Formation of nanostructured emitter for silicon solar cells using catalytic silver nanoparticles

Applied Surface Science 264 (2013) 621–624

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Formation of nanostructured emitter for silicon solar cells using catalytic silver nanoparticles Dan Li a , Lei Wang a , Dongsheng Li a , Ning Zhou a , Zhiqiang Feng b , Xiaoping Zhong b , Deren Yang a,∗ a b

State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Zhejiang Shuqimeng Photovoltaic Technology Co., Ltd., Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 26 July 2012 Received in revised form 9 October 2012 Accepted 13 October 2012 Available online 23 October 2012 Keywords: Silicon solar cell Nanoporous silicon Antireflection Ag nanoparticles Chemical etching

a b s t r a c t A simple process for nanotexturing on the emitter of silicon solar cells using catalyzed wet chemical etching by size-controlled silver nanoparticles was reported. A fine textured black surface was achieved to realize the low light reflectivity less than 5%. After screen printing and firing by the industrial standard fabrication protocol, we obtained the nanotextured Si solar cells with 15.7%-efficiency without any additional antireflection (AR) coating. This result suggests that the inexpensive metal-assisted wet chemical nanotexture method is prospective to be used in photovoltaic industry. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In order to maximize the amount of absorbed incident light which can be converted to electricity, minimization of surface reflection losses is required strongly for high efficiency silicon solar cells. Typically, the reflectance is normally reduced in crystalline silicon by applying pyramidal texturing [1,2] and antireflection (AR) layers deposition [3] on the surface. However, the average reflectance (>10%) in the wavelength range of 400–1100 nm after current pyramid texture process is still high, which prevents improving the conversion efficiency of solar cells further. Furthermore, the interference single-layer structure of AR coating works only in a narrow spectral range and a limited angular of incident photons and the fabrication is expensive due to using vacuum equipments compared to chemical methods. The nanoporous structures provide a promising approach to minimize surface reflection and replace conventional AR interference layers in Si solar cells [4]. In generally, the nanoporous surface can be formed by chemical etching in HF/HNO3 solutions or forward-biased electrochemical anodic etching [5–8]. In recent years, metal-assisted chemical etching has attracted increasing attention in the potential solar cell application for nanotexturing. Because it is a simple, fast and low cost process to reduce the reflectance of Si surface [9–13]. However, among the previous reports, the nanoporous layer was made on p-type silicon

surface directly or the emitter surface which the screen printing process had applied on [13–16]. In the former case, the complicated nanoporous structure would impede the formation of a p–n junction [13,17]. Furthermore, in the solar cell fabrication procedure (phosphorus diffusion and phosphorosilicate glass removing), the nanoporous structure would be damaged or even disappeared. In addition, residual noble metals would also be detrimental to minority carrier lifetime of silicon and then to the efficiency of solar cells. In the latter case, metal grid would be dissolved causing the degradation of the fill factor (FF) and then cell efficiency [14]. Therefore, development of new methods to realize the antireflection ability of nanoporous silicon on the solar cell device is required. In this paper, we develop a novel procedure to fabricate the nanoporous structures on the emitters of 125 mm × 125 mm silicon solar cells based on a standard industry process. Since our process could preserve the nanoporous structure of solar cells to a large extent and avoid damaging the metal grid compared to the peer’s work [15], measurements such as reflectance and current–voltage (I–V) have shown a significant improvement in nanotextured solar cell performance compared with reference solar cell. Our work suggests the nanoporous silicon solar cell is prospective for the practical application in photovoltaic industry. 2. Experimental details 2.1. Materials

∗ Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: [email protected] (D. Yang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.079

The p-type solar grade CZ (100) Si wafers (1–3  cm, 125 mm × 125 mm) were purchased from Shuqimeng Photovoltaic

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Fig. 1. Cross-sectional SEM micrographs of Si wafers etched by a size-controlled Ag catalyzed process for different times. (a) Before etching; (b) 2 min; (c) 3 min; (d) 5 min.

