Progress in the development of industrial nPERT cells

Abstract

This article addresses several issues and challenges that have been faced, when developing an industrially feasible n-type solar cell (n-PERT) process employing homogeneous gas-phase diffusions of front and back side. As a result of the presented developments and optimizations solar cell front side efficiencies close to 21% could be obtained.The first issue that has been investigated is the impact of rear side treatments, i.e. etching and diffusion, on the rear side contact formation. We have seen process instabilities in the rear side contacting step when moving to a back surface field with high sheet resistance, especially with current diamond sawed wafers becoming increasingly flat. The second challenge was to develop an edge isolation process that is superior to conventional laser edge isolation. Wet chemical edge isolation is used for the first time to obtain well defined reverse bias behavior. Less than -1.5 A were recorded at -12 V reverse bias. Furthermore, a major increase in efficiency was achieved by using Al-free Ag-paste for front- and back-side metallization. A significantly improved Voc demonstrates a reduced recombination current J0met. Finally we present the determination bifacial characteristics according to the Equivalent Irradiance method showing an effective efficiency ηeffective_BiFi20 of 24.6 % in applications with 20 % rear side illumination.