Federica Landucci1 Quentin Jeangros1 2 Ivan Marozau3 Esteban Rucavado1 Monica Morales-Masis1 Olha Sereda3 Christophe Ballif1 Cécile Hébert1 Aicha Hessler-Wyser1

1, EPFL, Neuchatel, , Switzerland
2, University of Basel, Basel, , Switzerland
3, CSEM, Neuchâtel, , Switzerland

Transparent conductive oxides (TCOs) are key materials for many applications, including inorganic and organic solar cells and light emitting diodes. In some cases, the TCO layer is subject to a high temperature processing step, as in the fabrication of silicon/mesoporous perovskite tandem solar cells1, which may deteriorate its electrical properties. To address this issue, the optoelectronic properties and microstructure of tin oxide (SnO2)-based TCOs was investigated as a function of temperature. Amorphous films of either SnO2 or SnO2 with a few at% of Zn (ZTO) were deposited by sputtering2 and annealed in an in situ X-ray diffractometer (XRD, in air or vacuum) before characterising their optoelectronic properties. Cross-sections of the annealed films were analysed by transmission electron microscopy (TEM) imaging and energy-dispersive X-ray spectroscopy (EDX). Separate in situ TEM experiments included the acquisition of images, electron energy-loss spectra, diffraction patterns (including fluctuation electron microscopy, FEM) as a function of temperature.

Crystallising SnO2-based TCOs that are deposited amorphous results in a sharp decrease in their electrical properties. Barriers formed by grain boundaries and fine changes in the film chemistry through interactions with the annealing atmosphere impede carrier transport and reduce the number of free carriers, respectively. Crystallisation is postponed by local compressive strain in the amorphous network, which is induced by smaller cations or oxygen deficiencies as these are thought to dampen vibrations and increase the energy barrier to crystallisation.3 Temperature-dependant XRD data acquired in air indicated that the addition of 5 at% of Zn postponed the crystallisation temperature from 350°C to 590 °C, while annealing these ZTO films in vacuum prevents the passivation of oxygen vacancies, resulting in a further increase in the crystallization temperature by ~400 °C. Annealing at higher temperatures or longer times to promote grain growth was found to be detrimental. Indeed, EDX revealed that Zn evaporates > 750 °C in air and > 950 °C in vacuum, creating voids that further hinder carrier transport. XRD, TEM and FEM data indicate that the amorphous structure of ZTO remains largely unchanged before crystallization.4

Through this multimodal approach, the sensitivity of the material to the annealing atmosphere and temperature could be investigated, while typical artefacts of each technique could be identified and accounted for in the results analysis.

[1] Werner et al. (2016), Applied Physics Letters 109 23
[2] Morales-Masis et al. (2016) Adv. Funct. Mater. 26, 384
[3] Zhu et al. (2014) J. Appl. Phys. 115, 033512
[4] Rucavado et al. (2017), Physical Review B 95, 245204