NOVELTY - Structure (S1) comprises: a first photovoltaic sub-cell including a first semiconductor material having a first lattice constant; a second photovoltaic sub-cell formed below the first photovoltaic sub-cell; a third photovoltaic sub-cell formed below the second photovoltaic sub-cell; and a defect-trapping layer disposed between the first and second photovoltaic sub-cells and/or between the second and third photovoltaic sub-cells, where the defect-trapping layer includes a crystalline material, and a non-crystalline material.    USE - The structure is useful as solar cell structure which is useful in a solar cell that converts sunlight to electricity.    DETAILED DESCRIPTION - Structure (S1) comprises: a first photovoltaic sub-cell including a first semiconductor material having a first lattice constant; a second photovoltaic sub-cell formed below the first photovoltaic sub-cell, where the second photovoltaic sub-cell includes a second semiconductor material having a second lattice constant different from the first lattice constant; a third photovoltaic sub-cell formed below the second photovoltaic sub-cell, where the third photovoltaic sub-cell includes a third semiconductor material having a third lattice constant different from the second lattice constant; and a defect-trapping layer disposed between the first and second photovoltaic sub-cells and/or between the second and third photovoltaic sub-cells, where the defect-trapping layer includes a crystalline material (1400) comprising defects arising from lattice-mismatch of the crystalline material with an adjacent semiconductor material, and a non-crystalline material having a trench (320). The trench has a width and a length in a plane of a surface of the adjacent semiconductor material, where the length is greater than the width, the crystalline material is disposed at least partially in the trench, and the defects terminating at the non-crystalline material such that a portion of the non-crystalline material is substantially free from defects. INDEPENDENT CLAIMS are also included for:    (1) a structure (S2) comprising: a semiconductor substrate (300); a first dielectric layer on a first surface of the semiconductor substrate, where the first dielectric layer has a first trench extending to the first surface of the semiconductor substrate; a first solar sub-cell disposed in the first trench, where the first sub-cell comprising a first crystalline material comprises defects terminating at respective sidewalls of the first trench, the first sub-cell further comprises a second crystalline material, where the first crystalline material is disposed between the first surface of the semiconductor substrate and the second crystalline material, and the second crystalline material is substantially free from defects; and a first contact on the first solar sub-cell, where at least a portion of the first contact extends into the first trench; and    (2) a structure (S3) comprising: a semiconductor substrate; a first dielectric layer on a first surface of the semiconductor substrate, where the first dielectric layer has first many trenches extending to the first surface of the semiconductor substrate; a first trapping region comprising a first crystalline material disposed in each of the first trenches, where the first crystalline material comprising defects arise from a lattice-mismatch terminating at respective sidewalls of the first trenches; a second crystalline material on the first trapping region, where the first trapping region is disposed between the first surface of the semiconductor substrate and the second crystalline material, the second crystalline material coalesce between neighboring ones of the first trenches and on a surface of the first dielectric layer distal from the semiconductor substrate to form a first continuous layer, and the first continuous layer is substantially free from defects; a first solar sub-cell on the first continuous layer; and a first contact on the first solar sub-cell, where the first solar sub-cell is disposed between the first continuous layer and the first contact.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram illustrating a three junction solar-cell structure.    Semiconductor substrate (300)    Dielectric layer (310)    Trench (320)    Wetting layer (1140)    Crystalline material (1400)