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porous silicon thin films for plasmonic solar cells
Accepted Manuscript Viewpoint paper Hybrid gold/porous silicon thin films for plasmonic solar cells S. Sánchez de la Morena, G. Recio-Sánchez, V. Torres-Costa, R.J. Martín-Palma PII: DOI: Reference:
S1359-6462(13)00325-4 http://dx.doi.org/10.1016/j.scriptamat.2013.06.015 SMM 9966
To appear in:
Scripta Materialia
Please cite this article as: S. Sánchez de la Morena, G. Recio-Sánchez, V. Torres-Costa, R.J. Martín-Palma, Hybrid gold/porous silicon thin films for plasmonic solar cells, Scripta Materialia (2013), doi: http://dx.doi.org/10.1016/ j.scriptamat.2013.06.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hybrid gold/porous silicon thin films for plasmonic solar cells. S. Sánchez de la Morena, G. Recio-Sánchez, V. Torres-Costa and R. J. Martín-Palma Departamento de Física Aplicada, Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spain
Abstract
Here we present a study on the viability of using gold nanoparticles infiltrated into porous silicon (PS) aiming at the development of plasmonic thin-film solar cells. For this objective, hybrid structures consisting of PS thin films and gold nanoparticles were fabricated. Optical, electrical, and photocurrent measurements were carried out, from which it was found that the presence of gold nanoparticles into PS results in increased responsivity. This effect can be associated to increased light absorption and increased carrier collection efficiency.
Keywords: Porous silicon; Gold; Nanostructure; Plasmonics; Photocurrent; Responsivity; Solar cell.
Given the abundance of silicon in earth‟s crust, together with its long-term stability, wellestablished technology (which has its origin in semiconductor industry), and relatively low cost, silicon is and probably will remain the dominant photovoltaic material [1]. During the past years, silicon thin-film solar cells have attracted wide attention mainly due to a significant cost advantage over their bulk crystalline counterparts. In fact, around 40% of the cost of a solar module made from crystalline silicon is the cost of the silicon wafers [2]. However, a major problem of Si thin-film solar cells is related to ineffective light absorption given that Si is an indirect-bandgap semiconductor and low carrier collection efficiency due to short carrier diffusion lengths [3].
In order to increase the overall efficiency of thin-film solar cells, boosting light absorption while still keeping the active layer thin would be an optimal solution. Among others, an attractive method to achieve this objective is to take advantage of light scattering from metal nanoparticles near their localized plasmon resonance frequency [2]. Plasmon resonance can be defined as a collective oscillation of the conduction electrons in the metal. Incident light of wavelength close to the resonance wavelength of metal particles is strongly scattered or absorbed depending on the size, shape, and material of the particles, as well as on the refractive index of the surrounding medium [4]. Accordingly, light absorption can be enhanced by engineering of metallic nanostructures [5], which is a key factor in the case of thin-film solar cells. Photon absorption can be increased by optimizing the coupling between the absorbing layer and incident light, especially in the spectral range where a given material shows weak absorption [6]. Photocurrent generation has been experimentally found to improve by placing or scattering metallic nanoparticles on the top of, within, or at the bottom of photovoltaic devices [5].
The particular morphology and physico-chemical properties of porous silicon (PS) greatly depend on the fabrication process and materials [7]. In particular, pore dimensions can be precisely controlled and are highly tunable from sizes below 2 nm to several microns [8]. Additionally, porosity has been reported to vary from about 1% to 95% [9]. Moreover, unlike any other porous material, patterns of porosity can also be generated laterally and vertically. Electrochemical etching of monocrystalline silicon wafers in HF-based solutions is the most widely used and versatile method for the fabrication of porous silicon. Given its good optical properties, PS has been used in a wide range of photonic applications (see for example references 10 and 11). In addition to its tunable properties, the relatively simplicity and low cost of PS processing, make PS a promising material for its use in the field of solar cells. However, in the field of photovoltaics PS has seen limited applications and has been mainly used as an antireflection coating in silicon-based solar cells [12,13,14,15].
In this work hybrid structures consisting of porous silicon thin films and gold nanoparticles were fabricated by a combination of electrochemical etching and electrodeposition. The electrical and optical behavior of these structures was determined. Additionally, the spectral behavior of the photocurrent generated by these devices was measured from which the spectral responsivity was determined.
