Photovoltaic effect is the key physical mechanism for light-to-electricity conversion and energy harvesting in commercialized solar cells and plays an important role in the field of green technology. Two-dimensional (2D) materials, with layered van der Waals structures, atomically-thin thickness, good compatibility with silicon technology, and rich configurations of heterostructures, offer a unique platform for investigating photovoltaic effect and novel optoelectronic devices. In this talk, I will focus on our recent works on tuning photovoltaic effect in 2D materials and related devices and applications. Firstly, I will show that one-dimensional quantum well structures existing in 2D materials can break the inversion symmetry of their lattice, which supports strong bulk photovoltaic effect (a second order nonlinear optical effect) along the 1D quantum well direction. Secondly, we construct van der Waals twisted structures and systematically investigated their optoelectronic properties. Spontaneous polarization and bulk photovoltaic effect in twisted interfaces are observed. Finally, we nonlinearly and widely tuned the photoresponse profile of 2D materials and their heterostructures under an out-of-plane electric field. This can be attributed to quantum Stark effect, quantum confined Franz-Keldysh effect, and Burstein Moss effect. Based on these effects, we realized an on-chip single-detector-based miniaturized spectrometer which is capable of operating at room temperature with active sensing area of 1500 um² and photodetection range from 1.7 to 3.6 um.