Realization of ZnO/PVK transparent heterojunction by solution processing routes
Prakash Thanigainathan
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Chandrasekar Paramasivan
1
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Department of Medical Bionanotechnology, Chettinad Hospital and Research Institute
, Kelambakkam, Tamilnadu 603103,
India
1
Department of Physics, Saveetha Engineering College
, Saveetha Nagar, Thandalam, Chennai, Tamil Nadu 602 105,
India
In this work, transparent heterojunction between zinc oxide (ZnO) and poly(N-vinyl carbazole) (PVK) was fabricated by solution processing techniques such as spin-coating and dip-coating techniques; then, its performance was studied using current (I)-voltage (V) measurement at room temperature. Before fabricating the heterojunction, initially, the growth characteristics of both thin films were independently optimized on a well-cleaned glass substrate, then its structural properties, optical properties, and surface topography were characterized using an Xray diffractometer, UV-VIS-NIR spectrophotometer, and atomic force microscope, respectively. The structural analysis confirms the existence of a PVK thin film in amorphous nature and ZnO thin film in hexagonal crystal structure. The transparent nature of the heterojunction was found to be more than 85% in the visible and NIR regions with the absorption onset in the ultraviolet region. The observed experimental results explored the possibilities of fabricating ZnO/PVK transparent heterojunction by solution-based routes on a transparent fluorine-doped tin oxide substrate for transparent electronics applications.
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Background
Materials which are displaying the remarkable
combination of high electrical conductivity and optical
transparency already form the basis of many important
technological applications including flat panel displays,
solar energy capture, and other optoelectronic devices
[1]. Transparent electronic devices such as bipolar
transistors and diodes can be fabricated with the help of
p-n transparent junction. After the discovery of the first
transparent semiconductor delafossite CuAlO2 in 1997
by Kawazoe et al. [2], the development of a series of
ptype transparent materials and transparent p-n junction
devices such as UV-light-emitting diodes was triggered.
Very recently, Snure et al. [3] discovered another p-type
transparent oxide (CuBO2) that also belongs to the
delafossite family but exhibits better room-temperature
electrical conductivity (1.65 S/cm). Zinc oxide (ZnO) is a
transparent n-type semiconductor material having a
wide bandgap (e.g., approximately 3.37 eV at room
temperature) with large exciton binding energy of
approximately 60 meV [4]. Such properties make them
well suited for the realization of many optoelectronic
applications. Poly(N-vinyl carbazole) (PVK) is one of the
famous holes conducting polymeric material with
amorphous nature and widely utilized in the fabrication
of blue light-emitting diodes [5]. It has, by linear chains
of repeated molecular groups (H2CHC)n with pendant
carbazole side groups [(C6H4)2NH], arranged randomly
around the same chain. Since the electrical and optical
properties of films strongly depend on their
microstructure, stoichiometric nature, and impurities present, each
deposition technique with its associated parameters
yields films of different properties. Depending on the
deposition method, the substrate can have a significant
influence on the properties of the films. Particularly, the
thermal expansion coefficient of the substrate affects the
orientation of crystallites in the films and its electrical
properties. Solution processing is envisioned as a key
technology in the low-cost production of electronic
devices on both conducting and non-conducting
substrates. In this report, the formation of transparent
heterojunction between ZnO and PVK thin films by
solution processing routes and their performance were
discussed.
Methods
All the precursor and solvent materials were used in
asreceived condition without any further purification.
Transparent PVK solution was prepared in dichloromethane
(DCM) by dissolving 0.5 g of PVK in 20 ml of DCM at
room temperature. The well-cleaned substrates (glass or
fluorine-doped tin oxide (FTO)) were dip-coated using
freshly prepared polymer sol and then baked at 80C for 15
min to get the PVK films. In the case of ZnO thin films,
the sol was prepared [6] using the following procedure:
isopropyl alcohol (136.4 ml) and diethanolamine (5.2 ml)
were stirred for 15 min at room temperature; then, zinc
acetate dehydrate (13.38 g) was added and stirred once
again for 60 min then refluxed at 70C for 30 min, cooled,
and aged for 6 h. The use of metal acetate instead of the
commonly used alkoxides will reduce the problem of
moisture sensitivity like significant factors in assessing the
effectiveness of a process. A commercially available flat
motor with a fan which is able to rotate at 2,000 rpm on
12 V DC supply was used to spin coat the ZnO sol on
the substrates. After deposition, the films were baked in
a box furnace at 450C for 10 min and allowed to reach
room temperature before int (...truncated)