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With ever-increasing world energy demands, the development of solar-cell technology has never been so relevant. With the aim of reducing production costs, researchers have, for some time now, investigated thin films as an alternative to bulk materials. One of the main obstacles to creating efficient thin-film silicon solar cells is the poor absorption of long-wavelength light. At a wavelength of 1,200 nm, the absorption length of light in silicon is as long as 3 mm, a distance clearly incompatible with the thickness of the thin layers (100s of micrometres). A number of light-trapping techniques have been demonstrated that increase the effective thickness of the absorbing layer; however, these techniques are often limited by severe reflection losses. Now, a new design demonstrated by a team at MIT can reduce these losses1.

Their technique consists of etching a grating onto the substrate and then depositing a distributed Bragg reflector (DBR) on top. The DBR has a reflectivity of over 99.8%, whereas the grating diffracts most of the light into an oblique angle such that, when it returns to the solar-cell surface, it is totally internally reflected. In this way, the light is effectively confined to the silicon active region until it can be absorbed. The researchers demonstrated experimentally a significant enhancement in external quantum efficiency of up to 135 times between the wavelengths of 1,000 nm and 1,200 nm, as well as a considerable increase in overall power-conversion efficiency. With further improvements already suggested, silicon thin-films could represent a viable alternative material to more-expensive GaAs in solar cells of the future.