Speaker
Behzad Esfandiarpour
Title
Integration of Nanostructured Antireflection Coatings to Spherical Silicon Solar Cells
Abstract
Crystalline Si solar cells have remained a dominant technology in the photovoltaic (PV) industry due to their high reliability and high conversion efficiency. Although the single-crystal cells offer high efficiencies, due to the high cost of solar-grade Si wafers, cost reduction through technological advances is required to make PV a major energy source. During past decades, various alternatives such as thin film solar cells, organic PV devices, solar cells based on nanostructured material, etc., have been widely considered to develop less expensive technologies.
The spherical Si solar cell is relatively new technology based on crystalline silicon spheres with a diameter of about 1mm. This technology has been a research goal due to the low-cost fabrication process, high performance, and mechanical flexibility when embedded into flexible aluminum foils. In this technology, small single-crystal Si spheres with different qualities from metallurgical to semiconductor grade are produced by the melting and recrystallization of Si feedstock. Therefore, silicon spheres can be obtained directly from molten Si without a cutting or polishing process, which makes them a promising candidate for low-cost PV modules. Furthermore, Si spheres can be embedded in transparent substrates to manufacture semi-transparent silicon solar cells.
Optimization of the optical losses of spherical silicon solar cells is one of the most important techniques for improving the short-circuit current density and hence the efficiency of these cells. However, due to the shape of the silicon spheres embedded in aluminum foil, conventional deposition methods cannot easily be applied to make a uniform antireflection coating (ARC) layer on spherical silicon solar cells.
We have successfully developed and integrated a nanostructured antireflection coating layer into spherical silicon solar cells. The nanostructured porous layer consists of graded-size silicon nanocrystals and quantum-size Si nanoparticles embedded in an oxide matrix. This layer has been characterized by the means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunneling TEM, energy filtered TEM, dark-field TEM, transmission electron diffraction (TED), electron energy loss spectroscopy (EELS), energy dispersive x-ray (EDX), Raman spectroscopy and photoluminesence spectroscopy (PL). Due to the quantum confinement effects, this ARC layer shows incredible properties such as enhanced luminescence, light trapping, photon-shifting and bandgap opening. Quantum-size silicon nanoparticles with dimensions less than 2nm showed significantly high luminescence in the visible range of spectral response. Band-gap opening in the emitter structure minimizes the absorption of photons in the dead layer and improves the spectral response. The external quantum efficiency of silicon spheres integrated with graded-density n-PS layers exhibited significant improvement in a wide range of the spectral response.