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Abstract
Light trapping has now been recognized as an essential element of highly efficient solar cells. A large number of sophisticated nanostructures have been developed and optically characterized, many of which have been aimed at thin-film silicon technology. It is still an open question whether such nanostructures are beneficial for thick devices, however, especially, since highly efficient solar cells employ >100 μm thick absorber materials and wet etched micron-sized pyramids for light trapping. In this paper, we study and compare the optical and electrical performances of binary quasirandom nanostructures with pyramidal structures to address this question. We show that, while simulations indicate that pyramids have better optical performance, the best overall performance observed experimentally was achieved with binary nanostructures. We found that the experimental short-circuit current for a solar cell patterned with a quasirandom nanostructure is 3.2 mA/cm2 higher than the current observed with pyramids. We attribute this higher current to a better balance between optical performance and surface recombination achieved by the binary nanostructures. This result indicates that binary nanostructures may be beneficial even for thick solar cells.
Highlights
As these pyramidal features are typically larger than the wavelength of sunlight, they are not applicable to thin film devices that may have an active layer thickness of only a few microns or less
Very few light trapping nanostructures have been implemented and characterised in real devices, and we are not aware of any comparison including electrical measurements in real thick solar cells between light trapping binary nanostructures and the standard pyramidal structures. Such a comparison is important for addressing the following two questions: 1 – is the optical performance of binary nanostructures comparable to that of pyramids, especially in thick solar cells? 2 – is the electrical performance of binary nanostructures better or worse than that of pyramids? This second question is generally overlooked but is important because smaller feature sizes result in smaller surface area for recombination, offering a route for improving the electrical properties of silicon solar cells
In order to address this question, we have investigated nanostructures that have been optimised for light trapping in thin film devices, have improved their antireflection property by applying a standard AR coating and have applied them to thick silicon solar cells
Summary
As these pyramidal features are typically larger than the wavelength of sunlight, they are not applicable to thin film devices that may have an active layer thickness of only a few microns or less. All high efficiency silicon solar cell designs demonstrated to date employ light trapping structures that typically consist of wet etched pyramids [1], inverted pyramids [2], [3] or honeycomb [4], [5] structures.
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