**1.1 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 11–13% 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).

### **1.2 Aluminum antimony thin films**

342 Solar Cells – New Aspects and Solutions

tandem solar cells and moderately efficient organic based photovoltaic solar cells (Sean and Ghassan, 2005; Currie et al., 2008). The technology and science for third generation solar

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

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).

limited to levels of a few GW per year (Aberle, 2009).

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

cells are still immature and subject of widespread research area in PV.

**1.1 Thin film solar cells** 

in industry (Aberle, 2009).

Aluminum Antimony is a binary compound semiconductor material with indirect band gap of 1.62 eV thus ideal for solar spectrum absorption (Chandra et al., 1988). This also has become the material of interest due to relatively easy abundance and low cost of Al and Sb. AlSb single crystal has been fabricated from the Czochralski process but the AlSb thin film was prepared by Johnson et al. by co-evaporation of Al and Sb. They also studied the material properties to find out its donor and acceptor density and energy levels (Johnson, 1965). Francombe et al. observed the strong photovoltaic response in vacuum deposited AlSb for the first time in 1976 (Francombe et al., 1976). Number of research groups around the world prepared thin AlSb thin film and studied its electrical and optical properties by vacuum and non vacuum technique. These include, Leroux et al. deposited AlSb films on number of insulating substrate by MOCVD deposition technique in 1979 (Leroux et al., 1980). Dasilva et al. deposited AlSb film by molecular beam epitaxy and studied its oxidation by Auger and electron loss spectroscopy in 1991 (Dasilva et al., 1991). Similarly, AlSb film was grown by hot wall epitaxy and their electrical and optical properties was studied by Singh and Bedi in 1998 (Singh and Bedi, 1998). Chen et al. prepared the AlSb thin film by dc magnetron sputtering and studied its electrical and optical properties (Chen et al. 2008) in 2007. Gandhi et al. deposited AlSb thin film on by the alternating electrical pulses from the ionic solution of AlCl3 and SbCl3 in EMIC (1-methyl-3-ethylimidazolium chloride) and studied its electrical and optical characters (Gandhi et al. 2008). However, AlSb thin film had never been successfully employed as an absorber material in photovoltaic cells. We, electro-deposited AlSb thin film on the TiO2 substrate by using the similar technique as Gandhi et al. and had observed some photovoltaic response in 2009 (Dhakal et al., 2009). Al and Sb ions are extremely corrosive and easily react with air and moisture. Thus it becomes very difficult to control the stoichiometry of compound while electroplating. Thus, vacuum deposition techniques become the first choice to prepare AlSb thin films for the solar cell applications. In this work, we fabricated AlSb thin film cosputtering of Al and Sb target. The film with optimized band-gap was used to fabricate p-n and p-*i*-n device structures. The photovoltaic response of the devices was investigated.

AlSb Compound Semiconductor as Absorber Layer in Thin Film Solar Cells 345

In DC sputtering, the target must be electrically conductive otherwise the target surface will charge up with the collection of Ar+ ions and repel other argon ions, halting the process. RF Sputtering - Radio Frequency (RF) sputtering will allow the sputtering of targets that are electrical insulators (SiO2, etc). The target attracts Argon ions during one half of the cycle and electrons during the other half cycle. The electrons are more mobile and build up a negative charge called self bias that aids in attracting the Argon ions which does the sputtering. In magnetron sputtering, the plasma density is confined to the target area to increase sputtering yield by using an array of permanent magnets placed behind the sputtering source. The magnets are placed in such a way that one pole is positioned at the central axis of the target, and the second pole is placed in a ring around the outer edge of the target (Ohring, 2002). This configuration creates crossed *E* and *B* fields, where electrons drift perpendicular to both *E* and *B.* If the magnets are arranged in such a way that they create closed drift region, electrons are trapped, and relies on collisions to escape. By trapping the electrons, and thus the ions to keep quasi neutrality of plasma, the probability for ionization is increased by orders of magnitudes. This creates dense plasma, which in turn leads to an increased ion bombardment of the target, giving higher sputtering rates and, therefore,

We employed dc magnetron sputtering to deposit AlSb thin films. Fig. 2 shows the schematic diagram of Meivac Inc sputtering system. Al and Sb targets were placed in gun 1 and 2 while the third gun was covered by shutter. Both Al and Sb used were purchase from Kurt J. Lesker and is 99.99% pure circular target with diameter of 2.0 inches and thickness of

Fig. 2. Schematic diagram of dc magtron sputtering of Al and Sb targets.

Firstly, a separate experiment was conducted to determine the deposition rate of aluminum and antimony and the associated sputtering powers. Al requires more sputtering power than Sb does for depositing the film at same rates. Next Al and Sb was co-sputtered to

higher deposition rates at the substrate.

0.250 inches.
