Tunable Optical Properties and Charge Separation in CH3NH3SnxPb1–xI3/TiO2-Based Planar Perovskites Cells

Composite of formamidinium lead bromide perovskite FAPbBr3 with reduced graphene oxide (rGO) for efficient H2 evolution from HBr splitting

Solar hydrobromic acid (HBr) splitting using perovskite photocatalysts provides an attractive avenue to store solar energy into hydrogen (H2) and bromine (Br2), while an efficient photocatalytic system is still demanded. As for the semiconductor photocatalyst, formamidinium perovskites show some superiorities in structural stability, light adsorption and charge dynamics compared to their methylammonium counterparts, which are fitter for the photocatalysis process. Herein, the composite of formamidinium lead bromide perovskite (FAPbBr3) with reduced graphene oxide (rGO) is prepared using a facile photoreduction method. Under simulated sunlight irradiation (AM1.5G, 100 mW cm-2), this FAPbBr3/rGO composite (100 mg) demonstrates a noteworthy enhancement in photocatalytic H2 evolution activity of 386.7 μmol h-1, and it exhibits a notable stability with no significant decrease after 50 h of repeated tests. The single particle PL (photoluminescence) microscope is employed to study the charge dynamics, revealing that rGO in the composite effectively promotes the carrier separation. This work provides a highly efficient and stable photocatalyst for HBr splitting, and offers an effective modification strategy on lead bromide perovskites.

Surface Modification in CsPb0.5Sn0.5I2Br Inorganic Perovskite Solar Cells: Effects of Bifunctional Dipolar Molecules on Photovoltaic Performance

Inorganic tin-lead binary perovskites have piqued the interest of researchers as effective absorbers for thermally stable solar cells. However, the nonradiative recombination originating from the surface undercoordinated Sn²⁺ cations and the energetic offsets between different layers cause an excessive energy loss and deteriorate the perovskite device's performance. In this study, we investigated two thioamide derivatives that differ only in the polar part connected to their common benzene ring, namely, benzenecarbothioamide and 4-fluorophenylcarbothioamide (F-TBA). These two molecules were implemented as modifiers onto the inorganic tin-lead perovskite (CsPb_0.5Sn_0.5I₂Br) surface in the perovskite solar cells. Modifiers that carry C═S and NH₂ functional groups, equipped with lone electron pairs, can autonomously associate with surface Sn²⁺ through coordination and electrostatic attraction mechanisms. This interaction serves effectively to passivate the surface. In addition, due to the permanent dipole moment of the intermediate layer, an interfacial dipole field appears at the PCBM/CsPb_0.5Sn_0.5I₂Br interface, reducing the electron extraction potential barrier. Consequently, the planar solar cell with an ITO/PEDOT:PSS/CsPb_0.5Sn_0.5I₂Br/PCBM/BCP/Ag layered structure featuring an F-TBA surface post-treatment demonstrated a noteworthy power conversion efficiency of 14.01%. Simultaneously, after being stored for 1000 h in an inert atmosphere glovebox, the non-encapsulated CsPb_0.5Sn_0.5I₂Br solar cells managed to preserve 94% of their original efficiency.

Nickel Single-Atom Catalyst-Mediated Efficient Redox Cycle Enables Self-Checking Photoelectrochemical Biosensing with Dual Photocurrent Readouts

Developing a self-checking photoelectrochemical biosensor with dual photocurrent signals could efficiently eliminate false-positive or false-negative signals. Herein, a novel biosensor with dual photocurrent responses was established for the detection of acetylcholinesterase activity. To achieve photocurrent polarity-switchable behavior, the iodide/tri-iodide redox couple was innovatively introduced to simultaneously consume the photoexcited electrons and holes, which circumvents the inconvenience caused by the addition of different hole- and electron-trapping agents in the electrolyte. Importantly, benefiting from the high catalytic activity, the enhanced photoelectric responsivity can be realized after decorating the counter electrode with nickel single-atom catalysts, which promotes a more efficient iodide/tri-iodide redox reaction under low applied voltages. It is envisioned that the proposed photocurrent polarity switching system offers new routes to sensitive and reliable biosensing.

