Directing Energy Transfer in Halide Perovskite-Chromophore Hybrid Assemblies

Interplay between photoinduced charge and energy transfer in manganese doped perovskite quantum dots

A comprehensive study on the photo-excited relaxation dynamics in semiconducting perovskite quantum dots (PQDs) is pivotal in realizing their extensive potential for optoelectronics applications. Among different competing photoinduced relaxation kinetics, energy transfer and charge transfer (CT) in PQDs need special attention, as they often influence the device efficacy, particularly with the donor-acceptor hybrid architecture. In this work, we explore a detailed investigation into photoinduced CT dynamics in mixed halide undoped CsPb(Br/Cl)3 and Mn2+ doped CsPb(Br/Cl)3 PQDs with a quinone molecule, p-benzoquinone (BQ). The energy level alignment of undoped PQDs with BQ allows an efficient CT, whereas Mn2+ doping reduces the CT efficiency, experiencing a competition between energy transfer from host to dopant and CT to BQ. The conductive atomic force microscopy measurements unveil a direct correlation with the spectroscopic studies by showing a significant improvement in the conductance of undoped PQDs in the presence of BQ, while an inappreciable change is observed for doped PQDs. A much-reduced transition voltage and barrier height in the presence of BQ further validate faster CT for undoped PQD than the doped one. Furthermore, Mn2+ doping in PQDs is observed to enhance their stability, showing better air and thermal stability compared to their undoped counterparts. These results reveal that doping strategy can regulate the CT dynamics in these PQDs and increase their stability, which will be beneficial for the development of desired optoelectronic devices with long-term stability.

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.

Quantifying Förster Resonance Energy Transfer from Single Perovskite Quantum Dots to Organic Dyes

Colloidal quantum dots (QDs) are promising regenerable photoredox catalysts offering broadly tunable redox potentials along with high absorption coefficients. QDs have thus far been examined for various organic transformations, water splitting, and CO2 reduction. Vast opportunities emerge from coupling QDs with other homogeneous catalysts, such as transition metal complexes or organic dyes, into hybrid nanoassemblies exploiting energy transfer (ET), leveraging a large absorption cross-section of QDs and long-lived triplet states of cocatalysts. However, a thorough understanding and further engineering of the complex operational mechanisms of hybrid nanoassemblies require simultaneously controlling the surface chemistry of the QDs and probing dynamics at sufficient spatiotemporal resolution. Here, we probe the ET from single lead halide perovskite QDs, capped by alkylphospholipid ligands, to organic dye molecules employing single-particle photoluminescence spectroscopy with single-photon resolution. We identify a Förster-type ET by spatial, temporal, and photon-photon correlations in the QD and dye emission. Discrete quenching steps in the acceptor emission reveal stochastic photobleaching events of individual organic dyes, allowing a precise quantification of the transfer efficiency, which is >70% for QD-dye complexes with strong donor-acceptor spectral overlap. Our work explores the processes occurring at the QD/molecule interface and demonstrates the feasibility of sensitizing organic photocatalysts with QDs.

Discriminatory fluorescence and FRET in the chiral-perovskite/RhB system

Förster resonance energy transfer (FRET) holds a significant position in various natural and artificial systems, especially within donor-acceptor systems encompassing chiral components. Despite extensive investigations, a clear understanding of the effects of chirality and FRET on discriminatory fluorescence remains elusive. Here, chiral perovskite nanowires (CPNWs) and achiral rhodamine B (RhB) are employed to examine the FRET and discriminatory fluorescence behavior in a donor-acceptor system involving a chiral nanostructure. A notable FRET from the CPNWs to RhB is observed, along with circular dichroism (CD) and circularly polarized luminescence (CPL) activities in RhB. Although the FRET interaction remains consistent over time, a notable inversion in the polarity preference of the CD and CPL of RhB is observed. This reveals that the discriminatory fluorescence of the acceptor arises from the electromagnetic influence of the chiral donor. These findings elucidate that "chirality", as a property related to spatial orientation, cannot accompany the transfer of energy (which is a scalar) from chiral nanostructures to achiral molecules, which helps advance the understanding of the discriminatory fluorescence in the donor-acceptor system with a chiral nanostructure.

