High Efficiency Solar Cells
L. Zeng, Y. Yi, C.-Y. Hong, J. Liu, N.-N. Feng, X. Duan, L. C. Kimerling
A broad-area Si solar cell can made from an out-of-plane p-i-n design, where the intrinsic thickness dimension is on the order of the Si wafer thickness. This design can be a readily deployable source of DC current, built on a processing technology that leverages the knowledge base and processing capital of the Si IC industry. However, the weak absorption coefficient in the visible and IR range of Si (absorption length ~3 mm) implies generation of a weak photo-current, from the single-pass absorption of sunlight within a ~500 micron thick Si wafer. Since the photo-generated carriers must be collected in the same direction as light absorption, this intrinsic thickness dimension must be kept small, in order to optimize excess photo-current extraction through the p- and n-type regions. If the intrinsic thickness dimension is too long, recombination mechanisms may reduce the excess number of photo-carriers, before they can drift to the p- and n-type regions; as a result, extracted photo-current will decrease. A novel device solution is needed that optimizes sunlight absorption, without creating excessively thick intrinsic regions that compromise photo-current extraction.
Our proposed solution is a backside textured Bragg Reflector (see Fig. 1(a)), which can be more generally be understood as a 2D photonic crystal. In the direction of light absorption, a periodic 1D Bragg Reflector reflects 99.8% of incident light in the visible and near-IR spectrum. Perpendicular to this direction, i.e. in-plane, a one-dimensional periodic grating creates a textured pattern for the deposited layers of the Bragg Reflector. The periodicity of this texture is designed to scatter light the reflected light at highly oblique angles into the Si wafer. The majority of these oblique, or grazing, angles lie outside the angle of total internal reflection between Si and air; the incident sunlight has thus been reflected and re-directed into the in-plane direction of the solar cell, where the cell's broad area dimensions (>> cm) easily ensure complete absorption of the sunlight and transduction into photo-current. This approach has currently demonstrated a 5-10´ enhancement in external quantum efficiency, for the near-IR spectrum (see Fig. 1(b)).
Fig. 1(a) Schematic of a surface-textured Bragg Reflector, in the backside of a p-i-n Si solar cell. The textured surface acts to scatter incident sunlight into oblique, or grazing, angles.
