Article in HTML

Author(s): Anil Kumar Verma*, Swati Sahu, Mohan Patel, Sanjay Tiwari

Email(s): akverma.prsu@gmail.com

Address: Photonics Research Laboratory, School of Studies in Electronics & Photonics, Pt. Ravishankar Shukla University, Raipur-492010 Chhattisgarh, India
State Forensic Science Laboratory, Raipur- 492001 Chhattisgarh, India

Published In:   Volume - 33,      Issue - 1,     Year - 2020


Cite this article:
Verma et al. (2020). Preparation, Fabrication and Characterization of Sol-Gel ZnO Thin Films for Organic Solar Cells. Journal of Ravishankar University (Part-B: Science), 33(1), pp. 24-30.



                                               Journal of Ravishankar University–B, 33 (1), 24-30 (2020)

 

 

 



Preparation, Fabrication and Characterization of Sol-Gel ZnO Thin Films for Organic Solar Cells

Anil Kumar Vermaa*, Swati Sahua, Mohan Patelb and Sanjay Tiwaria

aPhotonics Research Laboratory, School of Studies in Electronics & Photonics, Pt. Ravishankar Shukla University, Raipur-492010 Chhattisgarh, India

bState Forensic Science Laboratory, Raipur- 492001 Chhattisgarh, India

 

*Corresponding author: akverma.prsu@gmail.com

[Received: 10 December 2019; Revised version:  30 August 2020; Accepted: 08 September 2020]

Abstract: In this work, ZnO has been prepared by the sol-gel method and thin films have been deposited onto the ITO (Indium-Tin-Oxide) coated glass substrates by spin coating method at different ZnO concentration and spin parameters. For this, Sol-gel ZnO was synthesized by Zinc acetate dehydrate, 2-methoxethanol and ethanolamine as a starting material, solvent and stabilizer respectively. The study of deposition parameters on the structural, optical and electrical properties of the ZnO thin films was carried out. The Roughness and thickness were calculated by Profilometer. X-ray diffraction (XRD) analysis of the films showed the polycrystalline nature of the prepared films. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) was used to describe the surface morphology and optical properties were studied using UV-VIS-IR Spectroscopy.  The fabricated results showed that ZnO thin films is crystalline and low-cost techniques with good features that will be useful for Organic Solar Cells (OSCs) device as an electron transport layer.

Keywords: Sol-gel ZnO, XRD, SEM, AFM, Organic Solar Cells, OSCs.

Introduction

For deposition of materials, the sol-gel process is a technology for producing solid materials from small molecules. The technique is very useful for the fabrication of various metal oxides layers. The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles. Zinc oxide (ZnO) is an inexpensive, inorganic semiconductor with a wide band gap having optical transparency in the visible range. It crystallizes in a hexagonal wurtzite structure (zincite) and ionization of excess zinc atoms in interstitial positions and the oxygen vacancies. Surface defects play an important role in the photo catalytic activities of metal oxides as they increase the number of the active sites The structure of ZnO can be described as a number of alternating planes composed of tetrahedral coordinated O2− and Zn2+ stacked alternately along three-axis, as shown in Figure 1. Various techniques for the preparation of ZnO thin films have been used: sol–gel method, co-precipitation techniques etc. among various techniques, the sol-gel approach to be one of the most promising techniques to prepare ZnO thin films. Some of the most important merits of the sol-gel techniques are: Easy for synthesis at very low temperature for chemical reaction. These merits make the sol-gel technique a very attractive preparation method (Yu et al., 2016; Farooq et al., 2012; Jurablu et al., 2015; Znaidi et al., 2016; Kim et al., 2014; Poongodi et al., 2015). By applying ZnO thin layer in Organic Solar Cells, Quantum dot solar cells and Dye-sensitized solar cells (DSSCs) is a dominant technology that has potential as a suitable candidate for the development of third generation solar cells. It has many advantages of low cost, flexibility, transparency and ease of fabrication over the traditional silicon solar cells (Verma et al., 2016; Lai et al,. 2015; Patel et al., 2016; Sahu et al,. 2016; Liao et al., 2014)

The performance and stability of conventional and inverted bulk-heterojunction solar cells and perovskite solar cells  with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO thin films on indium tin oxide plastic substrates showed high power conversion efficiency. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after many days while the conventional one showed negligible photovoltaic activity after few days (Morgenstern et Al., 2011; Yuan et al., 2015; Li et al., 2014; Sun et al., 2011).