Engineering Stable Lead-Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn-PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn-PSCs, including early developments to the latest state-of-the-art approaches benefitting the stability of devices. The degradation pathways associated with Sn-PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn-based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn-PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn-PSCs.

Boosting Reversibility and Stability of Li Storage in SnO2 -Mo Multilayers: Introduction of Interfacial Oxygen Redistribution

Among the promising high-capacity anode materials, SnO₂ represents a classic and important candidate that involves both conversion and alloying reactions toward Li storage. However, the inferior reversibility of conversion reactions usually results in low initial Coulombic efficiency (ICE, ≈60%), small reversible capacity, and poor cycling stability. Here, it is demonstrated that by carefully designing the interface structure of SnO₂ -Mo, a breakthrough comprehensive performance with ultrahigh average ICE of 92.6%, large capacity of 1067 mA h g^-1 , and 100% capacity retention after 700 cycles can be realized in a multilayer Mo/SnO₂ /Mo electrode. Furthermore, high capacity retentions are also achieved in pouch-type Mo/SnO₂ /Mo||Li half cells and Mo/SnO₂ /Mo||LiFePO₄ full cells. The amorphous SnO₂ /Mo interfaces, which are induced by redistribution of oxygen between SnO₂ and Mo, can precisely adjust the reversible capacity and cycling stability of the multilayers, while the stable capacities are parabolic with the interfacial density. Theoretical calculations and in/ex situ investigation reveal that oxygen redistribution in SnO₂ /Mo heterointerfaces boosts Li-ion transport kinetics by inducing a built-in electric field and improves the reaction reversibility of SnO₂ . This work provides a new understanding of interface-performance relationship of metal-oxide hybrid electrodes and pivotal guidance for creating high-performance Li-ion batteries.

Stable CsPbX3/ZnO Heterostructure Nanocrystals for Light-Emitting Application

Lead halide perovskite nanocrystals (PNCs) are attractive in light-emitting applications with their extraordinary photoluminescent property. However, the inherent instability of mixed Br/I ions poses a major hurdle to the practical application of yellow-red emitting PNCs. In this work, we report the growth of crystalline ZnO on the surface of CsPb(Br/I)₃ nanocrystals through a ligand-mediated in situ surface reaction route, which forms monodispersed CsPb(Br/I)₃/ZnO heterostructure nanocrystals (PZNCs). The unique heterostructure endows the PZNCs with largely enhanced stability against air, moisture, and polar solvents. Notably, the red-emitting PZNCs exhibit high color quality and superior stability compared with classical CsPb(Br/I)₃ PNCs. The PZNCs retain 95% of initial luminescence after 500 h of illumination, and the PZNC-converted red-light LEDs show no obvious change in the peak position and light intensity after continuous operation for 100 h, demonstrating the promising prospect of the materials in light-emitting applications.

Extremely large static and dynamic nonlinear optical response of small superalkali clusters NM3M' (M, M'=Li, Na, K)

Exploring novel nonlinear optical (NLO) materials with excess electron properties is essential for advancing the use of excess electron compounds in optics. The studied superalkali clusters NM₃M' (M, M' = Li, Na, K) are thermodynamically stable and their binding energies range from -27.10 to -53.84 kcal mol^-1. The observed significant values for VIPs suggest their electronic stabilities. Being excess electron candidate these clusters show significant β_o value (3.9 × 10⁷ au), which nicely correlates the hyperpolarizability reported by a two-level model (β_tl). Furthermore, these clusters exhibit a remarkable static second hyperpolarizability (γ_o) value of 1.1 × 10¹⁰ au for the NK₄ superalkali cluster. The hyper Rayleigh scattering (β_HRS) is also computed where the highest value of 2.9 × 10⁷ is recorded for NNa₃K superalkali. The obtained values of β_vec values (projection of hyperpolarizability on dipole moment vector) also signify the excellent nonlinearity of clusters. Besides, the calculated electro-optica pockel's effect β(-ω; ω,0) and second harmonic generation β(-2ω; ω, ω) values are much pronounced at larger dispersion frequency ω = 1064 nm. Moreover, the frequency-dependent second hyperpolarizability γ(ω) with dc-Kerr effect γ(-ω; ω,0,0) and electric field induced second harmonic generation γ(-2ω; ω,ω,0) show larger values at ω = 1064 nm. Thus the highest value of the dc-Kerr constant increases up to 1.0 × 10¹¹ au which also signifies the larger nonlinear refractive index of the studied cluster. We hope this work could open up new possibilities using superalkali clusters as NLO materials for optoelectronics, laser, second harmonic generation and as frequency doubler.

