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Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells
The presence of unreacted lead iodide in organic-inorganic lead halide perovskite solar cells is widely correlated with an increase in power conversion efficiency. We investigate the mechanism for this increase by identifying the role of surfaces and interfaces present between methylammonium lead iodide perovskite films and excess lead iodide. We show how type I and II band alignments arising under different conditions result in either passivation of surface defects or hole injection. Through first-principles simulations of solid-solid interfaces, we find that lead iodide captures holes from methylammonium lead iodide and modulates the formation of defects in the perovskite, affecting recombination. Using surface-sensitive optical spectroscopy techniques, such as transient reflectance and time-resolved photoluminescence, we show how excess lead iodide affects the diffusion and surface recombination velocity of charge carriers in methylammonium lead iodide films. Our coupled experimental and theoretical results elucidate the role of excess lead iodide in perovskite solar cells.
One-Step Deposition of Photovoltaic Layers Using Iodide Terminated PbS Quantum Dots
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Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study
Femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy based on a high-harmonic generation source is used to study the 266 nm induced A-band photodissociation dynamics of allyl iodide (CH{sub 2} =CHCH{sub 2}I). The photolysis of the C—I bond at this wavelength produces iodine atoms both in the ground ({sup 2}P{sub 3/2}, I) and spin-orbit excited ({sup 2}P{sub 1/2}, I*) states, with the latter as the predominant channel. Using XUV absorption at the iodine N{sub 4/5} edge (45–60 eV), the experiments constitute a direct probe of not only the long-lived atomic iodine reaction products but also the fleeting transition state region of the repulsive n{sub I}σ{sup ∗}{sub C—I} excited states. Specifically, three distinct features are identified in the XUV transient absorption spectrum at 45.3 eV, 47.4 eV, and 48.4 eV (denoted transients A, B, and C, respectively), which arise from the repulsive valence-excited nσ{sup ∗} states and project onto the high-lying core-excited states of the dissociating molecule via excitation of 4d(I) core electrons. Transients A and B originate from 4d(I) → n(I) core-to-valence transitions, whereas transient C is best assigned to a 4d(I) →σ{sup ∗}(C—I) transition. The measured differential absorbance of these new features along with the I/I* branching ratios known from the literature is used to suggest a more definitive assignment, albeit provisional, of the transients to specific dissociative states within the A-band manifold. The transients are found to peak around 55 fs–65 fs and decay completely by 145 fs–185 fs, demonstrating the ability of XUV spectroscopy to map the evolution of reactants into products in real time. The similarity in the energies of transients A and B with analogous features observed in methyl iodide [Attar et al. J. Phys. Chem. Lett. 6, 5072, (2015)] together with the new observation of transient C in the present work provides a more complete picture of the valence electronic structure in the transition state region. The results provide a benchmark for theoretical calculations on the nature of core-excited states in halogenated hydrocarbons, especially in the transition state region along the C—I reaction coordinate.
Hydrothermal synthesis and solid-state structures of polynuclear technetium iodide compounds
Two new technetium iodide compounds, Tc-2(O2CCH3)(4)I and K[Tc-8(mu-I)(8)I-4] I, were synthesized in an autoclave from the reaction of KTcO4 in glacial acetic acid with hydroiodic acid and/or alkali metal iodide salts at 210 degrees C under 60-70 atm hydrogen pressure. The structures of Tc-2(O2CCH3)(4)I and K[Tc-8(mu-I)(8)I-4] I were solved by single-crystal X-ray diffraction. The compound Tc-2(O2CCH3)(4)I crystallizes in the monoclinic space group C2/m with a = 7.1194(6) angstrom, b = 14.5851(13) angstrom, c = 7.1586(6) angstrom, and beta = 110.9540(10)degrees. The structure of Tc-2(O2CCH3)(4)I consists of infinite chains of Tc-2(O2CCH3)(4)(+) units linked by bridging iodides, an arrangement similar to the one found in Tc-2(O2CCH3)(4)X (X = Cl, Br). The TcATc separation in Tc-2 (O2CCH3)(4)I (i.e., 2.1146(4) angstrom) is consistent with the presence of a TcATc bond of order 3.5. Magnetic susceptibility measurements reveal Tc-2(O2CCH3)(4)I to be paramagnetic (mu(eff) = 1.84 B.M.) and support the electronic configuration sigma(2)pi(2)delta(2)delta*(1) for the Tc-2(5+) unit in the compound. The compound K[Tc-8(mu-I)(8)I-4]I crystallizes in the monoclinic space group P2(1)/n with a = 8.0018(5) angstrom, b = 14.5125(10) angstrom, c = 13.1948(9) angstrom, and beta = 102.3090(10) degrees, and is the first octanuclear technetium iodide cluster to be reported. The Tc-Tc separations in the [Tc-8(mu-I)(8)I-4] cluster (i.e., 2.164(3) angstrom, 2.5308(8) angstrom and 2.72(3) angstrom suggest the presence of Tc equivalent to Tc triple bonds, Tc=Tc double bonds and Tc-Tc single bonds.
