Improved diffusion profiles in back-contacted back-junction Si solar cells with an overcompensated boron-doped emitter

摘要

The performance of n-type back-contacted back-junction silicon solar cells where the boron-doped emitter diffusion on the rear side is locally overcompensated by a phosphorus-doped base-type back surface field (BSF) diffusion has been analysed theoretically and experimentally. By overcompensating the emitter diffusion the noncollecting base-type region can be reduced significantly allowing electrical shading losses to be minimized. It has been found that for solar cells with a lowly doped BSF diffusion the local external quantum efficiency and the short-circuit current density J_(sc) could be improved significantly. For reference solar cells with an undiffused gap between emitter and BSF diffusion and a large noncollecting base-type region, a maximum J_(sc) of 40.9 mA/cm~2 could be achieved and for solar cells with a locally overcompensated boron-doped emitter diffusion featuring a small noncollecting base-type region a maximum J_(sc) of 41.4 mA/cm~2 has been measured. The reduction of J_(sc) losses caused by free carrier absorption (FCA) in highly doped silicon at near-infrared wavelengths is also shown. Furthermore, theoretical investigations are performed by one-dimensional device simulations and the influence of highly doped and lowly doped emitter and BSF diffusions on the open-circuit voltage V_(oc) is presented. For solar cells with a locally overcompensated boron-doped emitter diffusion V_(oc) could be improved from 629 to 652 mV when lowly-doped diffusions and thermally grown SiO_2 and antire-flection plasma enhanced chemical vapour deposited (PECVD) SiN_x passivation stacks are applied. For the reference solar cells with an undiffused gap between the lowly doped emitter and BSF diffusions V_(oc) of 693 mV could be achieved for a plasma enhanced atomic layer deposited (PEALD) Al_2O_3 passivation layer.