Aluminum thin films (about 1 micron thick) were deposited by electron beam evaporation on the back side of boron-doped (p-type) monocrystalline Si wafers (orientation <100> and resistivity 0.01-0.02 Ω∙cm). In order to create low-resistivity ohmic contacts, annealing at 400ºC for 5 min. was performed. Porous silicon (PS) thin films were fabricated by the electrochemical etching of silicon wafers into HF(48%wt.):ethanol (1:2 concentration in volume) solutions. The anodization time was 4 seconds under a constant current density of 60 mA·cm-2.
Electrodeposition of gold nanoparticles into PS was performed immediately after the formation of the porous thin films following an experimental method previously described [16,17]. For this task, a solution consisting of HAuCl4, 0.42 M Na2S2O3, and 0.42 M Na2SO3 was used. The concentration of HAuCl4 was 1 mM, the current 1 µA·cm-2, and the duration of the immersion/deposition step was 60 s and 600 s. The overall fabrication process results in hybrid Al/Si/PS+Au structures which are named Al/Si/PS+Au(60 s) and Al/Si/PS+Au(600 s), depending on the length of the immersion/deposition step.
Finally, semitransparent indium-tin oxide (ITO) layers were grown on top of the PS thin films by magnetron sputtering, at a typical pressure of 5×10-3 Torr. The sputtering time was fixed at 60 minutes, resulting in layers typically 0.5 µm thick. After the deposition process, the structures were subjected to rapid thermal processing at 550ºC for 600 s in vacuum aiming at improving the conductivity and transparency of the ITO layers.
The previously described process results in a four-layer structure schematically depicted in Figure 1.
Scanning Electron Microscopy (SEM) images of the different structures were obtained using a Philips XL30 S field emission microscope operated at 10 kV.
Electrical characterization, i.e., measurement of the current-voltage (I-V) curves, was carried out in the dark by using a Hewlett Packard pA meter/dc voltage source, Model 4140B. The reflectance spectra in the 350-900 nm wavelength range were taken using a Jasco V-560 double beam spectrophotometer equipped with an integrating sphere, which avoids scattering losses. The photometric accuracy was better than 0.3 %.
Photocurrent measurements were carried out at 0 V bias using a dual digital lock-in amplifier (Signal Recovery 7225) at a chopper frequency of 300 Hz. Illumination was provided by an Acton Research Corporation Tungsten-Deuterium dual light source (model TDS-429) and a SpectraPro 150 monochromator equipped with three interchangeable diffraction gratings (1200 lines/mm) was used to select the wavelength.
Field-emission scanning electron microscopy (FESEM) was used to directly analyze the morphology of the PS thin films and ITO layers, as well as the characteristic dimensions of the gold nanoparticles electrodeposited into the porous structure. For this purpose, the structure of the samples was analyzed at different stages of the fabrication process. Figure 2(top) shows a cross-sectional view of the typical morphology of PS layers before electrodeposition of gold nanoparticles. It is observed that the thickness of the PS thin films, which show a columnar structure, is about 180 nm with typical pore diameter ranging from around 30 to 60 nm. This particular thickness was selected for the PS thin films in order to create quarter wavelength anti-reflection coatings in the near infrared. It is worth noting that the electrochemical fabrication process used throughout this work provides good control over the etching process resulting in good reproducibility and homogeneous thickness. Figures 2(middle) and 2(bottom) show cross-sectional views of the PS thin films upon electrodeposition of gold nanoparticles for two different times: 60 and 600 seconds. FESEM analyses demonstrate that the gold nanoparticles show a spherical shape with characteristic dimensions in the tens of nanometers range.
Figure 3 shows cross-sectional images of the Si/PS/ITO areas of the four-layer structures. In the case of Figure 4(top), gold was not electrodeposited into the PS thin film. In the case of Figures 4(middle) and 4(bottom), electrodeposition of gold nanoparticles was performed at 60 and 600 seconds respectively. It is observed that the diffusion of ITO into the porous structure is negligible in all cases.
