Trap-Limited Dynamics of Excited Carriers and Interpretation of the Photoluminescence Decay Kinetics in Metal Halide Perovskites

Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Weak Dispersion of Exciton Landé Factor with Band Gap Energy in Lead Halide Perovskites: Approximate Compensation of the Electron and Hole Dependences

The optical properties of lead halide perovskite semiconductors in vicinity of the bandgap are controlled by excitons, so that investigation of their fundamental properties is of critical importance. The exciton Landé or g-factor gX is the key parameter, determining the exciton Zeeman spin splitting in magnetic fields. The exciton, electron, and hole carrier g-factors provide information on the band structure, including its anisotropy, and the parameters contributing to the electron and hole effective masses. Here, gX is measured by reflectivity in magnetic fields up to 60 T for lead halide perovskite crystals. The materials band gap energies at a liquid helium temperature vary widely across the visible spectral range from 1.520 up to 3.213 eV in hybrid organic-inorganic and fully inorganic perovskites with different cations and halogens: FA0.9Cs0.1PbI2.8Br0.2, MAPbI3, FAPbBr3, CsPbBr3, and MAPb(Br0.05Cl0.95)3. The exciton g-factors are found to be nearly constant, ranging from +2.3 to +2.7. Thus, the strong dependences of the electron and hole g-factors on the bandgap roughly compensate each other when combining to the exciton g-factor. The same is true for the anisotropies of the carrier g-factors, resulting in a nearly isotropic exciton g-factor. The experimental data are compared favorably with model calculation results.

Enhanced Spontaneous Emission of CsPbI3 Perovskite Nanocrystals Using a Hyperbolic Metamaterial Modified by Dielectric Nanoantenna

In this work, we demonstrate, theoretically and experimentally, a hybrid dielectric-plasmonic multifunctional structure able to provide full control of the emission properties of CsPbI₃ perovskite nanocrystals (PNCs). The device consists of a hyperbolic metamaterial (HMM) composed of alternating thin metal (Ag) and dielectric (LiF) layers, covered by TiO₂ spherical MIE nanoresonators (i.e., the nanoantenna). An optimum HMM leads to a certain Purcell effect, i.e., an increase in the exciton radiative rate, but the emission intensity is reduced due to the presence of metal in the HMM. The incorporation of TiO₂ nanoresonators deposited on the top of the HMM is able to counteract such an undesirable intensity reduction by the coupling between the exciton and the MIE modes of the dielectric nanoantenna. More importantly, MIE nanoresonators result in a preferential light emission towards the normal direction to the HMM plane, increasing the collected signal by more than one order of magnitude together with a further increase in the Purcell factor. These results will be useful in quantum information applications involving single emitters based on PNCs together with a high exciton emission rate and intensity.

Spray-driven halide exchange in solid-state CsPbX3 nanocrystal films

CsPbI₃ perovskite nanocrystals (NCs) are promising building blocks for photovoltaics and optoelectronics. However, they exhibit an essential drawback in the form of phase stability: α-phase, with a ∼1.80 eV bandgap, can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment. This leads to the need to carry out the CsPbI₃-based device fabrication in an inert atmosphere, which is technologically inconvenient and expensive. One of the most successful approaches proposed to overcome this problem is synthesizing mixed halide CsPbBr_3-xI_x NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBr_3-xI_x NCs with high PLQY is challenging owing to the degradation of their optical properties after deposition on a substrate. This work presents spray coating to carry out a solid-state anion exchange in CsPbBr₃ NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBr_3-xI_x NC thin films with high optical quality and record high photoluminescence quantum yields (PLQY) based on spray-driven halide (Br^- to I^-) anion exchange in a solid-state phase. Besides, tunable emission wavelengths between 520 and 670 nm can be obtained from CsPbBr_3-xI_x NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr₃ film to provide the halide exchange. The optical quality of the halide-exchanged PNCs films remains practically identical to that of initial Br-containing layers, with a remarkable PLQY enhancement after anion exchange, from ∼61% for CsPbBr₃ thin films emitting at 520 nm to ∼84% for mixed halide CsPbBr_3-xI_x film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.

