Compared to near-infrared (NIR) organic light-emitting devices, solid-state NIR light-emitting electrochemical cells (LECs) could possess several superior advantages such as simple device structure, low operating voltages and balanced carrier injection. However, intrinsically lower luminescent efficiencies of NIR dyes and self-quenching of excitons in neat-film emissive layers limit device efficiencies of NIR LECs. In this work, we demonstrate a tandem device structure to enhance device efficiencies of phosphorescent sensitized fluorescent NIR LECs. The emissive layers, which are composed of a phosphorescent host and a fluorescent guest to harvest both singlet and triplet excitons of host, are connected vertically via a thin transporting layer, rendering multiplied light outputs. Output electroluminescence (EL) spectra of the tandem NIR LECs are shown to change as the thickness of emissive layer varies due to altered microcavity effect. By fitting the output EL spectra to the simulated model concerning microcavity effect, the stabilized recombination zones of the thicker tandem devices are estimated to be located away from the doped layers. Therefore, exciton quenching near doped layers mitigates and longer device lifetimes can be achieved in the thicker tandem devices. The peak external quantum efficiencies obtained in these tandem NIR LECs were up to 2.75%, which is over tripled enhancement as compare to previously reported NIR LECs based on the same NIR dye. These efficiencies are among the highest reported for NIR LECs and confirm that phosphorescent sensitized fluoresce combined with a tandem device structure would be useful for realizing highly efficient NIR LECs.