International Journal of Photoenergy
Volume 2018 (2018), Article ID 6062390, 7 pages
https://doi.org/10.1155/2018/6062390

Research Article

Fabrication of Affordable and Sustainable Solar Cells Using NiO/TiO₂ P-N Heterojunction

Kingsley O. Ukoba, Freddie L. Inambao, and Andrew C. Eloka-Eboka

Mechanical Engineering, University of KwaZulu-Natal, Durban, South Africa

Correspondence should be addressed to Kingsley O. Ukoba; ukobaking@yahoo.com

Received 7 December 2017; Revised 22 January 2018; Accepted 28 January 2018; Published 29 March 2018

Academic Editor: Ahmad Umar

Copyright © 2018 Kingsley O. Ukoba et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The need for affordable, clean, efficient, and sustainable solar cells informed this study. Metal oxide TiO₂/NiO heterojunction solar cells were fabricated using the spray pyrolysis technique. The optoelectronic properties of the heterojunction were determined. The fabricated solar cells exhibit a short-circuit current of 16.8 mA, open-circuit voltage of 350 mV, fill factor of 0.39, and conversion efficiency of 2.30% under 100 mW/cm² illumination. This study will help advance the course for the development of low-cost, environmentally friendly, and sustainable solar cell materials from metal oxides.

1. Introduction

The need for affordable and sustainable electricity in developing nations has been an issue of concern to all stakeholders. Renewable energy has been identified as a viable solution to ending global electricity problems as its availability exceeds world energy demand [1]. Solar energy is a good source of renewable energy [2]. The hourly solar influx on the surface of the earth surpasses annual human energy needs [3]. Photovoltaic energy has received increasing interest caused by a decrease in module prices in countries like China [4]. Interest in these devices is due to improved reliability, efficiency, and costs in generating electricity [5]. Solar cells of high efficiency have been achieved with inorganic materials [6], but they require expensive materials of high purity and a technique that is energy intensive. There is, therefore, a need to explore ways of manufacturing solar cells that can scale-up to large volumes at low cost.

Metal oxide solar cells offer a good replacement for conventional silicon solar cells. This is because metal oxides are low-cost materials, have flexible optical properties, can be deposited using low-cost techniques, and are simple to scale-up to large volume production. They also display quantum confinement effects in two dimensions [7]. Nanostructured metal oxides are used in a wide range of device applications because of their broad composition and band structures [8–14]. The widely used oxides are ZnO [15], CuO [16], In₂O₃ [17], and TiO₂ [18], to mention but a few. They are widely applied in optoelectronic devices such as humidity sensors [19], photodiodes [20], solar cells [21], and photocatalysts [22].

NiO is a P-type semiconductor with a wide bandgap between 3.5 eV and 4.0 eV [23]. The excellent properties of NiO make it a promising material for solar cells [24]. Similarly, TiO₂ is a desirable material for harvesting solar energy because of its optoelectronic properties, high resistance to photocorrosion, affordability, stability in a wide range of pH, and nonpoisonous nature [25]. Various techniques are available for depositing metal oxides [26–31]. Low cost of equipment, ease of control of deposited film structure, and the ability to coat large areas in thin layers with uniform thickness [32–35] influenced the choice of the technique used in this study.

Heterojunctions are known to be the most competitive method of solar cell fabrication on account of being the simplest [36]. A P-N junction is created when P-type (NiO) and N-type (TiO₂) semiconductor materials are placed in contact with one another. A solar cell is basically a P-N junction with a large surface area. Figure 1 depicts generation of electricity by a solar cell using a P-N junction.

Figure 1: Solar cell generation of electricity using a P-N junction.

The overall aim of this research is the provision of affordable and sustainable solar panels for developing and low-income countries. This was achieved by fabricating nanostructured TiO₂/NiO heterojunction thin-film solar cells using the spray pyrolysis technique.

2. Methodology

2.1. Deposition

The chemicals used are of analytical reagent grade and were used without further purification. Distilled and deionized pure water were used during the course of the experiment.