A Nanocrystal Catalyst Incorporating a Surface Bound Transition Metal to Induce Photocatalytic Sequential Electron Transfer Events

Heterogeneous photocatalysis is less common but can provide unique avenues for inducing novel chemical transformations and can also be utilized for energy transductions, i.e., the energy in the photons can be captured in chemical bonds. Here, we developed a novel heterogeneous photocatalytic system that employs a lead-halide perovskite nanocrystal (NC) to capture photons and direct photogenerated holes to a surface bound transition metal Cu-site, resulting in a N-N heterocyclization reaction. The reaction starts from surface coordinated diamine substrates and requires two subsequent photo-oxidation events per reaction cycle. We establish a photocatalytic pathway that incorporates sequential inner sphere electron transfer events, photons absorbed by the NC generate holes that are sequentially funneled to the Cu-surface site to perform the reaction. The photocatalyst is readily prepared via a controlled cation-exchange reaction and provides new opportunities in photodriven heterogeneous catalysis.

The spin-orbit interaction controls photoinduced interfacial electron transfer in fullerene-perovskite heterojunctions: C60versus C70

Here, we used collinear and noncollinear density functional theory (DFT) methods to explore the interfacial properties of two heterojunctions between a fullerene (C₆₀ and C₇₀) and the MAPbI₃(110) surface. Methodologically, consideration of the spin-orbit interaction has been proven to be required to obtain accurate energy-level alignment and interfacial carrier dynamics between fullerenes and perovskites in hybrid perovskite solar cells including heavy atoms (such as Pb atoms). Both heterojunctions are predicted to be the same type-II heterojunction, but the interfacial electron transfer process from MAPbI₃ to C₆₀ is completely distinct from that to C₇₀. In the former, the interfacial electron transfer is slow because of the associated large energy gap, and the excited electrons are thus trapped in MAPbI₃ for a while. In contrast, in the latter, the smaller energy gap induces ultrafast electron transfer from MAPbI₃ to C₇₀. These points are further supported by DFT-based nonadiabatic dynamics simulations including the spin-orbit coupling (SOC) effects. These gained insights could help rationally design superior fullerene-perovskite interfaces to achieve high power conversion efficiencies of fullerene-perovskite solar cells.

Spin-Orbit Coupling Accelerates the Photoinduced Interfacial Electron Transfer in a Fullerene-Based Perovskite Heterojunction

In this work, we explore the interfacial properties of the C₆₀-Py@MAPbI₃ heterojunction of the PbI-terminated MAPbI₃(001) surface and pyridine-functionalized C₆₀-Py fullerene derivative through both collinear and noncollinear density functional theory calculations with and without spin-orbit coupling (SOC) effects. C₆₀-Py is bound to the MAPbI₃ surface through interfacial Pb-O and Pb-N bonds. Although C₆₀-Py@MAPbI₃ is predicted to be the same type II heterojunction at all of the computational levels considered, the SOC effects largely decrease the energy gap of the first conduction bands of C₆₀-Py and MAPbI₃, thereby accelerating the interfacial electron transfer. Further dynamics simulations show that the inclusion of the SOC effects induces the transfer of approximately 80% of electrons from MAPbI₃ to C₆₀-Py within 1 ps. The work demonstrates that the SOC effects are indispensable for the interfacial properties of C₆₀-Py@MAPbI₃ and could also play a non-negligible role in tuning the optoelectronic properties of fullerene-based or similar perovskite devices.

