**1. Introduction**

340 Solar Cells – New Aspects and Solutions

[7] JF Geisz, S Kurz, MW Wanlass, JS Ward, A Duda, DJ Friedman, JM Olson, WE McMahon, TE Moriarty, and JT Kiehl, Applied Phys. Letters 91, 023502 (2007)

[11] GFX Strobl at al, Proc. 7th European Space Power Conference, 9-13 May, Italy, 2005 [12] H. L. Cotal, D. R. Lillington, J. H. Ermer, R. R. King, S.R. Kurtz, D. J. Friedman, J. M.

[22] AC Varonides, RA Spalletta, WA Berger, WREC-X and Exhibition, 19-25 July 2008,

[22] R. King, *Multijunction Cells*, Industry Perspective, Technology Focus, Nature photonics

[23] T Kirchartz, BE Pieters, K Taretto, U Rau, Journal of Appl. Physics 104, 094513 (2008)

[6] Es Yang, Microelectronic Devices, McGraw-Hill, 1988

Olson, et al, 28th IEEE PVSC, 2000, p. 955

[17] E Istrate, EH Sargent, Rev Mod Phys, 78, 455 (2006) [18] H E Runda et al, Nanoscale Res Lett (2006) 1:99

 [19] W Li, BE Kardynal, et al, Appl. Phys Lett 93, 153503 (2008) [20] AC Varonides, Thin Solid Films, Vol. 511-512, July 2006, pp 89-92 [21] BL Stein and ET Yu, Appl. Phys. Lett. 70 (25), 23 June, 1997

[25] K Jandieri, S D Baranovskii, W Stolz, F Gebhard, W Guter, [33] M Hermle and A W Bett, J. Phys. D: Appl. Phys. 42 (2009) 155101

[9] T Mei, Journal of Appl Phys 102, 053708 (2007)

[16] AC Varonides, Physics E 14, 142 (2002)

Glasgow, Scotland, UK.

[34] R Jones, CPV Summit, Spain, 2009

[8] T Kirchartz, Uwe Rau, et al, Appl. Phys Lett. 92, 123502 (2008)

[10] AC Varonides and RA Spalletta, Physica Stat. Sol. 5, No. 2 441 (2008)

[13] T Kieliba, S Riepe, W Warta, Journal of Appl. Phys. 100, 093708 (2006) [14] JF Geisz and DJ Friedman, Semicond. Sci. Technol. 17, 789 (2002) [15] W Hant, IEEE Trans Electron Devices, VOL ED-26, NO 10, 1573 (1979)


[24] AC Varonides and RA Spalletta, Thin Solid Films 516, 6729-6733 (2008)

Since industrial revolution by the end of nineteenth century, the consumption of fossil fuels to drive the economy has grown exponentially causing three primary global problems: depletion of fossil fuels, environmental pollution, and climate change (Andreev and Grilikhes, 1997). The population has quadrupled and our energy demand went up by 16 times in the 20th century exhausting the fossil fuel supply at an alarming rate (Bartlett, 1986; Wesiz, 2004). By the end of 2035, about 739 quadrillion Btu of energy (1 Btu = 0.2930711 Whr) of energy would be required to sustain current lifestyle of 6.5 billion people worldwide (US energy information administration, 2010). The increasing oil and gas prices, gives us enough region to shift from burning fossil fuels to using clean, safe and environmentally friendly technologies to produce electricity from renewable energy sources such as solar, wind, geothermal, tidal waves etc (Kamat, 2007). Photovoltaic (PV) technologies, which convert solar energy directly into electricity, are playing an ever increasing role in electricity production worldwide. Solar radiation strikes the earth with 1.366 KWm-2 of solar irradiance, which amounts to about 120,000 TW of power (Kamat 2007). Total global energy needs could thus be met, if we cover 0.1% of the earth's surface with solar cell module with an area 1 m2 producing 1KWh per day (Messenger and Ventre, 2004).

There are several primary competing PV technologies, which includes: (a) crystalline (c-Si), (b) thin film (a-Si, CdTe, CIGS), (c) organic and (d) concentrators in the market. Conventional crystalline silicon solar cells, also called first generation solar cells, with efficiency in the range of 15 - 21 %, holds about 85 % of share of the PV market (Carabe and Gandia, 2004). The cost of the electricity generation estimates to about \$4/W which is much higher in comparison to \$0.33/W for traditional fossil fuels (Noufi and Zweibel, 2006). The reason behind high cost of these solar cells is the use of high grade silicon and high vacuum technology for the production of solar cells. Second generation, thin film solar cells have the lowest per watt installation cost of about \$1/W, but their struggle to increase the market share is hindered mainly due to low module efficiency in the range of 8-11% ((Noufi and Zweibel, 2006; Bagnall and Boreland, 2008). Increasing materials cost, with price of Indium more than \$700/kg (Metal-pages, n.d.), and requirements for high vacuum processing have kept the cost/efficiency ratio too high to make these technologies the primary player in PV market (Alsema, 2000). Third generation technologies can broadly be divided in two categories: devices achieving high efficiency using novel approaches like concentrating and

AlSb Compound Semiconductor as Absorber Layer in Thin Film Solar Cells 343

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

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

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.

PV modules (Aberle, 2009).

in literature (Zheng et al., 2009).

**1.2 Aluminum antimony thin films** 

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