2, Taiyo Nippon Sanso Corporation, Tsukuba, Ibaraki, Japan
Efficiency of III–V solar cells has increased with the optimization of the device design , though the use of these cells is limited to space and high-concentration terrestrial systems owing to their high manufacturing cost using metal-organic vapor-phase epitaxy (MOVPE) growth. Therefore, it is crucial to develop low-cost, high-throughput growth techniques for implementing large-scale terrestrial modules operating at 1 sun. Hydride vapor-phase epitaxy (HVPE), in contrast to MOVPE, can reduce the cost due to a higher growth rate, use of cheaper group III metal sources, and the capability to grow crystals under low arsenic overpressure . However, the properties of GaAs cells grown by HVPE are not understood well in contrast to those of MOVPE- and molecular beam epitaxy (MBE)-grown devices. In the present study, a comparison of HVPE and MBE growth for GaAs cells was studied. In HVPE, we fabricated the cell on p-GaAs(001) substrates in a custom-built, hot-wall reactor at atmospheric pressure. The quartz reactor tube was designed to have three chambers, two growth chambers and a preparation chamber, with a horizontal flow. Detailed reactor design and growth sequences have been described previously . The temperature of the source and substrate region were set to 850 and 680 °C. For the growth of GaAs, the flow rates of HCl(Ga) and AsH3 were 10 and 50 sccm, respectively, resulting in a growth rate of 14 μm/h. For the growth of InGaP, the flow rates of HCl(Ga), HCl(In), and PH3 were 1.5, 20, and 50 sccm, respectively, resulting in a growth rate of 14 μm/h. The DMZn and H2S were used for p- and n-type dopants. The cell consists of 200 nm-thick p-InGaP BSF / 2 μm-thick p-GaAs base / 100 nm thick n-GaAs emitter / 150 nm-thick n-InGaP window / 200 nm-thick n-GaAs contact layers. An identical cell structure was fabricated at 510 °C at a growth rate of 1 μm/h by MBE. A SiO2/TiO2 antireflection coating was deposited on the cell after the deposition of ohmic contacts. The cell size was 0.08 cm2. The short-circuit current density, open-circuit voltage, fill factor, and efficiency was 23.59 mA/cm2, 0.914 V, 0.768, and 16.56% for the HVPE-grown cell, which was found to be smaller than those of 26.81 mA/cm2, 0.960 V, 0.848, and 21.82% for the MBE-grown cell. The anomarous extra inter layer was clearly observed at the heterointerface between GaAs base layer and InGaP BSF layer for HVPE-grown devices. As a result, recombination of photo-generated carriers was enhanced at the heterointerfaces, resulting in the reduction in the short-circuit current density. In addition, incorporation of iron atoms in wide range of the growth region for HVPE-grown cell, which may come from the reaction of HCl gas with the stainless-steel pipes, degraded its open-circuit voltage and fill factor.  F. Dimroth, et al., IEEE J. Photovolt. 6, 343 (2016).  J. Simon et al., J. Photovolt. 7, 157 (2017).  R. Oshima et al., Proceedings of IEEE PVSC 2017, Washington D.C.