The stable hydrogen isotope deuterium (D), which is released during the annealing of deuterated silicon nitride films, diffuses through the crystalline silicon and is captured by a thin, amorphous layer of silicon sputtered on the rear surface. We report on the measurement of the concentration of “penetrated” D by secondary ion mass spectrometry to monitor the flux of D diffusing through single-crystalline silicon wafers. The penetrated D content in the trapping layer increases with the annealing time. However, the flux of D injected into the silicon from the silicon nitride layer decreases as annealing time increases.

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In this paper we report on the impact of mc-Si wafer thickness on efficiency. We have obtained 16.8%, 16.4%, 16.2% and 15.7% efficient screen printed 4 cm 2 solar cells on 280 µm, 170 µm, 140 µm and 115 µm thick cast mc-Si respectively. Analysis of these cells showed that the efficiency of the 115 µm thick cell is limited by a BSRV of 750 cm/s, FSRV of 120,000 cm/s and a BSR of 67%. A module manufacturing cost model for a 25 MW plant was used to demonstrate that 15.7% efficient cells on 115 µm thick wafers are more cost effective than 16.8% cells on 280 µm wafers. The module manufacturing cost reduced from $1.82/W to $1.63/W when the wafer thickness was reduced from 280 µm (efficiency 16.8%) to 115 µm (efficiency 15.7%). A roadmap is developed for 115 µm thick wafers to demonstrate how cell efficiency can be increased to greater than 18% resulting in a module cost of less than $1.40/W.

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A low resistivity of 0.2-0.3 Ω.cm has been shown to be optimum for high quality single crystal silicon for solar cells. However, for lower quality cast mc-Si, this optimum resistivity increases owing to a dopant-defect interaction, which reduces the bulk lifetime at lower resistivities. In this study, solar cells fabricated on 225 µm thick cast multicrystalline silicon wafers showed very little or no enhancement in efficiency with the decrease in resistivity. However, V oc enhancement was observed for the lower resistivity cells despite significantly lower bulk lifetimes compared to higher resistivity cells. After gettering (during P diffusion) and hydrogenation (from SiN x ) steps used in cell fabrication, the bulk lifetime in 225 µm thick wafers from the middle of the ingot decreased from 253 µs to 135 µs when the resistivity was lowered from 1.5 Ω.cm to 0.6 Ω.cm. This paper shows that solar cells fabricated on 175 µm thick, 1.5 Ω.cm, wafers showed no appreciable loss in the cell performance when compared to the 225 µm thick cells, consistent with PC1D modeling.

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