Photovoltaics is a key technology within alternative energies. Solar cells enable generation of electricity for a wide range of applications: large scale utilities, commercial and residential roof top installations and various other on-grid and off-grid application. Improving the cell efficiency and reducing the manufacturing cost of solar cells will even increase this broad range of applications.

Multi-junction solar cells can achieve far higher efficiencies compared to single junction cells, because they use several cells to absorb different parts of the solar spectrum. Wafer bonding is a key process step for a variety of process flows for multi-junction solar cells.

Layer transfer
Monolithic multi-junction cells consist of a stack of various epitaxial layers. A big challenge for manufacturing is that the growth of the various epitaxial layers requires that either the crystal lattices are matched or that buffer layers are being integrated. Typically substrates with optimal properties for epitaxial layer growth do not have optimal properties (electrical, thermal) as a PV cell substrate.
Layer transfer by wafer bonding allows growing the individual layers on a growth substrate with optimal crystal lattice. The grown layer is then bonded to the final substrate and the growth substrate is removed. Examples are GaN on Si substrates as well as compound semiconductor multijunction cells e.g. GaInP/GaInAs/Ge.

Inverted metamorphic cells
For monolithic multi-junction cells it is beneficial to grow the solar cell upside down on a GaAs substrate. After processing of all the layers the whole stack is bonded to the device substrate and released from the growth substrate thereby flipping the stack.
For terrestrial CPV applications the solar cell is typically bonded to a rigid substrate with a metal-metal wafer bond. The substrate acts as heat sink. For aeronautic or space applications weight is a major concern. The solar cell can be bonded to very thin, even flexible substrate enabling ultra-light cells.

Mechanically stacked multi-junction cells
In monolithic multi-junction cells all the individual cells are connected in series, which results in the need for tunnel junctions and current matching. Mechanically stacking is a promising alternative approach. Each cell has individual electrical contacts and interconnects, which allows harvesting of all the power generated by each cell.
Mechanically stacked multi-junction cells are manufactured by aligned wafer bonding. High precision wafer alignment allows to minimize shading due to the electrical contacts. With this approach any combination of cells with different bandgaps can be stacked. Even stacking of monolithic multi-junction cells is possible.



cell transfer 
Inverted metamorphic multijunction cell: transfer of the layers from growth substrate to the device substrate.

Scanning Acoustic Image (SAM) of Au-Sn eutectic wafer bond

SEM cross section of Au/Sn eutectic alloy interface.

Monolithic integration vs. mechanically stacking. Courtesy of IMEC