Impact of Cesium Concentration on Optoelectronic Properties of Metal Halide Perovskites

Performance of a perovskite solar cell is largely influenced by the optoelectronic properties of metal halide perovskite films. Here we study the influence of cesium concentration on morphology, crystal structure, photoluminescence and optical properties of the triple cation perovskite film. Incorporation of small amount (x = 0.1) of cesium cations into Csx(MA0.17FA0.83)1−x Pb(I0.83Br0.17)3 leads to enhanced power conversion efficiency (PCE) of the solar cell resulting mainly from significant rise of the short-current density and the fill factor value. Further increase of Cs concentration (x > 0.1) decreases the film’s phase purity, carrier lifetime and correspondingly reduces PCE of the solar cell. Higher concentration of Cs (x ≥ 0.2) causes phase segregation of the perovskite alongside with formation of Cs-rich regions impeding light absorption.

Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr₃) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.

Interpreting time-resolved photoluminescence of perovskite materials

Time-resolved photoluminescence (TRPL) spectroscopy is a powerful technique to investigate excited charge carrier recombinations in semiconductors and molecular systems. The analysis of the TRPL decays of many molecular systems (e.g. molecules and organic materials) is usually fairly straightfoward and can be fitted with an exponential function allowing extraction of the rate constants. Due to the non-excitonic nature of charge carriers in lead halide perovskite materials coupled with the presence of localised trap states in their band-gap, the TRPL of these materials is much more complicated to interpret. Here we discuss two models used in the literature to simulate charge carrier recombinations and TRPL in perovskites. These models consider the bimolecular nature of direct electron-hole recombination but differ in their treatment of trap-mediated recombination with one model describing trapping as a monomolecular process whereas the other as a bimolecular process between free carriers and the available trap states. In comparison, the classical analysis of perovskite TRPL decay curves (using a sum of exponentials) can lead to misinterpretation. Here we offer some recommendations for meaningful measurements of lead halide perovskite thin-films. The fluence dependence as well as charge carrier accumulation due to incomplete depopulation of all photoexcited carriers between consecutive excitation pulses are discussed for both models.

Purcell Enhancement and Wavelength Shift of Emitted Light by CsPbI3 Perovskite Nanocrystals Coupled to Hyperbolic Metamaterials

Manipulation of the exciton emission rate in nanocrystals of lead halide perovskites (LHPs) was demonstrated by means of coupling of excitons with a hyperbolic metamaterial (HMM) consisting of alternating thin metal (Ag) and dielectric (LiF) layers. Such a coupling is found to induce an increase of the exciton radiative recombination rate by more than a factor of three due to the Purcell effect when the distance between the quantum emitter and HMM is nominally as small as 10 nm, which coincides well with the results of our theoretical analysis. Besides, an effect of the coupling-induced long wavelength shift of the exciton emission spectrum is detected and modeled. These results can be of interest for quantum information applications of single emitters on the basis of perovskite nanocrystals with high photon emission rates.

Hot carriers perspective on the nature of traps in perovskites

Amongst the many spectacular properties of hybrid lead halide perovskites, their defect tolerance is regarded as the key enabler for a spectrum of high-performance optoelectronic devices that propel perovskites to prominence. However, the plateauing efficiency enhancement of perovskite devices calls into question the extent of this defect tolerance in perovskite systems; an opportunity for perovskite nanocrystals to fill. Through optical spectroscopy and phenomenological modeling based on the Marcus theory of charge transfer, we uncover the detrimental effect of hot carriers trapping in methylammonium lead iodide and bromide nanocrystals. Higher excess energies induce faster carrier trapping rates, ascribed to interactions with shallow traps and ligands, turning these into potent defects. Passivating these traps with the introduction of phosphine oxide ligands can help mitigate hot carrier trapping. Importantly, our findings extend beyond photovoltaics and are relevant for low threshold lasers, light-emitting devices and multi-exciton generation devices.

The benign nature of defects in lead halide perovskites is widely regarded as the basis for their outstanding optoelectronic properties. Here Righetto et al. overthrew this perception, revealing the defects’ surprising potency to hot carriers and devised a strategy to suppress them.

