NOVELTY - The solid state device comprises a first solar cell and a second solar cell of first composition and second composition respectively, and a third region of third composition separating the first solar cell from the second solar cell. The first composition is different from the second composition. The third region comprises a first and second rare-earth such that the incident radiation passing through the first solar cell is converted into radiation of different wavelength by the third region. The first composition comprises elements from group IV.    USE - The solid state device is useful for converting incident radiation into electrical energy.    ADVANTAGE - The solid state device ensures economical and efficient conversion of incident radiation to electrical energy with minimal increase in size and reduced thickness of spectral conversion material.    DETAILED DESCRIPTION - The solid state device comprises a first solar cell and a second solar cell of first composition and second composition respectively, and a third region of third composition separating the first solar cell from the second solar cell. The first composition is different from the second composition. The third region comprises a first and second rare-earth such that the incident radiation passing through the first solar cell is converted into radiation of a different wavelength by the third region. The first composition comprises elements from group IV. The second composition comprises elements from groups III and V, and groups II and VI. The composition of the third region is described by (RE1)v(RE2)w(RE3)x(J1)y(J2)z, where (RE) is a rare earth, and (J1) and (J2) are oxygen, nitrogen and phosphorus, where x, y, v, w and z are 0-5. The third region comprises a first portion of fourth composition adjacent to the first solar cell, a second portion of fifth composition, and a third portion of sixth composition separated from the first portion by the second portion and adjacent to the second solar cell. The fifth composition is different from the fourth and sixth compositions. The second portion comprises a first surface adjacent to the first portion, a second surface adjacent to the third portion, and a superlattice with a structure comprising two layers of different composition. The fifth composition varies between the first surface and the second surface. The first portion is present in a first state of stress, and the third portion is present in a second state of stress different than the first state of stress. The first solar cell is composition X(1-n)Ym, and the second solar cell is composition X(1-n)Yn, where n is different from m, and X and Y are group IV semiconductor such that n is 0 and m is 1. The rare earth layer is a composition described by (RE1)v(RE2)w(RE3)x(J1)y(J2)z, where (RE) is a rare earth and (J1) and (J2) are oxygen, nitrogen, and phosphorus, and w and z are 0-5 and v, x and y are 0-5 for the rare earth layer. The solid state device further comprises a first semiconductor layer operable as a photovoltaic cell, a second semiconductor layer operable as a photovoltaic cell, a first rare earth layer of a first rare earth composition separating the first semiconductor layer from the second semiconductor layer, and a second rare earth layer of a second rare earth composition separated from the first rare earth layer by the second semiconductor layer. The first rare earth composition is described by (RE1)v(RE2)w(RE3)x(J1)y(J2)z and the second rare earth composition is described by (RE1)a(RE2)b(RE3)c(J1)d(J2)e, where (RE) is a rare earth, and (J1) and (J2) are oxygen, nitrogen, and phosphorus, and a, b, e, v, w and z are 0-5, and c, d, x and y are 0-5 such that the first rare earth composition is different than the second rare earth composition. The incident radiation passing through the first semiconductor layer is converted into radiation of a different wavelength by the rare earth layer. The first and second semiconductor materials are group IV elements, group III-V elements or group II-VI elements.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a three terminals, dual back contact, and tandem solar cell.