Solar cells capable of light‐harvesting during daytime and light‐emission at night are multifunctional semiconductor devices with many potential applications. Here, it is reported that halide perovskite heterojunction interfaces can be refined to yield stable and efficient solar cells. The cell can also operate effectively as an ultralow‐voltage light‐emitting diode (LED) with a peak external quantum efficiency of electroluminescence (EQEEL) of 3.3%. Spectroscopic and microscopic studies reveal that double‐heterojunction refinement with wide‐bandgap salts is key to densifying the packing of perovskite grains and enlarging the bandgaps of the perovskite surfaces that are in contact with charge‐transport semiconductors. The refined perovskite enables a simple device with dual actions of solar cells and LEDs. This type of all‐in‐one device has the potential to be used in multifunctional harvesting–storage–utilization (HSU) systems. A double‐heterojunction refinement strategy with wide‐bandgap salts is used to engineer the bottom interfaces and the top interfaces associated with a perovskite. The double‐heterojunction refinement allows both efficient light‐harvesting through a photovoltaic response and light‐emission under ultralow bias in the same device. By connecting to an electric battery, a multifunctional harvesting–storage–utilization system is realized.