Sprayed and Spin-Coated Multilayer Antireflection Coating Films for Nonvacuum Processed Crystalline Silicon Solar Cells

1Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan 2Department of Electrical and Electronics Engineering, Graduate School of Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey 3Specialty Materials Research Laboratory, Nissan Chemical Industries Ltd., 11-1 Kitasode, Sodegaurashi, Chiba 299-0266, Japan 4Daiwa Sangyo Co. Ltd., 3-4-11, Nakayasui, Sakai, Sakai, Osaka, Japan


Introduction
In order to achieve high efficiency silicon solar cells, reducing the optical losses and improving the absorption properties of the cell are one of the key factors.To minimize the reflection of the solar cell surface, a range of material and thin films have been used for ARC purposes for solar cells (SiO, SiO 2 , SiN x , TiO 2 , Al 2 O 3 , ZnS thin films, etc.) depending to the type of solar cell.Although a single ARC coating layer can minimize the reflection less than 1% only at a single wavelength, a multilayer structure with gradual increase of refractive index on the light path is necessary in order to achieve improved reflectivity along the spectrum [1].In this case, it is necessary to build multilayer structure by considering the gradual increase order of refractive indexes as air < 1st layer < 2nd layer < ⋅⋅⋅ < th layer < silicon.Considering the optimum refractive index for a single ARC layer between air and silicon according to  1 = ( 0   ) −1/2 (  ,  0 , and  1 : refractive index of silicon, air, and ARC thin film, resp.), one could consider that widely in use SiN x ( = 1.97) might be the optimum material for ARC layer for silicon solar cells [1].Quality forming of ARC coating materials mostly utilizes a type of chemical vapor deposition (CVD) technique.In case of SiN x films, for instance, typically a plasma-enhanced chemical vapor deposition (PECVD) technique [2,3] is used.Taking the advantage of tunability of the refractive index of SiN x by changing the deposition parameters of PECVD [4], better reflection performances with multilayer stacks of SiN x are achievable.Although, it is a well-established technology and has advantages including the high deposition rate, the need of toxic and hazardous gases such as SiH 4 (silane), SiH 2 Cl 2 (dichlorosilane), and NH 3 (ammonia) with high vacuum processing for CVD operation and considerably high costs come along as the major drawbacks.Considering these 2 International Journal of Photoenergy disadvantages and the fact that low-cost and high throughput are a continuous demand for silicon solar cell industry, simple low-cost technologies with high throughput, cost-effective methods need to be investigated and adapted to the solar cell manufacturing process.
In literature, TiO 2 has been widely investigated as an alternative ARC layer for crystalline silicon solar cells [5,6], due to its good optical properties that enhance the light absorption capability because of the high refractive index for silicon solar cell.Solution processed materials for silicon solar cells including TiO 2 , TiO 2 -SiO 2 [5,6], and Al 2 O 3 based Ti doped mixed sol-gel sources have been introduced as well [7,8].Although there is a considerable amount of research regarding such alternative materials for ARC, mainly based on TiO 2 and SiO 2 solutions, the availability of proper material and combined structures is still limited.Various techniques can be used in principle to form such precursors on the substrate homogenously, including spin coating [9,10], spray pyrolysis, or dip-coating [11][12][13].These practical and low-cost techniques have advantages including simple deposition of thin films from the prepared precursor solutions.Utilizing such techniques also leads an opportunity of using a broad range of chemicals with flexible amounts for both high or low temperature applications.
Additionally, ZrO 2 thin films with good optical properties and both chemical and mechanical stabilities [14,15] have been investigated initially for dye synthesized solar cells to enhance the properties of polymers [14], as well as improve the antireflection property of solar module glasses [16].With the attractive optical properties, high refractive index and good thermal stability ZrO 2 films can be another promising candidate for ARC layer of silicon solar cells [17,18].
In this work, as a low temperature and nonvacuum method, spin-coating deposition was performed for ZrO 2polymer composite films and spray pyrolysis deposition method was performed for TiO 2 -compact film and ZrO 2polymer composite/TiO 2 -compact multilayer films were introduced as an alternative ARC film for silicon solar cells.After confirming the structure of TiO 2 and ZrO 2 polymer based thin films separately, ⟨air/ZrO 2 /TiO 2 /Si⟩ multilayer structure was also built considering the gradual increase order of refractive indexes.And after the experimental evaluations, silicon solar cells were fabricated with and without using TiO 2 -compact film or ZrO 2 -polymer composite/TiO 2compact film multilayer films.

