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Improved light absorption in thin-film silicon solar cells by integration of silver nanoparticles
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Journal of Non-Crystalline Solids 354 (2008) 2488–2491 www.elsevier.com/locate/jnoncrysol
Improved light absorption in thin-film silicon solar cells by integration of silver nanoparticles E. Moulin a,*, J. Sukmanowski b, P. Luo a, R. Carius a, F.X. Royer c, H. Stiebig a a Research Center Juelich, Institute of Energy Research (Photovoltaics), D-52425 Juelich, Germany Institute of Experimental Physics, Saarland University, P.O. Box 151150, D-66041 Saarbruecken, Germany c Laboratoire de Physique des Milieux Denses (LPMD), Universite´ de Metz, IUT de Thionville – Yutz, F-57 970 Yutz, France b
Available online 31 January 2008
Abstract Silver nanoparticles, produced by thermal evaporation and a subsequent annealing treatment, were integrated at the back side of thinfilm silicon solar cells. Metallic nanoparticles can lead to (i) a strong enhancement of the electric field in their surrounding when they are irradiated by light and (ii) significant scattering of the light when they have the proper diameter (>100 nm). In this study, we investigated the optical properties of two types of substrates, one with large and well separated ellipsoidal silver nanoparticles (with average lateral size of 300 nm), and the other with silver nanostructures connected to each other. Furthermore, these substrates were used as back reflectors in microcrystalline silicon solar cells in substrate (n–i–p) configuration. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 73.40.Jn; 61.46.Df; 73.20.Mf Keywords: Solar cells; Nanoparticles
1. Introduction Due to their unique optical properties, metallic nanoparticles are of particular interest for a large number of applications, such as chemical and biological sensing [1] or optical filtering [2]. For metallic nanoparticles with a diameter between 10 and 100 nm optical absorption, which is accompanied by a strong enhancement of the electric field in the vicinity of the particle, occurs in the visible and the near infrared part of the spectrum. This enhancement is caused by the collective oscillation of the electrons (plasmons), which are excited by incident electromagnetic radiation. Small nanoparticles (with average lateral size of 20 nm) have been integrated in amorphous solar cells. Despite a significant amplification of the Raman intensity, which increases with the electric field, *
Corresponding author. Tel.: +49 (0) 2461 612069; fax: +49 (0) 2461 613735. E-mail address: [email protected] (E. Moulin). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.031
and a reduction of the cell reflectance, even a decrease of the photocurrent has been found for cells with nanoparticles in comparison to cells without nanoparticles [3]. Metallic nanoparticles can also lead to significant light scattering, when they have a proper diameter (>100 nm) [4]. This effect makes them attractive for solar cell applications [5–7]. In this study, two different types of substrate, one with large and well separated Ag nanoparticles (with average lateral sizes of 300 nm) and the other with Ag nanostructures connected to each other, were integrated at the back side of microcrystalline silicon (lc-Si:H) n–i–p solar cells to investigate their influence on the optoelectronic device properties. 2. Experimental details The two types of novel back reflectors with nanoparticles/nanostructures consist of a glass/Ag/TCO/nanoparticles (substrate 1) and a glass/nanostructures/Ag (substrate 2) layer-stacks and were prepared on 10
E. Moulin et al. / Journal of Non-Crystalline Solids 354 (2008) 2488–2491
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10 cm2 corning glass. The Ag layer and the thin transparent and conductive oxide (TCO) layer of ZnO were deposited at room temperature (RT) by rf-sputtering with a layer thickness of 500 nm and 80 nm, respectively. The nanoparticles and nanostructures are obtained by thermal evaporation of a thin Ag nano-layer at a pressure of approximately 10 5 mBar and subsequent annealing treatment at a temperature of 180 °C for several hours. For substrate 1, the thickness of the Ag layer was 20 nm. During annealing treatment, well separated ellipsoidal Ag nanoparticles with average lateral sizes of 300 nm and height of 50 nm, were formed, as shown in the atomic force microscopy (AFM) image (Fig. 1, left). In the case of substrate 2, the Ag layer was directly deposited on glass. The thickness of the nanolayer was varied between 15 nm and 40 nm. After annealing, nanoparticles are formed for Ag nano-layer thinner than 20 nm due to both surface diffusion of Ag and the surface tension of the ellipsoidal spheres. For larger thickness, the Ag nano-layer shows a structure close to the percolation threshold after annealing. In the case of a nano-layer of 40 nm, nanostructures with an average height and lateral size of around 80 nm and 800 nm, respectively and a root mean square roughness of 27 nm. The substrate was subsequently covered with a 500 nm thick Ag layer which roughly follows the morphology of the nanostructures. Afterwards, the lc-Si:H n-, i- and p-layers were deposited by PECVD on the substrates 1 and 2. For comparison, cells with (left) and without (right) nanoparticles were prepared on each substrate. A sketch of the device structures is shown in Fig. 2. The cells have an area of 1 cm2 and the thickness of the lc-Si:H absorber layer is 1 lm. As a front contact, we use a 80 nm thick TCO layer, deposited at 160 °C in combination with an Ag grid, evaporated through a mask. The non-optimized Ag grid design leads to a shading of the cell by approximately 25%. More details concerning the PECVD and sputtering process are given elsewhere [8]. Reflection measurements were carried out in an integral sphere (Ulbricht) on cells without the front grid. For reflection simulations of the glass/Ag/TCO/Ag nano-layer system, bulk properties of the materials were used.
