Laser fired back contact for silicon solar cells

Abstract

To get high efficiency c-Si solar cells reduction of surface recombination losses and good surface passivation and/or Back Surface Field (BSF) formation are needed. Most industrial solar cells are made covering the back area with screen-printed Al, forming an Al–BSF upon firing step, with a Back Reflectance of 65% and a Back Surface Recombination Velocity (BSRV) of 1000 cm/s on 1 Ωcm Si wafer. Simulations reveal that PV efficiency can increase up to 18% after improving the BSRV to ≤ 200 cm/s and the BR to > 95%. The aim of this work is to get these goals by a laser fired back contact with low temperature passivation of the remainder of the back. This can be obtained by a double layer of PECVD Amorphous Silicon and Silicon Nitride, on which a spin-on Boron dopant layer is deposited. The structure is completed by 2 µm thick e-beam evaporated Al. The formation of an improved local BSF is obtained using a Nd:YAG pulsed laser, which promotes an Al & B simultaneous diffusion trough the passivation layers. Several cells, using this structure, have been fabricated on different substrates. By fitting procedure of cell Internal Quantum Efficiency we have extracted several parameters as surface recombination velocity, diffusion length and internal reflection that are comparable with the state of art of the cells having effective back surface field.