NOVELTY - A solar cell comprises at least one sub-cell (12, 15) containing several specific indium gallium nitride nanocolumns (13, 14).    USE - As solar cell for forming solar panel, which is useful for forming solar concentrator, which is useful for forming power plant; where solar cell, solar panel, solar concentrator and power plant are useful for producing electricity; and for manufacturing electrical device e.g. photovoltaic cell or solar cell (claimed).    ADVANTAGE - In solar cell comprising sub-cell(s) with several InxGa1-xN nanocolumns/nanorods, each nanocolumn/nanorod is substantially defect free; as nanocolumns/nanorods vary in composition along their length, and have adjacent portions separated by tunnel junctions formed by doping nanocolumn, such that tunnel junctions are defect free. Further, under lateral growth conditions, continuous layer is grown on top of nanocolumns/nanorods. The precursor layer and overlayer are doped. Continuous InxGa1-xN layers formed by lateral growth of nanocolumns/nanorods have much lower defect densities than continuous epilayers e.g. 107 to 108 cm-2, as opposed to 109 to 1010 cm-2. The nanorod/nanocolumn devices have several key advantages over devices based on continuous epilayers. The nanorods are perfect single crystals, free of threading defects (less than 108 cm-2), and remain free of defects as they grow laterally until coalescence. The nanorods are compliant structures and have right geometry such that misfit stresses can be eliminated by lateral relaxation, in content to be grown pseudomorphically on GaN base, where composition change is graded. Misfit dislocations and layer stresses which lead to phase separation can thus be avoided. This is not possible with continuous epilayers as there is 7% mismatch between GaN and InN. There is also good evidence that high crystal quality InGaN nanorods can be grown by molecular beam epitaxy (MBE) for all compositions up to pure InN. If threading dislocations are generated e.g. in misfit dislocation source, they are eliminated on nanorod sides; where driving force is relaxation of strain energy. Further, threading defects can be generated in nanorods and are propagated, where they provide top surface growth step. However, for solar cell probability of such defect generation is low (less than 5-10%). Stress relaxation in nanorods also means that composition can be reversed to give InxGa1-xN overlayers with low x. Thus, enables p-type layers (needed in two-junction device) having low x value. This provides crucial flexibility in doping needed to integrate sub-cells into multi-junction device. The solar cell offers advantage of using optimum combination of band gaps for improved overall efficiency, combined with lower epitaxy and processing costs, and without use of toxic materials. The manufacturing is maskless and does not require use of etching chemicals or only in relatively small amounts. InxGa1-xN also has intrinsic properties which are advantageous for solar cells, including high optical absorption of 2x 105 cm-1, giving greater than 90% absorption in 200 nm, compared with greater than or equal to 10 microns for Si, and high carrier mobility and drift velocity which can reduce carrier recombination rates. Moreover, high piezoelectric and spontaneous electric fields present in GaN heterostructures can be used to enhance tunneling through reverse biased junctions. For x greater than 0.43, nanorod/nanocolumn surfaces show electron accumulation, although depth of electron accumulation layer should be 2-3 nm. This suggests that electrons and holes may become spatially separated, and reduce probability of electron-hole recombination.    DETAILED DESCRIPTION - A solar cell comprises at least one sub-cell (12, 15) containing several indium gallium nitride nanocolumns (13, 14) of formula InxGa1-xN (where x is 0-1, preferably greater than or equal to 0.1-1, especially greater than or equal to 0.4-1). The solar cell comprises several sub-cells containing nanocolumns, and tunnel junction is present between each pair of adjacent sub-cells. The x has different values for nanocolumns in sub-cell(s) from value x in other sub-cells, or x takes different value for nanocolumns in each sub-cell. The solar cell comprises first sub-cell and second sub-cell, and tunnel junction is present between first and second sub-cell, and x for nanocolumns in first and second sub-cells is 0.4-0.5 and 0.65-0.8 respectively. The tunnel junction(s) are provided within continuous layer, and within nanocolumns. The nanocolumns have diameter of greater than or equal to 5 nm. The spacing from a nanocolumn to its nearest neighbor(s) is 5-100 nm. The sub-cell(s) is/are located between precursor layer (11) and overlayer. The precursor layer and/or overlayer is continuous epilayer. The solar cell comprises electrical contact(s), and wire(s) are connected to each electrical contact. INDEPENDENT CLAIMS are included for following:    (1) manufacturing electrical device e.g. photovoltaic cell or solar cell, involving: growing precursor layer on substrate, and growing several InxGa1-xN nanocolumns or nanorods on precursor layer;    (2) solar panel comprising: several solar cells, at least one of which comprises the solar cell comprising at least one sub-cell containing several indium gallium nitride nanocolumns of formula InxGa1-xN;    (3) solar concentrator comprising: the solar cell(s) comprising at least one sub-cell containing several indium gallium nitride nanocolumns of formula InxGa1-xN and/or the solar panel(s);    (4) power plant comprising: the solar cell comprising at least one sub-cell containing several indium gallium nitride nanocolumns of formula InxGa1-xN, the solar panel and/or the solar concentrator; and    (5) generating electricity involving: exposing the solar cell comprising at least one sub-cell containing several indium gallium nitride nanocolumns of formula InxGa1-xN to sunlight, preferably via solar concentrator, to generate electric current; and transmitting electric current along transmission line e.g. wire, to location remote from solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows the multi-junction solar cell.    Epitaxial gallium nitride precursor layer (11)    Lower sub-cell (12)    N-doped indium gallium nitride layer (13)    P-doped indium gallium nitride layer (14)    Upper sub-cell (15)    Top layer (16)