3. Results
Fig. 1. 2 2 lm2 AFM picture of Ag nanoparticles on a glass/Ag/TCO substrate produced by evaporation of a 20 nm Ag nano-layer and subsequent annealing at 180 °C for 90 min (left). 7 7 lm2 AFM scan of a 40 nm Ag nano-layer deposited on a glass substrate after annealing at 180 °C for several hours (right).
Fig. 3. Total (bold lines) and diffuse (thin lines) reflectivity of: (top) large silver nanoparticles on glass/Ag/TCO substrate before (triangles) and after (circles) HCl dip, (bottom) the modified glass/nanostructures/Ag back reflector. The reflectivity of the 20 nm Ag layer on glass/Ag/TCO before annealing is plotted as well (top, line).
Fig. 2. Sketch of two co-deposited lc-Si:H n–i–p solar cells deposited on (left) substrate 1 with and without nanoparticles, (right) substrate 2 with and without nanostructures.
The optical properties of a novel back contact system with ellipsoidal Ag nanoparticles deposited on a glass/ Ag/TCO layer-stack (substrate 1) were investigated before and after annealing (Fig. 3, top). Before annealing, reflection measurements and simulations of a glass/Ag(500 nm)/TCO(80 nm)/Ag(20 nm) layer-stack are in good agreement. The distinct minimum at around 550 nm originates
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from an interference effect and depends on the TCO layer thickness. After annealing, large nanoparticles with a lateral diameter of around 300 nm are formed (Fig. 1, left). The substrate exhibits a strong enhancement of scattered light and a reduction of the total reflection. The latter can be likely attributed to (i) light confinement in the TCO due to the increased fraction of diffuse scattered light or (ii) absorption in or at the nanoparticles. The minimum in the diffuse scattered light at around 550 nm is also caused by the interference effect. By etching the modified back contact system with HCl, the TCO is removed at areas which are not covered by the nanoparticles as verified by SEM measurements (not shown). The nanoparticles act as etching masks. Since their diameter can be varied over a wide range, this approach may be useful for other applications. Reflection measurements before and after the HCl dip are shown in Fig. 3 (top). After etching of the TCO layer, the two maxima in the diffuse scattered light disappear and a single peak at around 500 nm occurs. By removing the TCO (nTCO 2), the optical path length of the light which is reflected at the Ag back reflector is reduced. Thus, the phase shift between the light which is reflected at the nanoparticle surface and the light which is reflected at the Ag back contact, is decreased. According to the Rayleigh criterion for rough surfaces, the fraction of scattered light decreases [9]. On the other hand, the total reflectivity increases in the long wavelength range after etching. This indicates that most of the absorption losses observed for the system glass/Ag/TCO/nanoparticles (substrate 1) originate from the trapping of light in the TCO layer rather than the absorption in or at the nanoparticles. In order to avoid light trapping in the thin TCO layer, we developed the substrate 2. The diffuse part of the light increases with the roughness of the nanostructured surface (thickness of the Ag nano-layer). For an Ag nano-layer thickness of 40 nm, the total reflectivity exceeds 95% from 450 nm to 1300 nm (Fig. 3, bottom) and the diffuse part of the light reaches 95% at 500 nm. The absorption peaks found at 320 nm and 350 nm can be explained by the bulk and surface plasmon absorption of Ag, respectively [10].
