- Email: [email protected]
Formation of porous silicon for large-area silicon solar cells: A new method
Solar Energy Materials & Solar Cells 59 (1999) 377}385
Formation of porous silicon for large-area silicon solar cells: A new method M. Saadoun , H. Ezzaouia , B. BessamK s *, M.F. Boujmil , R. Bennaceur Institut National de Recherche Scientixque et Technique, Laboratoire de Photovoltan( que et des Mate& riaux Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisia Laboratoire de Physique de la Matie% re Condense& e, Faculte& des Sciences de Tunis, De& partement de Physique, 1006 Le Belve& de% re, Tunis, Tunisia Received 29 December 1998
Abstract Luminescent porous silicon (PS) was prepared for the "rst time using a spraying set-up, which can di!use in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very "ne HF drops. The front side of N>/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N>/P, 78.5 cm) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion e$ciency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and #ow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface re#ectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I ), and the "ll factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion e$ciency. The dramatic improvement of the latter is discussed throughout the in#uence of HF concentration and spraying time on the I}< characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-re#ection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer
* Corresponding author. Tel.: 216-1-430 044; fax: 216-1-430 934. E-mail address: [email protected] (B. BessamK s) 0927-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 5 7 - 4
378
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
wavelengths (that are more suitable for conversion in a Si cell) throughout quantum con"nement into the PS layer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Porous silicon; Solar cells
1. Introduction In 1986, Yablonovitch et al. [1] showed that oxidation of silicon followed by an Fluorhydric acid (HF) etch results in the formation of Si-H bonds, which made passive the recombination centres. Preparation in this manner gives the lowest carrier recombination velocity value ever reported for any semiconductor. Since this date, a brief HF etching has been used to improve the carrier lifetime and hence ameliorate the current density of silicon solar cells. In solar cells processing, the HF etching could be applied after sintering the metallic contacts, just before encapsulation of the cells, to avoid possible degradation. However, in all cases the HF etching time cannot exceed 10 s, otherwise the metallic contacts are damaged. It is evident that in an industrial solar cells processing, an HF etching step may involve some risks overall regarding the adhesion and the quality of the ohmic contacts. Recently, it has been shown that the electrochemical etching of silicon in HF produces a porous silicon (PS) layer rich in hydrogen [2]. Since this event, several workers searched to take bene"ts from the Si}H-rich layer forming PS. Thus many attempts have been done to introduce PS in photovoltaic devices. Primitive solar cells using PS have been demonstrated. However, few higher-e$ciency cells based on PS were reported. In forming PS, highly textured surfaces are obtained, enhancing light trapping and its potential use as an anti-re#ection coating [3]. The use of PS as an optimised emitter has been shown to be possible in a Si solar cell [4]: the PS layer is formed onto the top surface of the emitter by electrochemical etching of the surface, then the emitter consists of a top layer of PS and a bulk layer of N> Si. The quantum e$ciency measurements show the e!ectiveness of the PS layer in Si solar cells. The main problems are due to the series resistance which limit the "ll factor (FF), limiting the conversion e$ciency. But, in all these attempts PS was formed using the conventional electrochemical etching process in an HF solution. This technique is known to be aggressive, so to take bene"ts from PS as an anti-re#ection coating or as a passivating layer, very short anodisation time ((5 s) must be applied to avoid destroying the junction and damaging the front grid contacts. Recently, encouraging results have been reported by Schirone et al. [5], who produced large-area solar cells by converting the Si surface into PS by etching in controlled solutions; they reported a 100 cm cell with an e$ciency of 10.4% (AM1.5), with improved photon absorption at near infrared radiation and surface passivation. In this work, we demonstrate, for the "rst time, that PS may be formed on the top surface of large area N>/P monocrystalline silicon solar cells by spraying in an homogeneous manner concentrated HF solutions for long periods. From I}< characteristics, we optimise the HF concentration and the HF-spray-etching time. The e!ects of both parameters on the Photovoltaic (PV) features of the cells (current density, "ll factor (FF) etc.) are discussed. The bene"ts of forming thin PS layer on
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
379
