Krajewska Chantalle J, Kavanagh Seán R, Zhang Lina, Kubicki Dominik J, Dey Krishanu, Gałkowski Krzysztof, Grey Clare P, Stranks Samuel D, Walsh Aron, Scanlon David O, Palgrave Robert G
Department of Chemistry, University College London 20 Gordon Street London WC1H 0AJ UK
Thomas Young Centre, University College London Gower Street London WC1E 6BT UK.
Chem Sci. 2021 Oct 5;12(44):14686-14699. doi: 10.1039/d1sc03775g. eCollection 2021 Nov 17.
Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite CsBiBr, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr to the solution-phase synthesis of CsBiBr leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, Cs and Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.
具有钙钛矿相关结构的无铅卤化物，如空位有序钙钛矿CsBiBr，在光伏和光电子应用中具有重要意义。我们发现，在CsBiBr的溶液相合成中添加SnBr会导致高达7%的Bi(iii)离子被等量的Sn(ii)和Sn(iv)取代。通过X射线衍射、Cs和Sn固态核磁共振、X射线光电子能谱以及密度泛函理论计算研究了替代缺陷的性质。所得的混合价化合物由于价间电荷转移以及在带隙内与局域化Sn基态之间的电子跃迁，表现出强烈的可见光和近红外吸收。Sn(ii)和Sn(iv)缺陷优先占据相邻的B阳离子位点，形成双取代络合物。与Sn(ii)化合物不同的是，该材料在空气中储存12个月后，光学和结构性能变化极小。我们的计算表明，双取代络合物中Sn(ii)的稳定性有助于这种异常的稳定性。这些结果将无机混合价卤化物的研究扩展到一种新的层状结构，并为无铅空位有序钙钛矿结构的调控、掺杂机制以及结构-性能关系提供了见解。
