Photonic crystal back-reflectors for light management and enhanced absorption in a-Si:H solar cells
Vikram L. Dalal
Thin film hydrogenated amorphous silicon (a-Si:H) solar cells suffer from weak absorption of long wavelength photons, which have absorption lengths that are far greater than the absorber layer thickness. Light trapping schemes utilizing photonic crystal based back-reflectors can strongly diffract long wavelength photons and increase their optical path length. Photonic crystal back-reflectors were fabricated to investigate optical absorption enhancement in a-Si:H solar cells.
Photonic crystal back-reflectors were patterned with photolithography and etched in crystalline silicon wafers using reactive-ion etching. The etched photonic crystals were then coated with silver and aluminum doped zinc oxide for a highly reflective back contact. These structures had triangular lattice symmetry, an etch depth between 200 nm and 250 nm, and a pitch of 760 nm. Scanning electron microscopy images demonstrate high quality long-range periodicity. A-Si:H n-i-p solar cells were deposited on these back-reflectors using standard plasma enhanced chemical vapor deposition techniques.
Diffuse and total reflection measurements indicate high diffuse reflectance and strong absorption within the photonic crystal. Normalized external quantum efficiency (EQE) measurements demonstrated considerable absorption enhancement at near-infrared wavelengths. The photonic crystal back-reflector increased optical absorption by a factor of 6 to 7 at certain wavelengths, relative to flat reference devices on the same substrate. An enhancement of 8 times was also observed for a reference device on stainless steel coated with flat silver. Short circuit current, as determined by weighting EQE against the AM1.5 solar spectrum, increased by over 7% with the photonic crystal back-reflectors.