3.  Low-cost tandem solar cells

Multijunction solar cells are the solar cell technology with the highest efficiency of any type of solar cell, presently with 47.1% confirmed efficiency under concentration. Monolithic multijunction (MJ) cells have dominated power generation for satellites in space for decades, as well as terrestrial concentrator photovoltaics, both applications in which the very high efficiencies of multijunction cells strongly reduce the overall system costs. The powerful concept of dividing up the solar spectrum in multijunction cells is receiving increasing attention for one-sun, flat-plate solar cells as well, as single-junction silicon cell efficiencies have plateaued and further large jumps in efficiency will likely require a new one-sun cell architecture.

We are investigating the interconnect layers which connect top and bottom cells in low-cost, flat-plate tandem solar cells designs, for their electrical conductance, optical transmittance, and robustness to thermal processing. We are also studying the effect of the growth of top cell materials on silicon bottom cell substrates in monolithic multijunction cell designs, such as the effect of II-VI top cell growth on silicon bulk minority-carrier lifetime in II-VI/Si tandem solar cells. Top cell bandgaps in the 1.7-2.0 eV range are needed in optimum 2- and 3-junction cells in tandem with silicon, higher than prototypical defect-and-interface tolerant (DIT) semiconductors such as low-Ga CIGS with 1.0-1.1 eV, and CH3NH3PbI3 with 1.55 eV bandgap. One of the best ways to gauge the quality of semiconductors with new preparation methods and widely varying bandgap is the bandgap-voltage offset (Woc) formulation of semiconductor device equations. DIT materials with 1.7-2.0 eV bandgap, enabled by discovery of new compositions, interface combinations and treatments, and new processes for bulk defect modification and integration with silicon, could lead to 30%-efficient flat-plate multijunction solar cells at one sun, with a potentially game-changing effect on solar electricity generation costs.