Performance evaluation of thin film silicon solar cell based on dual diffraction grating
© Dubey et al.; licensee Springer. 2014
Received: 5 November 2014
Accepted: 12 December 2014
Published: 19 December 2014
Light-trapping structures are more demanding for optimal light absorption in thin film silicon solar cells. Accordingly, new design engineering of solar cells has been emphasized and found to be effective to achieve improved performance. This paper deals with a design of thin film silicon solar cells and explores the influence of bottom grating and combination of top and bottom (dual) grating as a part of back reflector with a distributed Bragg reflector (DBR). Use of metal layer as a part of back reflector has found to be promising for minimum requirement of DBR pairs. The effect of grating and anti-reflection coating thicknesses are also investigated for absorption enhancement. With optimization, high performance has been achieved from dual grating-based solar cell with a relative enhancement in short-circuit current approximately 68% while it was approximately 55% in case of bottom grating-based solar cell. Our designing efforts show enhanced absorption of light in UV and infrared part of solar spectrum.
Nowadays, non-conventional energy sources are demanding alternatives to overcome the problem of power scarcity at worldwide. This need has generated a huge scope of research in fundamental and advanced branches of science and technology. Severe work on silicon thin film solar cells including design and fabrication has been focused by scientific community. Silicon technology is well known which is safe, non-toxic, and cheaper for thin film solar cells; however, weak absorption in longer wavelength is a major issue which needed to be attained to a maximum possibility. To overcome this problem, new design engineering of solar devices are to be spotlighted which includes an efficient light-trapping structure. For efficient light-trapping structures, one-dimensional photonic crystal also known as distributed Bragg reflector and diffraction grating are the components of bottom reflector and have been explored for thin film silicon solar cells. Distributed Bragg reflector provides total internal reflection of longer wavelength light that passes through absorbing layer of solar cell; however, diffraction grating diffracts and scatters the light. Diffraction grating can increase the light absorption by a factor of 4n2 (n is medium's refractive index) through diffraction of the light at higher angles so that light can propagate through medium or couple the guided lights. The combination of both DBR and grating as back reflectors enforces the light towards active region and as a result enhances the absorption.
Several ideas of light trapping in solar cells have been reported by the researchers such as random or periodic pyramids, gratings, plasmonic nanoparticles, one-dimensional photonic crystals, etc [1–4]. Zeng et al. have demonstrated a combined effect of one-dimensional photonic crystal as a distributed Bragg reflector and diffraction grating on the performance of thin film solar cells. They have observed enhanced light absorption in active region due to high reflectivity and large angle diffraction. Experimentally, short-circuit current density was observed to be increased by 19% as comparison to simulated results . Rao et al. have presented a modeling of a solar cell based on silicon diffraction grating . A solar cell structure with 500 nm period and 150 nm depth has produced 76.5% enhancement of short-circuit current density. They have concluded that the positioning of grating on the rear surface can reduce the short wavelength losses. Further, the use of high refractive index material (silicon) can result in strong diffraction while light absorbed in the grating pillar can yield better performance of the device. Kuo et al. have demonstrated an amorphous silicon-based solar cell with a backside distributed Bragg reflector . They have suggested using of more than one DBR structure as a part of back reflector in order to utilize wider bandwidth of solar spectrum. A solar cell with a combination of three DBR pairs has produced optimal quantum efficiency due to transmission of shorter wavelength and reflection of longer wavelength of light. Gjessing et al. have presented a numerical investigation of a light-trapping structure consisting of a two-dimensional back diffraction grating in combination with an aluminum reflector . An enhancement in short-circuit current density from 30.4 mA/cm2 to 35.5 mA/cm2 has been achieved from 20 μm thick silicon solar cell. This enhancement in current density has been attributed to the increased path length due to in-coupling of light with decreased parasitic absorption in aluminum medium due to a spacer layer used. Recently, there is an intensive research that can be seen on multiple material-based grating structure with a combination of DBR. Abass et al. have numerically presented a study of complex dual interface grating system for thin film silicon solar cells which is just the combination of a plasmonic grating at the back side and a dielectric grating at the front side of the cell . Such structures have shown a strong coupling of higher-order guided modes in addition to coupling of lower-order modes. Further, combination of blazing and dual interface grating structures were observed to be more accessible modes with a strong coupling efficiency which ultimately enhanced the light absorption. Chriki et al. have presented the solar cell architecture design with the use of two periodic layers of metallic and dielectric grating and analyzed that both layers can couple the incident light to photonic and plasmonic modes; as a result, enhanced absorption can be assured . The importance of the relative shift between these two gratings has been explored and shown to be crucial in enhancing the current density via the mechanism of coupling to dark modes providing additional absorption. Schuster et al. have reported a simple layer transfer fabrication technique to enhance light trapping which allows independent patterning of thin film at both the sides. The fabricated dual grating structure showed an improvement over single grating pattern either on the top or bottom of the film . Zhao et al. have demonstrated a design of solar cell with an indium tin oxide diffraction grating, a DBR of a-Si:H/ITO and an Ag reflector . With the use of metal reflector, they have observed 69% and 72% weighted absorptance of solar cell in cases 4 and 8 pairs of a-Si:H/ITO DBR, respectively. It is claimed that the use of metal reflector is helpful to trap light in a better way with reduced number of DBR pairs and hence makes easy fabrication. Mutitu et al. have presented a light-trapping design which can be applied to stand alone and multiple junction thin film silicon solar cells . This design includes diffraction grating and one-dimensional photonic crystals as band pass filters that reflect short light wavelengths and transmit longer wavelengths at the interface between two adjacent cells. Enhanced short-circuit current approximately 30.25 mA/cm2 was achieved with 5 μm cell thickness solar cell based on top and bottom triangular grating with a DBR.
In this paper, we present a design of thin film silicon solar cell by employing FDTD method and have found promising for absorption enhancement in UV and infrared part of solar spectrum. With the influence of bottom and top grating, it is observed that the bottom grating enhances short-circuit current by 55% whereas combination of top and bottom (dual) grating yields 68% due to enhanced absorption. In the second section, simulation approach is presented and simulated results are discussed in the third section. Finally, the fourth section concludes the paper.
Results and discussion
Among four above discussed solar cells, double grating with one DBR pair and metal layer-based solar cell (cell D) gave 46% relative enhancement in short-circuit whereas it is 40% for cell B. Figure 2b shows comparison of short-circuit current of best performed solar cells as a function of number of DBR pairs. The short-circuit current is found to be maximum approximately 26.95 and 27.17 mA/cm 2 for the cells A and B with two DBR pairs; however, cells C and D show enhancement in current approximately 27.65 and 28.33 mA/cm 2 with the use of only one DBR pair. The use of double grating as a part of back reflector yields enhanced performance of the devices whereas metal layer plays a promising role to reduce the requirement of DBR pairs and hence, makes easy fabrication of solar cell.
We have designed and analyzed the performance of solar cells by considering various parameters. It is observed that the combination of metal layer at the bottom with grating and DBR reduces the requirement of DBR pairs. The double bottom grating-based solar cell showed better performance against to single grating. Our results showed that the use of bottom double grating is helpful to diffract longer wavelength whereas top grating is found to be a promising one to diffract shorter wavelength of solar spectrum. A relative enhancement in short-circuit current approximately 68% and 55% is achieved with dual grating and double bottom grating-based solar cells, respectively. This presented designing of solar cell is helpful to trap UV and infrared part of solar spectrum whereas fabrication challenges remain.
The financial support provided by the Defence Research and Development Organisation (DRDO), New Delhi, INDIA is highly acknowledged.
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