Solar cell stack

A solar cell stack comprising:

• a first semiconductor solar cell having a p-n junction made of a first material which has a first lattice constant, the first semiconductor solar cell consisting of Ge or GaAs;
• a second semiconductor solar cell having a p-n junction made of a second material which has a second lattice constant, the first lattice constant being at least 0.008 Å smaller than the second lattice constant, the second semiconductor solar cell consisting of InGaAs or InGaP;
• and
• a metamorphic buffer formed between the first semiconductor solar cell and the second semiconductor solar cell, the metamorphic buffer including a series of at least five layers, each lattice constant of the at least five layers of the metamorphic buffer increasing in a direction of the second semiconductor solar cell,
• wherein each lattice constant of the at least five layers of the metamorphic buffer is larger than the first lattice constant,
• wherein the metamorphic buffer adjoins the first semiconductor solar cell and the second semiconductor solar cell,
• wherein the dopant concentration within the at least five layers of the metamorphic buffer is less than 1E19 N/cm3 throughout the metamorphic buffer,
• wherein a lattice constant of a last layer one of the series of at least five layers of the metamorphic buffer in a direction of the second semiconductor solar cell is greater than the lattice constant of the second semiconductor solar cell,
• wherein the metamorphic buffer comprises an additional layer disposed between the last layer of the series of layers of the metamorphic buffer and the second semiconductor solar cell,
• wherein the lattice constant of the additional layer is equal to a lattice constant of the second semiconductor solar cell,
• wherein the hardness increases in steps from a first layer to a second layer and from the second layer to a third layer, and from the third layer to a fourth layer of the metamorphic buffer, and
• wherein the first semiconductor solar cell, the second semiconductor solar cell and the metamorphic buffer are monolithically integrated.