Lead-based Halide Perovskites Research Articles

Barium as doping element tuning both toxicity and optoelectric properties of lead-based halide perovskites

Organic-inorganic halide perovskites <i>ABX</i><sub>3</sub> (<i>A</i> = CH<sub>3</sub>NH<sub>3</sub>, HC(NH<sub>2</sub>)<sub>2</sub>; <i>B</i> = Pb; <i>X</i> = Cl, Br, I) have recently attracted increasing attention due to their advanced optoelectronic properties. However, the poor stability and toxicity of organic lead halogen perovskites are still a major challenge for deploying the outdoor solar cells. Element substitution is a simple and effective strategy to solve these problems. For example, the substitution of the I ions with Cl and Br has been regarded as a reliable method to improve the device stability. <i>A</i>-site engineering, i.e., replacing organic ions with inorganic cations (such as Cs<sup>+</sup>, Rb<sup>+</sup>), has also been reported. The <i>B</i>-site alloying approach has been demonstrated with Zn, Sr, Sn, etc. Inorganic halide perovskites can be synthesized by the low-cost solution spin-coating method and have similar optoelectronic properties and improved stability to their organic counterparts. Here in this paper, we report a comprehensive study of the alloyed perovskite CsPb<sub>1–<i>x</i></sub>Ba<sub><i>x</i></sub><i>X</i><sub>3</sub> (<i>X</i> = Cl, Br, I) by combining the disorder alloy structure search method with first-principles energy calculations. We find that it is not easy to dope barium into the perovskite lattice when Ba concentration is low and the stable disordered solid solution can exist in the high Ba concentration case. Carrier effective mass and bandgap increase with the increase of Ba concentration and the bandgap change range is wide, owing to the difference in both electronegativity and ionic radius between Pb and Ba. After inducing Ba into CsPb<sub>1–<i>x</i></sub>Ba<sub><i>x</i></sub><i>X</i><sub>3</sub> (<i>X</i> = Cl, Br, I), the higher electron concentration on the I sites also enhances the Coulomb interaction of the Pb—I bonds. Moreover, the electrons and holes tend to be located on Pb sites, and this may give rise to the formation of local potential wells, which would further induce the large lattice deformation to accommodate the self-trapped excitons. Especially, CsPbI<sub>3</sub>-<i>Pnma</i> perovskite is metastable in the ambient environment with a suitable photon absorption threshold. The CsPb<sub>1–<i>x</i></sub>Ba<sub><i>x</i></sub>I<sub>3</sub> can be used as a capping layer on CsPbI<sub>3</sub> in solar cells, thereby significantly improving the power conversion efficiency and long-term stability. Overall, the alloyed perovskite CsPb<sub>1–<i>x</i></sub>Ba<sub><i>x</i></sub><i>X</i><sub>3</sub> (<i>X</i> = Cl, Br, I) with high Ba concentration can be stable and less-toxic, and they can be used in short wave light-emitting diodes, radiation detectors or other fields because of their large bandgaps (> 2.8 eV).

Unveiling the Role of tBP-LiTFSI Complexes in Perovskite Solar Cells.

