal the physical processes determining the photoelectric characteristics and the factors limiting the efficiency of the devices. In this chapter, we present the results of studying the losses accompanying the photoelectric conversion in the thin-film CdS/CdTe heterostructures and hence reducing the efficiency of modules on glass substrate coated with a semitransparent ITO or SnO2 conducting layer. We discuss the main parameters of the material used and the barrier structure determining the photoelectric conversion efficiency in CdS/CdTe solar cell: (i) the width of the space-charge region, (ii) the lifetime of minority carriers, (iii) their diffusion length and drift length, (iv) the surface recombination velocity, and (v) the thickness of the CdTe absorber layer. Among other factors, one of the important characteristics determining the efficiency of a solar cell is the spectral distribution of the quantum efficiency which accounts for the formation of the drift and diffusion components of the photocurrent and ultimately the short-circuit current density. In the paper particular attention is given to this aspect of solar cell. We demonstrate the possibility to describe quantitatively the quantum efficiency spectra of the thin-film CdS/CdTe solar cells taking into account the recombination losses at the CdS-CdTe interface and the back surface of the CdTe absorber layer. Charge collection efficiency in thin-film CdS/CdTe solar cells are also discussed taking into consideration losses caused by a finite thickness of the p-CdTe layer, recombination losses at the front and back surfaces as well as in the space-charge region. The dependences of the Source: Solar Energy, Book edited by: Radu D. Rugescu, ISBN 978-953-307-052-0, pp. 432, February 2010, INTECH, Croatia, downloaded from SCIYO.COM