Thin film solar cells

Second Generation thin film solar cells (TFSC) are a promising approach for both the terrestrial and space PV application and offer a wide variety of choices in both device design and fabrication. With respect to single crystal silicon technology, the most important factor in determining the cost of production is the cost of 250-300 micron thick Si wafer (Chopra et al., 2004). Thin-film technologies allow for significant reduction in semiconductor thickness because of the capacity of certain materials for absorbing most of the incident sunlight within a few microns of thickness, in comparison to the several hundred microns needed in the crystalline silicon technology (Carabe and Gandia, 2004). In addition, thin-film technology has an enormous potential in cost reduction, based on the easiness to make robust, large-area monolithic modules with a fully automatic fabrication procedure. Rapid progress is thus made with inorganic thin-film PV technologies, both in the laboratory and in industry (Aberle, 2009).

Amorphous silicon based PV modules have been around for more than 20 years Chithik et al. first deposited amorphous silicon from a silane discharge in 1969 (Chittik et al., 1969) but its use in PV was not much progress, until Clarson found out a method to dope it n or p type in 1976 (Clarson, n. d.) Also, it was found that the band gap of amorphous silicon can be modified by changing the hydrogen incorporation during fabrication or by alloying a-Si with Ge or C (Zanzucchi et al., 1977; Tawada et al. 1981). This introduction of a-Si:C:H alloys as p-layer and building a hetero-structure device led to an increase of the open-circuit voltage into the 800 mV range and to an increased short-circuit current due to the "window" effect of the wideband gap p layer increasing efficiency up to 7.1% (Tawada et al. 1981, 1982). Combined with the use of textured substrates to enhance optical absorption by the "light trapping" effect, the first a-Si:H based solar cell with more than 10% conversion efficiency was presented in 1982 (Catalakro et al., 1982). However, there exists two primary reasons due to which a-Si:H has not been able to conquer a significant share of the global PV market. First is the low stable average efficiency of 6% or less of large-area single-junction PV modules due to "Staebler-Wronski effect", i. e. the light-induced degradation of the initial module efficiency to the stabilized module efficiency (Lechner and Schad, 2002; Staebler and Wronski, 1977). Second reason is the manufacturing related issues associated with the processing of large (>1 m2) substrates, including spatial non-uniformities in the Si film and the transparent conductive oxide (TCO) layer (Poowalla and Bonnet, 2007). Cadmium Telluride (CdTe) solar cell modules have commercial efficiency up to 10-11% and are very stable compound (Staebler and Wronski, 1977). CdTe has the efficient light absorption and is easy to deposit. In 2001, researches at National Renewable Energy Laboratory (NREL) reported an efficiency of 16.5% for these cells using chemical bath deposition and antireflective coating on the borosilicate glass substrate from CdSnO4 (Wu et al., 2001). Although there has been promising laboratory result and some progress with commercialization of this PV technology in recent years (First Solar, n. d.), it is questionable whether the production and deployment of toxic Cd-based modules is sufficiently benign environmentally to justify their use. Furthermore, Te is a scarce element and hence, even if most of the annual global Te production is used for PV, CdTe PV module production seems limited to levels of a few GW per year (Aberle, 2009).

The CIGS thin film belongs to the multinary Cu-chalcopyrite system, where the bandgap can be modified by varying the Group III (on the Periodic Table) cations among In, Ga, and Al and the anions between Se and S (Rau and Schock, 1999). This imposes significant challenges for the realization of uniform film properties across large-area substrates using high-throughput equipment and thereby affects the yield and cost. Although CIGS technology is a star performer in laboratory, with confirmed efficiencies of up to 19.9% for small cells (Powalla and Bonnet, 2007) however the best commercial modules are presently

II – 13% efficient (Green et al. 2008). Also there are issues regarding use of toxic element cadmium and scarcity of indium associated with this technology. Estimates indicate that all known reserves of indium would only be sufficient for the production of a few GW of CIGS PV modules (Aberle, 2009).

This has prompted researchers to look for new sources of well abundant, non toxic and inexpensive materials suitable for thin film technology. Binary and ternary compounds of group III-V and II-VI are of immediate concern when we look for alternatives. AlSb a group

III – V binary compound is one of the most suitable alternatives for thin film solar cells fabrication because of its suitable optical and electrical properties (Armantrout et al., 1977). The crystalline AlSb film has theoretical conversion efficiency more than 27% as suggested in literature (Zheng et al., 2009).