NOVELTY - A glass substrate (5) contains (in mol%) silicon dioxide (55-70), alumina (6.5-12.6), boron trioxide (0-1), magnesium oxide (3-10), calcium oxide (0-4.8), strontium oxide (0-2), barium oxide (0-2), zirconium oxide (0-2.5), titanium dioxide (0-2.5), sodium oxide (5.3-10.9), and potassium oxide (0-10). The glass substrate has glass transition temperature of 650-750 degrees C, average coefficient of thermal expansion of 75x 10-7-95x 10-7/ degrees C at 50-350 degrees C, density of 2.6 g/cm3 or less and brittleness index of less than 7000 m-1/2.    USE - Glass substrate is used for copper-indium-gallium selenium solar cell (claimed).    ADVANTAGE - The glass substrate has high glass transition temperature, low density and favorable devitrification characteristics, and provides solar cell with high electric power generation efficiency.    DETAILED DESCRIPTION - A glass substrate contains (in mol%) silicon dioxide (55-70), alumina (6.5-12.6), boron trioxide (0-1), magnesium oxide (3-10), calcium oxide (0-4.8), strontium oxide (0-2), barium oxide (0-2), zirconium oxide (0-2.5), titanium dioxide (0-2.5), sodium oxide (5.3-10.9), and potassium oxide (0-10). The total amount of magnesium oxide, calcium oxide, strontium oxide and barium oxide is 7.7-17 mol%. The total amount of sodium oxide and potassium oxide is 10.4-16 mol%. The molar ratio of magnesium oxide and alumina is 0.9 or less. The ratio of total amount of sodium oxide, potassium oxide, strontium oxide and barium oxide with respect to total amount of alumina and zirconia is 2.2 or less. The product of molar ratio of total amount of sodium oxide and potassium oxide with respect to amount of alumina, and molar ratio of sodium oxide and potassium oxide is 0.9 or more. The glass substrate has glass transition temperature of 650-750 degrees C and average coefficient of thermal expansion of 75x 10-7-95x 10-7/ degrees C at 50-350 degrees C. The difference of temperature of glass substrate at which viscosity is 104 dPa.second and devitrification temperature is -30 degrees C or more. The glass substrate has density of 2.6 g/cm3 or less and brittleness index of less than 7000 m-1/2. An INDEPENDENT CLAIM is included for solar cell (1).    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of solar cell.    Solar cell (1)    Glass substrate (5)    Positive electrode (7)    Buffer layer (11)    Transparent conductive film (13)