Properties of mixed phase silicon-oxide-based passivating contacts for silicon solar cells

Abstract

We investigate the properties of an electron selective front contact based on a phosphorous doped mixed-phase SiOx/Si layer stack at device level. The addition of the SiOx phase to the Si layer targets reduced optical absorption, pursuing the goal of a broad-band transparent full-area passivating contact for front-side application. To demonstrate the validity of our approach we realised a planar hybrid solar cell with the mixed-phase SiOx/Si-based passivating contact on the front side and a hydrogenated amorphous (i/p) silicon heterojunction as rear hole-selective contact. With this structure, we obtained a VOC of 691mV, a JSC of 33.9mA/cm2, a fill factor of 79.4% and an efficiency of 18.6% on a planar n-type FZ Si-wafer.Temperature-dependent IV-measurements at solar cell level were performed in order to understand the physical mechanisms behind charge carrier transport and surface passivation of the mixed-phase SiOx/Si layer stack. The results were compared to those of a standard silicon heterojunction (SHJ) cell on a similar planar substrate. The temperature dependence of the IV-curves in the range from –100°C to +75°C reveals that the hybrid cell is less temperature sensitive with respect to the SHJ cell. Furthermore, at low temperatures, the analysis reveals a reduction of the VOC temperature coefficient of the hybrid cell, whereas for the SHJ cell a saturation occurs. This behaviour hints that the barrier imposed by the SiOx/Si-based contact is less pronounced than the barrier imposed by a standard SHJ contact.