Tuning the bandgap of CsAgBiBr through dilute tin alloying

• Published in: Chemical science (Cambridge) 2019-11, Vol.1 (45), p.162-1628
• Main Authors: Lindquist, Kurt P, Mack, Stephanie A, Slavney, Adam H, Leppert, Linn, Gold-Parker, Aryeh, Stebbins, Jonathan F, Salleo, Alberto, Toney, Michael F, Neaton, Jeffrey B, Karunadasa, Hemamala I
• Format: Article
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c9sc02581b

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs 2 AgBiBr 6 ( 1 ) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ∼2 : 1 substitution of Sn 2+ and Sn 4+ for Ag + and Bi 3+ , respectively, with Ag + vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn 2+ substitution for both Ag + and Bi 3+ . Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, 119 Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs 2 AgBiBr 6 ( 1:Sn ) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner. Sn alloying tunes a halide double perovskite to absorb visible light, in a nontoxic composition.