Comparison of front - and back - junctions monocrystalline silicon solar cell
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Date
2008
Authors
Bagudo, Ibrahim Muhammad
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Abstract
This thesis undertakes fabrication, characterization and optimization of front and back junction solar cells. As part of our effort on cost reduction strategy, a simple technologies of conventional furnace processing (CFP) and rapid thermal processing (RTP) has been compared. A monocrystalline silicon wafer with an area of 19.64cm2 and effective active area of 12.6cm2 resistivity of (0.75-1.25) Ωcm was used to find an optimal emitter for both front and back junction solar cells. The front junction cell utilizes CFP for its diffusion. It shows an average FF of 69 % with short-circuit current density Jsc= 35.0mA/cm2 and an excellent open circuit voltage of 0.640V with an efficiency of η= 15.6 %. We also developed a back junction cell whose design is compatible with a wider range of silicon material qualities. For back junction solar cell more specifically, its efficiency strongly depends on material quality, the ratio of device thickness to minority carrier diffusion length and highly passivated surfaces. The back junction solar cell demonstrated an efficiency of 19.3 % with FF 72 %, open circuit voltage VOC of 0.67V and a short-circuit current density of 40.0mA/cm2. We found that emitter formation for back junction cell requires a very long time by using CFP, which might not be compatible with cost reduction. High –efficiency silicon solar cells requires texturing of the 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 and pollutant. The K+ and Na ions contaminate the passivation layer SiO2 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
show the influence of different parameters (concentration, temperature and time) for
the texturing processes. We found that 3 % TMAH concentration at 45 min and 90
0C gave an optimized condition for high efficient solar cells. The solar cells were
excellently passivated by the growth of SiO2 layers. The deposition temperature of
the passivation layer was kept at 850 0C, in order to maintain high lifetime of
minority carriers. This passivation layer reduces surface recombination velocity of
the minority carriers.
Description
Master
Keywords
Science Physic , Monocrystalline , Solar cell