Crystal silicon cells have made great progress in the past 20 years, and the adoption and introduction of many new technologies have greatly improved the efficiency of solar cells. In the early days of silicon cell research, people explored a variety of cell structures and techniques to improve cell performance, such as back surface field, shallow junctions, suede, oxide film passivation, Ti/Pd metalized electrodes, and anti-reflective films. Wait. Later high-efficiency batteries were developed on the basis of these early experiments and theories. 1.2.1 Single crystal silicon high efficiency battery The typical representative of high efficiency single crystal silicon battery is the back point contact battery (PCC) of University of Stanford, passivation emission area battery of UNSW (UNSW), PESC, PERC, PERL and the localized Back Surface Field (LBSF) battery of Fraumhofer Solar Energy Research Institute in Germany, etc. China also conducted highly efficient battery research during the “Eighth Five-Year Plan†and “Ninth Five-Year Plan†period, and achieved promising results in recent years. One important advancement has come from the improvement of surface passivation technology, which has evolved from the thin oxide layer (<10 nm) of passivated emission cell solar cells (PESC) to PCC/PERC/PER1. The thick oxide layer (110 nm) of the cell. Passivation surface technology has reduced the density of surface states to less than 10 Bucm2, and the surface recombination velocity has dropped below 100 cm/s.In addition, surface V-grooves and inverted pyramid techniques, dual-layer anti-reflection coating technology enhancement and trapping theory Perfection also reduces the reflection of the surface of the battery and the absorption of infrared light.The low-cost and high-efficiency silicon battery has also been rapidly developed.
(1) The University of New South Wales High-efficiency Battery (A) Passivated Emitter Battery (PESC): The PESC battery was introduced in 1985. In 1986, the V-groove technology was applied to the battery and the efficiency exceeded 20%. The contribution of the V-groove to the electrode is: to reduce reflection on the surface of the cell; Vertical rays enter the wafer at a 41†angle after refracted at the surface of the V-groove, making the photogenerated carriers closer to the emitter junction, improving the collection efficiency and the low lifetime. The substrate is particularly important; the V-groove reduces the emitter's lateral resistance by a factor of 3. Since PESC's optimal emitter square resistance is above 150Ω/s, lowering the emitter resistance increases the cell fill factor.
After the phosphorous diffusion in the emitter is diffused, a ...m thick Al layer is deposited on the backside of the electrode, and then a 10 nm surface passivation oxide layer is thermally grown, and the back Al and Si are alloyed. The front oxide layer can greatly reduce the surface recombination rate. The alloy can absorb impurities and defects in the body, so the open circuit voltage is improved. Early PESC cells used shallow junctions, but later studies showed that shallow junctions were only effective for electrodes without surface passivation, were unnecessary for cells with good surface passivation, and oxide layer passivation performance and aluminum absorption The effect can be enhanced at higher temperatures, so that the best PEsC emitter junction depth increases to about 1 μm. It is worth noting that all current batteries with efficiencies greater than 20% use deep junctions instead of shallow junctions. Shallow junction batteries have become history.
The metallization of the PEsC cell is formed by a lift-off method to form Ti-pd contacts and then electroplated with Ag. This metallization has a considerable thickness/width ratio and a very small contact area, so the battery can achieve an 83% fill factor and a 20.8% (AM1.5) efficiency.
(B) Passivated Emission Zone and Back Surface Battery (PERC): Aluminum backside gettering is a key technology for PEsC batteries. However, due to the high recombination and low reflection of the back surface, it has become a major factor limiting the further improvement of the PESC cell technology. PERC and PERL batteries successfully solve this problem. It uses back surface contact instead of PEsC to electrically contact its entire backside aluminum alloy, and passivates its front and back surfaces with a 110 nm thick oxide layer grown with TCA (chloroethane). TCA oxidation produces extremely low interface state densities, while also eliminating metal impurities and reducing surface layer errors, thereby maintaining the original minority carrier lifetime of the substrate. Due to the high minority lifetime of the substrate and the high recombination at the back metal contacts, the back contact points are designed with a large pitch of 2 mm and a contact aperture of 2001 Lm. The contact point spacing needs to be greater than the minority diffusion length to reduce recombination. This battery achieves an open circuit voltage of approximately 700mV and an efficiency of 22.3%. However, since the contact pitch is too large, the series resistance is high and therefore the fill factor is low.
