Chemical Texturing Used to Reduce The Reflectance of the Multi-Crystalline Silicon Wafer For Improving Optical Properties and Solar Cell Efficiency

doi:10.21203/rs.3.rs-934808/v1

The aim of this work is to improve the optical properties in the multi-crystalline silicon (mc-Si) by acid texturization. Generally, HF and HNO₃are using the mc-Si wafer acid texturization process and it is toxic chemical acids. In this work, H₂O₂is used instead of HNO₃ because of H₂O₂less toxic chemical compared to HNO₃. In this work, we have used the different types of chemical acids in different ratios for etching. Here, we have used HF: H₂O₂: CH₃COOH=3:2:2, HF: H₂O₂: KMnO₄ =3:2:0.2 M and HF: H₂O₂: HNO₃: KMnO₄ =3:2:2:0.2M for etching with the etching time of 60 sec. The HF: H₂O₂: KMnO₄ =3:2:0.2M gives the better results as obtained from optical microscope, UV- Visible reflectance studies and X-ray diffraction (XRD) studies. The etched mc-Silicon wafer surface was analyzed by the optical microscope and Scanning Electron Microscope (SEM). The FTIR results indicate the reduction of oxidation in the etched samples. Moreover, the HF: H₂O₂: KMnO₄ =3:2:0.2M textured wafers have the advantages of lower reflectance and increased etching of the mc-Silicon wafer.

Electronic Materials and Devices

Acid and base Texturization

Multicrystalline silicon

Light trapping

Solar cell

Surface reflections are considered as a key process to achieve higher efficiencies in solar cells. Mc-Silicon has occupied larger percent of the silicon solar cell market due to its low cost. However, the effective surface texturing in mc-Silicon is a challenging task for solar cell application. The different type of chemical textures was used for the etching process. Acid mediated texture on etching is a key technology to fabricate mc-Silicon for solar cell application. Because, it is a cost effective technique to get high efficiency. The different types of etching process were used to reduce the light reflectance. Reactive Ion Etching (RIE) [1], Metal-catalyzed chemical etching (MACE) [2], Plasma texturing, plasma-less atmospheric pressure dry [3] and acidic texturing [4] were used to reduce the reflectance in mc-Silicon wafer. RIE and MACE methods give less than 5% reflectance. But, these are inadmissible in the photovoltaic field due to low throughput and process complexity [4]. These are also expensive etching methods. The chemical etching is most widely used to remove unbounded molecule and impurities. In general, alkaline solution based anisotropic etching was adopted for the single crystalline - silicon and in the case of mc-silicon the acid solution based isotropic etching has been performed. Usually, random sized pyramid structures are obtained in c-silicon and “worm like” structures are obtained in mc-Silicon. More specifically, the texturization of the (100) oriented C- Silicon wafers have already been commercialized based on alkali or isopropyl alcohol etching process [5].

Mc-Silicon is different from c-Silicon. Randomly oriented crystalline grains make the grain boundaries and it is unavoidable in the mc-Silicon. Mechanical etching and laser etching textures gave regular microstructure surface. But, it reduces photoelectric conversion. Further, it is highly expensive and not suitable for mass production [8]. Acidic etching is a key technology on Texturing the mc-Silicon wafer to reducing the reflectance of the wafer. In addition, it gives higher efficiency, increasing the optical path length and it is least expensive method. Sheng et al., [9] have investigated the crystal orientation and crystalline peak intensity of the etched silicon wafers. The X-ray diffractogram result indicates that the crystalline peak intensity of the etched silicon wafer is slightly shifted to lower 2θ compared to without etched wafer. Shuai Z et al., [10] have optimized chemical etching (MCCE) of the mc-Silicon wafers with different orientation with both alkali and metal-catalyzed chemical etching.

Gangopadhyay et al., [11] have analyzed the different ratios of sodium hydroxide–sodium hypochlorite (NaOH–NaOCl) texturing solution with mc-Silicon. They have showed the formation of Si-Cl bonds through the FTIR imaging and the bonds have improved the quality of the diffused junction.

In this paper, the reaction behavior of the mc-Silicon in HF –H₂O₂– CH₃COOH/ KMnO₄/ HNO₃–KMnO₄ etching mixtures is studied and the appropriate composition of the texturing reaction mixture was found. It is obtaining the effective isotropic surface texture of mc-Silicon solar cells in a short time of 60 secs and it is helpful in improving the optical properties. The weight loss, reducing the reflection, surface morphology and crystal structure of the mc-Silicon wafer were determined by using the digital micrometre, UV-Vis-NIR Spectrophotometer, optical microscope and X-Ray diffractometer (XRD), respectively. The etching mechanism is studied for different grains of mc-Silicon wafers. The reduction of the oxidation is also studied by using FTIR.