both surface re#ectivity and internal quantum e$ciency are also shown. This work does not consist, from a technological point of view, to ameliorate the e$ciency of monocrystalline silicon solar cells. Our aim is simply to demonstrate qualitatively the dramatic improvement that can provide the application of a thin PS layer, formed within an HF-spray-etching process, on the characteristics of silicon solar cells. 2. Experimental The N>/P monocrystalline silicon solar cells (78.5 cm, p-type solar-grade cells) were prepared by the usual phosphorus di!usion technology. The phosphorus source is a POCl /Propanol-2 solution. The latter is spread by the spinning technique onto p-type NaOH-textured Si wafers. Both front grid and back side metallic contacts were realised by screen printing a silver paste and an aluminium/silver paste, respectively. The front grid contact must be realised before forming PS to limit the high series resistance. The metallic contacts undergo a co-"ring in an Infrared furnace, in air, without any further precautions (such as ventilation and controlled atmosphere...). Owing to I}< characteristics, we have optimised the ratio POCl /Propanol-2 to and temperature and time exposure within di!usion to 9253C and 20 min, respectively. The edges of the cells were mechanically etched. This low-cost technique allows us to achieve expected poor quality cells having a conversion e$ciency of about &7.5%. This poor e$ciency is needed, to show the dramatic bene"ts that can provide a thin PS top layer on the characteristics of the Si cells. A speci"c HF-spraying set-up was built to supply "ne HF drops in order to treat the top surface of the cells for long periods. The speci"c spraying nozzle is mounted so that it can execute an automated X}> scanning. 3. Results and dicussion 3.1. Formation of porous silicon In the preparation of PS by stain etching, it was reported that pure HF cannot produce alone the required holes to start forming PS. It was always recommended to add HNO to the etching solution. However, a new method was reported recently [6] to prepare thin PS layers (&1000 As ) with HF/HNO : a thin Al "lm was deposited by evaporation prior to etching, the reaction between Al and HNO produces the very fast start of the chemical etching of Si. Following these recent results, we have intentionally contaminated the front of the N> side of the cells by performing a simple co-"ring of the metallic contacts, so that Al vapour contamination occurs, prior to form PS by HF-spray-etching. The advantage of our technique is to use HF solution only, instead of HF/HNO , because of the aggressive etch of the latter, which may seriously damage the front metallic contacts of the cells. The HF-spray-etching that we have performed consists to treat for long periods the front-side surface of the cells to make it passive by Si-H bonds, following the statements of Yablonovitch et al. [1]. As previously said, this HF treatment should be done after screen printing and sintering the metallic contacts, to take bene"ts from Al
380
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
vapour contamination. The co-"ring is a key step in the formation of the PS layer. Indeed, we have noticed that when the front side of the cells undergo an HF-sprayetching after screen printing and sintering the contacts, a good and uniform surface darkening occurs, leading to the formation of a homogeneous PS layer. On the contrary, if the HF-spray-etching is performed before sintering the contacts, one may obtain at the most dispersed darkish stains, due to very weak Al contamination of the front side of the cells, and hence the formation of an inhomogeneous and poor PS layer. During the HF-spray-etching step, the cells are uniformly heated with a temperature-regulated hot plate. The temperature of the hot plate is determinant to obtain an homogeneous frontal surface darkening. When the darkish surface is excited with an UV light, it emits bright red}orange luminescence which can be seen with the naked eye. On the other hand, the darkish aspect of the surface of the cells is not removed when etching is performed in concentrated HF solution, while removed when the wafers are etched in NaOH (1 N) solution. These two latter "ndings con"rm the formation of PS. Fig. 1 shows the PL spectrum emitted by a thin PS layer prepared by HF-spray-etching. One can notice that the forming PS layer emit in the red-orange spectral region with a typical peak at 1.72 eV. Thus, the di!erence of results between HF-spray-etching the cells before and after sintering the metallic contacts seems to be due to the co-"ring step where we have intentionally contaminated the N>-type front layer by Al vapours (no ventilation done during the co-"ring). Paradoxically this simple and low-cost co-"ring step contributes (intentionally) to the poor quality of the junction (i.e., conversion e$ciency), but plays an important role in the formation of PS and in the darkish aspect of the front surface of the cells. Indeed, in optimising the HF #ow rate and the temperature of the hot plate (which may have a weak oxidising e!ect) HF may attack at a very low rate the Al-based aggregates formed at the front N>-type surface during the co-"ring, leading to the formation a thin PS layer rich in passivating species (i.e., Si}H species) V and acting as a good anti-re#ecting coating (Fig. 2). Fig. 2 depicts the re#ectivity of the
Fig. 1. Room temperature PL spectra of a thin PS layer prepared by HF-spray-etching the N> emitter of N>/P monocrystalline silicon solar cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
381
Fig. 2. (a) Re#ectivity of a textured monocrystalline silicon solar cell; (b) Re#ectivity of a textured monocrystalline silicon solar cell in the presence of a thin porous silicon layer onto the front-side surface.
