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This study demonstrates the efficacy of combining a matrix of silver nanoparticles (Ag-NPs) with indium nanoparticles (In-NPs) to improve the electric and optical performance of plasmonic silicon solar cells. We examined the excitation of localized surface plasmons of Ag-NPs and In-NPs using surface enhanced Raman scattering measurements. Optical reflectance and external quantum efficiency (EQE) measurements demonstrate that the light scattering of Ag-NPs at short wavelengths can be improved by surrounding them with In-NPs. This also leads to high EQE band matching in the high energy band of the AM1.5G solar energy spectrum. Impressive improvements in optical reflectance and EQE response were also observed at short wavelengths. Cells with a matrix of Ag-NPs (20% surface coverage) surrounded by In-NPs (80% surface coverage) increased the overall efficiency of the cell by 31.83%, as confirmed by photovoltaic current density-voltage characterization under AM 1.5 G illumination.
© 2016 Optical Society of America
1. Introduction
Metallic nanoparticles (NPs) exhibit unique optical properties, including strong light scattering and the absorption of incident light, resulting from the excitation of surface plasmons [1]. The localized surface plasmon resonance (LSPR) of metallic NPs also enhances the optical near-field around the particle [2–4]. The interactions between photons and metallic NPs are studied in terms of the peak in plasmonic resonance, which is strongly influenced by the size, shape, and the dielectric environment [5–9]. This makes it possible to tailor SPR to a particular range of wavelengths through the selection of metallic nanoparticles of a specific geometry. The plasmonic resonance of most noble metal NPs, such as gold (Au) or silver (Ag), ranges from the visible to the infrared region of the electromagnetic spectrum, which makes them applicable to photovoltaic applications [3]. Researchers have been seeking the means to exploit the light scattering of metallic NPs in order to promote the coupling of light in the active region of photovoltaic devices. Several groups have employed the deposition of Ag NPs on the surface of photovoltaic devices: Ag NPs in Si-based solar cell [10–14], Ag NPs on organic solar cell [15], Ag NPs on dye-sensitives solar cell [16], and Ag on GaAs solar cell [17]. However, substantial losses have been observed when using Ag NPs at wavelengths below a cross-over point, typically in a region where the solar spectrum still provides very high solar energy. This represents a poor tradeoff between gains and losses in efficiency. In [18–20], it was reported that this cross-over point is closely related to the position of the LSPR in the particle. This means that inducing a blue shift in the LSPR could improve enhance the benefits of using NPs. The LSPR can be blue shifted considerably by reducing the size of the particles; however, this decreases the scattering efficiency. Achieving a blue shift sufficient to move the cross-over point to 500 nm while maintaining reasonable scattering efficiency of Ag-NPs is extremely difficult, which generally means that far too much of the available solar energy cannot be exploited. In contrast, Indium (In) NPs produce strong optical plasma-resonance absorption at 276-335 nm [9]. Combining In NPs with Ag NPs on the surface of solar cells could theoretically provide broadband plasmonic light scattering from the near infrared to the ultraviolet [21–24]. In this study, we explore the plasmonic light scattering provided by a matrix of Ag-NPs patterns surrounded by In-NPs.
Cells with various surface profiles, including a uniform distribution of Ag-NPs, a matrix-pattern of Ag-NPs, and a matrix-pattern of Ag-NPs surrounded by In-NPs, were characterized according to optical reflectance and external quantum efficiency induced by plasmonic light scattering. In the cell with a matrix-pattern of Ag-NPs surrounded by In-NPs, impressive improvements in optical reflectance and EQE response were observed at short wavelengths. We then evaluated the electrical and optical performance of plasmonic silicon solar cells based on a matrix of silver and indium nanoparticles using photovoltaic current density-voltage (J-V) under AM 1.5 G simulation.
2. Experiment details
Small metallic particles with surface plasmons can be used to enhance Raman scattering [25] and manipulate the light for photovoltaic devices to enhance its efficiency. In this work, the excitation of localized surface plasmons of the proposed Ag-NPs and In-NPs was examined by surface enhanced Raman scattering measurement. Next, plasmonics light scattering modulation of a matrix Ag-NPs pattern on Si solar cells was characterization. Finally, light scattering modulation of the cells with the matrix Ag-NPs pattern by the surrounding In-NPs was characterized and compared.