Technology Co., Ltd., Hangzhou, China. The HF (49%, AR), potassium hydroxide (KOH, AR), silver nitrate (AgNO3 , AR), formaldehyde solution (CH2 O, AR), ammonium hydroxide (NH3 ·H2 O, 28–30% NH3 , AR), isopropyl alcohol (IPA, (CH3 )2 CHOH, AR) and the polyvinylpyrrolidone (PVP, K-30) were purchased from Sinopharm Chemical Reagent Co., Ltd.

process, after edge isolation, we fabricated the nanotextured cells with/without SiNx coating separately while the reference cells were treated only through a standardized alkali texturing. The rear (Al/Ag) and front metal (Ag) for electrical contacts were printed by a conventional screen-printing method. 2.3. Characterization

2.2. The preparation of silver nanoparticles The size-controlled colloidal Ag nanoparticles were synthesized by the method reported in the previous work from our group [18]. 37% CH2 O was added into AgNO3 water solution using polyvinylpyrrolidone (PVP) as a surfactant, after that concentrated ammonia (28%) was injected to initiate the reaction. The reaction mixture was stirred for 30 min before centrifuging. Fabrication of nanoporous silicon layers and nanoporous silicon solar cells. After removing the oxide layers on the surfaces by a 1% HF solution, pyramid structures were formed in a mixture of 3 wt% KOH and 7 vol% IPA solution at 80 ◦ C for 60 min. After that the wafers were washed in an ultrasonic bath with deionized water for 5 min. Then, the emitters (n-type layers) were thermally generated using POCl3 as the phosphorous-doped source. After removing the remaining phosphorous silicate glass, the wafers were immersed into the deionized water (DIW) containing Ag NPs while the rear surfaces were protected by the insulating tape from Ag NPs deposition. The solvent was evaporated in the atmosphere to make the Ag NPs firmly fixed on the Si wafer surfaces. Then, the wafers were etched in the HF:H2 O2 :DIW = 1:5:10 (vol.) solution in the dark at room temperature at different time durations (2–5 min) for Ag-catalyzed texturing. The residual Ag NPs on the nanotextured surfaces were removed by HNO3 and the etched wafers were finally rinsed with DIW. Followed by a normal solar cell fabrication

The morphology of Ag nanoparticles (Ag NPs) was characterized by a transmission electron microscope (TEM, Philips CM200). The morphology and structures of the etched samples were characterized by a field emission scanning electron microscope (FESEM Hitachi U-70). Hemispherical reflectance spectra were measured on a spectrometer (Hitachi U-4100 Spectrophotometer) with an integrating sphere. Electrical characterization of the solar cells was carried out using an I–V and spectral response system (SEIKO NPC Co, Model: NCT-180AA-T14) under AM 1.5G spectral irradiance (100 mW/cm2 at 25 ◦ C). 3. Results and discussion 3.1. Structure characterization and the reflectance characteristics Fig. 1S(a) shows the TEM micrograph of the Ag NPs prepared in our experiment. Obviously, the Ag NPs are sphere-like. The size uniformly distributes in 60–70 nm range resulting from PVP regulating the Ag NPs growth process. Fig. 1S(b) shows the distribution of Ag NPs deposited on the pyramid structure. We can observe that the Ag NPs distribute uniformly on the silicon wafer surface. Fig. 1(a) shows the cross-sectional microstructure of Si wafer before Ag-catalyzed texturing. The obtained average sheet resistance is about 20 /sq, which is due to a heavily doped diffusion

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the average R() of the nanotextured Si wafers decreases gradually with the increasing etching time. After 2 min etching, the reflectivity dramatically decreases especially in the short wavelengths. The average reflectance (Rave ) below 5% is achieved in the Si wafer etched for 5 min. In this case, the depth of prorus structures is about 100 nm. The morphology of the etched Si surface as shown in Fig. 1 has a direct relation with the low surface reflectance. By forming the nanoporous structure on the pyramid surface of solar cells, a refractive index gradient is introduced between air and silicon wafer, which precludes the Fresnel reflection at a sharp surface interface with large refractive index difference. According to the numerical simulation by Hitoch et al. [16], when the aspect ratio of the nanotexture is high, the great suppression of reflectance over a wide spectral bandwidth happens. What’s more, the formation of hierarchical structures composed of nano- and micro-scale roughness features could reduce the reflectance more effectively [19,20]. For these reasons, the Rave of the Si wafer etched for 5 min is below 5% while the etched depth is only about 100 nm. Fig. 2. The thickness of nanoporous layer in different etching times.