front-side surface of a textured monocrystalline silicon solar cell without (Fig. 2a) and with (Fig. 2b) a thin porous silicon layer. After PS formation, the re#ectivity of the cell decreases from &12% (textured cell without PS layer) to &3% (textured cell with a PS layer). It should be noted that the darkish colour of the surface occurs after 5 mn of HF-spray-etching. Beyond 5 mn of HF treatment the front surface re#ectivity is independent of HF-spraying time whatever the HF concentration may be. Now, it is important to optimise the HF concentration and the HF-spray-etching time to obtain the maximum bene"ts from the thin PS layer, without destroying the N>/P junction and damaging the metallic contacts. 3.2. Ewect of HF concentration on the I}< characteristics Fig. 3 shows the evolution of the I}< characteristics taken under AM1 illumination for sprayed HF concentration varying between 0% and 40%, for a "xed spraying time of 5 mn. In Table 1, we report the variation of the PV parameters (< , I , J , FF and conversion e$ciency g) with HF concentration. From Table 1, one may notice that the current density (J ) increases with HF concentration. Since the re#ectivity (Fig. 2) is independent of HF concentration and spraying time (as previously said), the increase of the current density with HF concentration is mainly due to the presence of further Si}H bonds. So, this phenomenon seems to be due to further surface passivation as HF concentration increases. However, we note an improvement of FF and the conversion e$ciency (g) with HF concentration up to 20% of HF concentration. Beyond this value, we notice that FF and g decrease.
382
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
Fig. 3. E!ect of the sprayed HF concentration on the I}< characteristics. Spraying time is "xed to 5 min. The I}< characteristics are measured under AM1 illumination.
Table 1 In#uence of HF concentration on the photovoltaic parameters of the cells. Spraying time was "xed to 5 mn HF concentr.
Without HF
10%
20%
30%
40%
0.580 1.665 21.26 60.11 7.83
0.573 1.883 23.96 64.32 9.48
0.584 2.012 25.64 66.82 10.03
0.561 2.106 26.84 56.31 8.07
0.538 2.214 28.21 42.25 7.68
Parameters < (V) I (A) J (mA/cm) FF(%) g(%)
The deterioration of these characteristics beyond 20% of HF concentration may be due to a notable increase of the series resistance (cf. Fig. 3) and to the damaging of the contact resistance. The open circuit voltage (< ) breaks down at 40% HF; probably at this concentration the N>/P junction begins to degrade. As shown in Table 1, the better performances are achieved for an HF concentration of 20%. Now, we search to optimise the HF-spray-etching time that gives the best I}< characteristic at 20% HF concentration. 3.3. Ewect of HF-spray-etching time on the I}< characteristics Fig. 4 depicts the in#uence of HF-spray-etching time on the I}< characteristics taken under AM1 illumination for an optimised HF concentration of 20%.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
383
Fig. 4. E!ect of Spraying time on the I}< characteristics. HF concentration was optimised to 20%. The I}< characteristics are measured under AM1 illumination.