Fig. 4. Diffuse (thin lines) and total (bold lines) reflection measurements of cells deposited on: (top) substrate 1 with (circles) and without (lines) nanoparticles, (bottom) substrate 2 with (circles) and without (lines) nanostructures.
4. Discussion The back reflector systems (substrates 1 and 2) were used as substrates for lc-Si:H solar cells (see Fig. 2). Reflection measurements of both types of cell structures show similar trends (Fig. 4). For solar cells with nanoparticles, the fraction of diffuse scattered light (thin lines, with circles) is increased in the whole spectrum range in comparison to cells without nanoparticles (lines). Due to light scattering at the back contact, the light path in the solar cells is enhanced and the total reflection (bold lines with circles) is reduced. Fig. 5 depicts the corresponding quantum efficiency (QE) curves. The QE is increased in the long wavelength range for cells with nanoparticles. For the cell deposited on substrate 2 with nanostructures, a higher blue response is also measured (k < 600 nm). Due to the nearly
Fig. 5. Quantum efficiency of cells based on: (top) substrate 1 with (circles) and without (lines) nanoparticles, (bottom) substrate 2 with (circles) and without (lines) nanostructures. In the bottom, the thin/bold lines represent the quantum efficiencies of cells without/with TCO.
conformal deposition of the thin-film layer-stack the topology of the front surface is similar to the topology of the back reflector. Therefore, the nanostructured substrate 2 can lead to an improved light incoupling, due to the enhanced roughness and the varied shape of its texture [11]. The higher blue response of this cell correlates with
E. Moulin et al. / Journal of Non-Crystalline Solids 354 (2008) 2488–2491
the lower cell reflectivity (Fig. 4, bottom bold line with circles). However, for both types of cells, the reduction of the cell reflectance in the long wavelength range is not totally reflected in QE. Independent of the structure of the Ag containing back contact (smooth Ag layer, small nanoparticles [3], large nanoparticles or nanostructures connected to each other), optical absorption losses are found. Ag directly connected to the silicon layer leads to significant optical losses, as observed also by other groups [10,12–14]. These absorption losses are caused by surface plasmon absorption and are influenced by (i) the nucleation region of the Ag film (for p–i–n cells), (ii) the deposition conditions used for Ag, (iii) the substrate heating step before the thin-film silicon system is deposited and (iv) the surface texture of the substrate [10,15]. The spectral range of the observed absorption losses fits with the losses of individual Ag nanoparticles. Depending on the size of the particles and the embedding material, plasmon absorption losses can be found between 500 nm and 2 lm [16,17]. When the Ag surface is embedded in a material with a smaller refractive index in comparison to silicon (n 4), the surface plasmons of Ag can be shifted to smaller wavelengths and the absorption losses are reduced. Therefore, a thin TCO (n 2) or a dielectric layer can be incorporated between Ag and the silicon layers [18]. For substrate 2, the incorporation of a TCO layer leads to an increase of QE between 500 nm and 700 nm for cells without nanostructures (Fig. 5, bottom bold lines) likely due to a decrease of plasmon absorption in Ag. For cells with nanostructures, the incorporation of a TCO layer leads to an enhanced QE between 500 nm and 1.1 lm which is also caused by a reduction of optical losses in Ag. However in comparison to the flat structure also an enhancement of the QE for longer wavelengths is measured. This can be likely explained by multiple light reflection within the cell due to the enhanced scattering. Furthermore the texture of substrate with nanostructures can behave as large particles. From nanoparticles it is known that the absorption spectrum broadens and shifts to