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Effective industrially feasible BSF formation by simultaneous Al and B diffusion

TThe BSF is a component of the solar cell structure, which suppresses losses due to carrier recombination on the cell back surface. Current multi-crystalline Si solar cell production technology includes an Al alloying operation for BSF formation, which also provides a low resistance back contact. The quality of this BSF structure does not satisfy the requirements for next generation high efficiency cells.

Previous experiments with single crystal FZ Si cells demonstrated that an improvement in the BSF could be achieved by simultaneous Al & B instead of Al only, alloying/diffusion. The application of this process to mc Si solar cell fabrication technology improves the quality of the BSF and increases the cell parameters dependant on back surface recombination (long wavelength spectral response and consequently photo generated current, open circuit voltage, fill factor, FF) and conversion efficiency Eff., as a result of the improvement of the above parameters.

The cell fabrication technology was developed to evaluate the BSF improvement on a structure similar to that of future industrially produced high efficiency cells.

Parameters of these cells were: thin solar cells (215 +/-10mm), uniformly low doped emitter with sheet resistance 60-100/square(instead of a selectively doped emitter with the analogous low doped regions), passivated front surface (by a thermally grown SiO2 layer instead of a SiN coating), vacuum deposited and plated Ti-Pd-Ag front contact (instead of fine screen printed contacts).

BSF structures were formed at a temperature of 950 oC for 1 or 2 hrs using for each experiment wafers with an Al layer covered by the spin-on Boron dopant composition and, for comparison, with an Al only layer.Simultaneously with Al and B diffusion P drive-in and front surface oxidation took place.

Several batches of mc-Si solar cells with Al only and Al & B doped backs were fabricated using the above process with 2 hrs BSF doping. The principal positive effect of additional B doping for BSF structure formation was demonstrated: increase of long wavelength response, short circuit current and open circuit voltage.
Considerable improvement in performances of mc Si solar cells with BSF formed by Al & B doping was shown: an increase in efficiency at least of 0.5% abs. up to 17.5 17.8% was demonstrated.

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Reported by

Non Conventional Optics & Solar Energy Labs. Jerusalem College of Technology
Havaad Haleumi Street 21
91160 Jerusalem
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