2.1 Raman scattering produced by metallic nanoparticles
A 4 nm-thick Ag film and a 3.8 nm-thick In film were respectively deposited on a 250 nm-thick layer of TiO2 on a glass substrate using E-beam evaporation, in order to characterize the plasmon-enhanced Raman scattering produced by metallic nanoparticles. The samples were then annealed at 200 °C for 30 min under ambient H2 to form metal nanoparticles (Ag-NPs and In-NPs). We also fabricated reference samples of a TiO2/glass-substrate without metallic NPs for comparison. Raman scattering spectra were collected using a micro-Raman system (Ramboss 500i Micro – Raman/PL Spectroscopy, DONGWOO) with a semiconductor laser operating at 473 nm with an output power of 0.09 mW as the excitation source.
2.2 Fabrication of plasmonic silicon solar cells
Boron doped crystalline silicon (C-Si) wafers with a (100) orientation and resistivity of 5 Ω-cm were polished on one side to a thickness of 525 μm for use as a base material for solar cell devices after being cut to samples of 1 × 1 cm2. Following standard RCA cleaning, the C-Si samples were coated with a phosphorus liquid source (Phosphorofilm, Emulsitone Co., New Jersey, US) using spin-on coating at a speed of 3000 rpm for 20 s. They were then prebaked at 200 °C for 5 min followed by further baking at 400 °C for 10 min in a rapid thermal annealing (RTA) chamber for the removal of organic species. The front and back-sides of the samples were then capped using a layer of SiO2 (200-nm thick) using e-beam evaporation before being heated in an RTA chamber at 900 °C under ambient N2 with 1-2% O2 for 2 min to initiate the diffusion of phosphorus and thereby obtain an n+-Si emitter layer. The samples were then soaked in a solution of hydrofluoric acid to remove the layers of SiO2 and phosphosilicate-glass before undergoing isolation etching using a photolithographic process using a solution of HNO3:HF:H2O at a ratio of 1:1:2, which resulted in individual cells 4 × 4 mm2 in size. Ohmic contact electrodes were produced by depositing an aluminum (Al) film to a thickness of 200 nm on the back side and a titanium (Ti)/Al film (20-nm-Ti/200-nm-Al) on the front side using e-beam evaporation. The final step in the fabrication of the bare Si solar cells involved annealing under ambient N2 at 450°C for 5 min to ensure a good ohmic contact between the metallic electrodes and Si semiconductor.
Plasmonic Si solar cells were fabricated by depositing a TiO2 spacer layer (20-nm thick) on the top surface of the bare solar cells using e-beam evaporation. The TiO2-film was evaporated under pressure of 1.50 × 10−4 Pa with an emission current of 22 mA, which resulted in a deposition rate of 0.32 Å/s. This layer of TiO2 was then annealed in an RTA chamber at 200 °C for 15 mins under ambient H2 to improve the quality of the TiO2 film. As shown in Fig. 1, a variety of Si solar cell samples were produced from the bare substrate to investigate the light-scattering of Ag-NPs in various particle-distribution schemes: uniformly deposited Ag-NPs (Fig. 1(a)), a matrix-pattern of Ag-NPs with various degrees of coverage (Fig. 1(b)), and a matrix-pattern of Ag-NPs/In-NPs with various degrees of coverage (Fig. 1(c)). In this study, the unit area of 0.8 cm × 0.8 cm and a single matrix-pattern of Ag-NPs of 20 μm × 20 μm are chose firstly. Then, the designed distance between the matrix-patterns of Ag-NPs of 24.56 μm (11.57 μm) on two dimension scale in the unit area with coverage of 20% (40%) can be obtained. Thus the distance between the matrix-patterns of Ag-NPs on a unit area is a parameter of change. After the formation of the matrix-pattern of Ag-NP with coverage of 20% (40%), In-NPs deposited on the other part of the unit area was 80% (60%).The morphology and dimensions of the metallic NPs were characterized using electron scanning microscopy (SEM; JEOL JSM-6500F) and the size distribution of the particles was calculated from SEM images using J-image software (National Institute of Mental Health, Bethesda, MD, USA). We then characterized the optical and electrical performance of the resulting plasmonic Si solar cells based on optical reflectance (Lambda 35, PerkinElmer, Inc., Waltham, Massachusetts, USA), EQE response at wavelengths between 300 nm and 1000 nm (Enli Technology Co., Ltd.), and photovoltaic current density-voltage (J-V) under AM 1.5G (1000 mW/cm2 at 25 °C) solar simulation. The solar simulator (XES-151S, San-Ei Electric Co., Ltd.) was calibrated using a crystalline silicon reference cell (PVM-236) certified by the National Renewable Energy Laboratory (NREL) prior to measurement.
