World's highest efficiency triple-junction solar cells fabricated by inverted layers transfer process

A world record-setting efficiency of 35.8% at AM1.5G (x1) has been demonstrated by an InGaP (1.88 eV)/GaAs/InGaAs (0.97 eV) triple-junction solar cell fabricated using the inverted layer transfer process. Lattice-matched top and middle cells are grown first. Then, a lattice-mismatched bottom cell is grown to attain good crystal quality for the top and middle cells. A large stress caused by the increasing lattice constant is successfully released in a buffer layer between the middle and bottom cells, and a high Voc close to Eg/q-0.4 V has been achieved for the lattice-mismatched InGaAs bottom cell. The high Voc of over 3.0 V contributed to the record efficiency. After epitaxial growth, the cell layers on the GaAs substrate are transferred on a handling substrate. As the cell layer is transferred onto a film substrate, a lightweight flexible cell can be fabricated. New triple-junction cells will be applied to a flexible module, called a “space solar sheet,” after optimization of the cell structure for the AM0 spectrum and radiation tolerance. A heat-resistant concentrator cell can be obtained by transferring the cell layer onto a heat sink substrate. A terrestrial concentrator cell using the new structure is also attractive, because a high efficiency of close to 45% can be expected.