Photoredox Organic Synthesis Employing Heterogeneous Photocatalysts with Emphasis on Halide Perovskite

Lately, heterogeneous semiconductor materials have been explored as an emerging type of efficient photocatalyst for photoredox organic synthesis. Among these semiconductors, lead halide perovskite materials demonstrate unique properties towards excellent charge separation and charge transfer, extremely long charge carrier migration, high efficiency in visible light absorption, and long excited states lifetimes, etc., as proved in ground-breaking solar cell applications, garnering necessary merits for an efficient catalytic system for photoredox organic reactions. Here, the latest progress in heterogeneous semiconductor materials towards this endeavor is examined, with particular emphasis on lead halide perovskite nanocrystals (NCs) in photocatalytic organic synthesis.

High Efficiency Mesoscopic Solar Cells Using CsPbI3 Perovskite Quantum Dots Enabled by Chemical Interface Engineering

All-inorganic α-CsPbI₃ perovskite quantum dots (QDs) are attracting great interest as solar cell absorbers due to their appealing light harvesting properties and enhanced stability due to the absence of volatile organic constituents. Moreover, ex situ synthesized QDs significantly reduce the variability of the perovskite layer deposition process. However, the incorporation of α-CsPbI₃ QDs into mesoporous TiO₂ (m-TiO₂) is highly challenging, but these constitute the best performing electron transport materials in state-of-the-art perovskite solar cells. Herein, the m-TiO₂ surface is engineered using an electron-rich cesium-ion containing methyl acetate solution. As one effect of this treatment, the solid-liquid interfacial tension at the TiO₂ surface is reduced and the wettability is improved, facilitating the migration of the QDs into m-TiO₂. As a second effect, Cs⁺ ions passivate the QD surface and promote the charge transfer at the m-TiO₂/QD interface, leading to an enhancement of the electron injection rate by a factor of 3. In combination with an ethanol-environment smoothing route that significantly reduces the surface roughness of the m-TiO₂/QD layer, optimized devices exhibit highly reproducible power conversion efficiencies exceeding 13%. The best cell with an efficiency of 14.32% (reverse scan) reaches a short-circuit current density of 17.77 mA cm^-2, which is an outstanding value for QD-based perovskite solar cells.

Thermodynamic Stability and Structural Insights for CH3NH3Pb1-xSixI3, CH3NH3Pb1-xGexI3, and CH3NH3Pb1-xSnxI3 Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations

The recent reaching of 20% of conversion efficiency by solar cells based on metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide, CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the photovoltaic materials. However, the toxicity of Pb is a hindrance for large scale commercial of MHP and motivates the search of another congener eco-friendly metal. Here, we employed first-principles calculations via density functional theory combined with the generalized quasichemical approximation to investigate the structural, thermodynamic, and ordering properties of MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures. The inclusion of a smaller second metal, as Si and Ge, strongly affects the structural properties, reducing the cavity volume occupied by the organic cation and limitating the free orientation under high temperature effects. Unstable and metaestable phases are observed at room temperature for MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in ordered phases even at very low temperatures. Conversely, the high miscibility of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a pseudo-cubic random alloy with tunable properties at room temperature.

Effects of Transition Metal Substituents on Interfacial and Electronic Structure of CH3NH3PbI3/TiO2 Interface: A First-Principles Comparative Study

To evaluate the influence of transition metal substituents on the characteristics of CH3NH3PbI3/TiO2, we investigated the geometrical and electronic properties of transition metal-substituted CH3NH3PbI3/TiO2 by first-principles calculations. The results suggested that the substitution of Ti4+ at the five-fold coordinated (Ti5c) sites by transition metals is energetically favored. The substituted interface has enhanced visible light sensitivity and photoelectrocatalytic activity by reducing the transition energies. The transition metal substitution can effectively tune the band gap of the interface, which significantly improves the photo-reactivity. The substituted systems are expected to be more efficient in separating the photo-generated electrons-holes and active in the visible spectrum.

