RTWCG SiOx ARC/SE/TO Solar Cell Design

RTWCG patented process developed by SPECMAT can be used to fabricate high-efficiency silicon solar cells at a very low cost.   RTWCG replaces multiple expensive manufacturing steps used by traditional silicon solar cells manufacturers with a 60-second chemical bath immersion that creates four important features known to increase photovoltaic cell efficiency.

1. The process grows a graded index of refraction SiOX antireflective coating (ARC), with a 5% to 9% AM1.5 weighted average reflectance.

2. The process creates a selective emitter (SE) concomitantly with the ARC by etching back the emitter surface. It requires no additional steps such as lithography since the screen printed metallization grid acts as a mask.

3. Passivates the surface.

4. Cleans the surface and contacts.

 Using the RTWCG process, SPECMAT has developed a proprietary silicon solar cell design and technology that enables a significant increase in cell efficiency at an ultra-low fabrication cost.  The target average efficiency for large area c-Si solar cells is 21%, compared to the industry’s average of about 15%.  The estimated fabrication cost of a silicon solar cell is 25% to 40% lower than that of conventional high efficiency cells.

RTWCG SiOX ARC/SE/TO cell structure and technology competitive merit:

q       Low SiOX reflectance;

q       Low SiOX absorption;

q       Highly effective selective emitter (SE) with a high surface concentration below

the front grid lines and near-optimal surface concentration between the front

grid lines;

q       Good collection efficiency;

q       Good surface passivation;

q       Improved gettering and removal  of unwanted bulk  impurities;

q       Low resistive losses of smooth SE/textured SiOX (TO) concept;

q       Low series resistance;

q       Low (screen printed) front grid contact resistance;

q       Low sheet resistance;

q        High shunt resistance.

RTWCG SiOX ARC

The RTWCG antireflection coatings (ARC) with graded index of refraction are applied in one easy step on finished (bare) photovoltaic cells. The RTWCG coatings grown on planar, textured and porous Si, as well as on other substrates, show excellent uniformity irrespective of a photovoltaic cell's surface topography, crystal orientation, size, and shape.

The weighted AM 1.5 average reflectance of the  RTWCG SiOX coating in the example above, is below 1.8% from 600 nm to 950 nm and below 3.3% from 400 nm to 1100 nm.
 

Selective Emitter:

In order to gain the full benefit of improved emitter surface passivation on cell performance, it is necessary to tailor the emitter doping profile so that the emitter is lightly doped between the gridlines, yet heavily doped under them. This is especially true for screen-printed gridlines, which require very heavy doping beneath them for acceptably low contact resistance.

RTWCG SiOX ARC/SE Technique:

During the growth of our RTWCG SiOX ARC film, a certain thickness (about 0.2 microns) of the emitter's large defect density surface is etched off from the areas between the front screen-printed collection grid lines. Consequently, the SE is formed

Reflectance of  a Si wafer (a) prior to and, (b) after RTWCG SiOX coating.

simultaneously with the growth of the SiOX ARC. This new SE concept, which has been successfully tested on production-type single crystal (c-Si), multicrystalline (mc-Si) and ribbon Si large solar cells, is by far the biggest contributor to efficiency gain.

Surface SiOX Texturing:

Using an additional room temperature wet chemical process step, within 10 to 15 seconds the surface of the SiOX film can be textured. This novel surface oxide texturing (TO) concept will increase the efficiency gain by further reducing the reflectance of the SiOX ARC, and increasing the collection efficiency of the diffused portion of the Sun's rays.

Advantages of the RTWCG SiOX ARC/SE versus conventional Si solar cell design:

The environmentally friendly RTWCG SiOx ARC/SE is a one wet chemical process step that requires about one minute, during which it:  

q       In-situ cleans silicon and metallization surfaces

q       Grows a graded index of refraction SiOx ARC, with 5% to 9% AM1.5 weighted average reflectance (depending on the type of silicon substrate and SiOx thickness)

q       RTWCG SiOx growth is compatible with front grid lines

q       SiOx does not cover the contacts (interconnect ready), and at the same time

q       Creates an excellent quality SE, compatible with large-volume, low-cost product.

Click on the following link for a general description of a photovoltaic cell.

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