NOVELTY - The photovoltaic junction comprises first and second bulk semiconductor regions, and an intrinsic region disposed between the bulk regions. The intrinsic region comprises twenty quantum wells formed of indium gallium phosphide (InGaAsP) and separated by quantum barriers. Some of the barriers are formed of a semiconductor material. One of the bulk semiconductor regions is formed of a doped semiconductor material. The quantum wells and quantum barriers have compositions and thicknesses such that the quantum wells and barriers provide compensating compressive and tensile stresses.    USE - The photovoltaic junction of a solar cell is useful in a photovoltaic system for generating electrical power (all claimed).    ADVANTAGE - The photovoltaic junction of a solar cell provides an alternative barrier/well structure based on gallium indium phosphide with an extended wavelength absorption edge, and allows a relatively greater proportion of quantum well thickness within the structure and a greater flexibility for choice of well and barrier thickness thus allowing defect free growth and reliably disordered mix of group III species and increasing the efficiency of the cell.    DETAILED DESCRIPTION - The photovoltaic junction comprises first and second bulk semiconductor regions, and an intrinsic region disposed between the bulk regions. The intrinsic region comprises twenty quantum wells formed of indium gallium phosphide (InGaAsP) and separated by quantum barriers. Some of the barriers are formed of a semiconductor material. One of the bulk semiconductor regions is formed of a doped semiconductor material. The quantum wells and quantum barriers have compositions and thicknesses such that the quantum wells and barriers provide compensating compressive and tensile stresses that balance at a common lattice spacing matched to that of an underlying substrate and/or that of at least one of the bulk semiconductor regions. The absorption band edge of the quantum wells has a wavelength of 700-740 nm. Some or all of the InGaAsP quantum wells are less than 10 nm thick. INDEPENDENT CLAIMS are included for:    (1) a solar cell;    (2) a photovoltaic system for generating electrical power from solar illumination;    (3) a method of forming a photovoltaic junction using epitaxial growth; and    (4) a method of providing an extended absorption edge in a GalnP based photovoltaic junction lattice matched to gallium arsenide or germanium.    DESCRIPTION OF DRAWING(S) - The figure shows a structure of a tandem junction solar cell.    Substrate (20)    Lower photovoltaic junction (22)    Tunnel junction (24)    Upper photovoltaic junction (26)    Window layer (28)    Electrodes. (30)