The design of antireflection coating (ARC) for multijunction solar cells is challenging due to the broadband absorption and the need for current matching of each subcell. Silicon nitride, which is deposited by plasma-enhanced chemical vapor deposition (PECVD) using standard conditions, is widely used in the silicon wafer solar cell industry but typically suffers from absorption in the short-wavelength range. We propose the use of silicon nitride deposited by low-frequency PECVD (LFSiN) optimized for high refractive index and low optical absorption as a part of the ARC design for III–V/Ge triple-junction solar cells. This material can also act as a passivation/encapsulation coating. Simulations show that the SiO $_{{\rm 2}}$ /LFSiN double-layer ARC can be very effective in reducing the reflection losses over the wavelength range of the limiting subcell for top subcell-limited, as well as middle subcell-limited, triple-junction solar cells. We also demonstrate that the structure’s performance is stable over expected variations in the layer parameters (thickness and refractive index) in the vicinity of the optimal values.