Phase Stability and Electronic Structure of Prospective Sb-Based Mixed Sulfide and Iodide 3D Perovskite (CH 3 NH 3 )SbSI 2
Lead-free antimony-based mixed sulfide and iodide perovskite phases have recently been reported to be synthesized experimentally and to exhibit reasonable photovoltaic performance. In this work, through a combination of experimental validation and computational analysis, we show no evidence of the formation of the mixed sulfide and iodide perovskite phase, MASbSI 2 (MA = CH 3 NH 3 + ), and instead that the main products are a mixture of the binary and ternary compounds (Sb 2 S 3 and MA 3 Sb 2 I 9 ). Density functional theory calculations also indicate that such a mixed sulfide and iodide perovskite phase should be thermodynamically less stable compared with binary/ternary anion-segregated secondary phases and less likely to be synthesized under equilibrium conditions. Additionally, band structure calculations show that this mixed sulfide and iodide phase, if possible to synthesize (e.g., under nonequilibrium conditions), should have a suitable direct band gap for photovoltaic application.
Barium iodide and strontium iodide crystals and scintillators implementing the same
In one embodiment, a material comprises a crystal comprising strontium iodide providing at least 50,000 photons per MeV, where the strontium iodide material is characterized by a volume not less than 1 cm.sup.3. In another embodiment, a scintillator optic includes europium-doped strontium iodide providing at least 50,000 photons per MeV, where the europium in the crystal is primarily Eu.sup.2+, and the europium is present in an amount greater than about 1.6%. A scintillator radiation detector in yet another embodiment includes a scintillator optic comprising SrI.sub.2 and BaI.sub.2, where a ratio of SrI.sub.2 to BaI.sub.2 is in a range of between 0:1 and 1.0, the scintillator optic is a crystal that provides at least 50,000 scintillation photons per MeV and energy resolution of less than about 5% at 662 keV, and the crystal has a volume of 1 cm.sup.3 or more; the scintillator optic contains more than about 2% europium.
Fabrication of efficient low-bandgap perovskite solar cells by combining formamidinium tin iodide with methylammonium lead iodide
Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn-Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI 3 ) and methylammonium lead iodide (MAPbI 3 ). The best-performing cell fabricated using a (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 absorber with an absorption edge of ~1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799) V, a short-circuit current density of 26.86(26.82) mA/cm 2 , and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. In conclusion, the average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility.
Electron and hole drift mobility measurements on methylammonium lead iodide perovskite solar cells
We report nanosecond domain time-of-flight measurements of electron and hole photocarriers in methylammonium lead iodide perovskite solar cells. The mobilities ranged from 0.06 to 1.4 cm{sup 2}/Vs at room temperature, but there is little systematic difference between the two carriers. We also find that the drift mobilities are dispersive (time-dependent). The dispersion parameters are in the range of 0.4–0.7, and they imply that terahertz domain mobilities will be much larger than nanosecond domain mobilities. The temperature-dependences of the dispersion parameters are consistent with confinement of electron and hole transport to fractal-like spatial networks within nanoseconds of their photogeneration.
History and current status of strontium iodide scintillators
Eu-doped strontium iodide single crystal growth has reached maturity and prototype SrI 2 (Eu)-based gamma ray spectrometers provide detection performance advantages over standard detectors. SrI 2 (Eu) offers a high, proportional light yield of >80,000 photons/MeV. Energy resolution of <3% at 662 keV with 1.5” x 1.5” SrI2(Eu) crystals is routinely achieved, by employing either a small taper at the top of the crystal or a digital readout technique. These methods overcome light-trapping, in which scintillation light is re-absorbed and re-emitted in Eu 2+ -doped crystals. As a result, its excellent energy resolution, lack of intrinsic radioactivity or toxicity, and commercial availability make SrI 2 (Eu) the ideal scintillator for use in handheld radioisotope identification devices. A 6-lb SrI 2 (Eu) radioisotope identifier is described.
Substitutional Growth of Methylammonium Lead Iodide Perovskites in Alcohols
Methylammonium lead iodide (MAPbIx) perovskites are organic-inorganic semiconductors that serve as the light-harvesting component in photovoltaic devices. Their long carrier diffusion lengths yield power conversion efficiencies in excess of 22%. Conventional growth techniques apply spin-coated precursors on an oxide or polymer substrate followed by annealing; however, use of toxic solvents and high temperature hinder device stability and performance. Through a nucleophilic substitutional growth of MAPbIx in polar protic alcohols, we show evidence for their substrate- and annealing- free production. These variable-sized crystals (~100 nm-10 µm) are found to be tetragonally single-phased in alcohols, and precipitated as powders that are metallic-lead-free with a negligibly small amount of unreacted PbI2 (<10%). On the basis of this chemical approach, we obtain chemically stable MAPbIx in air up to ~1.5 months with thermal stability at ≤300°C. This method is amendable for large-area deposition, large-scale manufacturing, and ultimately leads to energy-efficient, low-cost, and stable optoelectronics.
