TECHNOLOGY FOCUS - ELECTRONICS - Preferred Component: The solar cell includes tunnel junctions formed within the subcells, and an electrically conductive substrate upon which the subcells are disposed. Each subcell has effective bandgap energy. It is for absorbing same fraction of solar photons. The tunnel junctions electrically connect the subcells, which are disposed to increase effective bandgap energy. A subcell with lowest effective bandgap energy is closest to the substrate. Each subcell consists of three subcells, in which a first subcell has the lowest effective bandgap energy, a third subcell has a highest effective bandgap energy, and a second subcell includes self-assembled quantum dot material. The first subcell is an interdiffused portion of the substrate. The substrate is n-doped or p-doped. Each first, second and third subcell includes an n-p or an n-i-p junction, an n-side of junction closer to the substrate than a p-side of the junction; or a p-n or a p-i-n junction, a p-side junction closer to the substrate than the n-side of the junction. The subcells may contain four subcells.Preferred Parameter: The first, second, and third subcells have effective bandgap energy of 0.7, 1.16, and 1.8 eV, respectively. The four subcells have effective bandgap energy of 0.7, 1.0, 1.4, and 1.8 eV, respectively.Preferred Method: The first subcell is epitaxially grown.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Material: The substrate is a germanium (Ge), gallium arsenide (GaAs), or silicon substrate. The first subcell includes Ge. The second subcell includes indium gallium arsenide (InGaAs) quantum dot layers intercalated with GaAs or aluminum gallium arsenide (AlGaAs) layers. The third subcell includes gallium indium phosphide (GalnP), AlGaAs or aluminum gallium indium phosphide (AlGaInP). The fourth subcell includes GaInP or AlGaAs.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Material: The substrate is a germanium (Ge), gallium arsenide (GaAs), or silicon substrate. The first subcell includes Ge. The second subcell includes indium gallium arsenide (InGaAs) quantum dot layers intercalated with GaAs or aluminum gallium arsenide (AlGaAs) layers. The third subcell includes gallium indium phosphide (GalnP), AlGaAs or aluminum gallium indium phosphide (AlGaInP). The fourth subcell includes GaInP or AlGaAs.