Electrical characterization of ALD Al_2O_3 - HfO_2 and PECVD Al_2O_3 passivation layers for p-type CZ-Silicon PERC solar cells

摘要

This work characterizes p-type Silicon surface passivation using a high-k material (Al_2O_3 or HfO_2) combining capacitance voltage (CV) and lifetime measurements. For Al_2O_3 samples, the Silicon substrate bulk and surface quality is equivalent to CZ Silicon used in industrial solar cell processing. While Al_2O_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-/OH- terminated 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 Al_2O_3 thicknesses (5 to 20 nm) and post-deposition annealing temperatures (300 to 800 0q. 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 Al_2O_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).