Open access peer-reviewed chapter

Fabrication of ZnO Thin Film through Chemical Preparations

Written By

Ersan Y. Muslih and Badrul Munir

Submitted: September 26th, 2017 Reviewed: February 6th, 2018 Published: August 1st, 2018

DOI: 10.5772/intechopen.74985

From the Edited Volume

Emerging Solar Energy Materials

Edited by Sadia Ameen, M. Shaheer Akhtar and Hyung-Shik Shin

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Zinc oxide (ZnO) is a compound that has unique physical and chemical properties. It has a direct band gap at 3.4 eV (without dopant), a high bonding energy (60 meV), and a high thermal and mechanical stability at room temperature. Thus, ZnO thin film can be suitably applied in many fields, and it also has many functions such as UV light emitters, hydrophobic coating, transparent thin film in electronic devices, piezoelectric material, transducers, gas-sensing, and a transparent conductive oxide (TCO) layer in thin film solar cells. ZnO thin film could be prepared by many chemical preparations such as chemical bat deposition (CBD), chemical vapor deposition (CVD), sol–gel spin coating, doctor blade, printing deposition, and electrochemical deposition (ED). This chemical process is a low-cost, simple, and easy preparation process to be adjusted or doped by other elements.


  • zinc oxide
  • thin film
  • chemical preparations
  • TCO
  • solar cells

1. Introduction

Zinc oxide (ZnO) is a II-VI semiconductor group which has a wurtzite as the most stable compound structure and has unique physical and chemical properties. In physical properties, ZnO has a high thermal stability, which starts to melt and boil at 1975°C; in optical properties, it has a high transparency and a direct band gap at 3.4 eV, which can be tuned by doping several elements such as Al, Ga, or In [1]. ZnO also has a low resistivity, which is about 1–2 × 10−4 Ωcm [2], a high thermal stability, a high electrochemical coupling coefficient, and a high bonding energy (60 meV) [1]. Due to all these unique properties, ZnO becomes a promising material, which can be applied in many fields as a sensor, converter, catalyst, and even as an important part in solar cells. This study aims to review ZnO as a transparent conductive oxide (TCO) in solar cells.


2. Zinc oxide as thin film in solar cells

In solar cells, ZnO plays important roles in collecting the energy from sunlight in various solar cells such as silicone-based solar cell (first-generation solar cell), thin film (second-generation), organic, multijunction, dye-sensitized (third-generation), hybrid and perovskite solar cells (fourth generation). In the first to the third generation of solar cell, ZnO plays a role as transparent conductive oxide (TCO), except in organic solar cell in which ZnO acts as a junction for exciton separation [3].

Transparent conductive oxide (TCO) is very important for harvesting electricity from sunlight and can improve solar cell efficiency. This layer is used to place on top of thin film solar cells and has a high transmittance (greater than 80% in the visible spectrum area) but has a low resistivity (less than 10−4 Ωcm). These properties will minimize the probability of the received photon absorbed by the layers before the absorber layer or reflected back. Table 1 shows several properties of ZnO as TCO on solar cells.

Table 1.

Properties of ZnO [1].

In addition, TCO should have a higher band gap than the absorber layer and the buffer layer, approximately 3.2 eV. If all of the layers are stacked together, they will make a gradation of band gap energy from low band gap energy (absorber layer) to the high energy (the top contact). This gradation can help electrons of the absorbed photons in the absorber layer to move past the other layers to the top layer. On the other hand, the holes move to the back-contact as a hole summation in solar cells. Since the electrons and holes move to the opposite side, this makes a difference potential on either side and produces electricity. If the band gap gradation is not sequentially arranged, it will disturb electron and hole movement. As a consequence, the electricity will not appear. Figure 1 shows illustrations of electrons and holes, movement through several layers with different band gaps in the CIGSe (CuInGeSe4) solar cell.

Figure 1.

A schematic layer structure of a classical CIGSe solar cell [4] and a schematic band profile of a typical CIGSe solar cell under zero bias voltage condition [5].

