We present a light-trapping design capable of significantly enhancing the photon absorption in any subcell of a multi-junction solar cell. The design works by coupling incident light into waveguide modes in one of the subcells via a diffraction grating, and preventing these modes from leaking into lower subcells via a low-index layer and a distributed Bragg reflector, which together form an omnidirectional mirror. This allows the thickness of the target subcell to be reduced without compromising photon absorption, which improves carrier collection, and therefore photocurrent. The paper focuses on using the composite structure to improve the radiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown via simulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700 nm, whilst maintaining strong photon absorption, and that this leads to a significantly improved end-of-life photocurrent in the Ga(ln)As middle cell. However, the design can in general be applied to a wide range of multi-junction solar cell types. We discuss the principles of operation of the design, as well as possible methods of its fabrication and integration into multi-junction solar cells. (C) 2016 Elsevier B.V. All rights reserved.