(Invited) Advanced Photon Management in Printed High-Efficiency Multijunction Solar Cells

High efficiency photovoltaic (PV) cells represent a clean and efficient method to utilize the abundant solar energy in both space and terrestrial applications. Advances in solar cell technology and associated light management systems are the primary determinants of improvements in efficiency. Single junction (SJ) solar cells are already near theoretical efficiency limits defined by thermalization losses and sub-bandgap transparency. Devices that incorporate multiple junctions (i.e. sub-cells) in monolithic stacks, known as multijunction (MJ) cells, provide one of the most attractive routes to ultrahigh efficiency. Over the last decade, increases in the efficiency of MJ cells correspond to nearly 1% (absolute) per year, reaching values that are presently ~44%. Further improvements, however, will require solutions to daunting challenges in achieving lattice-matched or metamorphic epitaxial growth in complex stacks and in maintaining current matched outputs from each of the sub-cells. Here we present materials and strategies to make printed multijunction solar cell structures, to bypass the challenges in conventional multijunction cell design. In the first part, I will talk about printing-based assembly of microscale, quadruple junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, I will introduce an alternative device architecture, which uses low refractive index interface materials for enhanced photon recycling in printed multijunction solar cell stacks. These results establish routes to ultrahigh efficiency cells and modules, with potential to approach thermodynamic efficiency limits and realize large-scale photovoltaic energy production.