In the following the effect of the addition of gold nanoparticles to the PS thin films on the optical, electrical and photocurrent generation properties of the different devices will be studied and discussed. As such, for the measurement of reflectance, current-voltage characteristics and photocurrent, an indium tin oxide (ITO) layer was deposited on top of the hybrid gold/PS structures, leading to Al/Si/PS/ITO and Al/Si/PS+Au/ITO structures such as those shown in Figure 3.
Aiming at determining the optical properties of the different PS-based devices (before and after the electrodeposition of gold nanoparticles into the PS thin films at two different times, 60 and 600 s), their characteristic reflectance (R) spectra were measured in the 300 to 900 nm wavelength range. The experimental results are plotted in Figure 4, from which a typical interference pattern for the Al/Si/PS/ITO structures is observed. It is also found that the typical interference pattern of these structures is lost upon electrodeposition of gold nanoparticles into PS, indicating strong absorption within the PS thin films. Additionally, a reduction of the average reflectance after electrodeposition of gold nanoparticles into PS is clearly observed. Increased light absorption is a key factor to increase the efficiency of the PS-based devices. The average reflectance in the 300 to 900 nm wavelength range for the Al/Si/PS/ITO structures is around 12.7 %. This value is notably reduced when Au nanoparticles are grown into the porous structure: 5.9 % for Al/Si/PS+Au(60 s)/ITO structures and 6.1 % for Al/Si/PS+Au(600 s)/ITO structures. However, increased Au nanoparticle deposition time does not lead to reduced average reflectance but results in a shift of the main reflectance peak. Finally, after electrodeposition of gold nanoparticles a broad absorption band is observed rather that well-defined absorption peaks. This effect is most likely due to a relatively large dispersion in size of the gold nanoparticles, given the strong effect that size has in optical absorption.
Current–voltage (I–V) measurements of the Al/Si/PS/ITO and Al/Si/PS+Au/ITO structures described previously were taken in the -5.5 to 2.5 V range in order to determine their electrical conduction behavior. The results are shown in Figure 5. In the case of the Al/Si/PS/ITO structures a rectifying behavior is observed, which is consistent with our previous studies in which we have determined that this type of structures show a similar behavior to that of metal–insulator–semiconductor (MIS) diodes [18]. Additionally, the experimental results show that the two Al/Si/PS+Au/ITO structures show an almost symmetrical rectifying behavior for forward and reverse biasing (Figure 5). Furthermore, a direct relationship between gold deposition time and electric conductance is clearly observed: the larger the concentration of gold nanoparticles, the larger current for a given voltage, i.e., conductivity.
Photocurrent was measured for the Al/Si/PS/ITO and the hybrid Al/Si/PS+Au/ITO structures and from the experimental results the spectral responsivity was determined (Figure 6). For
this task, the photocurrent spectra were divided by the spectral emissive power of the light source. The current photogenerated by the Al/Si/PS/ITO structures increases with increased photon energy and varies between about 10-6 and 9×10-4 A/W. These values are comparable to previous results using multilayer PS structures [19]. As Figure 6 shows, the electrodeposition of gold nanoparticles into the porous silicon thin films results in an overall increment of the photocurrent generated by the device. Moreover, it is observed that the deposition time of gold nanoparticles into PS has a direct effect in the responsivity of the PSbased devices, i.e., the longer the electrodeposition time, the larger the photocurrent generated for a given wavelength. Accordingly, this behavior is directly related to the presence of a larger amount of gold into PS (see Figures 2 and 3). At wavelengths around the point of maximum of solar irradiance (550 nm), the Al/Si/PS+Au/ITO structures for which the electrodeposition time was 60 s produce a ten-time increment over the photocurrent generated by PS thin films. In the case of Al/Si/PS+Au structures for which the electrodeposition time was 600 s, the photocurrent measured under the same experimental conditions is 200 times larger. It is worth noting that although the photocurrent measured for Al/Si/PS+Au(600 s)/ITO structures is much larger than that of Al/Si/PS+Au(60 s)/ITO structures, reflectance remains almost the same. Accordingly increased photocurrent generation cannot be associated just to improved optical performance (larger absorption), but to enhanced carrier collection.
We have investigated the effect of the addition of gold nanoparticles on the light absorption efficiency of porous silicon (PS) thin films. For this task, gold nanoparticles were electrodeposited into PS thin films at two very different times, resulting in different concentration of gold nanoparticles into the porous layer. The preliminary experimental results indicate that the presence of gold nanoparticles results in increased photocurrent generated, and that photocurrent increases for increasing concentration of nanoparticles.
