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

Anil Kumar Verma^a*, Swati Sahu^a, Mohan Patel^b and Sanjay Tiwari^a

^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

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

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

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.

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