Electronic quality improvement of crystalline silicon by stain etching-based PS nanostructures for solar cells application

Available online at www.sciencedirect.com

ScienceDirect Solar Energy 129 (2016) 38–44 www.elsevier.com/locate/solener

Electronic quality improvement of crystalline silicon by stain etching-based PS nanostructures for solar cells application Lotfi Khezami a, Abdelbasset Bessadok Jemai b, Raed Alhathlool c, Mohamed Ben Rabha d,e,⇑ a

Department of Chemistry, College of Sciences, Al Imam Mohammad Ibn Saud Islamic University (IMSIU), P.O. Box 5701, Riyadh 11432, Saudi Arabia b Department of Chemical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia c Department of Physics, College of Sciences, Al Imam Mohammad Ibn Saud Islamic University (IMSIU), P.O. Box 5701, Riyadh 11432, Saudi Arabia d Riyadh College of Technology, Technical and Vocational Training Corporation, P.O. Box 42826, Riyadh 11551, Saudi Arabia e LPV, Research and Technology Center of Energy, BP 95, 2050 Hammam-Lif, Tunisia Received 23 June 2015; received in revised form 20 December 2015; accepted 5 January 2016 Available online 12 February 2016 Communicated by: Associate Editor Arnulf Jaeger-Waldau

Abstract In the present work, the impact of stain etching-based Porous Silicon (PS) nanostructures on both the effective lifetime and photoluminescence were examined. Results showed that stain etching-based PS nanostructures increase the effective lifetime from 3 ls (un-etched samples) to 48 ls (following 8 min etching time) at a minority carrier density (Dn) of 1015 cm
3 and drastically decrease the silicon surface reflectivity from about 28% to approximately 7%. These results allowed to correlate between the rise of the effective lifetime values and the photoluminescence intensity; which in turn depends on the effects of hydrogen and oxygen passivation. Consequently, the electronic quality of the multi-crystalline silicon improved significantly leading to high photovoltaic quantum efficiency response. This low-cost stain etching-based PS nanostructures process could potentially be applied in the photovoltaic cell technology and applications. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Stain etching duration; Porous silicon; Silicon solar cells; Effective lifetime

1. Introduction The electronic quality of multi-crystalline silicon (mc-Si) has improved significantly in recent years due to the reduction of impurity contamination by gettering and bulk passivation techniques (Khedher et al., 2005; Perichaud, 2002); These essentially have some influence on the electrical ⇑ Corresponding author.

E-mail addresses: [email protected] (A. Bessadok Jemai), [email protected] (M. Ben Rabha). http://dx.doi.org/10.1016/j.solener.2016.01.034 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

parameters of the material, resulting in an improved minority carrier diffusion length. Therefore, the processing step like gettering and surface passivation are the most suitable, as they either remove or block the impurities and defects away from the device-active regions leading to extended minority carrier diffusion length. Such techniques have to be incorporated in the device-processing step to improve the electronic quality of the mc-Si substrate (Kruger et al., 2000). Efficiencies of mc-Si solar cells with aluminum back surface field or silicon nitride have been reported (Mittelsta et al., 2002; Ben Rabha et al., 2012;

