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

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 J_0e for both emitters formed by either diffusion or epitaxy was investigated. It was found that epitaxy on a textured surface provides J_0e values even better than boron diffusion and J_0e for both surfaces (textured rear and polished rear) using ALD-Al₂O₃/PECVD SiO_x as dielectric passivation are much lower than the ones by wet oxidation. A small difference in V_oc 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 (J_sc) exceeding 20.5% and 39 mA/cm², respectively were achieved on 238.9 cm² 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.