The wikipedia article is a good starting point.
ITO, Indium Tin Oxide
Typically 90% In₂O₃, 10% SnO₂ by weight, density ≈7140 kg/m³ , bulk resistivity 1e-5 Ω-m . So ITO is 75% indium by weight, and the density of indium in ITO films is 5300 kg/m³. A 1Ω/□ sheet of ITO is 10μm thick and uses 53 g/m² of indium. At $520/kg (year end 2012) this is $28/m² .
For comparison, the bulk resistivity of aluminum is 2.8e-8 Ω-m, and the density is 2700 kg/m³. A 1Ω/□ sheet of Al is 28nm thick and uses 76μg/m² of aluminum. At $2/kg (year end 2012) this is $1.5e-4/m², about 200,000 times cheaper than ITO, and 1 million times lighter.
AZO, Aluminum Zinc Oxide
Aluminum Zinc Oxide is difficult to pattern, and is damaged by moisture, but is far cheaper than ITO. There is no moisture in the vacuum of space, and front-side solar cell patterns can be large planes without fine linewidth features. According to Park et. al. (Figure 5), the resistivity of AZO (2% Al) in 450nm films is less than 2e-6 Ω-m and the transmittance greater than 90% when prepared with high temperature RF magnetron sputtering. If the material is that good, it is surprising that it hasn't completely replaced expensive, higher resistivity ITO. More study needed!
Alternatives to low resistivity front-side conductors
In the vacuum of space, the sun is a highly collimated source half a degree wide. Sunlight can be slightly focused by cylindrical glass microlenses onto narrow ( < 1mm? ) strips of InP, with fat aluminum conductors in the gaps between the strips.