L. Khezami et al. / Solar Energy 129 (2016) 38–44

Gindner et al., 2014). The possibility of improving the electronic quality of silicon wafers by extracting impurities from them using phosphorus diffusion is well known (Goetzberger and Shockley, 1960; Kang and Schroeder, 1989; Cuevas et al., 1997). It has been shown that a porous etched silicon surface could be associated to excellent antireflection properties (Tsuo et al., 1994). Porous silicon may also serve as a wide bandwidth absorber in a multiplejunction cell structure, with crystalline silicon as the substrate or as gettering centers to reduce the impurity levels in a silicon substrate (Menna and Tsuo, 1997). However, silicon surface treatment with acid (HF–HNO3–H2O) solution (formation of Porous Silicon (PS)) can lower the reflectivity of the surface of c-Si. PS layers also allow diffusion of hydrogen into the device to passivate defects and impurities for improved device performance. In crystalline silicon-based solar cells, the effective lifetime is one of the most important electrical parameters used to quantify the electronic quality of the crystalline silicon. In this work, we report on the effect of stain etchingbased PS nanostructures on the minority carrier lifetime of mc-Si wafers. The surface composition and morphology of the PS layer were investigated and correlated to the carrier lifetime. The influence of PS surface treatment on surface reflectance, photo-luminescence and effective minority carrier lifetime were compared to untreated mc-Si substrate. 2. Experimental details The present research work was geared toward the improvement of: (i) the mc-Si electronic quality by reducing the front surface reflection, and (ii) the minority carrier lifetime. The etching experiments were carried out on p-type mc-Si wafers with resistivity 0.5–2 X cm and thickness 330 lm. Before stain etching-based PS, the wafers were rinsed in 5% hydrofluoric acid (HF) during 30 s to remove

39

native oxide and rinsed in deionized water to produce a uniform PS layer. Then, the wafers were etched in HF (48 wt%)/ HNO3 (65 wt%)/H2O solutions at different times at temperature 30 °C and dried under N2 flux, to avoid the PS film degradation. After the surface treatment process, the morphology of the PS layers was analyzed using Atomic Force Microscopy (AFM). To characterize the surface reflectance, an integrating sphere/spectrophotometer (PerkinElmer Lambda 950) was used with light in the wavelength range from 350 to 1150 nm. Fourier Transform Infrared spectrometer (FTIR) model Nicolet MAGNA 560 was used to analyze the composition of the produced PS. Photoluminescence (PL) measurements were performed at room temperature using the 476.5 nm line of an argon (Ar+) laser as the excitation source. Finally, the effective minority carrier lifetimes was measured by photo-conductance using the WCT-120 Silicon Wafer Lifetime Tester. 3. Result and discussion Fig. 1 exposes the AFM images of the surface morphology of the silicon wafers etched in HF (48 wt%)/HNO3 (65 wt%)/H2O solutions at 30 °C for 4, 8, 12 and 16 min, respectively. The moment that the sample is immersed in the solution the etching begins and the porous layer is propagated on the whole surface of the sample; subsequently, one can observe that porous silicon layer disappears and a new etching process will start again. For instance, Fig. 1(a) depicts the PS formed during the initial stages (for 4 min). Fig. 1(b) shows that the PS layer structure is grown in pore size after an etching time of 8 min. However, in Fig. 1(c) it is observed that after 12 min the porous silicon layer vanishes and a new PS layer is reformed (Fig. 1(d)). The dependence of the PS layers at different etching times and the reflectance of the silicon surfaces on the

Fig. 1. AFM images of PS layers obtained for samples S1 (a), S2 (b), S3 (c) and S4 (d) as a function of Stain etching time.

L. Khezami et al. / Solar Energy 129 (2016) 38–44

Etching time Etching time Etching time Etching time

40

Total reflectance (%)

35

0.020

04 min 08 min 12 min 16 min

0.016

30

Bare mc-Si

25

(c)

0.018

PL intencity (a. u.)

40

20 15

0.014

(e)

0.012

(d)

0.010 0.008

(b)

0.006

(a)

0.004

10

0.002

5 400

500

600

700

800

900

1000

1100

wavelength (nm)

0.000 500

550

600

650

700

750

800

Wavelength (nm)

Fig. 2. Total reflectivity as a function of wavelength measured on mc-Si surface before and after different etching times.

