Electrical characterization of ALD Al2O3 - HfO2 and PECVD Al2O3 passivation layers for p-type CZ-Silicon PERC solar cells

Abstract

This work characterizes p-type Silicon surface passivation using a high-k material (Al 2 O 3 or HfO 2 ) combining capacitance voltage (CV) and lifetime measurements. For AI 2 O 3 samples, the Silicon substrate bulk and surface quality is equivalent to CZ Silicon used in industrial solar cell processing. While AI 2 O 3 has been proven to provide high quality surface passivation on p-type doped Silicon surfaces, the influence of the growth conditions and the post-deposition annealing is not yet completely understood. The dielectric thin film has been deposited by common techniques (ALD, PECVD) on H-/OHterminated Silicon surfaces (hydrophobic and hydrophilic, respectively). The impact of the roughness of the surface prior to the deposition has been also considered. Then, the passivation of each layer has been investigated as a function of different AI 2 O 3 thicknesses (5 to 20 nm) and post-deposition annealing temperatures (300 to 800°C). CV measurements have been used to characterize chemical passivation (= interface trap density, D it ) and field effect passivation (= fixed charge density, Q f ). Lifetime measurements have been used to assess the effective surface passivation. The results of both types of electrical characterization fit well together. (i) Prior post-deposition anneal, only either chemical passivation (ALO) or field effect passivation (PECVD) is adequate, resulting in lower effective lifetimes. (ii) At higher annealing temperatures, a negative net charge in the AI 2 O 3 and a low D it at the interface are measured, ideal for p-type CZ Silicon passivation and causing maximal effective lifetimes. (iii) At too high annealing temperatures, chemical passivation is destroyed resulting in decreasing effective lifetimes even though negative field effect remains in many cases. Another candidate as passivation layer on Silicon is HfO 2 . Being a new material in photovoItaics, it has been studied on FZ Silicon substrates and its electrical characterization has demonstrated interesting passivation properties at low anneal temperatures (also without thermal treatment).