Enhanced absorption and short circuit current density of selective emitter solar cell using double textured structure

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ScienceDirect Solar Energy 116 (2015) 265–271 www.elsevier.com/locate/solener

Enhanced absorption and short circuit current density of selective emitter solar cell using double textured structure Changheon Kim a,b,1, Jonghwan Lee a,1, Sangwoo Lim b,2, Chaehwan Jeong a,⇑ a

Solar Cell R&D Center, Applied Optics & Energy R&D Group, Korea Institute of Industrial Technology, Gwangju 500-480, South Korea Department of Chemical and Bio-Molecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea

b

Received 14 April 2014; received in revised form 31 March 2015; accepted 6 April 2015 Available online 29 April 2015 Communicated by: Associate Editor Elias K. Stefanakos

Abstract A double textured selective emitter (DTSE) solar cell was fabricated using Si wafer. The 40  40 mm2 silicon substrates were textured to form a pyramid-shaped surface, and nanowires were fabricated by a metal-assisted chemical etching process using Ag nanoparticles. All surface modifications of the micro and nanostructures were done by a wet-based process. The heavily doped and shallow emitters for selective emitter solar cells were prepared through the POCl3 diffusion and a chemical etch-back process, respectively. The front and rear electrodes were prepared with a conventional screen printing method. The optical properties were enhanced through the double textured (DT) structure, and additional enhancement of the electrical properties was realized through the selective emitter concept. The DTSE solar cell achieved a higher conversion efficiency of 17.9% with improved absorption and short circuit current density compared to a DT solar cell. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Silicon solar cells; Silicon nanowires; Selective emitter solar cells; Metal assisted chemical etching

1. Introduction Silicon (Si) is one of the most important materials in the photovoltaic (PV) industry because of its natural abundance and low toxicity. Thus, an opportunity for a lowcost manufacturing process is available (Conibeer et al., 2006; Green, 2003). Recently, the developments of many energy sources are in progress, and more innovations and technological advances are needed in the PV research area. Hence, maximizing the conversion efficiency of solar cells is

⇑ Corresponding author. Tel.: +82 626006380; fax: +82 626006179. 1 2

E-mail address: [email protected] (C. Jeong). Tel.: +82 626006380; fax: +82 626006179. Tel.: +82 221235754.

http://dx.doi.org/10.1016/j.solener.2015.04.010 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

a key parameter while avoiding increased producing cost (Foldyna et al., 2013). In order to enhance the conversion efficiency of the solar cell, there are two typical ways: optical and electrical enhancements. To improve the optical characteristics of solar cells, the modification of the surface morphology is one of the simple ways for more absorption of the incident light. Many approaches have been developed to enhance the optical aspects, such as texturing (Park et al., 2009) by using both wet and dry processes (Moreno et al., 2014), periodic gratings (Deceglie et al., 2012), nanostructures (Du et al., 2011; Han and Chen, 2010; Li et al., 2011) and luminescent downshifting (Griffini et al., 2015). In particular, Si nanowire arrays are good candidates for harvesting sunlight because of the light scattering effect. It has been reported that

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nanowire arrays realize low reflectance, and strong broadband optical absorption has been measured from the structure (Peng et al., 2005; Hu and Chen, 2007; Muskens et al., 2008). Based on these advantages, Si nanowires have been fabricated randomly or periodically through various methods such as the vapor–liquid–solid (VLS) method (Gunawan and Guha, 2009; Stelzner et al., 2008; Maiolo III et al., 2007), reactive ion etching (RIE) (Spurgeon et al., 2008), electrochemical etching, and metal-assisted chemical etching (Jaballah et al., 2012; Zhu et al., 2008; Sinakov et al., 2009; Garnett and Yang, 2008). Among these methods, metal-assisted chemical etching has attracted great attention because of its simple and low-cost process. Metal-assisted chemical etching methods enable Si nanowire arrays to be fabricated in a wet hood without expensive vacuum equipment. Another advantage is that there is no obvious size limitation (Huang et al., 2011). One of the electrical modified models for the enhancement of conversion efficiency is the selective emitter solar cell (Lee et al., 2012). A highly doped emitter surface lowers the contact resistance with the front metal contact, while the lowly doped region, which is a shallow emitter, produces higher open circuit voltage (VOC) and lowers losses of the converted charge carriers due to the reduced recombination in the emitter. Thus, combining these two concepts of metal-assisted chemical etching for optical aspects and a selective emitter for electrical aspects has a chance to improve the cell performance. In this paper, we fabricated Si nanowires on a pyramidal textured surface for double textured (DT) solar cells and double textured selective emitter (DTSE) solar cells. The specific flow for the fabrication of the solar cell is depicted in Fig. 1. Their optical and electrical properties, including the cell performance, were examined following the manufacturing process.

