高效硅太阳电池的制造工艺（赵建华）
CEEG CEEG Nanjing PV-Tech Co. NJPV CEEG Nanjing PV-Tech Co. PRODUCTION TECHNOLOGIES FOR HIGH EFFICIENCY CRYSTALLINE SILICON SOLAR CELLS Jianhua Zhao CEEG Nanjing PV-Tech Co. Ltd Focheng Rd(w), Jiangning Economic & Technical Development Zone Nanjing, CHINA email: zhaojh@njpv-tech.com CEEG CEEG Nanjing PV-Tech Co. NJPV CEEG Nanjing PV-Tech Co. CEEG Nanjing PV-Tech Co. has started production of standard screen printed cells since June with a capacity of 30 MW/y. Two more cell lines should be installed by the end of 2005. This will increase the production capacity to about 100 MW/y. CEEG CEEG Nanjing PV-Tech Co. NJPV TABLE OF CONTENT 1. High Efficiency Silicon Solar Cell Fabrication Requirements 2. High Efficiency Silicon Solar Cell Production Ready Technologies 3. Material Developments by Silicon Wafer Manufacturers CEEG CEEG Nanjing PV-Tech Co. NJPV 1. High Efficiency Silicon Solar Cell Fabrication Requirements: 1. A high quality substrate with a long carrier lifetime in millisecond range. 2. Good surface passivation with low surface recombination velocity. 3. Low surface reflection with a good light trapping scheme. 4. A proper emitter design to collect all the light generated carriers and a proper metal contact scheme for minimum series resistance CEEG CEEG Nanjing PV-Tech Co. NJPV 1.1. High effective carrier lifetimes have been measured in n-type silicon materials Alneal: FGA: n-Cz 5.5 SEH Wafer Ω⋅cm Manuf. n-Cz 1.3 SEH n-Cz 0.4 SEH n-FZ 1.5 Topsil n-FZ 0.9 SEH 350 Storage: 3 28 28 days 380 400 420 440 390 390 ¢C Eff. carrier lifetime [ms] … and . 1 2 3 4 5 6 7 8 P-type FZ(B), MCZ(B), CZ(Ga) and CZ(In) have also demonstrated long carrier lifetimes. CEEG CEEG Nanjing PV-Tech Co. NJPV 1.2. Good surface passivation methods 1. Chlorine based (TCA) oxidation, or dry oxidation have demonstrated excellent surface passivation qualities. This surface passivation can be enhanced by Alneal process. 2. PECVD SiNx has also demonstrated very low surface recombination velocities. 3. A light surface diffusion can enhance the surface passivation quality. BBr 3 liquid source diffusion is a good choice for such purpose. 4. A hetero-junction can give extremely low surface recombination if carefully designed. CEEG CEEG Nanjing PV-Tech Co. NJPV 1.2.1. Good Surface Passivation with TCA Oxidation and Alneal (aluminum anneal) Process The measured effective minority carrier lifetimes at different processing stages for an 1.5 Ω-cm FZ wafer with 1100Å TCA grown oxide. Alneal and forming gas anneal significantly improved the minority carrier lifetimes. Processing Stage after TCA oxidation after sinter in forming gas after alneal τ 14 µ s 400 µ s 40 µ s P. Balk, The Si-SiO2 System, (Elsevier, Amsterdam, 1988, p.234). The performance of PERL cells with different oxide thickness. The oxide has been grown in a TCA ambient and annealed in forming gas. Cell ID W4-19-2E Z4-16-2E W4-6-1H Oxide Thickness (? Jsc (mA/cm 2 ) Voc (mV) 200 600 1100 36.5 37.5 40.7 682 697 703 (Å) CEEG CEEG Nanjing PV-Tech Co. NJPV One hetero-junction cell structure was realized using n-SIPOS and p-SIPOS to sandwich the high quality silicon substrate. SIPOS is a mixture of microcrystalline silicon and silicon dioxide. Such structure has very low saturation current density of Jo=3.5E-14 A/cm 2 for a bifacial structure. It also achieved 720 mV of Voc under 1.3 suns light intensity. E. Yablonovitch, T. Gmitter, R.M. Swanson and Y.H. Kwark, “A 720 mV open circuit voltage SiOx:c-Si:SiOx double heterostructure solar cell”, Appl. Phys. Lett., Vol. 47, pp. 1211, 1985. Another example of hetero-junction cell structure was Sanyo’s HIT cell, which demonstrated a remarkable 717 mV of Voc on n- type CZ substrate. 