Highly transparent and luminescent gel glass based on reabsorption-free gold nanoclusters

Luminescent and transparent composites formed by embedding luminophores in a solid matrix are essential components for several photonic applications, such as luminescent solar concentrators (LSCs) and luminescent down-shifting/conversion layers. For these applications, the optical losses, including reabsorption and scattering need to be minimized, while the photoluminescence (PL) emission must be stable against outdoor environments. Here, highly transparent and luminescent aluminosilicate glass doped with surface-engineered gold nanoclusters (AuNCs) was prepared without involving toxic elements and hazardous solvents. Such an AuNC@glass composite with a high loading (∼14 wt%) exhibits a unique absorption profile; near-unity absorptance in the absorption range but near-zero reabsorption in the emission region, and thus generates bright PL emission with negligible reabsorption losses. Meanwhile, the PL quantum yield was enhanced (from ∼1% to ∼14%) without sacrificing the Stokes shift, while still maintaining high optical transparency. In addition, they have high stability due to the effective protection of rigid inorganic matrices, and thus would be eco-friendly candidates for further preparation of efficient and reabsorption-free LSCs.

Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites

Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films. At low temperatures, continuous thin films of the perovskite can exhibit strain inhomogeneities associated with the formation of different 'T'-defective domains leading to an energy spread of states over more than 200 meV. On the other hand, a single photoluminescence line peak related to the perovskite 'O'-phase (associated with the distortion of the [PbI3]- anion) is observed in the island-like sample that we attribute to strain relaxation for this morphology. Moreover, the predominantly radiative recombination dynamics of the continuous-like sample mainly originates from nongeminate electron-hole formation of excitons in the 'O'-phase and the internal dynamics with carrier trapping levels. This observation is in strong contrast to the free exciton recombination dominantly found in the island-like sample.

Improved Performance of Planar Perovskite Solar Cells Using an Amino-Terminated Multifunctional Fullerene Derivative as the Passivation Layer

Organic-inorganic metal-halide perovskite solar cells (PSCs) have been revolutionizing the photovoltaic (PV) community in the past decade. However, the trap states in TiO₂ as the electron-transport layer seriously affect the device PV performance and stability. Here, we design and synthesize a fullerene derivative C60NH₂ featuring an amino-terminated group. We use C60NH₂ as a passivation layer between TiO₂ and perovskite in planar PSCs with a standard configuration to improve the quality of the obtained perovskite film as well as the electron-transfer efficiency, resulting in an obvious increment of PV performance and stability of the devices. The champion power conversion efficiency of 18.34% is achieved under 100 mW cm^-2 illumination utilizing C60NH₂ as the passivation layer with much less hysteresis. Planar PSCs demonstrate superior stability under natural sunlight and 40-50% relative humidity after C60NH₂ passivation. This work enriches the choices of materials for interface engineering toward fabrication of planar PSCs with high performance.

Reduced-Dimensional Perovskite Enabled by Organic Diamine for Efficient Photovoltaics

Reduced-dimensional (RD) perovskite solar cells (PSCs) are emerging as highly attractive alternatives to three-dimensional (3D) PSCs due to their dramatically improved environmental stability and photostability. Diamine-based RD perovskites with a single organic amine interlayer possess orderly inorganic sheets and a smaller insulation area, indicating great potential in combining high efficiency and long-term stability. Here, we report an efficient and stable RD PSC based on 1,4-butanediamine (BDA). We found that the BDA-based RD perovskite exhibits improved crystallinity, reduced trap-state densities, and enhanced charge mobility compared to those of butylamine (BA)-based RD (BA-RD) perovskite. A high power conversion efficiency of 17.91% was achieved with negligible hysteresis. Moreover, the device showed improved stability compared to those of BA-RD and 3D films and devices. The findings may inspire new developments in introducing organic diamine for efficient and stable RD PSCs.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.

Influence of shape on the carrier relaxation dynamics of CsPbBr3 perovskite nanocrystals

Shape dependent carrier relaxation dynamics of lead halide perovskite nanocrystal (NCs) is an important issue for efficient light harvesting system.