A vertical multi-junction photovoltaic device includes a structured substrate including a plurality of substantially vertical elongated structures protruding from a planar surface of the structured substrate. An areal density of the elongated structures at a first sliced plane parallel to the planer surface is different than an areal density of the elongated structures at a second sliced plane parallel to the planar surface. The device further includes least a first sub-cell and a second sub-cell, each having a corresponding vertical p-n or p-i-n junction formed of conformal layers, the first sub-cell being formed in a first region incorporating the first sliced plane and the second sub-cell being formed above the first sub-cell in a second region incorporating the second sliced plane.