1 Introduction

Solar cell should absorb light and convert into electricity, in order to get high power output, reduce surface reflection is very important. For crystalline-Si solar cell, texturing surface becomes standard procedure by Si etching, either by wet chemical etching or dry ion etching. Solar cells based on mc-Si currently occupy for more than 70% of global PV production [1]. However mc-Si cells have lower efficiencies than single crystalline counterpart, one of the main reasons for it is the lack of feasible industrial-scale cost effective texturing method of mc-Si [2]. After proper wet acid isotropic etching, the mc-Si wafer surface becomes rough and reflection is reduced from above 40% down to around 20–24% which is still lower than the reflectance of texturing single crystalline Si wafer etched by alkaline [3, 4].

Dry etching by reactive ion (also called RIE) and acidic wet etching by chemicals are two main methods so far for texturing mc-silicon surface in the solar cell process, no matter for slurry cutting-based wafer or diamond wire cutting-based wafer [5, 6]. RIE is rather complex and needs expensive vacuum system and hazardous chemical gas compared with the isotropic texturing process by acidic solution. Wet acidic texturing, performed with solutions containing HF–HNO3–H2O, either HF rich or HNO3 rich, tends to result in features with rough Si surfaces. The essential mechanism for etching Si with HF–HNO3 is also a chemical redox reaction, whereby the Si is oxidized by the HNO3, whilst the HF serves to remove the oxide so formed [7]. Acidic etchants based on the HF:HNO3 system are isotropic in nature, certain byproduct gas bubble, like yellow toxic NO, NO2 is generated, so reaction environment control will play key role for optimized texturing structure [8].

To further decrease surface reflectance of mc-Si solar cell, various texturing technologies have been investigated. One of the most promising methods to reduce reflection is producing nanostructure surface which is often termed as “black silicon” [9]. Torres et al. have produced black silicon by femtosecond laser and demonstrated that the electrical performances were improved compared to the solar cell with non-textured surface [10]. However, for the industrial mass production, black silicon is still a high cost method of low reflectance, wet HF–HNO3–H2O system is widely used so far for solar cell fabrication. In the Si MEMS production, wet chemical etching is used very popular, to get controlled etching rate for desired Si structure, better Si-aqueous solutions interface contact is needed, surfactants are often used as wetting agents in chemical etching solutions [11]. Poor wetting of the Si wafer surface by the acid etchant during semiconductor device fabrication can result in inhomogeneous etching rate and air entrapment through the formation of small bubbles. These bubbles can mask a portion of the area to be etched, which can result in bridging over of the fine openings in the resistor surface [12]. Surfactants have very low surface tensions, and cause homogeneous Si-aqueous solutions interface contact. A uniform etch can be made using this type of surfactant. The elimination of air entrapment and the stability of the surfactant leads to an extended life of the etch solution. In the alkaline single crystalline Si etching process, the use of additives has been standard for several years. For industrial-scale multicrystalline texturing however, no such product had been available, although many non-ion surfactants can be selected as additive for acid etching, the stability of these surfactants in the acid solutions and sequent wafer surface contamination are big issues for further solar cell process [13].

In this work, we changed the wetting property of multicrystalline Si (mc-Si) wafers by simply spray of surfactant containing solution on the surface prior to etching, following texturing process becomes gentle, measured total reflectance of mc-Si wafer is less than 20%, keep the rest solar cell processes identical, we could get about 0.1% cell efficiency improvement, specially its short circuit current Isc is increased about 40–50mA.

2 Experimental

2.1 Wafer surface pretreatment and etching

Here commercial available P-type multicrystalline Si (mc-Si) wafers from same batch were employed. All investigated wafers have the same technical specifications, with thickness of 180 ± 20 μm, area of 156 mm × 156 mm, and resistivity of 1–3 Ω cm. It is well known that pure Si is hydropholic [14], and has non-wetting surface after top thin native oxide removed. The purpose of the wafer pretreatment is to enhance the wetting property of Si surface, since all acids etching used are of water-miscible. Very diluted surfactant solution was prepared: a 1.25 ml non-ion surfactant Tween 80 was added into 1000 ml DI water used as pretreatment liquid.

One drop of above liquid with about 0.5 ml volume is deposited on the Si wafer and brushed, then dispersed homogenously on the whole surface. After this simple pretreatment, Si wafer with wet downside surface is transferred into acid bath for texturing etching. Here the texturing etching happened immediately after pre-wetted, so the stability of surfactant is not a key issue, compare with pre-mixing solution of acids with surfactants. This extends the selection of surfactants.

Texturing process was done on the Schmid inline acid etching system at Red Sun photo-electricity company, Changsha, Hunan. Acidic solution contains HF(~49%), HNO3(~69%), and DI water in the volume ratio 51:170:125, respectively. Etching time was 60 s, sequentially porous structure on the wafer was etched away by alkaline NaOH etching, impurities on the wafer was removed by HCl etching, and finished wafer was dried by N2 flush.

Meanwhile a group of wafers as a control sample without this pretreatment finish their texturing process as the same procedure. The morphology of typical sample from these 2 groups was analyzed using scanning electron microscopy (SEM) JSM-6700F. Reflectance curves of all etched samples were also measured by D8 reflector.

2.2 Solar cell fabrication

After texturing, we make the same rest steps to finish solar cell fabrication for these 2 groups of wafers (total 400pcs neighboring wafers divided into 2 groups), which have used conventional POCl3 diffusion, PECVD for silicon nitride deposition as antireflection coating (ARC) and screen-printed metallization. All finished solar cells are sorted according to their performances.