The solar cell was fabricated using a modified spray pyrolysis technique (SPT) as reported by Ukoba et al. [37] and represented pictorially in Figure 2. Prior to sample preparation, the indium tin oxide- (ITO-) coated glass and soda lime glass used as substrate were clean ultrasonically as reported by Adeoye Abiodun and Salau [38]. The precursor for the window layer titanium oxide (TiO₂) nanostructure thin film was prepared by mixing 3 ml of titanium ethoxide with 30 ml of distilled water and ethanol mixture, and three droplets of acetic acid. This was stirred for one hour before spraying on cleaned indium tin oxide- (ITO-) coated glass substrates and soda lime glass substrates maintained at about 350°C. Also, deposition parameters such as substrate temperature, carrier gas flow rate, and pressure were optimized to obtain quality films.

Figure 2: Pictorial representation of the experimental set-up of the spray pyrolysis technique.

The nanostructured nickel oxide (NiO) absorber layer was deposited on the prepared ITO/TiO₂ layers and empty soda lime glass using SPT, as shown in Figure 3. The precursor for NiO was obtained by preparing 0.05 M nickel acetate tetrahydrate in double distilled water.

Figure 3: Experimental set-up of spray pyrolysis technique.

The precursors were thoroughly stirred for several minutes prior to spraying onto preheated substrates maintained at about 350°C. Other deposition parameters were maintained to obtain good quality thin films. The optimized parameters used in the deposition of the NiO films are tabulated in Table 1. To complete the TiO₂/NiO heterojunction solar cell illustrated in Figure 4, gold (Au) metal contact was deposited as a back contact using DC magnetron sputtering.

Table 1: Optimum deposition parameters of SPT NiO film.

Deposition parameter
Value

Substrate to nozzle height
20 cm

Rate of spray
1 ml/min

Spray time
1 min

Sprays interval
30 sec

Carrier gas
1 bar of filled compressed air

Figure 4: Schematic of the fabricated NiO/TiO₂ heterojunction solar cells.

2.2. Testing

The TiO₂ and NiO prepared on soda lime glass were used to study the elemental, morphological, and structural characteristics of TiO₂ and NiO using energy dispersive X-ray spectrometer (EDS or EDX: “AZtec Oxford Detector”), a ZEISS Ultra Plus field emission gun scanning electron microscope (FEGSEM), and Bruker AXS D8 Advance X-ray diffractometer (XRD) with Cu-Kα radiation, respectively. The J-V characteristics of the fabricated TiO₂/NiO heterojunction cell in dark and under illumination were done using the Keithley SourceMeter 2400, coupled with a two-point probe. Newport solar simulator of intensity (100 mW/cm²) was used as the source of illumination.

3. Results and Discussion

3.1. Morphological Studies

Figures 5(a) and 5(b) show the scanning electron micrograph of the NiO thin film at lower and higher magnification, respectively. The micrograph reveals scattered distribution of the NiO particles across the surface of the film. The film has even distribution, is adherent to the film surface, and has no cracks. This represents a better surface morphology compared to that of NiO films reported by Sriram and Thayumanavan [39]. The SEM shows the potential of NiO as an absorber layer in solar cell fabrication. Figures 5(c) and 5(d) show the scanning electron micrograph of the heterojunction of NiO/TiO₂. This micrograph was obtained by the SEM at the junction or point of interaction between the TiO₂ and NiO. It shows a polycrystalline structure. The micrograph shows the P-type NiO and N-type TiO₂ of the thin film with their polycrystalline structures. It shows complete penetration at the heterojunction.

(a)

(b)

(c)

(d)

Figure 5: SEM of (a) NiO thin film at lower magnification, (b) NiO thin film at higher magnification, (c) fabricated NiO/TiO₂ heterojunction solar cell at lower magnification, and (d) fabricated NiO/TiO₂ heterojunction solar cell at higher magnification.

3.2. Elemental Composition

Figure 6 shows the elemental composition of the NiO/TiO₂ heterojunction solar cell deposited on the ITO-coated glass substrate. Figure 6 shows the presence of Ti, O, and Ni for the TiO₂ and NiO, respectively, and the indium (In) representing the ITO-coated glass substrate. This confirms the presence of the metal oxides in the heterojunction.

