Nanoparticle Scattering, Absorption, and Interface Effects for Surface Plasmon Enhanced Thin Silicon Solar Cells: Theory, Past Findings, and Future Directions

Thin Film Crystalline Silicon Solar Cells

Silicon is the second most abundant material in the earth’s crust. It accounts for the vast majority of electronic devices and lies at the heart of the computer industry. Despite its being abundant, environmentally benign, and at this point in our scientific evolution precisely grown and engineered, it is—to put it somewhat poetically—not particularly fond of our sun’s offerings. Its absorption coefficient in much of the solar spectrum is so weak that crystalline substrates 300 micrometers thick are required to adequately absorb the bulk of photons whose energy is greater than or equal to its bandgap. Though a thickness less than third of a millimeter may seem nominal by everyday standards, it has in fact proven to be a central factor compromising the adoption of silicon photovoltaics. Semiconductors of direct bandgap materials require thicknesses of a few micrometers, expediting their production and reducing their costs. As is seen in Figure 1, 300 µm of silicon with an adequate anti-reflective coating will absorb 90% of above bandgap photons, while a 2 µm c-Si film with no front or rear reflection absorbs only one third; for a film on a glass substrate without any anti-reflective coatings, that number drops to barely about one fifth.¹ Yet the long-term benefits of silicon cannot be understated: it is inexhaustible and nontoxic. For this reason, much of silicon-based photovoltaic research now focuses on methods of increased absorption and light trapping within silicon itself, so that thin crystalline silicon solar cells, less than a few micrometers thick, may attain efficiencies as high as bulk wafers.

Figure 1.

Absorbed photon flux for 300 µm (upper solid line) and 2 µm (lower solid line) thick c-Si wafers, assuming no reflection on the front or rear of the cell. Also shown is the absorbed flux for a 2 µm c-Si film with no antireflective coatings situated upon on a glass substrate (dotted line), and the incident AM1.5 solar flux (dashed line).