Effect of annealing temperature on the characteristics of the modified spray deposited Li-doped NiO films and their applications in transparent heterojunction diode

doi:10.1016/j.solmat.2014.09.017

Solar Energy Materials and Solar Cells

Volume 132, January 2015, Pages 492-498

https://doi.org/10.1016/j.solmat.2014.09.017 Get rights and content

Highlights

•
Non-vacuum method of modified spray pyrolysis was used to fabricate transparent 8 at%-lithium-doped NiO films.
•
The NiO:8-Li films were deposited on ITO/glass substrates, the rectifying current–voltage properties confirmed that a p–n heterojunction diode characteristic was successfully formed in a NiO:8-Li/ITO structure.
•
The ideality factor of the NiO:8-Li/ITO heterojunction diodeof 3.3, barrier height of ~0.72 eV, and the series resistance of~0.21 kΩ were estimated using I–V characteristics.

Abstract

Modified spray method (m-SM) was used to fabricate 8 at%-lithium-doped NiO (NiO:8-Li) films using the nickel nitrate [Ni(NO₃)₂·6H₂O] and lithium nitrate (LiNO₃) solutions. The NiO:8-Li solutions were sprayed on glass substrates and annealed at various temperatures. It found that the resistivity was decreased and the optical bandgap value were increased as annealing temperature of the NiO:8-Li films was increased from 400 °C to 600 °C. As the annealing temperature increased, the ratio (fitting area) of Ni³⁺/Ni²⁺ in the NiO:8-Li films decreased, which was caused by the increasing in carrier concentration. When the NiO:8-Li films was deposited on ITO glass substrates, the rectifying current–voltage (I–V) properties confirmed that a p–n heterojunction diode characteristic was successfully formed in a NiO:8-Li/ITO structure. The NiO:8-Li/ITO heterojunction parameters such as ideality factor (n), barrier height (φ_b), and series resistance (R_s) were determined using conventional forward bias I–V characteristics, Cheung׳s and Norde׳s methods. The ideality factor of 3.3, barrier height of ~0.72 eV, and the series resistance of ~0.21 kΩ were estimated using I–V characteristics.

Introduction

Transparent conducting oxides (TCOs) have been extensively studied in recent years since they not only exhibit high optical transparency in the visible region but also have the high electrical conductivity. At present, TCOs, such as tin oxide (SnO₂), indium tin oxide (ITO), and zinc oxide (ZnO), are routinely used as transparent electrodes and window coatings for optoelectronic devices [1], [2], [3]. In contrast to n-type TCOs (like SnO₂, ITO, and ZnO), nickel oxide (NiO) shows p-type semiconductivity and has attracted much attention due to its excellent chemical stability and unique optical, electrical, and magnetic properties. NiO films have a band gap ranging from 3.6 eV to 4.0 eV and they are transparent to ultraviolet, visible, and near infrared radiation [4]. The resistivity of NiO films can be decreased by doping with monovalent impurities, such as copper (Cu), lithium (Li) and so on [5], [6]. For that, NiO films have found important applications in electrochromic devices [7], organic light emitting diodes [8], gas sensors [9], dye sensitized solar cells [10], and p–n heterojunction junctions [11].

According to the literatures, NiO films can be prepared by sputtering [12], sol–gel, [13] and spray pyrolysis method (SPM) [14]. SPM is a very important method to fabricate the TCO films because spray pyrolysis is a relatively simple and atmospheric pressure deposition process, and it is an inexpensive technique for large-area coating. However, the traditional spray pyrolysis method is sprayed nickel nitrate solution onto the preheated glass substrates (>300 °C). As the substrates are heated at higher temperature, the evaporation ratio of the solution on glass substrates is too swift, resulting in the formation inferior of the NiO films. In this research, a modified spray method (m-SM) was used to develop the 8 at%-Li doped NiO (NiO:8-Li) films. The structural, optical, and electrical characterizations of NiO:8-Li films were studied in detail, and we found that the 600 °C-annealed films had better optical and electrical properties. In addition, the p–n heterojunction diodes were fabricated by depositing NiO:8-Li films on ITO glass substrates. In the past, the surveyed literatures revealed that there were no detailed reports on heterojunction parameter analysis using different methods including Sato and Yasamona׳s [15], Cheung׳s [16] and Norde׳s [17] methods for p-type NiO and n-type ITO. Norde proposed a method for the evaluation of the series resistance (R_s) from the forward current–voltage (I–V) characteristics, in which an ideal diode is sought, namely, with ideality factor (n=1). For n>1, the Sato and Yasamona method used a function F(V) similar to that of the Norde method, taking into account that n can be greater than unity. Therefore, different diode parameters such as ideality factor, barrier height, and series resistance were determined by using Cheung׳s and Norde׳s methods in this research.