1.2.4. Hetero structure for surface passivation CEEG CEEG Nanjing PV-Tech Co. NJPV 1.3. Surface structure to reduce reflection Textured cell surface Inverted pyramid cell surface These surface structures utilise the “double bunce” effect to reduce the surface reflection CEEG CEEG Nanjing PV-Tech Co. NJPV 1.3. Light trapping effect c-Si i n c i de nt l i ght rear surface mirror c-Si in cid e n t lig h t rear surface mirror The front surface structure combined with rear surface mirror may trap the light in the substrate for many passes up to 4 n 2 times. The rear surface mirror reflects internal light into the cell substrate at the rear surface. CEEG CEEG Nanjing PV-Tech Co. NJPV 1.3. Calculated Light Trapping Performance of Different Cell Structures P. Campbell and M. A. Green, J. Appl. Phys. 62 (1987) 243. CEEG CEEG Nanjing PV-Tech Co. NJPV 1.3. Two new light trapping schemes rear metallisation inc i d e n t lig h t total internal reflection SiO rear metallisation SiO 2 Quiltwork Pattern Bi-Dimensional Skew CEEG CEEG Nanjing PV-Tech Co. NJPV 0 0.1 0.2 0.3 0.4 0.5 900 1000 1100 1200 Wavelength, nm Reflection Quiltwork Pattern Bi-Dimensional Skew Inv. Pyr. Reduced surface reflection for the new light trapping structures than standard inverted pyramid structure CEEG CEEG Nanjing PV-Tech Co. NJPV 1.4. A proper emitter design to collect all the light generated carriers and a proper metal contact scheme for minimum series resistance n + p-silicon thin oxide (~200Å) oxiderear contact finger “inverted“ pyramids p + p + p + double layer antireflection coating SunPower’s high efficiency backside point contact cells. 24.7% efficient PERL (passivated emitter, rear locally-diffused) cell structure. CEEG CEEG Nanjing PV-Tech Co. NJPV 1.4. The double diffusion under metal contact (as used in PERL cells) can reduce recombination at the contact Ti/Pd/Ag SiO 2 n, emitter n + , contact passivation p-Si substrate Emitter contact area is passivated by double n-type diffusions. This structrue allows the emitter profile to be optimized for collection of minority carriers. Phosphorus doping profiles for emitter and the emitter contact areas. CEEG CEEG Nanjing PV-Tech Co. NJPV Strong electroluminescence spectra from a PERL cell (Nature, 412, 805-808, 2001) 0 1 2 3 4 5 6 7 1,000 1,050 1,100 1,150 1,200 1,250 1,300 Wavelength (nm) Photon flux (arbitrary units) . Planar x 10 Textured x 1 Calculated Space cell x 100 Planar x 1 Space cell x 1 Due to its indirect band structure, silicon used to be thought not capable to emit light. However, with the excellent surface passivation and light trapping, the PERL cells can emit light with very high efficiency up to 1%. Hence, it was published in Nature magazine. CEEG CEEG Nanjing PV-Tech Co. NJPV 2. High Efficiency Silicon Solar Cell Production Ready Technologies: 1. Rear emitter n-PERT (passivated emitter, rear totally-diffused) cell. 2. LFC or PERC cell. 3. Rear Point Contact cell. 4. HIT MT cell. 5. OECO cell. CEEG CEEG Nanjing PV-Tech Co. NJPV 2.1. Rear Boron Emitter PERT (passivated emitter, rear totally-diffused) Cells on N-Type Silicon Substrates n + n n-silicon thin oxide (~200Å) oxiderear contact finger “inverted“ pyramids p + p + p + double layer antireflection coating p p Advantages: •Large area scribed cells. •High Voc and High Efficiency. •No performance degradation problem. •Thinner substrates. •Higher substrate resistivity. Requirements: CEEG CEEG Nanjing PV-Tech Co. NJPV Performance of FZ N-Type Rear Emitter PERT Cells The performances of scribed 22 cm 2 larger area rear emitter PERT cells on FZ n-type silicon substrates as measured at Sandia National Laboratories under global AM1.5 spectrum (100 mW/cm 2 ) at 25ºC. Cell ID Thickness ( µ m) Jsc (mA/cm 2 ) Voc (mV) FF Efficiency (%) Wnrj1-1a 400 39.1 0.696 0.774 21.0 Wnrj4-3b 270 40.4 0.705 0.785 22.3 Wnrj7-1a 170 40.1 0.706 0.791 22.4 Wnrj7-2a 170 40.1 0.702 0.805 22.7 Wnrj7-2b 170 40.1 0.701 0.802 22.6 CEEG CEEG Nanjing PV-Tech Co. NJPV Equal world best efficiency for a silicon cell on an n-type substrate (R. King, 21st IEEE PVSC, p. 227, 1990) 0.0 0.2 0.4 0.6 0.8 1.0 0.00.10.20.30.40.50.60.70.8 volts am ps Cell ID: Wnrj7-2a Cell Area: 22.04 cm 2 Temperature: 25.1 °C Voc = 702 mV Isc = 0.884 A Jsc = 40.1 mA/cm 2 FF = 0.805 Eff = 22.7% The latest Wnrj7 batch of rear emitter cells were fabricated with confined rear emitter areas to reduce the scribing damage to the emitter edge, and hence reduce the edge shunting loss. This had significantly improved the cell fill factor to over 80%. CEEG CEEG Nanjing PV-Tech Co. NJPV The early cell, Wnrj1-1a, has a 400 µm thick substrate and a low resistivity of 0.9 Ω-cm. These resulted in a low IQE of ~93% and a low Jsc of 39.1 mA/cm 2 . 