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BORON DIFFUSION OF THE SILICON SOLAR CELL WITH BBr3Le Yang, Jing Yang, Xin Fang, Yang Shi, Jingxiao Wang,Qinghao YeShanghai Jiaotong University Solar Energy InstituteShanghai 200240, Chinale_yang@ Jianhua Huang, Xiang Li, Chunjian Wu SJTU-Solarfun PV R&D CenterShanghai 201109, ChinaABSTRACTBoron diffusion for the passivation of silicon solar cell is a crucial element of high efficiency solar cells. Comparing with the traditional screen-printed aluminum back surface field (Al-BSF), boron diffusion back surface field has its advantage on improving the surface passivation of silicon solar cell and increasing the effective minority carrier lifetime. For traditional P-type solar cells, the silicon material is becoming less and less. However, the N-type silicon material as a substitute is noticeable. For N-type silicon material, the pn junction is directly formed by boron diffusion. I n this paper, comparing with spin-on diffusion method, we mainly focus on the boron diffusion with BBr3. Diffusion temperature, N2 volume and diffusion time etc. elements have the remarkable influence on the number and equality of the sheet resistance, junction depth etc. of the silicon solar cell after boron-diffusion. Masked boron diffusion, one-side erode after boron diffusion also improve the solar cell manufacture technique a lot.1. INTRODUCTIONWe have already known the boron diffusion had provided a new approach to improve the efficiency of the traditional solar cells. The selective boron diffusion has applied successfully on 24%-efficient passivated emitter, real local-diffused (PERL) [1] and 22.5%-efficient front surface field-interdigitated back contact (FSF-IBC) [2] solar cells. Nowadays comparing to the P-type silicon material N-type silicon material is richer. The pn junction is directly formed by boron diffusion for N-type silicon material. However, for p-type silicon material the boron diffusion may substitute the boron back surface field for the AL-BSF. The solid solubility of boron in silicon is one order of magnitude high than aluminum (the surface concentration of boron on silicon solar cells is around 1020/cm3), which is more efficient in decreasing the surface recombination ratio and increasing the minority carrier lifetime. Our experiments mainly focused on several key elements influencing the boron diffusion effect of the mono-crystalline, which included diffusion time, temperature, liquid source flux and location in quartz carrier etc, in order to find the best technical parameter for boron diffusion. Furthermore we will design our industrialized bifacial solar cell manufacture progress referred to existed techniques [3].2. EXPERIMENTS AND ANALYSIS2.1 BBr3 DiffusionIn the experiments, we use 103 P-type CZ mono-crystalline wafer (103mm×103mm), average thickness is 310μm, average resistivity is 1Ωcm, the minority carrier lifetime is around 10μs. The experiments mainly divided into two steps: First, using BBr3 as the liquid source for the boron diffusion. With measuring the primary elements: diffusion time, temperature, liquid source flux, location in quartz carrier etc., we can understand the BBr3 diffusion4 PV TECHNOLOGI ES, SYSTEMS AND APPLI CATI ONS1149techniques; Second, we are going to complete the wholeprocess of manufacturing the solar cells by using pure SiO 2 latex source for masking or one-side erosion after diffusion. Design bifacial solar cell manufacture techniques and do the solar cells sample. Afterwards, test the sample and use the feedback information to adjust techniques.Before we start the diffusion, the wafer should be marked in order to recognize the boron diffusion side, then cleaned and grinded by NaOH. Afterwards, spin pure SiO 2 latex on the wafer and drying till the water of organic solvent volatilized and wafer surface stabled.The diffusion process sequence is as following: TCA clearing;Furnace tube saturation;Place the cells back-to-back in quartz carrier; Start diffusion;Close the liquid source of diffusion;Open the tube after 10 minutes and take the wafer out; Rinse;Testing (minority carrier lifetime, sheet resistance etc.);The Furnace structure is in Fig. 1.Fig. 1: Diffusion furnace structure.At beginning, we have decided the proportion and flux of three main source: Small-N 2(2.05L/min, it is short as S-N 2, and takes the BBr3 into tube), O 2 (1.45L/min) and Large-N 2 (18L/min), which referred to the existed POCl 3 diffusion parameters and the phenomenon (fawn-colored borosilicate glass) after diffusion. After basic verification, we have fixed the source flux, and start to change the temperature and time.As we aim to get high boron surface concentration to testify boron back surface field and practical industry utilization,the temperature too high or diffusion time too long is out of our consideration. The temperature is changing from the 1000°C to 1150°C , and the diffusion time is fixed around 30 minutes for comparison. We can easily get the most suitable temperature (lowest deviation of sheet resistance and longest minority carrier lifetime) is 1070°C for our experiments from analysis the Fig. 2 andFig. 