Besides its physical properties, ZnO is a common material as TCO for thin film solar cell because this material is non-toxic and abundant. However, ZnO still lacks in optical and electrical (opto-electrical) properties. Thus, some of elements or molecules have been successfully developed to improve its optoelectrical properties such as Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, Hf, In2O3 and SnO2 [6]. Some of elements provide a significant change in optical and electrical properties of ZnO. Table 2 shows some changes of ZnO optical and electrical properties as TCO after being doped by some elements.

DopantsOptimum content in target (%)Thickness (nm)Resistivity (Ωcm)Transmittance (%)References
Al25004.5 × 10−488[7]
Ga52008.12 × 10−4>90[8]
In40>10004.02 × 10−4>85[9]
F22004.83 × 10−4>90[10]
Si2∼1506.2 × 10−4∼80[11]

Table 2.

Properties of ZnO films with different dopants.

ZnO thin film could be prepared by physical or vacuum methods such as radio frequency (RF) magnetron sputtering or direct current (DC) sputtering, pulsed laser deposition (PLD), and e-beam evaporation [12, 13, 14, 15]. However, all of these methods need particular vacuum equipments and investment. Contrary from physical or vacuum methods, chemical methods are well known as a low-cost method because it does not require particular equipment or expensive investment. Thus, chemical process becomes a promising and low-cost process.

There are several processes for depositing ZnO by using chemical process, such as chemical bath deposition (CBD)/dip coating, chemical vapor deposition (CVD)/spray pyrolysis, sol-gel spin coating, doctor blade coating, printing deposition, and electrochemical deposition. Even though there are various deposition techniques, the steps of each technique are the same. The first step is preparing the precursor which can be in a solution, sol-gel, colloidal, or even in a dispersion precursor form. The second step is depositing using various techniques, and the last step is drying or heat treatment to remove the solvent and to develop the ZnO thin film. Common steps in the preparation of ZnO by chemical process are shown in Figure 2.

Figure 2.

A common preparation step to fabricate ZnO thin film by chemical processes.

In the chemical preparation process, to adjust ZnO thin films into p or n semiconductor, it is done by adding some particular element of salt such as aluminum salt or boron salt into the precursor mixture.

Some of experiments only need a single step of heat treatment to develop the ZnO thin film, but in spin-coating method, several repetition steps of coating and heat treatment are required in order to get the desired thickness, and this repetition step is called as preheat treatment. Contrary with the spin coating, chemical vapor deposition (CVD) or spray pyrolysis method may not have specific drying or heat treatment steps because in this method, the heat treatment is done simultaneously with deposition. To adjust the thickness in CVD or spray pyrolysis, it is adjusted by the deposition time.


3. Chemical deposition methods

3.1. Chemical bath deposition/dip coating

Chemical bath deposition (CBD) or dip coating is the simplest and low-cost deposition method because this method does not need expensive and special equipment. In addition, it also has a simple deposition principle. The arrangement of equipment is shown in Figure 3.

Figure 3.

The arrangement of CBD equipment [16].

At the precursor preparation, zinc oxide is prepared from zinc-salt compound such as zinc acetate dihydrate, zinc nitrate, zinc chloride, and zinc sulfate [17, 18, 19, 20, 21]. After these kinds of zinc salts were dissolved in the solution, zinc in the salt becomes a cation which can react with anion from basic compound, as well as form a seed of nuclei which adhere on the substrate. Besides that, zinc cation also reacts with other compounds in the precursor mixture such as surfactants which act as a binder and form zinc complex compound or in order to get desire properties of the mixture such as viscosity or homogeneity. Table 3 shows various zinc salts and other compounds as precursor mixture.

Zinc saltOther additive compoundsReferences
Zn(CH3COO)22-methoxyethanol, mono-ethanolamine[17]
Zn(NO3)2NaOH, sodium n-dodecyl sulfate, triethanolamine[18]
ZnCl2NH4OH, hexadecyl(trimethyl)azanium bromide[19]

Table 3.

Various zinc salts and other compounds as precursor mixture.

Further, this seed of nuclei will develop as zinc-salt precipitation which adheres on the substrate, and after drying or heat treatment under various atmospheres, it becomes oxidized and forms a zinc oxide as thin film. The schematic of this preparation is shown in Figure 4.