Finally, the typical solar cells were tested using Solar Simulator of model Quick Sun 125 C under standard condition of AM 1.5 Global Spectrum at 25 °C, and typical I–V curve of selected solar cells was measured by Halm Solar Simulator, performance parameters were recorded and analyzed. Quantum efficiency (IQE and EQE) are measured by QEX10 with 300–1200 nm wavelength range.

3 Results and discussion

3.1 Morphology characterization

Under different magnifications, the SEM morphology comparison of two type etched Si wafers was shown at Fig. 1. HF/HNO3 system without any addictive or diluent is an aggressive chemical etching system which is hardly controllable, and surface show a typical worm structure, this can be confirmed from the comparisons in Fig. 1 that the pre-wetted wafer in Fig. 1b, d, f showed relatively even and small etch pit during the texturing process, comparing with Fig. 1a, c, e. Surface roughness in AFM images in Fig. 2 also proves this difference. Reflection increase with etched deep and large dimension in correspondence with the increased flatness of the texture as showing for normal wafer, meanwhile lower R of pretreated wafer correspondence for the shallow and small etches with correspondingly the highest etch pit curvature [15]. This may mean that, if saw-damage is sufficiently removed, as low an etch depth as possible should be used for acidic texture, to maximize curved surface and thereby getting lower reflectivity, with the advantage of minimal usage of corrosive chemical agents, like HF and HNO3. So this pre-wetted method could be more suitable for the new type Si wafer, like direct wafer from 1366 technology [16], since no saw-damage on the wafer during the production.

Fig. 1
figure 1

SEM morphology photography of normal mc-Si wafer as shown in (a), (c), (e) and pre-wetted mc-Si wafer as shown in (b), (d), (f) after acid texturing etching, under different magnifications

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Fig. 2
figure 2

Texturing wafer surface roughness analysis by AFM for normal mc-Si wafer (a) and pre-wetted mc-Si wafer (b). The etched pits in (a) are larger and deeper than pits in (b)

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Figure 3 shows a set of reflectivity (R) versus wavelength measurements of the pretreated mc-Si wafer and controlled normal wafer with error bar. The reflection spectrum of the controlled normal wafer shows the typical industrial accepted average R (20–24%), whereas average R achieved for pretreated mc-Si wafer is 18–20% in the 350–1050 nm wavelength range. This clearly shows that the average R of pretreated mc-Si wafer is about 2% lower than average R of routine mc-Si. The origin of this lower reflectivity from the etched pretreated mc-Si wafer is already elucidated from the SEM morphology comparison in Fig. 1. There have several parameters could influence the etching quality of mc-Si in the wet process, typically like temperature, acid solutions, various acids ratio of solution and process time. One commonly used diluent for the HF/HNO3 system is acetic acid CH3COOH, but CH3COOH is not an economic solution of it. Except above parameters, the wetting property of Si wafer surface is also important factor for better texturing structure as we show here, so intentionally switch intrinsic hydropholic feature of Si wafer into the hydrophilic prior to etching, this will benefit etching process, also is simple to implement for industrial-scale mass production [17].

Fig. 3
figure 3

Reflectivity versus wavelength curves of normal and pretreated mc-Si wafer. The curves are showing the reflectivity difference of them

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3.2 Solar cell characterizations

Table 1 list the average performance of these two groups solar cells, obviously, solar cells from pre-wetted wafer have averagely higher short circuit current Isc than normal ones, as well as efficiency, the Isc of pre-wetted one is 7.756 A and efficiency is 17.95%, respectively. Isc is enhanced about 40–50mA, correspondingly efficiency is enhanced about 0.12%. Rest parameters, like Voc, FF, summarized in Table 1 are almost comparable, the variations in FF of the cells could be attributed to the different values of parasitic resistances such as shunt and series resistances, indicates that quality electrical contacts of pretreated wafer-based solar cell still have room to improve. Figure 4 shows a set of illuminated I–V measurements of the finished control solar cell and solar cells from pretreated wafer with error bar. It shows pre-wetted cell has higher efficiency performance, because of relatively high Isc.

Table 1 Comparison of solar cell performance parameter for the control cells with pretreated wafer based cell
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Fig. 4
figure 4

Typical illuminated I–V curve measurements of pre-wetted based solar cell and normal solar cell. Here has the Isc difference among of them

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Here we also measured and compared the internal quantum efficiency (IQE) curve of these 2 type solar cells, as shown in the Fig. 5, pre-wetted one has overall better spectra response in the blue and visible range, but the IQE of pre-wetted based cell is decreased slightly in the IR wavelength region, which could be attributed to following: the silicon surface’s dangling bonds and traps that will play as effective recombination centers, pre-wetted solar cell actually has homogeneous and shallow front texturing surface, it should be good for PECVD surface passivation, front surface recombination velocity is lower than control sample. The texturing surface of pre-wetted solar cell presents an optimized texturing condition.

Fig. 5
figure 5

The internal quantum efficiency (IQE) of the solar cells and comparison of pre-wetted one and normal one

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4 Conclusion

We have developed a simple and easy integrated chemical process for silicon surface texturing wherein all reflectance <20% can be achieved in the usable spectral range for solar cells. Obvious improvement in Isc and efficiency has been demonstrated in the pretreated ones compared to the normal ones. Wetting property of etched wafer is the key reason to get such better reflection. The quantum efficiency measurement also reveals a better response in short wavelength region for solar cell that may be due to the homogeneous front surface passivation of fine-textured surface compared with the rough-textured surface. Switching to wetting surface of Si wafer prior acid etching is a simple, effective process to realize an effective low reflective Si surface, which has potential for silicon solar cells. This could be a good solution to upgrade the current inline acid etching facilities for PV efficiency improvement.