Semiconductor monolayers exhibit excellent device characteristics, as well as novel electrical and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation of the layered chalcogenides, and the development of a monolayer light-emitting device using a new device concept. We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS2. The work presents the first demonstration of an optoelectronically perfect monolayer. Moving forward to the device level, forming n- and p-type contacts is a necessary step for developing a light emitting device but it is challenging for the monolayers. We circumvent this challenge via a new device design that allows for high-level injection of electrons or holes without the requirement of ohmic contacts. Taking advantage of the device simplicity, we have been able to show the first millimeter-scale light emission and transparent digital display using semiconductor monolayers. Additionally, I will discuss the challenges of the materials system and present possible solutions towards achieving high efficient monolayer light emitting devices.