- Email: [email protected]
Characterization and properties of a modified Si solar cell emitter by a porous Si layer
Materials Science and Engineering B101 (2003) 291 /296 www.elsevier.com/locate/mseb
Characterization and properties of a modified Si solar cell emitter by a porous Si layer Z. Swiatek a,*, E. Beltowska a, W. Maziarz a, F. Krok b a
b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str., 30-059 Cracow, Poland Regional Laboratory for Physicochemical Analyses and Structural Research, Jagiellonian University, 3 Ingardena Str., 30-060 Cracow, Poland
Abstract Porous Si (PS) has become an interesting material owing to its potential applications in many fields including microelectronics, optoelectronics and photovoltaics. PS layers on the front surface of n//p monocrystalline, textured Si solar cells have been investigated with the aim of improving the performance of standard screen-printed cells, because an antireflection coating and a surface passivation can be obtained simultaneously in one chemical process. The results obtained could be useful in optimising the Si surface chemical treatment process. The surface morphology and microstructure of PS layers were investigated using SEM, TEM and non-contact AFM methods. The surface morphology of a PS layer depends strongly on the region where the pores are formed. The structure of PS layer is composed of macro-pores formed in p type Si (sizes vary over a large range up to 250 nm) and mesopores formed in the n region of the p /n junction. The meso-pores of average size 20 nm on the pyramid slope elongate preferentially along the 111 direction. The interface between the PS layer and the substrate as well as the surface roughness are clearly defined. The results show that the PS layer on the pyramids is formed uniformly along the walls. Meso-pores created on the macro-pore surface are a characteristic feature of the surface between pyramids. Such a surface modification allows improving the Si solar cell characteristics. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Solar cells; Surface morphology; Texturization
1. Introduction Porous silicon (PS) has become a very interesting material owing to its potential applications in many fields like microelectronics, optoelectronics and photovoltaics. Recently, several reports concerning the use of PS in photovoltaic devices have been published [1 /6]. Extremely low reflection coefficients of ARC on the bases of PS were reached, which are comparable with the most efficient double ARC layer of ZnS/MgF2 deposited on pretextured silicon surface by means of expensive vacuum methods [7]. Despite the large number of studies devoted to this question, the problem of commercial application has not been solved yet. The parameters of the used PS layers have not been fully optimised and the adaptation to the technology has not
* Corresponding author. Tel.: /48-12-6374200; fax: /48-126372192. E-mail address: [email protected] (Z. Swiatek).
been satisfactory so far. Nevertheless, it is widely believed that PS can be adapted to mass production of solar cells, because of the simple and cheap technology. It is known that apart from the antireflection properties the PS layer based silicon solar cells have other advantages including a new kind of passivation and a surface texturization (light trapping), a band-gap control (1.5 /1.8 eV) and a solar light transformation from ultraviolet (due to its luminescent properties) to longer wavelengths [8]. The mechanisms of the complex microstructural changes in near surface areas of the silicon wafers occurring during PS layer formation are not yet fully understood. The structural and morphological studies are currently the subject of many investigations, but they concern mainly PS layer formation on the platelet silicon surface [9 /11]. Thus, the presented work is aimed at the investigation of the inhomogenity and microstructure of a thin PS layer formed in the n -type region of the texturized Si. Texturing is one of the most important and difficult processing step in silicon solar
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00718-3
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
292
cell fabrication. The monocrystalline silicon can be textured by alkaline solutions, which etch preferentially the 111 direction, resulting in random pyramid structures. However, production problems still exist including e.g. repeatability, lack of control over pyramid size, etching of pyramid peaks as well as the presence of untextured regions between the pyramids. The PS layers formation on the top textured surface of n/p monocrystalline Si solar cells could reduce some of anisotropic texturization disadvantages.
