Comparison Between Front-And Backjunction Monocrystalline Silicon Solarcell

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Date
2009-06
Authors
Bagudo, Ibrahim Muhammad
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This research thesis was undertaken to fabricate, characterize and optimize the front and back junction solar cells. As part of an effort towards cost reduction, simple technologies of conventional furnace processing (CFP) and rapid thermal processing (RTP) were compared. A monocrystalline silicon wafer with an area of 19.64cm2 and resistivity of (0.75-1.25) Qcm was used to find an optimal emitter for both the front and back junction solar cells. The front junction cell utilized CFP for diffusion. It showed an average FF of 69 % with short-circuit current density Jsc= 35.0mAicm2 and a good open circuit voltage of 0.640V, with an efficiency of 1] = 15.6±1 % (l00mW/cm2, 250C).We also developed a back junction cell, the design of which was compatible with a wide range of silicon quality materials. For back junction solar cell specifically, its efficiency strongly depended on material quality, ratio of device thickness to minority carrier diffusion length and highly passivated surfaces. The back junction solar cell demonstrated an efficiency of 19 .3± 1 % with FF 72 %, open circuit voltage Voc of 0.67V and a short-circuit current density of 40.0mAicm2. We found that by using CFP, emitter formation for back junction cell required a long time, which might not be compatible with the cost reduction strategy. High -efficiency silicon solar cells require a textured front surface to reduce reflectance and to improve light trapping. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are commonly used, but these solutions are toxic pollutants. The K+ and Na + ions contaminate the passivation layer of Si02 deposited on the surface of the cell after texturing. In this study an alternative solution containing tetramethyl ammonium hydroxide «CH3)4 NOH), TMAH) was used .This research shows the influence of different parameters (co~centration, temperature and time) on the texturing processes. We found that 3 % TMAH concentration at 45 min and 90°C were the optimal conditions for highly efficient solar cells. The solar cells were well passivated by the growth of Si02 layers. The deposition temperature of the passivation layer was kept at 850°C, to maintain high lifetime of minority carriers. This passivation layer reduced surface recombination velocity of the minority earners.
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A monocrystalline silicon wafer , was used to find an optimal emitter
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