Abstract
Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect band gap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 mum thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.
Highlights
One of the foremost challenges in designing silicon photovoltaic cells is devising an efficient light-trapping scheme
The first solar cell design consists of an anti-reflection coating made of a transparent dielectric on top of a slab of Crystalline silicon (c-Si)
It is found that wave optics can vastly outperform geometrical optics within the range of wavelengths requiring enhancement in thin films of c-Si
Summary
One of the foremost challenges in designing silicon photovoltaic cells is devising an efficient light-trapping scheme. Crystalline silicon (c-Si) have an indirect band gap, which gives rise to weak absorption of light in the near infrared (near-IR), with an absorption length that increases from just over 10 μm for λ =800 nm to over 1 mm for λ =1108 nm [1]. That range of wavelengths contains 36.2% of solar photons with energies above the band gap of c-Si [2]. As a result, advanced light trapping schemes are needed in order to create thin yet efficient solar cells, made from c-Si and other, closely related materials, such as nanocrystalline silicon (which has the same bandgap and similar absorption characteristics [4]). There are two distinctive approaches to light trapping: geometrical optics, which is commonly used in solar cells today, and wave optics, which represents a new approach to the problem that has just begun to be explored
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