Figure 4.

Mechanism of ZnO thin film growth [21].

This is the reaction mechanism of ZnO thin film fabrication.


At drying or heat treatment step, the heat treatment can be done at various temperatures and atmospheres. These various processes have the effect to the shape, morphology, optical, and electrical properties of zinc oxide thin film [22]. Table 4 shows various annealing temperatures and atmospheres or heat treatment step.

Temperature (°C)Duration (hour)AtmosphereReferences

Table 4.

Various annealing temperatures and atmospheres/heat treatment step.

In addition to these, the concentration of precursor of zinc salt as zinc source and other additive concentrations also give significant role to ZnO thin film [22]. The effect of zinc and surfactant concentrations can be seen in Figure 4. The increase of zinc and surfactant concentration shows an increase of ZnO thin film crystallinity. Besides crystallinity, the effect of the increase of the concentration also affects the reaction rate of nucleation between zinc and anion, in which it becomes faster. However, besides concentration, the reaction rate is also affected by the temperature reaction.

3.2. Chemical vapor deposition/spray pyrolysis

The preparation of chemical vapor deposition (CVD) or spray pyrolysis is similar with CBD. The zinc precursor must be solved in a solution and has to be evaporated. Evaporation of precursor can be done by several ways. First, zinc precursor must be easy to evaporate or in other words it should be a volatile precursor. Second, by using high temperature, the zinc ingot as precursor can evaporate, and the last step is by using devices such as an atomizer or an ultrasonic transducer that can atomize the precursor or make a mist of it. The arrangement of CVD equipment is shown in Figure 5.

Figure 5.

CVD using the ultrasonic transducer to produce a precursor mist [27].

In Figure 5, zinc precursor solution has changed into mist by ultrasonic apparatus and carried by inert gas into the heat furnace chamber with a substrate placed inside the chamber as a target to develop the ZnO thin film. The temperatures of CVD in several experiments are shown in Table 5.

Temperature (°C)Zinc sourceAtmosphereReferences
90Zinc acetateO2, N2O, H2O[28]
135–235DiethylzincHe-Diborane 2%[29]
300–375Zinc acetateN2, O3[31]

Table 5.

Several ZnO thin film CVD condition.

After the precursor mist or vapor is carried and arrived at the heater chamber, some of the zinc precursor particles undergo a reaction with a molecule that contains oxygen in the chamber and forms a larger molecule. Then, this molecule is attached on to the substrate. Due to the high temperature or enough energy, this molecule decomposes into zinc that reacts with the oxygen and then forms a zinc oxide behind. The solvent is evaporated and leaves the chamber with the carrier gas, and at the same time, zinc oxide gets developed as zinc oxide thin film. Or the reaction mechanism could undergo another possibility, when zinc precursor is arrived at the chamber, and it will attach on the substrate and then undergo a reaction or bonding with another molecule which contains oxygen. Due to the high temperature and enough energy to decompose, the zinc precursor molecule and oxygen containing molecule react to the zinc oxide and release some decomposed solvents, which get carried out with gas, and finally the zinc oxide molecule becomes zinc oxide thin film. This schematic is shown in Figure 6.

Figure 6.

ZnO thin film fabrication. Mechanism with CVD [32].

3.3. Spin coating

Spin-coating method uses the energy of the substrate rotation to remove excess or unattached precursor and flatten the thin film. While the precursor drops on the substrate, it will attach on the substrate but the spread is uneven. When the substrate starts to spin, the precursor will spread along the substrate surface due to the centripetal force. A high spin velocity means a high centripetal force to remove the excess of precursor from the substrate. The amount of precursor in the substrate depends on the spin velocity and the precursor’s ability to adhere or attach on the substrate, which is usually called as adhesivity or wet ability. It makes the precursor to attach strongly on the substrate. Besides that, the viscosity also has a role on it. If the precursor has a high viscosity, it will slowly move due to the centripetal force. Consequently, it takes a longer spinning time or a higher spin velocity. The schematic of the spin coater is shown in Figure 7.

