15 research outputs found
Recommended from our members
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
Recommended from our members
The Harvard Clean Energy Project: Large-Scale Computational Screening and Design of Organic Photovoltaics on the World Community Grid
This Perspective introduces the Harvard Clean Energy Project (CEP), a theory-driven search for the next generation of organic solar cell materials. We give a broad overview of its setup and infrastructure, present first results, and outline upcoming developments. CEP has established an automated, high-throughput, in silico framework to study potential candidate structures for organic photovoltaics. The current project phase is concerned with the characterization of millions of molecular motifs using first-principles quantum chemistry. The scale of this study requires a correspondingly large computational resource, which is provided by distributed volunteer computing on IBM’s World Community Grid. The results are compiled and analyzed in a reference database and will be made available for public use. In addition to finding specific candidates with certain properties, it is the goal of CEP to illuminate and understand the structure–property relations in the domain of organic electronics. Such insights can open the door to a rational and systematic design of future high-performance materials. The computational work in CEP is tightly embedded in a collaboration with experimentalists, who provide valuable input and feedback to the project.Chemistry and Chemical Biolog
Chlorine in PbCl<sub>2</sub>‑Derived Hybrid-Perovskite Solar Absorbers
Chlorine in PbCl<sub>2</sub>‑Derived Hybrid-Perovskite
Solar Absorber
Mechanism of Tin Oxidation and Stabilization by Lead Substitution in Tin Halide Perovskites
The recent development
of efficient binary tin- and lead-based
metal halide perovskite solar cells has enabled the development of
all-perovskite tandem solar cells, which offer a unique opportunity
to deliver high performance at low cost. Tin halide perovskites, however,
are prone to oxidation, where the Sn<sup>2+</sup> cations oxidize
to Sn<sup>4+</sup> upon air exposure. Here, we identify reaction products
and elucidate the oxidation mechanism of both ASnI<sub>3</sub> and
ASn<sub>0.5</sub>Pb<sub>0.5</sub>I<sub>3</sub> (where A can be made
of methylammonium, formamidinium, cesium, or a combination of these)
perovskites and find that substituting lead onto the B site fundamentally
changes the oxidation mechanism of tin-based metal halide perovskites
to make them more stable than would be expected by simply considering
the decrease in tin content. This work provides guidelines for developing
stable small band gap materials that could be used in all-perovskite
tandems
Transformation from crystalline precursor to perovskite in PbCl2-derived MAPbI3
Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices
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
Band Gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics
Tin and lead iodide
perovskite semiconductors of the composition
AMX<sub>3</sub>, where M is a metal and X is a halide, are leading
candidates for high efficiency low cost tandem photovoltaics, in part
because they have band gaps that can be tuned over a wide range by
compositional substitution. We experimentally identify two competing
mechanisms through which the A-site cation influences the band gap
of 3D metal halide perovskites. Using a smaller A-site cation can
distort the perovskite lattice in two distinct ways: by tilting the
MX<sub>6</sub> octahedra or by simply contracting the lattice isotropically.
The former effect tends to raise the band gap, while the latter tends
to decrease it. Lead iodide perovskites show an increase in band gap
upon partial substitution of the larger formamidinium with the smaller
cesium, due to octahedral tilting. Perovskites based on tin, which
is slightly smaller than lead, show the opposite trend: they show
no octahedral tilting upon Cs-substitution but only a contraction
of the lattice, leading to progressive reduction of the band gap.
We outline a strategy to systematically tune the band gap and valence
and conduction band positions of metal halide perovskites through
control of the cation composition. Using this strategy, we demonstrate
solar cells that harvest light in the infrared up to 1040 nm, reaching
a stabilized power conversion efficiency of 17.8%, showing promise
for improvements of the bottom cell of all-perovskite tandem solar
cells. The mechanisms of cation-based band gap tuning we describe
are broadly applicable to 3D metal halide perovskites and will be
useful in further development of perovskite semiconductors for optoelectronic
applications