Figure 6: EDX of fabricated ITO/TiO₂/NiO heterojunction solar cell.

3.3. Structural Analysis

Figure 7 shows the X-ray diffraction patterns of the fabricated ITO/TiO₂/NiO heterojunction solar cell. The peaks corresponding to NiO and TiO₂ were determined with JCPDS patterns. The XRD spectrum indicates strong NiO peaks with (1 1 1), (2 0 0), and (2 2 0) preferential orientation. The patterns of the NiO thin film have peak diffractions at (2θ = 37°, 43°, and 64°) for the (1 1 1), (2 0 0), and (2 2 0) plane. The XRD analysis confirms Bunsenite, which corresponds to the JCPDS card: 04-0835 for nickel oxide [40] confirming it as a good absorber layer of solar cells [41]. The TiO₂ spectrum also shows strong spectrum and polycrystalline structures typical of N-type in heterojunction solar cells. The structure of the heterojunction indicates that the film is polycrystalline and chemically pure.

Figure 7: XRD of fabricated ITO/TiO₂/NiO heterojunction solar cell.

3.4. Current Density-Voltage (J-V) Characterization

The J-V characteristic curve of the prepared TiO₂/NiO heterojunction thin-film solar cell under illumination and in the dark is depicted in Figure 8. The J-V characteristic at room temperature in the dark shows that the forward current of the cells increases slowly with increasing voltage. The solar cell has rectification properties since the dark J-V plots were similar to the Shockley diode characteristics, which can be expressed by the standard diode equation

where is the electronic charge, is the diode quality factor (ideality factor), is Boltzmann’s constant, is the absolute temperature, and is the reverse saturation current.

Figure 8: Typical I-V curve for the prepared TiO₂/NiO heterojunction thin-film solar cell under illumination and in the dark.

The solar cell parameters evaluated from the J-V curve are presented in Table 2. The fabricated solar cell exhibits the short-circuit current (Jsc) of 16.8 mA, the open-circuit voltage (Voc) of 350 mV, the fill factor (FF) of 0.39, and the conversion efficiency (η) of 2.30%. This is a marked improvement on the values of 0.33 V and 0.29 recorded by Georgieva and Tanusevski [42] for the open-circuit voltage and fill factor, respectively. It also showed improvement in the fill factor of 0.28 reported by Noda et al. [43].

Table 2: Solar cell parameters of the fabricated solar cell.

Sample
Jsc (mA)
Voc (mV)
Jmax (mV)
Vmax (mV)
FF (%)
η (%)

TiO₂/NiO
16.8
350.0
13.10
173.57
39
2.30

4. Solar Cell Parameters

The primary parameters that describe the performance of a photovoltaic device are discussed below.

4.1. Open-Circuit Voltage (Voc)

Open-circuit voltage is the applied voltage relative to an open circuit where no current flows through the device (i.e., the voltage across the device at zero current). is obtained at the point of intersection of the I-V curve under illumination at the voltage axis. Under open-circuit conditions, the structure has to bias itself to some voltage in order to counter the light-beam-induced current. The open-circuit voltage arises as a result of the built-in electric field present in the materials system and can be expressed as

This quantity is left unaffected by series resistance losses in the cell but is sensitive to shunt losses.

4.2. Short-Circuit Current Density (Jsc)

is the current that flows through the junction under illumination at zero applied voltage, that is, . In the ideal case, it equals the photogenerated current density () and is proportional to the incident number of photons or alternatively the intensity of illumination.

is represented as the intersection of the J-V curve under illumination at the current axis. For an ideal solar cell (RS = 0 and RSH = ∞), the short-circuit current is given by

4.3. Fill Factor (FF)

The fill factor is defined as the inverse of the ratio of the ideal power to the maximum power in operating conditions. It can be defined also as the area of the maximum power rectangle to the product of the short-circuit current and the open-circuit voltage. This is shown as

4.4. Efficiency (η)

The most important parameter of a solar cell in terms of its ultimate function is the photovoltaic conversion efficiency. This is defined as the ratio of the output power (electricity) to the input power (light) and can be calculated as

5. Conclusion

In this study, TiO₂ and NiO thin films were used to fabricate ITO/TiO₂/NiO heterojunction solar cells. It shows that NiO can be used in thin-film solar cells. The conversion efficiency, open-circuit voltage, short-circuit current, and fill factor were 2.30%, 350 mV, 16.8 mA, and 0.39 under 100 mW/cm² illumination, respectively. This is an improvement on existing values. This will open up frontiers in affordable and sustainable solar cell fabrication in developing and low-income countries.