Experimental
Spray-deposited TiO 2 -compact films and spin-coating ZrO 2polymer composite films were formed on alkaline textured crystalline silicon (c-Si) substrates for primary evaluation of the coated films in terms of optical and physical analysis.Afterwards, textured surface p-type CZ-Si solar cells were fabricated using TiO 2 -compact films and ZrO 2 -polymer composite/TiO 2 -compact multilayer film as an antireflection coating (ARC) layer on the front surface of the c-Si wafers and were compared to those of solar cells without ARC.
For the optical measurements, square-shaped p-type silicon wafers (25 × 25 mm 2 ) were cut out from 6-inch CZ-Si p-type wafers.All wafers were dipped into the 20% diluted HF for 1 min and rinsed in distilled water.Afterwards, UV/O 3 surface treatment was carried out for a complete cleaning process.ZrO 2 -polymer composite film was deposited on silicon substrate by spin coating using the zirconium sol (ZR-30AH, provided by Nissan Chemical Industry Co. Ltd., Chiba, Japan).The investigation and optimization of ZrO 2 based solutions were introduced in our previous works [19] in order to use them for silicon solar cell applications with flat or textured surfaces.According to these works, optimized ZrO 2 solution composed by ethyl cellulose, ethanol, and ZR-30AH in ratio of 2 : 16 : 1 (in volume) was used in this work as well.Deposition of the ZrO 2 film was carried out by spin coating with a spin speed of 1500 rpm for 25 s (acceleration time is 5 s).
TiO 2 compact films were formed on the silicon wafers by spray pyrolysis deposition using precursor solutions which was sprayed on to the surface of silicon wafers using a glass atomizer.The crystalline silicon wafers were set on a hot plate heated at deposition temperature of 450 ∘ C. The TiO 2 precursor solution was prepared by the ten-time dilution of titanium di-isopropoxidebis-acethylacetone (TAA) (prepared by mixing titanium (VI) isopropoxide and acetylacetone by 1 : 2 in mole) to ethanol.Annealing temperature was optimized for TiO 2 as 450 ∘ C and as 125 ∘ C for ZrO 2 , respectively.Analyses were mainly carried out by reflection analysis (by ultraviolet-visible spectroscopy, Lambda 750 UV/VIS Spectrometer, Perkin Elmer, Waltham, MA, USA) and ellipsometer (UviselErAgms-nds, Horiba Jobin Yvon, Kyoto, Japan) and finally by the performance of the fabricated silicon solar cells with or without ARC.
Silicon solar cells were fabricated with a variety of final surface conditions: without ARC on textured surface, with TiO 2 -compact film on textured surface, and with ZrO 2polymer composite/TiO 2 -compact multilayer ARC film on textured surface.For the fabrication of silicon solar cells, 25 mm × 25 mm p-type CZ-Si wafers were used as well.For textured surface, alkaline texturing was performed in KOH (5.19 g) solution in H 2 O (100 mL) with Alka-Tex (0.28 mL, GP Solar, Konstanz, Germany) at 80 ∘ C (set: 100 ∘ C) for 30 min.The silicon wafers were dipped into the 20% diluted HF for 10 min and rinsed in distilled water.Then, RCA cleaning [20,21] was carried out to remove contaminant particles on the surface of the wafers by using a NH 4 OH/H 2 O 2 /H 2 O (1 : 1 : 5 in volume) solution, for 10 min at 80 ∘ C.After removal of the natural oxide films by 20% HF, a mix solution of HCl/H 2 O 2 /H 2 O (1 : 1 : 5 in volume) was used to remove metallic contaminations and mobile ions on the surface by dipping the wafers for 10 min at 80 ∘ C. In order to prevent phosphorous diffusion and to prevent the texture on the back side of the silicon wafer, polysilazane was coated using spincoating method by two times at 1500 rpm for 20 s, succeeded by annealing at 600 ∘ C for 60 min in O 2 gas flow.POCl 3 diffusion was performed at 930 ∘ C for 35 min with 0.2 l/min N 2 flow, to form n + emitter.Afterwards, wafers were rinsed in 10% HF and pure water by dipping, successively.SiO 2 film was formed by thermal oxidation process at temperature of 800 ∘ C for 10 min under O 2 gas.Before the metallization process, wafers were set on a hot plate heated at deposition temperature (450 ∘ C) and deposition of TiO 2 films were carried out by spray pyrolysis.Then, front and back contacts were formed by screen-printing Ag and Al, respectively.
Cofiring was carried out at 780 ∘ C for 1 min in an oven.After evaluating the solar cells with a single layer TiO 2 -compact films on the wafer surfaces, ZrO 2 -polymer composite films were deposited on the finished cells and compared to those of the cells without ARC and with single layer TiO 2 -compact films.Deposition of ZrO 2 -polymer composite ARC films was carried out by the annealing step after spin-coating process (1000 rpm on flat surface, 1500 rpm on textured surface, 5 s acceleration + 25 s, annealing at 125 ∘ C, 5 min).