3.Fig. 2: Average deviation of sheet resistance after borondiffusion at different temperature.Fig. 3: Average minority carrier lifetime of the wafer afterboron diffusion at different temperature. After finding out the right diffusion temperature, we do more experiments on changing the diffusion time. The result shows in Fig. 4, so we reconfirm our diffusion time is 30 minutes.I n order to testify the source flux that the experiments based on, we compare the deviation of sheet resistance onProceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement1150Fig. 4: Average deviation of sheet resistance after borondiffusion with different diffusion time. different Large-N 2 source flux. The result form Fig. 5testifies the assumption.Fig. 5: Average deviation of sheet resistance after borondiffusion on different Large-N 2 source flux.From the several experiments above, we have realized that the three most important boron diffusion parameters (temperature, time and liquid source flux) decided the diffusion result. When we fixed two parameters and change the third one, we understand its behavior which influences the diffusion effect. For example, when we fixed the liquid source flux and diffusion time and then started to change the temperature, the deviation of sheet resistance start todecrease, fluctuation and increase. These threeparameters influence each other, but there is an optimization point.2.2 Design the Bifacial Solar CellsAfter successfully getting the boron diffusion phenomenon(the equality of sheet resistance and fawn borosilicate grass),We are going to design the bifacial solar cells in order to not only examine the boron diffusion effect but also get the feed back to adjust the techniques. We have designed the manufacture process of the bifacial solar cells, and that is: Cleaning;SiO 2 Latex Spin-on Mask; BBr3 Diffusion (back-to-back); Rid the Borosilicate glass off; Texture Etching; Rinse (HF);SiO 2 Latex Spin-on Mask (on Boron side); POCl3 Diffusion (back-to-back); 2nd Rinse; Edge Isolation;PECVD SiN x ARC 2-side; Screen Printing Front Side (Ag); Drying;Screen Print Back Side (Ag); Drying; Firing; Testing;We make the bifacial solar cells following the before-mentioned process. Both sides of the solar cells arethe same grid-line structure (Fig. 6).Fig. 6: The boron BSF bifacial solar cell. The solar cell results (100mm ×100mm P-type CZ mono-crystalline wafer with average thickness of 310μm) are listed below:4 PV TECHNOLOGI ES, SYSTEMS AND APPLI CATI ONS1151TABLE 1: SOLAR CELL RESULTS ON 100mm ×100mmWAFERSNO. 1 2 3V oc (mV) 0.575 0.569 0.567Isc (A) 2.937 2.917 2.786FF (%) 55.6 65.1 49.4 η (%) 9.39 10.81 7.81 τ (μs) [wafer] 25.2 27.6 21.8 τ (μs) [after diffusion]38.446.627.5The highest efficiency of our three solar cells is 10.81%. The open-circuit voltage is only 569 mV and the fill factor is 0.651, which is much lower than the average solar cell’s level. The most possibility arise this problem is intercross-diffusion, which means part of the boron- diffusion side is doped by phosphorus. The pure SiO 2 latex mask can not protect the boron side from phosphorus diffusion, so the boron back surface field is not worked. This can also be proved by our V oc value that is lower than average solar cells 30-40mv, which is just the increase value of back surface field. However, we have also realized that all the minority carrier lifetime is high. For example the minority carrier lifetime of the NO.2 sample is 46.6μs, which is much higher than the value of wafer. I t means that the boron diffusion actually improve the characteristics of the silicon wafer.3. CONCLUSION AND OUTLOOKDuring our study, we have demonstrated that there was an optimized point of BBr3 diffusion temperature, time and liquid source flux. We have used pure SiO 2 latex asdiffusion mask and designed a whole manufacture process for bifacial solar cells. Although the performance of these bifacial solar cells needs to be improved a lot, we still find that the minority carrier lifetime increasing a lot after theboron diffusion. In the future research, we will first focuson the performance of the pn junction and try to improvethe pure SiO 2 latex mask. I n the mean time, we have already started to use one-side erosion method anddesigned a simple equipment to substitute SiO 2 latex. 4. ACKNOWLEDGMENTSI would like to thank my supervisor Professor. Jingxiao Wang, Rongqiang Cui and Qinghao Ye for so many valuable instruction and discussion. Furthermore, truly thank all the staff in the SJTU-Soarfun PV R&D center providing me so many support to encourage me continue my research.5. REFERENCES(1) Jianhua Zhao, Aihua Wang, Pietro P, Altermatt, Stuart R. Wenham, Martin A. Green, “24% Efficient perl silicon solar cell: Recent improvements in high efficiency silicon cell research”, Solar Energy Materials and Solar cell 41/42(1996) 87-99 (2) Ajeet Rohatgi, “Designs and Fabrication Technologies for Future Commercial Crystalline Si Solar Cells”, 15th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes pp.11-22, (2005) (3) Andres Cuevas, “The early history of bifacial solar cells”, 20th EPVSEC Barcelona (2005)。