Periodically Aligned Si Nanopillar Arrays as Efficient Antireflection Layers for Solar Cell Applications
© The Author(s) 2010
Received: 24 May 2010
Accepted: 13 July 2010
Published: 28 July 2010
Periodically aligned Si nanopillar (PASiNP) arrays were fabricated on Si substrate via a silver-catalyzed chemical etching process using the diameter-reduced polystyrene spheres as mask. The typical sub-wavelength structure of PASiNP arrays had excellent antireflection property with a low reflection loss of 2.84% for incident light within the wavelength range of 200–1,000 nm. The solar cell incorporated with the PASiNP arrays exhibited a power conversion efficiency (PCE) of ~9.24% with a short circuit current density (J SC ) of ~29.5 mA/cm2 without using any extra surface passivation technique. The high PCE of PASiNP array-based solar cell was attributed to the excellent antireflection property of the special periodical Si nanostructure.
Climate change, energy crisis and increasing monetary cost arisen from the nonrenewable fossil fuels have attracted extensive broad public attentions. The demands for developing renewable clean energy resources have been greatly increased in recent years [1, 2]. Among the various energy projects in progress, photovoltaic (PV) is an almost maintenance-free and truly renewable clean energy and is considered as the most promising candidate for future energy resources [3, 4]. Although many Si-based PV devices have been developed during the past decades, single-crystal Si wafer–based PV modules still show the highest efficiency. However, more than 30% of incident light is reflected back due to the high reflective index of Si, which greatly reduces the PCE of the photovoltaic device. For traditional Si wafer–based solar cells, the pyramidal or inverted pyramidal structures were generally constructed on Si surface to reduce the reflection loss for incident light [5, 6]. Extra single-layer antireflection coating (SLARC) or double-layer antireflection coating (DLACR), such as Si3N4, MgF2 and Si3N4/MgF2 DLACR, was also needed to further suppress the reflection loss . Unfortunately, these complex processes often make the rigorous requirements for experimental condition and limit the practical application of Si-based solar cells.
Recent studies on the optical and electrical characteristics of Si nanostructures, including Si nonowires (SiNWs) [8–12], Si nanopillars (SiNPs)  and Si nanocones/Si nanotips (SiNCs/SiNTs) [14–16], demonstrate their promising applications in solar cell. These typical structures involve the utilization of SiNWs/NPs/NTs that are long enough to absorb most of incident light. Meanwhile, their small diameters provide a short collection length for excited carriers in a direction normal to the light absorption, even for relatively impure absorber materials . Theoretical studies have indicated that PASiNP or PASiNW arrays are beneficial for light reflection suppression than the disordered ones [12, 16, 17]. Inspired by these promising applications, many methods have been developed to synthesize the SiNW and PASiNP arrays. In these methods, electroless chemical etching is a simple method to fabricate large-area SiNWs/NPs arrays without using any special equipment [18–22]. However, this wet etching process is hard to precisely control the position and diameter of SiNWs, thus producing the disordered SiNWs bundles and limiting their applications in solar cells. It is worth mentioning that the reports on photovoltaic applications utilized highly ordered NWs/NPs arrays are still rare .
In this study, a simple floating–transferring technique was adopted to create large-scale PS sphere monolayer without using any special equipment . Combined with a dry catalyst deposition as well as the previous wet chemical etching process, large-area PASiNP arrays were fabricated on Si surface using the diameter-reduced PS spheres as mask. Due to their typical sub-wavelength structure, the PASiNP arrays show a low reflectance loss of 2.84% within the wavelength range of 200–1,000 nm. Based on their excellent antireflection property, the solar cell incorporated with PASiNP shows a high PCE of ~9.24% without any further modification for light trapping scheme and structural optimization, indicating their great potential in photovoltaic application.
P-type Si (100) wafers with thickness of ~750 μm and resistivity of 1–30 Ω cm were used as substrates. The Si wafers were cut into 2 × 2 cm2 squares and precleaned with Piranha solution (H2SO4/H2O2 = 3:1, v:v) at 90°C and RCA solution (NH3/H2O2/H2O = 1:1:5, v:v) at 75°C for 1 h in turn, to obtain a hydrophilic surface. The Si substrates were then rinsed in deionized water for several times.
The fabrication of PASiNP-based solar cell is similar to the traditional Si solar cell technology. After removal of residual PS spheres and silver particles on the surface of PASiNP arrays, a thin layer of aluminum film with thickness of ~250 nm was deposited on the backside of Si substrate and annealed at 600°C to form an ohmic contact with Si wafers. Then, a thin layer of Ti/Pd/Ag (60/60/100 nm) multifilm was deposited on the surface of PASiNP arrays via a mask evaporation process. Finally, the samples were annealed in N2 atmosphere at 200°C for 6 h and cut into 1 × 1 cm2 for PCE measurement.
The morphologies of the samples were characterized by LEO 1550 field emission scanning electron microscopy (FESEM) and JEOL 2010 high-resolution transmission electron microscopy (HRTEM). The TEM samples were prepared by dispersing the as-fabricated SiNWs in ethanol under ultrasonication and transferred to a carbon-coated copper grid. Optical reflectance spectra were recorded by a PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer. The PEC measurement of PASiNP array–based solar cell was performed using a solar simulator under Air Mass (AM) 1.5 G illumination with intensity of 100 mW/cm2.
Results and Discussion
In summary, large-area PASiNP arrays were fabricated by the silver-catalyzed chemical etching process using reduced PS spheres as mask. The diameter, length and periodicity of SiNPs were precisely controlled. The PASiNP arrays show excellent antireflection property and give a low reflection loss of 2.84% within the wavelength range of 200–1,000 nm. The solar cell based on the PASiNP arrays shows a power conversion efficiency of ~9.24% with J SC of ~29.5 mA/cm2 under illumination. The large J SC of the PASiNP array–based solar cell is attributed to the excellent antireflection of the PASiNP arrays for incident light. It is expected that this special PASiNP structure will have great potential in various applications in the near future, not just as the antireflection layer of solar cell.
The authors thank the financial support of the MOE Tier II project of Singapore (Grant No. ARC 13/08).
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