Performance Optimization of ZnO QDs/F8BT Heterojunction-Based UV–Visible Photodetectors Using MoO<i> _x </i> Hole Transport Layer

Abstract

This article investigates the effect of the MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> /Ag (Device-2) structure. The MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}{)}$ </tex-math></inline-formula> , specific detectivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${D}^{\,\ast }{)}$ </tex-math></inline-formula> , EQE, rise time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\text {on}}{)}$ </tex-math></inline-formula> , and fall time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\text {off}}{)}$ </tex-math></inline-formula> of Device-2 (Device-1) were obtained as 44 A/W (24 A/W), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.5\times 10^{{12}}$ </tex-math></inline-formula> Jones ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.3\times 10^{{12}}$ </tex-math></inline-formula> Jones), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 14171% (7729%), 0.016 s (0.026 s), and 0.018 s (0.030 s), respectively, under −1 V bias voltage and 25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> light intensity of 385-nm wavelength. Furthermore, the MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> HTL in Device-2 introduced the self-biased feature of the photodetector with the maximum values of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${D}^{\ast }$ </tex-math></inline-formula> , EQE, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\text {on}}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{\text {off}}$ </tex-math></inline-formula> as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 59 mA/W, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.70\times 10^{{10}}$ </tex-math></inline-formula> Jones, 18.98%, 0.012 s, and 0.017 s, respectively, under zero-bias operation and 25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> intensity of 385 nm.

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