Elusive Solar Light-Trapping Limit Nearly Reached

The theoretical limit of light-trapping in solar cells has eluded researchers for decades. But a group from Delft University of Technology (TU Delft) has come closer to it than anyone else.

The researchers, members of TU Delft’s Photovoltaic Materials and Devices (PVMD) group, have experimentally demonstrated the theoretical limit of the enhancement of light absorption using an advanced metal-free light-trapping scheme for the ultrathin crystalline silicon wafers that they developed.

A 20-µm-thick flexible absorber extracted from the crystalline silicon wafer support. Photos courtesy of TU Delft (Photovoltaic Materials and Devices Group).

On the front of the silicon wafers, they applied a nanotexture known as black silicon. On the back, they implemented a random pyramidal texture coated with a photonic dielectric back reflector, designed to exhibit maximal and omnidirectional internal reflectance.

Wafers thinner than 35 µm allowed the group to achieve more than 99 percent of the theoretical classical light absorption limit in the 400- to 1200-nm spectral range (with the photonic reflector), and up to 99.8 percent with the silver back reflector.

Successful implementation of the light-trapping scheme in crystalline silicon solar cells required an adequate surface passivation of the front nanotexture.

Cross-sectional scanning electron microscopy image of optimized dielectric distributed Bragg reflector coating randomly-etched pyramids of crystalline silicon.

For this purpose, the researchers also developed thermal silicon oxide and aluminum oxide passivation layers.

These developments essentially pave the way for the next generation of high-efficiency, cost-effective ultrathin crystalline silicon solar cells, the researchers said.

The work was funded by Agentschap NL, with project partners Solland Solar Cells and the Energy Research Centre of the Netherlands. The research is published in ACS Photonics (doi: 10.1021/ph4001586).