NOVELTY - A glass substrate (2) comprises (in mole%) silica (60-75), alumina (1-7.5), boron trioxide (0-1), magnesium oxide (8.5-12.5), calcium oxide (1-6.5), strontium oxide (0-3), barium oxide (0-3), zirconium oxide (0-3), sodium oxide (1-8) and potassium oxide (2-12) as matrix composition. The total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide is 10-24 mole%. The total content of sodium oxide and potassium oxide is 5-15 mole%. The ratio of magnesium oxide content to alumina content is 1.3 or more.    USE - Glass substrate is used for cadmium telluride solar cell (claimed) and copper indium gallium selenide solar cell.    ADVANTAGE - The glass substrate has excellent transmittance, weather resistance, strength, high glass transition temperature, ultraviolet-ray shielding, refractive index, average coefficient of thermal expansion, solubility during plate glass production, moldability and loss-of-clarity prevention characteristic, and low glass density. The solar cell using the glass substrate has excellent electric power generation efficiency.    DETAILED DESCRIPTION - A glass substrate comprises (in mole%) silica (60-75), alumina (1-7.5), boron trioxide (0-1), magnesium oxide (8.5-12.5), calcium oxide (1-6.5), strontium oxide (0-3), barium oxide (0-3), zirconium oxide (0-3), sodium oxide (1-8) and potassium oxide (2-12) as matrix composition. The total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide is 10-24 mole%. The total content of sodium oxide and potassium oxide is 5-15 mole%. The ratio of magnesium oxide content to alumina content is 1.3 or more. The ratio of total content of sodium oxide, potassium oxide, strontium oxide and barium oxide to total content of alumina and zirconium is 3.3 or less. The ratio of sodium oxide content to potassium oxide content is 0.2-2. The content of alumina is satisfies the relation: aluminum content greater than or equal to (-0.94x magnesium content)+11. The content of calcium oxide greater than or equal to (-0.48x magnesium content)+6.5. The glass substrate has glass transition temperature of 640 degrees C or more, average thermal expansion coefficient of 70x 10-7-90x -7/ degrees C at 50-350 degrees C and density of 2.7 g/cm3 or less. The viscosity of glass substrate at 1230 degrees C or less and 1650 degrees C or less is 104 dPa.second and 102 dPa.second, respectively. The difference of temperatures when viscosity reaches 104 dPa.second and 102 dPa.second is (-30) degrees C or more.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of cadmium telluride solar cell.    Cadmium telluride solar cell (1)    Glass substrate (2)    Transparent conductive film (3)    Buffer layer (4)    Cadmium telluride layer (5)