Fabrication of Antireflective Sub-Wavelength Structures on Silicon Nitride Using Nano Cluster Mask for Solar Cell Application
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We have developed a simple and scalable approach for fabricating sub-wavelength structures (SWS) on silicon nitride by means of self-assembled nickel nanoparticle masks and inductively coupled plasma (ICP) ion etching. Silicon nitride SWS surfaces with diameter of 160–200 nm and a height of 140–150 nm were obtained. A low reflectivity below 1% was observed over wavelength from 590 to 680 nm. Using the measured reflectivity data in PC1D, the solar cell characteristics has been compared for single layer anti-reflection (SLAR) coatings and SWS and a 0.8% improvement in efficiency has been seen.
KeywordsSub-wavelength Structure Solar cell SWS fabrication Reflectance Anti-reflective coatings
The antireflection coating has become a key feature for solar cell design [1, 2, 3, 4]. Many researchers have investigated double-layer antireflection (DLAR) coatings because single-layer antireflection coatings (SLAR) are not able to cover a broad range of the solar spectrum [5, 6]. Unfortunately, multilayer ARCs are expensive to fabricate owing to the stringent requirement of high vacuum, material selection, and layer thickness control. Additionally, thermal mismatch induced lamination and material diffusion of the multilayer ARCs limit the device performance at high power densities.
An alternative to multilayer ARCs are the sub-wavelength structured (SWS) surface with dimensions smaller than the wavelength of light . In publications concerning broadband or solar anti-reflective surfaces, [8, 9, 10, 11] the principle to achieve the necessary low refractive indices is always the same: substrate material is mixed with air on a sub-wavelength scale. To date, a wide variety of techniques have been investigated for texturing multi-crystalline (mc) silicon cells . One of the promising options is surface texturing by dry etching technique. Some groups have succeeded in fabricating uniform textures with a submicron scale on mc-Si wafers by reactive ion etching and applied to the Si solar cells [13, 14]. Unfortunately, there is not much report on texturization of silicon nitride and the optical properties of submicron textures on silicon nitride for the application of solar cells.
In this study, we fabricated sub-wavelength structure on antireflection coating layers instead of semiconductor layer on solar cell. The main motivation behind this lies in the fact that the sub-wavelength structures will act as a second ARC layer with an effective refractive index so that the total structure can perform as a DLAR layer. Thus we can cost down the deposition of 2nd ARCs layer can be saved with better or comparable performance as that of a DLAR solar cell. We fabricate the silicon nitride sub-wavelength structures using the mask less RIE technique on silicon substrate and explore the reflection properties of the texturing structures through spectroscopic measurements .
Results and Discussion
Average residual reflectivity calculated by the Eq. 1for measured reflectance of bare silicon, 69.1 nm silicon nitride deposited on silicon, 69.1 nm silicon nitride and 56 nm MgF2double layer deposited on silicon and 140–150 nm silicon nitride SWS fabricated on silicon
Average residual reflectivity,Rav(%)
Silicon nitride SLARC
Silicon nitride SWS
In summary, we have developed an easy and scalable non-lithographic approach for creating sub-wavelength structured antireflection coatings directly on silicon nitride anti-reflection coatings for the first time to improve the solar cell efficiency. PC1D simulated solar characteristics inferred that the efficiency increase of 0.8% for a silicon solar cell can be achieved using silicon nitride SWS over a cell with silicon nitride SLAR and a comparable performance with a cell with silicon nitride and MgF2DLAR.
This work was supported in part by Taiwan National Science Council (NSC) under Contract NSC-97-2221-E-009-001-PAE, Motech Industries Inc. (MOTECH), Tainan, Taiwan, under 2008–2009 grants and by Laser application Department, Industrial Technology Research Institute, Hsinchu, Taiwan, under a 2008 grant.