optical wavelength, are shown in Fig. 2. It can be seen that the reflectivity drastically decreases as the etching time increases up to 8 min; in this case a reflectivity of about 8% is achieved at a wavelength of 700 nm. One may notice as the etching time increases beyond 8 min, that the PS layers merge into one another leading to a destruction of PS nanostructures. Indeed, the reflectivity increases again to a value of about 10%. After the treatment the silicon samples shows a high porous texturized surface suitable for light trapping and diffusion-based multi-reflection inside the PS surface caused an increase of the reflectivity after PS treatment. Results show that after the etching of mcSi surface, the total reflectivity drops from 28% to about 7% which is typical for an antireflection coating and comparable to that obtained with SiNx-based antireflection coating (Rabha et al., 2010) and better than that obtained by mechanical textured (Ben Rabha et al., 2011). The effect of etching times on the light emission properties of PS was also examined as shown in Fig. 3, which compares the PL spectra of the normal and PS layers for different etching times. The PL spectra reflects the size distribution of silicon nano-crystallites embedded within the PS structure (Canham, 1990; Cullis and Canham, 1991); where smaller crystallites emitting at shorter wavelengths is consistent with quantum confinement effects. On the other hand, the variation of the PL intensity with respect to the PS layers for different etching times is believed to result from the passivation of non-radiative Si dangling bond defects on the surfaces of mc-Si. To confirm that the increased PL intensity depends both on hydrogen and oxygen passivation, we performed Fourier transform infrared (FTIR) spectroscopy on the samples (before and after different etching time) (Fig. 4). From the FTIR data, it is clear that there are 2 major bonds: (i) one around 1025–1160 cm
1 and the second around 2050–2260 cm
1. The latter bonds are attributed to Si–H stretching modes of the form SiHx, where x = 1, 2 and 3 respectively. The band between 1025 and 1160 cm
1 is related to oxygen complexes, mostly SiO

Fig. 3. PL spectra of the porous silicon samples as a function of Stain etching time: (a) 0 min (control), (b) 4 min, (c) 8 min, (d) 12 min and (e) 16 min.

stretching in O–SiO and C–SiO (Bisi et al., 2000). Banerjee et al. (1994) noted from the oxide removal experiments and passive etching experiments that a better hydrogen passivation may not be entirely responsible for the enhancement of PL intensity. They succeeded at enhancing the PL intensity following chemical oxidation. Maruyama and Ohtani (1994) observed the formation of Si–O–Si bonds on the porous layers in the presence of humidity, thus enhancing the PL intensities. Considering the above discussions, it appears from the PL (Fig. 3) and FTIR (Fig. 4) spectra that samples having higher concentrations (see Table 1) in both bands attributed to hydrogen and oxygen related complexes, have more intense PL. This demonstrated that the nature of PL depends both on hydrogen and oxygen passivation, which plays a key role in reducing the non-radiative Si dangling bond defects. As a result, the increase in the PL intensity of the PS layer is believed to be caused by a reduction in the Si dangling bond density. The hydrogen and oxygen concentrations (N, in cm
3), in the IR absorption spectrum, were deduced using the following formula (Zanzucchi, 1984): Z aðxÞ dx N ¼A x where N, A, a and x are the concentration, the proportionality constant (in cm
2), the absorption coefficient, and the wave number (in cm
1), respectively. The integrated intensity of hydrogen and oxygen, in the IR absorption spectrum, were deduced using the equation: Z N aðxÞ dx I¼ ¼ A x As depicted in the AFM pictures (Fig. 1), the surface to volume ratio is shown to increase after stain etching time of 8 min; this allows significant number of hydrogen and oxygen related localized states to be present, as more surface

L. Khezami et al. / Solar Energy 129 (2016) 38–44

41

0 .3

Absorbance (a,u,)

S iH x

(x= 1 -3 )

04 08 12 16

0 .2

0 .1

S iO 2

m in m in m in m in

-------------

08 min 16 min 12 min 04 min

0 .0 1200

1500

1800

2100

2400

2700

3000

Wavenumber (cm -1 )

Fig. 4. FTIR spectra before and after PS treatment.

