磷酸湿化学蚀刻改进多晶硅片太阳能电池的方法

1. Introduction

 Silicon nitride (SiN) thin films are widely used in various  microelectronic and optoelectronic devices (Ino et al 1994). It is used as gate dielectric in MOS components  (Tsividis 1999) and also applied in the fabrication of microelectromechanical systems (MEMS) (Kaushik et al 2005). In photovoltaic field, SiN has the function of  defects passivation (Sopori et al 1996) and light antireflection (Barreraa et al 2008). SiN is applied as antireflective coating (ARC) to minimize reflections and  ensuring a higher photocurrent (El Amrani et al 2008). It  is also known that the hydrogen introduced in SiN film  has a crucial role in the bulk and surface passivation  (Duernickx and Szlufck 2002; Sopori et al 2005). Generally, for solar cells metallization a standard screen printing process is applied. Initially, this process was first  developed for monocrystalline silicon solar cells and then  adapted for multicrystalline silicon substrate. The application of this process on mc-Si wafers results in lower  performance solar devices. A classical hydrofluoric acid  (HF) dipping is well known for enhancing the efficiency  (η) and the fill factor (FF) and for long time was applied  on monocrystalline silicon solar cells. Unfortunately, HF  immersion was found to be less effective in enhancing the electrical characteristics (η and FF) of the mc-Si solar  cells. By contrast, we show in this paper that we can  boost mc-Si solar cells performances with our new and  cost effective chemical H3PO4 treatment.

2. Experimental  

Solar cells used in this study were based on structure of  n+ /p junction. The structure of this n+ /p multicrystalline  silicon cell is given in figure 1.

The starting material was a mc-Si wafer of p-type  (boron-doped) with ~ 3 Ω cm resistivity and having an  initial thickness of 380 μm. These wafers were dipped  into NaOH (30% at 85°C) polishing solution until a final  thickness of 350 μm was reached; followed neutralization  and RCA decontamination stages by putting them in a  bath made of H2O2/NH4OH/DI H2O at 70°C, then etched  in diluted HF (5%) immediately (Kern and Puotinen  1970).

3. Results and discussion  

The H3PO4 treatment was applied to n+ /p standard solar  cell that has been realized by the process described  above. Conceding that, the initial efficiency (η) and fill  factor (FF) were as low as 5⋅4% and 50⋅4%, respectively  for sample S1. Nevertheless, after H3PO4 treatment for  30 min at 85°C, we observed an improvement of η and  FF as shown in figure 2. The η reached 7⋅7% and FF  70⋅8%, this means a relative increase of about 40% up  from the initial values. In order to demonstrate the reproducibility of this process we have etched a second sample  (S2). Figure 3 shows the amelioration of η and FF. For the S2 solar cell, we have chosen the etching times: 10,  20, 30 and 40 min. Series resistance was calculated and  found to be decreasing after chemical attack as shown in  figure 2. In order to understand the physics behind the  boosting of η and FF, we made a full analysis of the  chemical composition of SiN thin layer before and after  etching.

4. Conclusions  

We have shown an effective amelioration of I(V) characteristics resulting from a new H3PO4 chemical treatment.  A boosting up to 40% of efficiency and fill factor has  been successfully accomplished by this treatment. AFM  study shows that SiN layer is starting to be locally etched  after 30 min of H3PO4 etch which facilitates the acidic  solution penetration to the SiN/Si interface. Thus, the  passivation by hydrogenation becomes more probable.  Finally, SEM–EDX study showed that after this chemical  treatment, the rate of Si content increases in all regions  (small grains, large grains and grain boundaries) of the  SiN thin layer and thus ameliorating the passivation  effect. Phosphoric acid passivation is an effective way to  make a better passivation of SiN/mc-Si interface at low  temperature.

Fig1

Finally, we used a Varian Cary 500 spectrophotometer  to extract the reflectance of thin SiN layer. The reflectance was measured for each of the samples that were  etched for different times. We can see from figure 7 that  a non-etched SiN layer is highly reflective in the visible  range as well as the near-UV. In contrast, the sample  etched for 25 min displays a low reflectance behaviour in  the near-UV range. The sample etched for 15 min in  H3PO4 possesses the best reflective characteristics in the  400–700 nm range i.e. it absorbs more light than the other  samples on a broader spectral array.