Abstract
Photovoltaic light trapping theory and experiment do not always clearly demonstrate how much useful optical absorption is enhanced, as opposed to parasitic absorption that cannot improve efficiencies. In this work, we develop a flexible flux plane method for capturing these parasitic losses within finite-difference time-domain simulations, which was applied to three classical types of light trapping cells (e.g., periodic, random and plasmonic). Then, a 2 µm-thick c-Si cell with a correlated random front texturing and a plasmonic back reflector is optimized. In the best case, 36.60 mA/cm2 Jsc is achieved after subtracting 3.74 mA/cm2 of parasitic loss in a 2-µm-thick c-Si cell slightly above the Lambertian limit.
Highlights
There has been a strong trend in the solar industry toward thinner photovoltaic cells, in the last decade
The upper limit of light trapping for purely random texturing is known as the Lambertian limit, which represents a maximum enhancement of path length of 4n2 in weakly absorptive materials [12]; for 2 μm thick crystalline silicon (c-Si) solar cells with perfect carrier collection, it would yield a short circuit current around 35 mA/cm2 [13]
To quantify the role of material thickness in the periodically textured design, and for ease of comparison with other experiments, we modeled the effect of increasing the average thickness of crystalline silicon from the experimental value of 1 μm to a value of 2 μm, which was used in the random texturing and plasmonic cells
Summary
There has been a strong trend in the solar industry toward thinner photovoltaic cells, in the last decade. The upper limit of light trapping for purely random texturing is known as the Lambertian limit, which represents a maximum enhancement of path length of 4n2 in weakly absorptive materials [12]; for 2 μm thick c-Si solar cells with perfect carrier collection, it would yield a short circuit current around 35 mA/cm2 [13] This gap between experiment and theory strongly implies that parasitic losses are occurring in these designs, which may be associated with metallic structures in the back [6, 9, 11]. Finite difference time domain (FDTD) methods can employ the integral of the power loss over the volume of the photovoltaic materials to quantify the fraction of useful and parasitic optical absorption at any layer of the solar cells [17]. The proposed simulation technique will be discussed in detail, and verified through a detailed comparison to experimental thin-film c-Si light trapping structures from
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