(C) Passive Emission Zone and Backside Local Diffusion Cell (PERL): Add a thick boron diffusion layer under the back contact to reduce metal contact resistance. Since the boron diffusion layer reduces the effective surface recombination, the contact point distance can be reduced to 250 μm and the contact aperture can be reduced to 10 μm without increasing the recombination of the back surface, thereby greatly reducing the electrical series resistance. The PERL battery achieved an open circuit voltage of 702mV and an efficiency of 23.5%. PERC and PER1. Another feature of the battery is its excellent light trapping effect. Since silicon is an indirect band gap semiconductor, the absorption coefficient to infrared is very low, and some infrared light can penetrate the battery without being absorbed. Ideally, incident light can travel back and forth through the substrate material 4n2 times, where n is the refractive index of silicon. PER1. The backside of the cell is formed of aluminum on SiO2 to form a good reflection surface, and the incident light is reflected back to the front surface on the back surface. Due to the inverted pyramid structure of the front surface, a large part of the reflected light is reflected back to the substrate. Repeatedly. Sandia National Laboratory P. Dr. Basore invented an infrared analysis method to measure the light trapping performance. The reflectance of the back surface of the PERL battery was greater than 95%, and the trapping coefficient was greater than 25 round trips. Therefore, PRER's infrared response is extremely high, and it is also particularly suitable for the absorption of monochromatic infrared light. Under monochromatic light with a wavelength of 1.02 μm, PER1. His conversion efficiency reached 45.1%. The efficiency of this battery AM0 also reached 20.8%.
(D) Buried gate cell: A laser-etched gate buried gate cell developed by UNSW. After the diffusion of the emitter junction, a trench of 20 μm wide and 40 μm deep was etched in front of the laser, and the phosphor was diffused after cleaning the bath. The metal electrode is then plated in the tank. The electrode is located inside the battery, reducing the area covered by the gate lines. The back of the battery is the same as the PESC. Since the groove will introduce damage, its performance is slightly lower than that of the PESC battery. He achieved an efficiency of 19.6%.
(2) Backside Contact Battery (PCC) at Stanford University
Touch his structure with PER1. Like the battery, using TCA to grow the oxide layer passivates the front and back of the battery. In order to reduce the shading effect of the metal strip, the metal electrode is designed on the back of the battery. The front of the cell uses a photolithographic pyramid (smear) structure. The emitters on the backside are designed as dots, 50 μm pitch, 10 μm diffused area, 5 μm contact aperture, and the base area is also of the same shape, which reduces backside recombination. The substrate uses an n-type low-resistance material (which takes advantage of low surface and in-vivo recombination), and the substrate is thinned to about 100 μm to further reduce in-body recombination. This electricity conversion efficiency is 22.3% under AM1.5.
(3) Deep junction partial back-field battery (LBSF) of Fraunhofer Solar Energy Research Institute, Germany
The structure of the LBSF is similar to that of a PERL battery, and also uses a TCA oxide passivation and inverted pyramid front structure. Because the backside boron diffusion generally causes high surface recombination, local aluminum diffusion is used to make the surface contact of the battery, and the cell efficiency of the 2cm×2cm cell reaches 23.3% (Voc=700mV, Isc−~41.3mA, FF−0.806).
(4) Japan sHARP C-Si/μc-Si heterogeneous pp+ junction high-efficiency battery SHARP energy conversion laboratory high-efficiency battery, in front of the use of suede texture, deposition of SiN on the SiO2 passivation layer behind A only B A RF-PECVD boron-doped μc-Si film was used as the back field, and a SiN film was used as the passivation layer on the back surface. The Al layer was in contact with the μcSi film through the holes on the SiN. The efficiency of 5cmX5cm was 21.4% at AM1.5 (Voc=669mV, Isc=40.5mA, FF=0.79).
