NOVELTY - A glass substrate (5) comprises (in mol%) 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), titanium oxide (0-3), sodium oxide (1-8), and potassium oxide (2-12). The glass substrate has glass transition temperature of 640 degrees C or more, average thermal expansion coefficient of 70x 10-7-90x 10-7/ degrees C at 50-350?Oc, viscosity of 104 dPa.second at 1230 degrees C or less, viscosity of 102 dPa.second at 1650 degrees C or less, and density of 2.7 g/cm3.    USE - Glass substrate is used for copper-indium-gallium-selenium solar cell (claimed).    ADVANTAGE - The solar cell containing the glass substrate has high power-generation efficiency, glass transition temperature and glass strength.    DETAILED DESCRIPTION - A glass substrate comprises (in mol%) 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), titanium oxide (0-3), sodium oxide (1-8), and potassium oxide (2-12). The total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide is 10-24 mol%. The total content of sodium oxide and potassium oxide is 5-15 mol%. The ratio of content of magnesium oxide to content of aluminum oxide is 1.3 or more. The ration of total content of sodium oxide, potassium oxide, strontium oxide and barium oxide to total content of aluminum oxide and zirconium oxide is 3.3 or less, which is calculated by relation of formula: (2Na2O+K2O+SrO+BaO)/(Al2O3+ZrO2). The ratio of content of sodium oxide to potassium oxide is 0.2-2. The ratio of content of aluminum oxide and magnesium oxide is calculated by relation of formula: Al2O3 less than or equal to -0.94MgO+11. The glass substrate has glass transition temperature of 640 degrees C or more, average coefficient of thermal expansion of 70x 10-7-90x 10-7/ degrees C at 50-350 degrees C, viscosity of 104 dPa.second at 1230 degrees C or less (T4), viscosity of 102 dPa.second at 1650 degrees C or less (T2), and density of 2.7 g/cm3. The devitrification temperature (Tl) is calculated by relation of formula: T4-Tl greater than or equal to -30 degrees C. An INDEPENDENT CLAIM is included for solar cell (1).    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic sectional view of solar cell.    Solar cell (1)    Glass substrates (5)    Positive electrodes (7)    Copper-indium-gallium-selenium layer (9)    Buffer layer (11)