Ultrafast energy transfer dynamics in CsPbBr3 nanoplatelets-BODIPY heterostructure

Understanding and directing the energy transfer in nanocrystals-chromophore heterostructure is critical to improve the efficiency of their photocatalytic and optoelectronic applications. In this work, we studied the energy transfer process between inorganic-organic molecular complexes composed of cesium halide perovskite nanoplatelets (CsPbBr3 NPLs) and boron dipyrromethene (BODIPY) by photoluminescence spectroscopy (PL), time-correlated single photon-counting (TCSPC) and femtosecond transient absorption spectroscopy. The quenching of PL in CsPbBr3 NPLs occurred simultaneously with the PL enhancement of BODIPY implied the singlet energy transfer process. The rate of energy transfer has been determined by transient absorption spectrum as kET = 3.8 × 109 s-1. The efficiency of Förster energy transfer (FRET) has been quantitatively calculated up to 70%. Our work advances the understanding of the interaction between BODIPY and perovskite nanoplatelets, providing a new solution based on their optoelectronic and photocatalytic applications.

Phase Control and Singlet Energy Transfer Enabled by Trimethylamine Modified Boron Dipyrromethene for Stable CsPbBr3 Quantum Wells

The phase distribution and organic spacer cations play pivotal roles in determining the emission performance and stability of perovskite quantum wells (QWs). Here, we propose a universal molecular regulation strategy to tailor phase distribution and enhance the stability of CsPbBr3 QWs. The capability of sterically hindered ligands with formidable surface binding groups is underscored in directing CsPbBr3 growth and refining phase distribution. With trimethylamine modified boron dipyrromethene (BDP-TMA) ligand as a representative, the BDP-TMA driven can precisely control phase distribution and passivate defects of CsPbBr3 . Notably, BDP-TMA acts as a co-spacer organic entity in obtained BDP-TMA-CsPbBr3 , facilitating efficient singlet energy transfer and tailoring the luminescence to produce a distinctive bluish-white emission. The BDP-TMA-CsPbBr3 demonstrates significant phase stability under water exposure, light irradiation, and moderate temperature. Interestingly, BDP-TMA-CsPbBr3 exhibits the thermally-induced dynamic fluorescence control at elevated temperatures, which can be achieved feasible for advanced information encryption. This discovery paves the way for the exploration of perovskite QWs in applications like temperature sensing, anti-counterfeiting, and other advanced optical smart technologies.

Tuning Energy Transfer Pathways in Halide Perovskite-Dye Hybrids through Bandgap Engineering

Lead halide perovskite nanocrystals, which offer rich photochemistry, have the potential to capture photons over a wide range of the visible and infrared spectrum for photocatalytic, optoelectronic, and photon conversion applications. Energy transfer from the perovskite nanocrystal to an acceptor dye in the form of a triplet or singlet state offers additional opportunities to tune the properties of the semiconductor-dye hybrid and extend excited-state lifetimes. We have now successfully established the key factors that dictate triplet energy transfer between excited CsPbI3 and surface-bound rhodamine dyes using absorption and emission spectroscopies. The pendant groups on the acceptor dyes influence surface binding to the nanocrystals, which in turn dictate the energy transfer kinetics, as well as the efficiency of energy transfer. Of the three rhodamine dyes investigated (rhodamine B, rhodamine B isothiocyanate, and rose Bengal), the CsPbI3-rose Bengal hybrid with the strongest binding showed the highest triplet energy transfer efficiency (96%) with a rate constant of 1 × 109 s-1. This triplet energy transfer rate constant is nearly 2 orders of magnitude slower than the singlet energy transfer observed for the pure-bromide CsPbBr3-rose Bengal hybrid (1.1 × 1011 s-1). Intriguingly, although the single-halide CsPbBr3 and CsPbI3 nanocrystals selectively populate singlet and triplet excited states of rose Bengal, respectively, the mixed halide perovskites were able to generate a mixture of both singlet and triplet excited states. By tuning the bromide/iodide ratio and thus bandgap energy in CsPb(Br1-xIx)3 compositions, the percentage of singlets vs triplets delivered to the acceptor dye was systematically tuned from 0 to 100%. The excited-state properties of halide perovskite-molecular hybrids discussed here provide new ways to modulate singlet and triplet energy transfer in semiconductor-molecular dye hybrids through acceptor functionalization and donor bandgap engineering.