Figure 1. The wurtzite structure model of ZnO

Methodology

Materials and Experimental section

ZnO thin films were synthesized by a new sol gel techniques as discuss in following processes. We were used to prepare zinc oxide thin films using zinc acetate dihydrate [Zn(CH3COO)2. 2H2O] of 99.9%, Sigma Aldrich. Zinc acetate dihydrate was dissolved in 2-Methoxyethanol anhydrous[C3H8O2], 99.8%, Merck to prepare ZnO sol and Ethanolamine(MEA) [NH2CH2CH2OH], 99.9%, Sigma Aldrich was added drop wise in MEA to become homogenous and transparent solution.

Indium Tin Oxide (ITO) coated glass slides were used as substrate for deposition of Zinc Oxide thin films. The size of the glass slides used as substrate is of 0.25 inch2. The substrate was cleaned with detergent water, DI water, Acetone and IPA for about 10 minutes each and then subsequently done Plasma Ashing to remove oxide contamination for the surface. After cleaning the substrate was put into the hot plate for heating at 100 0C for 10 minutes to remove containment (water or solvent) on the substrate. Magnetic stirring was done for 2hr at 50 0C then the solution was allowed to aged for 24 hrs in room temperature. Three different solution was prepare by varying the sol concentration of 0.1, 0.2, 0.5 Molar concentration respectively for studying of thickness optimization. After that the aged solution was spin coated on the glass substrate at different Speed and time for optimized thickness and surface morphology. After deposition the samples were transferred on Hot plate at 200 0C for evaporation of the residual solvent and any other containment. We were also done for more times to get thick, smooth and uniform surface of thin films for the application of the Organic Electronics devices like Organic Solar Cells (OSCs), Organic Light Emitting Diodes (OLEDs) etc.

Results and Discussion

Thickness Measurement

The thickness optimization is crucial for developments of efficient devices. So we studied the thickness variation on different concentration and speed. This was calculated and confirmed by DektakXT Profilometer to study uniform deposition of ZnO thin film on ITO coated glass. The results as summarized in Table-1-3. This shows that the thickness varies on Molar concentration of sol-gel. We have also optimized the average thickness from 20 nm to 180 nm which is suitable as electron transport layer for inverted organic solar cells.

Table 1. Molar: 0.1M Spin: 2000rpm /50sec Annealing: 2000C for 10 minute

Sample Name

 

Thickness-1

(nm)

Thickness-2

(nm)

Thickness-3

(nm)

Average Thickness

(nm)

ZnO_1

22

11

8

22

ZnO_2

23

8

30

21

ZnO_3

9

5

26

15

ZnO_4

27

28

28

27

 

 

 

 

 

Table 2. Molar: 0.2M Spin: 2000rpm /50sec Annealing: 2000C for 10 minutes

Sample Name

Thickness-1(nm)

Thickness-2(nm)

Thickness-3(nm)

Average Thickness (nm)

ZnO_1

181

187

181

184

ZnO_2

179

132

173

161

ZnO_3

169

175

172

172

ZnO_4

186

159

178

174

 

 

 

 


Table 3. Molar: 0.2M Spin: 2000rpm /50sec Annealing: 2000C for 10 minutes

Sample Name

Thickness-1

(nm)

Thickness-2

(nm)

Thickness-3

(nm)

Average Thickness

(nm)

ZnO_1

26

25

27

26

ZnO_2

47

19

45

37

ZnO_3

30

32

28

30

ZnO_4

27

43

27

33

 

 

 

 

 

 

Structural Analysis and Surface morphology

High Resolution X-ray diffraction (HRXRD) was used to identify crystalline phases and to estimate the crystalline sizes as shown in Figure 2 and 3. These results shows the XRD peaks of ZnO thin films on ITO glass with 0.1M and 0.5M concentration respectively at 200 0C annealed for 10 minutes each. This shows wurtzite zinc oxide structure.

Figure 3. XRD pattern of 0.1 Molar concentrations ZnO thin film annealed samples at 200 0C for 10 minute using Spin Coating on ITO coated glass substrate

Figure 4. XRD pattern of 0.5 Molar concentrations ZnO thin film annealed samples at 200 0C for 10 minute using Spin Coating on ITO coated glass substrate

We did the observation by Atomic force microscopy (AFM) to study the surface topography of ZnO thin films at different concentration for different surfaces to study the variation on the films. The result shows indeed an increase of the particle size, together with an increase of the overall roughness. RRMS (roughness root mean squared) and Raa (roughness arithmetic average of absolute values) values have been measured on the 2×2 μm2 and 4×4 μm2. Atomic force microscopy (AFM) were confirmed the surface topography of ZnO for different thickness as shown in Figure 4 for 0.1M and 0.5M concentration respectively.

