Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application

Abstract

In this work we investigated the optical and electrical performance of p-type epitaxial layers as the rear emitter of bifacial n-type PERT solar cells. In the first part of this paper, the surface morphology of epitaxial layers grown on textured surfaces is studied. Because of the epitaxial growth, a pyramids-rounding effect is observed as a result of {311} and {911} facet propagation. The growth pattern was quantified and modelled. In the second part of this paper, the optical performance of semi-device test structures is evaluated. The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25° without having to introduce any additional polishing steps to modify the morphology of the textured surface. In the last part of this paper, we present the electrical results for semi-device structures created to quantify the recombination losses in the passivated and metallized regions of those p-type epitaxial emitters. These results indicate that by introducing a rear epitaxial emitter in the bifacial n-type PERT cell structure, we can increase the implied Voc up to 17mV compared to a diffused emitter with the same sheet resistance.

Efficiency gain in plated bifacial n-type PERT cells by means of a selective emitter approach using selective epitaxy

“…2, the overall result is the growth of a silicon epitaxial layer solely on the bare silicon surface where the dielectric was opened during a previous patterning step. The remaining surface of the dielectric (stack), which will be kept for passivation and optical purposes at cell level, functions as a "mask" preventing the deposition of the epitaxial layer on that area [21][22][23][24][25][26][27][28][29]. An equivalent local polysilicon-based contact structure on the front side of a solar cell would require a more complex processing sequence [5].…”

“…The application of selective epitaxy to form the p-type local contact in bifacial PERT devices poses two main challenges: (1) the feasibility to selectively grow an epitaxial layer on the silicon surface opened during the dielectric patterning by laser ablation, and (2) the impact of the epitaxial thermal budget on the passivating properties of both front and rear dielectrics. Selective epitaxy has been successfully applied on blanket sawdamage-etched and textured surfaces as well as surfaces patterned by photolithography [21][22][23][24][25][26][27][28][29]. However, laser ablation of dielectrics to open the contact regions not only introduces damage, and therefore the presence of additional defects [30], but also gives rise to a very rough surface morphology as shown in FIG.…”