Chemical vapour etching-based porous silicon and grooving: Application in silicon solar cells processing

phys. stat. sol. (c) 2, No. 9, 3505–3509 (2005) / DOI 10.1002/pssc.200461237

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Chemical vapour etching-based porous silicon and grooving: Application in silicon solar cells processing

M. Ben Rabha, M. F. Boujmil, M. Saadoun, and B. Bessaïs*

Institut National de Recherche Scientifique et Technique, Laboratoire de Photovoltaïque et des Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisia

Received 13 July 2004, revised 19 October 2004, accepted 27 January 2005 Published online 9 June 2005

PACS 68.37.Hk, 78.30.Am, 78.40.Fy, 81.05.Rm

Sponge like porous silicon (PS) was formed by a simple and low cost chemical vapour etching (CVE) method and applied in polycrystalline silicon (mc-Si) solar cells processing. The CVE method consists of exposing Si wafers to HNO3/HF vapours. It was shown that 8 min of HNO3/HF CVE (volume ratio = 1/7) is sufficient to form optimized PS layers on the emitter of mc-Si cells. The CVE–based PS can simultane-ously passivate the Si surface and serves as an effective antireflection coating (ARC). As a result, the re-flectivity decreases by about 60% of its initial value and the internal quantum efficiency is improved, par-ticularly in the short wavelength region. For acid vapours rich in HNO3 (HNO3/HF >1/4), the CVE method favours the formation of a (NH4)2SiF6 powder, which is highly soluble in water. These findings let us achieve anisotropic grooving that enables to groove mc-Si wafers locally and in depth using an ade-quate anti-acid mask. The CVE - based grooving technique was used to form buried metallic contacts on the rear and frontal surface of the Si wafer in order to improve the current collection in mc-Si solar cells. No alteration of the spectral response in the long wavelength range was observed in mc-Si cells with rear-buried contacts. Adjustments of theoretical spectral responses to experimental ones show an increase in the effective electron diffusion length (Ln), which was attributed to Al gettering (passivation) at grain boundaries and to the reduction of the effective thickness of the base of the cells.


1 Introduction

Polycrystalline silicon (mc-Si)–based solar cells processing is always under permanent development due to its lower wafer cost compared to monocrystalline Si. The wafers have extended crystallographic de-fects like grain boundaries, dislocations, and stacking faults, which are often inhomogeneously distrib-uted. These defects results in a lower lifetime of the minority charge carriers in wafers. However, the lifetime can be enhanced by hydrogen passivation of crystal defects and by gettering of metallic impuri-ties during solar cell processing. Recently many attempts have been done to introduce porous silicon (PS) as an efficient antireflective coating (ARC) or as a passivating layer on the emitter of crystalline Si solar cells using electrochemical etching of Si wafers [1–3]. However, the electrochemical etching proc-ess is rather aggressive, and thus may limit the conversion efficiency of the cells. Recent investigations demonstrated that PS may be formed directly using a vapour etching (VE) method, which consists of exposing Si wafers to HNO3/HF acid vapours [4]. This new technique enabled us to achieve thin controllable PS layers and silicon grooving. We highlight the remarkable effect of VE-based PS and grooved buried contacts on the photovoltaic (PV) performances of mc-Si solar cells.

* Corresponding author: e-mail: [email protected] Phone: +216-71-430-160, Fax: +216-71-430-934

3506 M. Ben Rabha et al.: Chemical vapour etching-based porous silicon and grooving


2 Processing sequence for the solar cells

The mc-Si solar cells were realized from p-type solar grade wafers having a thickness of about 330 µm and a surface area of 2.5x2.5 cm2. The average grain size of the wafers is about 7 mm. Phosphorus diffusion technology was used to achieve the N+/P junction. Phosphorus is mixed with acetone and spread by the spinning technique onto the mc-Si wafers. Both front grid and back side metallic contacts were realized by screen printing a silver paste and an aluminum/silver paste, respectively. The front grid contact must be realized before forming PS to avoid the high series resistance. The metallic contacts undergo a sintering in an infrared furnace, in air, without any further precautions (such as ventilation and controlled atmosphere....) at a temperature of 750 °C. Owing to the I-V characteristics, we optimized the ratio POCl3/aceton to 1/5 and the diffusion temperature and exposure time to 925 °C and 30 min, respectively. The edges of the cells were mechanically etched. In fact, deep phosphorus diffusion is done to get a rather thick N+ zone in order to prevent the destruction of the N+/P junction, while forming PS on

the front side of the cells by the CVE technique. This induces in turn a rather significant subsurfacic dead

layer, which may limit the short circuit current of the cells. Thus, this low cost technique allows achieving poor quality cells as expected, having a conversion efficiency of about ~ 7.7 %.

