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International Journal of Engineering Science Invention ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726 www.ijesi.org Volume 2 Issue 7 ǁ July. 2013 ǁ PP.101-104

Silver nanoparticles to enhance the efficiency of silicon solar cells 1

Santanu Maity, 2Sahadev Roy, 3Abhishek Kumar

1,2

(Electronics and communication Engineering, National Institute of Technology, Arunachal Pradesh, Itanagar, India) 3 (Electrical and Electronics Engineering, National Institute of Technology, Arunachal Pradesh, Itanagar, India )

ABSTRACT: Three major criteria are to be fulfilled for achieving high conversion efficiency in silicon solar cells. They are injection of maximum optical power of the incident solar spectrum into the silicon, trapping of optical power for maximum absorption inside silicon and efficient collection of the photo generated carriers. Photon injection into the silicon solar cell is maximized with the help of anti reflection coatings on the silicon surface which balances the refractive index mismatch between air and silicon. Absorption inside the cell may be maximized by obliqueness of the light and multiple reflections inside the cell resulting in path length enhancement. The obliqueness of light also results in a higher collection efficiency of the photo generated carriers as the absorption takes place near the junction. To reduce the cost of such cells, there is a need for more efficient use of the active silicon material. Advancements in nanotechnology have led to a new research area for utilizing nano photonics for increasing the efficiency of existing solar cells. The use of metallic nanoparticles on the front surface of silicon solar cells fulfills all the three criteria for achieving high efficiency solar cells.

KEYWORDS: external quantum efficiency, plasma ionic effect, reflectance, nano particles I.

INTRODUCTION

Plasmonic structures can offer the possibility of reducing the physical thickness of the photovoltaic absorber layers while remaining optically thick. Metallic nanoparticles can be used as subwavelength scattering elements to couple freely propagating plane waves in sunlight into guided modes of an absorbing semiconductor thin film[1]. Thin film solar cells show promise as economic alternatives to conventional silicon-wafer-based photovoltaics, combining high efficiency with low processing and materials costs[2]. and thin film cells of CdTe[3], amorphous Si [4] and CuInxGa1-xSe2[5,6] (CIGS) have been fabricated with absorbing layers a few micrometers thick. In conventional cell designs, efficiencies of nanometer- thickness cells are strongly limited by decreased absorption, carrier excitation, and photocurrent generation, and so new strategies for enhanced absorption and light trapping are desirable[7,8] . Metallic nanostructured thin films, which support surface plasmon polaritons (SPPs), have the potential to confine and guide incident sunlight into wavelength-scale or subwave- length thickness absorber layer volumes[ 9] SPPs are collective oscillations of free electrons at the boundary of a metal and a nonconducting metalic or semiconductor material[10-12]. optical excitation of SPPs has been performed by prism coupling [13,14] and more recently by gratings[15] which can be highly efficient but limited in both wavelength and angular range. The diffraction and scattering of small apertures have been studied for a number of years [16, 17] with particular attention recently to the transmission properties of holes and slits. Plasmonic scattering from the metal nanoparticles when coupled to the underlined silicon results in increased injection. The angular spread of the scattering results in path length enhancement with better absorption and better collection. But the ohmic loss present in metallic nanoparticles is seemed to degrade the performance of conventional nitride coated silicon solar cells in the wavelength region of 300-1100nm. However, replacing metallic small size nanoparticles with metalic big size nanoparticles avoids the loss of energy due to joule heating as most conventional metalics are lossless in the wavelength region of 3001100nm. The application of such lossless metalic nanoparticles with optimum surface coverage on the top surface of the solar cell substantially improves the photon injection but do not produce large angular scattering for light trapping. However, light trapping inside the solar cell is achieved by embedding metalic nanoparticles in the active silicon layer. The angular scattering from the metallic nanoparticles is enhanced as the mismatch in refractive index with the embedding medium is maximized. In this paper, we study the comparative roles of metal and metalic nanoparticles atop and embedded in the silicon substrate in enhancing the efficiency of silicon solar cells. It is seen that a maximum relative improvement in efficiency of 28.4% is achieved for optimum design conditions in case of 2µm thick silicon solar cell with optimized metallic nanoparticle coating atop the silicon for maximized photon transmission and embedded metallic nanoparticles for large angular scattering.