2. Experimental 2.1. Preparation for nanowires structure The surface modification of pyramidal structures and Si nanowires was prepared through an all-wet-based process by texturing and electro-less etching using Ag nanoparticles. Chokralsky-grown 200-lm-thick p-type (100) solargrade Si wafers (q = 0.5–3.0 X cm) were cut to the size of 40  40 mm2 for substrates. The nanowires structure was prepared by a texturing and electro-less etching process after the conventional texturing process. First, substrates were dipped into the NH4OH and H2O2 solution with a mixing ratio of 4:1 at 80 °C for 10 min to remove impurities. After the removal of native oxide layer on wafers by dipping in 1:10 diluted HF solution, substrates were dipped into a mixed solution consisting of 2 vol.% NaOH and 5 vol.% isopropyl alcohol with deionized water for 30 min. Then, textured wafers were immersed into a solution of 10 mM AgNO3 and 4.8 M HF to precipitate Ag particles on the surface and etched for 45, 60, 75, and 90 s by dipping the wafers in an etching solution consisting of 4.8 M HF and 0.5 M H2O2. Ag nanoparticles were removed after the etching process by dipping the samples in HNO3 for 30 s. 2.2. Selective emitter solar cell process A solar cell was fabricated through emitter doping by a phosphorous oxychloride (POCl3) source using a diffusion furnace and conventional screen printing method. Samples were put into the diffusion furnace to form an n++ emitter layer using the POCl3 source. Thermal diffusion was applied in ambient N2, O2, and POCl3 gas at 860 °C for 30 min. After the doping process, the phosphorus silica glass was removed by dipping in separate 1:10 diluted

Fig. 1. The schematic diagram of process sequence for DTSE solar cell. Note that the schematic is not drawn to scale.

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Fig. 2. SEM images of Si nanowires on textured pyramid structure following the etching time of (a) 45, (b) 60, (c) 75 and (d) 90 s.

HF and HCl solutions for 60 s at room temperature. An acid barrier was pasted following grid lines to protect heavily doped surfaces that are directly in contact with the front metal grid. Then, samples were immersed into a 1:200 v/v mixed solution of HF and HNO3 at 20 °C to etch-back to form the shallow emitter. Next, in order to protect the surface of the cell and enlarge the absorption of incident light, an anti-reflection coating (ARC) layer was deposited on the emitter layer using plasma enhanced chemical vapor deposition. An 80-nm-thick SiNX:H thin film with refractive index of 2.1, which was optimized according to a previous report (Schmidt and Kerr, 2001), was deposited on the emitter layer. Finally, front and rear electrodes were printed using a conventional screen printing method with Ag and Al paste, respectively, and then a co-firing process was applied. 2.3. Characterization The DT structure on the Si wafer was observed using a field emission scanning electron microscope (FE-SEM, Quanta 200 FEG, FEI). Secondary ion mass spectrometry (SIMS, IMS 7f magnetic sector, CAMECA) measurement was carried out to analyze the depth profile of the POCl3 diffused junction. The four-point-probe method was applied to examine the impact of each doping and etchback step. The current–voltage (I–V) parameter of the solar cell was measured under an AM 1.5 G solar spectrum at 25 °C using a solar simulator (WXS-155S-L2, Wacom),

