  Two dimensional transition metal di-chalcogenides (TMDCs) are promising
candidates for ultra-low intensity photodetection. However, the performance of
these photodetectors is usually limited by ambience induced rapid performance
degradation and long lived charge trapping induced slow response with a large
persistent photocurrent when the light source is switched off. Here we
demonstrate an indium tin oxide (ITO)/WSe$_2$/SnSe$_2$ based vertical double
heterojunction photoconductive device where the photo-excited hole is confined
in the double barrier quantum well, whereas the photo-excited electron can be
transferred to either the ITO or the SnSe$_2$ layer in a controlled manner. The
intrinsically short transit time of the photoelectrons in the vertical double
heterojunction helps us to achieve high responsivity in excess of $1100$ A/W
and fast transient response time on the order of $10$ $\mu$s. A large built-in
field in the WSe$_2$ sandwich layer results in photodetection at zero external
bias allowing a self-powered operation mode. The encapsulation from top and
bottom protects the photo-active WSe$_2$ layer from ambience induced
detrimental effects and substrate induced trapping effects helping us to
achieve repeatable characteristics over many cycles.
