Chirped porous silicon reflectors for thin-film epitaxial silicon solar cells

Abstract

The studies of porous silicon as a one-dimensional photonic crystal have led to solutions allowing the fabrication of broad photonic band gaps as large as several hundreds nanometers for various types of applications. In this work we demonstrate the use of the chirping process, i.e., the gradual increase in the spatial period of the structure, as it is used in image processing, to design porous silicon broad-band reflectors for thin-film silicon solar cells. Modeling of those layers is done using linear design method and a simulation software. Such chirped structures are fabricated by an anodization process. Samples are prepared on mono and multicrystalline silicon substrates with 15, 40, 60, and 80 layers. The reflectance spectra of such prepared porous reflectors are evaluated and the results show an increase in bandwidth of over 50% of the total width in comparison with the conventional, unchirped reflectors. We demonstrate clear advantages of introducing chirped multilayer structures over conventional ones. Although some understanding of chirping exists mainly in the field of image processing and photonics, detailed study of chirped porous structures as broad-band reflectors is needed for their implementation in thin-film epitaxial silicon solar cells. A record efficiency of almost 14% was reached for those types of reflectors on a large area 71 cm2 epitaxial Si solar cell, giving a boost in efficiency of 0.6% absolute in comparison with the solar cells containing conventional reflectors.

Influence of Chirped DBR Reflector on the Absorption Efficiency of Multi-nanolayer Photovoltaic Structures: Wavelength-scale Analysis by the Method of Single Expression

“…DBRs made as non-periodic dielectric stacks, where the thicknesses of bilayers are monotonously changed by some linear, quadratic, exponential or other law, are called chirped DBRs. Absorption in thin-film PV cells using chirped DBR mirrors can be enhanced over a wider spectral range by providing higher efficiency in comparison with those using conventional reflectors [7], [14]- [16]. By tailoring the law of chirp, and the number and thickness of bilayers in chirped DBRs, the efficiency of such PV cells can essentially be enhanced.…”

“…Nowadays PV cells are thin-film cells with absorbing layer thicknesses that are of the same order or smaller than the operating wavelength [5], [6]. Thin films essentially reduce the amount of semiconductor material required for each PV cell compared to bulk PV cells and hence lower the cost of production [2], [7]. The probability of photon absorption increases with an increased thickness of the absorbing layer [8].…”

Improved Performance and Spectral Features of Complex Porous Silicon Structure Containing Silicon Solar Cells

“…A porous silicon layer has a lower reection and refractive indices than bulk silicon [15]. These circumstances allow the production of optical elements made of porous silicon, such as antireection coatings and the Bragg mirrors [11,16].…”

“…Recently, porous silicon structure has been mostly used to enhance the eciency of the epitaxial silicon solar cell [11,12]. The production of chirped Bragg reector was performed in two steps: at rst, the conventional p-type silicon substrate was made.…”

Thermal emission of macroporous silicon chirped photonic crystals

“…Porous materials created by electrochemical techniques are excellent candidates for fabricating chirped structures, since the porosity profile can be almost freely selected. Specifically, chirped bragg mirrors based on microporous silicon have been already produced [10,11].…”