Fig. 1 Schematic diagram showing cells with (a) uniformly deposited Ag-NPs, (b) matrix-pattern of Ag-NPs, and (c) matrix-pattern of Ag-NPs surrounded by In-NPs. (d) SEM image of the cell with Ag-NPs surrounding by In-NPs, which the formation of matrix-pattern of Ag-NPs with coverage of 20%.
2.3 Modulation of light scattering by matrix of Ag-NPs
Ag film was deposited to a thickness of 4-nm on the bare Si solar cells with a TiO2 spacer layer through patterned photolithographic photoresists via e-beam evaporation, followed by lift-off processing and annealing at 200 °C in an RTA chamber under ambient H2 for 20 mins. This resulted in the formation of matrix-pattern of Ag-NPs with coverage of either 20% or 40%. For comparing light scattering induced by matrix of Ag-NPs, the bare solar cell and the cell with a TiO2 spacer layer also involved in this study. We then measured the optical reflectance, EQE response, and photovoltaic J-V curves induced by the light-scattering effects of Ag-NPs with various particle distributions.
2.4 Modulation of light scattering by matrix combining Ag-NPs and In-NPs
The uniformly deposition of Ag-NPs resulted in relatively high reflectance and low EQE values at short wavelengths. To reduce reflectance at these wavelengths, we fabricated a cell with a matrix combining Ag-NP and In-NPs to take advantage of the superior SPR light scattering of In-NPs. This matrix was produced by applying matrix-pattern of Ag-NPs using photolithography in conjunction with Ag film evaporation and lift-off, followed by thermal annealing. Surrounding the matrix-pattern of Ag-NPs was deposited a film of In to a thickness of 3.8 nm using e-beam evaporation followed by lift-off. Final annealing at 200 °C in an RTA chamber under ambient H2 for 20 mins resulted in the formation of Ag-NPs and In-NPs. The ratio of Ag-NP/In-NP either 20/80% or 40/60% was calculated by the area of deposited of Ag-NP and In-NP, respectively, on a unit area. We then measured the optical reflectance, EQE response, and photovoltaic J-V curve.
3. Results and discussion
Figure 2(a) present SEM images of Ag-NPs deposited as a film to thicknesses of 4 nm, whereas Fig. 2(b) illustrates the size distribution and surface coverage, as calculated using J-image software from the corresponding SEM images. Figure 2(c) present SEM images of In-NPs deposited as a film to thicknesses of 3.8 nm, whereas Fig. 2(d) illustrates the size distribution and surface coverage, as calculated using J-image software from the corresponding SEM images. Measuring the plasmonic light scattering induced by Ag-NPs or In-NPs required samples with NPs of similar average diameter and coverage. The average surface coverage was as follows: Ag-NPs (29.41%) and In-NPs (36.67%). The average diameter was as follows: Ag-NPs (27.13 nm) and In-NPs (19.38 nm). The size distribution and surface coverage formed a base line for the fabrication of cells with a matrix of Ag-NPs or cells with a matrix of Ag-NPs surrounded by In-NPs.
Fig. 2 (a) SEM image and (b) size distribution and surface coverage of Ag-NPs, (c) SEM image and (d) the size distribution and surface coverage of In-NPs.
