A solar cell stack having a first semiconductor solar cell that has a p-n junction of a first material with a first lattice constant and a second semiconductor solar cell that has a p-n junction of a second material with a second lattice constant. The solar cell stack has a metamorphic buffer that includes a sequence of a first, lower layer and a second, center layer, and a third, upper layer, and includes an InGaAs or an AlInGaAs or an InGaP or an AlInGaP compound. The metamorphic buffer is formed between the first and second semiconductor solar cells and the lattice constant in the metamorphic buffer changes along the buffer's thickness dimension. The lattice constant of the third layer is greater than the lattice constant of the second layer, and the lattice constant of the second layer is greater than the lattice constant of the first layer.

A multijunction solar cell and its method of fabrication, having an upper first solar subcell composed of a semiconductor material including aluminum and having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a first and second DBR structure adjacent to the third solar subcell; and a fourth solar subcell adjacent to the DBR structures and lattice matched with said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV.

A growth structure having a lattice transition (or graded buffer) or an engineered growth structure with a desired lattice constant, different from a lattice constant of conventional substrates like GaAs, Si, Ge, InP, under a release layer or an etch stop layer is used as a seed crystal for growing optoelectronic devices. The optoelectronic device can be a photovoltaic device having one or more subcells (e.g., lattice-matched or lattice-mismatched subcells). The release layer can be removed using different processes to separate the optoelectronic device from the growth structure, which may be reused, or from the engineered growth structure. When using the etch stop layer, the growth structure or the engineered growth structure may be grinded or etched away. The engineered growth structure may be made from a layer transfer process between two wafers or from a ternary and/or a quaternary material. Methods for making the optoelectronic device are also described.

A growth structure having a lattice transition (or graded buffer) or an engineered growth structure with a desired lattice constant, different from a lattice constant of conventional substrates like GaAs, Si, Ge, InP, under a release layer or an etch stop layer is used as a seed crystal for growing optoelectronic devices. The optoelectronic device can be a photovoltaic device having one or more subcells (e.g., lattice-matched or lattice-mismatched subcells). The release layer can be removed using different processes to separate the optoelectronic device from the growth structure, which may be reused, or from the engineered growth structure. When using the etch stop layer, the growth structure or the engineered growth structure may be grinded or etched away. The engineered growth structure may be made from a layer transfer process between two wafers or from a ternary and/or a quaternary material. Methods for making the optoelectronic device are also described.

A multijunction solar cells that include one or more graded-index structures disposed directly above the growth substrate beneath a base layer of a solar subcells. In some embodiments, the graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through a solar cell above the graded-index reflector structure is reflected back into the solar subcell by the graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the solar cell above the graded-index reflector structure is transmitted through the graded-index reflector structure to layers disposed beneath the graded-index reflector structure. The second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

A method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell comprising providing first substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on said substrate having a first band gap; forming a second solar subcell over said first subcell having a second band gap smaller than said first band gap; and forming a grading interlayer over said second sub cell having a third band gap larger than said second band gap forming a third solar subcell having a fourth band gap smaller than said second band gap such that said third subcell is lattice mis-matched with respect to said second subcell.

A solar cell stack having a first semiconductor solar cell that has a p-n junction of a first material with a first lattice constant, and having a second semiconductor solar cell that has a p-n junction of a second material with a second lattice constant. The first lattice constant is smaller than the second lattice constant. The solar cell stack has a metamorphic buffer that includes a sequence of a first, lower AlInGaAs or AlInGaP layer and a second, center AlInGaAs or AlInGaP layer, and a third, upper AlInGaAs or AlInGaP layer, and the metamorphic buffer is formed between the first semiconductor solar cell and the second semiconductor solar cell. The lattice constant in the metamorphic buffer changes along the thickness dimension of the metamorphic buffer, and the lattice constant and the In content increase and the Al content decreases between at least two layers of the metamorphic buffer.

A multijunction solar cells that include one or more graded-index reflector structures disposed beneath a base layer of one or more solar subcells. The graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through a solar cell above the graded-index reflector structure is reflected back into the solar subcell by the graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the solar cell above the graded-index reflector structure is transmitted through the graded-index reflector structure to layers disposed beneath the graded-index reflector structure. The second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

A multijunction solar cell includes an InGaAs buffer layer and an InGaAlAs grading interlayer disposed below, and adjacent to, the InGaAs buffer layer. The grading interlayer achieves a transition in lattice constant from one solar subcell to another solar subcell.

A multijunction solar cell having an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a fourth solar subcell adjacent to and lattice mismatched from said third solar subcell and composed of germanium grown on a growth substrate. In some embodiments of a five junction solar cell, the growth substrate forms a bottom solar subcell and is composed of germanium.