2. Experimental procedure Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in standard alkaline solution [12]. Then, emitter was thermally generated using POCl3 as phosphorous doped source and BSF (Back Surface Field) was created from boron-emulsion. Both junctions were produced in one technological cycle. The electrical contacts (Ag front and Ag/Al back) were produced by the screen printing process and fired at 700 8C, while the PS layers were formed by electroless chemical etching in solutions containing concentrated HF, HNO3 and some surface active additives. The maximum etching time was 6 s, corresponding to a PS thickness of about 200 nm. The grid contacts of solar cells were not protected before PS formation. For the applied time of stain etching the metallization was not destroyed. The structure of PS layer was investigated using SEM (Philips XL30), TEM (Philips CM20), and AFM (Park Scientific Instruments Autoprobe CP). The photovoltaic characteristics of the solar cells (short-circuit current density ISC, open-circuit voltage VOC, fill factor FF, h efficiency) with and without PS layer were compared.
3. Results It is generally accepted that pore initiation occurs at surface active sites defects or irregularities. The proper-
ties of PS layer (porosity, thickness, pore diameter and microstructure) mainly depend on operating parameters of layer formation, including the HF/HNO3 concentration ratio, the presence of surface active compound additives, the duration and the temperature as well as the wafer type and its resistivity. The investigations have been focused on visualisation techniques, because the optical, electrical and physical properties of PS layer depend strongly on its microstructure. Information about pore size and their shape are not easy to obtain, therefore different experimental techniques such as SEM, TEM and AFM have been used. Fig. 1 shows SEM images of Si solar cell with PS layer on the front and back surface of solar cell. The difference in morphology of the obtained PS layer in both cases is evident. For p-type doped Si (the back surface of the solar cell) the pore sizes vary in the large range from 20 up to 250 nm (Figs. 1 and 2). For n-type doped Si the situation is more complicated. Generally, pore dimensions are smaller than the SEM resolution and they could only be observed by means of TEM and AFM. The surface morphology (SM) of PS layer depends strongly on the area where the pores were formed. That can be seen in Figs. 3 and 4 where non-contact AFM pictures of the PS layer morphology at the vicinity of three pyramids as well as in the regions between pyramids, respectively, are presented. Figs. 3 and 4 show the heterogeneity of the SM of the PS layer. Detailed observations (part B and C of Fig. 3) show that average ‘macro-pores’ of 100 nm formed by meso-pores are characteristic for the wall C, whereas craters of average diameter 200 nm and the meso-pores are seen at the B wall. The characteristic feature of the surface between the pyramids is meso-pores created on the macro-pore surface (part A, B in Fig. 4). TEM images gave further details about the morphology of stain-etched layer. The meso-pores of average size of 20 nm on the pyramid slope elongate preferentially along the 111 direction, as can be seen on the transmission electron micrographs (Fig. 5). The TEM
Fig. 1. The scanning electron micrographs of the front (a) and back (b) surface of silicon solar cell with PS layer.
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 2. The transmission electron micrograph of the front (a) and back (b) surface of silicon solar cells with PS layer.
Fig. 3. Non-contact AFM images of the front surface of silicon solar cell with PS layer on the slope pyramids.
293
294
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 4. Non-contact AFM images of the front surface of silicon solar cell with PS layer in the regions between pyramids.
structural characterisation confirms the existence of a porosity gradient, with decreasing porosity in depth. The pores are randomly distributed and have columnar shapes. A small disorientation of the separate columns takes place. The interface between the porous layer and the substrate as well as the interface roughness is clearly defined. The obtained results show that PS layer on the pyramids is formed uniformly along their walls. For the above investigations the solar cells with the efficiency about 12% were chosen, because larger changes in their parameters, caused by PS were expected. The photovoltaic characteristics of the solar cells with and without PS layer are shown in Table 1. The increase of the short circuit current density and the efficiency of the n/p c-Si solar cell with PS layer were obtained.
