  Hybrid heterostructure based phototransistors are attractive owing to their
high gain induced by photogating effect. However, the absence of an in-plane
built-in electric field in the single channel layer transistor results in a
relatively higher dark current and require a large operating gate voltage of
the device. Here, we report novel air-stable cesium lead iodide/tungsten
di-sulfide (CsPbI$_3$/WS$_2$) mixed dimensional heterostructure based
photo-field-effect-transistors (photo-FETs) with asymmetric metal electrodes
(Cr/WS$_2$/Au), exhibiting extremely low dark current (~10-12 A) with a
responsivity of ~102 A/W at zero gate bias. The Schottky barrier (WS$_2$/Au
interface) induced rectification characteristics in the channel accompanied by
the excellent photogating effect from solution-processed $\alpha$-phase
CsPbI$_3$ NCs sensitizers, resulting in gate-tunable broadband photodetection
with a very high responsivity (~104 A/W) and excellent sensitivity (~106). Most
interestingly, the device shows superior performance even under high humidity
(50-65%) conditions owing to the formation of cubic $\alpha$-phase CsPbI$_3$
nanocrystals with a relatively smaller lattice constant (a = 6.2315 {\AA}) and
filling of surface vacancies (Pb2+ centres) with the sulfur atoms from WS$_2$
layer, thus protecting it from environmental degradation. These results
emphasise a novel strategy for developing mixed dimensional hybrid
heterostructure based phototransistors for futuristic integrated
nano-optoelectronic systems.
