  Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction
solar cells on Ge|Si virtual substrates has been attempted by thinning the
structure, namely the Ge bottom cell and the GaInAs middle cell. The
theoretical analysis performed using realistic device parameters indicates that
the GaInAs middle cell can be drastically thinned to 1000 nm while increasing
its In content to 8% with an efficiency loss in the 3-junction cell below 3%.
The experimental results show that the formation of macroscopic cracks is
prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells.
These prototype crack-free multijunction cells demonstrate the concept and were
used to rule out any possible component integration issue. The performance
metrics are limited by the high threading dislocation density over 2e7cm-2 in
the virtual substrates used, but an almost current matched, crack-free, thinned
3-junction solar cell is demonstrated, and the pathway towards solar cells with
higher voltages identified.
