Researchers at the Solarenergieforschung Hameln ( ISFH ) Institute have developed a process for manufacturing solar cells, the Backcross Single Evaporation (RISE) process. With the help of laser processing technology, the photoelectric conversion efficiency of the back contact silicon solar cell manufactured by this process reaches 22%.
At present, solar cells made of crystalline silicon have dominated the market for solar photovoltaic products. The optical-to-electrical conversion efficiency of general industrial crystalline silicon solar cells is 14% to 16%, and the new laser processing technology can improve the light-to-electricity conversion efficiency of solar cells. Many manufacturers use laser processing technology to produce silicon solar cells. BP Solar (Frederick, MD) uses laser-grooved buried gate technology, which means that laser technology is used to groove the silicon surface and then fill the metal to make the front surface electrically contact the gate. This technology has the advantage of reducing shielding losses compared to standard front surface plated metal layers.
Advent Solar uses another technology called emitter wrapthrough. The laser is used to drill a through hole on the silicon wafer, and the highly doped wall conducts the current on the front surface of the emitter region to the metal contact layer on the back surface, thereby further reducing the shielding loss and improving the light-to-electric conversion efficiency.
Non-contact processing
In the process of producing solar cells by the R1SE process, a laser patterning method is used to fabricate a cross-pattern emitter region and a base region on the back side of the solar cell. Laser ablation also enables the self-aligned contact layer to be reliably separated after metal evaporation (see Figure 1). ). Non-contact processing (important for reducing wafer damage) firstly uses a port-switched triple-frequency 355 nmNd:YVO4 laser to ablate the finger-like interdigitated emitter regions in a thin layer of silicon nitride or silicon oxide on the backside of a crystalline silicon wafer. Base area graphic. Any damage to the wafer will shorten the lifetime of the carrier, as well as reduce the efficiency of light-to-electric conversion. It is then etched using potassium hydroxide (KOH) to remove the damaged portion caused by laser processing. After etching, the luminescent material diffuses to form an emitter region, leaving raised silicon nitride and oxidation as a base region. Prior to wet chemical etching, the next processing step is to separate the emitter and base regions by metal evaporation self-aligned contact layer separation processing.
In order to optimize the conversion efficiency of solar cells, ISFH Research Institute and Germany's Laser Zentrum Hannove: The company's researchers found that the lifetime of the carriers is related to the wafer damage and KOH corrosion depth caused by laser sources of different wavelengths. Moreover, the triple-frequency Nd:YAG 355 nm laser ablation results in a wafer damage depth of 3 μm; the double-frequency Nd:YAG 532 nm laser ablation results in a wafer damage depth of 4 μm; and the Nd:YAG 1 064 nm laser causes The damage depth will exceed 20 μm. As long as the damaged layer is removed to such a depth, the conversion efficiency of the solar cell is not affected by more than 20%, but the depth is closely related to the cost of the laser and the thickness of the silicon wafer, and needs to be fully considered in the production process.
Researchers at the Solar Cell Research Group believe that laser processing technology is the most critical technology in RISE processing. Since the non-contact laser processing technology they have used has been used in most other industries, the RISE processing technology is suitable for the production of RISE solar cells using the large-area crystalline silicon thin wafer industry, which will make its production cost less. Compared to the production cost of standard solar cells today.
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