 

Figure 5. AFM topography of ZnO thin film on ITO for 0.1M and 0.5M concentration

Figure 5 and Figure 6 shows the SEM image of the ZnO thin films prepared by sol-gel technique. In this figure, the particles prepared with formation of clusters. Figure 6 and Figures 7 shows the SEM image of the annealed ZnO thin films at 2000C for 10 minutes at different concentration. The ZnO thin films formed were less agglomerated. The crystallite size of annealed thin films is in the range of 20-80 nm in diameter.

 

Figure 5. SEM image of ZnO for 0.1M concentration

Figure 6. SEM image of ZnO for 0.5M concentration

Optical Characterization

The Optical properties were also studied using UV-VIS-IR Spectroscopy. The optical Absorbance was observed in the wavelength range of 300nm to 800nm as shown in Figure 7.

 

Figure 7. Absorbance spectra of ZnO thin films with different concentration

A broad spectrum is observed for the wavelength of about 400 nm in the absorption spectra which is characteristic of a pure ZnO. There is no other peaks were observed in the spectrum which shows that we have fabricated a pure ZnO thin films. The sharp absorption edge (400 nm), is near close to the intrinsic band gap of ZnO. The energy band gap of ZnO is around 2.4eV calculated from absorbance as shown in Figure 8. This confirmed that it will be great importance towards developments of organic solar cells and other devices.

Figure 8. Determination of Energy band gap of ZnO

Conclusion

In conclusion, we have prepared, fabricate and characterize ZnO thin films by a new sol gel techniques using different concentration and thickness. The XRD and SEM results show that these thin films are hexagonal wurtzite in phase ZnO with a mean grain size around 30nm. From AFM topography, it is clear that with increasing concentration the morphology of the particles changes. The absorbance peak of UV-VIS-IR Spectroscopy spectrum showed the wide band gap energy of 2.4 eV for as-prepared sol gel ZnO thin film. Therefore, this will be most applicable toward the developments of organic electronics device such as OSCs and OLEDs.

Acknowledgement

The facilities availed from Photonics Research Laboratory; School of Studies in Electronics & Photonics, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh (India) is thankful and acknowledged. The Fabrication and Characterization facilities provided from Indian Institute of Technology(IIT)-Bombay(India) under Indian Nanoelectronics User Program (INUP) which is support by DeitY, MCIT, Government of India, is gratefully acknowledged. The corresponding author  would like to thank Dr. Dinesh Kabra, Faculty Incharge, Organic Electronics Laboratory, CEN-IIT, Bombay (India) for his guidance and valuable support during the execution of the  INUP project.

References

Farooq, A. and Kamran, M., 2012. Effect of sol concentration on structural and optical behavior of ZnO thin films prepared by sol-gel spin coating. International Journal of Applied Physics and Mathematics2(6), p.430.

Jurablu, S., Farahmandjou, M. and Firoozabadi, T.P., 2015. Sol-gel synthesis of zinc oxide (ZnO) nanoparticles: study of structural and optical properties. Journal of Sciences, Islamic Republic of Iran26(3), pp.281-285.

Kim, J., Kim, G., Kim, T.K., Kwon, S., Back, H., Lee, J., Lee, S.H., Kang, H. and Lee, K., 2014. Efficient planar-heterojunction perovskite solar cells achieved via interfacial modification of a sol–gel ZnO electron collection layer. Journal of Materials Chemistry A2(41), pp.17291-17296.

Lai, L.H., Speirs, M.J., Chang, F.K., Piveteau, L., Kovalenko, M.V., Chen, J.S., Wu, J.J. and Loi, M.A., 2015. Increasing photon absorption and stability of PbS quantum dot solar cells using a ZnO interlayer. Applied Physics Letters, 107(18), p.183901.

Li, A., Nie, R., Deng, X., Wei, H., Zheng, S., Li, Y., Tang, J. and Wong, K.Y., 2014. Highly efficient inverted organic solar cells using amino acid modified indium tin oxide as cathode. Applied Physics Letters104(12), p.49_1.

Liao, S.H., Jhuo, H.J., Yeh, P.N., Cheng, Y.S., Li, Y.L., Lee, Y.H., Sharma, S. and Chen, S.A., 2014. Single junction inverted polymer solar cell reaching power conversion efficiency 10.31% by employing dual-doped zinc oxide nano-film as cathode interlayer. Scientific reports4, p.6813.