Figure 7.

Schematic representation of a home-built spin coater (a) and details of the rotor (b) [33].

After the precursor is attached on the substrate, the next stage is drying stage or evaporating the solvent. One thing that should be considered is that the evaporation of the solvent is affected by the viscosity of the precursor. Thus, it is really important to find out the optimum viscosity of the precursor. A high viscosity means difficult to spread well on the substrate surface, and it means that there is a high surface tension which makes it evaporate slowly. The evaporation of the precursor’s solvent is called as preheat treatment. Once the solvent evaporates, it will leave the zinc particle on the substrate and makes the layer thinner. In this method, the desired thickness cannot be achieved by a single process. It should be done in several preheat treatment stages to get the desired thickness. After the preheat treatment, the next stage is to fabricate the zinc oxide from the attached zinc on the substrate by oxidizing heating process. The schematic of alignment process on spin coating is shown in Figure 8.

Figure 8.

Schematic of the four stages of spin coating (a) Deposition, (b) Spin up, (c) Spin off, (d) Drying [34].

The precursor solution in spin-coating method is different from the chemical bath deposition and chemical vapor deposition. Precursor preparation in spin coating is more varied. It is not only a solution precursor but also a sol–gel or a colloidal form. Additionally, in this method, the quality on the thin film is affected not only by the concentration of zinc source and other additives but also by the rotation speed, time of spin coating, viscosity, and adhesivity of the precursor solution as mentioned earlier. Table 6 shows several conditions of ZnO spin coating.

Precursor’s formZinc sourceOther compoundsMixture conditionReferences
Sol-gelZinc acetate dihydrate2-methoxyethanol, monoethanolamineStirring for 30 min at 60°C, aged for 72 h at room temperature[35]
SolutionZinc acetate dihydrate2-methoxyethanol, monoethanolamineStirred at 65°C for 90 min[36]
SolutionZinc acetate dihydrateEthanol, NH4OHpH 10, room temperature[37]
SolutionZinc acetate dihydrateEthanol, acetylacetoneRoom temperature[38]

Table 6.

Precursor conditions.

Besides that, another thing that brings this method unique lies on the preheat treatment step. The function of this step is to make the coated film dried and attached on the substrate. The repetition of this step is to get a desired thickness. Table 7 shows the condition of several preheat treatments and heat treatments.

Spin conditionPreheat conditionHeat treatment conditionAtmosphereReferences
2000 rpm for 30 s200°C, 10 s400°C for 1 h[35]
3000 rpm for 30 s260°C, 10 min400°C for 3 hOxygen[36]
700 rpm for 10 s100°C for 10 min500°C for 2 hAir[37]
2000 rpm for 10 sRoom temperature,
3 min
500°C for 2 hOxygen[38]

Table 7.

Heat treatment condition.


4. Conclusions

In the thin film solar cells, zinc oxide (ZnO) layer acts as a transparent conductive oxide (TCO), which is an important part of solar cells. This thin film can be fabricated by various chemical processes as alternative of vacuum process, where the production is economically cost-effective but still has the same function and properties as vacuum methods.