and the internal quantum efficiency (IQE) was examined using a spectral response measurement system (QEX7, PV measurement). 3. Results and discussions The Ag nanoparticles precipitated from the mixed AgNO3 solution provide catalytic activity. When the Si substrates with Ag nanoparticles on the surface are dipped in the etchant, the oxidant diffuses into the Ag nanoparticles and the interface between Ag/Si is oxidized, which generates a wire structure. Following these procedures, Si nanowires were fabricated on the pyramid textured surface, and their SEM images of DT structures are depicted in Fig. 2. Four-different structures were fabricated by dipping the samples in etchant for 45, 60, 75 and 90 s. When the textured pyramids were etched for 45 s, Si nanowires were produced following electro-less etching and showed a height of 240 nm. However, as the electro-less etching time increased, Si nanowires tended to collapse with heights of 110, 95 and 80 nm for 60, 75 and 95 s, respectively. This trend is verified by the reflectance and comparison of average reflectance of the etched surface, as shown in Figs. 3(a) and (b). The textured sample and the sample etched for 90 s showed similar reflectance in the range of 750–1100 nm. However, the difference was noticeable when the wavelength was shortened from 300 to 750 nm. The sample etched for 45 s, which had the longest nanowires, showed

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Fig. 3. (a) The results of UV–Vis for reflectance of textured pyramid and Si nanowires on textured pyramid structure following the etching time of 45, 60, 75 and 90 s and (b) average percentages of reflectance between 300 and 1100 nm.

the lowest average reflectance of 6.18% in the range of 300– 1100 nm because of the light scattering caused by the wire array. The reflectance from the etched surface was increased with the etching time because of collapsed nanowires. Note that nanowires can improve the absorption of incident light, but a patterning process is always accompanied by surface defects, and the enlarged surface area may lead to surface recombination of the photo generated carriers (Kim et al., 2011). However, for the optical benefit of the nanowire structure, the sample etched for 45 s was selected for the fabrication of a solar cell in this work, because the low reflectance can improve the photo-current generation, which can lead to increased short circuit current density (Jsc). After the diffusion of POCl3, the pyramidal surface including nanowires were fully doped and the result of secondary ion mass spectrometry (SIMS) measurement is shown in Fig. 4(a). The doping concentration of the heavily doped emitter surface was measured to be 5.13  1020 cm 3, and the concentration of phosphorus which is from the Si wafer was measured to be 7.86  1015 cm 3. The emitter layer was confirmed to have a depth of 700 nm, which is suitable for applying the screen-printed conventional cell fabrication process (Jeon et al., 2011). A relatively uniform emitter was formed after the doping process, and the average sheet resistance (RSheet) at

Fig. 4. (a) The depth profile from secondary ion mass spectrometry (SIMS) analysis after POCl3 diffusion process and (b) the enlarged graph for confirming the concentration of the lowly doped emitter.

Fig. 5. The relationship between sheet resistance (RSheet) after dipping in etchant and etching time.

the surface was measured to be 34.75 X/sq. When the emitter is heavily doped, VOC can be increased due to the difference of the electrical potential, and the breakage of the emitter during the firing process at excessively high temperature could be prevented because of the heavily doped region. However, excessive dopants from the heavy doping process increase the recombination rate of carriers, thus leading to a loss of short circuit current (ISC). To minimize these circumstances, the heavily doped surface of the emitter was selectively etched back. The etchback process was controlled by the dipping time, and the

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Fig. 6. Sheet resistance (RSheet) of (a) heavily doped and (b) lightly doped emitter. 9-points were measured using a four point probe and vertical bar at the right side indicates RSheet in X/sq.