Such a surface modification allows to reduce the effective reflection coefficient in the wavelength range of 400 /1000 nm (Fig. 6a). This results in the increase of the short circuit current density and the efficiency of the n/ p c-Si solar cell (Table 1). The significant increase of the IR and UV wavelength ranges on the internal quantum efficiency has been observed (Fig. 6b).
4. Conclusions The blue coloured PS layers obtained by the simple stain etching were not homogenous (in macroscopic scale) on the whole emitter surface. The TEM structural characterisation reveals the existence of decreasing porosity in depth of the PS layer on the pyramid slope,
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 5. The transmission electron micrographs of the PS layer formed on the different parts of pyramid for the front surface of silicon solar cells: (a) near the top of pyramid; (b) on the slope of pyramid; (c) region between pyramids.
295
296
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 6. The reflectance of textured monocrystalline silicon without and with the PS layer (a) and internal quantum efficiency of textured monocrystalline silicon solar cell without and with the PS layer (b).
Table 1 Characteristics of solar cells measured at the AM 1.5 global spectrum Solar cells parameters
Initial
Modified by PS layer
IS.C. (A) VOC (V) Fill Factor Efficiency (%)
1.181 0.566 0.735 12.30
1.215 0.579 0.755 13.29
selective properties of PS layer (among others the light trapping in solar cells) has been observed. The technology of PS is simple, as well as cheap and it can be adapted to mass production of solar cells, because simultaneously, antireflection coating and surface passivation can be obtained in one chemical process.
References while AFM */the heterogeneity of the surface morphology of the PS layer at vicinity of pyramids as well as in the regions between pyramids. The PS layer formation on the top textured surface of n/p monocrystalline Si solar cells reduces some of anisotropic texturization disadvantages (mainly presence of untextured regions between pyramids) and contributes to reduction of the reflectance, thus effecting in improvement of solar cell characteristics. A porous surface layer might act as an antireflection coating effecting in the increase of short circuit current and efficiency of solar cell. The gain in short circuit current, open-circuit voltage, fill factor and cell efficiency relative to cell without PS has been obtained for solar cells on textured Cz-Si. The formation of the PS layer removes the highly doped dead layer between the fingers from the front surface of the emitter and it does not attack the metallic contacts and their adherence to the cell. The significant increase in the IR and UV ranges of wavelengths on the internal quantum efficiency due to
[1] P. Menna, G. Di Francia, V. La Ferrara, Solar Energy Mater. Solar Cells 37 (1997) 13. [2] C. Palsule, S. Liu, S. Gangopadhyay, M. Holtz, D. Lamp, M. Kristiansen, Solar Energy Mater. Solar Cells 46 (1997) 261. [3] V. Yerokhov, I. Melnyk, Renewable Sustainable Energy Rev. 3 (1999) 291. [4] S. Strehlke, S. Bastide, J. Guillet, C. Le´vy-Cle´ment, Mater. Sci. Eng. B69 /70 (2000) 81. [5] E. Ponomarev, C. Le´vy-Cle´ment, J. Porous Mater. 7 (2000) 51. [6] E. Ponomarev, C. Le´vy-Cle´ment, Solar Energy Mater. Solar Cells 58 (1999) 399. [7] Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, 23rd IEEE PVSC, Louisville, 1993, p. 287. [8] R.W. Fathauer, T. George, A. Ksendzov, R.P. Vasquez, Appl. Phys. Lett. 60 (1992) 995. [9] P. Vitanov, M. Delibasheva, E. Goranova, Ch. Angelov, V. Dimov, Vacuum 58 (2000) 351. [10] B. Bessais, O. Ben Younes, H. Ezzaouia, N. Mliki, M.F. Boujmil, M. Oueslati, R. Bennaceur, J. Luminescence 90 (2000) 101. [11] A. Ferreira da Silva, R.R. Rosa, P.W.A. Machado, F.M. Ramos, C. Rodrigues Neto, E.L.S. Roman, E. Veje, Physica A (2000) 223. [12] D.L. King, M.E. Buck, Proc. of the 22nd IEEE PVSPC, 1991, p. 303.
