Laser processing technology increases solar cell efficiency to 22%

Laser processing technology is the most critical technology in RISE machining programs. Since the non-contact laser processing technology they use has long been used in most other industries, the RISE process is suitable for the production of RISE solar cells using large-area crystalline silicon wafers, which will reduce the production costs. Comparing the production costs of today's standard solar cells.

At present, solar cells made of crystalline silicon have dominated the solar photovoltaic product market. The light-to-electric conversion efficiency of general industrial crystalline silicon solar cells ranges from 14% to 16%, and the adoption of new laser processing technology can improve the light-to-electric conversion efficiency of solar cells. Researchers at the Institut fur Solarenergieforschung Hameln (ISFH) Research Institute in Germany have developed a manufacturing process for solar cells, the Back Cross Single Evaporation (RISE) process. With the aid of laser processing technology, the photoelectric conversion efficiency of back-contact silicon solar cells manufactured using this process reaches 22%.

At present, many manufacturers use laser processing technology to produce silicon solar cells. BP Solar, Inc. (Frederick, MD) uses a laser-etched gated-gate technology, which uses laser technology to etch grooves in the silicon surface and then fill the metal to provide the front surface with electrical contact with the gate. The advantage of this technique is that it can reduce the shielding loss compared with the standard front-side plated metal layer. Advent Solar uses another technique called emitter wrap-through. A through-hole is drilled on the silicon wafer with a laser, 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-electric conversion efficiency.

Non-contact processing
In the process of producing solar cells using the R1SE process, a laser processing method is used to create a cross-pattern emission region and a base region on the back of a solar cell, and laser ablation also enables the self-aligned contact layer to be reliably separated after the metal evaporates (see FIG. 1). ). Non-contact processing (important for minimizing wafer damage) first utilizes an on-off triple frequency 355 nm Nd:YVO4 laser to ablate the interdigitated emitter region in a silicon nitride or silicon oxide layer on the backside of a crystalline silicon wafer. Base graphic. Any damage to the wafer will shorten the lifetime of the carrier and will also reduce the light-to-electricity conversion efficiency. Then, etching was performed using potassium hydroxide (KOH) to remove the damaged portion caused by laser processing. After etching, the luminescent material diffuses to form the emitter region, leaving the raised silicon nitride and oxidation as the base region. Before wet chemical etching, the next processing step is to separate the emitter and base regions using metal evaporation self-aligned contact layer separation processing.

In order to optimize the conversion efficiency of solar cells, the ISFH Research Institute and the German company Laser Zentrum Hannove: The company’s researchers found that the lifetime of carriers is related to wafer damage and KOH corrosion depth caused by different wavelength laser light sources. Moreover, the depth of the wafer damage caused by laser ablation of triple frequency Nd:YAG 355 nm is 3 μm; the depth of wafer damage caused by double-frequency Nd:YAG laser ablation of 532 nm is 4 μm; and that caused by Nd:YAG 1 064 nm laser The depth of damage will exceed 20 μm. As long as the removed damage layer reaches such a depth, it will not affect the conversion efficiency of more than 20% of the solar cell, but the depth is closely related to the cost of the laser cost and the thickness of the silicon wafer and needs to be fully considered in the production process.

Researchers in the solar cell research group believe that laser processing technology is the most critical technology in RISE processing programs. Since the non-contact laser processing technology they use has long been used in most other industries, the RISE process is suitable for the production of RISE solar cells using large-area crystalline silicon wafers, which will reduce the production costs. Comparing the production costs of today's standard solar cells.