near infrared with increasing particles size [17]. Since substrate 2 with TCO exhibits less plasmon absorption losses and the nanostructured surface shows a higher roughness than the substrates with nanoparticles, it is more suitable for solar cell applications. Despite the increased short circuit current, lc-Si:H cells deposited on substrate with nanostructures covered with TCO show a reduction in both FF and Voc of 7% and 50 mV, respectively. The lower cell performance may originate from an alteration of the lc-Si:H growth on the substrate due to the presence of nanostructures, because aSi:H cells deposited on the same type of back reflector do not show a significant deterioration in FF and Voc. 5. Conclusions The optical properties of two types of back contact systems with well separated ellipsoidal Ag nanoparticles (with
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lateral sizes of 300 nm) and with Ag nanostructures connected to each other were studied. Integrated in lc-Si:H n–i–p solar cells, both reflectors with large nanoparticles/ nanostructures lead to a decrease in the reflectivity in the long wavelength range, due to light scattering and a consequent increase of the quantum efficiency in comparison to cells without nanoparticles/nanostructures. For all types of back reflectors made with Ag (flat surface, substrate with well separated small and large Ag nanoparticles or nanostructures), plasmon absorption losses are always found. The Ag back reflector has to be covered with TCO or a dielectric layer to reduce the optical losses which partly compensate the gain in photocurrent originating from a rough nanotextured substrate. Acknowledgements The authors like to express their gratitude to J. Kirchhoff, W. Reetz, H. Siekmann and M. Huelsbeck for technical assistances. References [1] A.J. Haes, R.P. Van Duyne, Anal. Bioanal. Chem. 379 (2004) 920. [2] Y. Dirix, C. Bastiaansen, W. Caseri, P. Smith, Adv. Mater. 11 (1999) 223. [3] E. Moulin, J. Sukmanowski, F.X. Royer, H. Stiebig, in: 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, 2006, p. 1724. [4] M.A. van Dijk, A.L. Tchebotareva, M. Orrit, M. Lippitz, S. Berciaud, D. Lasne, L. Cognet, B. Lounis, Phys. Chem. Chem. Phys. 8 (2006) 3486. [5] D.M. Schaadt, B. Feng, E.T. Yu, Appl. Phys. Lett. 86 (2005) 063106. [6] D. Derkacs, S.H. Lim, P. Matheu, W. Mar, E.T. Yu, Appl. Phys. Lett. 89 (2006) 093103. [7] S. Pillai, K.R. Catchpole, T. Trupke, G. Zhang, J. Zhao, M.A. Green, Appl. Phys. Lett. 88 (2006) 161102. [8] B. Rech, T. Repmann, M.N. van den Donker, M. Berginski, T. Kilper, J. Hu¨pkes, S. Calnan, H. Stiebig, S. Wieder, Thin Solid Films 548 (2006) 511. [9] H. Stiebig, M. Schulte, C. Zahren, C. Haase, B. Rech, P. Lechner, SPIE (2006) 6197. [10] J. Springer, A. Poruba, L. Mu¨llerova, M. Vanecek, O. Kluth, B. Rech, J. Appl. Phys. 95 (2004) 3. [11] H. Stiebig, C. Haase, C. Zahren, B. Rech, N. Senoussaoui, J. NonCryst. Solids 352 (2006) 1949. [12] S. Yoshida, M. Yoshino, S. Kitahara, K. Seki, S. Katayama, K. Nabeshima, K. Nozue, A. Yamada, M. Konagai, in: 11th E.C. Photovoltaic Solar Energy Conference, 1992, p. 590. [13] H. Stiebig, A. Kreisel, K. Winz, M. Meer, N. Schultz, C. Beneking, Th. Eickhoff, H. Wagner, in: Proceedings of the First World Conference on Photovoltaic Energy Conversion (WCPEC), 1994, p. 603. [14] X. Deng, E.A. Schiff, in: Handbook of Photovoltaic Science and Engineering, 2003, p. 505. [15] D. Sainju, P.J. van den Oever, N.J. Podraza, M. Syed, J.A. Stoke, Jie Chen, Xiesen Yang, Xunming Deng, R.W. Collins, in: 4th World Conference on Photovoltaic Energy (WCPEC), 2006, p. 1732. [16] H. Mertens, J. Verhoeven, A. Polman, F.D. Tichelaar, Appl. Phys. Lett. 85 (2004) 1317. [17] C. Soennichsen, S. Geier, N.E. Hecker, G. von Plessen, J. Feldmann, H. Ditlbacher, B. Lamprecht, J.R. Krenn, F.R. Aussenegg, V.Z.-H. Chan, J.P. Spatz, M. Moeller, Appl. Phys. Lett. 77 (2000) 2949. [18] R.K. Roy, S. Bandyopadhyaya, A.K. Pal, Eur. Phys. J. 39 (2004) 491.
