Comparison and characterization of different tunnel layers, suitable for passivated contact formation

Abstract

Passivated contacts for solar cells can be realized using a variety of differently formed ultra-thin tunnel oxide layers. Assessing their interface properties is important for optimization purposes. In this work, we demonstrate the ability to measure the interface defect density distribution Dit(E) and the fixed interface charge density Qf for ultra-thin passivation layers operating within the tunnel regime (<2 nm). Various promising tunnel layer candidates [i.e., wet chemically formed SiOx, UV photo-oxidized SiOx, and atomic layer deposited (ALD) AlOx] are investigated for their potential application forming electron or hole selective tunnel layer passivated contacts. In particular, ALD AlOx is identified as a promising tunnel layer candidate for hole-extracting passivated contact formation, stemming from its high (negative) fixed interface charge density in the order of −6 × 1012 cm−2. This is an order of magnitude higher compared to wet chemically or UV photo-oxidized formed silicon oxide tunnel layers, while keeping the density of interface defect states Dit at a similar level (in the order of ∼2 × 1012 cm−2 eV−1). This leads to additional field effect passivation and therefore to significantly higher measured effective carrier lifetimes (∼2 orders of magnitude). A surface recombination velocity of ∼40 cm/s has been achieved for a 1.5 nm thin ALD AlOx tunnel layer prior to capping by an additional hole transport material, like p-doped poly-Si or PEDOT:PSS.