Application of Photonic Crystals as Back Reflectors in Thin-Film Silicon Solar Cells

Abstract

University of Ljubljana, Faculty of Electrical Engineering, Trzaska 25, SI-1000 Ljubljana, Slovenia ABSTRACT: Light trapping techniques are essential for increasing efficiency of thin-film silicon solar cells. Scattering at textured interfaces, introduced by surface-textured substrates, and highly reflective back reflectors have led to important improvements in solar cells performance. Beside metal, novel dielectric back reflectors are nowadays of great interest. One-dimensional photonic crystals in the role of distributed Bragg reflector can exhibit high reflectance in a broad wavelength range. Low temperature amorphous silicon/amorphous silicon nitride based one-dimensional photonic crystals eligible to act as dielectric back reflector in thin-film amorphous silicon solar cells have been developed. The conformal growth and optical properties of our photonic crystals on flat and on corrugated substrates (both randomly-textured and periodically-textured) are reported. Using one-dimensional photonic crystals as back reflectors we have obtained higher long-wavelength quantum efficiency of the solar cells with respect to the cells with metal back reflectors. Keywords: photon management, photonic crystal back reflector, surface texture, diffraction gratings, thin-film silicon solar cells 1 INTRODUCTION Thin-film solar cells present an important alternative to crystalline silicon photovoltaic technology. Due to the small thickness, the low deposition temperature and the use of advanced light management techniques, they offer attractive price/performance ratio, which results in one-fifth of the photovoltaic trade share [1]. Electrical and optical properties of the cells should be further improved, since there is still a lot of potential to increase their conversion efficiency in terms of band-gap engineering and light management [2]. By using surface-textured transparent conductive oxide (TCO) substrates [3, 4, 5], which enable efficient light scattering, and highly reflective metallic back contacts [6], light trapping in the absorber layers can be established. The excellent conductivity and the good reflectance of metal back reflectors are mediated by some disadvantage which constraints the cell performance. Textured silver reflectors suffer from parasitic absorption due to surface plasmon effects, which can reduce the reflectivity properties of the reflector and representing an important source of loss in the solar cell [7]. Furthermore, silver is an expensive material for manufacturing processes and it is sensible to moisture in the environment. In order to avoid these drawbacks, novel back reflectors based on dielectric materials such as white paints [8] and photonic crystal structures [9] are of interest. The implementation of a photonic crystal (PC) structure in the role of a Distributed Bragg reflector (DBR) as a back reflector in thin-film silicon solar cells has been already investigated [10, 11]. Recently we have successfully fabricated one-dimensional (1-D) photonic crystals deposited at a temperature consistent with the deposition of amorphous silicon layers [12]. Formed by alternating pair of amorphous silicon/amorphous silicon nitride, these photonic crystals can act like an ideal DBR in a broad long-wavelength range. The thickness, the optical constants of the individual layers, and the number of repeated layers of the desired photonic crystals can be designed by means of optical simulations [13]. In this paper we focus on the application of our low temperature a-Si:H/a-SiNx:H based 1-D PC deposited on periodic and random surface textures in order to combine the light scattering promoted by corrugated surfaces with the high reflective properties of the photonic crystal. To verify the conformal growth of our 1-D PC we have applied the spatial-frequency surface representation determined from atomic force microscopy (AFM) scans and to characterize the reflectivity properties spectral reflectance measurements were carried out. Amorphous silicon single-junction solar cells with PC-based back reflectors were fabricated. External quantum efficiencies of the cells are presented. We demonstrate the potential of 1-D photonic crystals as back reflectors for enhancing the absorption in the absorber layer with respect to metal contacts. We also show the performance of solar cells when photonic crystal is deposited on textured surfaces, both periodic and random. Using 1-D PC we have obtained higher long-wavelength quantum efficiency with respect to metal back reflectors. 2 EXPERIMENTAL The 1-D PC that was designed for the back reflector in a-Si:H solar cell consists of three pairs of alternating a-Si:H/a-SiN