Journal of Non-Crystalline Solids 354 (2008) 2488–2491 www.elsevier.com/locate/jnoncrysol
Improved light absorption in thin-film silicon solar cells by integration of silver nanoparticles E. Moulin a,*, J. Sukmanowski b, P. Luo a, R. Carius a, F.X. Royer c, H. Stiebig a a Research Center Juelich, Institute of Energy Research (Photovoltaics), D-52425 Juelich, Germany Institute of Experimental Physics, Saarland University, P.O. Box 151150, D-66041 Saarbruecken, Germany c Laboratoire de Physique des Milieux Denses (LPMD), Universite´ de Metz, IUT de Thionville – Yutz, F-57 970 Yutz, France b
Available online 31 January 2008
Abstract Silver nanoparticles, produced by thermal evaporation and a subsequent annealing treatment, were integrated at the back side of thinfilm silicon solar cells. Metallic nanoparticles can lead to (i) a strong enhancement of the electric field in their surrounding when they are irradiated by light and (ii) significant scattering of the light when they have the proper diameter (>100 nm). In this study, we investigated the optical properties of two types of substrates, one with large and well separated ellipsoidal silver nanoparticles (with average lateral size of 300 nm), and the other with silver nanostructures connected to each other. Furthermore, these substrates were used as back reflectors in microcrystalline silicon solar cells in substrate (n–i–p) configuration. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 73.40.Jn; 61.46.Df; 73.20.Mf Keywords: Solar cells; Nanoparticles
1. Introduction Due to their unique optical properties, metallic nanoparticles are of particular interest for a large number of applications, such as chemical and biological sensing [1] or optical filtering [2]. For metallic nanoparticles with a diameter between 10 and 100 nm optical absorption, which is accompanied by a strong enhancement of the electric field in the vicinity of the particle, occurs in the visible and the near infrared part of the spectrum. This enhancement is caused by the collective oscillation of the electrons (plasmons), which are excited by incident electromagnetic radiation. Small nanoparticles (with average lateral size of 20 nm) have been integrated in amorphous solar cells. Despite a significant amplification of the Raman intensity, which increases with the electric field, *
Corresponding author. Tel.: +49 (0) 2461 612069; fax: +49 (0) 2461 613735. E-mail address: [email protected] (E. Moulin). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.031
and a reduction of the cell reflectance, even a decrease of the photocurrent has been found for cells with nanoparticles in comparison to cells without nanoparticles [3]. Metallic nanoparticles can also lead to significant light scattering, when they have a proper diameter (>100 nm) [4]. This effect makes them attractive for solar cell applications [5–7]. In this study, two different types of substrate, one with large and well separated Ag nanoparticles (with average lateral sizes of 300 nm) and the other with Ag nanostructures connected to each other, were integrated at the back side of microcrystalline silicon (lc-Si:H) n–i–p solar cells to investigate their influence on the optoelectronic device properties. 2. Experimental details The two types of novel back reflectors with nanoparticles/nanostructures consist of a glass/Ag/TCO/nanoparticles (substrate 1) and a glass/nanostructures/Ag (substrate 2) layer-stacks and were prepared on 10
E. Moulin et al. / Journal of Non-Crystalline Solids 354 (2008) 2488–2491
2489
10 cm2 corning glass. The Ag layer and the thin transparent and conductive oxide (TCO) layer of ZnO were deposited at room temperature (RT) by rf-sputtering with a layer thickness of 500 nm and 80 nm, respectively. The nanoparticles and nanostructures are obtained by thermal evaporation of a thin Ag nano-layer at a pressure of approximately 10 5 mBar and subsequent annealing treatment at a temperature of 180 °C for several hours. For substrate 1, the thickness of the Ag layer was 20 nm. During annealing treatment, well separated ellipsoidal Ag nanoparticles with average lateral sizes of 300 nm and height of 50 nm, were formed, as shown in the atomic force microscopy (AFM) image (Fig. 1, left). In the case of substrate 2, the Ag layer was directly deposited on glass. The thickness of the nanolayer was varied between 15 nm and 40 nm. After annealing, nanoparticles are formed for Ag nano-layer thinner than 20 nm due to both surface diffusion of Ag and the surface tension of the ellipsoidal spheres. For larger thickness, the Ag nano-layer shows a structure close to the percolation threshold after annealing. In the case of a nano-layer of 40 nm, nanostructures with an average height and lateral size of around 80 nm and 800 nm, respectively and a root mean square roughness of 27 nm. The substrate was subsequently covered with a 500 nm thick Ag layer which roughly follows the morphology of the nanostructures. Afterwards, the lc-Si:H n-, i- and p-layers were deposited by PECVD on the substrates 1 and 2. For comparison, cells with (left) and without (right) nanoparticles were prepared on each substrate. A sketch of the device structures is shown in Fig. 2. The cells have an area of 1 cm2 and the thickness of the lc-Si:H absorber layer is 1 lm. As a front contact, we use a 80 nm thick TCO layer, deposited at 160 °C in combination with an Ag grid, evaporated through a mask. The non-optimized Ag grid design leads to a shading of the cell by approximately 25%. More details concerning the PECVD and sputtering process are given elsewhere [8]. Reflection measurements were carried out in an integral sphere (Ulbricht) on cells without the front grid. For reflection simulations of the glass/Ag/TCO/Ag nano-layer system, bulk properties of the materials were used.
