Impact of Tin Fluoride Additive on the Properties of Mixed Tin‐Lead Iodide Perovskite Semiconductors

Abstract

Mixed tin-lead halide perovskites are promising low-bandgap absorbers for all-perovskite tandem solar cells that offer higher efficiencies than singlejunction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin-lead perovskites. Here, the effects of the commonly used SnF 2 additive are revealed for FA 0.83 Cs 0.17 Sn x Pb 1−x I 3 perovskites across the full compositional lead-tin range and SnF 2 percentages of 0.1-20% of precursor tin content. SnF 2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein-Moss shifts, and higher charge-carrier mobilities. Such effects are optimized for SnF 2 addition of 1%, while for 5% SnF 2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF 2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

“…[ 79 ] However, this seems somewhat too high: an approximate model considering the reduced mass of either Sn–I or Pb–I diatomic chains would predict a factor of approximately 1.1 between the resulting phonon frequencies, and this has indeed been measured for the transverse optical modes in mixed tin‐lead iodide perovskites. [ 132 ] In any case, given that the Fröhlich coupling scales as , we would expect tin‐based MHPs to exhibit lower polaron couplings and higher charge‐carrier mobilities compared to their lead‐based counterparts—as reported in both theoretical [ 33 ] and experimental work [ 3,96,132 ] (see also Figure 6).…”

“…[ 79,130 ] Replacement of lead with tin might lead to changes in charge–lattice couplings, although the full impact is not yet clear, given that some of the fundamental changes in optoelectronic properties may still be obscured by extrinsic effects deriving from high defect densities and poor crystallinity in tin‐based MHPs. [ 96,132,133 ] The role of lattice anharmonicity in MHPs is still the subject of some debate, [ 24,32,60 ] and detailed experimental studies, for example using magneto‐absorption spectroscopy or electron paramagnetic resonance, which can provide valuable insights into polaron energy levels, transport and interactions with the local lattice structure, [ 49,115,192,193 ] could help to clarify which modeling approach is more suitable for understanding the fundamental properties of MHPs. Overall, polarons in metal‐halide perovskites have little detrimental impact, and thus high‐performance solar cell devices have indeed been developed based on these materials.…”

“…argue the opposite and find polaron binding energies to be largest for low Sn content in mixed‐cation MAPb 0.75 Sn 0.25 I 3 . Although replacement of lead with lighter tin indeed leads to higher phonon frequencies, [ 132 ] much of the deterioration in various optoelectronic properties for low tin content can be traced back to higher defect densities and increased non‐radiative recombination pathways, [ 132,133 ] without polaronic effects necessarily playing a role. Purely tin‐based MHPs have consistently shown higher charge‐carrier mobilities than their lead‐based counterparts, [ 3,96,134 ] which can be linked to the higher optical phonon frequencies in these materials, [ 90,132 ] as well as the lower effective charge‐carrier masses in tin‐based MHPs.…”

Highly Absorbing Lead-Free Semiconductor Cu2AgBiI6 for Photovoltaic Applications from the Quaternary CuI–AgI–BiI3 Phase Space

“… 57 However, through compositional tuning and more optimized processing, which resulted in reducing the crystalline disorder and background charge carrier density, this has been raised to over 80 cm 2 V –1 s –1 for tin iodide perovskites. 58 Mixed-cation, mixed-halide lead halide perovskites are similarly sensitive to extrinsic factors. 56 Given the already-promising value for Cu 2 AgBiI 6 measured here, an improved understanding of both the limits due to intrinsic factors and the influences of extrinsic factors could lead to further enhancements in charge-carrier mobilities in Cu 2 AgBiI 6 .…”

Optoelectronic Properties of Tin–Lead Halide Perovskites

“… 35 Easily formed, both tin vacancies and iodide interstitials generate defect levels just below the valence band edge, where they capture valence band electrons, effectively releasing free holes. Such unintentional p-type doping is therefore ubiquitous in tin iodide perovskites, for which our literature survey 26 − 29 , 31 , 39 − 42 finds experimentally reported hole densities to range predominantly around 10 17 –10 20 cm –3 (see Figure 1 for some examples). When metal content is reduced to 70–90% tin by substitution with lead, reported values 42 fall to around 10 17 –10 18 cm –3 , and to 10 15 –10 17 cm –3 for 50–60% tin content.…”