Figure 3 presents the Raman spectra of samples with the following configurations: TiO2/glass-substrate, Ag-NPs/TiO2/glass-substrate, and In-NPs/TiO2/glass-substrate. The intensity of the Raman signals was enhanced by surface plasmon resonance of the metallic NPs, compared that of the sample comprising only a TiO2/glass-substrate. The Ag-NPs/TiO2/glass-sample presented three Raman signal intensity peaks at 778, 1392, and 1628 cm−1. The In-NPs/TiO2/glass sample presented two peaks at 562 and 1120 cm−1, under an excitation of wavelength 473 nm. We observed a strong local electric field associated with the surface plasmon resonance of Ag-NPs and In-NPs in this study.
Fig. 3 Raman spectra of TiO2/glass, Ag-NPs/TiO2/glass, and In-NPs/TiO2/glass samples. A semiconductor laser operating at 473 nm with output power of 0.09 mW was used as the excitation source.
Figure 4(a) presents the optical reflectance of a bare solar cell, a cell with a TiO2 layer, and a cell with a layer of uniformly deposited Ag-NPs on TiO2. The reflectance of the cell with the layer of TiO2 was lower than that of the bare cell, due to the antireflective properties of the TiO2. At wavelengths of 350-550 nm, the reflectance of the cell with uniformly deposited Ag-NPs on the TiO2 layer was higher than that of the cell with only a TiO2 layer, due to optical reflectivity of the Ag-NPs. It should be noted that at wavelengths of 600-1100 nm, the reflectance of the cell with uniformly deposited Ag-NPs on the TiO2 layer decreased to a level below that of the cell with only a TiO2 layer, due to the plasmonic forward scattering of photons by the Ag-NPs. Figure 4(b) presents the optical reflectance of a cell with uniformly deposited Ag-NPs on a TiO2 as well as a cell with Ag-NPs deposited on the TiO2 layer in a matrix pattern with either 20% or 40% surface coverage. The matrix pattern of Ag-NPs significantly decreased reflectance at wavelengths of 350-550, well below that of the cell with uniformly deposited Ag-NPs on TiO2. More specifically, Ag-NP surface coverage of 20% had a greater effect than did surface coverage of 40% in reducing reflectance at wavelengths of 350-650 nm. Figure 4(c) presents the optical reflectance of cells with a Ag-NPs matrix and cells with an Ag-NPs matrix surrounded by In-NPs at an Ag-NP/In-NP ratio of either 20/80% or 40/60%. At wavelengths of 350-700 nm, the optical reflectance of cells with a combination of Ag-NPs and In-NPs was below that of samples with Ag-NPs only. We also calculated the average weighted reflectance (RW) of all samples at wavelengths (λ) from 350 to 1100 nm using Eq. (1).
where R(λ) is reflectance as a function of the wavelength and ϕph(λ) is the photon flux of AM 1.5 G solar energy spectrum as a function of wavelength. The resulting RW values are listed in Table 1.These results demonstrate that surrounding the matrix of Ag-NPs with In-NPs can reduce reflectivity at UV-wavelengths via strong localized surface plasmon resonance.Fig. 4 Optical reflectance: (a) bare solar cell, cell with a TiO2 layer, and cell with uniformly deposited Ag-NPs on a TiO2 layer; (b) cells with the matrix of Ag-NPs with either 20% or 40% surface coverage on TiO2 layer and cell with uniformly deposited Ag-NPs on TiO2 layer; (c) cells with matrix of Ag-NPs and cells with matrix of Ag-NPs surrounded by In-NPs (Ag-NPs surface coverage of 20% or 40%).