Results and Discussion
In order to evaluate the reflectivity performance of single layer TiO 2 -compact film, flat surface silicon wafers were used.Figure 1(a) shows the comparison of the reflectivity of flat samples w/or w/o TiO 2 -compact film on silicon surface.The thickness of the TiO 2 film was mainly controlled by the spraying amount of TiO 2 precursor solution in a range of 0∼12 mL.Depending the film thickness, minimum of the reflectance (<2%) shifts towards to the longer wavelengths.
A steep decrease before the minimum can be confirmed for all TiO 2 film coated surfaces.As can be confirmed in Figure 1(a), the minimum of the reflectance shifts to the wavelengths as high as around 1000 nm for the films coated with 10 and 12 mL TiO 2 precursors.The average reflectivity (the reflectance average between the wavelength of 300 nm and 1100 nm) was plotted by changing the spraying amount of the TiO 2 precursor as 0 mL, 2 mL, 4 mL, 6 mL, 8 mL, 10 mL, The bottom of reflectance valley shifted to higher wavelength with increasing the solution volume from 2 mL to 12 mL, gradually.Using the 8 mL precursor solution, the second valley arouse at the lower wavelength which was close to the valley bottom of 2 mL precursor solution.The second valley at the lower wavelength was also shifted to the larger wavelength with increasing the precursor solution.
ZrO 2 -polymer composite films were formed on textured silicon surface by a spin speed of 1500 rpm for 25 s (acceleration time is 5 s), based on our preliminary work of ZrO 2 -polymer composite films as an ARC layer for silicon solar cells [19].Such a ZrO 2 -polymer composite film has a thickness of 102 nm on flat silicon surface and was used in combination with TiO 2 -compact film in this work.The average reflectivity (the reflectivity average between 300 nm and 1100 nm) of ZrO 2 -polymer films deposited on textured surface with various spin speeds was given in Table 1 where a good optical performance can be confirmed with average  reflectivity of 6.91%.The slight increase of average reflectance for 2000 rpm was considered as intolerance due to the possible local inhomogeneity of the film.
In case of reflectivity measurement for ZrO 2 -polymer composite/TiO 2 -compact multilayer, first, TiO 2 -compact single layer was formed and measured and then ZrO 2 -polymer composite was formed on TiO 2 covered surface and remeasured.The comparison of reflectivity of TiO 2 -compact single layer with the ZrO 2 -polymer composite/TiO 2 -compact multilayer formed on textured silicon wafers is given in Figure 2. The average reflectance was improved further with applying ZrO 2 -polymer composite to the TiO 2 -compact film, offering between 5 and 12% reflectance gain from 300 nm to 450 nm and a steady lower reflectance tendency from wavelength of around 700 to 1000 nm can also be confirmed.
Finally, silicon solar cells were fabricated using textured CZ-Si p-type wafers with and without using the ZrO 2polymer antireflection composite layer on TiO 2 -compact film.Conversion efficiency of the cells improved from 15.2% to 15.9%, where the  sc also increased further from 35.3 mA cm −2 to 37.2 mA cm −2 owing to the ZrO 2 /TiO 2 multilayer ARC film (the  sc of the cell without ARC was 28.7 mA cm −2 ).The electrical characteristics of the cells are summarized in Table 2. 70 mV increase of  oc of the cells with ARC can be confirmed.Though SiO 2 film was formed by thermal oxidation process at temperature of 800 ∘ C for 10 min under O 2 gas, we did not compare the photovoltaic results with/without SiO 2 layer.Hence, the increase of  oc is due to the compact TiO 2 layer, which can prevent excessive fire of Ag into p-n junction.The greater FF of the cells with ARC layer can be attributed to the lower contact resistances in which the effect of additional SiO 2 layer can be considerable in order to avoid internal shunts.J-V characteristics of the silicon solar cells fabricated with TiO 2 -compact single layer and ZrO 2 -polymer composite/TiO 2 -compact multilayer ARC on surface of the cells are given in Figure 3.
One can conclude that a significant improvement on  sc and  could be confirmed owing to the ZrO 2 -polymer composite/TiO 2 -compact multilayer ARC when compared to the textured cells without ARC.The increase of the  sc was related to minimizing the reflectance loses which can boost the performance of the cell.These results suggest that ZrO 2 /TiO 2 based multilayer ARC films formed by spray pyrolysis deposition technique and spin-coating technique could be an attractive alternative as a low-cost, simple, and vacuum-less process for ARC coating for silicon solar cells.

