FisMat2017 - Submission - View

Abstract's title: Nanoscale electrical characterization and interface analysis of Si-based thin layers for photovoltaic applications
Submitting author: Maria Antonietta Fazio
Affiliation: University of Bologna - DIFA
Affiliation Address: Viale Berti Pichat, 6/II 40127 Bologna
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: Martina Perani (DIFA - University of Bologna), Bruna Dudda (DIFA - University of Bologna), Nils Brinkmann (Department of Physics - University of Konstanz), Barbara Terheiden (Department of Physics - University of Konstanz), Daniela Cavalcoli (DIFA - University of Bologna)

Silicon oxy-nitride (SiON) is an interesting material for several applications: it can be easily doped and exhibits low absorbance and a tunable refractive index [1]. These properties make SiON suitable not only for Si-based large-scale integration circuit technology or optoelectronic devices, but also for photovoltaic industry [1,2]. In fact, double-layered anti-reflection coatings with SiON and SiN boost the short circuit current of Si heterojunction solar cells (SHJ) compared to single-layered SiN coatings [2]. Moreover, nc-SiON and a-SiON could substitute a-Si:H, in the heteroemitter and passivation layers of the SHJ cells, as they already showed higher conductivity and higher optical gap the respect to a-Si:H [3,4]. In fact, a-SiON films as passivating layers suffer less from parasitic absorption than due to their higher bandgap [4].

Within this perspective, the present contribution aims to give a thorough study of macroscopic and nanoscale electrical properties, as well as of the interface structure in SiON thin films.

SiON films, deposited under different gas flows and subjected to different thermal treatments, have been analyzed by conductive-Atomic Force Microscopy (c-AFM). High-resolution current maps and localized current-voltage characteristics have been related to local changes in the crystallinity fraction and oxygen contents. The conductivity enhancements are linked to the annealing-induced redistribution of the conductive grains and doping activation, while conductivity decrease has been related to high O content which promotes crystal disorder [4].

Bandgap and interface properties of SiON layers have been investigated by Surface PhotoVoltage spectroscopy (SPV). The spectra clearly show features relevant to band-to-band, defects-band electronic transitions or electron-hole recombination at surface/interface. As-deposited samples show reduced interface recombination and thus good interface passivation properties, while the annealed ones show significant interface recombination likely due to annealing-induced relocation of oxygen and possible formation of oxygen precipitates at the interface. In addition, the extracted SiON bandgap values resulted to be dependent on the different oxygen content of the layers, correlated to structural disorder [5].

Due to their application in the heteroemitter stack in SHJ solar cells, high conductivity and good passivation properties are required in SiON thin films. Nanoscale electrical analyses (c-AFM) and surface photovoltage analyses (SPV) are useful to investigate these sample qualities, leading to identify the best deposition parameters to obtain the most suitable materials for SHJ solar cells application.

[1] S. Park et al. Curr. Appl. Phys. 17.4 (2017)

[2] J. Dupuis et al. Thin Solid Films 519.4 (2010)

[4] N. Brinkmann et al. Sol. Energ. Mat. Sol. C. 108 (2013)

[5] M. Perani et al. J. Phys. Chem. C 119 (2015)