Crystal and Electronic Structures of Complex Bismuth Iodides <i>A</i><sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> (<i>A</i> = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics
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Bibliographic record
Abstract
Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, especially given the significantly reduced toxicity of Bi when compared with Pb. The compounds A 3 Bi 2 I 9 ( A = K, Rb, Cs) examined here crystallize in two different structure types: the layered defect-perovskite K 3 Bi 2 I 9 type, and the Cs 3 Cr 2 Cl 9 type. The latter structure type features isolated Bi 2 I 9 3– anions. Here, the crystal structures of the ternary iodides are redetermined and a corrected structural model for Rb 3 Bi 2 I 9, as established by single crystal X-ray diffraction and solid state 87 Rb NMR spectroscopy and supported by density functional theory (DFT) calculations is presented. A variety of facile preparation techniques for single crystals, bulk materials, as well as solution-processed thin films are described. The optical properties and electronic structures are investigated experimentally by optical absorption and ultraviolet photoemission spectroscopy and computationally by DFT calculations. Absolute band positions of the valence and conduction bands of these semiconductors, with excellent agreement of experimental and calculated values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielectric properties, including the finding of anomalously large Born effective charge tensors on Bi 3+, suggest proximal structural instabilities arising from the Bi 3+ 6s 2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. The deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it