Section snippets

Experimental

Corning Eagle XG glass (Corning Incorporated, NY, USA) were used as the substrates to deposit NiO:8-Li films by using the modified spray method (m-SM). Lithium-doped nickel oxide films were prepared by SM with 1 M solution. The nickel nitrate [Ni(NO₃)₂·6H₂O, 99.9% in purity, Alfa Aesar, America] and lithium nitrate (LiNO₃, 99.6% in purity, J. T. Baker, America) were mixed with deionized (D. I.) water to form the 8 at% L-NiO solutions (NiO:8-Li). The NiO:8-Li solutions were baked at 140 °C and

Results and discussion

Fig. 1 shows the XRD patterns of the NiO:8-Li films corresponding to various annealing temperatures. Bragg peaks at 2θ=37.284^o, 43.184^o, and 62.986^o were indexed to the (111), (200), and (220) planes, respectively. In addition, diffraction peaks at (111) and (200) were indexed precisely to a bunsenite structure of NiO, which matched the JCPDS file no. 4-0835. The absence of impurity peaks revealed that the NiO:8-Li films exhibited high crystalline quality. The diffraction intensity of 600

Conclusions

In this study, the high transmittance NiO:8-Li films were deposited by using the non-vacuum process of modified spray method (m-SM). We had found that 600 °C-annealed NiO films had lower resistivity of 3.1×10⁻¹ Ω cm, higher surface roughness of 17.7 nm, higher transmittances of 82.9%, and higher optical bandgap (E_g) of 2.88 eV. Calculated by the Burstein–Moss shift theorey, the E_g value of the NiO:8-Li films increased from 2.75 to 2.83 eV as the annealing temperature increased from 400 °C to 600 °C;

Acknowledgments

The authors acknowledge the National Science Council of Taiwan and Ministry of Science and Technology, 102-2221-E-244-019, 102-2622-E-244-001-CC3 and 103-2221-E-244-018.

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Effect of annealing temperature on the characteristics of the modified spray deposited Li-doped NiO films and their applications in transparent heterojunction diode

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Mater. Lett.

Mater. Sci. Eng. B: Adv.

Thin Solid Films

Appl. Surf. Sci.

Thin Solid Films

Mater. Lett.

J. Eur. Ceram. Soc.

App. Surf. Sci.

Thin Solid Films

Physica E

Thin Solid Films

Preparation of transparent and semiconducting NiO films

Vacuum

Spray deposition and characterization of nanostructured Li doped NiO thin films for application in dye-sensitized solar cells

Nanotechnology

High-performance electrochromic device based on WO3/NiO complementary characteristic and highly porous structure

Nanosci. Nanotechnol. Lett.

Nanostructure modified gas sensor detection matrix for NO transient conversion of NO to NO2

J. Electrochem. Soc.

Improved photocurrents for p-type dye-sensitized solar cells using nano-structured nickel(II) oxide microballs

Energy Environ. Sci.

Orientation dependent band alignment for p-NiO/n-ZnO heterojunctions

J. Appl. Phys.

Electrical and optical properties of NiO films deposited by magnetron sputtering

Opt. Appl.

A sol–gel process for fabrication of NiO/NiCo2O4/Co3O4 composite with improved electrochemical behavior for electrochemical capacitors

ACS Appl. Mater. Interfaces

High-performance electrochromic device based on WO₃/NiO complementary characteristic and highly porous structure

Nanostructure modified gas sensor detection matrix for NO transient conversion of NO to NO₂

A sol–gel process for fabrication of NiO/NiCo₂O₄/Co₃O₄ composite with improved electrochemical behavior for electrochemical capacitors