Facile synthesis of Cr doped hierarchical ZnO nano-structures for enhanced photovoltaic performance

doi:10.1016/j.inoche.2020.107902

Inorganic Chemistry Communications

Volume 116, June 2020, 107902

https://doi.org/10.1016/j.inoche.2020.107902 Get rights and content

Highlights

•
Nanospheres of Cr doped ZnO were synthesized by using low cost hydrothermal method.
•
Growth mechanism and crystallinity of Cr doped ZnO nanospheres were discussed.
•
Cr doped hybrid solar cells gave better current densities as compared to undoped.
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An increase in the dopant concentration result in better optoelectronic property.

Abstract

Transition metal doped ZnO has been widely used in various research areas such as optoelectronics, chemical sensors, solar cells and photo catalysts. Herein, we fabricated Cr-doped ZnO nanospheres by a facile solvothermal treatment for enhanced photovoltaic performance. A systematic study of the structural and optical properties of Cr doped ZnO were investigated by scanning electron microscopy (SEM), X-ray diffraction technique (XRD), Ultra violet-Visible spectroscopy (UV–Vis) respectively. XRD patterns confirm that the samples have hexagonal (wurtzite) structure with no additional peaks, which suggests that Cr ions replaces the regular Zn sites in the ZnO crystal structure. Furthermore, doped ZnO nanospheres were employed in the hybrid solar cells in combination with P3HT and gave better current densities than their corresponding undoped counterparts. The increase in solar cell efficiency of doped ZnO nanospheres is solely due to the improvement in the charge separation efficiency in the 4% Cr doped ZnO nanospheres. Optoelectronic analysis of the Cr doped ZnO hybrid solar cell showed comparable results of J_SC −3.7 (mAcm⁻²), V_OC 0.44 (V), Fill Factor 0.49 and Efficiency (ƞ) 0.79 (%). The enhanced photovoltaic activity of the Cr doped hierarchical ZnO nanostructures provides an interesting grounds for the design and enhancement of the low cost, feasible synthesis of photovoltaic nanostructure materials.

Graphical abstract

Introduction

Nanostructures of zinc oxide (ZnO) such as, nanowires, nanospheres, nanorods, nanotubes and nanobelts [1] have got enormous attention by the scientific community because of their supreme optical, magnetic and electronic properties. It is also believed that such nanostructures provide unique morphology for both interconnects and functional units in fabricating nanodevices [2], [3], [4]. Due to direct band gap (3.35 eV) and large excitation binding energy (60 meV) of ZnO at room temperature, makes it an ideal candidate for the fabrication of ultraviolet laser diodes, ultraviolet light emitting devices, transparent conducting films, chemical sensors and solar cell [5], [6], [7], [8], [9], [10], [11], [12], [13]. Up till now, various methods have been used to synthesize the ZnO nanostructures like hydrothermal, sputtering, electrodeposition, pulsed laser deposition, thermal decomposition, chemical precipitation, sol-gel and microwave method [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Recently, many efforts have been devoted to synthesize ZnO nanostructures to enhance its electrical, optical and magnetic properties for its potential applications [14], [26], by introducing transition metal ions such as Mn, Cd, Ni and Co into ZnO crystal lattice to adjust the energy band structure [14], [27], [28]. Also the composites of ZnO and graphene oxide have been utilized for the photo-degradation of various dyes [26], [29]. Doping with suitable elements is an effective process to alter surface state, energy levels as well as transport performance of carriers in semiconductors, thus, improving their optoelectronic properties. Yet, to devise a new method for fabricating ZnO nanomaterials, specifically, transition metal-doped ZnO nanostructures, having features of ease in processibility, low cost and simple operation, presents a great challenge. Among various solutions to the aforementioned difficulty, soft chemistry route is a reliable and provides a practical implementation for ultrafine ZnO nanostructures synthesis at low temperature.