Table 2 In#uence of HF-spray-etching time on the photovoltaic parameters of the cells. HF concentration was optimised to 20% t(mn)
0
5
10
15
20
25
0.580 1.692 21.56 55.38 7.47
0.583 2.037 25.95 70.07 10.61
0.580 2.129 27.13 72.23 11.68
0.585 2.234 28.46 74.17 12.47
0.585 2.163 27.56 69.54 11.28
0.548 2.182 27.80 47.37 7.39
Parameters < (V) I (A) J (mA/cm) FF(%) g(%)
The PV parameters values versus HF-spray-etching time are shown in Table 2. We notice an improvement of the cell parameters up to 15 mn of HF-spray-etching. Beyond this time, we observe a degradation of the PV performances. We should note that the open-circuit voltage undergoes a little variation during the HF treatment. From Figs. 3 and 4 (i.e., Tables 1 and 2), we conclude that the dependence of the I}< characteristics on HF concentration and HF-spray-etching time are similar. Two steps characterise the evolution of the I}< characteristics. The "rst step corresponds to an improvement of the PV performances: when both HF concentration and HF-spray-etching time increase the PV performances attain a parametric limit corresponding to a maximum e$ciency of about 12.5% (for the maximum e$ciency HF concentration and HF-spray-etching time were optimised to 20% and 15 min, respectively). The second step corresponds to a degradation of the PV performances when
384
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
the treatment parameters go beyond the optimal limit. At "rst sight, the improvement of the PV parameters may be due to the existence of two simultaneous phenomena: surface passivation by Si}H bonds and formation of an anti-re#ecting coating. It is well known [1] that the hydrogen provided by HF improves the short-circuit current and the FF. This phenomenon is ampli"ed by the formation of a PS layer which, owing to its large internal surface contains an important quantity of hydrogen in surface as well as in volume. The formation of the PS layer decreases the N>-layer thickness and hence reduce the undesired dead layer. However, beyond a certain HF concentration and HF-spray-etching time limits (i.e., 20% and 15 min, respectively) HF vapours attack the metallic contacts and weakens their adherence to the cell. 3.4. Spectral response Fig. 5a and b shows the in#uence of incorporating PS on the spectral response (SR) (i.e., the internal quantum e$ciency) of a Si solar cell. The spectral response SR is expressed as J (j) , SR(j)" qN(j)(1!R(j)) where J (j) is the photocurrent density, N(j) is the monochromatic photon #ux and R(j) the re#ectivity (di!use and specular) at a given wavelength j. As shown in Fig. 2, when PS is formed, the surface re#ectivity fall down from &12% to &3%. The decrease of R(j) increases light absorption and hence J (j).
Fig. 5. A comparison between the spectral responses of (a) SiO }passivated monocrystalline Si solar cell and (b) in presence of a thin PS layer formed by HF-spray-etching the front surface of the cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
385
Fig. 5 clearly demonstrates that the internal quantum e$ciency (SR) of the HFspray-etched Si solar cell (formation of PS) is higher than that of SiO -passivated one. In fact, despite increasing light absorption from the highly textured surface that provides the thin PS layer, the photoluminescence property of the latter allows to convert (with a certain e$ciency) UV and blue light into longer absorbable wavelengths (in the red region). This may generate a small additional photo-current, in the UV and blue part of the solar spectrum, that may improve the internal quantum e$ciency in this spectral region, as shown in Fig. 5b. However, the main contribution to the improvement of the internal quantum e$ciency is signi"cantly due to the surface passivation of the cells by the hydrogen of the PS layer.
4. Conclusion We have shown for the "rst time that spraying for long periods the front side of N>/P monocrystalline silicon solar cells with "ne HF drops leads to the formation of a darkish thin porous silicon layer. It was shown that during the co-"ring step of the metallic contacts, Al contamination of the N> layer occurs and plays an important role in the start of the homogeneous chemical etching of Si, leading to PS formation. It was found that at optimised HF-spray-etching conditions, the presence of the thin PS layer signi"cantly improves the characteristics of the cells, allowing to improve the conversion e$ciency from 7.5% to 12.5% and the FF from &60% to &74%, suggesting that the thin Si}H rich PS layer acts not only as a good anti-re#ection coating, but also as a passivating layer. A signi"cant improvement of the internal quantum e$ciency is also shown, overall in the UV } blue spectral zone. This HF-spray-etching method seems to be promising in polycrystalline silicon solar cells technology.
Acknowledgement This work was supported by the SecreH tariat d'Etat a` la Recherche Scienti"que et a` la Technologie (P96EN01). The authors would like to thank M.Oueslati, Pr at the faculty of Sciences of Tunis for his help in PL measurements.
References [1] E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (1986) 249. [2] A. Borghesi, A. Sassella, B. Pivac, L. Pavesi, Solid State Commun. 87 (1993) 1. [3] S. Bastide, M. Cuniot, Q.N. Le, D. Sarty, C. Levy-CleH ment, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Kluwer Scienti"c, The Netherlands, 1994, pp. 780}783. [4] A.J. Mc Evoy, M. GraK tzel, Solar Energy Mater. (1994) 1779. [5] L. Schirone, G. Sotguiu, F. Rallo, F.P. Califano, 13th European Photovoltaic Solar Energy Conference H.S. Stephens & Assoc. UK, 1995, pp. 2447}2450. [6] D. Dimovamalinovska, M. Sendovavassileva, N. Tzenov, M. Kamenova, Thin Solid Films 297 (1997) 9.