3. Results
Fig. 1. 2 2 lm2 AFM picture of Ag nanoparticles on a glass/Ag/TCO substrate produced by evaporation of a 20 nm Ag nano-layer and subsequent annealing at 180 °C for 90 min (left). 7 7 lm2 AFM scan of a 40 nm Ag nano-layer deposited on a glass substrate after annealing at 180 °C for several hours (right).
Fig. 3. Total (bold lines) and diffuse (thin lines) reflectivity of: (top) large silver nanoparticles on glass/Ag/TCO substrate before (triangles) and after (circles) HCl dip, (bottom) the modified glass/nanostructures/Ag back reflector. The reflectivity of the 20 nm Ag layer on glass/Ag/TCO before annealing is plotted as well (top, line).
Fig. 2. Sketch of two co-deposited lc-Si:H n–i–p solar cells deposited on (left) substrate 1 with and without nanoparticles, (right) substrate 2 with and without nanostructures.
The optical properties of a novel back contact system with ellipsoidal Ag nanoparticles deposited on a glass/ Ag/TCO layer-stack (substrate 1) were investigated before and after annealing (Fig. 3, top). Before annealing, reflection measurements and simulations of a glass/Ag(500 nm)/TCO(80 nm)/Ag(20 nm) layer-stack are in good agreement. The distinct minimum at around 550 nm originates
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E. Moulin et al. / Journal of Non-Crystalline Solids 354 (2008) 2488–2491
from an interference effect and depends on the TCO layer thickness. After annealing, large nanoparticles with a lateral diameter of around 300 nm are formed (Fig. 1, left). The substrate exhibits a strong enhancement of scattered light and a reduction of the total reflection. The latter can be likely attributed to (i) light confinement in the TCO due to the increased fraction of diffuse scattered light or (ii) absorption in or at the nanoparticles. The minimum in the diffuse scattered light at around 550 nm is also caused by the interference effect. By etching the modified back contact system with HCl, the TCO is removed at areas which are not covered by the nanoparticles as verified by SEM measurements (not shown). The nanoparticles act as etching masks. Since their diameter can be varied over a wide range, this approach may be useful for other applications. Reflection measurements before and after the HCl dip are shown in Fig. 3 (top). After etching of the TCO layer, the two maxima in the diffuse scattered light disappear and a single peak at around 500 nm occurs. By removing the TCO (nTCO 2), the optical path length of the light which is reflected at the Ag back reflector is reduced. Thus, the phase shift between the light which is reflected at the nanoparticle surface and the light which is reflected at the Ag back contact, is decreased. According to the Rayleigh criterion for rough surfaces, the fraction of scattered light decreases [9]. On the other hand, the total reflectivity increases in the long wavelength range after etching. This indicates that most of the absorption losses observed for the system glass/Ag/TCO/nanoparticles (substrate 1) originate from the trapping of light in the TCO layer rather than the absorption in or at the nanoparticles. In order to avoid light trapping in the thin TCO layer, we developed the substrate 2. The diffuse part of the light increases with the roughness of the nanostructured surface (thickness of the Ag nano-layer). For an Ag nano-layer thickness of 40 nm, the total reflectivity exceeds 95% from 450 nm to 1300 nm (Fig. 3, bottom) and the diffuse part of the light reaches 95% at 500 nm. The absorption peaks found at 320 nm and 350 nm can be explained by the bulk and surface plasmon absorption of Ag, respectively [10].