Table 1 The integrated intensity of oxygen and hydrogen from FTIR spectra as a function of stain etching time. Etching times

4 min

8 min

12 min

16 min

Integrated intensity of oxygen in cm
1 Integrated intensity of hydrogen in cm
1

0.003 0.004

0.011 0.007

0.004 0.005

0.009 0.006

area is thereafter available to accommodate more of these species. As stated above, it can be concluded that the PL peak intensity depends on both the hydrogen and oxygen passivation, which in turn depends on the total surface area available for the hydrogen and oxygen complexes to be present. To compare and confirm the effect of the stain etching times on the effective minority carrier lifetime, similar wafers, which were vertically adjacent to each other in the ingot, were used with resistivity 0.5–2 X cm, thickness 330 lm and area 2  2 cm2. Therefore, differences between them, after different stain etching times, can be interpreted as being due to variations in the process parameters rather than to material variations. PS layers were formed by the stain-etching technique using HF/HNO3/H2O solution with a 1:3:5 volume composition at different times and without any heat treatment. It is potentially interesting after this treatment to examine the possibility of using etched silicon wafers for photovoltaic application. For this purpose one needs to quantify the variation of the electronic quality of the material by effective carrier lifetime measurements by photoconductance using the WCT-120 Silicon Wafer Lifetime Tester (Sinton Instruments, Boulder, CO, USA) under the quasi-steady-state lifetime measurement using the generalized analysis condition. The lifetimes reported here should be considered as effective lifetimes, which include both surface and volume recombination components. The effect of stain etching times on the effective lifetimes at a minority carrier density 1015 cm
3 is shown in Table 2 and plotted in Fig. 4. A rise of effective minority carrier lifetimes has been observed after PS formation compared to bare sample 4 ls. The maximum lifetime value was

Table 2 Effect of stain etching times on the effective lifetimes at a minority carrier density (Dn) of 1015 cm
3. Etching times Lifetime (ls)

Bare mc-Si 3

4 min 13

8 min 48

12 min 9

16 min 11

obtained at 8 min. The decrease of lifetime at 12 min is due to the PS layer destruction while the observed increase of lifetime at 16 min is caused by the reformed PS layer (see Fig. 5). The increase of the effective lifetime values can be attributed to the decrease of defects density as a consequence of hydrogen and oxygen passivation effect (Krotkus et al., 1997; Ben Rabha et al., 2009; Ben Rabha and Bessaı¨s, 2010); this plays a key role for the improvement of mc-Si electronic quality, which in turn is very critical for obtaining higher efficiency mc-Si solar cells. In this study, it was possible to correlate the rise of the lifetime values to the PL intensity (Fig. 6). As shown in Fig. 6(b), for 8 min etching time the increase of the lifetime values can be related to the decrease of defects density as a consequence of the passivation effect by the high amount of hydrogen. This is confirmed by FTIR analysis (Fig. 4). We observed for the same etching time (8 min) an increase of the band intensity related to Si–O vibration mode. This band can be attributed to the highly stressed SiO2–Si interface or defective silicon oxide at the porous silicon surface. This could explain the enhancement observed in PL signal as reported elsewhere (Banerjee et al., 1994; Maruyama and Ohtani, 1994). The light-beam-induced current LBIC-based silicon solar cell local characterization was used to qualitatively

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nanostructures by LBIC in the same area. One may notice that without any PS treatment (Fig. 7(a)), the IQE varies between 28% and 36%. However, after the PS treatment (Fig. 7(b)) the minimum IQE value becomes 57% and the maximum value reaches 70% (which represents an increase of about 35% on average). Consequently, the surface recombination decrease in grain and grain boundary (Dimassi et al., 2008; Khezami et al., 2015). Fig. 8 depicts the normalized LBIC profiles measured around the same GBs before and after PS formation on mc-Si. We notice that PS reduces the recombination velocity at the grain and GBs as a consequence of the passivating effect (Ben Rabha and Bessaı¨s, 2010). In the present work the fabrication of multicrystalline silicon solar cell starts with a 3.6 cm2, 300 lm thick, P-type and a resistivity in the range of 0.5–2.0 X cm. The creation of an n-type emitter to the p-type wafer (N+/P junction) was achieved using a simple phosphorus diffusion technique, which consists of spreading POCl3/acetone solution (ratio 1:5) on the wafer using the spin-on technique, where the temperature and time diffusion were optimized to 925 °C and 30 min, respectively (Ben Rabhaa et al., 2009). Both front grid and back side metallic contacts were realized by screen printing a silver paste and an aluminum/ silver paste at a suitable temperature (Ben Rabhaa et al., 2009). The front grid contact must be realized before