Table 1. Average weighted reflctance (RW) and average weighted EQE (EQEW) calculated for wavelengths (λ) from 350 to 1100 nm
Figure 5(a) presents the EQE response of a bare solar cell, a cell with a TiO2 layer, and a cell with Ag-NPs uniformly distributed on a TiO2 layer. The cell with a TiO2 layer presented higher EQE values than did the bare solar cell at wavelengths from 350 to 1100 nm. This finding is in agreement with the results of optical reflectance. At wavelengths below 575 nm, the cells with uniformly distributed Ag-NPs displayed EQE values lower than that of the cell with only a TiO2 layer due to the reflectivity of the Ag-NPs. This is in agreement with the results of optical reflectance obtained at wavelengths of 350-550 nm. The EQE values increased at incident wavelengths of exceeding 600 nm (> 65%), due to the forward light scattering of Ag-NPs at longer wavelengths. This is also in agreement with the results of optical reflectance, in which plasmonic forward scattering was induced by Ag-NPs. Figure 5(b) presents the EQE response of cells with a matrix of Ag-NPs on a layer of TiO2 (20% or 40% coverage) and a cell with uniformly deposited Ag-NPs on a TiO2 layer. At wavelengths of 350-650 nm, the EQE values of the cell with the Ag-NP matrix were significantly higher than those of the cell with uniformly deposited Ag-NPs, and lower surface coverage (20%) proved more effective than higher coverage (40% one). Beyond 650 nm, the EQE values of cells with uniformly distributed Ag-NPs increased slightly due to large-scale forward scattering. Figure 5(c) presents the EQE response of cells with a matrix of Ag-NPs and cells with a matrix of Ag-NPs surrounding by In-NPs). At short wavelengths, the EQE response of the cells with a 20/80% ratio of Ag-NPs/In-NPs was superior to that of cells with a ratio of 40/60% and cells with a matrix of Ag-NPs. In fact the Ag-Np/In-NP sample with ratio of 20/80% achieved an EQE value of >70%. We also compared the average weighted EQE (EQEW) of cells with uniformly deposited Ag-NPs, cells with a matrix of Ag-NPs, and cells with a matrix of Ag-NPs surrounded by In-NPs at wavelengths (λ) from 350 to 1100 nm. The EQEW results were calculated using Eq. (2).
where EQE(λ) is the EQE value as a function of wavelength and ϕph(λ) is the photon flux of AM 1.5 G solar energy spectrum as a function of wavelength. The EQEW values are listed in Table 1.Fig. 5 EQE response: (a) bare solar cell, a cell with a TiO2 layer, and cell with uniformly deposited Ag-NPs on a TiO2 layer; (b) cell with matrix of Ag-NPs on TiO2 layer (20% or 40% surface coverage) and cell with uniformly deposited Ag-NPs on TiO2 layer; (c) cell with matrix of Ag-NPs and cell matrix of Ag-NPs surrounded by In-NPs (20/80% or 40/60%).
EQE is the ratio of the number of photo-carriers collected by a solar cell to the number of photons of a given wavelength that strike the surface of a solar cell from outside. A high EQE value means that the incident photons at a particular wavelength were absorbed, such that a greater number of minority carriers were collected. The EQE of a solar cell also indicates the amount of current a cell will produce when irradiated by photons of a particular wavelength. Thus, integrating the EQE of a solar cell over the entire solar energy spectrum would make it possible to evaluate the amount of current that the cell could produce when exposed to sunlight. We obtained the short-circuit current density (JSC) of a solar cell by convolving the EQE with AM 1.5G solar energy spectrum using Eq. (3), as follows:
where q is the elementary charge, h is the Planck constant, c is the speed of light in a vacuum, and EAM1.5G is the spectral irradiance of AM 1.5G in Wm−2nm−1.Figure 6(a) presents the EQE curves of all cells evaluated in this study. The AM1.5G solar energy spectrum is also depicted in Fig. 6(a). The EQE curves show that the cells with a matrix of Ag-NPs and the cells with a matrix of Ag-NPs surrounded by In-NPs (particularly the 20/80% cell) achieved the highest wideband efficiency across the widest bandwidth. JSC is proportional to the product of EQE and EAM1.5G (see Eq. (3); therefore, achieving a high overall JSC depends largely on high EQE values across a wide proportion of the high energy band of the solar energy spectrum. As shown in Fig. 6(a), we obtained a high EQE band located in the high energy band of the solar spectrum (400-700 nm) from cells with a matrix of Ag-NPs surrounded by In-NPs. Figure 6(b) plots the enhancement factor of EQE for all cells compared in this study. The cells with a matrix of Ag-NPs surrounded by In-NPs achieved the best EQE enhancement factor (> 1.0, full wavelength) followed by cells with a matrix of Ag-NPs NPs (> 1.0; wavelength range of 350-800 nm) and then by cells with uniformly deposited Ag-NPs (> 1.0; wavelength range of 660-1100 nm).