3 Chemical vapour etching

The Chemical vapour etching (CVE) technique [4] was used to form PS layers on the frontal surface of the mc-Si material, just by exposing the wafers to acid vapours issued from an acid mixture of HNO3 (65%) and HF (40%). Conventional PS formation techniques, like anodisation or “Stain-Etching”, present some difficulties compared to the chemical vapour etching method [4]. In fact, forming PS on the frontal surface of mc-Si solar cells using conventional techniques, requires to put the wafer in direct contact with the acid solution; this may cause serious problems, like the destruction of the metallic front grid, thus supplementary and delicate operations like contact masking and etching time minimization should be taken into account. In the CVE method there are no deep contacts with the acid solutions, only a vapour etching is applied. This very simple and low cost method is easily adapted to cover large surfaces. The CVE parameters that need to be controlled are: the HNO3/HFvolume ratio, the temperature of the acid mixture and the exposure time of the Si substrate to the acid vapours. For acid vapours rich in HNO3, the CVE method favours the formation of a (NH4)2SiF6-like powder [5], highly soluble in water. In this case CVE process leads to an anisotropic groove formation in pc- Si wafers if an adequate anti-acid mask is used. The grooved areas may have a sufficient resolution to be applied in Si solar cells.

4 Results

Figure 1 shows a Scanning Electron Microscopy (SEM) view of a PS layer formed from VE of a mc-Si wafer at the following experimental conditions: the HNO3/HF volume ratio is 1/7, the temperature is 25 °C and the exposure time is 8 min.

Fig. 1 SEM top view image of PS made from VE a p-type mc-Si wafer. The HNO3/HF volume ratio is 1/7, the temperature of the acid mixture is 25 °C and the exposure time is 8 min. The cell surface area is 25 cm2.

phys. stat. sol. (c) 2, No. 9 (2005) / www.pss-c.com 3507


One may observe (Fig. 1) an interesting mesoporous sponge-like structure suitable for its use in light-trapping and light-diffusing and/or as an ARC. The surface texture of VE-based PS (Fig. 1) seems to be suitable as a good ARC for Si solar cells. Figure 2 depicts a comparison between the reflectivity of a PS-treated mc-Si wafer and an untreated one. We found that the reflectivity of the mc-Si wafers decreases by about 60 % of its initial value.

400 500 600 700 800 900 1000 11000

5

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After PS application

Before PS application

Tota

l R

efle

ctiv

ity (

%)

W avelength (nm) -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6

0,0

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(b)

(a)

I(A)

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Fig. 2 Total reflectivity of a mc-Si wafer, (a) before and (b) after PS treatment. The HNO3/HF volume ratio is 1/7, the temperature of the acid mixture is 25 °C and the exposure time is 8 min. The cell surface area is 25 cm2.

Fig. 3 I-V characteristics at AM1.5 illumination of pc- Si solar cells (a) before and (b) after PS treatment The HNO3/HF volume ratio is 1/7, the temperature of the acid mixture is 25 °C and the exposure time is 8 min.

From the I-V characteristic (Fig. 3), one may deduce an improvement of all electrical parameters after the formation of a PS layer onto the front surface of the mc-Si solar cells (cf. Table 1). The increase in the short circuit current density would essentially be due to an increase in light absorption (decrease of the reflectivity and increase of the light path inside the active material owing to the diffusing character of PS) and to a non negligible passivating role of the PS layer. The enhancement of the Fill Factor (FF) (cf. Table 1) could be due to the passivating character of PS, which in turn reduces the surface recombination velocity and then decreases the series resistance. The conversion efficiency rises from 7.7% to about 9.2% (Table 1). Clearly the improvement of the I-V characteristics is not only due to the effect of the ARC but eventually to the passivating character of the PS layer.

Table 1 Evolution of the electrical parameters of a mc-Si solar cell before and after VE-based PS treatment.

Electrical parameters (a) Before CVE treatment

(b) After CVE treatment

Short circuit current density: Isc(A) 0.52 0.62 Open circuit voltage: Voc(V) 0.53 0.54 Short circuit current density: Jsc(mA/cm2) 21 24.8 Fill Factor: FF(%) 63.2 65.5 Efficiency : η(%) 7.72 9.19

The current density (Jph) has a decreasingly function with the reflectivity. We found an enhancement of the internal quantum efficiency (Qint) after PS formation.