Characterization and properties of a modified Si solar cell emitter by a porous Si layer Z. Swiatek a,*, E. Beltowska a, W. Maziarz a, F. Krok b a
b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str., 30-059 Cracow, Poland Regional Laboratory for Physicochemical Analyses and Structural Research, Jagiellonian University, 3 Ingardena Str., 30-060 Cracow, Poland
Abstract Porous Si (PS) has become an interesting material owing to its potential applications in many fields including microelectronics, optoelectronics and photovoltaics. PS layers on the front surface of n//p monocrystalline, textured Si solar cells have been investigated with the aim of improving the performance of standard screen-printed cells, because an antireflection coating and a surface passivation can be obtained simultaneously in one chemical process. The results obtained could be useful in optimising the Si surface chemical treatment process. The surface morphology and microstructure of PS layers were investigated using SEM, TEM and non-contact AFM methods. The surface morphology of a PS layer depends strongly on the region where the pores are formed. The structure of PS layer is composed of macro-pores formed in p type Si (sizes vary over a large range up to 250 nm) and mesopores formed in the n region of the p /n junction. The meso-pores of average size 20 nm on the pyramid slope elongate preferentially along the 111 direction. The interface between the PS layer and the substrate as well as the surface roughness are clearly defined. The results show that the PS layer on the pyramids is formed uniformly along the walls. Meso-pores created on the macro-pore surface are a characteristic feature of the surface between pyramids. Such a surface modification allows improving the Si solar cell characteristics. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Solar cells; Surface morphology; Texturization
1. Introduction Porous silicon (PS) has become a very interesting material owing to its potential applications in many fields like microelectronics, optoelectronics and photovoltaics. Recently, several reports concerning the use of PS in photovoltaic devices have been published [1 /6]. Extremely low reflection coefficients of ARC on the bases of PS were reached, which are comparable with the most efficient double ARC layer of ZnS/MgF2 deposited on pretextured silicon surface by means of expensive vacuum methods [7]. Despite the large number of studies devoted to this question, the problem of commercial application has not been solved yet. The parameters of the used PS layers have not been fully optimised and the adaptation to the technology has not
* Corresponding author. Tel.: /48-12-6374200; fax: /48-126372192. E-mail address: [email protected] (Z. Swiatek).
been satisfactory so far. Nevertheless, it is widely believed that PS can be adapted to mass production of solar cells, because of the simple and cheap technology. It is known that apart from the antireflection properties the PS layer based silicon solar cells have other advantages including a new kind of passivation and a surface texturization (light trapping), a band-gap control (1.5 /1.8 eV) and a solar light transformation from ultraviolet (due to its luminescent properties) to longer wavelengths [8]. The mechanisms of the complex microstructural changes in near surface areas of the silicon wafers occurring during PS layer formation are not yet fully understood. The structural and morphological studies are currently the subject of many investigations, but they concern mainly PS layer formation on the platelet silicon surface [9 /11]. Thus, the presented work is aimed at the investigation of the inhomogenity and microstructure of a thin PS layer formed in the n -type region of the texturized Si. Texturing is one of the most important and difficult processing step in silicon solar
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00718-3
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
292
cell fabrication. The monocrystalline silicon can be textured by alkaline solutions, which etch preferentially the 111 direction, resulting in random pyramid structures. However, production problems still exist including e.g. repeatability, lack of control over pyramid size, etching of pyramid peaks as well as the presence of untextured regions between the pyramids. The PS layers formation on the top textured surface of n/p monocrystalline Si solar cells could reduce some of anisotropic texturization disadvantages.