The experimental results suggest that increased photocurrent generation in the Al/Si/PS+Au/ITO structures can be associated to (i) increased light absorption (as deduced from reduced reflectance) and (ii) increased conductivity given by the presence of metal nanoparticles, most likely given by improved carrier collection efficiency. The relative importance of each of these two mechanisms has yet to be determined in a subsequent study, as well as the effect of pore size and gold nanoparticle size and shape. Anyway, from reflectance measurements it was determined that a larger amount of gold nanoparticles into
porous silicon does not lead to increased light absorption. Additionally, the thickness of the porous silicon thin films has to be adjusted to further reduce the reflectance thus leading to increased overall efficiency of the PS-based devices.
Photovoltaics is a field in constant evolution, being the reduction of the cost per watt probably the major challenge in solar cell research aiming at making the price of solargenerated electricity lower than that of electricity generated by conventional sources of energy. As stated in the introduction, material costs represent a large part of the expense. Thin-film solar cell technology (i.e., second generation solar cells) has emerged as a way to reduce the amount of material required, although higher light absorption is a key factor. In this regard, the use of plasmonic nanostructures constitutes a powerful approach to improve both absorption of light and carrier collection in thin film solar cells, reducing at the same the overall manufacturing costs.
However, fabricating submicrometric metal structures generally involves the use of complex (and expensive) processing techniques, such as electron-beam lithography or focused ionbeam milling. These nanofabrication techniques might be difficult to implement over large surfaces, as required for solar cell manufacture. As such, it is highly desirable to achieve the proposed goal of costs reduction to identify a fabrication technique that would allow the inexpensive fabrication of metal nanostructures over large areas. At the same time, this nanofabrication technique has to be versatile enough to be used in combination with substrates of very different nature, including glasses and polymers. In the particular case of silicon solar cells, which constitute around 90% of the market share, we have demonstrated that hybrid metal/porous silicon thin films meet the basic physical and manufacturing requirements.
Acknowledgements
The authors gratefully acknowledge funding from Comunidad de Madrid under project „MICROSERES‟ and Ministerio Economía y Competitividad under research project MAT2011-28345-C02-01.
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Figure 3-middle
Figure 3-bottom
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Figure 6
S1359-6462(13)00325-4 http://dx.doi.org/10.1016/j.scriptamat.2013.06.015 SMM 9966
To appear in:
Scripta Materialia
Please cite this article as: S. Sánchez de la Morena, G. Recio-Sánchez, V. Torres-Costa, R.J. Martín-Palma, Hybrid gold/porous silicon thin films for plasmonic solar cells, Scripta Materialia (2013), doi: http://dx.doi.org/10.1016/ j.scriptamat.2013.06.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hybrid gold/porous silicon thin films for plasmonic solar cells. S. Sánchez de la Morena, G. Recio-Sánchez, V. Torres-Costa and R. J. Martín-Palma Departamento de Física Aplicada, Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spain
Abstract
Here we present a study on the viability of using gold nanoparticles infiltrated into porous silicon (PS) aiming at the development of plasmonic thin-film solar cells. For this objective, hybrid structures consisting of PS thin films and gold nanoparticles were fabricated. Optical, electrical, and photocurrent measurements were carried out, from which it was found that the presence of gold nanoparticles into PS results in increased responsivity. This effect can be associated to increased light absorption and increased carrier collection efficiency.
Keywords: Porous silicon; Gold; Nanostructure; Plasmonics; Photocurrent; Responsivity; Solar cell.
Given the abundance of silicon in earth‟s crust, together with its long-term stability, wellestablished technology (which has its origin in semiconductor industry), and relatively low cost, silicon is and probably will remain the dominant photovoltaic material [1]. During the past years, silicon thin-film solar cells have attracted wide attention mainly due to a significant cost advantage over their bulk crystalline counterparts. In fact, around 40% of the cost of a solar module made from crystalline silicon is the cost of the silicon wafers [2]. However, a major problem of Si thin-film solar cells is related to ineffective light absorption given that Si is an indirect-bandgap semiconductor and low carrier collection efficiency due to short carrier diffusion lengths [3].