(5) China's single-crystal silicon high-efficiency battery Tianjin Institute of Power Research carried out high-efficiency battery research under the support of the “8th Five-Year Plan†of the State Science and Technology Commission. Its battery structure is similar to UNSw's V-groove PEsC battery, with a battery efficiency of 20.4%. The Beijing Institute of Solar Energy conducted a high-efficiency battery research under the support of the Beijing Municipal Government during the “Ninth Five-Year Plan†period. The inverted pyramid structure of the battery was placed in front of the battery. The efficiency of the 2cmX2cm battery reached 19.8%. The large-surface (5cmX5cm) laser groove The buried grid cell efficiency reached 18.6%.
(1) The University of New South Wales High-efficiency Battery (A) Passivated Emitter Battery (PESC): The PESC battery was introduced in 1985. In 1986, the V-groove technology was applied to the battery and the efficiency exceeded 20%. The contribution of the V-groove to the electrode is: to reduce reflection on the surface of the cell; Vertical rays enter the wafer at a 41†angle after refracted at the surface of the V-groove, making the photogenerated carriers closer to the emitter junction, improving the collection efficiency and the low lifetime. The substrate is particularly important; the V-groove reduces the emitter's lateral resistance by a factor of 3. Since PESC's optimal emitter square resistance is above 150Ω/s, lowering the emitter resistance increases the cell fill factor.
After the phosphorous diffusion in the emitter is diffused, a ...m thick Al layer is deposited on the backside of the electrode, and then a 10 nm surface passivation oxide layer is thermally grown, and the back Al and Si are alloyed. The front oxide layer can greatly reduce the surface recombination rate. The alloy can absorb impurities and defects in the body, so the open circuit voltage is improved. Early PESC cells used shallow junctions, but later studies showed that shallow junctions were only effective for electrodes without surface passivation, were unnecessary for cells with good surface passivation, and oxide layer passivation performance and aluminum absorption The effect can be enhanced at higher temperatures, so that the best PEsC emitter junction depth increases to about 1 μm. It is worth noting that all current batteries with efficiencies greater than 20% use deep junctions instead of shallow junctions. Shallow junction batteries have become history.
The metallization of the PEsC cell is formed by a lift-off method to form Ti-pd contacts and then electroplated with Ag. This metallization has a considerable thickness/width ratio and a very small contact area, so the battery can achieve an 83% fill factor and a 20.8% (AM1.5) efficiency.
(B) Passivated Emission Zone and Back Surface Battery (PERC): Aluminum backside gettering is a key technology for PEsC batteries. However, due to the high recombination and low reflection of the back surface, it has become a major factor limiting the further improvement of the PESC cell technology. PERC and PERL batteries successfully solve this problem. It uses back surface contact instead of PEsC to electrically contact its entire backside aluminum alloy, and passivates its front and back surfaces with a 110 nm thick oxide layer grown with TCA (chloroethane). TCA oxidation produces extremely low interface state densities, while also eliminating metal impurities and reducing surface layer errors, thereby maintaining the original minority carrier lifetime of the substrate. Due to the high minority lifetime of the substrate and the high recombination at the back metal contacts, the back contact points are designed with a large pitch of 2 mm and a contact aperture of 2001 Lm. The contact point spacing needs to be greater than the minority diffusion length to reduce recombination. This battery achieves an open circuit voltage of approximately 700mV and an efficiency of 22.3%. However, since the contact pitch is too large, the series resistance is high and therefore the fill factor is low.