The random small size mc-Silicon wafers are cut from the commercial P-type multi-crystalline silicon wafers with 180-micron thickness. Before processing, the wafers were cleaned in RCA1 and the experiment was carried out in HF –H₂O₂– CH₃COOH/ KMnO₄/ HNO₃–KMnO₄ solutions. After etching, the mc-Silicon wafers are placed in deionized water for 1 minute. The weight of the wafer was taken before and after etching process. The different type of chemical etching solutions and the result of improved solar cell efficiency are shown in Table.1. It is indicating that the texturization of mc-Silicon wafer is leading to a significant reduction in reflectivity and gives high values of short circuit current [12]. The surface morphology of the mc-Silicon wafers was studied using Scanning Electron Microscopy (SEM) QUANTA 200 3D Dual Beam (FEI) and CMM-23 COSLAB Microscope. The crystal structure and orientation were studied using Analytical Empyrean X-ray diffractometer (XRD) with CuK_α radiation ((λ = 1.5406 Å). UV-Vis-NIR spectrophotometer (Lamda 35, PerkinElmer) was used to measure the surface reflectivity.

Table. 1. Literature survey of few chemical etching rates, the steps of the chemical etching process and the results of the efficiency.

S.no

Cleaning before etching

Etchant Name

Acid ratio

Cleaning after etching

Result

1

RCA1 cleaning

Other method:

(Mechanical etching

Laser etching)

(2014)

Isotropic Etching

HF + HNO₃ + H₂SO₄ 1: 1.6: 8.2 = 60 sec (Time)

HF + HNO₃ + CH₃COOH 2: 3 : 6 = 60 sec (Time)

Double distilled water

14.7 % Conversion efficiency

2

HF + HNO₃ + H₂O 3: 1: 2 = 60 sec (Time) (2017)

Isotropic Etching

KMnO₄ = 2.5 M HF = 4.5 M Time = 10 Minutes

NH₄OH and H₂O₂ Time = 90 sec After rinsed in DI water

18.3% Power Conversion efficiency

3

Acetone & HF = 7.5 M (2010)

Isotropic Etching

KMnO₄ = 0.2 M HF = 11.0 M HNO₃ = 0.2M

Different ratio etching (Time = 1 min / 120 min /180 minutes)

Double distilled water

Porous silicon

4

KOH + IPA + H₂O

80*C for 3 minutes

(2014)

Isotropic Etching

HF + HNO₃ + CH₃COOH/H₂O (7:1:2) Time one minute

(15:2.5:1) Time 60 sec

(10:1:1) Time 60 minutes

Double distilled water

22% Conversion efficiency

2.1 Etching mechanism and Etching of mc-Silicon wafers

Mc-Silicon wafers with lower reflectance and lower recombination defects enhances the solar cell conversion efficiency. This can be obtained by chemical etching process with low cost. The wet chemical etching process is used for the removal of metal impurities from texturing of the silicon wafer and silicon materials. Acid texturing in a mc-Silicon wafer has been performed using a Hydrofluoric (HF) and Hydrogen peroxide (HNO₃). The general etching mechanism is of two steps: (1) the SiO₂ is formed by nitric acid on the surface of the mc-Silicon wafer and then (2) SiO₂ is removed from the wafer surface via water-soluble complexes formed by HF [13].

The HNO₃ produces an SiO₂ layer

3Si + 4HNO₃ →3SiO₂ + 4NO + 2H₂O (1)

It is dissolved by HF

SiO₂ + 6HF →H₂SiF₆ + 2H₂O (2)

The overall reaction is:

3Si + 4HNO₃ + 18HF → 3H₂SiF₆ + 4NO + 2H₂O (3)

The described mechanism of silicon dissolution is an electrochemical process on the silicon surface. In this process, holes (h+) were injected in the valence band of the semiconductor by HNO₃ at local cathodic sites, followed by the formation of a SiO₂ layer which is dissolved by HF at local anodic sites. The formation of SiO₂ layers was confirmed by the X-ray Photoelectron Spectroscopy studies [14]. M. Steinert et al have [15] studied silicon etching process using HNO₃-rich HF/HNO₃ using XPS. The higher nitrite concentration reduces the etching rate slightly and lower nitrite concentration increases the etching rate linearly. Stirring is also increasing the etching rate. In this work, Hydrogen peroxide (H₂O₂) was used instead of the nitric acid (HNO₃) and etching mechanism of these combinations is as follows:

The H₂O₂ produces an SiO₂ layer

3Si + 2H₂O₂ →SiO₂ + 2H₂O (4)

It is dissolved by HF

SiO₂ + 6HF →H₂SiF₆ + 2H₂O (5)

The overall reaction is:

Si + 2H₂O₂ + 6HF → H₂SiF₆ + 4H₂O (6)

The electrochemical process works just like conventional operations. The mc-Silicon wafers were cleaned in RCA-1 process at 10 minutes and the purpose of the cleaning was to remove the organic residue on the silicon wafer surfaces. It is placed in deionized water for a few seconds. Three set of chemicals with different volume and molar ratios were selected for etching process at room temperature and etching duration was 60 sec. The first set of etching chemical solution is HF-H₂O₂-CH₃COOH with volume ratio of 3:2:2, second set of chemical solution is HF-H₂O₂-KMnO₄ with volume and mole ratio of 3:2:0.2M, third set of chemical solution is HNO₃-HF-H₂O₂-KMnO₄ with volume and mole ratio of 3:2:2:0.2M. the combination of the HF and HNO₃ were optimized to remove residual surface strain.