Conclusion
TiO 2 -compact and ZrO 2 -polymer composite antireflection coating layers were introduced as an alternative ARC layers for silicon solar cells.Films were formed by simple and cost-effective spray pyrolysis deposition and spincoating process.Reflectance decreased further with ZrO 2polymer composite/TiO 2 -compact layer comparing to the single TiO 2 -compact film formed surface.Less than 2% surface reflectance was confirmed after applying ZrO 2polymer composite/TiO 2 -compact multilayer in a wide range of spectra, from around 550 nm to 1050 nm.Silicon solar cells were fabricated using CZ-Si p-type wafers both with and without ARC layers on the illuminated surface.Conversion efficiency of the cells improved by a factor of 0.8% with ARC and reaches 15.9% efficiency where the  sc increased further by 2 mA cm −2 (from 35.3 mA cm −2 to 37.2 mA cm −2 ) owing to the ZrO 2 -polymer composite/TiO 2 -compact multilayer ARC.These results indicate that ZrO 2 -polymer composite films can be an attractive candidate as an ARC film and for multilayer ARC structures in order to produce cost-effective solar cells.

Figure 1 :
Figure 1: Comparison of the reflectivity of the flat samples with TiO 2 antireflection coatings by various amount TiO 2 precursor solution.(a) Reflectance spectra depending on spraying amount.(b) Average reflectance (300-1100 nm wavelength) depending on spraying amount.

Figure 2 :
Figure 2: Comparison of the reflectivity of the textured silicon wafers with TiO 2 single layer and spin-coated ZrO 2 /spray-deposited TiO 2 double layer.

Figure 3 :
Figure 3: J-V curve comparison of the best cells with or without antireflection coating.

Table 2 :
Photovoltaic characteristics of fabricated crystalline silicon solar cells with various surface structures with/without antireflection coating on textured surface.