Nomenclature

NiO: | Nickel oxide
Ni: | Nickel
Ti: | Titanium
O: | Oxygen
TiO₂: | Titanium oxide
ZnO: | Zinc oxide
CuO: | Copper oxide
In₂O₃: | Indium oxide
ITO: | Indium tin oxide
Au: | Gold
SPT: | Spray pyrolysis technique
FEGSEM: | Field emission gun scanning electron microscope
EDX: | Energy dispersive X-ray spectrometer
XRD: | X-ray diffractometer
J-V: | Current density-voltage
I-V: | Current-voltage
Voc: | Open-circuit voltage
Jsc: | Short-circuit current density
FF: | Fill factor
Pin: | Power in
Pmax: | Maximum power.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The financial assistance of the National Research Foundation and The World Academy of Science (NRF/TWAS) of South Africa under Grant number 105492 towards this research is acknowledged. The first author also wishes to thank EMDI Akure, Adeoye Eyitayo, and Remy Bucher of iThemba Lab for their assistance in the experiment analysis.

References

O. Ellabban, H. Abu-Rub, and F. Blaabjerg, “Renewable energy resources: current status, future prospects and their enabling technology,” Renewable and Sustainable Energy Reviews, vol. 39, pp. 748–764, 2014.

A. K. Hussein, “Applications of nanotechnology in renewable energies—a comprehensive overview and understanding,” Renewable and Sustainable Energy Reviews, vol. 42, pp. 460–476, 2015.

N. S. Lewis, “Research opportunities to advance solar energy utilization,” Science, vol. 351, no. 6271, article aad1920, 2016.

G.-N. Lee, M. P. Machaiah, W.-H. Park, and J. Kim, “Enhanced optical and electrical properties of ITO/Ag/AZO transparent conductors for photoelectric applications,” International Journal of Photoenergy, vol. 2017, Article ID 8315802, 9 pages, 2017.

S.-H. Hong, J.-H. Yun, H.-H. Park, and J. Kim, “Nanodome-patterned transparent conductor for highly responsive photoelectric device,” Applied Physics Letters, vol. 103, no. 15, article 153504, 2013.

M. A. Green, “Third generation photovoltaics: ultra-high conversion efficiency at low cost,” Progress in Photovoltaics: Research and Applications, vol. 9, no. 2, pp. 123–135, 2001.

S. Joshi, M. Mudigere, L. Krishnamurthy, and G. Shekar, “Growth and morphological studies of NiO/CuO/ZnO based nanostructured thin films for photovoltaic applications,” Chemical Papers, vol. 68, no. 11, pp. 1584–1592, 2014.

A. Czapla, E. Kusior, and M. Bucko, “Optical properties of non-stoichiometric tin oxide films obtained by reactive sputtering,” Thin Solid Films, vol. 182, no. 1-2, pp. 15–22, 1989.

A. Rahal, A. Benhaoua, M. Jlassi, and B. Benhaoua, “Structural, optical and electrical properties studies of ultrasonically deposited tin oxide (SnO₂) thin films with different substrate temperatures,” Superlattices and Microstructures, vol. 86, pp. 403–411, 2015.

W. Zhang, S. Ding, Z. Yang et al., “Growth of novel nanostructured copper oxide (CuO) films on copper foil,” Journal of Crystal Growth, vol. 291, no. 2, pp. 479–484, 2006.

M. Sharon and B. Prasad, “Preparation and characterization of iron oxide thin film electrodes,” Solar Energy Materials, vol. 8, no. 4, pp. 457–469, 1983.

