Metallization‒induced recombination losses of bifacial silicon solar cells

Abstract

In this study, we investigate the metallization‒induced recombination losses of high efficiency bifacial n‒type and p‒type crystalline Si solar cells. From the experimental data, we found that the most efficiency limiting parameter by the screen‒printed metallization is the open‒circuit voltage (VOC) of the cells. We investigated the mechanism responsible for this loss by varying the metallization fraction on either side of the cell and determined the local enhancement in the dark saturation current density beneath the metal contacts (J0(met)). Under optimum fabrication conditions, the J0(met) at metal‒p+ (boron) emitter interfaces was found to be significantly higher compared with the values obtained for metal‒n+ emitters. A two‒dimensional simulation model was used to get further insight into the recombination mechanism leading to these VOC losses. The model assumes that metal contacts penetrate (or etch) into the diffused region following the firing process and depassivate the interface. Applying this model to our n‒type solar cells with a boron p+ emitter, we demonstrated that the simple loss of passivated area beneath the metal contact cannot explain the degradation observed in the VOC of the cell without considering a significant etching or metal penetration into the emitter region. Copyright © 2014 John Wiley & Sons, Ltd.

2D-modelling of Metal-Si Emitter Interface Assuming Schottky or Ohmic Contact

“…Adding a tiny amount of Al in an Ag paste the contact resistance on p + emitter decreases significantly and the FF losses are reduced. However an AgAl firing through paste has been shown to degrade the metal-Si emitter interface to larger extends and yields a higher recombination current density under the contact than an Ag paste [3]. …”

“…The Al addition to an Ag paste is needed in order to achieve low contact resistance to p + emitters. However the role of Al in contact formation is not fully understood, but its presence seems to significantly increase contact recombination losses [3].…”

“…Theoretically calculated barrier height of Ag in contact with p + or n + surface based on Eq [1][2][3]. …”

Determination of metal contact recombination parameters for silicon wafer solar cells by photoluminescence imaging

“…There are several notable studies in recent years reporting on the recombination losses associated with metallisation based on specially designed experiments (Fellmeth et al, 2011;Hoenig et al, 2011;Edler et al, 2014;Helge Hannebauer et al, 2012). Fellmeth et al prepared samples each consisting of eight 2 cm  2 cm solar cells of varying metal contact fractions F M (F M is the ratio of area of metallised regions to the total area of solar cell) on the same wafer (Fellmeth et al, 2011).…”

“…They observed a clear rise in FF 0 À pFF, the difference between the ideal fill factor FF 0 and the pseudo fill factor pFF (pFF is only influenced by recombination and shunting effects and it is an useful parameter to evaluate the non-series resistance related limitation of a solar cells fill factor), as the coverage of the interrupted contact fingers increased. Edler et al studied both metal recombination at the phosphorus and boron diffused layers in bifacial solar cells (Edler et al, 2014). In this case, the difference between the finished cell V oc and the implied V oc determined from lifetime measurements on the samples prior to metallization was tracked as a function of the metal contact fractions on the emitter (boron) and back-surface-field (phosphorus) sides of the cell.…”