Formation of porous silicon for large-area silicon solar cells: A new method M. Saadoun , H. Ezzaouia , B. BessamK s *, M.F. Boujmil , R. Bennaceur Institut National de Recherche Scientixque et Technique, Laboratoire de Photovoltan( que et des Mate& riaux Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisia Laboratoire de Physique de la Matie% re Condense& e, Faculte& des Sciences de Tunis, De& partement de Physique, 1006 Le Belve& de% re, Tunis, Tunisia Received 29 December 1998
Abstract Luminescent porous silicon (PS) was prepared for the "rst time using a spraying set-up, which can di!use in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very "ne HF drops. The front side of N>/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N>/P, 78.5 cm) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion e$ciency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and #ow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface re#ectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I ), and the "ll factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion e$ciency. The dramatic improvement of the latter is discussed throughout the in#uence of HF concentration and spraying time on the I}< characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-re#ection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer
* Corresponding author. Tel.: 216-1-430 044; fax: 216-1-430 934. E-mail address: [email protected] (B. BessamK s) 0927-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 5 7 - 4
378
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
wavelengths (that are more suitable for conversion in a Si cell) throughout quantum con"nement into the PS layer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Porous silicon; Solar cells
1. Introduction In 1986, Yablonovitch et al. [1] showed that oxidation of silicon followed by an Fluorhydric acid (HF) etch results in the formation of Si-H bonds, which made passive the recombination centres. Preparation in this manner gives the lowest carrier recombination velocity value ever reported for any semiconductor. Since this date, a brief HF etching has been used to improve the carrier lifetime and hence ameliorate the current density of silicon solar cells. In solar cells processing, the HF etching could be applied after sintering the metallic contacts, just before encapsulation of the cells, to avoid possible degradation. However, in all cases the HF etching time cannot exceed 10 s, otherwise the metallic contacts are damaged. It is evident that in an industrial solar cells processing, an HF etching step may involve some risks overall regarding the adhesion and the quality of the ohmic contacts. Recently, it has been shown that the electrochemical etching of silicon in HF produces a porous silicon (PS) layer rich in hydrogen [2]. Since this event, several workers searched to take bene"ts from the Si}H-rich layer forming PS. Thus many attempts have been done to introduce PS in photovoltaic devices. Primitive solar cells using PS have been demonstrated. However, few higher-e$ciency cells based on PS were reported. In forming PS, highly textured surfaces are obtained, enhancing light trapping and its potential use as an anti-re#ection coating [3]. The use of PS as an optimised emitter has been shown to be possible in a Si solar cell [4]: the PS layer is formed onto the top surface of the emitter by electrochemical etching of the surface, then the emitter consists of a top layer of PS and a bulk layer of N> Si. The quantum e$ciency measurements show the e!ectiveness of the PS layer in Si solar cells. The main problems are due to the series resistance which limit the "ll factor (FF), limiting the conversion e$ciency. But, in all these attempts PS was formed using the conventional electrochemical etching process in an HF solution. This technique is known to be aggressive, so to take bene"ts from PS as an anti-re#ection coating or as a passivating layer, very short anodisation time ((5 s) must be applied to avoid destroying the junction and damaging the front grid contacts. Recently, encouraging results have been reported by Schirone et al. [5], who produced large-area solar cells by converting the Si surface into PS by etching in controlled solutions; they reported a 100 cm cell with an e$ciency of 10.4% (AM1.5), with improved photon absorption at near infrared radiation and surface passivation. In this work, we demonstrate, for the "rst time, that PS may be formed on the top surface of large area N>/P monocrystalline silicon solar cells by spraying in an homogeneous manner concentrated HF solutions for long periods. From I}< characteristics, we optimise the HF concentration and the HF-spray-etching time. The e!ects of both parameters on the Photovoltaic (PV) features of the cells (current density, "ll factor (FF) etc.) are discussed. The bene"ts of forming thin PS layer on
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
379
