NOVELTY - The gallium nitride solar cell (20) comprises a substrate (100) having a surface with a first kind of photonic crystals, a multilayer semiconductor layer disposed on the surface of the substrate, a second kind of photonic crystals on a top surface of the semiconductor layer, and a first electrode and a second electrode respectively disposed on different conduction-type portions of the multilayer semiconductor layer two terminals for forming ohmic contacts. The multilayer semiconductor layer has an active layer (204). The multilayer semiconductor layer comprises a p-n or a p-i-n structure.    USE - The gallium nitride solar cell is useful in a photovoltaic device of a bi-functional apparatus and a photovoltaic module (all claimed).    ADVANTAGE - The gallium nitride solar cell exhibits excellent opto-electric conversion efficiency and photoelectric performance, increases the absorption probability of the light, prevents the defect caused by mismatch of lattice constant in the growth of heterojunction, and can be economically manufactured with less weight.    DETAILED DESCRIPTION - The gallium nitride solar cell (20) comprises a substrate (100) having a surface with a first kind of photonic crystals, a multilayer semiconductor layer disposed on the surface of the substrate, a second kind of photonic crystals on a top surface of the semiconductor layer, and a first electrode and a second electrode respectively disposed on different conduction-type portions of the multilayer semiconductor layer two terminals for forming ohmic contacts. The multilayer semiconductor layer has an active layer (204). The multilayer semiconductor layer comprises a p-n or a p-i-n structure. An arrangement of the photonic crystals (102) comprises periodic, quasi-periodic or non-periodic arrangement. The periodic arrangement comprises four-fold rotational symmetry or sixfold rotational symmetry. The substrate comprises sapphire, gallium arsenide, germanium, silicon or silicon germanium. The first electrode and the second electrode comprise forked, concentric or circular type. A sapphire substrate is sandwiched by the first electrode and the multilayer semiconductor layer. A portion of the sapphire substrate is removed to let the multilayer semiconductor layer and the first electrode contact. INDEPENDENT CLAIMS are included for:    (1) a bi-functional photovoltaic device;    (2) a bi-functional apparatus;    (3) a method of manufacturing a gallium nitride based solar cell; and    (4) a photovoltaic module.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a solar cell.    Solar cell (20)    Substrate (100)    Photonic crystals (102)    N-type semiconductor layer (202)    Active layer (204)    P-type semiconductor layer. (206)