Lead halide perovskites for photocatalytic organic synthesis

Nature is capable of storing solar energy in chemical bonds via photosynthesis through a series of C–C, C–O and C–N bond-forming reactions starting from CO₂ and light. Direct capture of solar energy for organic synthesis is a promising approach. Lead (Pb)-halide perovskite solar cells reach 24.2% power conversion efficiency, rendering perovskite a unique type material for solar energy capture. We argue that photophysical properties of perovskites already proved for photovoltaics, also should be of interest in photoredox organic synthesis. Because the key aspects of these two applications are both relying on charge separation and transfer. Here we demonstrated that perovskites nanocrystals are exceptional candidates as photocatalysts for fundamental organic reactions, for example C–C, C–N and C–O bond-formations. Stability of CsPbBr₃ in organic solvents and ease-of-tuning their bandedges garner perovskite a wider scope of organic substrate activations. Our low-cost, easy-to-process, highly-efficient, air-tolerant and bandedge-tunable perovskites may bring new breakthrough in organic chemistry.

While photoredox catalysis provides organic chemistry new avenues for chemical reactions, typical photocatalysts require expensive noble metals and show modest stabilities. Here, authors examine lead halide perovskites nanocrystals as stable and tunable photoredox catalysts for organic synthesis.

Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors

Hybrid halide perovskites are now superstar materials leading the field of low-cost thin film photovoltaics technologies. Following the surge for more efficient and stable 3D bulk alloys, multilayered halide perovskites and colloidal perovskite nanostructures appeared in 2016 as viable alternative solutions to this challenge, largely exceeding the original proof of concept made in 2009 and 2014, respectively. This triggered renewed interest in lower-dimensional hybrid halide perovskites and at the same time increasingly more numerous and differentiated applications. The present paper is a review of the past and present literature on both colloidal nanostructures and multilayered compounds, emphasizing that availability of accurate structural information is of dramatic importance to reach a fair understanding of quantum and dielectric confinement effects. Layered halide perovskites occupy a special place in the history of halide perovskites, with a large number of seminal papers in the 1980s and 1990s. In recent years, the rationalization of structure-properties relationship has greatly benefited from new theoretical approaches dedicated to their electronic structures and optoelectronic properties, as well as a growing number of contributions based on modern experimental techniques. This is a necessary step to provide in-depth tools to decipher their extensive chemical engineering possibilities which surpass the ones of their 3D bulk counterparts. Comparisons to classical semiconductor nanostructures and 2D van der Waals heterostructures are also stressed. Since 2015, colloidal nanostructures have undergone a quick development for applications based on light emission. Although intensively studied in the last two years by various spectroscopy techniques, the description of quantum and dielectric confinement effects on their optoelectronic properties is still in its infancy.

Brightly luminescent and color-tunable green-violet-emitting halide perovskite CH3NH3PbBr3 colloidal quantum dots: an alternative to lighting and display technology

Organic-inorganic hybrid perovskite (CH3NH3PbX3, X = Cl, Br, or I) quantum dots have become one of the most promising materials for optoelectronic applications. We controllably synthesized CH3NH3PbBr3 quantum dots with a tunable spectrum with the emission peaks covering the range from green (523.6 nm), blue and eventually to deep violet (409.4 nm), which is wider than that of quantum dots obtained without changing the halide component. The mechanism of the blueshift was investigated. The purified quantum dots have allowed the fabrication of efficient electroluminescence devices having a simple glass/ITO/PEDOT:PSS/TFB/CH3NH3PbBr3 quantum dot/TPBi/LiF/Al structure. CH3NH3PbBr3 quantum dots with 5-30 μL n-octylamine showed an ideal color-saturated green emission with Commission Internationale de l'Eclairage color coordinates of (0.123, 0.744) and a narrow full width at half-maximum of 19-24 nm. The photoluminescence quantum yield was up to 90.2%. In addition, it is also worth noting that the chromaticity coordinates (x, y) of CH3NH3PbBr3 quantum dots with 50-100 μL n-octylamine are (0.300, 0.344), (0.305, 0.314) and (0.323, 0.318) in the white region. All these properties indicate that these MAPbBr3 quantum dots can provide effective data support for the application of white LEDs, and may potentially be used as single-component multicolor-emitting materials, which can be applied to lighting and display technology.