Ionic and Optical Properties of Methylammonium Lead Iodide Perovskite across the Tetragonal–Cubic Structural Phase Transition
Abstract Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal–cubic structural phase transition of methylammonium lead iodide (MAPbI 3 ). However, the ionic and optical properties of MAPbI 3 in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 °C in the ionic/electrical properties of MAPbI 3 owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. These observations can be explained by the “continuous” nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal–cubic structural phase transition occurs.
Electron–Rotor Interaction in Organic–Inorganic Lead Iodide Perovskites Discovered by Isotope Effects
We report on the carrier-rotor coupling effect in perovskite organic-inorganic hybrid lead iodide (CH3NH3PbI3) compounds discovered by isotope effects. Deuterated organic-inorganic perovskite compounds including CH3ND3PbI3, CD3NH3PbI3, and CD3ND3PbI3 were synthesized. Devices made from regular CH3NH3PbI3 and deuterated CH3ND3PbI3 exhibit comparable performance in band gap, current-voltage, carrier mobility, and power conversion efficiency. However, a time-resolved photoluminescence (TRPL) study reveals that CH3NH3PbI3 exhibits notably longer carrier lifetime than that of CH3ND3PbI3, in both thin-film and single crystal formats. Furthermore, the comparison in carrier lifetime between CD3NH3PbI3 and CH3ND3PbI3 single crystals suggests that vibrational modes in methylammonium (MA(+)) have little impact on carrier lifetime. In contrast, the fully deuterated compound CD3ND3PbI3 reconfirmed the trend of decreasing carrier lifetime upon the increasing moment of inertia of cationic MA(+). Polaron model elucidates the electron-rotor interaction.
Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites
The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI 3 ) and formamidinium lead iodide (FAPbI 3 ), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Herein, we determine the dynamics of the molecular cations in FAPbI 3 between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons with the reported temperature dependence of the dynamics of MAPbI 3 are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes. For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule–cage interaction in FAPbI 3 produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom–atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including those of large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.
In situ investigation of the formation and metastability of formamidinium lead tri-iodide perovskite solar cells
Organic–inorganic perovskites have emerged as an important class of next generation solar cells due to their remarkably low cost, band gap, and sub-900 nm absorption onset. Here, we show a series of in situ observations inside electron microscopes and X-ray diffractometers under device-relevant synthesis conditions focused on revealing the crystallization process of the formamidinium lead-triiodide perovskite at the optimum temperature of 175 °C. Direct in situ observations of the structure and chemistry over relevant spatial, temporal, and temperature scales enabled identification of key perovskite formation and degradation mechanisms related to grain evolution and interface chemistry. The lead composition was observed to fluctuate at grain boundaries, indicating a mobile lead-containing species, a process found to be partially reversible at a key temperature of 175 °C. Using low energy electron microscopy and valence electron energy loss spectroscopy, lead is found to be bonded in the grain interior with iodine in a tetrahedral configuration. At the grain boundaries, the binding energy associated with lead is consequently shifted by nearly 2 eV and a doublet peak is resolved due presumably to a greater degree of hybridization and the potential for several different bonding configurations. At the grain boundaries there is adsorption of hydrogen and OH¯ ions as a result of residual water vapor trapped as a non-crystalline material during formation. Lastly, insights into the relevant formation and decomposition reactions of formamidinium lead iodide at low to high temperatures, observed metastabilities, and relationship with the photovoltaic performance were obtained and used to optimize device processing resulting in conversion efficiencies of up to 17.09% within the stability period of the devices.
Surface hopping molecular dynamics simulation of ultrafast methyl iodide photodissociation mapped by Coulomb explosion imaging
Our simulation demonstrates the robustness of the Coulomb explosion imaging technique in studying methyl iodide photodissociation, and shows that it can be effectively used for imaging non-adiabatic transitions in coordinate space.
Optical response of mixed methylammonium lead iodide and formamidinium tin iodide perovskite thin films
Here, mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI 3 ) and formamidinium tin iodide (FASnI 3 ) precursors. Optical response in the form of complex dielectric function (ε = ε 1 + iε 2 ) spectra and absorption coefficient (α) spectra of (FASnI 3 ) 1-x (MAPbI 3 ) x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI 3 , MAPbI 3 , and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI 3 ) 1-x (MAPbI 3 ) x perovskites. (FASnI 3 ) 0.60 (MAPbI 3 ) 0.4 is found to have the minimum absorption band edge energy near ~1.2 eV.