Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit fascinating optical properties including strong light-matter coupling, prominent excitonic effects, and valley-selective circular dichroism. These properties render TMDs as a potential component for advanced optoelectronics devices. One of the most remarkable phenomena of 2D TMDs is the ultrafast charge transfer of photocarriers across van der Waals (vdW) interfaces, which potentially allows realization of hot carrier optoelectronic devices. Hot photocarrier and exciton dynamics in 2D TMDs depend on intrinsic and extrinsic factors such as many-body effects and interface electronic states. Thus, the prospects for exploiting the photophysical properties of 2D TMDs and their heterostructures for hot carrier optoelectronic devices remain largely unexplored. In this thesis, experimental studies on the photophysics and optoelectronic device properties of 2D TMD-based vdW heterostructures are presented. The first part of this thesis discusses an unconventional electro-optic light upconverter. It is shown that ultrafast interlayer transfer of photoexcited carriers can be electrically induced, allowing upconversion of relatively weak continuous wave laser light. The second part reports on efficient exciton-exciton annihilation (EEA) process in monolayer TMDs. We discover that encapsulation of TMD within hexagonal boron nitride (hBN) films strongly suppresses EEA. The last part reports on unusually robust photocurrent response in a metal-insulator-semiconductor (MIS) heterostructure. It is shown that generation of high energy free carriers via efficient EEA plays a fundamental role in the observed unusual photoresponse of the heterostructures.