50

Lifetime (µs)

40

30

20

10

0 0

4

8

12

16

Etching Time (min)

Fig. 5. Influence of stain etching time on the effective carrier lifetime.

and quantitatively evaluate the mc-Si solar cells before and after stain etching-based PS nanostructures. A He–Ne laser source (633 nm) with a maximum power of 5 mW was used for this purpose. The laser beam was focused at the surface of the sample in order to perform the local twodimensional quantum efficiency QE(x, y) at the wavelength of the He–Ne laser (633 nm) (Dimassi et al., 2011a,b). Fig. 7 shows the quantum efficiency (QE) distribution of mc-Si solar cells before and after stain etching-based PS

60

0.020

(a)

55 50

0.018 0.016

45 0.014

Lifetime (µs)

35

0.012

30

0.010

25

0.008

20

0.006

15

PL intencity (a. u.)

40

0.004

10

0.002

5

0.000

0 0

4

8

12

16

-1

(b)

0.010 0.008 0.006

0.010

0.004 0.005 0.002 0.000

0.000 0

4

8

12

Etching time (min)

16

0.020

0.008

(c)

0.007

0.015

0.006 0.005

0.010

0.004 0.003

0.005

0.002

PL intencity (a. u.)

0.015

Oxygen integrated intencity (cm )

-1

0.020

PL intencity (a. u.)

Oxygen integrated intencity (cm )

Etching Time (min)

0.001 0.000

0.000 0

4

8

12

16

Etching time (min)

Fig. 6. Correlation between PL intensity for different stain etching time versus (a) effective lifetime, (b) Oxygen Integrated intensity and (c) hydrogen Integrated intensity.

L. Khezami et al. / Solar Energy 129 (2016) 38–44

43

Fig. 7. LBIC maps: (a) before and (b) after PS treatment.

measured proving the feasibility of the front PS approach for incorporation in high-efficiency multicrystalline silicon solar cells via passivating the impurities and defects in surface and bulk silicon by the hydrogen-rich PS layer. It is interesting to note that the efficiency in the present work is similar to the ones of the commercial cell (Strehlke et al., 1997) and comparable to the result obtained by Vitanov et al. (2000) and Bilyalov et al. (2000).

1.00

After Etching time 8 min 0.95

GB

0.90

0.85

4. Conclusions

Bare mc-Si

0.80 110

112

114

116

118

120

122

124

126

128

130

Laser spot position (µm)

Fig. 8. Normalized LBIC profiles around the same selected GB before and after stain etching.

0.035 0.030

Current (mA/Cm2 )

0.025

After PS

Before PS

0.020 0.015

In this study it has been demonstrated that electronic quality of multicrystalline silicon improved by Stain etching-based PS nanostructures. PS leads to enhanced surface lifetime and PL, and reduces the reflectivity. Based on the integrated intensity calculations, it has been confirmed that PL peak intensity depends on both the effects of hydrogen and oxygen passivation, which in turn depends on the total surface area available for the hydrogen and oxygen complexes to reside. The results suggest that the stain etching-based PS reduces the average reflectance for treated mc-Si wafer to about 6%. The effective minority carrier lifetime dramatically increases from 3 ls to 48 ls at a minority carrier density (Dn) of 1015 cm
3. As a consequence, Stain etching-based PS nanostructures were found to be very effective in improving the performances of mc-Si cell.

0.010

Acknowledgments 0.005 0.000 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V)

Fig. 9. J–V characterization of solar cells before and after front PS at AM 1.5 illuminations.

forming PS to limit the high series resistance. Fig. 9 illustrates the J–V characterizations on solar cells with front PS as well as cells without front PS at AM1.5 illuminations. As shown in Fig. 9, the open circuit voltage (Voc) and the short circuit current (Jsc) are both improved in the solar cells with front PS and an efficiency of 13.5% was

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