The poor opto-electronic properties often associated with tin (Sn) halide perovskites are usually ascribed to the oxidation of Sn²⁺ to Sn⁴⁺.^1,2 Due to the generation of Sn²⁺ vacancies upon oxidation that limit the electron and hole diffusion lengths to 30 nm by acting as scattering centres, it remains challenging to obtain highly efficient Sn-based perovskite solar cells in planar as well as mesoporous device architectures. Theoretical calculations corroborate that reducing the background hole concentration by suppressing oxidation could allow favourable opto-electronic properties for these materials with diffusion lengths approaching 1 micron.¹ Nevertheless, innovative approaches based on reducing agents remain rather limited as only inadequate fractions of these compounds are tolerated in the formation of the desired efficient absorbers.³ Hence, it remains particularly challenging to maintain a stoichiometric solution for the development of highly efficient Sn-based perovskite solar cells.

Having demonstrated vast potential as a narrow bandgap rear-cell in perovskite-perovskite tandems, the optimisation of Sn (and thereby Pb-Sn mixed) perovskites is of particular relevance as currently all-perovskite multi-junction solar cells are being envisioned—with reports of theoretical performances surpassing 45%.^4,5 To that end, this work focuses on the extent of oxidation present in supposedly pristine precursor solutions—an aspect currently not profoundly reported on in literature. Based on observed changes in morphology, crystallinity and optoelectronic properties of films obtained from corresponding solutions, an efficient method is discussed to estimate the early-on degradation induced at the precursor stage. The quantification can be used as an effective tool to characterise and extend the effect of reducing agents, and thereby assist in enhancing the performance of Sn-based solar cells.

Literature

1. F. Hao, C. C. Stoumpos, D. H. Cao, R. P. Chang, M. G. Kanatzidis, Lead-free solid-state organic-inorganic halide perovskite solar cells. Nature Photonics 8, 489 (2014).

2. N. K. Noel et al., Lead-free organic-inorganic tin halide perovskites for photovoltaic applications. Energy & Environmental Science 7, 3061 (2014).

3. L. Ma et al., Carrier Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH3NH3SnI3 Films. Journal of the American Chemical Society 138, 14750 (2016).

4. G. E. Eperon et al., Perovskite-perovskite tandem photovoltaics with optimized bandgaps. Science 354, 6314 (2016).

5. G. E. Eperon, M. T. Hörantner, H. J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics. Nature Reviews Chemistry 1, 0095 (11/29/online, 2017).