NOVELTY - A photoelectrochemical (PEC) device (D1) comprises tandem PEC cell and counter electrode (110) connected to ohmic contact (106). The tandem PEC cell includes PEC semiconductor including photoactive material for absorbing portion of solar spectrum upon exposure to light to generate electron-hole pairs; photovoltaic (PV) cell connected in series with PEC semiconductor, where PV cell absorbs portion of solar spectrum upon exposure to light to generate biasing voltage for tandem PEC cell and includes conduction band and valence band having band gap; and ohmic contact connected to PV cell.    USE - Photoelectrochemical (PEC) device (D1) for generation of hydrogen and oxygen.    ADVANTAGE - The device (D1) provides spontaneous dissociation of water into hydrogen and oxygen.    DETAILED DESCRIPTION - Photoelectrochemical (PEC) device (D1) comprises tandem PEC cell and counter electrode (110) connected to ohmic contact (106). The tandem PEC cell includes PEC semiconductor including photoactive material for absorbing portion of solar spectrum upon exposure to light to generate electron-hole pairs, where PEC semiconductor includes conduction band and valence band having band gap; photovoltaic (PV) cell connected in series with PEC semiconductor, where PV cell absorbs portion of solar spectrum upon exposure to light to generate biasing voltage for tandem PEC cell and includes conduction band and valence band having band gap; and ohmic contact connected to PV cell. The conduction band and valence band of PEC semiconductor is aligned with corresponding valence band and conduction band of PV cell to form low resistance tunnel junction (114) between PEC semiconductor and the PV cell. The tandem PEC cell functions as a work electrode and is placed together with counter electrode in contact with aqueous solution, such that, upon exposure to light, the PV cell generates internal biasing voltage for tandem PEC cell while PEC semiconductor generates electron-hole pairs that interact with water present in aqueous solution for causing dissociation of water involving reduction process to generate hydrogen and process to generate oxygen.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic representation showing tandem photoelectrochemical cell.    Photoelectrochemical cell (100)    Ohmic contact (106)    Counter electrode (110)    Semiconductor electrolyte interface (112)    Tunnel junction (114)