Performance Optimization of ZnO QDs/F8BT Heterojunction-Based UV–Visible Photodetectors Using MoOx Hole Transport Layer

This article investigates the effect of the MoO
_x
hole transport layer (HTL) on the performance of a poly(9,9-dioctylfluorene-alcohol-benzothiadiazol) (F8BT) and ZnO colloidal quantum dots (CQDs) heterojunction-based ultraviolet-visible (UV-vis.) photodetector fabricated on indium-tin oxide (ITO) substrates. The performance of the conventional ITO/ZnO CQDs/F8BT/Ag (Device-1) structure is compared with that of the proposed ITO/ZnO CQDs/F8BT/MoO
_x
/Ag (Device-2) structure. The MoO
_x
HTL in the proposed device is used to enhance the hole transport and reduce the dark current by preventing electron injection from anode under reverse bias. On the other hand, the trapping of photogenerated electrons at the intrinsic defects of ZnO CQDs has been explored for enhancing the external quantum efficiency (EQE) beyond 100% by trap-assisted photomultiplication phenomenon. The maximum responsivity (
${R}{)}$
, specific detectivity (
${D}^{\,\ast }{)}$
, EQE, rise time (
$\tau _{\text {on}}{)}$
, and fall time (
$\tau _{\text {off}}{)}$
of Device-2 (Device-1) were obtained as 44 A/W (24 A/W),
$6.5\times 10^{{12}}$
Jones (
$1.3\times 10^{{12}}$
Jones),
$\sim $
14171% (7729%), 0.016 s (0.026 s), and 0.018 s (0.030 s), respectively, under −1 V bias voltage and 25-
$\mu \text{W}$
/cm
²
light intensity of 385-nm wavelength. Furthermore, the MoO
_x
HTL in Device-2 introduced the self-biased feature of the photodetector with the maximum values of
${R}$
,
${D}^{\ast }$
, EQE,
$\tau _{\text {on}}$
, and
$\tau _{\text {off}}$
as
$\sim $
59 mA/W,
$3.70\times 10^{{10}}$
Jones, 18.98%, 0.012 s, and 0.017 s, respectively, under zero-bias operation and 25-
$\mu \text{W}$
/cm
²
intensity of 385 nm.