Fig. 6 (a) EQE curves and AM1.5G solar energy spectrum of all cells evaluated in this study: (b) enhancement factor of EQE for cell with uniformly deposited Ag-NPs, cell with matrix of Ag-NPs and cells with matrix of Ag-NPs surrounded by In-NPs, compared that of cell with only TiO2 layer.
Figure 7(a) presents the photovoltaic J-V characteristics of a bare solar cell, a cell with a TiO2 layer, and a cell with uniformly distributed Ag-NPs on a TiO2 layer. The (11.46%) increase in Jsc in the cell with uniformly distributed Ag-NPs (compared to the cell with a TiO2 layer) can be attributed to plasmonic light scattering induced by Ag-NPs. Figure 7(b) presents photovoltaic J–V characteristics of cells with uniformly distributed Ag-NPs, cells with a matrix of Ag-NPs, and cells with a matrix of Ag-NPs surrounded by In-NPs. The photovoltaic performance of all evaluated solar cells is summarized in Table 2. The Jsc of the cells with a matrix of Ag-NPs is as follows: 20% Ag-NP coverage (32.65 mA/cm2), 40% Ag-NP coverage (32.24 mA/cm2). The Jsc of the cells with a matrix of Ag-NPs surrounded by In-NPs is as follows: 20% Ag-NP coverage (35.08 mA/cm2) and 40% Ag-NP coverage (33.08 mA/cm2). The Jsc of the cells with uniformly distributed Ag-NPs was 31.91 mA/cm2. The Jsc of these solar cells was shown to be strongly correlated to EQE response. Generally, the conversion efficiency (η) of solar cells depends on Jsc, open-circuit voltage (Voc), and fill factor (FF). However, in this study, the variation in Voc and FF was less than 2% among the cells with different surface structure profiles. The factors that made the greatest contribution to η were Jsc and EQE. We achieved a notable increase in absolute efficiency: 1.32% (from 13.04% to 14.36%) in sample with 20% Ag-NPs coverage surrounded by In-NPs and 1.09% (from 13.04% to 14.13%) in sample with 40% Ag-NPs coverage surrounded by In-NPs.
Fig. 7 Photovoltaic J-V characteristics (a) bare solar cell, cell with TiO2 layer, and cell with uniformly distributed Ag-NPs on TiO2 layer; (b) cell with uniformly distributed Ag-NPs, cell with matrix of Ag-NPs, and cell with matrix of Ag-NPs surrounded by In-NPs.
Table 2. Photovoltaic performance of all evaluated solar cells.
4. Conclusions
This paper reports the characterization of localized surface plasmons of Ag-NPs and In-NPs, using surface enhanced Raman scattering measurement. Three Raman signal intensity peaks were observed in Ag-NPs at 778, 1392, and 1628 cm−1 and two peaks were observed in In-NPs at 562 and 1120 cm−1, due to the plasmon-enhanced Raman scattering producing by metallic nanoparticles. The plasmonic light scattering of samples with uniformly distributed Ag-NPs, samples with a matrix of Ag-NPs, and samples with a matrix of Ag-NPs surrounded by In-NPs were compared using optical reflectance and EQE measurements. Experiment results demonstrate that the light scattering of Ag-NPs at short wavelengths can be improved when they are combined with In-NPs. This also leads to high EQE band matching in the high energy band of the AM1.5G solar energy spectrum. We achieved efficiency of 13.04% in a cell with uniformly distributed Ag-NPs and efficiency of 14.36% in a cell with a matrix of Ag-NPs surrounding by In-NPs.
Funding
Ministry of Science and Technology, Taiwan (MOST 103-2221-E-027-049-MY3).
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