NOVELTY - A multi-junction solar cell (400) comprises substrate (411) with metal layer (409); a first sub-cell (405) placed on metal layer; a second sub-cell (403) placed on first sub-cell; and tunnel junction layer (407) between the sub-cells. Each sub-cell includes semiconductor p/n junction, and is fabricated from same semiconductor. The surface of each sub-cell has indents/protrusions with a depth of greater than or equal to lambda /2 and width of greater than lambda , where lambda is the de Broglie wavelength corresponding to an electron of predetermined energy in semiconductor. The electron energy is different in each sub-cell.    USE - As a multi-junction solar cell for converting solar energy to electrical energy, which is used for powering electrical devices.    ADVANTAGE - The multi-junction solar cell converts solar energy to electrical energy (i.e. photocurrent and photovoltage) with improved efficiency. As each sub-cell is fabricated from the same semiconductor material, all sub-cells have identical lattice constant. Therefore, thermal energy losses due to imperfect lattice matching is prevented. Nanosized indentations/protrusions formed on the surface of each sub-cell, modifies the size of the semiconductor bandgap, and creates appropriate bandgaps to efficiently harness a larger portion of the solar spectrum. Thus, the solar cell exhibits improved solar energy conversion efficiency. In addition, the thickness of sub-cells can be selected, to optimize the solar to electrical energy conversion efficiency of the solar cell. The solar cell is grown monolithically in a single deposition process, rather than forming individual cells and then stacking the formed cells. The monolithic growth avoids the manufacturing and technical difficulties associated with stacking. The photocurrents generated by each sub-cell are precisely matched by varying cell thicknesses, thus harnessing all the current produced by the cell. The bandgap of the semiconductor materials forming the sub-cells, is optimized to allow each sub-cell to absorb the desired part of the solar spectrum. Larger portion of the solar spectrum is absorbed due to the presence of multiple sub-cells, each having a different bandgap, in the composite solar cell. As a result, the efficiency of solar cell is increased, and the cost of solar energy is reduced, thus making the solar cell a more competitive energy source.    DETAILED DESCRIPTION - A multi-junction solar cell (400) comprises a substrate (411); a metal layer (409) disposed on the substrate; a first sub-cell (405) positioned on the metal layer; a second sub-cell (403) positioned on the first sub-cell; and a tunnel junction layer (407) interposed between the two sub-cells, to facilitate current flow between the sub-cells. Each sub-cell includes a semiconductor p/n junction, and is fabricated from same semiconductor. The surface of each sub-cell has a periodically repeating structure of indents/protrusions, each having a depth of greater than or equal to lambda /2 and a width of greater than lambda , where lambda is the de Broglie wavelength corresponding to an electron of a predetermined energy in the semiconductor. The predetermined energy of the electron in first sub-cell is not equal to the predetermined energy of electron in second sub-cell. The solar cell includes a window layer (401) positioned on the second sub-cell, to allow solar radiation to enter the solar cell; and electrical contacts attached to the solar cell, to conduct current away from and into the solar cell. The substrate comprises a semiconductor or a polymer. The predetermined energy of the electron in the first sub-cell is preferably less than the predetermined energy of electron in the second sub-cell. The depth of the indents/protrusions in first sub-cell is preferably less than depth of the indents/protrusions in the second sub-cell. The semiconductor p/n junction of the second sub-cell preferably has a bandgap greater than that of the semiconductor p/n junction of first sub-cell.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a multi-junction solar cell.    Multi-junction solar cell (400)    Window layer (401)    Second sub-cell (403)    First sub-cell (405)    Tunnel junction layer (407)    Metal layer (409)    Substrate. (411)