In the sphere of transition metal atoms, Cr holds eminent significance as dopant, since Cr has many electron shells and its ionic radius is close to Zn²⁺. Hence, Cr³⁺ ion can easily penetrate into ZnO crystal lattice or substitute Zn²⁺ in ZnO crystal [14]. As a result, it is safe to assume that Cr doped ZnO materials may consequent in boosting the visible light absorption characteristics. In addition, the transition metals can generate energy states within the band gap, creating intermediate steps for the transition of electrons between the valence and conduction bands. Cr doping affects the defects and oxygen vacancies and thus helps in the band gap tuning of ZnO for their use in nano-optoelectronic devices, charge storage and nanoscale memory devices.

In this study, we present a simple approach to synthesize ZnO nanospheres with different doping concentrations of Cr, followed by the analysis of its effect on crystallinity, band gap and the efficiency of the hybrid solar cell. The morphological, structural, compositional, and optical properties of the synthesized Cr doped ZnO nanospheres were investigated by scanning electron microscopy (SEM), X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV–Visible absorption. The un-doped and Cr doped ZnO nanospheres were combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) to fabricate the hybrid solar cells and their working capacities are estimated as a function of the Cr concentration.

Section snippets

Chemical

Zinc acetate (99.9%), diethylene glycol and chromium nitrate Cr(NO₃)₃·9H₂O (99.9%), polystyrene sulfonate (PEDOT–PSS) (99.5%) and poly(3-hexylthiophene-2,5-diyl) (99.9%) were purchase from sigma Aldrich. Milli-Q ultra-pure water with a resistivity greater than 18.2 MΩ cm was used in all experiments. The chemicals discussed were of reagent grade and used as received.

Synthesis of Cr doped ZnO nanospheres

The Cr doped ZnO nanospheres were prepared according to the modified method, described elsewhere [30]. In a primary reaction,

Results and discussion

It is believed that for a superior optical, electrical and efficient band gap tuning, the well-defined crystallinity of the synthesized material is considered vital. For this purpose, Cr doped ZnO nanospheres having excellent optical properties were obtained by using easy and genial solvothermal method. Structural and optical properties of Cr doped ZnO NSs were investigated by SEM, XRD, UV–Vis respectively. XRD patterns exhibit the high crystallinity of ZnO nanostructures and further confirm

Conclusion

In summary, we have successfully synthesized Cr-doped ZnO nanospheres with excellent optical properties by using solvothermal method. Structural and optical properties of Cr doped ZnO NSs were investigated by SEM, XRD, UV–Vis respectively. XRD patterns exhibit the high crystallinity of ZnO nanostructures and further confirm that the samples have hexagonal (wurtize) structure with no additional peak which suggests that Cr ions go to the regular Zn sites in the ZnO crystal structure. The Nyquist

CRediT authorship contribution statement

Muzaffar Iqbal: Writing - original draft, Conceptualization, Methodology, Resources. Akbar Ali Thebo: Data curation, Writing - original draft, Resources. Wahid Bux Jatoi: Visualization, Investigation. Muhammad Tayyab Tabassum: Validation. Misbah Ur Rehman: Resources. Khalid Hussain Thebo: Supervision, Writing - original draft, Conceptualization, Methodology, Project administration. Muhammad Ali Mohsin: Supervision, Writing - review & editing, Software, Validation, Writing - original draft,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the University of Chinese Academy of Sciences, Beijing and Government of Pakistan.

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Facile synthesis of Cr doped hierarchical ZnO nano-structures for enhanced photovoltaic performance

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemical

Synthesis of Cr doped ZnO nanospheres

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Ceram. Int.

J. Hazard. Mater.

J. Alloy. Compd.

Electrochim. Acta

Mater. Res. Bull.

Ceram. Int.

Journal of Electroanalytical Chemistry

Mater. Chem. Phys.

Mater. Charact.

Mater. Lett.

Mater. Sci. Eng., B

Mater. Des.

Appl. Catal. B

Ceram. Int.

Appl. Surf. Sci.

Mater. Lett.

J. Alloy. Compd.