Front side improvements for n-Pasha solar cells

Summary We present a new approach to improve the efficiency of n-type solar cells by tuning the boron emitter doping profile and optimization of surface passivation. The boron emitter profile is tuned using a new method of just etching the surface by 10-30 nm. This resulted in a boron emitter without boron depletion at the surface, a higher Voc by 6 mV and a higher efficiency by 0.2% absolute. To improve the surface passivation, 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 AlOx capped with PECVD-SiNx. Purpose of the work Our aim is to improve the efficiency of our bifacial n-Pasha solar cells and modules using cost effective and industrial processing. In preceding years, the focus for improvements of n-Pasha cells was on the BSF and metallization [1] with which we have recently reached a highest efficiency of 20.4%, and average efficiencies of 20.2%. In present work, the focus shifted to the emitter and its passivation. 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. Scientific innovation and relevance Currently, one of the main limiting factors for high efficient n-type front junction solar cells is the boron emitter and its passivation [2]. We present a simple and effective method to tune the emitter profile such that the recombination is decreased while sheet resistance and contact resistance are unaffected. The emitter saturation current density J0E is decreased from 100 fA/cm 2 to 60 fA/cm 2 with this improved emitter profile, The surface recombination velocity SRV can be reduced from 10 4 cm/s towards 1000 cm/s by applying improved chemical surface pre-treatment and ALD AlOx passivation. Implementation of these modifications will result in a large step towards efficiencies of 21%. Approach Improved boron profiles were obtained by etching the surface by 10-30 nm to remove the boron depleted region. This step was followed by application of our patented technology of chemical oxidation. The lifetime and J0E of these emitters values were measured on symmetrically diffused wafers. Subsequently full cells with these improved emitters were made and characterized. In parallel, improved chemical pre-treatments and passivation schemes were applied on lifetime samples with a ‘standard’ non-etched diffused boron emitter and phosphor diffused BSF. Several parameters related to the surface cleaning, variations of chemical oxide passivation and use of dielectric layer AlOx have been investigated. The implied Voc and J0E are determined to characterize the quality of the passivation effect. Numerical simulations, using boron profiles measured with ECV, were used to estimate the improvement on the surface recombination velocity SRV.