S. K. Shaikh, S. I. Inamdar, V. V. Ganbavle, and K. Y. Rajpure, “Chemical bath deposited ZnO thin film based UV photoconductive detector,” Journal of Alloys and Compounds, vol. 664, pp. 242–249, 2016.

M. Jlassi, I. Sta, M. Hajji, and H. Ezzaouia, “Optical and electrical properties of nickel oxide thin films synthesized by sol–gel spin coating,” Materials Science in Semiconductor Processing, vol. 21, pp. 7–13, 2014.

R. Drevet, C. Legros, D. Bérardan et al., “Metal organic precursor effect on the properties of SnO₂ thin films deposited by MOCVD technique for electrochemical applications,” Surface and Coatings Technology, vol. 271, pp. 234–241, 2015.

B. Siwach, S. Sharma, and D. Mohan, “Structural, optical and morphological properties of ZnO/MWCNTs nanocomposite photoanodes for dye sensitized solar cells (DSSCs) application,” Journal of Integrated Science and Technology, vol. 5, no. 1, pp. 1–4, 2017.

O. Langmar, C. R. Ganivet, G. de la Torre, T. Torres, R. D. Costa, and D. M. Guldi, “Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study,” Nanoscale, vol. 8, no. 41, pp. 17963–17975, 2016.

Z. Yu, I. R. Perera, T. Daeneke et al., “Indium tin oxide as a semiconductor material in efficient p-type dye-sensitized solar cells,” NPG Asia Materials, vol. 8, no. 9, article e305, 2016.

M. Rokhmat, E. Wibowo, and M. Abdullah, “Performance improvement of TiO₂/CuO solar cell by growing copper particle using fix current electroplating method,” Procedia Engineering, vol. 170, pp. 72–77, 2017.

Z. Yuan, H. Tai, X. Bao, C. Liu, Z. Ye, and Y. Jiang, “Enhanced humidity-sensing properties of novel graphene oxide/zinc oxide nanoparticles layered thin film QCM sensor,” Materials Letters, vol. 174, pp. 28–31, 2016.

A. Kathalingam, D. Vikraman, H.-S. Kim, and H. J. Park, “Facile fabrication of n-ZnO nanorods/p-Cu₂O heterojunction and its photodiode property,” Optical Materials, vol. 66, pp. 122–130, 2017.

T. Shinagawa, M. Chigane, J. Tani, and M. Izaki, “Effect of oxide intermediate layers on pyramidally textured Cu2O/ZnO solar cells prepared by electrodeposition,” ECS Meeting Abstracts, vol. 2, no. 21, p. 1636, 2016.

A. Samad, M. Furukawa, H. Katsumata, T. Suzuki, and S. Kaneco, “Photocatalytic oxidation and simultaneous removal of arsenite with CuO/ZnO photocatalyst,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 325, pp. 97–103, 2016.

G. Boschloo and A. Hagfeldt, “Spectroelectrochemistry of nanostructured NiO,” The Journal of Physical Chemistry B, vol. 105, no. 15, pp. 3039–3044, 2001.

S. Kerli and Ü. Alver, “Preparation and characterisation of ZnO/NiO nanocomposite particles for solar cell applications,” Journal of Nanotechnology, vol. 2016, Article ID 4028062, 5 pages, 2016.

S. A. Rawool, M. R. Pai, A. M. Banerjee et al., “pn Heterojunctions in NiO:TiO₂ composites with type-II band alignment assisting sunlight driven photocatalytic H₂ generation,” Applied Catalysis B: Environmental, vol. 221, pp. 443–458, 2018.

A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, P. Poulopoulos, and S. N. Yannopoulos, “Synthesis and characterization of ZnO/NiO p–n heterojunctions: ZnO nanorods grown on NiO thin film by thermal evaporation,” Photonics and Nanostructures - Fundamentals and Applications, vol. 9, no. 2, pp. 132–139, 2011.

M. Boroujerdnia and A. Obeydavi, “Synthesis and characterization of NiO/MgAl₂O₄ nanocrystals with high surface area by modified sol-gel method,” Microporous and Mesoporous Materials, vol. 228, pp. 289–296, 2016.

