SIMULATION OF PLASMONIC CRYSTAL ENHANCEMENT OF THIN FILM SOLAR CELL ABSORPTION
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1153-A03-02
SIMULATION OF PLASMONIC CRYSTAL ENHANCEMENT OF THIN FILM SOLAR CELL ABSORPTION R. Biswas1,2, D. Zhou2, L. Garcia2 1 Department of Physics and Astronomy, Ames Laboratory, Iowa State University, Ames IA 50011 2 Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames IA 50011
ABSTRACT Light management and enhanced photon harvesting are critical areas for improving efficiency of thin film solar cells. Red and near infrared photons with energies just above the band edge have large absorption lengths in amorphous silicon and cannot be efficiently collected. We previously demonstrated that a photonic crystal back reflector involving a periodically patterned ZnO layer can enhance absorption of band edge photons. We propose and design alternative new plasmonic crystal structures that enhance absorption in thin film solar cell structures. These plasmonic crystals consist of a periodically patterned metal back reflector with a periodic array of holes An amorphous/nanocrystalline silicon layer resides on top of this plasmonic crystal followed by a standard anti-reflecting coating. We have found plasmonic crystal structures enhance average photon absorption by more than 10%, and by more than a factor of 10 at wavelengths just above the band edge, and should lead to improved cell efficiency. The plasmonic crystal diffracts band edge photons within the absorber layer, increasing their path length and dwell time. In addition there is concentration of light within the plasmonic crystal. Design simulations are performed with rigorous scattering matrix simulations where both polarizations of light are accounted for. INTRODUCTION Photovoltaics and solar cells have been an active area for research and development, driven by the world's constantly increasing demand for power. Amorphous silicon (a-Si:H) is among the most developed material for thin film solar cells. Light trapping is the standard technique for improving the thin film solar cell efficiencies and harvesting the spectrum of incoming sunlight. The conventional light trapping schemes unitize a random textured Ag/ZnO back reflector that scatters light within the absorber layer and increases the optical path length of solar photons [1]. However, those metallic back reflectors of silver coated with ZnO, suffers from intrinsic losses from surface plasmon modes generated at the granular metal-dielectric interface [2]. Periodic metallic gratings were also used to improve absorption of polymer based thin film solar cells [3]. Recently, we developed a light trapping scheme for a-Si:H thin film solar cells, where the back reflector was replaced by two dimensional photonic crystal on top of distributed Bragg reflector (DBR) [4]. Photonic crystals have been a major scientific revolution in manipulating and guiding light in novel ways [5]. The advantage of photonic crystals is to introduce diffraction, where the photon momentum
(k) can be scattered away from the specular direction with (k|| = ki|| + G), where G is
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