Fig. 4. Diffuse (thin lines) and total (bold lines) reflection measurements of cells deposited on: (top) substrate 1 with (circles) and without (lines) nanoparticles, (bottom) substrate 2 with (circles) and without (lines) nanostructures.
4. Discussion The back reflector systems (substrates 1 and 2) were used as substrates for lc-Si:H solar cells (see Fig. 2). Reflection measurements of both types of cell structures show similar trends (Fig. 4). For solar cells with nanoparticles, the fraction of diffuse scattered light (thin lines, with circles) is increased in the whole spectrum range in comparison to cells without nanoparticles (lines). Due to light scattering at the back contact, the light path in the solar cells is enhanced and the total reflection (bold lines with circles) is reduced. Fig. 5 depicts the corresponding quantum efficiency (QE) curves. The QE is increased in the long wavelength range for cells with nanoparticles. For the cell deposited on substrate 2 with nanostructures, a higher blue response is also measured (k < 600 nm). Due to the nearly
Fig. 5. Quantum efficiency of cells based on: (top) substrate 1 with (circles) and without (lines) nanoparticles, (bottom) substrate 2 with (circles) and without (lines) nanostructures. In the bottom, the thin/bold lines represent the quantum efficiencies of cells without/with TCO.
conformal deposition of the thin-film layer-stack the topology of the front surface is similar to the topology of the back reflector. Therefore, the nanostructured substrate 2 can lead to an improved light incoupling, due to the enhanced roughness and the varied shape of its texture [11]. The higher blue response of this cell correlates with
E. Moulin et al. / Journal of Non-Crystalline Solids 354 (2008) 2488–2491
the lower cell reflectivity (Fig. 4, bottom bold line with circles). However, for both types of cells, the reduction of the cell reflectance in the long wavelength range is not totally reflected in QE. Independent of the structure of the Ag containing back contact (smooth Ag layer, small nanoparticles [3], large nanoparticles or nanostructures connected to each other), optical absorption losses are found. Ag directly connected to the silicon layer leads to significant optical losses, as observed also by other groups [10,12–14]. These absorption losses are caused by surface plasmon absorption and are influenced by (i) the nucleation region of the Ag film (for p–i–n cells), (ii) the deposition conditions used for Ag, (iii) the substrate heating step before the thin-film silicon system is deposited and (iv) the surface texture of the substrate [10,15]. The spectral range of the observed absorption losses fits with the losses of individual Ag nanoparticles. Depending on the size of the particles and the embedding material, plasmon absorption losses can be found between 500 nm and 2 lm [16,17]. When the Ag surface is embedded in a material with a smaller refractive index in comparison to silicon (n 4), the surface plasmons of Ag can be shifted to smaller wavelengths and the absorption losses are reduced. Therefore, a thin TCO (n 2) or a dielectric layer can be incorporated between Ag and the silicon layers [18]. For substrate 2, the incorporation of a TCO layer leads to an increase of QE between 500 nm and 700 nm for cells without nanostructures (Fig. 5, bottom bold lines) likely due to a decrease of plasmon absorption in Ag. For cells with nanostructures, the incorporation of a TCO layer leads to an enhanced QE between 500 nm and 1.1 lm which is also caused by a reduction of optical losses in Ag. However in comparison to the flat structure also an enhancement of the QE for longer wavelengths is measured. This can be likely explained by multiple light reflection within the cell due to the enhanced scattering. Furthermore the texture of substrate with nanostructures can behave as large particles. From nanoparticles it is known that the absorption spectrum broadens and shifts to