9 research outputs found
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Tuning the bandgap of Cs2AgBiBr6 through dilute tin alloying.
The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs2AgBiBr6 (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 Sn2+ and Sn4+ for Ag+ and Bi3+, 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 Sn2+ substitution for both Ag+ and Bi3+. 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, 119Sn 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 Cs2AgBiBr6 (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
Tuning Defects in a Halide Double Perovskite with Pressure
Dopant defects in semiconductors can trap charge carriers or ionize to produce charge carriers playing a critical role in electronic transport. Halide perovskites are a technologically important semiconductor family with a large pressure response. Yet, to our knowledge, the effect of high pressures on defects in halide perovskites has not been experimentally investigated. Here, we study the structural, optical, and electronic consequences of compressing the small-bandgap double perovskites Cs2AgTlX6(X = Cl or Br) up to 56 GPa. Mild compression to 1.7 GPa increases the conductivity of Cs2AgTlBr6by ca. 1 order of magnitude and decreases its bandgap from 0.94 to 0.7 eV. Subsequent compression yields complex optoelectronic behavior: The bandgap varies by 1.2 eV and conductivity ranges by a factor of 104. These conductivity changes cannot be explained by the evolving bandgap. Instead, they can be understood as tuning of the bromine vacancy defect with pressure varying between a delocalized shallow defect state with a small ionization energy and a localized deep defect state with a large ionization energy. Activation energy measurements reveal that the shallow-to-deep defect transition occurs near 1.5 GPa, well before the cubic-to-tetragonal phase transition. An analysis of the orbital interactions in Cs2AgTlBr6illustrates how the bromine vacancy weakens the adjacent Tl s-Br p antibonding interaction, driving the shallow-to-deep defect transition. Our orbital analysis leads us to propose that halogen vacancies are most likely to be shallow donors in halide double perovskites that have a conduction band derived from the octahedral metal's s orbitals
Chemical Approaches to Addressing the Instability and Toxicity of Lead–Halide Perovskite Absorbers
The
impressive rise in efficiencies of solar cells employing the three-dimensional
(3D) lead–iodide perovskite absorbers APbI<sub>3</sub> (A =
monovalent cation) has generated intense excitement. Although these
perovskites have remarkable properties as solar-cell absorbers, their
potential commercialization now requires a greater focus on the materials’
inherent shortcomings and environmental impact. This creates a challenge
and an opportunity for synthetic chemists to address these issues
through the design of new materials. Synthetic chemistry offers powerful
tools for manipulating the magnificent flexibility of the perovskite
lattice to expand the number of functional analogues to APbI<sub>3</sub>. To highlight improvements that should be targeted in new materials,
here we discuss the intrinsic instability and toxicity of 3D lead–halide
perovskites. We consider possible sources of these instabilities and
propose methods to overcome them through synthetic design. We also
discuss new materials developed for realizing the exceptional photophysical
properties of lead–halide perovskites in more environmentally
benign materials. In this Forum Article, we provide a brief overview
of the field with a focus on our group’s contributions to identifying
and addressing problems inherent to 3D lead–halide perovskites
Charge Carrier Dynamics in Cs<sub>2</sub>AgBiBr<sub>6</sub> Double Perovskite
Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.ChemE/Opto-electronic Material
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Defect-Induced Band-Edge Reconstruction of a Bismuth-Halide Double Perovskite for Visible-Light Absorption
Halide double perovskites
have recently been developed as less
toxic analogs of the lead perovskite solar-cell absorbers APbX<sub>3</sub> (A = monovalent cation; X = Br or I). However, all known
halide double perovskites have large bandgaps that afford weak visible-light
absorption. The first halide double perovskite evaluated as an absorber,
Cs<sub>2</sub>AgBiBr<sub>6</sub> (<b>1</b>), has a bandgap of
1.95 eV. Here, we show that dilute alloying decreases <b>1</b>’s bandgap by ca. 0.5 eV. Importantly, time-resolved photoconductivity
measurements reveal long-lived carriers with microsecond lifetimes
in the alloyed material, which is very promising for photovoltaic
applications. The alloyed perovskite described herein is the first
double perovskite to show comparable bandgap energy and carrier lifetime
to those of (CH<sub>3</sub>NH<sub>3</sub>)ÂPbI<sub>3</sub>. By describing
how energy- and symmetry-matched impurity orbitals, at low concentrations,
dramatically alter <b>1</b>’s band edges, we open a potential
pathway for the large and diverse family of halide double perovskites
to compete with APbX<sub>3</sub> absorbers