Forthe greater development of the world’s renewable energy system, there is plentyof research is going on around the world.
Increasing the solar cell efficiencyby light trapping of the solar cell is now becomes a matter of great interestresearchers. As diffusion process is one of the most important processes forfabricating solar cell, many researchers are interested in diffusion technique.Forming an emitter in solar-grade silicon wafer in different ways by varyingthe diffusion time and temperature is now a quit interesting research topic forresearchers.Hanet.al. fabricated a monocrystalline silicon solar cell. The doping temperaturewas 8500C.
They found that the reflectance for bare and texturedsilicon wafer is 31.1% and 14.1%. The efficiency of their solar cell was 16.7%.
42Hongzhaoet.al studied the impact of deposition gas flow ratio, drive in temperature andduration, drive in O2 flow rate, and thermal oxidation temperature, on emitterformation and electrical performance. POCl3 diffusion can be broken down intotwo steps. 1.
Deposition of PSG layer. 2.a subsequent drive in to move thephosphor-silicate deeper into the silicon. In their study they identified theprevious parameters and given an overall guideline to demonstrate how emitterformation could be controlled to fulfill different device application, ensuringglow recombination and high electrical conductivity emitters.
43Mandelkornet.al. fabricated phosphorus diffused solar cell. Solar cells having the sheetresistance of 10 ohm/sq, high long-wavelength collection efficiency, andefficiencies above 10% were fabricated by diffusion at 9750C.Diffusions carried out at 8750C for 0.5 to 1-hour periods resultedin cells having high short and long wavelength collection efficiency. Griddedcontacts applied to the cells to minimize the sheet resistance raised theefficiencies of the cells to values above 10%. The high short- wavelengthcollection efficiency of the 8750C diffused cells results inincreased radiation resistance of these cells.
Cells having efficiencies above10% were made from 13 ohm-cm material and found to have higher radiationresistance than cells made from 1 ohm-cm material. Low junction reversecurrents and contact resistances of approximately 0.2 ohms have also beenachieved in their research. 44Ghembazaet.
al presented a theoretical model on study of the effects of the temperature,diffusion time, surface concentration and doping profile on the crystallinesilicon solar cell performances by using new parameters. They performed anestablished phosphorus diffusion model to show that practical diffusion timesand temperatures can control the junction depth as well as the formed emitterquality. Their output data delineate direction for solar cells efficiencyimprovement through a focus on the exact SiO2 barrier layerthickness and phosphorus profile management. They improved the emitterefficiency and solar cell performance by 2.78%. 45Asrafusjamanet.al.
fabricated monocrystalline silicon solar cell at Bangladesh AtomicEnergy Commission. They used 150*150 mm2 monocrystalline siliconwafer with 200 ?m thickness. The reflectance of raw wafer and textured waferwas also measured. The difference of surface reflectance at wavelength 975nm is30.4%. They diffused the wafer with liquid POCl3 in the diffusionchamber. They carried out their diffusion process at 8500 C – 9000C.
The sheet resistance for the diffused wafer is 0.88 ?-cm which was measured bythe four-point probe. They found the efficiency of their solar cell is about6.89%. 46Shiraziet.
al. showed the influence of the phosphorus precipitation during at thePSG/Si interface during the pre-deposition phase on the passivation quality ofthe corresponding emitter. In the second step, they used the results to createemitters with a reduced density of phosphorus precipitates. In a last step, theoptimized emitter structure was transferred to screen-printed solar cellprocesses, whereby efficiencies up to 19.4%.
They also showed that the PSG can not only serve as a diffusion sourceof phosphorus, but also as a source for diffusion of O into Si. This fact mayalso be responsible for the additional precipitate formation in the Sisubstrate. The increase in the POCl3-N2 gas flow during pre-depositionclearly leads to an increased P precipitate formation on the emitter surfaceand in the emitter volume, which was confirmed by comparing ECV with SIMSmeasurements. 47Akandet. al. fabricated a monocrystalline silicon solar cell with 200 ?mthick silicon wafer.
They fabricated the emitter on the silicon surface attemperature 8750C on the diffusion chamber. When the phosphosilicatelayer was formed at 8500C they remove the PSG layer by chemicaltreatment of HF solution. After doing RTA they found the efficiency of theirsolar cell is 7%. 48