both surface re#ectivity and internal quantum e$ciency are also shown. This work does not consist, from a technological point of view, to ameliorate the e$ciency of monocrystalline silicon solar cells. Our aim is simply to demonstrate qualitatively the dramatic improvement that can provide the application of a thin PS layer, formed within an HF-spray-etching process, on the characteristics of silicon solar cells. 2. Experimental The N>/P monocrystalline silicon solar cells (78.5 cm, p-type solar-grade cells) were prepared by the usual phosphorus di!usion technology. The phosphorus source is a POCl /Propanol-2 solution. The latter is spread by the spinning technique onto p-type NaOH-textured Si wafers. Both front grid and back side metallic contacts were realised by screen printing a silver paste and an aluminium/silver paste, respectively. The front grid contact must be realised before forming PS to limit the high series resistance. The metallic contacts undergo a co-"ring in an Infrared furnace, in air, without any further precautions (such as ventilation and controlled atmosphere...). Owing to I}< characteristics, we have optimised the ratio POCl /Propanol-2 to and temperature and time exposure within di!usion to 9253C and 20 min, respectively. The edges of the cells were mechanically etched. This low-cost technique allows us to achieve expected poor quality cells having a conversion e$ciency of about &7.5%. This poor e$ciency is needed, to show the dramatic bene"ts that can provide a thin PS top layer on the characteristics of the Si cells. A speci"c HF-spraying set-up was built to supply "ne HF drops in order to treat the top surface of the cells for long periods. The speci"c spraying nozzle is mounted so that it can execute an automated X}> scanning. 3. Results and dicussion 3.1. Formation of porous silicon In the preparation of PS by stain etching, it was reported that pure HF cannot produce alone the required holes to start forming PS. It was always recommended to add HNO to the etching solution. However, a new method was reported recently [6] to prepare thin PS layers (&1000 As ) with HF/HNO : a thin Al "lm was deposited by evaporation prior to etching, the reaction between Al and HNO produces the very fast start of the chemical etching of Si. Following these recent results, we have intentionally contaminated the front of the N> side of the cells by performing a simple co-"ring of the metallic contacts, so that Al vapour contamination occurs, prior to form PS by HF-spray-etching. The advantage of our technique is to use HF solution only, instead of HF/HNO , because of the aggressive etch of the latter, which may seriously damage the front metallic contacts of the cells. The HF-spray-etching that we have performed consists to treat for long periods the front-side surface of the cells to make it passive by Si-H bonds, following the statements of Yablonovitch et al. [1]. As previously said, this HF treatment should be done after screen printing and sintering the metallic contacts, to take bene"ts from Al
380
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
vapour contamination. The co-"ring is a key step in the formation of the PS layer. Indeed, we have noticed that when the front side of the cells undergo an HF-sprayetching after screen printing and sintering the contacts, a good and uniform surface darkening occurs, leading to the formation of a homogeneous PS layer. On the contrary, if the HF-spray-etching is performed before sintering the contacts, one may obtain at the most dispersed darkish stains, due to very weak Al contamination of the front side of the cells, and hence the formation of an inhomogeneous and poor PS layer. During the HF-spray-etching step, the cells are uniformly heated with a temperature-regulated hot plate. The temperature of the hot plate is determinant to obtain an homogeneous frontal surface darkening. When the darkish surface is excited with an UV light, it emits bright red}orange luminescence which can be seen with the naked eye. On the other hand, the darkish aspect of the surface of the cells is not removed when etching is performed in concentrated HF solution, while removed when the wafers are etched in NaOH (1 N) solution. These two latter "ndings con"rm the formation of PS. Fig. 1 shows the PL spectrum emitted by a thin PS layer prepared by HF-spray-etching. One can notice that the forming PS layer emit in the red-orange spectral region with a typical peak at 1.72 eV. Thus, the di!erence of results between HF-spray-etching the cells before and after sintering the metallic contacts seems to be due to the co-"ring step where we have intentionally contaminated the N>-type front layer by Al vapours (no ventilation done during the co-"ring). Paradoxically this simple and low-cost co-"ring step contributes (intentionally) to the poor quality of the junction (i.e., conversion e$ciency), but plays an important role in the formation of PS and in the darkish aspect of the front surface of the cells. Indeed, in optimising the HF #ow rate and the temperature of the hot plate (which may have a weak oxidising e!ect) HF may attack at a very low rate the Al-based aggregates formed at the front N>-type surface during the co-"ring, leading to the formation a thin PS layer rich in passivating species (i.e., Si}H species) V and acting as a good anti-re#ecting coating (Fig. 2). Fig. 2 depicts the re#ectivity of the
Fig. 1. Room temperature PL spectra of a thin PS layer prepared by HF-spray-etching the N> emitter of N>/P monocrystalline silicon solar cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
381
Fig. 2. (a) Re#ectivity of a textured monocrystalline silicon solar cell; (b) Re#ectivity of a textured monocrystalline silicon solar cell in the presence of a thin porous silicon layer onto the front-side surface.
