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

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 D it(E) and the fixed interface charge density Q f for ultra-thin passivation layers operating within the tunnel regime (<2 nm). Various promising tunnel layer candidates [i.e., wet chemically formed SiO x , UV photo-oxidized SiO x , and atomic layer deposited (ALD) AlO x ] are investigated for their potential application forming electron or hole selective tunnel layer passivated contacts. In particular, ALD AlO x 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 D it 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 AlO x tunnel layer prior to capping by an additional hole transport material, like p-doped poly-Si or PEDOT:PSS.