In order to increase the overall efficiency of thin-film solar cells, boosting light absorption while still keeping the active layer thin would be an optimal solution. Among others, an attractive method to achieve this objective is to take advantage of light scattering from metal nanoparticles near their localized plasmon resonance frequency [2]. Plasmon resonance can be defined as a collective oscillation of the conduction electrons in the metal. Incident light of wavelength close to the resonance wavelength of metal particles is strongly scattered or absorbed depending on the size, shape, and material of the particles, as well as on the refractive index of the surrounding medium [4]. Accordingly, light absorption can be enhanced by engineering of metallic nanostructures [5], which is a key factor in the case of thin-film solar cells. Photon absorption can be increased by optimizing the coupling between the absorbing layer and incident light, especially in the spectral range where a given material shows weak absorption [6]. Photocurrent generation has been experimentally found to improve by placing or scattering metallic nanoparticles on the top of, within, or at the bottom of photovoltaic devices [5].
The particular morphology and physico-chemical properties of porous silicon (PS) greatly depend on the fabrication process and materials [7]. In particular, pore dimensions can be precisely controlled and are highly tunable from sizes below 2 nm to several microns [8]. Additionally, porosity has been reported to vary from about 1% to 95% [9]. Moreover, unlike any other porous material, patterns of porosity can also be generated laterally and vertically. Electrochemical etching of monocrystalline silicon wafers in HF-based solutions is the most widely used and versatile method for the fabrication of porous silicon. Given its good optical properties, PS has been used in a wide range of photonic applications (see for example references 10 and 11). In addition to its tunable properties, the relatively simplicity and low cost of PS processing, make PS a promising material for its use in the field of solar cells. However, in the field of photovoltaics PS has seen limited applications and has been mainly used as an antireflection coating in silicon-based solar cells [12,13,14,15].
In this work hybrid structures consisting of porous silicon thin films and gold nanoparticles were fabricated by a combination of electrochemical etching and electrodeposition. The electrical and optical behavior of these structures was determined. Additionally, the spectral behavior of the photocurrent generated by these devices was measured from which the spectral responsivity was determined.
Aluminum thin films (about 1 micron thick) were deposited by electron beam evaporation on the back side of boron-doped (p-type) monocrystalline Si wafers (orientation <100> and resistivity 0.01-0.02 Ω∙cm). In order to create low-resistivity ohmic contacts, annealing at 400ºC for 5 min. was performed. Porous silicon (PS) thin films were fabricated by the electrochemical etching of silicon wafers into HF(48%wt.):ethanol (1:2 concentration in volume) solutions. The anodization time was 4 seconds under a constant current density of 60 mA·cm-2.
Electrodeposition of gold nanoparticles into PS was performed immediately after the formation of the porous thin films following an experimental method previously described [16,17]. For this task, a solution consisting of HAuCl4, 0.42 M Na2S2O3, and 0.42 M Na2SO3 was used. The concentration of HAuCl4 was 1 mM, the current 1 µA·cm-2, and the duration of the immersion/deposition step was 60 s and 600 s. The overall fabrication process results in hybrid Al/Si/PS+Au structures which are named Al/Si/PS+Au(60 s) and Al/Si/PS+Au(600 s), depending on the length of the immersion/deposition step.
Finally, semitransparent indium-tin oxide (ITO) layers were grown on top of the PS thin films by magnetron sputtering, at a typical pressure of 5×10-3 Torr. The sputtering time was fixed at 60 minutes, resulting in layers typically 0.5 µm thick. After the deposition process, the structures were subjected to rapid thermal processing at 550ºC for 600 s in vacuum aiming at improving the conductivity and transparency of the ITO layers.
The previously described process results in a four-layer structure schematically depicted in Figure 1.
Scanning Electron Microscopy (SEM) images of the different structures were obtained using a Philips XL30 S field emission microscope operated at 10 kV.
Electrical characterization, i.e., measurement of the current-voltage (I-V) curves, was carried out in the dark by using a Hewlett Packard pA meter/dc voltage source, Model 4140B. The reflectance spectra in the 350-900 nm wavelength range were taken using a Jasco V-560 double beam spectrophotometer equipped with an integrating sphere, which avoids scattering losses. The photometric accuracy was better than 0.3 %.