0 10 20 30 40 50 60 70 80 90 100 350 450 550 650 750 850 950 1050 1150 Wavwlength, nm IQ E , E Q E , a n d R, % Wnrj1-1a Ig=38.1, Isc=39.05 CEEG CEEG Nanjing PV-Tech Co. NJPV The later cell, Wnrj7-2a, has a 170 µm thin substrate and a higher resistivity of 1.5 Ω-cm. These resulted in a high IQE of ~98% and a higher Jsc of 40.1 mA/cm 2 . 0 10 20 30 40 50 60 70 80 90 100 350 450 550 650 750 850 950 1050 1150 Wavwlength, nm IQ E , E Q E , a n d R, % Wnrj7-2a, Ig=40.0, Isc=40.09 Wnrj1-1a Ig=38.1, Isc=39.05 CEEG CEEG Nanjing PV-Tech Co. NJPV PC-1D simulation to spectral response 75 80 85 90 95 100 300 400 500 600 700 800 900 1000 1100 1200 Wavelength, nm Inter n al Q u antum E f fi c i enc y , % Wnrj1-1a 400um, calculated The measured IQE of a 400 um thick cell, and the simulation result of the same thickness. CEEG CEEG Nanjing PV-Tech Co. NJPV PC-1D simulation to spectral response 75 80 85 90 95 100 300 400 500 600 700 800 900 1000 1100 1200 Wavelength, nm Inter n al Q u antum E f fi c i enc y , % Wnrj1-1a 400um, calculated 270um, calculated The measured IQE of a 400 um thick cell, and the simulation result of the same thickness. The simulated IQE of a 270 um cell. CEEG CEEG Nanjing PV-Tech Co. NJPV PC-1D simulation to spectral response 75 80 85 90 95 100 300 400 500 600 700 800 900 1000 1100 1200 Wavelength, nm Inter n al Q u antum E f fi c i enc y , % Wnrj1-1a 170um, calculated 400um, calculated Wnrj7-2a 270um, calculated The measured IQE of a 400 um thick cell, and the simulation result of the same thickness. The simulated IQE of a 270 um cell. The 170 um cell gives the highest simulated and the experimental IQE. CEEG CEEG Nanjing PV-Tech Co. NJPV Calculated thickness effect on rear emitter cells PC-1D program was used to simulate the thickness effect on the rear emitter n-PERT cells. The calculation results show that the optimum thickness for these rear emitter cells is below 200 µm. The following parameters were used in the simulation : Substrate resistivity: 0.9 Ω-cm Bulk minority carrier lifetime: 1.1 ms Front surface recombination velocities: 300 cm/s Rear surface recombination velocities: 400 cm/s 37 37.1 37.2 37.3 37.4 37.5 37.6 37.7 37.8 37.9 0100200300400500 Cell Thickness, um Jsc, m A / c m 2 694 696 698 700 702 704 706 708 710 712 714 716 0100200300400500 Cell Thickness, um Voc , m V 21.4 21.5 21.6 21.7 21.8 21.9 22 22.1 22.2 22.3 22.4 0100200300400500 Cell Thickness, um Efficiency, % CEEG CEEG Nanjing PV-Tech Co. NJPV Stable cell performance under illumination The performance of rear boron emitter n-type PERT cells during 1 – 2 days one-sun illumination by ELH lamp. All parameters of Wnrj1 cells were improved with illumination time. Most possibly, the edge recombination may have been reduced by illumination process. However, for the later cell Wnrj71-b with confined emitter area, its performance was basically stable under illumination. 0 O F T V O * M M V N J O B U J P O 5 J N F I S 7 P D N 7 8 O S K C 8 O S K B 8 O S K C 8 O S K B 8 O S K C 0 O F T V O * M M V N J O B U J P O 5 J N F I S 8 O S K C 8 O S K B 8 O S K C 8 O S K B 8 O S K C 0 O F T V O * M M V N J O B U J P O 5 J N F I S & G G J D J F O D Z 8 O S K C 8 O S K B 8 O S K C 8 O S K B 8 O S K C 0 O F T V O * M M V N J O B U J P O 5 J N F I S + T D N B D N 8 O S K C 8 O S K B 8 O S K C 8 O S K B 8 O S K C CEEG CEEG Nanjing PV-Tech Co. NJPV 2.2. 21.6% efficient LFC - PERC cells suitable for production PERC (passivated emitter and rear cell) structure was originally developed at the UNSW. At Fraunhofer ISE, laser is used to fire the rear point contacts for LFC (laser fired contact) cells. 21.6% efficiency has been demonstrated by these LFC cells. n + n p-silicon thin oxide (~200Å) oxide rear contact finge r “inverted“ pyramids SiO 2 antireflection coating CEEG CEEG Nanjing PV-Tech Co. NJPV 2.2. Processing of LFC cells at Fraunhofer ISE S. W. Glunz, et al., “Laser-fired contact silicon solar cells on p- and n-substrates”, 19th European Photovoltaic Solar Energy Conference, Paris, pp. 408, 2004. 1. No masking required. 2. High speed process. 3. Insensitive to substrate resistivity and type. Advantages: CEEG CEEG Nanjing PV-Tech Co. NJPV 2.2. Performance of LFC cells by Fraunhofer ISE
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