As semiconductors’ thickness approaches the monolayer limit, excitonic quasiparticles could be observed at room temperature due to the strong Coulomb interaction between space-confined charge carriers. 2D layered semiconductors in monolayer form (e.g., transition metal dichalcogenide monolayers) provide an ideal platform to investigate the transport properties of excitons and their complexes because surface issues in 2D monolayers are not as severe as in bulk semiconductors. I will show that such thickness-determined excitonic systems are promising for optoelectronic applications due to their near-unity photoluminescence (PL) quantum yields (QYs), a key figure of merit dictating the maximum efficiency achievable in light-emitting diodes, lasers, and solar cells. I will discuss the comprehensive recombination mechanisms of excitons in monolayers and showed that the non-radiative recombination pathways can be fully suppressed by electrostatic gating, despite the presence of native defects. This research reveals that room-temperature excitons are robust and bright regardless of monolayer quality, indicating the potential of achieving highly efficient excitonic devices. To deal with the injection inefficiency caused by Schottky contacts, I will show an AC carrier injection architecture, a device concept that is capable of efficiently injecting carriers in various excitonic systems, including monolayer semiconductors, with demonstrations of bright light-emitting devices from infrared to ultraviolet regimes