3.1 Reflectance of the mc-Silicon wafers

The acid etching is sufficient to create isotropic surface texture of the wafer and to avoid the saw damages. The reflectance of the mc-Silicon textured surfaces are investigated from wavelength between 300–1000 nm and shown in Figure.1. Three set of etched samples and reference silicon sample reflectance results were compared and shown in Figure.1. It is clearly seen that Set-2 (HF-H₂O₂-KMnO₄) chemical ratio reduces the reflectivity and the reduction up to 12.5 %. The Set- 2 chemical ratios in etching process strongly influences the effective reflectance. It has higher light absorbance compared to other chemical textured wafers since, the higher light absorbance leads to increase the solar cell efficiency. Kulesza et al., have optimized [12] the time efficient texturization of mc-Silicon in the HF/HNO₃ solution and its effect on the optoelectonic parameters of the solar cells. The optimal chemical etching ratio has increased the solar cell efficiency up to 13.9 %.

3.2 Crystalline structure of the grain orientation

Figure. 2 shows the XRD pattern before and after chemically etching the samples. The crystalline structure of the wafers is investigated by using Analytical Empyrean X-ray diffractometer (XRD) with CuK_α radiation ((λ = 1.5406 Å). The XRD pattern of silicon wafer shows the characteristic peak 28.40 and 56.10 (JCPDS file 271402) which correspond to (111) and (311). From the Fig. 2(a), HF: H₂O₂: CH₃COOH etched wafer gives slight peak shift to a lower value of 2θ in the (311) orientations. Sheng et al., [18] have optimized the mixture solution of HF: HNO₃: CH₃COOH with different time and concentration. From the Fig. 2(b), HF: H₂O₂: KMnO₄ etched wafer gives higher intensity in the (111) orientations and lower light reflection. Zou et al., [17] have done the alkali and Ag- MCCE etching of the (111) orientated mc-Silicon wafers expose dark appearance owing to the excellent light-trapping ability with Pyramid structure. From the Fig. 2(c), HF: H₂O₂:HNO₃: KMnO₄ etched wafer gives higher intensity in the (311) orientations and appear shiny. This refers to the higher inter planer spacing values of the atomic layers in silicon. In XRD results, the etched silicon wafers have given the higher intensity with slight peak shift in lower 2θ angle side. The X-ray diffraction (XRD) result of HF: H₂O₂:KMnO₄ indicates that the crystalline peak intensity of the etched mc-Silicon is higher than non- etched wafer and HF: H₂O₂: CH₃COOH = 3:2:2 etched results indicates the peak shift slightly to lower 2θ. Moreover, the HF: H₂O₂:KMnO₄ = 3:2:0.2M textured wafers have the advantages of lower reflectance and increased etching of the mc-Silicon wafer.

3.3 Surface morphology of the mc-Silicon wafers:

The surface structures of the etched mc-Silicon wafers were investigated by using an optical microscope and SEM. Figure. 3, shows the changes occurred on the mc-Silicon wafers after etching by using an optical microscope at a resolution of 500X. Different etched patterns were produced by using three different chemical etching solution in the silicon surface. The comparison in results, the surface structures are considerably changed by using HF:H₂O₂: KMnO₄ chemical solution. This chemical solution induces the small size uneven worm-like trenches and it has given lower reflectance compared to other two chemical solutions etched samples. The HF: H₂O₂:HNO₃: KMnO₄ chemical solution induces the medium size worm-like trenches structures. The HF: H₂O₂: CH₃COOH chemical solution gives very small size worm-like trenches structures and it has given slightly lower reflectance compared to non-etched wafer.

An optimization of the three different textured solutions were helpful to achieving lower reflectance. The SEM images of silicon wafer surface are shown in Figure.4. The saw damage has been removed and the mc-Silicon defects have disappeared by the etching process. Images show the formation of oval pits smoothened surface after etching the wafers at in HF/ H₂O₂/CH₃COOH and HF/H₂O₂/ KMnO₄ textured solutions. HF/ H₂O₂/CH₃COOH gives 3–6 µm size oval pits and HF/H₂O₂/ KMnO₄ textured solutions gives 2–4 µm size oval pits which are shown in figure.5. HF/H₂O₂/ KMnO₄ results is reduced loss of weight compared to HF/ H₂O₂/CH₃COOH textured solution and it is shown in Table.2.

