A photovoltaic cell , comprising: a first p-n junction layer having a first energy band gap between a valence band and a conduction band of said first p-n junction layer , wherein the first p-n- junction layer comprises an intrinsic layer between the n-doped and the p-doped layers of the first p-n- junction layer ; a second p-n junction layer having a second energy band gap between a valence band and a conduction band of said second p-n junction layer , wherein the second p-n junction layer comprises an intrinsic layer between the n-doped and the p-doped layers of the second p-n junction layer , said first energy band gap being greater than said second energy band gap; a tunnel junction layer positioned between said first and second p-n junctions; a plurality of quantum dots , which are nanocrystal structures formed of semiconductor material; a first electrical conductor layer contiguous with said first p-n junction layer ; and a second electrical conductor layer contiguous with said second p-n junction layer , characterized in that the plurality of quantum dots has a distribution confined to a surface of an n-doped layer and a p-doped layer of at least said first p-n layer, wherein said quantum dots are disbursed throughout the n-doped and p-doped layer at a volume density up to 10 21 cm -3 or an areal density from about 10 8 cm -2 to about 10 11 cm -2 wherein the size of the quantum dots , their density and spacing from one another are coordinated so as to provide effective quantum mechanical tunneling of charge carriers energized by incident photons to the conductor layer ; wherein the quantum dots are distributed substantially at the surface of the n-doped and p-doped layers or throughout the volume of the layers in a three dimensional array . comprising: a first p-n junction layer having a first energy band gap between a valence band and a conduction band of said first p-n junction layer , wherein the first p-n- junction layer comprises an intrinsic layer between the n-doped and the p-doped layers of the first p-n- junction layer ; a second p-n junction layer having a second energy band gap between a valence band and a conduction band of said second p-n junction layer , wherein the second p-n junction layer comprises an intrinsic layer between the n-doped and the p-doped layers of the second p-n junction layer , said first energy band gap being greater than said second energy band gap; a tunnel junction layer positioned between said first and second p-n junctions; a plurality of quantum dots , which are nanocrystal structures formed of semiconductor material; a first electrical conductor layer contiguous with said first p-n junction layer ; and a second electrical conductor layer contiguous with said second p-n junction layer , characterized in that the plurality of quantum dots has a distribution confined to a surface of an n-doped layer and a p-doped layer of at least said first p-n layer, wherein said quantum dots are disbursed throughout the n-doped and p-doped layer at a volume density up to 10 21 cm -3 or an areal density from about 10 8 cm -2 to about 10 11 cm -2 wherein the size of the quantum dots , their density and spacing from one another are coordinated so as to provide effective quantum mechanical tunneling of charge carriers energized by incident photons to the conductor layer ; wherein the quantum dots are distributed substantially at the surface of the n-doped and p-doped layers or throughout the volume of the layers in a three dimensional array . <EOS>