( )( )

( )( )int 1

phJQ

qN R

ll

l=

-

(1)

3508 M. Ben Rabha et al.: Chemical vapour etching-based porous silicon and grooving


where q is the electron charge, N the number of incident photons per surface unit and per time unit, λ is the wavelength, Jph is the photonic current density, and R(λ) is the frontal surface reflectivity. Figure 4 depicts the internal quantum efficiency of a mc-Si solar cell before and after formation of a PS layer onto the front surface of the cell. It is obvious that PS formation is done at the expense of the N+ zone, which may lead to a significant reduction of the dead layer and to a thinning of the effective emit-ter. Then, the improvement of the internal quantum efficiency after a front PS formation would come from surface passivation by the hydrogen-rich PS layer and to a reduction of the dead layer. This would increase the minority carrier diffusion length Lp (in the N zone) and contribute efficiently to the im-provement of Qint in the 400-700 nm spectral range. The enhancement of Qint in the 700-1100 nm spectral range of the base region could be due to the light diffusing nature of the PS layer.

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Fig. 4 Internal quantum efficiency of mc-Si solar cells (a) without and (b) with a CVE-based PS front layer.

Increasing the HNO3/HF volume ratio (HNO3/HF > 1/4) will transform the PS layer into a (NH4)2SiF6 powder [5]. The high solubility of the powder in water enables to achieve different groove patterns on the pc- Si wafers. The grooved areas may be adjusted in width and in depth. We used this grooving technology to achieve rear buried contacts (RBC). For this purpose a specific masking was used. Figure 5 shows a model of grooved patterns that were used for rear buried contacts (RBC). The grooves shown in Fig. 5 lead to an effective thickness of the base of the cells of about 180 µm (the thickness of the starting wafer is 330 µm). In order to realise a RBC, an Aluminum paste was screen printed on the grooved rear area of the wafers and fired at 850 °C. In that case, a non-negligible increase of the quantum efficiency of the mc-Si solar cells was observed, particularly at the long wavelength range (Fig. 6).

Fig. 5 Photo of CVE grooved patterns achieved for rear buried contacts in mc-Si solar cells

Grooved areas

7 mm

phys. stat. sol. (c) 2, No. 9 (2005) / www.pss-c.com 3509


Taking into account some approximations, and using root mean square minimizations, we extract the intrinsic parameters of the mc-Si cells by adjusting theoretical spectra to experimental ones. This adjustment let us estimate the effective minority carrier diffusion length Ln, which was found to be improved from 14 µm to 30 µm. The reduction of the effective thickness of the base of the cells could play a key role in the Ln enhancement. Furthermore, the RBC can also favour Aluminum passivation at the grain boundaries, which may also contribute to the increase of the effective minority carrier diffusion length.

Fig. 6 Internal quantum efficiency (Qint ) of mc-Si solar cells before and after realising a RBC grooving; dotted and solid lines indicate experimental and computed internal quantum efficiency, respectively. The RBC was achieved at the following experimental conditions: the HNO3/HF volume ratio is 1/3, the temperature of the acid mixture is 45°C and the time exposure is 5h.

5 Conclusion

The application of vapour etching based Porous silicon (PS) onto mc-Si solar cells was investigated. We found that the VE-based PS acts as a good antireflection coating in mc-Si solar cells, whose surface reflectivity decreases of about 60% of its initial value. The internal quantum efficiency of PS treated mc-Si cells improves, particularly in the short wavelengths range. The CVE technique allows the groove of the mc-Si wafers and the achievement of low cost rear buried contacts (RBC). As a result, we found an enhancement of the effective minority carrier diffusion lengths (Ln), as a consequence of the reduction of the effective thickness of the base of the cells, and probably to an Al passivation at the grain boundaries.

Acknowledgements This work was supported by the « Secrétariat d’Etat à la Recherche Scientifique et à la Technologie ».

References

[1] R. Bilyalov, R. Lüdermann, W. Wettling, L. Stalmans et al., Solar Energy Mater. Solar Cells 60, 391 (2000). [2] L. Schirone, G. Sotgiu, M. Montecchi, and A. Parisini, Proceedings of 14th EC Photovoltaic Solar Energy

Conference, 1997, pp. 1479-1482. [3] S. Strehlke, S. Bastide, and C. Lévy-Clément, Solar Energy Mater. Solar Cells 58, 399 (1999). [4] M. Saadoun, N. Mliki, H. Kaabi, K. Daoudi, B. Bessaïs et al., Thin Solid Films 405, 29 (2002) [5] M. Saadoun, B. Bessaïs, N. Mliki, M. Ferid, H. Ezzaouia, and R. Bennaceur, Appl. Surf. Sci. 240, 88 (2003).

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Chemical vapour etching-based porous silicon and grooving: Application in silicon solar cells processing