front-side surface of a textured monocrystalline silicon solar cell without (Fig. 2a) and with (Fig. 2b) a thin porous silicon layer. After PS formation, the re#ectivity of the cell decreases from &12% (textured cell without PS layer) to &3% (textured cell with a PS layer). It should be noted that the darkish colour of the surface occurs after 5 mn of HF-spray-etching. Beyond 5 mn of HF treatment the front surface re#ectivity is independent of HF-spraying time whatever the HF concentration may be. Now, it is important to optimise the HF concentration and the HF-spray-etching time to obtain the maximum bene"ts from the thin PS layer, without destroying the N>/P junction and damaging the metallic contacts. 3.2. Ewect of HF concentration on the I}< characteristics Fig. 3 shows the evolution of the I}< characteristics taken under AM1 illumination for sprayed HF concentration varying between 0% and 40%, for a "xed spraying time of 5 mn. In Table 1, we report the variation of the PV parameters (< , I , J , FF and conversion e$ciency g) with HF concentration. From Table 1, one may notice that the current density (J ) increases with HF concentration. Since the re#ectivity (Fig. 2) is independent of HF concentration and spraying time (as previously said), the increase of the current density with HF concentration is mainly due to the presence of further Si}H bonds. So, this phenomenon seems to be due to further surface passivation as HF concentration increases. However, we note an improvement of FF and the conversion e$ciency (g) with HF concentration up to 20% of HF concentration. Beyond this value, we notice that FF and g decrease.
382
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
Fig. 3. E!ect of the sprayed HF concentration on the I}< characteristics. Spraying time is "xed to 5 min. The I}< characteristics are measured under AM1 illumination.
Table 1 In#uence of HF concentration on the photovoltaic parameters of the cells. Spraying time was "xed to 5 mn HF concentr.
Without HF
10%
20%
30%
40%
0.580 1.665 21.26 60.11 7.83
0.573 1.883 23.96 64.32 9.48
0.584 2.012 25.64 66.82 10.03
0.561 2.106 26.84 56.31 8.07
0.538 2.214 28.21 42.25 7.68
Parameters < (V) I (A) J (mA/cm) FF(%) g(%)
The deterioration of these characteristics beyond 20% of HF concentration may be due to a notable increase of the series resistance (cf. Fig. 3) and to the damaging of the contact resistance. The open circuit voltage (< ) breaks down at 40% HF; probably at this concentration the N>/P junction begins to degrade. As shown in Table 1, the better performances are achieved for an HF concentration of 20%. Now, we search to optimise the HF-spray-etching time that gives the best I}< characteristic at 20% HF concentration. 3.3. Ewect of HF-spray-etching time on the I}< characteristics Fig. 4 depicts the in#uence of HF-spray-etching time on the I}< characteristics taken under AM1 illumination for an optimised HF concentration of 20%.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
383
Fig. 4. E!ect of Spraying time on the I}< characteristics. HF concentration was optimised to 20%. The I}< characteristics are measured under AM1 illumination.