M. El-Nahass, M. Emam-Ismail, and M. El-Hagary, “Structural, optical and dispersion energy parameters of nickel oxide nanocrystalline thin films prepared by electron beam deposition technique,” Journal of Alloys and Compounds, vol. 646, pp. 937–945, 2015.

H. Wang, Y. Wang, and X. Wang, “Pulsed laser deposition of the porous nickel oxide thin film at room temperature for high-rate pseudocapacitive energy storage,” Electrochemistry Communications, vol. 18, pp. 92–95, 2012.

C. E. Morosanu and G. Siddall, Thin Films by Chemical Vapour Deposition, Elsevier, Amsterdam, 2016.

I. Sta, M. Jlassi, M. Hajji, and H. Ezzaouia, “Structural, optical and electrical properties of undoped and Li-doped NiO thin films prepared by sol–gel spin coating method,” Thin Solid Films, vol. 555, pp. 131–137, 2014.

F. Garcés, N. Budini, J. Schmidt, and R. Arce, “Highly doped ZnO films deposited by spray-pyrolysis. Design parameters for optoelectronic applications,” Thin Solid Films, vol. 605, pp. 149–156, 2016.

M. G. Faraj, “Effect of aqueous solution molarity on the structural and electrical properties of spray pyrolysed lead sulfide (PbS) thin films,” International Letters of Chemistry, Physics and Astronomy, vol. 57, pp. 122–125, 2015.

A. Enigochitra, P. Perumal, C. Sanjeeviraja, D. Deivamani, and M. Boomashri, “Influence of substrate temperature on structural and optical properties of ZnO thin films prepared by cost-effective chemical spray pyrolysis technique,” Superlattices and Microstructures, vol. 90, pp. 313–320, 2016.

A. E. Adeoye, E. Ajenifuja, B. A. Taleatu, and A. Fasasi, “Rutherford backscattering spectrometry analysis and structural properties ZnxPb1–xS of thin films deposited by chemical spray pyrolysis,” Journal of Materials, vol. 2015, Article ID 215210, 8 pages, 2015.

D. Chen, X. Zou, H. Yang et al., “Effect of annealing process on CH₃NH₃PbI₃-XClX film morphology of planar heterojunction perovskite solar cells with optimal compact TiO₂ layer,” International Journal of Photoenergy, vol. 2017, Article ID 7190801, 9 pages, 2017.

O. K. Ukoba, A. C. Eloka-Eboka, and F. L. Inambao, “Review of nanostructured NiO thin film deposition using the spray pyrolysis technique,” Renewable and Sustainable Energy Reviews, vol. 82, pp. 2900–2915, 2018.

E. Adeoye Abiodun and A. Salau, “Effect of annealing on the structural and photovoltaic properties of cadmium sulphide: copper sulphide (Cds:Cuxs) heterojunction,” International Journal of Scientific and Research Publications, vol. 5, no. 8, pp. 1–5, 2015.

S. Sriram and A. Thayumanavan, “Structural, optical and electrical properties of NiO thin films prepared by low cost spray pyrolysis technique,” International Journal of Material Science Engineering, vol. 1, pp. 118–121, 2013.

M. Gabal, “Non-isothermal decomposition of NiC₂O₄–FeC₂O₄ mixture aiming at the production of NiFe₂O₄,” Journal of Physics and Chemistry of Solids, vol. 64, no. 8, pp. 1375–1385, 2003.

N. A. Bakr, S. A. Salman, and A. M. Shano, “Effect of co doping on structural and optical properties of NiO thin films prepared by chemical spray pyrolysis method,” International Letters of Chemistry, Physics and Astronomy, vol. 41, pp. 15–30, 2015.

V. Georgieva and A. Tanusevski, “Characterization of p-Cu₂O/n-ZnO heterojunction solar cells,” AIP Conference Proceedings, vol. 1203, pp. 1074–1078, 2010.

S. Noda, H. Shima, and H. Akinaga, “Cu₂O/ZnO heterojunction solar cells fabricated by magnetron-sputter deposition method films using sintered ceramics targets,” Journal of Physics: Conference Series, vol. 433, pp. 1–10, 2013.