Physics / mechanism
Tandem solar refers to stacking two or more photovoltaic absorbers with different band gaps in a single device so that each layer converts a different part of the solar spectrum. The practical bottleneck is materials: the wide-gap top absorber must combine a suitable optical gap, processability, and long-term stability with the narrow-gap bottom cell. Halide perovskites are the main candidate class for the top absorber, and much of the current work is screening compositions computationally before synthesis.
The supplied source base covers this materials-screening stage rather than device stacks. A density functional theory study of the lead-free halide double perovskite series β2SnGeX6 (β = K, Rb; X = Cl, Br, I) computes structural, mechanical, electronic, optical and thermoelectric properties for all six compositions. The double perovskite structure replaces the single divalent B-site cation with a pair (here Sn and Ge), which removes lead from the lattice while retaining the corner-sharing octahedral framework.
Screening criteria used are formation energy, the Goldschmidt tolerance factor and the octahedral factor; all six compounds are reported as thermodynamically stable in a highly symmetric cubic geometry on these measures. Elastic constants place the whole series in the ductile regime, which the authors link to processing elasticity. Halide substitution across Cl, Br and I is the primary tuning handle in such series, and the same calculations also address thermoelectric response, so the compositions are not proposed exclusively as photovoltaic absorbers.
Competitive landscape
The only comparison the sources support is within perovskite chemistry: lead-free double perovskites such as β2SnGeX6 are positioned against lead-containing halide perovskites, with the motivation being removal of lead rather than any demonstrated device-level advantage. No source here compares tandem architectures against single-junction silicon, thin films, or III-V multijunctions, and no efficiency figures for any tandem device appear in the supplied material.
Evidence base
- 18 Jun 2026: DFT study of β2SnGeX6 (β = K, Rb; X = Cl, Br, I) reports structural, mechanical, electronic, optical and thermoelectric properties for the full six-compound series.
- Formation energies, tolerance factors and octahedral factors are reported to confirm robust thermodynamic stability in a highly symmetric cubic geometry for all six compounds.
- Elastic-parameter analysis characterises the entire series as fundamentally ductile.
- The work is computational only (arXiv:2606.18903v1, cond-mat.mtrl-sci); no synthesis or measured device data is reported.
- The other supplied source, a 2026 orbital data centre analyst report, does not address tandem photovoltaics.
Frontier (open questions)
- What band gaps do the β2SnGeX6 compositions have, and does any halide variant fall in the 1.6-1.8 eV window needed for a top cell over silicon?
- Can any member of the series be synthesised as a phase-pure thin film, and do measured lattice parameters and elastic moduli match the DFT predictions?
- Do the Sn(II)/Ge(II) oxidation states survive ambient and operational conditions, or does oxidation dominate degradation as in other tin-based lead-free perovskites?
- Has any lead-free double perovskite been integrated into a two-terminal tandem stack, and what open-circuit voltage and fill factor result?
Synthesised 2026-08-31 from 2 KB sources by the resynth pipeline; citations are KB source slugs.