- Nano Express
- Open Access
External quantum efficiency response of thin silicon solar cell based on plasmonic scattering of indium and silver nanoparticles
© Ho et al.; licensee Springer. 2014
- Received: 26 June 2014
- Accepted: 5 September 2014
- Published: 11 September 2014
This study characterized the plasmonic scattering effects of indium nanoparticles (In NPs) on the front surface and silver nanoparticles (Ag NPs) on the rear surface of a thin silicon solar cell according to external quantum efficiency (EQE) and photovoltaic current–voltage. The EQE response indicates that, at wavelengths of 300 to 800 nm, the ratio of the number of photo-carriers collected to the number of incident photons shining on a thin Si solar cell was enhanced by the In NPs, and at wavelengths of 1,000 to 1,200 nm, by the Ag NPs. These results demonstrate the effectiveness of combining the broadband plasmonic scattering of two metals in enhancing the overall photovoltaic performance of a thin silicon solar cell. Short-circuit current was increased by 31.88% (from 2.98 to 3.93 mA) and conversion efficiency was increased by 32.72% (from 9.81% to 13.02%), compared to bare thin Si solar cells.
- Conversion efficiency
- External quantum efficiency (EQE)
- Plasmonic scattering
- Silicon solar cell
Photovoltaic energy is a viable renewable source of energy for coming generations. Unfortunately, the cost per unit of electricity generated by a photovoltaic system is higher than the retail price of electricity generated using conventional methods. Making power from photovoltaic devices competitive with other technologies, such as fossil fuels, will require considerable reductions in the cost of manufacturing. Current photovoltaic technology is based on bulk wafer-based crystalline silicon (C-Si) technology, which depends on the cost of Si materials and processing. Thus, the easiest way to reduce the costs of these devices is to reduce the amount of materials by producing thinner devices (thin Si solar cells; approximately 100 to 150 μm-thick) rather than traditional silicon solar cells (approximately 300-μm-thick). Many light-trapping methods have been proposed to achieve high efficiency without incurring high costs. Metallic nanoparticle plasmonic applications have been widely studied to enhance photovoltaic performance [1–4]. The resonance of most metallic nanoparticles is in the visible or infrared regions of the electromagnetic spectrum; however, this also depends on size, shape, and spacing of the metallic particles as well as the dielectric properties of the surrounding medium [5–7]. Most previous studies have shown that silver (Ag) and gold (Au) nanoparticles (NPs) can be used in bulk wafer-based C-Si solar cells or thin-film Si solar cells where the NPs are deposited on one surface of the solar cells [1, 3, 4, 8–12]. However, few studies have examined the effects of metallic NPs deposited on the front and back surfaces of a thin Si solar cell .
This study fabricated solar cells with indium (In) NPs [14, 15] on the front surface and Ag NPs on the rear surface. We then examined the degree to which photovoltaic performance was enhanced by the plasmonic scattering of In NPs and Ag according to external quantum efficiency (EQE) and measurements of photovoltaic current–voltage (I-V). EQE was enhanced at wavelengths of 300 to 800 nm thanks to In NPs and at 1,000 to 1,200 nm thanks to the Ag NPs. Short-circuit current was increased by 31.88% (from 2.98 to 3.93 mA), and conversion efficiency was increased by 32.72% (from 9.81% to 13.02%), compared to those of bare thin Si solar cells.
A 250-μm-thick p-type (boron doped) Si wafer with resistivity of 1 to 10 Ωcm and (100) orientation was cut into small samples (1 × 1 cm2) and polished on one side for the fabrication of solar cells. The back side of the Si samples was then ground down to obtain Si samples of 120-μm-thick. After standard RCA cleaning, the thin Si samples were coated with a phosphorus liquid source (Phosphorosilicafilm; Emulsitone Co., Washington, NJ, USA) on the front surface using a spin-on film (SOF) technique at a speed of 6,000 rpm for 20 s. This was followed by prebake processing on a hot plate at 200°C for 5 min for the removal of solvents and 400°C for 10 min to promote cross-linking. Both sides of the samples were then capped with a 250-nm-thick SiO2 layer using e-beam evaporation and heated in a rapid thermal annealing (RTA) chamber under an N2 atmosphere at 900°C for 2 min in order to diffuse the phosphorus resulting in an n+-Si emitter approximately 0.4 μm in thickness. Following phosphorus diffusion, the samples were soaked in an HF solution to remove the SiO2 caps as well as the layer of phosphorus oxide. The diffusion profile was examined using secondary ion mass spectrometry (SIMS). We then deposited 20-nm-Ti/200-nm-Al films on the front surface using patterns of photo-resist. Finally, the samples were isolation etched in KOH solution using a photolithography process to obtain individual areas 4 × 4 mm2.
To examine the electrical and optical properties of the proposed solar cell, we measured the photovoltaic current-voltage (I-V) and EQE in each stage of processing. The contribution of the plasmonic scatterings of In NPs and Ag NPs was characterized according to EQE response at wavelengths between 300 and 1,000 nm (Enli Technology Co., Ltd., Kaohsiung City, Taiwan). The short-circuit current (Isc), open-circuit voltage (Voc), and conversion efficiency (η) were obtained using photovoltaic I-V measurements under one-sun AM 1.5 G (1,000 mW/cm2 at 25°C) solar simulation. The solar simulator (XES-151S, San-Ei Electric Co., Ltd., Osaka, Japan) was calibrated using a National Renewable Energy Laboratory (NREL)-certified crystalline silicon reference cell (PVM-236) prior to measurements.
where q is the elementary charge, h is Planck's constant, c is the speed of light in a vacuum, and EAM1.5G is the spectral irradiance of AM 1.5 G in Wm-2 nm-1.
This paper focused on the enhancement of photovoltaic performance in a thin Si solar cell (120-μm-thick) through the introduction of plasmonic scattering using In NPs and Ag NPs. We measured improvements in the EQE of a thin Si solar cell resulting from the respective deposition of metallic nanoparticles on the front and rear surfaces at wavelengths of between 300 and 1,200 nm. Besides, the increased EQE leading to promote Jsc and η is also revealed in this study step by step due to Jsc proportional to EQE and η proportional to Jsc × Voc.
Photovoltaic I-V and EQE response of plasmonic solar cell with Ag NPs on the rear surface
Photovoltaic I-V and EQE response of plasmonic solar cell with TiO2 spacing layer on the front surface
Photovoltaic I-V and EQE response of plasmonic thin silicon solar cell with In NPs/TiO2 layer on the front surface and Ag NPs on the rear surface
Summary of photovoltaic performance of samples A, B, C, and D
A: Bare cell
B: Cell with Ag NPs
C: Cell with Ag NPs and TiO2
D: Cell with Ag NPs and In NPs/TiO2
Enhancement (D-B/B) × 100
Enhancement (D-A/A) × 100
This study fabricated and characterized thin silicon solar cells with different metallic nanoparticles deposited on the front and rear surfaces. The EQE response revealed plasmonic scattering at short wavelengths by the In NPs on the front surface and at long wavelengths by the Ag NPs on the rear side. Overall improvements in short-circuit current and conversion efficiency were in strong agreement with the EQE response resulting from the broadband plasmonic scattering produced by the different metallic NPs on each surface.
The authors would like to thank the National Science Council of the Republic of China for financial support under Grant NSC-100-2221-E-027-053-MY3.
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