3.2. Performance of the nanotextured silicon solar cells

Fig. 3. Reflectance spectra of the nanotextured Si wafers with different etching times.

condition applied on the sample. Fig. 1(b)–(d) displays the crosssectional microstructures of Si solar cells after nanotexture for different etching times. It can be seen that, with the increase of etching time, the depth of nanoporous structures increases. And this trend is showed clearly in Fig. 2. To insure the performance of nanoporous silicon solar cells, the nanoporous layers should not penetrate cross the p–n junction. Fig. 3 displays the reflectance (R()) spectra for the pyramidnanoporous samples etched for different time. It clearly shows that

Fig. 4(a) shows the current–voltage characteristics of the experimental solar cells with the nanotextured surface and the reference cell without nanotexturing under 1 sun illumination (AM 1.5). Both of the cells are not coated with SiNx . The corresponding electrical parameters include the short circuit current (Isc ), open circuit voltage (Voc ), fill factor (FF), efficiency (Eff ), series (Rs ) and shunt (Rsh ) resistances and emitter dark saturation current density (Joe ) are given in Table 1. It shows that the highest efficiency of 15.7% has been obtained for the nanotextured solar cells without any passivation coating. The result is better than those previous results obtained from nanopores made on p-type substrate or completed solar cells [13–16]. The efficiency of reference solar cell without AR layer deposition is only 13.72%. The significant enhancement of photocurrent could be explained by the AR property of the nanoporous surface and the short wavelength (blue light) response for the dead layer partly removed during the etching process and the relatively low Rs indicates that good quality electrical contacts could be realized on nanotextured surfaces. However, the larger surface area due to nanotexture will increase the surface recombination, which causes the leakage current increasing and then a decrease of Rsh . In order to passivate the nanoporous layers, we apply SiNx coatings on the emitter surfaces by PECVD and the thickness of the SiNx layers is about 80 nm. As shown in Fig. 4(b) and Table 1, most of the electrical parameters of the nanotextured solar cell with SiNx coating, compared to the one without SiNx coating, are improved except the Rs , Rsh and FF. The

Fig. 4. I–V characteristics of the nanotextured solar cells (a) without SiNx , and (b) with SiNx under 1 sun illumination.

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Table 1 Photovoltaic parameters of the experimental cell with nanotextured surface (without/with SiNx coating): reference cell 1 and experimental cell 1 refer to the samples without SiNx coating. Reference cell 2 and experimental cell 2 refer to the samples with SiNx coating. Category

Isc (A)

Voc (V)

Rs (m)

Rsh ()

FF

Eff (%)

Joe (A)

Reference cell 1 Experimental cell 1 Reference cell 2 Experimental cell 2

4.514 5.101 4.984 5.439

0.6054 0.6117 0.6164 0.6187

6.790 8.200 7.430 10.07

54.65 77.05 166.6 10.64

0.7773 0.7813 0.7927 0.7277

13.72 15.70 15.73 15.82

2.335 0.5365 0.06439 0.07881

reason may be that the residual Ag NPs diffused into the body of the silicon during the relatively long-time high-temperature PECVD process. But the performance of experimental solar cells remains superior to the reference cell. This is mainly owing to the significantly increasing of short-circuit current (0.455 A absolute, the effective area of the cell is 154.83 cm2 ). 4. Conclusion In summary, for the first time, we have fabricated a 15.7%efficiency solar cell with nanoporous structures on a full size, solar grade CZ Si wafer on basis of a standard solar cell process except for SiNx coating. The nanoporous structures were formed by the assistant of size-controlled Ag NPs as catalyst. After 5 min etching, the Rave below 5% in the visible spectrum could be achieved. The new process clearly shows the conversion efficiency of Si solar cells increases from 13.72% to 15.7% due to the antireflection effect of the nanoporous layer even without additional AR layer deposition (SiNx ). By applying SiNx passivation coating on the surface the efficiency of the nanotextured Si solar cells increased by 1.98% absolutely. This process is prospective to be used in photovoltaic industry. Acknowledgments This work is supported by the 863 Project (No. 2011AA050517), the Fundamental Research Funds for the Central Universities and Innovation Team Project of Zhejiang Province (No. 2009R50005) for the financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2012.10.079. References [1] P. Campbell, M.A. Green, Light trapping properties of pyramidally textured surfaces, J. Appl. Phys. 62 (1987) 243–249. [2] P.K. Singh, R. Kumar, M. Lal, S.N. Singh, B.K. Das, Effectiveness of anisotropic etching of silicon in aqueous alkaline solutions, Sol. Energy Mater. Sol. Cells 70 (2001) 103–113.

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