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Kevin Schulte1 John Simon1 Aaron Ptak1

1, National Renewable Energy Laboratory, Lakewood, Colorado, United States

We report the development of GaInP/GaAs monolithic tandem solar cells grown by hydride vapor phase epitaxy (HVPE). HVPE is a route to reduced III-V growth costs because the technique takes advantage of low cost inputs (elemental metals and HCl) and high source utilization, and exhibits high growth rates (up to 5 µm/min). The tandem device consists of three main components: a 1.90 eV Ga0.5In0.5P top cell, a p-Ga0.5In0.5P/n-GaAs tunnel junction, and a 1.41 eV rear heterojunction GaAs cell. The open circuit voltage (VOC) of the tandem is 2.41 V, indicating high material quality in both subcells, and voltage addition through the tunnel junction. Electroluminescence measurements indicate that the individual VOC’s of the top and bottom cells are 1.40 and 1.01 V, respectively, at short circuit. This yields a band gap (EG) voltage offset, WOC = EG/q-VOC = 0.50 and 0.40 V, respectively, where q is the elementary charge. The WOC of the top cell is higher in part because the structure contains an unpassivated front surface, and because the back surface field (BSF) consists of p+ GaInP rather than a higher- EG heterobarrier. The top cell limits the current of this series-connected device for these reasons, with a short-circuit current density, JSC, of 10.1 ± 0.2 mA/cm2. The overall efficiency is 20.3 ± 0.4% (uncertified). We measured the device under concentration to look for signs of tunnel junction breakdown, which we did not observe up to ~1000x, which was the highest concentration tested. Importantly, our dynamic-HVPE reactor enables us to deposit this device, which requires multiple abrupt changes in composition and doping, in a short growth time. We discuss next steps to improve the current result, with a clear pathway towards 30% efficiency. The potential for terrestrial applications of this lower cost III-V technology is also discussed.

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