Photocurrent measurements were carried out at 0 V bias using a dual digital lock-in amplifier (Signal Recovery 7225) at a chopper frequency of 300 Hz. Illumination was provided by an Acton Research Corporation Tungsten-Deuterium dual light source (model TDS-429) and a SpectraPro 150 monochromator equipped with three interchangeable diffraction gratings (1200 lines/mm) was used to select the wavelength.
Field-emission scanning electron microscopy (FESEM) was used to directly analyze the morphology of the PS thin films and ITO layers, as well as the characteristic dimensions of the gold nanoparticles electrodeposited into the porous structure. For this purpose, the structure of the samples was analyzed at different stages of the fabrication process. Figure 2(top) shows a cross-sectional view of the typical morphology of PS layers before electrodeposition of gold nanoparticles. It is observed that the thickness of the PS thin films, which show a columnar structure, is about 180 nm with typical pore diameter ranging from around 30 to 60 nm. This particular thickness was selected for the PS thin films in order to create quarter wavelength anti-reflection coatings in the near infrared. It is worth noting that the electrochemical fabrication process used throughout this work provides good control over the etching process resulting in good reproducibility and homogeneous thickness. Figures 2(middle) and 2(bottom) show cross-sectional views of the PS thin films upon electrodeposition of gold nanoparticles for two different times: 60 and 600 seconds. FESEM analyses demonstrate that the gold nanoparticles show a spherical shape with characteristic dimensions in the tens of nanometers range.
Figure 3 shows cross-sectional images of the Si/PS/ITO areas of the four-layer structures. In the case of Figure 4(top), gold was not electrodeposited into the PS thin film. In the case of Figures 4(middle) and 4(bottom), electrodeposition of gold nanoparticles was performed at 60 and 600 seconds respectively. It is observed that the diffusion of ITO into the porous structure is negligible in all cases.
In the following the effect of the addition of gold nanoparticles to the PS thin films on the optical, electrical and photocurrent generation properties of the different devices will be studied and discussed. As such, for the measurement of reflectance, current-voltage characteristics and photocurrent, an indium tin oxide (ITO) layer was deposited on top of the hybrid gold/PS structures, leading to Al/Si/PS/ITO and Al/Si/PS+Au/ITO structures such as those shown in Figure 3.
Aiming at determining the optical properties of the different PS-based devices (before and after the electrodeposition of gold nanoparticles into the PS thin films at two different times, 60 and 600 s), their characteristic reflectance (R) spectra were measured in the 300 to 900 nm wavelength range. The experimental results are plotted in Figure 4, from which a typical interference pattern for the Al/Si/PS/ITO structures is observed. It is also found that the typical interference pattern of these structures is lost upon electrodeposition of gold nanoparticles into PS, indicating strong absorption within the PS thin films. Additionally, a reduction of the average reflectance after electrodeposition of gold nanoparticles into PS is clearly observed. Increased light absorption is a key factor to increase the efficiency of the PS-based devices. The average reflectance in the 300 to 900 nm wavelength range for the Al/Si/PS/ITO structures is around 12.7 %. This value is notably reduced when Au nanoparticles are grown into the porous structure: 5.9 % for Al/Si/PS+Au(60 s)/ITO structures and 6.1 % for Al/Si/PS+Au(600 s)/ITO structures. However, increased Au nanoparticle deposition time does not lead to reduced average reflectance but results in a shift of the main reflectance peak. Finally, after electrodeposition of gold nanoparticles a broad absorption band is observed rather that well-defined absorption peaks. This effect is most likely due to a relatively large dispersion in size of the gold nanoparticles, given the strong effect that size has in optical absorption.
Current–voltage (I–V) measurements of the Al/Si/PS/ITO and Al/Si/PS+Au/ITO structures described previously were taken in the -5.5 to 2.5 V range in order to determine their electrical conduction behavior. The results are shown in Figure 5. In the case of the Al/Si/PS/ITO structures a rectifying behavior is observed, which is consistent with our previous studies in which we have determined that this type of structures show a similar behavior to that of metal–insulator–semiconductor (MIS) diodes [18]. Additionally, the experimental results show that the two Al/Si/PS+Au/ITO structures show an almost symmetrical rectifying behavior for forward and reverse biasing (Figure 5). Furthermore, a direct relationship between gold deposition time and electric conductance is clearly observed: the larger the concentration of gold nanoparticles, the larger current for a given voltage, i.e., conductivity.
