CR1220 battery

　　Development status and summary of photovoltaic thin film CR1220 batterypreparation technology

　　The technologies for preparing thin film solar cells currently include the following:

　　The first technique is to make epitaxial thin-film solar cells, starting from highly doped crystalline silicon wafers (such as premium metallurgical silicon or scrap), and then using chemical vapor deposition (CVD) methods to deposit epitaxial layers. In addition to advantages such as cost and availability, this approach could enable the gradual transition of silicon solar cells from wafer-based to thin-film technology. Because it has a process similar to the traditional bulk silicon process, this technology is easier to implement on existing process lines than other thin film technologies.

　　The second is thin-film solar CR1220 batterytechnology based on layer transfer, which epitaxially deposits a single-crystal silicon layer on a porous silicon film, so that the single-crystal silicon layer can be separated from the substrate at a certain point in the process. The idea behind this technology is to reuse the mother substrate multiple times, resulting in a very low cost for the final silicon wafer per solar cell. An interesting option under investigation is the possibility of isolating the porous silicon film prior to epitaxy and exploring the possibility of a support-free film process.

　　The last type is thin-film polysilicon solar cells, which deposit a layer of crystalline silicon with a thickness of only a few microns on a cheap heterogeneous substrate, such as ceramics or high-temperature glass. Polycrystalline silicon films with grain sizes between 1-100mm are a good choice. We have demonstrated that high-quality polycrystalline silicon solar cells can be obtained using aluminum-induced crystallization of amorphous silicon. This process can produce a very thin polysilicon layer with an average grain size of about 5mm. Then, using high-temperature CVD technology with a growth rate exceeding 1 mm/min, the seed layer is epitaxially grown into an absorption layer several microns thick, and the substrate is ceramic alumina or glass ceramic. Thermal CVD was chosen because of its high growth rate and the ability to obtain high-quality crystals. However, this choice limits the use of heat-resistant substrate materials such as ceramics. This technology is not as mature as other thin film technologies, but it has shown great potential to reduce costs.

　　Adopting thin-film PV technology has been able to improve the efficiency of solar cells or simplify their processes and will reduce their costs. But no one has been able to combine these two aspects at the same time. However, some recent findings have taken a necessary step in the right direction.

　　Improvements in epitaxial cells

　　The efficiency of epitaxial thin-film silicon solar cells is not very high (cells produced by semi-industrial screen printing technology are about 12%), which limits the photovoltaic industry's attention to this CR1220 batterytype. It can achieve an open circuit voltage and fill factor comparable to bulk silicon solar cells (monocrystalline silicon solar cells are ±77.8%). However, the short-circuit current (Jsc) is limited by thin optical active layers (<20mm). Light penetrating the epitaxial layer is collected and lost by the highly doped, low-quality substrate. Therefore, it is not uncommon to see a 7mA/cm2 short-circuit current difference between these two solar CR1220 batterytechnologies. The typical Jsc value for bulk silicon solar cells is about 33mA/cm2, while the average value for epitaxial thin film cells is about 26mA/cm2.

　　However, two independent battery-level developments have improved the situation2. By increasing the optical path length within the thin active layer, we reported a screen-printed epitaxial CR1220 batterywith a Jsc of 30 mA/cm2 and an efficiency of 13.8%.

　　The first improvement that contributed to these results was surface light scattering using fluorine-based plasma roughening. Ideally, this roughened surface of the active layer would diffuse the light 100% (i.e., a Lambertian refractor). This allows photons to pass through the active layer at an average angle of 60°, doubling the optical path length. In other words, the optical performance of a 20mm thin layer is equivalent to that of a 40mm thick active layer. We found that this total light scattering can be obtained by removing just 1.75mm of silicon. The advantages of plasma roughening are many, including lower reflection (down from 35% before roughening to 10%), oblique incidence light coupling, and lower contact resistance (because of the smaller contact area between the silicon substrate and the silver electrode). larger). We observe an absolute increase in Jsc of 1.0-1.5, while an efficiency increase of 0.5-1.0%.

　　The second improvement is internal light trapping through the introduction of porous silicon Bragg reflectors. To reduce the transmission of long wavelength light into the substrate, an intermediate reflector is placed at the interface between the substrate and the epitaxial layer. In this way, photons reaching this interface are reflected and pass through the active layer a second time. Since light begins to diffuse the moment it enters the CR1220 battery(this is determined by the Lambertian characteristics of plasma roughening), a large proportion of photons will hit the front surface at an angle greater than the escape angle. Therefore, most of the photons are reflected inward again and pass through the active layer a third time. This situation is repeated continuously, making it possible for photons to pass through the epitaxial layer multiple times.

　　In practice, such reflectors are produced by electrochemically growing stacks of porous silicon with alternating high and low porosity (multiple Bragg reflectors).

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