Electroluminescent devices using organic semiconductors offer advantages over their inorganic counterparts such as processability, transparency, and the potential of lower-cost, large-area devices. They are becoming a serious alternative to conventional inorganic technology as their efficiencies and stabilities have improved dramatically over the last years. The most efficient and stable organic light-emitting devices (OLEDs) are based on a multi-stack of small molecular-weight components that use air-sensitive injection layers or metals for efficient electron injection. The multi-layer architecture is obtained by sequentially evaporating the active species under high-vacuum conditions. These devices require rigorous encapsulation to prevent degradation of the electron-injecting layers. Another type of electroluminescent device, referred to as a light-emitting electrochemical cell (LEC), has a much simpler architecture and does not rely on air-sensitive charge-injection layers or metals for electron injection. This greatly simplifies their preparation and makes them more cost efficient. In its simplest form, it consists of a single active layer composed of an ionic transition-metal complex (iTMC). The presence of mobile ions facilitates the formation of ionic junctions that lower the barrier for electron and hole injection and makes these devices independent of the work function of the electrode material. Thus, electroluminescent devices based on iTMCs are simple devices, easy to prepare, and do not require rigorous encapsulation. These characteristics make them suitable for low-cost lighting and signing applications.