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Ge1-x-ySixSny alloys have reached a level of maturity that permits the creation of prototype devices on group-IV platforms. Here, we compare the optical and electrical properties of GeSiSn diodes with similar target compositions (Si; 10-11%, Sn; 1.7-2.3%) grown directly on Si(100) and Ge(100) using low-temperature ultrahigh vacuum chemical vapor deposition reactions of designer hydrides. The diodes grown on Ge substrates have relatively low ideality factors in the 1.3-1.4 range and low dark currents with a sizable diffusion component. The corresponding characteristics of the analogous devices grown on Si are significantly degraded due to mismatch-induced reduction in crystal quality. Quantum efficiency measurements show that both sets of diodes have absorption edges near 1 eV with collection efficiencies reaching at least 76% in the devices grown on Ge. Collectively, these results suggest that Ge1-x-ySixSny alloys represent a viable alternative as the long sought photovoltaic material with a lattice constant equal to that of Ge and a bandgap around 1 eV.