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Silver nanoparticles to enhance the efficiency …… II.

EXPERIMENTAL DISCUSSION

Silver nitrate AgNO3 (Sigma Aldrich, UK) and trisodium citrate C6H5O7Na3 (Sigma Aldrich, UK) of analytical grade purity, were used as starting materials without further purification. For making solution deionised water (DI -water) of 18.2 MΩ-cm is taken. The silver colloid was prepared by using chemical reduction method according to the description of Lee and Meisel [18].

Figure 1: Experimental setup for Ag nano particle reduction

The AgNO3 was dissolved in de-ionized water in a beaker. In experiment 250 ml of 20 mg AgNO3 was heated to boiling. To this solution 5 ml of 1 % trisodium citrate was added drop by drop. During the process solution was mixed vigorously using magnetic starrer. Solution was heated until color’s change is evident (pale yellow). Then it was removed from the heating element and stirred until cooled to room temperature. For controlling the size of silver nano particles hydroxylamine hydrochloride (ClH4NO) of 5 x 10-3M is added with different amount like 0.1 ml, 0.2ml, 0.3ml, 0.4ml, 0.5 ml in the original solution. As OH increases then pH increases as a result it controls the size nano particles. Reaction of Ag nano particle is describe bellow 4Ag+ + C6H5O7Na3 + 2H2O → 4Ag0 + C6H5O7H3 + 3Na++ H++ O2↑

III.

RESULT AND DISCUSSION

Size dependent metal nanoparticles are strong scatterers of light at wavelengths near the plasmon resonance for collective oscillation of the conduction electrons in the metal. For particles with diameters well below the wavelength of light, a point dipole model describes the absorption and scattering of light well but the size not less than 50 nm. On scattering different types of theory is introduced like Mie
 theory
 describes
 the
 scattering
 and
 absorption
 of
 electromagnetic
 radiation
 by
 spherical
 particles
 through
 solving
 the
 Maxwell
 equations. The scattering and absorption cross-sections are described nicely by Bohren et al. below [19]

1

2 Where, is the polarizability of the particle. Here V is the particle volume, is the dielectric function of the particle and is the dielectric function of the embedding medium. We can see that when the particle polarizability will become very large. This is known as the surface plasmon resonance. At the surface plasmon resonance the scattering cross-section can well exceed the geometrical cross section of the particle. For example, at resonance a small silver nanoparticle in air has a scattering cross-section that is around ten times the cross-sectional area of the particle. In such a case, to first-order, a substrate covered with a 10 % areal density of particles could fully absorb and scatter the incident light. .

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Silver nanoparticles to enhance the efficiency ‌‌ 0

1

1

28 26

After adding Ag Nano particles Before adding Ag Nano particles

Reflectance(%)

24 22 20 18 16 14 200

400

600

800

1000

0 1200

Wavelength

Figure 2: Reflectance of Si surface before and after the treatment It is seen in figure 2 that before depositing nano particle the reflectance was near about 20% but after depositing the nano particle it is reduces to 15% because when light signed on the top of the solar cell it excite the silver particle depending on size. The excited nano particle then creates oscillating dipole which causes the plasmonic effect and it enhance the path length of input light which shown in fig. 4. 0

1

1

90

External Quantum Efficiency

80 70 60 50 40 30

Before adding Ag Nano particles After adding Ag Nano particles

20 10 200

400

600

800

1000

0 1200

Wavelength (nm)

Figure 3: EQE of Si surface before and after the treatment Due to the plasmonic effect electron hole pair generation increased which caused the enhancement of solar cell efficiency. It is seen in figure 3 that the external quantum efficiency is increased after depositing the silver nano particles that means the electron hole pair generation and collection is increased due to plasmonic effect.

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Silver nanoparticles to enhance the efficiency ……

Figure 4: Plasmonic effect on silver nano particles

IV.

CONCLUSION

Path length enhanced in the solar cell due to the properties of silver nanoparticles placed on the top of the solar cell. Light trapping inside the solar cell is achieved by embedding nanoparticles in the active silicon layer. The angular scattering from the nanoparticles is enhanced as the mismatch in refractive index with the embedding medium is maximized. It is also seen that the size of the nanoparticle is also another vital issue as <50nm size particle reduces the efficiency. In further study by taking different size and shape is investigated.

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