Inserting a dielectric between the absorber and rear metal electrode of a solar cell increases rear internal reflectance by both limiting the transmission cone and suppressing the plasmonic absorption of light arriving outside of the cone. We fabricate rear reflectors with low-refractive-index magnesium fluoride (MgF2) as the dielectric, and with local electrical contacts through the MgF2 layer. These MgF2/metal reflectors are introduced into amorphous silicon/crystalline silicon heterojunction solar cells in place of the usual transparent conductive oxide/metal reflector. An MgF2/Ag reflector yields an average rear internal reflectance of greater than 99.5% and an infrared internal quantum efficiency that exceeds that of the world-record UNSW PERL cell. An MgF2/Al reflector performs nearly as well, enabling an efficiency of 21.3% and a short-circuit current density of nearly 38 mA/cm(2) in a silicon heterojunction solar cell without silver or indium tin oxide at the rear.