Electrochemical oxidation pre-treatment for wet texturing of monocrystalline silicon solar cells

Highlights

• The average reflectivity of textured surface was lowered.

• The uniformity of pyramidal textured structure was improved.

• An ultra-thin oxide silicon layer could improve uniform growth of pyramid nuclei.

• SiO_x layer with thickness of 1.5–2 nm was formed by electrochemical oxidation.

• The electrolyte could be an acid, alkali or a neutral salt.

Abstract

To improve the textured structure uniformity of monocrystalline silicon solar cells, an electrochemical anodic oxidation pretreatment technology was implemented to the silicon surface before the alkali texturing process. The effects of pretreatment on the textured structure were investigated at different oxidation conditions (voltage and duration) in three typical aqueous electrolytes with different pH values. The structure and thickness of the silicon oxide layer on the silicon wafer surface were analyzed by X-ray photoelectron spectroscopy. It was found that the uniform oxide layer of 1.5–2 nm formed on the Si surface can promote the uniform growth of pyramidal nuclei in the initial texturing stage, while the excessive thicker oxide layer will reduce their density and uniformity. The average pyramid sizes were 1.4 ± 0.7 μm and 1.6 ± 1.7 μm with and without electrochemical oxidation pretreatment, respectively. The distribution of pyramid size became more uniform because the fraction of extremely large pyramids was greatly reduced. Compared with the traditional texturing process, the average reflectivity of the textured surface decreased from 13.1 % to 10.4 %. This technique is expected to be a suitable substitute for the H₂O₂ pre-cleaning process in the Si texturization for the industrial production of solar cells.

Introduction

Surface texturization is an essential step to reduce the light reflectance for the fabrication of silicon solar cells. In recent years, a number of texturing methods have been proposed to reduce the surface reflectance of Si solar cells, such as metal-assisted chemical etching [1], [2] and masked dry or wet etching [3], [4], [5]. The inverted pyramid structure has also attracted increasing interest due to its excellent light-trapping properties [6], [7], [8]. However, anisotropic wet etching with sodium hydroxide (NaOH) or potassium hydroxide (KOH) basic solutions has still been a standard texturing technology for monocrystalline silicon (c-Si) solar cells in industrial production due to the low fabrication cost and high throughput. The wet etching process can produce the textured surface consisting of upright random pyramid structures which reduces the optical reflectance due to the double-reflection of the incident ray [9]. An excellent textured surface of solar cells should meet the following requirements: low light surface reflectivity, low carrier surface recombination rate and low contact resistance with the silver grid electrode. The size, distribution, and uniformity of the pyramid textured structure seriously affect the reflectivity and carrier lifetime of solar cells [10], [11], [12], [13], [14].

Chemical additives are commonly used in the alkali solution to improve the wettability on the wafer surface, regulate the nucleation process of the pyramid, control the etching rate and tune the pyramid size to achieve optimized uniformity of the pyramids across the Si wafer surface rapidly [15], [16]. Early alkali wet texturing not only took a long time, but also formed pyramids with an average size of > 7–8 μm [17]. The wide use of new texturing additives reduces pyramid size and its dispersion greatly [15]. Recent studies have shown that pyramid textured structures with an average size of 1–2 μm have not only low reflectivity, but also a low surface carrier recombination rate, which contributes to the improvement of solar cell performance [12], [13]. Although sub-micrometer pyramid structure may be more conductive to HIT solar cells because the interface between the a-Si:H layer and Si substrate needs small hills and valleys [18], 1–2 μm pyramid structure is the mainstream choice for PERC solar cells.

Though there are many studies related to the pyramid texturing, it is still difficult to achieve a uniform size and shape distribution of pyramids in the whole Si surface in industrial production because of a random growth of pyramids. Silicates (Na₂SiO₃ or K₂SiO₃) are one of the most well-known additives and have been studied for a long time [19]. Minkyu Ju et al. have proposed controlling pyramid size and its uniformity by adding different concentrations of silicate into alkali etching solution [13]. Na₂SiO₃ was used as an effective micro-mask for anisotropic etching and the small pyramid textured surface was generated. During alkali etching, SiO₃^2- increased the nucleation rate and density but decreased the growth rate of pyramids. However, SiO₃^2- was the reaction product in the texturing process. The concentration of silicate in the etching solution was constantly changing, which would lead to variations in the pyramid size. The application of silicates in continuous industrial production needs further study.

In addition to the use of additives, oxidation of the Si wafer was also an effective method to improve the pyramid structure. For example, a cleaning process with hydrogen peroxide (H₂O₂) and a low concentration alkali solution before texturing was reported to reduce the larger pyramids and to make the size distribution more uniform [20]. The etching process of c-Si using H₂O₂, O₃ or other oxidation agents can reduce Si surface roughness by restraining the generation of pyramids at the etched surface [21], [22], [23]. It was because that Si-O-Si or Si-OH bands band generated from the oxidation agents in the alkaline solution could change the isotropic etching rate of Si (1 0 0) and Si (1 1 1) face and inhibit pyramid nucleation. Although there are few related reports, we hold the opinion that the silicon oxide layer on the Si surface can affect the growth of the pyramid nuclei, which can be used to influence the size and uniformity of the pyramid textured structure.

However, strong oxidizing agents such as H₂O₂ and O₃ are environmentally toxic and hazardous. It is widely known that electrochemical oxidation is also an effective approach to generate silicon oxide film on the Si surface [24], [25], [26]. It is challenging to use anodic oxidized SiO₂ film in solar cells or other semiconductor devices as passivation or mask layers because of its high structure defects [25], [26]. However, if an ultra-thin silicon oxide layer plays as a sacrificial or buffer layer in the initial stage of the texturing process, the electrochemical oxide may be an attractive option compared with other methods of wet-chemical oxidation of the Si surface because it has little to do with the carrier traps at the interface.

This paper demonstrates an effective pre-treatment method to optimize the textured structure for industrial-scale c-Si solar cells. The electrochemical oxidation of c-Si wafer at a very low voltage based on the oxygenolysis of water was used rather than strong oxidizing agents. The effects of the oxidization pre-treatment on the textured structure of c-Si solar cells were investigated and compared with those of H₂O₂ pretreatment. The influence of the pyramid textured structure on solar cell performance was also estimated. Compared with the dangerous and unstable hydrogen peroxide, the electrochemical oxidation with low voltage and low concentration is beneficial to production safety and cost reduction.