  1. 1. Pearton SJ et al. Recent progress in processing and properties of ZnO. Progress in Materials Science. 2005;50:293-340. DOI: 10.1016/j.pmatsci.2004.04.001
  2. 2. Kim KH, Wibowo RA, Munir B. Properties of Al-doped ZnO thin film sputtered from powder compacted target. Materials Letters. 2006;60:1931-1935. DOI: 10.1016/j.matlet.2005.12.055
  3. 3. Steiger P et al. Hydrothermally grown ZnO electrodes for improved organic photovoltaic devices. Thin Solid Films;2017:417-423. DOI: 10.1016/j.tsf.2017.11.021
  4. 4. Azimi H, Yi H, Brabec CJ. Towards low-cost, environmentally friendly printed chalcopyrite and kesterite solar cells. Energy & Environmental Science. 2014;7:1829-1849. DOI: 10.1039/C3EE43865A
  5. 5. Chirilă A et al. Highly efficient Cu (In, Ga) Se2 solar cells grown on flexible polymer films. Nature Materials. 2011;10:857-861. DOI: 10.1038/nmat3122
  6. 6. Minami T. Transparent conducting oxide semiconductors for transparent electrodes. Semiconductor Science and Technology. 2005;20:35-44. DOI: 10.1088/0268-1242/20/4/004
  7. 7. Gondoni P et al. Structural and functional properties of Al: ZnO thin films grown by pulsed laser deposition at room temperature. Thin Solid Films. 2012;520:4707-4711. DOI: 10.1016/j.tsf.2011.10.072
  8. 8. Park S-M, Ikegami T, Ebihara K. Effects of substrate temperature on the properties of Ga-doped ZnO by pulsed laser deposition. Thin Solid Films. 2006;513:90-94. DOI: 10.1016/j.tsf.2006.01.051
  9. 9. Socol G et al. High quality amorphous indium zinc oxide thin films synthesized by pulsed laser deposition. Thin Solid Films. 2011;520:1274-1277. DOI: 10.1016/j.tsf.2011.04.196
  10. 10. Cao L et al. Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition. Solar Energy Materials and Solar Cells. 2011;95:894-898. DOI: 10.1016/j.solmat.2010.11.012
  11. 11. Das AK, Misra P, Kukreja LM. Effect of Si doping on electrical and optical properties of ZnO thin films grown by sequential pulsed laser deposition. Journal of Physics D: Applied Physics. 2009;42:165405. DOI: 10.1088/0022-3727/42/16/165405
  12. 12. Mahdhi H, Djessas K, Ben Ayadi Z. Synthesis and characteristics of ca-doped ZnO thin films by rf magnetron sputtering at low temperature. Materials Letters. 2018;214:10-14. DOI: 10.1016/j.matlet.2017.11.108
  13. 13. Seawsakul K et al. Effects of sputtering power toward the Al-doped ZnO thin film prepared by pulsed DC magnetron sputtering. Materials Today: Proceedings. 2017;4:6466-6471. DOI: 10.1016/j.matpr.2017.06.154
  14. 14. Look DC. Mobility vs thickness in n+-ZnO films: Effects of substrates and buffer layers. Materials Science in Semiconductor Processing. 2016;69:2-8. DOI: 10.1016/j.mssp.2016.11.026
  15. 15. Shettigar N et al. Tuning the third-order nonlinear optical properties of in: ZnO thin films by 8 MeV electron beam irradiation. Journal of Physics and Chemistry of Solids. 2017;110:260-265. DOI: 10.1016/j.jpcs.2017.06.020
  16. 16. Wang X, Ahmad M, Sun H. Three-dimensional ZnO hierarchical nanostructures: Solution phase synthesis and applications. Materials. 2017;10:1304. DOI: 10.3390/ma10111304
  17. 17. Saleem M et al. Effect of zinc acetate concentration on the structural and optical properties of ZnO thin films deposited by sol-gel method. International Journal of Physical Sciences. 2012;7:2971-2979. DOI: 10.5897/IJPS12.219
  18. 18. Li P et al. Growth of well-defined ZnO microparticles with additives from aqueous solution. Journal of Solid State Chemistry. 2005;178:855-860. DOI: 10.1016/j.jssc.2004.11.020
  19. 19. Ma C-l, Sun X-d. Preparation of nanocrystalline metal oxide powders with the surfactant-mediated method. Inorganic Chemistry Communications. 2002;5:751-755. DOI: 10.1016/S1387-7003(02)00546-4
  20. 20. Cao Z et al. Synthesis and UV shielding properties of zinc oxide ultrafine particles modified with silica and trimethyl siloxane. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2009;340:161-167. DOI: 10.1016/j.colsurfa.2009.03.024
  21. 21. Kumar PS et al. Growth and characterization of ZnO nanostructured thin films by a two step chemical method. Applied Surface Science. 2008;255:2382-2387. DOI: 10.1016/j.apsusc.2008.07.136
  22. 22. Saleem M et al. Effect of zinc acetate concentration on the structural and optical properties of ZnO thin films deposited by sol-gel method. International Journal of Physical Sciences. 2012;7:2971-2979. DOI: 10.5897/IJPS12.219
  23. 23. Baviskar PK et al. Wet chemical synthesis of ZnO thin films and sensitization to light with N3 dye for solar cell application. Journal of Physics D: Applied Physics. 2009;42:125108. DOI: 10.1088/0022-3727/42/12/125108
  24. 24. Urgessa ZN et al. Patterned growth of ZnO nanorods by chemical bath deposition. Physica B: Condensed Matter. 2017. DOI: 10.1016/j.physb.2017.06.061
  25. 25. Kumar N, Kaur R, Mehra RM. Photoluminescence studies in sol–gel derived ZnO films. Journal of Luminescence. 2007;126(2):784-788. DOI: 10.1016/j.jlumin.2006.11.012
  26. 26. Wang M et al. Influence of annealing temperature on the structural and optical properties of sol–gel prepared ZnO thin films. Physica Status Solidi. 2006;203:2418-2425. DOI: 10.1002/pssa.200521398
  27. 27. Liu L et al. Effects of intermittent atomization on the properties of Al-doped ZnO thin films deposited by aerosol-assisted chemical vapor deposition. Thin Solid Films. 2016;605:163-168. DOI: 10.1016/j.tsf.2015.09.011
  28. 28. Kim JS et al. Characterization of high quality c axis oriented ZnO thin films grown by metal organic chemical vapor deposition using zinc acetate as source material. Thin Solid Films. 1992;217:133-137. DOI: 10.1016/0040-6090(92)90619-M
  29. 29. Faÿ S et al. Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells. Solar Energy Materials & Solar Cells. 2006;90:2960-2967. DOI: 10.1016/j.solmat.2006.06.003
  30. 30. Rowlette PC et al. Plasma-enhanced atomic layer deposition of semiconductor grade ZnO using dimethyl zinc. Chemical Vapor Deposition. 2009;15:15-20. DOI: 10.1002/cvde.200806725
  31. 31. Pacio M et al. Study of (100) orientated ZnO films by APCVD system. Materials Science and Engineering B. 2010;174:38-41. DOI: 10.1016/j.mseb.2010.04.030
  32. 32. Marchand P et al. Aerosol-assisted delivery of precursors for chemical vapour deposition: Expanding the scope of CVD for materials fabrication. Dalton Transactions. 2013;42:9406-9422. DOI: 10.1039/C3DT50607J
  33. 33. Bianchi RF et al. Spin coater based on brushless dc motor of hard disk drivers. Progress in Organic Coating. 2006;57:33-36. DOI: 10.1016/j.porgcoat.2006.05.004
  34. 34. Daniel TW. Toolan et al. Development of in situ studies of spin coated polymer films. Journal of Materials Chemistry C. 2013;4:603-616. DOI: 10.1039/c2tc00026a
  35. 35. Saleem M et al. Effect of zinc acetate concentration on the structural and optical properties of ZnO thin films deposited by sol-gel method. International Journal of Physical Sciences. 2013;7:2971-2979. DOI: 10.5897/IJPS12.219
  36. 36. Khan ZR et al. Influence of zinc concentration on band gap and sub-band gap absorption on ZnO nanocrystalline thin films sol-gel grown. Materials Science - Poland. 2017;35:246-253. DOI: 10.1515/msp-2017-0039
  37. 37. Smirnov M, Baban C, Rusu GI. Structural and optical characteristics of spin-coated ZnO thin films. Applied Surface Science. 2010;256:2405-2408. DOI: 10.1016/j.apsusc.2009.10.075
  38. 38. Muslih EY, Kim KH. Preparation of zinc oxide (ZnO) thin film as transparent conductive oxide (TCO) from zinc complex compound on thin film solar cells: A study of O2 effect on annealing process. IOP Conference Series: Materials Science and Engineering. 2017;214. DOI: 10.1088/1757-899X/214/1/012001

Written By

Ersan Y. Muslih and Badrul Munir

Submitted: September 26th, 2017 Reviewed: February 6th, 2018 Published: August 1st, 2018