Effects of transition metals incorporated into perovskite crystals on the electronic structures and magnetic properties by first-principles calculation

Additive effects of transition metals (M = Cr2+, Co2+, Cu2+ and Y3+) on the electronic structures and magnetic properties of formamidinium lead halide perovskite compounds (FAPbI3, where FA = NH2CHNH2+) were investigated by first-principle calculation using density functional theory. In the case of Cr2+, Cu2+ and Y3+-incorporated FAPbI3 perovskite crystals, the electron density distribution of d-p hybrid orbital on the transition metal and iodine halogen-atoms were delocalized at frontier orbital. The total and partial density of state appeared the 3d-p hybrid orbital near the frontier orbital with narrowing band gap, yielding the wide broad absorption in the near-infrared region. The electronic correlation worked in between the localized spin on 3d orbital of the metal, and the itinerant carriers on the 5p orbital of the iodine halogen ligand and the 6p orbital of the lead atom in the perovskite crystal. The vibration behavior of the Raman and Infrared spectra were associated with change of polarization and slight distortion near the coordination structure. The considerable splitting of chemical shift of 127I-NMR and 207Pb-NMR in the Co2+ and Cu2+-incorporated FAPbI3 crystals were caused by crystal field splitting as Jahn-Teller effect with nearest-neighbor nuclear quadrupole interaction based on the charge distribution.

Effect of water on the effective Goldschmidt tolerance factor and photoelectric conversion efficiency of organic-inorganic perovskite: insights from first-principles calculations

Water is often believed to be the leading killer of perovskite solar cells' efficiency. However, recent experimental results show that perovskite solar cells have higher photoelectric conversion efficiency in a suitably moist environment. In this study, the relationship between the interstitial water molecule and the theoretical maximum efficiency of the perovskite absorber layer is discussed based on density functional theory calculations. Our calculated results show that an interstitial water molecule can enlarge the effective Goldschmidt tolerance factor, which is an empirical structural parameter for the structure of the perovskite material. The primitive MAPbI₃ structure is not the ideal perovskite structure with the highest photoelectric conversion efficiency. Surprisingly, appropriate interstitial water molecules are beneficial to perovskite absorbers in terms of increasing photoelectric conversion efficiency. This can be attributed to the relatively larger effective Goldschmidt tolerance factor of the perovskite structure with an interstitial water molecule, which affects the photoelectric conversion efficiency of the perovskite structure. Our calculations indicate that the perovskite absorbers with a H₂O : MAPbI₃ ratio of 1/4-1/2 have a relatively higher photoelectric conversion efficiency. This study helps us understand the role of the interstitial molecule in the perovskite structure deeply, which is very useful in the design and optimization of the perovskite absorbers for high-efficiency perovskite cells.

First-Principles Study of Electron Injection and Defects at the TiO2/CH3NH3PbI3 Interface of Perovskite Solar Cells

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO₂(001) and a tetragonal CH₃NH₃PbI₃(110) (MAPbI₃(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI₃ and the TiO₂ conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO₂ layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI₃ layer produced no additional states in the MAPbI₃ gap, a Pb vacancy (V_Pb) at the interface created an energy level below the MAPbI₃ CB edge and had a lower energy of formation than the V_Pb defect in bulk because of the interaction with the TiO₂ surface.

