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ECN publication
Towards 21% : front side improvements for n-Pasha solar cells
Published by: Publication date:
ECN Solar Energy 20-5-2014
ECN report number: Document type:
ECN-M--14-029 Conference Paper
Number of pages: Full text:
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The purpose of the work presented in this paper is to show cost effective and industrial solutions for the tuning of boron emitters, surface preparation and passivation of n-Pasha solar cells. The n-Pasha cells now reach average efficiencies of 20.4%, with op efficiencies of 20.5%. One of the main limiting factors for n-Pasha cells currently is the boron emitter and its passivation [1]. We present a simple and effective method to tune the emitter profile such that the recombination is decreased while the sheet conductance and contact ability is largely unaffected. The boron emitter profile is tuned using a new method of etching the surface by 10-20 nm, resulting in a boron emitter without boron depletion at the surface. The emitter recombination current J0E is decreased from 100 fA/cm2 to 60 fA/cm2 while maintaining the sheet resistance at 60 ohm/sq. n-Pasha cells made with these profiles exhibit a higher Voc by 6 mV and a higher efficiency by 0.2% absolute. Several parameters related to the surface cleaning, variations of chemical oxide passivation and dielectric layers have also been investigated. Implied Voc and J0 are determined to characterize the quality of the passivation effect, and combined with the emitter profiles in 2D Atlas simulations to estimate the improvement on the surface recombination velocity SRV. We found that a very high implied Voc of 680±2 mV can be obtained with an improved pre-cleaning followed by a wet chemical surface oxidation and ALD Al2O3 capped with PECVD-SiNx. The associated values for SRV are reduced from above 104 cm/s towards 1000 cm/s. Combining the improved emitter profile and the improved surface passivation, values of J0E of 40 – 50 fA/cm2 for contactable boron emitters are within reach. Implementation of these modifications is expected to result in a large step towards efficiencies of 21%.

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