Band Gap Engineering in Cs 2 (Na x Ag 1– x )BiCl 6 Double Perovskite Nanocrystals

Wang

Advanced Optical Materials

et al. 2020

126

strategy is to replace Pb 2+ cations with the divalent Sn 2+ or Ge 2+ cations by equivalent substitution during the nanocrystal (NC) synthetic process. [4] However, the Sn 2+ and Ge 2+ ions in the perovskite NCs are easily oxidized to their tetravalent states under ambient conditions, resulting in defects induced photoluminescence (PL) quenching of this kind of lead-free halide perovskite NCs. [5] Another strategy to synthesize lead-free halide perovskite NCs is the substitution of three Pb 2+ ions by two trivalent ions such as Sb 3+ and Bi 3+ to form low-dimensional perovskite NCs. [6] Unfortunately, these kinds of bismuth/ antimony-based perovskite NCs have poor optical properties due to reduced band dispersion and low carrier mobility. [7] Beyond above single metal ion substitution, a promising strategy for the synthesis of lead-free halide perovskite NCs is using one monovalent M + and one trivalent M 3+ cations to replace two divalent Pb 2+ ions. This strategy leads to the formation of lead-free double perovskite (DP) AMMʹX 6 (A = Cs, CH 3 NH 3 ; M = Ag, Au, Na; Mʹ = Bi, Sb, In; X = Cl, Br, I) structure. The lead-free DP maintains both the 3D perovskite structure and the charge neutrality, which will have a great potential to explore new optoelectronic properties. [8] For instance, the metal halide DP Cs 2 AgBiCl 6 and Cs 2 AgBiBr 6 NCs with good stability under ambient conditions have been synthesized, but the optical properties of the pure phase lead-free double halideThe exploration of lead-free halide perovskite nanocrystals (NCs) with intriguing optical properties is highly desirable owing to the toxicity and instability of lead halide perovskite NCs. Here, a new kind of uniform lead-free double perovskite Cs 2 NaBiCl 6 NCs are reported as versatile hosts to accommodate ionic dopants for improving optical properties especially the photoluminescence (PL). In contrast to the low deep-blue PL with a quantum yield of only 1.7% of the as-synthesized pristine Cs 2 NaBiCl 6 NCs, the PL of the Cs 2 NaBiCl 6 NCs can be impressively regulated and enhanced via doping Ag + , Mn 2+ , or Eu 3+ ions in the double perovskite lattices. The femtosecond time-resolved transient absorption spectroscopy is adopted to unravel the PL enhancement mechanism of the ion doping in the Cs 2 NaBiCl 6 NCs. For the Ag + -doping, the excitonic absorption energy of the Cs 2 NaBiCl 6 NCs can be tuned from 3.82 to 3.48 eV with the significant improvement of the PL quantum yield (PLQY) from 1.7% to 20%. The Mn 2+ -doped Cs 2 NaBiCl 6 NCs show broad orange-red emission peak centered at 585 nm with a PLQY of 3%, owing to the 4 T 1 → 6 A 1 transition of octahedrally coordinated Mn 2+ . Eu 3+ -doped Cs 2 NaBiCl 6 NCs are endowed with strong Eu 3+5 D 0 → 7 F J ( J = 1, 2) orange-red emission at 591 and 615 nm.

Section: Figuresupporting

confidence: 87%

Section: Figurementioning

confidence: 74%

Improving Lead‐Free Double Perovskite Cs₂NaBiCl₆ Nanocrystal Optical Properties via Ion Doping

Wang

Advanced Optical Materials

et al. 2020

126

“…The three different DFT functionals employed for the relaxation of the geometry of A 2 AgRhCl 6 were SCAN-rVV10 (Sun et al, 2015;Sun J. et al, 2016;Buda et al, 2017), PBE (Perdew et al, 1996) and PBEsol (Perdew et al, 2008). The reason for choosing three functionals is that we were interested in determining the extent to which the latter two functionals underestimate the bandgaps of the systems under investigation compared to SCAN-rVV10, since they generally underestimate the bandgap of halide single and double perovskites compared to both experiment and the computationally expensive GW and HSE06 (Volonakis et al, 2017;Lamba et al, 2019;Umadevi and Watson, 2019;Wang H.-C. et al, 2019). We note that the newly-proposed SCAN-rVV10 functional is one of the strongly constrained and appropriately normed metageneralized gradient approximation (meta-GGA) functionals that is considered to model well metallic, insulating and semiconducting materials (Sun et al, 2015;Sun J. et al, 2016;Buda et al, 2017).…”