Enhanced optical absorption via cation doping hybrid lead iodine perovskites

The suitable band structure is vital for perovskite solar cells, which greatly affect the high photoelectric conversion efficiency. Cation substitution is an effective approach to tune the electric structure, carrier concentration, and optical absorption of hybrid lead iodine perovskites. In this work, the electronic structures and optical properties of cation (Bi, Sn, and TI) doped tetragonal formamidinium lead iodine CH(NH₂)₂PbI₃ (FAPbI₃) are studied by first-principles calculations. For comparison, the cation-doped tetragonal methylammonium lead iodine CH₃NH₃PbI₃ (MAPbI₃) are also considered. The calculated formation energies reveal that the Sn atom is easier to dope in the tetragonal MAPbI₃/FAPbI₃ structure due to the small formation energy of about 0.3 eV. Besides, the band gap of Sn-doped MAPbI₃/FAPbI₃ is 1.30/1.40 eV, which is considerably smaller than the un-doped tetragonal MAPbI₃/FAPbI₃. More importantly, compare with the un-doped tetragonal MAPbI₃/FAPbI₃, the Sn-doped MAPbI₃ and FAPbI₃ have the larger optical absorption coefficient and theoretical maximum efficiency, especially for Sn-doped FAPbI₃. The lower formation energy, suitable band gap and outstanding optical absorption of the Sn-doped FAPbI₃ make it promising candidates for high-efficient perovskite cells.

Investigation of Fe2+-incorporating organic–inorganic hybrid perovskites from first principles and experiments

The structural, electronic, magnetic and optical properties of the perovskite CH₃NH₃(Pb:Fe)I₃have been systematically studied by using the first-principle calculation.

Polyethyleneimine High-Energy Hydrophilic Surface Interfacial Treatment toward Efficient and Stable Perovskite Solar Cells

The interfacial contact is critical for the performance of perovskite solar cells (PSCs), leading to dense perovskite thin films and efficient charge transport. In this contribution, an effective interfacial treatment solution using polyethyleneimine (PEI) was developed to improve the performance and stability of PSCs. Inserting PEI between the s-VO_x and perovskite layers can produce a high-energy hydrophilic surface to facilitate the formation of a high-quality perovskite layer by the solution method. Accordingly, the surface coverage of perovskite film on the s-VO_x layer increased from 80% to 95%, and the PCE of the device improved from 12.06% (with an average of 10.16%) to 14.4% (with an average value of 12.8%) under an irradiance of 100 mW cm^-2 AM 1.5G sunlight. More importantly, the stability of PSCs was further improved after adding another PEI layer between the electron transport layer and LiF/Al layer, less than 10% decay in efficiency during a 10-days observation. Since all layers of the PSCs were fabricated at low temperature (<150 °C), these PEI-treated PSCs based on the amorphous VO_x layer have the potential to contribute significantly toward the development of efficient and stable solar cells on flexible substrates.

Stable Low-Bandgap Pb-Sn Binary Perovskites for Tandem Solar Cells

A low-bandgap (1.33 eV) Sn-based MA_0.5 FA_0.5 Pb_0.75 Sn_0.25 I₃ perovskite is developed via combined compositional, process, and interfacial engineering. It can deliver a high power conversion efficiency (PCE) of 14.19%. Finally, a four-terminal all-perovskite tandem solar cell is demonstrated by combining this low-bandgap cell with a semitransparent MAPbI₃ cell to achieve a high efficiency of 19.08%.

Optical analysis of CH3NH3Sn x Pb1-x I3 absorbers: a roadmap for perovskite-on-perovskite tandem solar cells

Organic-inorganic perovskite structures in which lead is substituted by tin are exceptional candidates for broadband light absorption. Herein we present a thorough analysis of the optical properties of CH3NH3Sn x Pb1-x I3 films, providing the field with definitive insights about the possibilities of these materials for perovskite solar cells of superior efficiency. We report a user's guide based on the first set of optical constants obtained for a series of tin/lead perovskite films, which was only possible to measure due to the preparation of optical quality thin layers. According to the Shockley-Queisser theory, CH3NH3Sn x Pb1-x I3 compounds promise a substantial enhancement of both short circuit photocurrent and power conversion efficiency in single junction solar cells. Moreover, we propose a novel tandem architecture design in which both top and bottom cells are made of perovskite absorbers. Our calculations indicate that such perovskite-on-perovskite tandem devices could reach efficiencies over 35%. Our analysis serves to establish the first roadmap for this type of cells based on actual optical characterization data. We foresee that this study will encourage the research on novel near-infrared perovskite materials for photovoltaic applications, which may have implications in the rapidly emerging field of tandem devices.