relationship between the etching time and RSheet of the emitter is shown in Fig. 5. Since the emitter RSheet of 120 X/sq. is regarded as a suitable condition to achieve highly efficient solar cells (Lee et al., 2012), the sample was dipped in etchant for 45 s, and an average RSheet of 134.82 X/sq. was realized in this study. Based on the etch rate of the etchant, the etch-back depth was calculated to be 22.5 nm, and the doping concentration of the surface of lightly doped emitter was confirmed as 4.76  1020 cm 3 which is shown in Fig. 4(b). Fig. 6 shows the distribution of RSheet at the surface before and after the etch-back process. Table 1 also presents the corresponding results of Fig. 6. The standard deviation of 1.28, which is the RSheet before the etch-back process, verifies the uniform diffusion, but after dipping in etchant, the value increased to 10.97. This may be due to the non-uniform etching from the wet process. The photo current–voltage (I–V) characteristics of DT and DTSE solar cells were measured for comparison. The I–V curve and the corresponding results are depicted in

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Fig. 7. (a) Comparison of current–voltage (I–V) curves with real feature of the DTSE solar cell and (b) internal quantum efficiency (IQE) of DT and DTSE solar cells. The effective area of incident light in both DT and DTSE solar cells was 10.8 cm2.

Fig. 7(a) and Table 2. The conversion efficiencies of DT and DTSE solar cells were obtained as 17.2% and 17.9%, respectively. The effective area of incident light in both DT and DTSE solar cells was 10.8 cm2. The emitter surface of the DTSE solar cell has relatively fewer defects than that of the DT solar cell, which is a heavily doped region, and this improved the collection of carriers in the DTSE solar cell. In addition, the relatively higher conversion efficiency of the DTSE solar cell can be explained by comparing values of Jsc and VOC. The increased Jsc could be caused by the etched-back (i.e., high sheet resistance) surface, which improved the quantum efficiency in the short wavelength. These results can be compared with a previous report (Lee et al., 2013). The same trends of micro and nanostructures by texturing and electro-less etching process were presented. However, enhanced electrical and optical properties produced better conversion efficiencies in this study.

Table 1 Calculated sheet resistances corresponding to results of Fig. 5. RSheet (X/sq.)

Average

Maximum

Minimum

Standard deviation

Heavily doped region Etched back region

34.75 134.82

37.69 148.85

32.39 120.33

1.28 10.97

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Name

Jsc (mA cm 2)

VOC (mV)

Eff. (%)

FF (%)

Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090093823).

DT DTSE

36.7 37.3

613 628

17.2 17.9

76.6 76.7

References

Table 2 Comparative results of fabricated DT and DTSE solar cells.

Enhanced properties were also verified by the measurement of internal quantum efficiency (IQE), as shown in Fig. 7(b). The quantum efficiency of the DTSE solar cell showed a bigger percentage than that of the DT solar cell for the whole range. In particular, at the short wavelength, the surface recombination at the emitter surface of the DT solar cell caused the reduction of quantum efficiency. On the other hand, the etched back emitter surface of the DTSE solar cell enhanced the quantum efficiency at the short wavelength region, which is called a blue response because of the decreased defects, and this contributed to a significant gain in Jsc. 4. Conclusions In this study, selective emitter solar cells using a DT structure have been fabricated. For the optical aspects, enhanced light absorption has been realized by nanowires on the textured pyramid structures, which was verified by reflectance analysis. The electro-less etching process using Ag metal particles was applied to prepare the double textured Si nanowires. The electrical enhancement has been realized through the introduction of a selective emitter. After the etch-back process to fabricate the shallow emitter, the improved conversion efficiency was 17.9% because of the enhanced Jsc, which is the dominating reason for this improvement. In addition, three different widths of finger electrodes were applied for DTSE solar cells, and the cell with 50-lm-sized fingers showed improved Jsc because of its enlarged area for light absorption. Finally, the Si nanowire structure, which has great possibility for advanced solar cells, can be applied to many manufacturing processes, and the selective emitter concept was an effective method for improving efficiency. Acknowledgements This work was supported by the New & Renewable Energy Technology Development Program as well as the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry & Energy (20143020010860, 20132020102110). Also, this research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2008788) as well as by the Priority Research Centers Program through the National

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