The effect of surface morphology on the performance of 21% n-type PERT solar cells with an epitaxial rear emitter

Abstract

In this work, we evaluate by means of simulations and measurements the effect of rear surface morphology on the performance of n-type rear-junction Passivated Emitter Rear Totally-diffused (PERT) solar cells with epitaxial emitter. A comparison of the electrical and optical performance between a rear polished and a rear textured surface was conducted on both test structures and solar cells. The effect of surface morphology on J0e for both emitters formed by either diffusion or epitaxy was investigated. It was found that epitaxy on a textured surface provides J0e values even better than boron diffusion and J0e for both surfaces (textured rear and polished rear) using ALD-Al2O3/PECVD SiOx as dielectric passivation are much lower than the ones by wet oxidation. A small difference in Voc between both surfaces was also predicted in our electrical simulations. Overall, the measurements show that n-type PERT solar cells with textured rear epitaxial emitter can perform at almost the same efficiency level to the ones with polished rear. For both types of n-PERT cells (textured rear and polished rear), average efficiencies (η) and short-circuit current densities (Jsc) exceeding 20.5% and 39mA/cm2, respectively were achieved on 238.9cm2 cells. Moreover, from our optical simulations it was found that either a textured rear surface or a non-perfect polished rear surface are ideal for light trapping at standard illumination conditions, with light impinging perpendicular onto the cell. However, a perfectly polished rear surface or one with pyramids at angles of 45° should be avoided. Finally, from a cost of ownership (CoO) point of view it was found that the potential efficiency gain originating from a rear polished surface is not cost effective. For polishing to become cost effective, the cell in the module should have an efficiency of 21.05% which is 0.45% more than the textured case.