Table 2 In#uence of HF-spray-etching time on the photovoltaic parameters of the cells. HF concentration was optimised to 20% t(mn)
0
5
10
15
20
25
0.580 1.692 21.56 55.38 7.47
0.583 2.037 25.95 70.07 10.61
0.580 2.129 27.13 72.23 11.68
0.585 2.234 28.46 74.17 12.47
0.585 2.163 27.56 69.54 11.28
0.548 2.182 27.80 47.37 7.39
Parameters < (V) I (A) J (mA/cm) FF(%) g(%)
The PV parameters values versus HF-spray-etching time are shown in Table 2. We notice an improvement of the cell parameters up to 15 mn of HF-spray-etching. Beyond this time, we observe a degradation of the PV performances. We should note that the open-circuit voltage undergoes a little variation during the HF treatment. From Figs. 3 and 4 (i.e., Tables 1 and 2), we conclude that the dependence of the I}< characteristics on HF concentration and HF-spray-etching time are similar. Two steps characterise the evolution of the I}< characteristics. The "rst step corresponds to an improvement of the PV performances: when both HF concentration and HF-spray-etching time increase the PV performances attain a parametric limit corresponding to a maximum e$ciency of about 12.5% (for the maximum e$ciency HF concentration and HF-spray-etching time were optimised to 20% and 15 min, respectively). The second step corresponds to a degradation of the PV performances when
384
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
the treatment parameters go beyond the optimal limit. At "rst sight, the improvement of the PV parameters may be due to the existence of two simultaneous phenomena: surface passivation by Si}H bonds and formation of an anti-re#ecting coating. It is well known [1] that the hydrogen provided by HF improves the short-circuit current and the FF. This phenomenon is ampli"ed by the formation of a PS layer which, owing to its large internal surface contains an important quantity of hydrogen in surface as well as in volume. The formation of the PS layer decreases the N>-layer thickness and hence reduce the undesired dead layer. However, beyond a certain HF concentration and HF-spray-etching time limits (i.e., 20% and 15 min, respectively) HF vapours attack the metallic contacts and weakens their adherence to the cell. 3.4. Spectral response Fig. 5a and b shows the in#uence of incorporating PS on the spectral response (SR) (i.e., the internal quantum e$ciency) of a Si solar cell. The spectral response SR is expressed as J (j) , SR(j)" qN(j)(1!R(j)) where J (j) is the photocurrent density, N(j) is the monochromatic photon #ux and R(j) the re#ectivity (di!use and specular) at a given wavelength j. As shown in Fig. 2, when PS is formed, the surface re#ectivity fall down from &12% to &3%. The decrease of R(j) increases light absorption and hence J (j).
Fig. 5. A comparison between the spectral responses of (a) SiO }passivated monocrystalline Si solar cell and (b) in presence of a thin PS layer formed by HF-spray-etching the front surface of the cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
385
Fig. 5 clearly demonstrates that the internal quantum e$ciency (SR) of the HFspray-etched Si solar cell (formation of PS) is higher than that of SiO -passivated one. In fact, despite increasing light absorption from the highly textured surface that provides the thin PS layer, the photoluminescence property of the latter allows to convert (with a certain e$ciency) UV and blue light into longer absorbable wavelengths (in the red region). This may generate a small additional photo-current, in the UV and blue part of the solar spectrum, that may improve the internal quantum e$ciency in this spectral region, as shown in Fig. 5b. However, the main contribution to the improvement of the internal quantum e$ciency is signi"cantly due to the surface passivation of the cells by the hydrogen of the PS layer.
4. Conclusion We have shown for the "rst time that spraying for long periods the front side of N>/P monocrystalline silicon solar cells with "ne HF drops leads to the formation of a darkish thin porous silicon layer. It was shown that during the co-"ring step of the metallic contacts, Al contamination of the N> layer occurs and plays an important role in the start of the homogeneous chemical etching of Si, leading to PS formation. It was found that at optimised HF-spray-etching conditions, the presence of the thin PS layer signi"cantly improves the characteristics of the cells, allowing to improve the conversion e$ciency from 7.5% to 12.5% and the FF from &60% to &74%, suggesting that the thin Si}H rich PS layer acts not only as a good anti-re#ection coating, but also as a passivating layer. A signi"cant improvement of the internal quantum e$ciency is also shown, overall in the UV } blue spectral zone. This HF-spray-etching method seems to be promising in polycrystalline silicon solar cells technology.
Acknowledgement This work was supported by the SecreH tariat d'Etat a` la Recherche Scienti"que et a` la Technologie (P96EN01). The authors would like to thank M.Oueslati, Pr at the faculty of Sciences of Tunis for his help in PL measurements.
References [1] E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (1986) 249. [2] A. Borghesi, A. Sassella, B. Pivac, L. Pavesi, Solid State Commun. 87 (1993) 1. [3] S. Bastide, M. Cuniot, Q.N. Le, D. Sarty, C. Levy-CleH ment, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Kluwer Scienti"c, The Netherlands, 1994, pp. 780}783. [4] A.J. Mc Evoy, M. GraK tzel, Solar Energy Mater. (1994) 1779. [5] L. Schirone, G. Sotguiu, F. Rallo, F.P. Califano, 13th European Photovoltaic Solar Energy Conference H.S. Stephens & Assoc. UK, 1995, pp. 2447}2450. [6] D. Dimovamalinovska, M. Sendovavassileva, N. Tzenov, M. Kamenova, Thin Solid Films 297 (1997) 9.