Morgenstern, F.S., Kabra, D., Massip, S., Brenner, T.J., Lyons, P.E., Coleman, J.N. and Friend, R.H., 2011. Ag-nanowire films coated with ZnO nanoparticles as a transparent electrode for solar cells. Applied Physics Letters, 99(18), p.242.

Patel, M., Sahu, S., Verma, A.K. and Tiwari, S., 2017, August. Solution processed solar cells based on in-situ synthesis of CdSe quantum dots. In 2017 International Conference on Energy, Communication, Data Analytics and Soft Computing (ICECDS) (pp. 1683-1687). IEEE.Physics Letters107(18), p.183901.

Poongodi, G., Anandan, P., Kumar, R.M. and Jayavel, R., 2015. Studies on visible light photocatalytic and antibacterial activities of nanostructured cobalt doped ZnO thin films prepared by sol–gel spin coating method. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy148, pp.237-243.

Sahu, S., Awasthy, R., Patel, M., Verma, A., Agnihotri, P. and Tiwari, S., 2016, December. Fabrication and characterization of nanoporous TiO2 layer on photoanode by using doctor blade method for dye-sensitized solar cells. In International Conference on Fibre Optics and Photonics (pp. Th3A-9). Optical Society of America.

Sun, Y., Seo, J.H., Takacs, C.J., Seifter, J. and Heeger, A.J., 2011. Inverted polymer solar cells integrated with a low‐temperature‐annealed sol‐gel‐derived ZnO film as an electron transport layer. Advanced Materials23(14), pp.1679-1683.

Verma, A.K., Agnihotri, P., Patel, M., Sahu, S. and Tiwari, S., 2016, December. Fabrication and Characterization of Novel Inverted Organic Solar Cells Employing ZnO ETL and MoO3 HTL. In International Conference on Fibre Optics and Photonics (pp. Tu4A-18). Optical Society of America.

Yu, X., Marks, T.J. and Facchetti, A., 2016. Metal oxides for optoelectronic applications. Nature materials15(4), pp.383-396.

Yuan, T., Zhu, X., Zhou, L., Zhang, J. and Tu, G., 2015. Efficient inverted polymer solar cells based on conjugated polyelectrolyte and zinc oxide modified ITO electrode. Applied Physics Letters106(8), p.24_1.

Znaidi, L., Chauveau, T., Tallaire, A., Liu, F., Rahmani, M., Bockelee, V., Vrel, D. and Doppelt, P., 2016. Textured ZnO thin films by sol–gel process: Synthesis and characterizations. Thin Solid Films617, pp.156-160.

 

 

 



Related Images:

Recomonded Articles:

Author(s): Anil Kumar Verma*; Swati Sahu; Mohan Patel; Sanjay Tiwari

DOI:         Access: Open Access Read More

Author(s): Neelu Singhai; Sati Prasad Banerjee

DOI:         Access: Open Access Read More

Author(s): Anil Kumar; A.P. Mishra

DOI:         Access: Open Access Read More

Author(s): A.B. Soni; H. Kumar

DOI:         Access: Open Access Read More

Author(s): Deepti Tikariha; Jyotsna Lakra; Srishti Dutta Roy; Toshikee Yadav; Kallol KGhosh

DOI:         Access: Open Access Read More

Author(s): R Javdani Yekta and BB Waghmode

DOI:         Access: Open Access Read More

Author(s): A.B. Soni; A.K. Dwivedi; H. Kumar

DOI:         Access: Open Access Read More

Author(s): Rajeeva Guhey; N. P.Wadhwa; M.W.Y Khan

DOI:         Access: Open Access Read More

Author(s): Rashmi Swami; Sanjay Tiwari

DOI:         Access: Open Access Read More

Author(s): A.K. Agrawal; P.C. Mahajan; K. Worwankar

DOI:         Access: Open Access Read More

Author(s): Dinesh Kumar Sahu; R.C. Agrawal

DOI:         Access: Open Access Read More

Author(s): S. Bhushan; R.B. Sahu

DOI:         Access: Open Access Read More

Author(s): P. Bose; S. Bhushan

DOI:         Access: Open Access Read More

Author(s): M.R. Khan; Vineeta Shukla; Richa Khetan

DOI:         Access: Open Access Read More

Author(s): Gitanjali Sahu; Anubha S. Gour

DOI:         Access: Open Access Read More

Author(s): Sudhir Kumar Shrivastava

DOI:         Access: Open Access Read More

Author(s): Jitendra Kumar Sharma;Sanjay Tiwari;Vivek Kant Jogi;Rajesh Kumar Awasthy

DOI:         Access: Open Access Read More