Ryan Hool
University of Illinois at Urbana-Champaign
My proposal aims to advance solar technology for both space applications and global energy production. This proposal focuses on NASAs objectives in the STR TABS: specifically, within Space Power & Energy Storage (TA03), Power Generation (3.1), Solar (3.1.3). Through addressing outstanding challenges of monolithic integration of III-V on Si, I aim to demonstrate a III-V with Si tandem solar cell with high efficiency.
High specific power and low cost are crucial for enabling a greater variety of space missions and for increasing the cost competitiveness of solar energy in the global market. The ability to leverage investments in space solar technology to benefit terrestrial power generation and vice versa is one of the great strengths of III-V on Si tandems. Si single-junction (1J) cells have dominated the terrestrial solar market due to low material cost and high 1J efficiency, but little room remains for further improvements toward its theoretical efficiency limit. III-V multi-junction (MJ) cells achieve much higher efficiencies through tailoring bandgaps of sub-cells, but are hindered by small-diameter, high-cost III-V substrates. Through monolithic integration of III-Vs with Si, there is great potential to dramatically reduce the cost of III-V MJ cells and to realize competitive, high-efficiency cells for both space and terrestrial power.
My proposal focuses on the monolithic integration of ~1.7eV GaAsP on Si through metamorphic buffers. Metamorphic growth gradually relaxes lattice mismatch in buffer layers by concentrating defect nucleation outside the active solar cell layers. In our experiments we have found that relaxed GaP on Si can possess nearly the same threading dislocation density (TDD) as relaxed GaAsP on Si, which is a remarkable given that the lattice mismatch of GaAsP on Si is >6x higher than GaP on Si. The primary objective of this proposal is therefore to understand the formation of threading dislocations as a function of thickness and composition through multiple characterization techniques and to optimize growth conditions for reduced TDD.
For metamorphic solar cells, threading dislocations are typically thought of as the primary contributor to reduced efficiency. However, we have recently discovered that point defects play an increasing role in limiting GaAsP device performance as TDD decreases. Thus, the second objective of this proposal is to determine whether rapid thermal annealing can eliminate point defects in GaAsP using time-resolved photoluminescence and other optical techniques.
Lastly, this proposal aims to develop a GaAsP/Si metamorphic tandem achieving higher efficiency than 1J Si. My proposal seeks to grow and optimize a low-loss tunnel junction to realize two-terminal tandem operation and to optimize the overall growth and design for the best combined performance of GaAsP and Si. Achieving high tandem efficiencies will also require me to design high-performance anti-reflection coatings and to develop optimized device processing methods.