Design and demonstration of ultra-compact microcell concentrating photovoltaics for space

Optical concentration can improve the efficiency and reduce the cost of photovoltaic power but has traditionally been too bulky, massive, and unreliable for use in space. Here, we explore a new ultra-compact and low-mass microcell concentrating photovoltaic (mu CPV) paradigm for space based on the monolithic integration of transfer-printed microscale solar cells and molded microconcentrator optics. We derive basic bounds on the compactness as a function of geometric concentration ratio and angular acceptance, and show that a simple reflective parabolic concentrator provides the best combination of specific power, angular acceptance, and overall fabrication simplicity. This architecture is simulated in detail and validated experimentally with a mu CPV prototype that is less than 1.7 mm thick and operates with six, 650 mu m square triple-junction microcells at a geometric concentration ratio of 18.4x. In outdoor testing, the system achieves a terrestrial power conversion efficiency of 25.8 +/- 0.2% over a +/- 9.50 degrees angular range, resulting in a specific power of approximately 111 W/kg. These results lay the groundwork for future space mu CPV systems and establish a realistic path to exceed 350 W/kg specific power at >33% power conversion efficiency by scaling down to even smaller microcells. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement