Keywords – antireflective coating, hydrothermal method, solar cell, seed layer, ZnO nanoparticles. [311126]



Abstract – The paper proposes a new and complex process for the synthesis of ZnO nanoparticles for antireflective coating corresponding to silicone solar cells applications. The process consists of two major steps: preparation of seed layer and hydrothermal growth of ZnO nanoparticles. [anonimizat]. [anonimizat]. [anonimizat].

Keywords – [anonimizat], [anonimizat], ZnO nanoparticles.

INTRODUCTION

ZInc oxide (ZnO) is characterized by a wide band gap of ~3.37 eV as well as by a high excitation binding energy of 60 meV. [anonimizat], [anonimizat] [1-4]. [anonimizat]-quality ZnO nanoparticles such as aqueous hydrothermal growth, [5],

metal – organic chemical vapor deposition, [6], vapor phase epitaxial, [7], vapor phase transport, [8], [anonimizat] – solid method [9]. The advantages of the hydrothermal method of ZnO nanowires synthesis over the previously mentioned methods regard the fact that no special equipment is necessary. Also, another important aspect relates to the reduced cost of the process and to the uniformity of the nanoparticles on a large surface. The properties of the ZnO seed layer play a significant role in obtaining high quality ZnO nanowires by using the hydrothermal method. The film can be deposited by a variation of methods: sol–gel method, [10], spin coating, [11], magnetron sputtering, [12], or spray pyrolysis [13].

[anonimizat]-efficiency solar cells. [anonimizat].

The paper presents a new and complex process designed for deposition of ZnO seed layers. [anonimizat]: the prevention of the nanoparticles agglomeration and the enhancement of the ZnO crystallinity by spray pyrolysis. [anonimizat].

[anonimizat] a two-step process. The first step was achieved by deposition of the seed layer through a [anonimizat]. 1 [14].

[anonimizat] a complex process that involves two stages: spray pyrolysis and spin coating followed by thermal treatment.

The ZnO seed layers were prepared by using zinc acetate dihydrate [Zn(CH3COOH)2·2H2O, Spolek Pro Chemickou] as a precursor, dissolved in 1-propanol [C3H8O, Acros Organics] by stirring at 50C for 30 minutes. The glass substrates were cleaned by using an ultrasonic bath. The cleaning was firstly performed by using acetone and methanol afterwards. In the followings, there was used deionized water. The glass substrates were then dried by blowing nitrogen on their surface.

The deposition of the ZnO seed layer has been undertaken by using spray pyrolysis of 10 mM zinc acetate solution in 1-propanol. The glass substrate was placed on a hot plate heated at 100°C. By using an airbrush, the layers were sprayed on the substrate every 20 seconds. The diameter of the used nozzle was 0.5 mm while the distance between the nozzle and the substrate was 15 cm. During the deposition process, the rate of the solution flow was maintained constant at 0.05 ml/min. Also, the gas pressure was equal to 3 bars. The number of layers has ranged from 1 to 10 layers. After spraying, the substrate was annealed at 100°C for 30 minutes.

The second step regards the spin coating of the zinc acetate solution on the substrate. The spin coating was achieved at a rotation speed of 2000 rpm for 30 seconds. The substrate was then annealed at a temperature of 300C for 30 minutes in ambient air.

Synthesis of the ZnO nanostructures

The ZnO nanowires synthesis on the ZnO seed layer substrates was developed through the hydrothermal process at a temperature of 90°C by using zinc nitrate hexahydrate [Zn(NO3)2 6H2O CHIMOPAR] and hexamethylenetetramine [C6H12N4 REACTIVUL] as source materials. For synthesis, 20 mmol Zn(NO3)2 6H2O was dissolved in 25 mL of deionized water. Also, 20 mmol C6H12N4 was dissolved in 25 mL of deionized water. In the followings, the final solution was mixed and heated at 90°C for 2 hours in an electric oven along with samples of ZnO seed layer substrates.

Fig. 1. Technological process regarding the ZnO nanoparticles [14].

Finally, the samples of ZnO nanowires were washed thoroughly with deionized water, dried with nitrogen and placed in a container at 25°C for 24 hours.

The structural analysis of the ZnO nanoparticles was performed, by grazing incident X-ray diffraction using an X-ray diffractometer (Bruker AXS D8 Discover) with Cu and Kα irradiation, 40 kV/40 mA, 20°- 60°, 2 Theta domain, 2 seconds/step scan speed and 0.04° step. The surface morphology and structure of the nanoparticles were examined by employing a scanning electron microscope (FESEM, Carl Zeiss Auriga) at an accelerating voltage of 2.00 kV. The imaging was performed at a high magnification of 100 kx while the optical transmission and reflection spectra was recorded in the wavelength range of 400-800 nm by using a double beam UV-Vis-NIR spectrophotometer (UV-VIS Spectrophotometer 570 Jasco).

In the following stages, a commercial polycrystalline silicone solar cell manufactured by Conrad Electronic SE was selected and covered by a nanostructured ZnO glass in order to be tested. The technical data related to the considered polycrystalline solar panel (123 cm2) consist of 1.35 W output power, 9 V nominal voltage, 10.5 V open circuit voltage and 150 mA short-circuit current. The photovoltaic module was tested for standard test conditions (1000 W/m2, 25°C, AM 1.5) as well as for reduced solar irradiance by using the Pasan Meyer Burger HighLight 3 solar simulator shown in Fig. 2. There were used 4 masks for the solar irradiance attenuation (100 W/m2, 200 W/m2, 400 W/m2 and 700 W/m2) in order to achieve the comparison between the generated power along with varying the operation conditions. Accordingly, IV characteristics resulted for each of the tested modules for various operating conditions.

Fig. 2. Pasan Meyer Burger HighLight 3 solar simulator, view from radiation source.

Results and Discussion

The ZnO seed layers were prepared by using zinc acetate dihydrate as a precursor, dissolved in 1-propanol. The zinc acetate is a salt of amphoteric zinc oxide and a weak acid like the acetic acid [15]. The decomposition or hydrolysis of zinc salts represents an established route to the formation of ZnO colloids and nanocrystals in aqueous solution [16-18].

Zinc acetate decomposes at 100-280oC to form Zn4O(CH3COO)6, which ultimately transforms into ZnO [19]. During the decomposes process it is form gaseous products (water, carbon dioxide , acetone and acetic acid) witch highest concentration it is obtained at 250 – 270°C, according to equations (1)–(4):

(1)

(2)

(3)

(4)

The synthesis of ZnO nanostructures or microstructures from the Zn(NO3)2-HMTA system has been reported in recent years [20-21]. It is generally considered as a very simple and novel process. Understanding the growth mechanism of different varieties of ZnO morphologies still needs further improvement. Based on other researches [22], the growth process of ZnO crystallites is generally accepted via the following mechanism:

(5)

(6)

(7)

(8)

(9)

(10)

In the aqueous solution of diluted ammonia, Zn(NH3)4 and Zn(OH)42- are able to coexist. Therefore, Zn(OH)42- is more stable and can be formed from the transformation of Zn(NH3)42+, as shown in (8). In the dilute ammonia solution, the amount of OH- is reduced (pH value around 10-11) and the formation of ZnO nuclei largely follows the reaction steps (5) – (10). In this case, the formation of ZnO nuclei is slow and it becomes the controlling step for synthesis of the ZnO films [23].

The X-ray diffraction analysis was performed for the ZnO seed layer as well as for the ZnO nanowires growth by the hydrothermal method as shown within Fig. 3a and Fig. 3b. In the case of the ZnO seed layer, there were identified only the specific peaks of ZnO, confirming the higher purity of the film. ZnO from seed layer presented wurtzite hexagonal structure P63mc as well as structure parameters a = b = 3.242 nm and c = 5.176 nm. The intensity of the diffraction peaks corresponding to (002) and (110) plans displayed low broad peaks in the case of all the analyzed seed layer samples.

The XRD analysis showed wurtzite hexagonal structure P63mc and structure parameters a = b = 3.242 nm and c = 5.176 nm when also considering the nanowires.The diffraction pattern highlighted peaks associated to (100), (002), (101) and (102) plans and the correspondence of ZnO. The (002) plan displayed a higher intensity peak in comparison to the corresponding plans (100), (101) and (102), indicating that the ZnO nanowires are predominantly c-axis orientated. Other peaks were not observed, leading to the fact that no other structures besides ZnO were formed. It was confirmed that high purity ZnO is obtained.

During the experimental work, there was also conducted a study regarding the influence of the seed layer morphology on the nanowires properties. A different number of depositions (spray pyrolysis and spin coating) were achieved in order to determine the optimal thickness and morphology of the ZnO seed layer. The optimal seed layer was obtained by 3 application stages of spray pyrolysis at a temperature of 100°C, 3 stages of spin coating followed by treatment at 300°C for a period of 30 minutes.

The microscopy micrographs shown within Fig. 4 were recorded by using a field scanning electron microscope or by employing the annular in-lens detector for a second set of electron images with magnification of 100.000 X and an accelerating voltage of 2000 V.

The morphology of the ZnO seed layer surface influences the morphology of the ZnO nanowire. These layers operate as seed crystals in order to ensure the epitaxial growth of ZnO nanowires [24]. In the case of thicker films, ZnO clusters are observed (grains with dimensions larger than 100 nm) consisting of agglomerations that influence the nanowires growth by a reduced order, scattered across the surface and random orientated (Fig. 4 a and b). The morphology and growth of the zinc oxide nanowires are influenced by the thickness and geometry of the seed layer (uniform grain, 30-55 nm) (as seen from Fig. 4 c and d). In this case, due to the seed layer uniformity and lack of agglomerations, the nanowires growth was orientated, with homogenous dimensions as well as displayed on the entire substrate surface. In this case, there was obtained a perfectly balanced seed layer and also a homogenous nanowire growth with lengths of ~200 nm and 50 nm diameter. Besides the high density of the ZnO nanowire arrays, other nanostructures are not observed.

The variation of the optical transmission is shown within Fig. 5 a, with wavelength found in the range of 400 – 800 nm for glass, ZnO seed layer and ZnO nanowires. The ZnO seed layer has presented a good transparency of approximately 80%, similar to the glass value due to the reduced thickness (50 nm) and surface uniformity. The samples of ZnO nanowires present a good transparency in the visible range (400-800 nm), with a lower average value of 76% (approximately 5% lower than in the case of glass). This decrease is due to the fact that the transmitted radiation by light diffusion increases the occurrence of the light scattering phenomenon of the ZnO nanowires.

Following the spectrophotometric analysis, the variation of the optical reflection with wavelength in the range of 400-800 nm is presented in Fig. 5b. The graph confirms that the reflection is reduced in comparison to the values obtained for simple glass. The ZnO seed layer presents an intermediate value between glass and ZnO nanowires, with an average of 11%. The average value of ZnO nanowires sample for the visible optical reflection is equal to 9%, 5% lower than the simple glass value (14.5%). By summarizing these optical characteristics, it is concluded that the ZnO nanowire films can be considered as a solution to the antireflective coatings in the solar cells field due to the optical proprieties and low price manufacturing.

The solar cells have been tested, Fig. 6-9, by using the SIM 3C HIGHLIGHT PASAN solar simulator for efficiency and output power analyses. Also, the functional parameters (efficiency, output power, open-circuit voltage and short-circuit current) have been determined, as shown within Fig. 10-13, achieving analyses on the photovoltaic cells provided with glass and ZnO nanowires. The used simulator is able to adjust the irradiance value between 100 W/m2 and 1000 W/m2, with both the light uniformity and light stability below 1%.

The influence of the solar irradiance on photovoltaic cells provided with glass and ZnO nanowires, related to efficiency, output power, open circuit voltage and short circuit current is shown in Fig. 10 – 13, suggesting that the functional parameters depend on the solar irradiance and ZnO antireflective coating, as suggested by the available literature [25-26] .

Fig. 6. Conrad Electronic SE module IV curve for 200 W/m2 solar irradiance.

Fig. 7. Conrad Electronic SE module IV curve for 400 W/m2 solar irradiance.

Fig. 8. Conrad Electronic SE module IV curve for 700 W/m2 solar irradiance.

Fig. 9. Conrad Electronic SE module IV curve for 1000 W/m2 solar irradiance, [14].

The performance variation in the case of cells with ZnO nanowires placed on glass was larger than the efficiency variation in the case of the cells covered with glass. Consistently, the ZnO coated samples had a larger efficiency, as shown within Fig. 10 suggesting that the reduced reflection is correlated to more light being coupled into the active material. The efficiency was increased from 7.79% for 100 W/m2 to 9.23% for 1000 W/m2 in the case of solar cells with simple glass. The efficiency has varied between 8.06% and 9.26% for solar cells with ZnO nanowires on glass, for the same values of irradiance. This increase is due to the reflectivity of glass which is reduced by ~5% when applying ZnO nanowires on glass.

Fig. 10. Efficiency of the solar cells with ZnO nanowires on glass (red), as compared to solar cells with simple glass (black), for various irradiance values.

Fig. 11. Short circuit current of the solar cells with ZnO nanowires on glass (red), as compared to solar cells with simple glass (black), for various irradiance values.

All the solar cells parameters have increased along with the irradiance (values ranging from 100 W/m2 to 1000 W/m2) and are superior in the case of the solar cells with ZnO nanowires on glass. The short circuit current increases from 0.015 A to 0.140 A for solar cells with simple glass while in the case of the solar cells with ZnO nanowires on glass to values from 0.016 A to 0.141 A (Fig. 11); the open circuit voltage ranges from 9.58 V to 10.92 V for solar cells with simple glass and from 9.66 V to 10.94 V for the solar cells with ZnO nanowires on glass (Fig. 12).

Fig. 12. Open circuit voltage of the solar cells with ZnO nanowires on glass (red), as compared to solar cells with simple glass (black), for various irradiance values.

Also, the output power increased from 0.105 W to 1.134 W for solar cells with simple glass while for the solar cells with ZnO nanowires on glass from 0.109 W to 1.14 W (Fig. 13).

Fig. 13. Output power of the solar cells with ZnO nanowires on glass (red), as compared to solar cells with simple glass (black), for various irradiance values.

For each value in the range of irradiation of 100 W/m2 – 1000 W/m2, Fig. 11-13, the values obtained in the case of the solar cells with ZnO nanowires on glass are higher as compared with the case to solar cells with simple glass.

Fig. 14. The photovoltaic conversion system, functional model.

In Fig. 14 shows the photovoltaic conversion system, functional model, in which used solar cells are with ZnO nanowires on glass.

Conclusion

The ZnO nanowires were prepared using the hydrothermal method of deposition on the seed layer by a new and complex process. There are applied the advantages of two methods, namely: the prevention of the nanoparticles agglomeration and the enhancement of the ZnO crystallinity. During the experimental work, a study was performed on the influence of ZnO seed layer morphology on the proprieties of ZnO nanowires. The resulting seed layer presented suitable growth proprieties by the hydrothermal method of uniform and vertical ZnO nanowires. The performed XRD analysis on the seed layer and nanowires confirmed a higher quality of ZnO with wurtzite hexagonal structure. The SEM images of the samples proved that the morphology of the ZnO seed layer has a major influence on the dimensions and geometry of the nanowires. Also, the samples dispose of a uniform and continuous film on the substrate. The ZnO nanowires present uniform dimensions and geometry. The UV-VIS transmission and reflection spectra of the ZnO nanowires samples exhibit a good transparency and a reduction of the reflection by approximately 5%. The determination of the solar cells functional parameters (efficiency, short circuit current, open circuit voltage and output power) has led to the conclusion that all the values are superior in the case of the solar cells with ZnO nanowires on glass showing that the performance of the solar cell depends on the irradiance and antireflective coating. The results confirm the advantages of using the ZnO nanowires in solar cells applications for antireflection coatings.

References

X. G. Han, H. Z. He, Q. Kuang., X. Zhou, X. H. Zhang, et. al., “Controlling Morphologies and Tuning the Related Properties of Nano/Microstructured ZnO Crystallites”, J. Phys. Chem. C, vol. 113, no. 2, pp. 584 – 589, 2009.

M. Law, L. E. Greene, J.C. Johnson, “Nanowire dye-sensitized solar cells. Nature Mater”, Nature Materials, vol. 4, no. 6, pp. 455 – 459, 2005.

J. H. Jun, H. Seong, K. Cho, B. M. Moon, S. Kim, “Ultraviolet photodetectors based on ZnO nanoparticles”, Ceramics International, vol. 35, no. 7, pp. 2797 – 2801, Sept. 2009.

H. Chen, Y. Liu, C. Xie, J. Wu, D. Zeng, Y. Liao, “A comparative study on UV light activated porous TiO2 and ZnO film sensors for gas sensing at room temperature”, Ceramics International, vol. 38, no. 1, pp. 503 –509, Jan. 2012.

E Chitanu, G Ionita, Hydrothermal growth of ZnO nanowires, The Scientific Bulletin of Valahia University – Materials and Mechanics – nr. 7, pp. 9-13, 2012

W. I. Park, D. H. Kim, S. W. Jung, G. C. Yi, “Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods”, Appl. Phys. Lett., vol. 80, no. 22, pp. 4232 – 4234, June 2002.

A. Bakin, A. Che Mofor, A. El-Shaer, A. Waag, “Vapour phase transport growth of ZnO layers and nanostructures”, Superlattices and Microstructures, vol. 42, no. 1–6, pp. 33 – 39, July – Dec. 2007.

D. I. Suh, C. C. Byeon, C. L. Lee, “Synthesis and optical characterization of vertically grown ZnO nanowires in high crystallinity through vapor–liquid–solid growth mechanism”, Applied Surface Science, vol. 257, no. 5, pp. 1454–1456, Dec. 2010.

A. Fulati A, S. M. Usman Ali, M. H. Asif, N. H. Alvi, M. Willander, C. Brännmark, et al., “An intracellular glucose biosensor based on nanoflake ZnO”, Sensors and Actuators B: Chemical, vol. 150, no. 2, pp. 673–680, Oct. 2010.

Zhifeng Liu, Jing Ya, Lei E, Effects of substrates and seed layers on solution growing ZnO nanorods, Journal of Solid State Electrochemistry, vol. 14, no. 6, pp. 957 – 963, June 2010.

K. H. Kim , K. Utashiro, Y. Abe, M. Kawamura, Growth of Zinc Oxide Nanorods Using Various Seed Layer Annealing Temperatures and Substrate Materials, Int. J. Electrochem. Sci., vol. 9, no. 4, pp. 2080 – 2089, Febr. 2014.

E Chitanu, G Ionita, RF magnetron sputtering deposition of AZO thin films,  Metalurgia International nr.16, vol.12, pp.32-35, 2011.

M. Breedon, M. B. Rahmani, S. H. Keshmiri, W. Wlodarski, K. Kalantar-zadeh, Aqueous synthesis of interconnected ZnO nanowires using spray pyrolysis deposited seed layers, Materials Letters, vol. 64, no. 3, pp. 291–294, Febr. 2010.

L. Pîslaru-Dănescu, E. Chițanu, R. A. Chihaia, D. Marin, L. A. El-Leathey, V. Marinescu, B. G. Sbârcea, C. A. Băbuțanu, New Harvesting System Based on Photovoltaic Cells with Antireflexive ZnO Nanoparticles Coatings and DC/DC Isolation Conversion, Proceeding of 4th International Symposium EFEA 2016 – Environmental Friendly Energies and Applications, Belgrade, Serbia, 14-16 Sept. 2016, E-ISBN: 978-1-5090-0749-3, DOI: 10.1109/EFEA.2016.7748798.

H. Bahadur, A. K. Srivastava, R. K. Sharma, S. Chandra, Morphologies of Sol-Gel Derived Thin Films of ZnO Using Different Precursor Materials and their Nanostructures, Nanoscale Research Letters, vol. 2, pp. 469 – 475, Oct. 2007.

E. A. Meulenkamp, Synthesis and Growth of ZnO Nanoparticles, J. Phys. Chem. B, vol. 102, no. 29, pp. 5566 – 5572, 1998.

S. Sakohara, M. Ishida, M. A. Anderson, Visible Luminescence and Surface Properties of Nanosized ZnO Colloids Prepared by Hydrolyzing Zinc Acetate, J. Phys. Chem. B, vol. 102, no. 50, pp. 10169 – 10175, Nov. 1998.

R. A. McBride, J. M. Kelly, D. E. McCormack, Growth of well-defined ZnO microparticles by hydroxide ion hydrolysis of zinc salts, J. Mater. Chem., vol. 13, no. 5, pp. 1196 – 1201, Febr. 2003.

S. H. Jeong, B. Na Park, D. G. Yoo, J. H. Boo, Al-ZnO thin films as transparent conducting oxides: Synthesis, Characterization and Application tests, Journal of the Korean Physical Society, vol. 50, no. 3, pp. 622 – 625, March 2007.

C. C. Lin, Y. Y. Li, Synthesis of ZnO nanowires by thermal decomposition of zinc acetate dehydrate, Materials Chemistry and Physics, vol. 113, no. 1, pp. 334 – 337, Jan. 2009.

A. Umar, M. M. Rahman, A. Al-Hajry, H. B. Hahn, Highly-sensitive cholesterol biosensor based on well-crystallized flower-shaped ZnO nanostructures, Talanta, vol. 78, no. 1, pp. 284 – 289, April 2009.

J. P. Cheng, X. B. Zhang, Z. Q. Luo, Oriented growth of ZnO nanostructures on Si and Al substrates, Surface and Coatings Technology, vol. 202, no. 19, pp. 4681 – 4686, June 2008.

P. K. Samanta, S. K. Patra, P. R. Chaudhuri, Violet emission from flower-like bundle of ZnO nanosheets, Physica E: Low-dimensional Systems and Nanostructures, vol. 41, no. 4, pp. 664 – 667, Febr. 2009.

H. Wang, J. Xie, K. Yan, M. Duan, Growth Mechanism of Different Morphologies of ZnO Crystals Prepared by Hydrothermal Method, Journal of Materials Science & Technology, vol. 27, no. 2, pp. 153 – 158, Febr. 2011.

P. Aurang, O. Demircioglu, F. Es, R.Turan, H. E. Unalan, ZnO Nanorods as Antireflective Coatings for Industrial-Scale Single-Crystalline Silicon Solar Cells, Journal of the American Ceramic Society, vol. 96, no. 4, pp. 1253–1257, April 2013.

X. Yu, D.Wang, D.Lei  G. Li, D.Yang, Efficiency Improvement of Silicon Solar Cells Enabled by ZnO Nanowhisker Array Coating, Nanoscale Res Lett. vol.7, pp. 306-310, Jun 2012.