Lead halide-based perovskite materials have been applied as an intrinsic layer for next-generation photovoltaic devices. However, the stability and performance reproducibility of perovskite solar cells (PSCs) needs to be further improved to match that of silicon photovoltaic devices before they can be commercialized. One of the major bottlenecks that hinders the improvement of device stability/reproducibility is the additives in the hole-transport layer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4- tert-butylpyridine (tBP). Despite the positive effects of these hole-transport layer additives, LiTFSI is hygroscopic and can adsorb moisture to accelerate the perovskite decomposition. On the other hand, tBP, the only liquid component in PSCs, which evaporates easily, is corrosive to perovskite materials. Since 2012, the empirical molar ratio 6:1 tBP:LiTFSI has been wildly applied in PSCs without further concerns. In this study, the formation of tBP-LiTFSI complexes at various molar ratios has been discovered and investigated thoroughly. These complexes in PSCs can alleviate the negative effects (decomposition and corrosion) of individual components tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change in tBP:LiTFSI ratio results in huge influences on the stability of perovskite. Due to the existence of uncomplexed tBP in the 6:1 tBP:LiTFSI mixture, this empirical tBP-LiTFSI molar ratio has been demonstrated not as the ideal ratio in PSCs. Instead, the 4:1 tBP:LiTFSI mixture, in which all components are complexed, shows all positive effects of the hole-transport layer components with dramatically reduced negative effects. It minimizes the hygroscopicity of LiTFSI, while lowering the evaporation speed and corrosive effect of tBP. As a result, the PSCs fabricated with this tBP:LiTFSI ratio have the highest average device efficiency and obviously decreased efficiency variation with enhanced device stability, which is proposed as the golden ratio in PSCs. Our understanding of interactions between hole-transport layer additives and perovskite on a molecular level shows the pathway to further improve the PSCs' stability and performance reproducibility to make them a step closer to large-scale manufacturing.

How SnF2 Impacts the Material Properties of Lead-Free Tin Perovskites

Lead-based halide perovskites (APbX3) are fascinating optoelectronic materials. Because of toxicity issues of Pb, Sn-based halide perovskites are studied, although less so, as an alternative. Adding SnF2 often improves the properties of Sn halide perovskite-based devices. This effect is usually ascribed to suppression of Sn2+ → Sn4+ oxidation and/or decreased Sn vacancy concentration. These effects will change the doping, sometimes in opposite directions. Here we review the effect of addition of SnF2 during the formation of ASnX3 layers as observed by different groups, both to the properties of the layers themselves and to photovoltaic cells made from these layers. SnF2 can affect many different properties of the ASnX3 perovskites, including film morphology, doping, control over formation of unwanted crystal phases, material stability to various factors, and energy level positions. It also improves (in general) the performance of photovoltaic cells made with these layers. Besides focusing on all these iss...

Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals.

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic–inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Poor Photovoltaic Performance of Cs3Bi2I9: An Insight through First-Principles Calculations

Bismuth-based halide perovskite derivatives have now attracted huge attention for photovoltaic (PV) applications after the unparalleled success of lead-based halide perovskites. However, the performances of PV devices based on these compounds are poor, despite theoretical predictions. In this Article, we have investigated the electronic structure and defect formation energies of Cs3Bi2I9 using density functional theory (DFT) calculations. The calculated electronic bandstructure indicates an indirect bandgap and high carrier effective masses. Our calculations reveal a large stability region for this compound; however, deep level defects are quite prominent. Even the varying chemical potentials from the stoichiometric region do not eliminate the presence of deep defects, ultimately limiting photovoltaic efficiencies.

High-Pressure Study of Perovskite-Like Organometal Halide: Band-Gap Narrowing and Structural Evolution of [NH3-(CH2)4-NH3]CuCl4.

Searching for nontoxic and stable perovskite-like alternatives to lead-based halide perovskites for photovoltaic application is one urgent issue in photoelectricity science. Such exploration inevitably requires an effective method to accurately control both the crystalline and electronic structures. This work applies high pressure to narrow the band gap of perovskite-like organometal halide, [NH3-(CH2)4-NH3]CuCl4 (DABCuCl4), through the crystalline-structure tuning. The band gap keeps decreasing below ∼12 GPa, involving the shrinkage and distortion of CuCl42-. Inorganic distortion determines both band-gap narrowing and phase transition between 6.4 and 10.5 GPa, and organic chains function as the spring cushion, evidenced by the structural transition at ∼0.8 GPa. The supporting function of organic chains protects DABCuCl4 from phase transition and amorphization, which also contributes to the sustaining band-gap narrowing. This work combines crystal structure and macroscopic property together and offers new strategies for the further design and synthesis of hybrid perovskite-like alternatives.