Kulesza et al have performed [12] the time efficient texturization process with HF/HNO₃ solutions. They reported that this etching process is improving the optoelectronic parameters of the solar cell. Higher concentration of HF/ HNO₃ results in the formation of round pits with varying diameters. Optimized etching solution gives lower reflectance and higher efficiency. HF/H₂O₂/ KMnO₄ textured solution led to formation of round pits in some places and MnO₄ is glazed. The solution HF/H₂O₂/ KMnO₄/ HNO₃ is causing more corrosion and it is shown in Figure.6. It reduces more weight and it is shown in Table.2. The HF/H₂O₂/ KMnO₄ solution is showing the optimal oval pit structure with the lowest reflectance.

Table. 2 Weight of wafer before and after chemical etching

Chemical Etchant	Before etching (g)	After etching for 60 sec (g)	Loss of Weight (g)
HF + H₂O₂ + CH₃COOH	0.14708	0.14675	0.00033
HF + H₂O₂ + KMnO₄	0.14903	0.14890	0.00013
HF + H₂O₂ + HNO₃ + KMnO₄	0.22467	0.22420	0.00040

3.4 FTIR Studies for Analysing impurities:

Wet etching of the mc- Silicon wafers is a critical step in the solar cell making process. Optimized wet etching process improves the optical properties [19, 20]. The recombination properties of mc-Silicon solar cells are more complex owing to the structural defects, point defects and interaction among defects. Oxygen segregation in structural defects affects electrical activity and further complicates it. Pizzini et al., [21] confirmed an oxygen segregation in the structural defects and they showed its influence on dislocation electrical activity. IR absorption spectroscopy is an important tool to study the nature of HF etched silicon. Reduction of oxidation is confirmed in three different textured solutions and it is shown in the IR spectrum of Figure.7. The peak position of Si–O–Si bonds at 1107 cm^− 1 and its reflectance are decreased in Figure.7. The combination of HF/ H₂O₂/CH₃COOH etching solution significantly decreases the Si–O–Si reflectance. The combination of HF/H₂O₂/ KMnO₄ reduces the Si–O–Si reflectance and C-H reflectance. The combination of HF/H₂O₂/ KMnO₄/ HNO₃ greatly decreases the Si–O–Si reflectance and C-H reflectance compared to the combination of HF/H₂O₂/ KMnO₄. But, the combination of HF/H₂O₂/ KMnO₄/ HNO₃ is causing more corrosion and it is more reflecting the light source.

Acid texturization based on HF: H₂O₂: (CH₃COOH or KMnO₄ or KMnO₄:HNO₃) mixture was demonstrated. The result of the two mixtures solutions HF: H₂O₂: (CH₃COOH or KMnO₄) were led to formation of round pits with different diameter in the single step and HF: H₂O₂: KMnO₄:HNO₃ texturization solution has given uneven surface. Silicon wafers engraved with HF: H₂O₂: KMnO₄ have high optoelectronic parameters. HF: H₂O₂: KMnO₄ solution gives high intensity in the (111) orientations and lower reflectance compared to other two solutions. It reduced the oxidation (Si–O–Si) reflectance and improved the electrical activity. The reduced surface reflectance and reduced oxidation were reducing the surface recombination loss and increasing the short circuit current (J_sc). In solar cell making process, HF: H₂O₂: KMnO₄ based textured wafer can give higher conversion efficiency.

Funding

This work was supported by the Department of science and technology Government of India (Order No. DST/TMD/CERI/RES/2020/7(c)dated on 31/12/2020).

Conflicts of interest/Competing interests

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Code availability

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Authors' contributions

All the authors are equal contribution in the manuscript.

Madhesh Raji - Writing - Original Draft
Kesavan Venkatachalam- Formal Analysis, Investigation,
Srinivasan Manikkam- Supervision
Ramasamy Perumalsamy - review & editing

Disclosure of potential conflicts of interest

Not applicable

Research involving Human Participants and/or Animals

Not applicable

Acknowledgements

This work was supported by the Department of science and technology Government of India (Order No. DST/TMD/CERI/RES/2020/7(c)dated on 31/12/2020).

Compliance with ethical standards

This study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee.

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Informed consent was obtained from all individual participants included in the study

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Consent for publication was obtained from all individual participants included in the study

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Chemical Texturing Used to Reduce The Reflectance of the Multi-Crystalline Silicon Wafer For Improving Optical Properties and Solar Cell Efficiency

Status:

Version 1

Abstract

Figures

Introduction

Experimental Section

Results And Discussion

Conclusion

Declarations

References

Status:

Version 1