near infrared with increasing particles size [17]. Since substrate 2 with TCO exhibits less plasmon absorption losses and the nanostructured surface shows a higher roughness than the substrates with nanoparticles, it is more suitable for solar cell applications. Despite the increased short circuit current, lc-Si:H cells deposited on substrate with nanostructures covered with TCO show a reduction in both FF and Voc of 7% and 50 mV, respectively. The lower cell performance may originate from an alteration of the lc-Si:H growth on the substrate due to the presence of nanostructures, because aSi:H cells deposited on the same type of back reflector do not show a significant deterioration in FF and Voc. 5. Conclusions The optical properties of two types of back contact systems with well separated ellipsoidal Ag nanoparticles (with
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lateral sizes of 300 nm) and with Ag nanostructures connected to each other were studied. Integrated in lc-Si:H n–i–p solar cells, both reflectors with large nanoparticles/ nanostructures lead to a decrease in the reflectivity in the long wavelength range, due to light scattering and a consequent increase of the quantum efficiency in comparison to cells without nanoparticles/nanostructures. For all types of back reflectors made with Ag (flat surface, substrate with well separated small and large Ag nanoparticles or nanostructures), plasmon absorption losses are always found. The Ag back reflector has to be covered with TCO or a dielectric layer to reduce the optical losses which partly compensate the gain in photocurrent originating from a rough nanotextured substrate. Acknowledgements The authors like to express their gratitude to J. Kirchhoff, W. Reetz, H. Siekmann and M. Huelsbeck for technical assistances. References [1] A.J. Haes, R.P. Van Duyne, Anal. Bioanal. Chem. 379 (2004) 920. [2] Y. Dirix, C. Bastiaansen, W. Caseri, P. Smith, Adv. Mater. 11 (1999) 223. [3] E. Moulin, J. Sukmanowski, F.X. Royer, H. Stiebig, in: 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, 2006, p. 1724. [4] M.A. van Dijk, A.L. Tchebotareva, M. Orrit, M. Lippitz, S. Berciaud, D. Lasne, L. Cognet, B. Lounis, Phys. Chem. Chem. Phys. 8 (2006) 3486. [5] D.M. Schaadt, B. Feng, E.T. Yu, Appl. Phys. Lett. 86 (2005) 063106. [6] D. Derkacs, S.H. Lim, P. Matheu, W. Mar, E.T. Yu, Appl. Phys. Lett. 89 (2006) 093103. [7] S. Pillai, K.R. Catchpole, T. Trupke, G. Zhang, J. Zhao, M.A. Green, Appl. Phys. Lett. 88 (2006) 161102. [8] B. Rech, T. Repmann, M.N. van den Donker, M. Berginski, T. Kilper, J. Hu¨pkes, S. Calnan, H. Stiebig, S. Wieder, Thin Solid Films 548 (2006) 511. [9] H. Stiebig, M. Schulte, C. Zahren, C. Haase, B. Rech, P. Lechner, SPIE (2006) 6197. [10] J. Springer, A. Poruba, L. Mu¨llerova, M. Vanecek, O. Kluth, B. Rech, J. Appl. Phys. 95 (2004) 3. [11] H. Stiebig, C. Haase, C. Zahren, B. Rech, N. Senoussaoui, J. NonCryst. Solids 352 (2006) 1949. [12] S. Yoshida, M. Yoshino, S. Kitahara, K. Seki, S. Katayama, K. Nabeshima, K. Nozue, A. Yamada, M. Konagai, in: 11th E.C. Photovoltaic Solar Energy Conference, 1992, p. 590. [13] H. Stiebig, A. Kreisel, K. Winz, M. Meer, N. Schultz, C. Beneking, Th. Eickhoff, H. Wagner, in: Proceedings of the First World Conference on Photovoltaic Energy Conversion (WCPEC), 1994, p. 603. [14] X. Deng, E.A. Schiff, in: Handbook of Photovoltaic Science and Engineering, 2003, p. 505. [15] D. Sainju, P.J. van den Oever, N.J. Podraza, M. Syed, J.A. Stoke, Jie Chen, Xiesen Yang, Xunming Deng, R.W. Collins, in: 4th World Conference on Photovoltaic Energy (WCPEC), 2006, p. 1732. [16] H. Mertens, J. Verhoeven, A. Polman, F.D. Tichelaar, Appl. Phys. Lett. 85 (2004) 1317. [17] C. Soennichsen, S. Geier, N.E. Hecker, G. von Plessen, J. Feldmann, H. Ditlbacher, B. Lamprecht, J.R. Krenn, F.R. Aussenegg, V.Z.-H. Chan, J.P. Spatz, M. Moeller, Appl. Phys. Lett. 77 (2000) 2949. [18] R.K. Roy, S. Bandyopadhyaya, A.K. Pal, Eur. Phys. J. 39 (2004) 491.