Photocurrent was measured for the Al/Si/PS/ITO and the hybrid Al/Si/PS+Au/ITO structures and from the experimental results the spectral responsivity was determined (Figure 6). For
this task, the photocurrent spectra were divided by the spectral emissive power of the light source. The current photogenerated by the Al/Si/PS/ITO structures increases with increased photon energy and varies between about 10-6 and 9×10-4 A/W. These values are comparable to previous results using multilayer PS structures [19]. As Figure 6 shows, the electrodeposition of gold nanoparticles into the porous silicon thin films results in an overall increment of the photocurrent generated by the device. Moreover, it is observed that the deposition time of gold nanoparticles into PS has a direct effect in the responsivity of the PSbased devices, i.e., the longer the electrodeposition time, the larger the photocurrent generated for a given wavelength. Accordingly, this behavior is directly related to the presence of a larger amount of gold into PS (see Figures 2 and 3). At wavelengths around the point of maximum of solar irradiance (550 nm), the Al/Si/PS+Au/ITO structures for which the electrodeposition time was 60 s produce a ten-time increment over the photocurrent generated by PS thin films. In the case of Al/Si/PS+Au structures for which the electrodeposition time was 600 s, the photocurrent measured under the same experimental conditions is 200 times larger. It is worth noting that although the photocurrent measured for Al/Si/PS+Au(600 s)/ITO structures is much larger than that of Al/Si/PS+Au(60 s)/ITO structures, reflectance remains almost the same. Accordingly increased photocurrent generation cannot be associated just to improved optical performance (larger absorption), but to enhanced carrier collection.
We have investigated the effect of the addition of gold nanoparticles on the light absorption efficiency of porous silicon (PS) thin films. For this task, gold nanoparticles were electrodeposited into PS thin films at two very different times, resulting in different concentration of gold nanoparticles into the porous layer. The preliminary experimental results indicate that the presence of gold nanoparticles results in increased photocurrent generated, and that photocurrent increases for increasing concentration of nanoparticles.
The experimental results suggest that increased photocurrent generation in the Al/Si/PS+Au/ITO structures can be associated to (i) increased light absorption (as deduced from reduced reflectance) and (ii) increased conductivity given by the presence of metal nanoparticles, most likely given by improved carrier collection efficiency. The relative importance of each of these two mechanisms has yet to be determined in a subsequent study, as well as the effect of pore size and gold nanoparticle size and shape. Anyway, from reflectance measurements it was determined that a larger amount of gold nanoparticles into
porous silicon does not lead to increased light absorption. Additionally, the thickness of the porous silicon thin films has to be adjusted to further reduce the reflectance thus leading to increased overall efficiency of the PS-based devices.
Photovoltaics is a field in constant evolution, being the reduction of the cost per watt probably the major challenge in solar cell research aiming at making the price of solargenerated electricity lower than that of electricity generated by conventional sources of energy. As stated in the introduction, material costs represent a large part of the expense. Thin-film solar cell technology (i.e., second generation solar cells) has emerged as a way to reduce the amount of material required, although higher light absorption is a key factor. In this regard, the use of plasmonic nanostructures constitutes a powerful approach to improve both absorption of light and carrier collection in thin film solar cells, reducing at the same the overall manufacturing costs.
However, fabricating submicrometric metal structures generally involves the use of complex (and expensive) processing techniques, such as electron-beam lithography or focused ionbeam milling. These nanofabrication techniques might be difficult to implement over large surfaces, as required for solar cell manufacture. As such, it is highly desirable to achieve the proposed goal of costs reduction to identify a fabrication technique that would allow the inexpensive fabrication of metal nanostructures over large areas. At the same time, this nanofabrication technique has to be versatile enough to be used in combination with substrates of very different nature, including glasses and polymers. In the particular case of silicon solar cells, which constitute around 90% of the market share, we have demonstrated that hybrid metal/porous silicon thin films meet the basic physical and manufacturing requirements.
Acknowledgements
The authors gratefully acknowledge funding from Comunidad de Madrid under project „MICROSERES‟ and Ministerio Economía y Competitividad under research project MAT2011-28345-C02-01.
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