碳酸氢钠溶液氧化单晶硅太阳能电池的研究

1 INTRODUCTION  

The photovoltaic technology is well established as a reliable and economical one for small  sources of electricity and one of the most important factors is the optoelectronic conversion efficiency. In order to improve the optoelectronic conversion efficiency of monocrystalline silicon  solar cells, the texturization was used to reduce surface reflectivity of silicon wafers and enhance light absorption. In the case of monocrystalline silicon, many studies were reported about  the mixture of sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution with isopropyl alcohol (IPA), because alkaline solutions etch the silicon anisotropically and generate small  pyramids on the silicon surface.  

2 EXPERIMENTS  

Monocrystalline silicon wafers of P-type <100> orientation with a resistivity of 2 Ω-4 Ω. cm  were used for this work. The size of the wafers was quasi-squared 15 mm × 15 mm which cut  from the 125 mm × 125 mm. Before texturing, all wafers were handled by the following procedures. The first step was to degrease the samples by cleaning the wafers with the absolute ethanol in the ultrasonic cleaner for 10 minutes. In the next step, the samples were immersed in diluted hydrofluoric acid (HF, 4 wt%) for 1 minutes to remove the native oxide and rinsed in  deionized water for several times. When all the cleaning work is over, the samples were heated  by the drying oven to remove moisture of their surface.  

The cleaned wafers were held vertically in a specially designed device, which is self-made.  Then the wafers were etching in different concentration of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3).During the experiments, the reaction temperature and time should  be well controlled. When the reaction is over, all the eroded wafers were washed again into the  absolute ethanol solution and deionized water for 10 minutes. After that, they were dried in the  drying oven for tests. In the en, we obtained the silicon wafers surface reflectance data using  UV-Visible spectrophotometer. In addition, images of surface morphology was measured by a  scanning electron microscopy (SEM).

3 RESULTS AND DISCUSSION 

3.1 The effect of different concentration of sodium carbonate on the texture.  

Primarily, the effect of the Na2CO3 concentration on the surface reflectivity was investigated.  The solution temperature is 85°C and the etching time was carried out at 30 min. The reflectivity was measured for samples textured using 15 wt%, 20 wt%, 25 wt% and 30 wt% Na2CO3 individually. As shown in Fig. 1, the reflectivity was found to decrease with increasing Na2CO3  concentration up to 20 wt%, when the concentration of Na2CO3 is over 20 wt%, the reflectivity  start to rise. Fig. 2 shows the weighted average reflectivity in the range of 400 nm-800 nm spectrum range, we can see the lowest value is 14.98%. Fig. 3 is the SEM images of monocrystalline  silicon wafers texture, which has shown the pyramids structure. All these experiments results  mean that it is better to use 20 wt% Na2CO3 when we use the single reagent.  

3.2 The effect of different concentration of sodium bicarbonate on the texture.  

Besides the existing 20 wt% concentration of Na2CO3, now we add some different concentration  of NaHCO3 (2 wt%, 4 wt%, 6 wt% and 8 wt%). Samely, we fix the solution temperature is 85℃ and the etching time is 30 min. From the reflectivity curves of Fig. 4, we can see a minimum reflectivity when the NaHCO3 concentration was increased to 4 wt%. Fig. 5 shows the lowest weighted average reflectivity is 12.84%. Fig. 6 is the SEM photograph. It seems that this kind of  combined reagent system can successfully texture monocrystalline silicon for solar cells.  

Fig1

4 CONCLUSIONS 

In this work, a texturization technique for crystalline silicon solar cells using sodium carbonate  and sodium bicarbonate

solutions has been investigated. We can found that it is possible to optimize the reflectivity values of the samples by varying the Na2CO3 and NaHCO3 concentration  of the solution, the solution temperature and the etching time, thus obtaining low reflectivity  values comparable to the ones obtained in IPA-based solutions.  

It is shown that our optimized process lasted for 30 min at 85°C using a solution containing 20  wt% Na2CO3 and 4 wt% NaHCO3. After texturing, for monocrystalline silicon wafers, we can  obtain reliable and uniform pyramidal texturing surfaces with an average weighted reflectivity  of 12.84%. Our approach is giving high hopes for the industrial production of high efficient solar cells.