NOVELTY - A photovoltaic system comprises a photovoltaic device comprising a lower photovoltaic cell (22) fabricated from semiconductor material having a first bandgap, an electrically insulating layer (26) monolithically fabricated on the lower cell, and an upper photovoltaic cell (24) monolithically fabricated on the insulating layer from semiconductor material having a larger second bandgap; and two photon sources to supply photons to the photovoltaic device.    USE - As photovoltaic cell, e.g. solar cell.    ADVANTAGE - The device is operable over two separate wavelength regimes, but which removes the requirement for current matching between individual cells and does not require any tunnel junctions. The electrical isolation of the upper and lower cells allows each to be designed and operated with optimum efficiency for wholly different spectral ranges. The device has an improved dependence of efficiency on spectral and temperature variations. The independent electrical operation of the cells renders degradation less important than for series-connected cells since each cell can give continuous optimum conversion without being affected by the other. The monolithic structure provides good optical connection between the upper and lower cells and good thermal connection throughout the parts of the device so that excess heat can be effectively coupled to a heatsink. An overall efficiency is increased.    DETAILED DESCRIPTION - A photovoltaic system comprises:    (1) a photovoltaic device comprising a lower photovoltaic cell fabricated from semiconductor material having a first bandgap and having first electrical contacts for extraction of current from the lower cell, an electrically insulating layer monolithically fabricated on the lower photovoltaic cell, and an upper photovoltaic cell monolithically fabricated on the insulating layer from semiconductor material having a second bandgap larger than the first bandgap, and having second electrical contacts for extraction of current from the upper cell;    (2) first photon source(s) to supply photons to the photovoltaic device and having wavelengths in a first wavelength range associated primarily with the first bandgap; and    (3) second photon source(s) to supply photons to the photovoltaic device.    The photons have wavelengths in a second wavelength range primarily associated with the second bandgap. An INDEPENDENT CLAIM is included for a method of generating electricity via the photovoltaic effect.    DESCRIPTION OF DRAWING(S) - The figure shows schematic representation of systems incorporating photovoltaic devices.    Lower photovoltaic cell (22)    Upper photovoltaic cell (24)    Insulating layer (26)    Sun (80, 82)