2. Experimental procedure Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in standard alkaline solution [12]. Then, emitter was thermally generated using POCl3 as phosphorous doped source and BSF (Back Surface Field) was created from boron-emulsion. Both junctions were produced in one technological cycle. The electrical contacts (Ag front and Ag/Al back) were produced by the screen printing process and fired at 700 8C, while the PS layers were formed by electroless chemical etching in solutions containing concentrated HF, HNO3 and some surface active additives. The maximum etching time was 6 s, corresponding to a PS thickness of about 200 nm. The grid contacts of solar cells were not protected before PS formation. For the applied time of stain etching the metallization was not destroyed. The structure of PS layer was investigated using SEM (Philips XL30), TEM (Philips CM20), and AFM (Park Scientific Instruments Autoprobe CP). The photovoltaic characteristics of the solar cells (short-circuit current density ISC, open-circuit voltage VOC, fill factor FF, h efficiency) with and without PS layer were compared.
3. Results It is generally accepted that pore initiation occurs at surface active sites defects or irregularities. The proper-
ties of PS layer (porosity, thickness, pore diameter and microstructure) mainly depend on operating parameters of layer formation, including the HF/HNO3 concentration ratio, the presence of surface active compound additives, the duration and the temperature as well as the wafer type and its resistivity. The investigations have been focused on visualisation techniques, because the optical, electrical and physical properties of PS layer depend strongly on its microstructure. Information about pore size and their shape are not easy to obtain, therefore different experimental techniques such as SEM, TEM and AFM have been used. Fig. 1 shows SEM images of Si solar cell with PS layer on the front and back surface of solar cell. The difference in morphology of the obtained PS layer in both cases is evident. For p-type doped Si (the back surface of the solar cell) the pore sizes vary in the large range from 20 up to 250 nm (Figs. 1 and 2). For n-type doped Si the situation is more complicated. Generally, pore dimensions are smaller than the SEM resolution and they could only be observed by means of TEM and AFM. The surface morphology (SM) of PS layer depends strongly on the area where the pores were formed. That can be seen in Figs. 3 and 4 where non-contact AFM pictures of the PS layer morphology at the vicinity of three pyramids as well as in the regions between pyramids, respectively, are presented. Figs. 3 and 4 show the heterogeneity of the SM of the PS layer. Detailed observations (part B and C of Fig. 3) show that average ‘macro-pores’ of 100 nm formed by meso-pores are characteristic for the wall C, whereas craters of average diameter 200 nm and the meso-pores are seen at the B wall. The characteristic feature of the surface between the pyramids is meso-pores created on the macro-pore surface (part A, B in Fig. 4). TEM images gave further details about the morphology of stain-etched layer. The meso-pores of average size of 20 nm on the pyramid slope elongate preferentially along the 111 direction, as can be seen on the transmission electron micrographs (Fig. 5). The TEM
Fig. 1. The scanning electron micrographs of the front (a) and back (b) surface of silicon solar cell with PS layer.
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 2. The transmission electron micrograph of the front (a) and back (b) surface of silicon solar cells with PS layer.
Fig. 3. Non-contact AFM images of the front surface of silicon solar cell with PS layer on the slope pyramids.
293
294
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 4. Non-contact AFM images of the front surface of silicon solar cell with PS layer in the regions between pyramids.
structural characterisation confirms the existence of a porosity gradient, with decreasing porosity in depth. The pores are randomly distributed and have columnar shapes. A small disorientation of the separate columns takes place. The interface between the porous layer and the substrate as well as the interface roughness is clearly defined. The obtained results show that PS layer on the pyramids is formed uniformly along their walls. For the above investigations the solar cells with the efficiency about 12% were chosen, because larger changes in their parameters, caused by PS were expected. The photovoltaic characteristics of the solar cells with and without PS layer are shown in Table 1. The increase of the short circuit current density and the efficiency of the n/p c-Si solar cell with PS layer were obtained.
Such a surface modification allows to reduce the effective reflection coefficient in the wavelength range of 400 /1000 nm (Fig. 6a). This results in the increase of the short circuit current density and the efficiency of the n/ p c-Si solar cell (Table 1). The significant increase of the IR and UV wavelength ranges on the internal quantum efficiency has been observed (Fig. 6b).
