  Strong light absorption, coupled with moderate carrier transport properties,
makes two-dimensional (2-D) layered transition metal dichalcogenide (TMD)
semiconductors promising candidates for low intensity photodetection
applications. However, the performance of these devices is severely
bottlenecked by slow response with persistent photocurrent due to long lived
charge trapping, and nonreliable characteristics due to undesirable ambience
and substrate effects. Here we demonstrate ultra-high specific detectivity (D*)
of 3.2x10^14 Jones and responsivity (R) of 5.77x10^4 AW-1 at an optical power
density (P_op) of 0.26 Wm-2 and external bias (V_ext) of -0.5 V in an indium
tin oxide (ITO)/MoS2/copper oxide (Cu2O)/Au vertical multi-heterojunction
photodetector exhibiting small carrier transit time. The active MoS2 layer
being encapsulated by carrier collection layers allows us to achieve negligible
trap assisted persistent photocurrent and repeatable characteristics over large
number of cycles. We also achieved a large D*>10^14 Jones at zero external bias
due to the built-in field of the asymmetric photodetector. Benchmarking the
performance against existing reports in literature shows a pathway for
achieving reliable and highly sensitive photodetectors for ultra-low intensity
photodetection applications.