High Efficiency Pb-In Binary Metal Perovskite Solar Cells

Mixed Pb-In perovskite solar cells are fabricated by using lead(II) chloride and indium(III) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of perovskites with multiple ordered crystal orientations.

Novel CdS Hole-Blocking Layer for Photostable Perovskite Solar Cells

Currently, the stability issue of the perovskite solar cells (PSCs) is one of the most critical obstacles in the commercialization of PSCs. Although incredible advances in the photovoltaic efficiencies of PSCs have been achieved in the past few years, research on the stability of PSCs has been relatively less explored. In this study, a new kind of CdS hole-blocking layer replacing the traditional compact TiO2 layer is developed to improve the photostability of PSCs because the intrinsic oxygen vacancies of the TiO2 surface are suspected to be the main cause for the photoinduced degradation of PSCs. As a result, PSCs with the CdS layer exhibit considerably improved photostability, maintaining over 90% of the initial efficiency after continuous sunlight illumination for 12 h, while the TiO2 PSC retains only 18% of the initial efficiency under the same conditions. Charge-transfer characteristics related to photodegradation are investigated by various analyses including electrochemical impedance spectroscopy and open-circuit voltage decay and time-resolved photoluminescence decay measurements. the CdS PSC exhibits negligible degradation in the charge-carrier dynamics, while the TiO2 PSC suffers from severely damaged characteristics like increased charge recombination rate, charge-transfer resistance, and reduced charge extraction rate.

Theoretical insights into a potential lead-free hybrid perovskite: substituting Pb(2+) with Ge(2.)

In recent years, perovskite solar cells have been considerably developed, however the lead in the absorber MAPbI3 is a potential threat to the environment. To explore potential alternatives, the structural and electronic properties of MAGeX3 (X = Cl, Br, I) were investigated using different density functional theory methods, including GGA-PBE, PBE-SOC, HSE06 and HSE-SOC. The results implied that MAGeI3 exhibits an analogous band gap, substantial stability, remarkable optical properties, and significant hole and electron conductive behavior compared with the so far widely used absorber MAPbI3. Moreover, the calculations revealed that the energy splitting resulting from the spin-orbit coupling is evident on Pb, moderate on Ge, I and Br, and negligible on Cl. Our work not only sheds some light on screening novel absorbers for perovskite solar cells but also deepens the understanding of these functional materials.

Manipulation of a neutral and nonpolar nanoparticle in water using a nonuniform electric field

The manipulation of nanoparticles in water is of essential importance in chemical physics, nanotechnology, medical technology, and biotechnology applications. Generally, a particle with net charges or charge polarity can be driven by an electric field. However, many practical particles only have weak and even negligible charge and polarity, which hinders the electric field to exert a force large enough to drive these nanoparticles directly. Here, we use molecular dynamics simulations to show that a neutral and nonpolar nanoparticle in liquid water can be driven directionally by an external electric field. The directed motion benefits from a nonuniform water environment produced by a nonuniform external electric field, since lower water energies exist under a higher intensity electric field. The nanoparticle spontaneously moves toward locations with a weaker electric field intensity to minimize the energy of the whole system. Considering that the distance between adjacent regions of nonuniform field intensity can reach the micrometer scale, this finding provides a new mechanism of manipulating nanoparticles from the nanoscale to the microscale.

The origin of enhanced optical absorption of the BiFeO3/ZnO heterojunction in the visible and terahertz regions

Optical absorption is improved for the BiFeO3/ZnO heterostructure prepared by a sol-gel process, especially, in the terahertz energy region. A dipole-corrected slab model is used to describe the bilayer film, and first-principles calculations agree with the experiments which present unambiguous explanation for the enhancement of the optical properties. Two-dimensional electrons in the ZnO side of the heterostructure are found to play an essential role in forming the photoinduced carriers and the enhancement of the absorption. The conducting layers tend to penetrate into the interface and decrease the band gap, leading to the transport of carriers through the interface to the BiFeO3 side. The photoinduced carriers can be separated by the ferroelectric domains in BiFeO3, and this mechanism makes the heterostructure an ideal candidate for BiFeO3-based ferroelectric photovoltaic cells.