2D materials come with a variety of properties including metallic, semiconducting and insulating. This enables them to be assembled into ultrathin devices that have lateral and vertical geometries. Advances in materials chemistry over the recent years means that the building blocks of 2D crystals can be synthesized by bottom up methods to achieve large scale area coverage using chemical vapour deposition. In this talk, I will discuss how we have created ultrathin all 2D opto-electronic devices using graphene as the semi-metal electrode, MoS2 and WS2 as the active semiconductor, and hBN as the insulator. Atomic resolution structural studies are performed using scanning transmission electron microscopy to understand defects, grain boundaries, dopants and interlayer stacking. Lateral devices are constructed to achieve transistor operation, and double up as photodetectors. Vertical devices that utilize tunneling interfaces are assembled and operated as photodetectors and light emitting devices. I will discuss how the thickness of the semiconductor layers and using type II heterojunctions of MoS2 and WS2 can lead to photovoltaic responses. Recent work on the scalable production of arrays of red light emitting devices using only CVD grown 2D crystals will be presented.