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Chirping boosts solar cell efficiency

11 Nov 2008

Chirped silicon reflectors have been integrated into thin-film silicon solar cells for the first time.

Chirped reflectors can boost the efficiency of thin-film silicon cells to a record level of 14%, say researchers at IMEC in Belgium. The team believes that this is the first time that chirped reflectors have been used in solar cells and say that their system outperforms other devices of the same type (Journal of Applied Physics 104 073529).

"We have successfully designed and implemented a novel type of reflector, which has proven to be much more efficient than conventional Bragg reflectors," Izabela Kuzma-Filipek, a researcher at IMEC, told optics.org. "When incorporated into a silicon solar cell, we report yield currents up to 30 mA/cm2 for an active layer thickness of only around 20 µ and overall solar efficiencies above 14%. This narrows the gap between the thin-film solar cell and industrial bulk technology."

Conventional thin-film silicon solar cells are limited by a thin active layer and efficiency values are typically around 13%. According to Filipek, introducing a chirped reflector, which incorporates a gradual increase in the spatial period of the structure, not only increases the efficiency of solar cells to 14%, but leads to other advantages.

"Chirped reflectors are more robust for both high-efficiency solar cells processing by photolithography and industrial solar cells processing on multicrystalline silicon substrates," she said. "It also leads to significant bandwidth enlargement and high reflectivity.

In fact, the bandwidth of a chirped reflector is 50–80% larger than that of a conventional Bragg reflector. The chirped pattern means that the reflector is designed not just for a single wavelength, but the operational wavelength varies with the structure. This means that more photons – particularly photons with low energy – are absorbed and reflected multiple times before escaping the active layer. Filipek and colleagues report a path length enhancement of up to 11 times.

In the set-up, the reflector is sandwiched between the active silicon layer and a silicon substrate. The reflector comprises a stack of porous silicon layers with alternating refractive indices (high and low) where the thickness of each layer increases with depth.

The next challenge for the team is industrialization of its epitaxial thin-film solar cell concept, taking into account the need for a high throughput epitaxial reactor.

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