NOVELTY - The cell has a set of dilute nitride sub-cells e.g. top dilute nitride sub-cells (1), among which each dilute nitride sub-cell has an emitter, a base, an emitter-base junction (1-2) and a graded doping profile formed in the emitter and the base. The doping profile is dependent upon depth position such that dopant concentration is lowest at the emitter-base junction and increased with a distance from the emitter-base junction, where the dopant concentration is about 1x 1015-1x 1019per cubic centimeter. A doping process is performed at the emitter-base junction.    USE - Multi-junction solar cell e.g. lattice-matched, multi-junction solar cell such as triple junction solar cell, 4-junction solar cell and 5-junction solar cell. Can also be used for a 6-junction solar cell.    ADVANTAGE - The graded doping profile formed in the emitter and the base is exponentially doped so as to improve a set of performance characteristics of the solar cell. The doping profile is dependent upon the depth position such that the dopant concentration is lowest at the emitter-base junction and increased with the distance from the emitter-base junction, where a doping process and doping distribution are selected to improve and ultimately optimize a short circuit current and an open circuit voltage in a dilute nitride layer, thus enhancing efficiency of the solar cell. The dopant concentration is dependant on the depth so as to improve collection of minority carriers and increase a photocurrent. An exponential doping profile introduces a linear and constant electric field in the emitter and creates a set of fields of differing geometries such that variation in the profile is possible to change the electric field as a function of the position to improve current collection without an accompanying loss of short circuit current and/or open circuit voltage.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for forming a graded doping profile.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an exemplary triple junction solar cell with three sub-cells of which the bottom cell is a dilute nitride sub-cell.    Top dilute nitride sub-cell (1)    Emitter-base junctions (1-2, 2-3)    Middle dilute nitride sub-cell (2)    Lower dilute nitride sub-cell (3)