Silevo, the solar startup formerly known as Sierra Solar Power (Sierra), has developed a “hybrid cell,” so called because it is a combination of three materials: crystalline silicon n-type substrates, thin-film passivation (non-reactive) layers that allow higher voltages, and a tunneling oxide layer.
Sierra is the owner of record of several published patent applications, including U.S. Publication Nos. 2010/0229927 (’927 Application) and 2011/0068367 (’367 Application), relating to a heterojunction solar cell and a double-sided heterojunction solar cell, respectively.
The ’927 Application is entitled “Heterojunction solar cell based on epitaxial crystalline silicon thin film on metallurgical silicon substrate design” and directed to methods of making a heterojunction solar cell and a cell comprising a crystalline silicon thin film (202) epitaxially grown on a metallurgical-grade silicon substrate (200).
A backside electrode (206) is formed on the back side of the substrate (200). A crystalline silicon base film (204) is epitaxially grown on top of the thin film (202), and a passivation layer (208) is deposited on top of the base film.
A heavily doped amorphous silicon layer is deposited on the passivation layer (208) to form an emitter layer (210), and a layer of transparent conducting oxide (TCO) is deposed on the emitter layer to form an anti-reflective layer (212).
The ’367 Application is entitled “Double-sided heterojunction solar cell based on thin epitaxial silicon” and directed to double-sided heterjunction solar cells and fabrication methods.
The cell has a layered structure similar to that taught in the ’927 Application with some of the layers deposited on both sides of the cell.
For instance, ultra-thin passivation layers (206, 208) are deposited on the front and back surfaces of the base layer (204).
Heavily doped amorphous silicon layers are deposited on the surfaces of passivation layers (206, 208) to form a front side emitter layer (210) and a back surface field (BSF) layer (212).
TCO layers form conductive anti-reflection layers (214, 216) on the surface of both the emitter layer (210) and the BSF layer (212).
The cell may also have a front and back side electrode (218, 220) on a respective TCO layer (216, 218) and metal strips (222) on the back side of the cell to complete the back side heterojunction.
According to the ’927 Application, the epitaxial mono-crystalline silicon thin film grown on metallurgical grade silicon solves the problem of higher cost associated with thicker crystalline silicon layers.
The ’367 Application notes the advantage of having ultra-thin passivation layers between the silicon film and substrate for higher voltage and lower resistance:
The thickness of passivation lawyer 206 and 208 can be between 2 nm and 8 nm. Note that such thickness is thin enough to allow tunneling of majority carriers, thus ensuring low series resistance of the solar cell.
The technology seems to be performing. According to these articles by Greentech Media and PhysOrg.com, Silevo’s thin film hybrid cells demonstrate 20-21% conversion efficiency on full-size substrates, and the company says they provide the best performance-to-cost ratio for solar modules in the industry.