4. Conclusions The blue coloured PS layers obtained by the simple stain etching were not homogenous (in macroscopic scale) on the whole emitter surface. The TEM structural characterisation reveals the existence of decreasing porosity in depth of the PS layer on the pyramid slope,
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 5. The transmission electron micrographs of the PS layer formed on the different parts of pyramid for the front surface of silicon solar cells: (a) near the top of pyramid; (b) on the slope of pyramid; (c) region between pyramids.
295
296
Z. Swiatek et al. / Materials Science and Engineering B101 (2003) 291 /296
Fig. 6. The reflectance of textured monocrystalline silicon without and with the PS layer (a) and internal quantum efficiency of textured monocrystalline silicon solar cell without and with the PS layer (b).
Table 1 Characteristics of solar cells measured at the AM 1.5 global spectrum Solar cells parameters
Initial
Modified by PS layer
IS.C. (A) VOC (V) Fill Factor Efficiency (%)
1.181 0.566 0.735 12.30
1.215 0.579 0.755 13.29
selective properties of PS layer (among others the light trapping in solar cells) has been observed. The technology of PS is simple, as well as cheap and it can be adapted to mass production of solar cells, because simultaneously, antireflection coating and surface passivation can be obtained in one chemical process.
References while AFM */the heterogeneity of the surface morphology of the PS layer at vicinity of pyramids as well as in the regions between pyramids. The PS layer formation on the top textured surface of n/p monocrystalline Si solar cells reduces some of anisotropic texturization disadvantages (mainly presence of untextured regions between pyramids) and contributes to reduction of the reflectance, thus effecting in improvement of solar cell characteristics. A porous surface layer might act as an antireflection coating effecting in the increase of short circuit current and efficiency of solar cell. The gain in short circuit current, open-circuit voltage, fill factor and cell efficiency relative to cell without PS has been obtained for solar cells on textured Cz-Si. The formation of the PS layer removes the highly doped dead layer between the fingers from the front surface of the emitter and it does not attack the metallic contacts and their adherence to the cell. The significant increase in the IR and UV ranges of wavelengths on the internal quantum efficiency due to
[1] P. Menna, G. Di Francia, V. La Ferrara, Solar Energy Mater. Solar Cells 37 (1997) 13. [2] C. Palsule, S. Liu, S. Gangopadhyay, M. Holtz, D. Lamp, M. Kristiansen, Solar Energy Mater. Solar Cells 46 (1997) 261. [3] V. Yerokhov, I. Melnyk, Renewable Sustainable Energy Rev. 3 (1999) 291. [4] S. Strehlke, S. Bastide, J. Guillet, C. Le´vy-Cle´ment, Mater. Sci. Eng. B69 /70 (2000) 81. [5] E. Ponomarev, C. Le´vy-Cle´ment, J. Porous Mater. 7 (2000) 51. [6] E. Ponomarev, C. Le´vy-Cle´ment, Solar Energy Mater. Solar Cells 58 (1999) 399. [7] Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, 23rd IEEE PVSC, Louisville, 1993, p. 287. [8] R.W. Fathauer, T. George, A. Ksendzov, R.P. Vasquez, Appl. Phys. Lett. 60 (1992) 995. [9] P. Vitanov, M. Delibasheva, E. Goranova, Ch. Angelov, V. Dimov, Vacuum 58 (2000) 351. [10] B. Bessais, O. Ben Younes, H. Ezzaouia, N. Mliki, M.F. Boujmil, M. Oueslati, R. Bennaceur, J. Luminescence 90 (2000) 101. [11] A. Ferreira da Silva, R.R. Rosa, P.W.A. Machado, F.M. Ramos, C. Rodrigues Neto, E.L.S. Roman, E. Veje, Physica A (2000) 223. [12] D.L. King, M.E. Buck, Proc. of the 22nd IEEE PVSPC, 1991, p. 303.