(C) Passive Emission Zone and Backside Local Diffusion Cell (PERL): Add a thick boron diffusion layer under the back contact to reduce metal contact resistance. Since the boron diffusion layer reduces the effective surface recombination, the contact point distance can be reduced to 250 μm and the contact aperture can be reduced to 10 μm without increasing the recombination of the back surface, thereby greatly reducing the electrical series resistance. The PERL battery achieved an open circuit voltage of 702mV and an efficiency of 23.5%. PERC and PER1. Another feature of the battery is its excellent light trapping effect. Since silicon is an indirect band gap semiconductor, the absorption coefficient to infrared is very low, and some infrared light can penetrate the battery without being absorbed. Ideally, incident light can travel back and forth through the substrate material 4n2 times, where n is the refractive index of silicon. PER1. The backside of the cell is formed of aluminum on SiO2 to form a good reflection surface, and the incident light is reflected back to the front surface on the back surface. Due to the inverted pyramid structure of the front surface, a large part of the reflected light is reflected back to the substrate. Repeatedly. Sandia National Laboratory P. Dr. Basore invented an infrared analysis method to measure the light trapping performance. The reflectance of the back surface of the PERL battery was greater than 95%, and the trapping coefficient was greater than 25 round trips. Therefore, PRER's infrared response is extremely high, and it is also particularly suitable for the absorption of monochromatic infrared light. Under monochromatic light with a wavelength of 1.02 μm, PER1. His conversion efficiency reached 45.1%. The efficiency of this battery AM0 also reached 20.8%.
(D) Buried gate cell: A laser-etched gate buried gate cell developed by UNSW. After the diffusion of the emitter junction, a trench of 20 μm wide and 40 μm deep was etched in front of the laser, and the phosphor was diffused after cleaning the bath. The metal electrode is then plated in the tank. The electrode is located inside the battery, reducing the area covered by the gate lines. The back of the battery is the same as the PESC. Since the groove will introduce damage, its performance is slightly lower than that of the PESC battery. He achieved an efficiency of 19.6%.
(2) Backside Contact Battery (PCC) at Stanford University
Touch his structure with PER1. Like the battery, using TCA to grow the oxide layer passivates the front and back of the battery. In order to reduce the shading effect of the metal strip, the metal electrode is designed on the back of the battery. The front of the cell uses a photolithographic pyramid (smear) structure. The emitters on the backside are designed as dots, 50 μm pitch, 10 μm diffused area, 5 μm contact aperture, and the base area is also of the same shape, which reduces backside recombination. The substrate uses an n-type low-resistance material (which takes advantage of low surface and in-vivo recombination), and the substrate is thinned to about 100 μm to further reduce in-body recombination. This electricity conversion efficiency is 22.3% under AM1.5.
(3) Deep junction partial back-field battery (LBSF) of Fraunhofer Solar Energy Research Institute, Germany
The structure of the LBSF is similar to that of a PERL battery, and also uses a TCA oxide passivation and inverted pyramid front structure. Because the backside boron diffusion generally causes high surface recombination, local aluminum diffusion is used to make the surface contact of the battery, and the cell efficiency of the 2cm×2cm cell reaches 23.3% (Voc=700mV, Isc−~41.3mA, FF−0.806).
(4) Japan sHARP C-Si/μc-Si heterogeneous pp+ junction high-efficiency battery SHARP energy conversion laboratory high-efficiency battery, in front of the use of suede texture, deposition of SiN on the SiO2 passivation layer behind A only B A RF-PECVD boron-doped μc-Si film was used as the back field, and a SiN film was used as the passivation layer on the back surface. The Al layer was in contact with the μcSi film through the holes on the SiN. The efficiency of 5cmX5cm was 21.4% at AM1.5 (Voc=669mV, Isc=40.5mA, FF=0.79).
(5) China's single-crystal silicon high-efficiency battery Tianjin Institute of Power Research carried out high-efficiency battery research under the support of the “8th Five-Year Plan†of the State Science and Technology Commission. Its battery structure is similar to UNSw's V-groove PEsC battery, with a battery efficiency of 20.4%. The Beijing Institute of Solar Energy conducted a high-efficiency battery research under the support of the Beijing Municipal Government during the “Ninth Five-Year Plan†period. The inverted pyramid structure of the battery was placed in front of the battery. The efficiency of the 2cmX2cm battery reached 19.8%. The large-surface (5cmX5cm) laser groove The buried grid cell efficiency reached 18.6%.
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