Ion‐Charged Dielectric Nanolayers for Enhanced Surface Passivation in High Efficiency Photovoltaic Devices

Abstract

limitation in achieving high efficiency is the recombination of electrons and holes at the silicon surface. For high efficiency PERC and TOPCon structures to exploit their full performance, superb passivation techniques are required at surfaces and interfaces in the cell. [1] A common method to reduce surface recombination is to deposit a dielectric thin film, typically a double layer of SiO 2 /SiN x , upon the silicon surface. This serves to chemically passivate the surface, while the dielectric's intrinsic charge provides an electric field that modulates charge carrier population and further reduces recombination. [2] The latter effect is commonly termed field effect or charge assisted passivation and can be exploited in a variety of solar cell technologies. [3,4] The effectiveness of such dielectrics can be further improved by extrinsic methods that modify the film properties after deposition. This work explores how field effect surface passivation in thin film dielectrics can be enhanced by adding extrinsic ionic charge. We term this kind of charged thin film an "ion-charged dielectric."In ion-charged dielectrics (ICDs), cations or anions are purposely embedded inside of the dielectric with the purpose of producing a large and permanent electric field. Such charge can add functionality and improve the performance of electronic devices, especially the surface passivation of silicon solar cells. [5] To date, the electric field in ICDs has been primarily demonstrated using embedded potassium and sodium cations, [6][7][8][9] with some work also using cesium ions. [10,11] While the field effect provided by such ions has shown promising results, it is possible that alternative positive and negative ions can provide greater control and stability. Methods of incorporating ions into dielectrics include ion implantation, [12] in situ delivery during oxidation, [6,8] or wet delivery prior to chemical vapor deposition. [10,11] Recently, a novel method of introducing ions to dielectrics extrinsically, after deposition, was proposed by the authors. [13,14] In this method an aqueous precursor containing KCl is thermally evaporated onto the SiO 2 surface followed by elevated temperature annealing to drive the ions to the Si-SiO 2 interface. Relying on pure diffusion kinetics, it was found that the transport time of K + ions across SiO 2 varied from several minutes at 500 °C to over an hour at 400 °C. Later work evaluated the transport time of K + ions in the presence of a surface electric field generated by a corona discharge. [7] It was foundThe power conversion efficiency of solar cells is strongly impacted by an unwanted loss of charge carriers occurring at semiconductor surfaces and interfaces. Here the use of ion-charged oxide nanolayers to enhance the passivation of silicon surfaces via the field effect mechanism is reported. The first report of enhanced passivation from rubidium and cesium ion-charged oxide nanolayers is provided. The charge state and formation energy of ioncharged silicon dioxide are calculated from ...