Tuning the gap of lead-based halide perovskites by introducing superalkali species at the cationic sites of ABX3-type structure.

We study with scalar relativistic density functional theory (DFT) calculations the effect of changes in the ionicity of the bonding mechanism and charge donation on the structure and electronic properties of new lead halide perovskites which show promising performance in optoelectronic applications as long-wave infrared detectors and thermoelectrics. Our results provide evidence that the band gap of these compounds can be tuned upon the introduction of appropriate superalkali moieties at the cationic A-sites in the CsPbI3-type structure. The computed band gap is 0.36 eV (direct) and 0.41 eV (indirect) for [Li3O]PbI3 and [Li3S]PbI3, respectively. By changing the chemical environment in the Pb-halide perovskite structure, we see drastic changes in the shape of both valence and conduction bands, as compared to CsPbI3. Introducing superalkali cations produces extra electronic states close to the Fermi level which arise from the formation of delocalized energy states, where a strong hybridization is identified between Pb and Li s-states near the top of the valence band. This can promote the hole mobility and increase the exciton diffusion length at longer wavelengths. Berry phase calculations show rather significant spontaneous polarization of 34 and 15 μC cm-2 along the x-axis in both [Li3O]PbI3 and [Li3S]PbI3, indicative of ferroelectric behavior.

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer.

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.

Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals.

Lead-based halide perovskites are emerging as the most promising class of materials for next-generation optoelectronics; however, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, perform structural refinement using single-crystal X-ray diffraction, and characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm3̅m space group and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-vis and photoluminescence measurements we obtain an indirect gap of 2.2 eV.

Perovskite Crystals for Tunable White Light Emission

A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost-effective means. Here, we synthesize organic–inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5–50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the p...

Tunable Optical Properties and Charge Separation in CH3NH3Sn(x)Pb(1-x)I3/TiO2-Based Planar Perovskites Cells.

A sharp potential drop across the interface of the Pb-rich halide perovskites/TiO2 heterostructure is predicted from first-principles calculations, suggesting enhanced separation of photoinduced charge carriers in the perovskite-based photovoltaic solar cells. The potential drop appears to be associated with the charge accumulation at the polar interface. More importantly, on account of both the β phase structure of CH3NH3Sn(x)Pb(1-x)I3 for x < 0.5 and the α phase structure of CH3NH3Sn(x)Pb(1-x)I3 for x ≥ 0.5, the computed optical absorption spectra from time-dependent density functional theory (TD-DFT) are in very good agreement with the measured spectra from previous experiments. Our TD-DFT computation also confirms the experimental structures of the mixed Pb-Sn organometal halide perovskites. These computation results provide a highly sought answer to the question why the lead-based halide perovskites possess much higher power conversion efficiencies than the tin-based counterparts for solar-cell applications.

Vacuum-assisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells.

Solar cells incorporating lead halide-based perovskite absorbers can exhibit impressive power conversion efficiencies (PCEs), recently surpassing 15%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, both of which are necessary for further advancements. Here we demonstrate vacuum-assisted thermal annealing as an effective means for controlling the composition and morphology of the CH(3)NH(3)PbI(3) films formed from the precursors of PbCl(2) and CH(3)NH(3)I. We identify the critical role played by the byproduct of CH(3)NH(3)Cl on the formation and the photovoltaic performance of the perovskite film. By completely removing the byproduct through our vacuum-assisted thermal annealing approach, we are able to produce pure, pore-free planar CH(3)NH(3)PbI(3) films with high PCE reaching 14.5% in solar cell device. Importantly, the removal of CH(3)NH(3)Cl significantly improves the device stability and reproducibility with a standard deviation of only 0.92% in PCE as well as strongly reducing the photocurrent hysteresis.

Elucidating the charge carrier separation and working mechanism of CH3NH3PbI3−xClx perovskite solar cells

Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p- and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.