Section: Computational Detailsmentioning

confidence: 99%

“…Similarly, others (Yang et al, 2018) have observed that the bandgap of Cs 2 AgIn x Bi 1−x Cl 6 can be tuned from indirect (x = 0, 0.25, and 0.5) to direct (x = 0.75 and 0.9) by manipulating the percentage of doping, and that they exhibited 3 times greater absorption cross section, lower sub-bandgap trap states, and more than 5 times the photoluminescence quantum efficiency (PLQE) compared to those observed for indirect bandgap nanocrystals such as Cs 2 AgBiCl 6 . Bandgap tuning by alloying of Cs 2 AgBiCl 6 nanocrystals resulted in a series of Cs 2 Na x Ag 1−x BiCl 6 (x = 0, 0.25, 0.5, 0.75, and 1) double perovskite nanocrystals that showed an increase in optical bandgap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and a 30-fold increment in weak photoluminescence (Lamba et al, 2019). Other materials generated by replacing the B ′ -site species in A 2 BB ′ X 6 with transition metals such as Mn 3+ (Locardi et al, 2018;Nandha and Nag, 2018;, Cr 3+ (Zhao et al, 2019), etc., via partial or heavy doping play a significant role in the discovery of innovative halide double perovskite materials for optoelectronics (Jain et al, 2017;Bartel et al, 2019;Cai et al, 2019;Li and Yang, 2019).…”

Section: Introductionmentioning

confidence: 99%

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

Varadwaj

Marques

2020

Front. Chem.

A-Site doping with alkali ions, and/or metal substitution at the B and B ′-sites, are among the key strategies in the innovative development of A 2 BB ′ X 6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2 AgRhCl 6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2 AgRhCl 6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs 2 AgBiX 6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2 AgRhCl 6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2 AgRhCl 6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs 2 AgRhCl 6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable. Keywords: A 2 AgRhCl 6 halide double perovskites, first-principles studies, optoelectronic properties, geometrical, dynamical and mechanical stabilities, DOS and band structures Varadwaj and Marques Stable Halide Double Perovskites

“…It should be noted that band gap engineering can also be performed for low‐dimensional HDPs. For example, the band gap of Cs 2 AgBiCl 6 NCs is enlarged through alloying with Na cations [66] . If the content of Na cations increases from 0 to 1, the band gap rises from 3.39 to 3.82 eV, possibly resulting from a reduction of the Ag contribution near the VBM upon incorporation of Na ions.…”

Section: Preparation Methods For Hdpsmentioning

confidence: 99%

Recent Advances and Optoelectronic Applications of Lead‐Free Halide Double Perovskites

Lü

Pan

Luo

et al. 2020

Chemistry A European J

Organic–inorganic metal halide perovskites (most notably CH3NH3PbI3) have demonstrated remarkable physical attributes for photovoltaic and diverse optoelectronic applications, whereas concerns about toxicity owing to the use of lead in the chemical composition still motivate further exploration of new, nontoxic candidates. Lead‐free halide double perovskites (HDPs), designed by the rational chemical substitution of Pb2+ with other nontoxic candidate elements, have recently attracted interest as a fascinating alternative to their Pb‐based counterparts. Herein, recent advances in crystal structures, physical properties, and versatile optoelectronic applications of lead‐free HDPs, such as solar cells, photodetectors, X‐ray detectors, and light‐emitting diodes, are reviewed. Perspectives to improve the physical and photoelectric properties of existing HDP materials are also discussed and will favor future development of new, lead‐free HDP candidates.