Ultra-thin atomic layer deposited aluminium oxide tunnel layer passivated hole-selective contacts for silicon solar cells

Abstract

In this work, we use ultra-thin thickness-controllable spatial atomic layer deposited (ALD) aluminium oxide (AlOx) tunnel layers, which contain high negative fixed charges (Qf), capped by highly boron-doped polysilicon layers to form tunnel layer passivated contacts. The high Qf of the tunnel layers is expected to enhance the carrier selectivity of these passivated hole-extracting contacts. The dependence of the ALD AlOx tunnel layer contact passivation performance on its thickness is investigated. Furthermore, two different thermal charge activation conditions, i.e., fast firing using a belt furnace at 700 and 800 °C are compared. The best measured recombination current density J0 and implied open-circuit voltage iVoc of the developed AlOx/polysilicon passivated contacts with a symmetrical AlOx/SiNx stack passivation are 6.6 fA/cm2 and 723 mV, respectively. Based on the measured J0 and on the measured total contact resistivity of the passivated contact, the practical efficiency limit of a rear-side full area passivated contact solar cell with a conventionally diffused front side is calculated to be as high as 23.2%. Additionally, three rear-side metallization schemes: (1) thermally evaporated full-area silver contacts; (2) screen-printed non-firing-through aluminium contacts and (3) screen-printed firing-through silver-aluminium contacts, are compared. Finally, rear-emitter solar cells, using a rear-side hole-selective AlOx tunnel layer passivated contact, are fabricated, which shows an efficiency of up to 20.5%. While the proposed hole-selective passivated contact scheme appears to be promising based on the simulation prediction, the efficiency of the fabricated cells is largely limited by the non-optimized front-side reflectance and recombination losses as well as the use of non-optimized rear-side metallization schemes.