NOVELTY - A solar cell (1) is formed by laminating lower electrode layer (3), silicon film layer (4) and transparent electrode layer (5) on a hard transparent substrate (2). The hard transparent substrate is obtained by hardening a hardening resin (1). The hardening resin has metal-oxide dense structural unit and organic material and organometallic oxide-containing sparse structural unit in the weight ratio of 0.01-5. The hardening resin has average molecular weight of 800-60000.    USE - Solar cell.    ADVANTAGE - The solar cell has excellent strength, transparency, heat resistance, light resistance and dimensional stability.    DETAILED DESCRIPTION - A solar cell is formed by laminating lower electrode layer, silicon film layer and transparent electrode layer on a hard transparent substrate. The hard transparent board is obtained by hardening a hardening resin of formula (1). The hardening resin has metal-oxide dense structural unit and organic material and organometallic oxide-containing sparse structural unit in the weight ratio of 0.01-5. The hardening resin contains unsaturated bond(s) and has average molecular weight of 800-60000. The metal-oxide dense structural unit has packing coefficient (Kp) of 0.68-0.8 measured by free-volume fraction. The organic material and organometallic oxide-containing sparse structural unit has packing coefficient of less than 0.68. The packing coefficient is calculated by using the formula: Kp=An-Vw-p/Mw, where An is Avogador's number, Vw is Vander Waals volume, p is density and Mw is molecular weight. The vander Waals volume is calculated using the formula: Vw=( Sigma )Va, in which Va is 4( pi )/R3-( Sigma )1/3( pi )hi2(3Ra-hi), where hi is Ra-(Ra2+di2-Ri2)/2di, in which Ra is atomic radius, Ri is combined atomic radius and di is interatomic distance. The hardening resin is of formula: -((A)-(B')m)n- (1).    A=metal-oxide dense structural unit;    B'=organic material and organometallic oxide-containing sparse structural unit;and    m,n=1 or more.    DESCRIPTION OF DRAWING(S) - The drawing shows cross-sectional view of the structure of solar cell.    Solar cell (1)    Hard transparent substrate (2)    Lower electrode layer (3)    Silicon film layer (4)    Transparent electrode layer (5)