NOVELTY - A glass substrate (5) comprises 45-70 wt.% silica, 11-20 wt.% alumina, 0.5 wt.% or less boron trioxide, 0-6 wt.% magnesium oxide, 4-12 wt.% calcium oxide, 5-20 wt.% strontium oxide, 0-6 wt.% barium oxide, 0-8 wt.% zirconia, 4.5-10 wt.% sodium oxide, and 3.5-15 wt.% potassium oxide, based on oxide. The glass substrate has glass transition temperature of 640-700 degrees C, average thermal expansion coefficient of 60x 10-7-110x 10-7/( degrees C) and density of 2.45-2.9 g/cm3.    USE - Glass substrate is used for solar cell (claimed) e.g. copper-indium-gallium-selenium solar cell.    ADVANTAGE - The glass substrate has high glass transition point temperature and glass strength, favorable moldability, loss-of-clarity property and average coefficient of thermal expansion, and reduced glass density. The solar cell containing the glass substrate has high electric power generation efficiency.    DETAILED DESCRIPTION - A glass substrate comprises 45-70 wt.% silica, 11-20 wt.% alumina, 0.5 wt.% or less boron trioxide, 0-6 wt.% magnesium oxide, 4-12 wt.% calcium oxide, 5-20 wt.% strontium oxide, 0-6 wt.% barium oxide, 0-8 wt.% zirconia, 4.5-10 wt.% sodium oxide, and 3.5-15 wt.% potassium oxide, based on oxide. The total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide is 10-30 wt.%. The sum of sodium oxide and potassium oxide is 8-20 wt.%. The weight ratio of sodium oxide and potassium oxide is 0.7-2. The content (X) of sodium oxide, magnesium oxide, calcium oxide and potassium oxide satisfies relation: (2x Na2O(%mass)-2x MgO(%mass)-CaO (%mass))x (Na2O (%mass)/K2O (%mass))=3-22. The glass substrate has glass transition temperature of 640-700 degrees C, average thermal expansion coefficient of 60x 10-7-110x 10-7/( degrees C) and density of 2.45-2.9 g/cm3. An INDEPENDENT CLAIM is included for solar cell (1).    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the solar cell containing the glass substrate.    Solar cell (1)    Glass substrate (5)    Buffer layer (11)    Transparent conductive layer (13)    Negative electrode (15)