Energy transfer enhanced photoluminescence of 2D/3D CsPbBr3 hybrid assemblies

Energy transfer has been proven to be an effective method to optimize optoelectronic conversion efficiency by improving light absorption and mitigating nonradiative losses. We prepared 2D/3D CsPbBr3 hybrid assemblies at different reaction temperatures using the hot injection method and found that the photoluminescence quantum yields (PLQYs) of these hybrids were greatly enhanced from 53.4% to 72.57% compared with 3D nanocrystals (NCs). Femtosecond transient absorption measurements were used to study the PLQY enhancement mechanisms, and it was found that the hot carrier lifetime improved from 0.36 to 1.88 ps for 2D/3D CsPbBr3 hybrid assemblies owing to the energy transfer from 2D nanoplates to 3D NCs. The energy transfer benefits the excited carrier accumulation and prolonged hot carrier lifetime in 3D NCs in hybrid assemblies, as well as PLQY enhancement in materials.

Ru(II)-Ru(II) and Ru(II)-Os(II) Homo-/Heterodinuclear Complexes and Ru3(II)-Ru(II) Homotetranuclear Complexes Based on Heteroditopic Bridging Ligands: Synthesis, Photophysics, and Effective Energy Transfer

In this paper, the synthesis, photophysics, electrochemistry, and intramolecular energy transfer of two series of dinuclear and tetranuclear metallic complexes [(bpy)2M1LxM2(bpy)2]4+ (x = 1, 2; M1 = Ru, M2 = Ru/Os; M1 = Os, M2 = Ru) and {[Ru(bpy)2(Lx)]3Ru}8+ based on new heteroditopic bridging ligands (L1 = 6-phenyl-4-Hpip-2-2'-bipyridine, L2 = 6-Hpip-2-2'-bipyridine, Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline) are reported. The dimetallic and tetrametallic complexes exhibit rich redox properties with successive reversible metal-centered oxidation and ligand-centered reduction couples. All complexes display intense absorption in the entire ultraviolet-visible spectral regions. The mononuclear [LxRu(bpy)2]2+ and homodinuclear [(bpy)2RuLxRu(bpy)2]4+ complexes display strong Ru-based characteristic emission at room temperature. Interestingly, the optical studies of heterodinuclear complexes reveal almost complete quenching of the RuII-based emission and efficient photoinduced energy transfer, resulting in an OsII-based emission in the near-infrared region. As a result of the intramolecular energy transfer from the center to the periphery and steric hindrance quenching of the peripheral RuII-centered emissive triplet metal-to-ligand charge transfer states, the tetranuclear complexes exhibit weak RuII-based emission with a short lifetime. Since the light absorbed by several chromophores is efficiently directed to the subunit with the lowest-energy excited state, the present multinuclear complexes can be used as well-visible-light-absorption antennas.

Energy transfer in metal-exchange binuclear complexes covalently linked by asymmetric ligands

Nitrogen complexation with π-conjugated ligands is an effective strategy for synthesizing luminescent molecules. The asymmetric bridging ligands L (L1 and L2) have been designed. The terminal chelating sites of the L1 and L2 bridging ligands consisted of 2,2'-bipyridine (bpy) and 1,10-phenanthroline moieties (where L = L1 and L2; L1 = 2-(3-((4-([2,2'-bipyridin]-6-yl)benzyl)oxy)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline and L2 = 2-(3-((4-(6-phenyl-[2,2'-bipyridin]-4-yl)benzyl)oxy)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline). The full use of the synthetic strategy of the "complexes as ligands and complexes as metals" was expected to successfully design and synthesize a series of conjugated metal-exchange complexes linked by the asymmetric bridging ligands L1 and L2. These compounds included monometallic complexes Ru(L) and (L)Ru (C1, C2, C7, and C8), homometallic complexes Ru(L)Ru (C3 and C4), and heterometallic complexes Os(L)Ru and Ru(L)Os (C5, C6, C9, and C10) with Ru- or Os-based units. C3-C10 complexes exhibited various degrees of octahedral distortion around the Ru(II) or Os(II) center, which was consistent with the optimized geometry of the coordination complexes based on density functional theory calculation. These complexes exhibited intense spin-allowed ligand-centered transitions with high absorbance at around 288 nm upon absorbing visible light. Notably, all complexes exhibited spin-allowed metal-to-ligand charge transfer absorption of the Ru-based units in the 440-450 nm range. In addition, the heterometallic C5, C6, C9, and C10 complexes showed absorption of the Os-based units in the range of 565-583 nm. The intramolecular energy transfer of C3 and C5 was briefly discussed by comparing the emission intensity of monometallic C1 and C2 to that of binuclear complexes C3 and C5, respectively. The results indicated that the intramolecular energy transfer of the Ru(II)/Os(II) polypyridine complexes proceeded from the Ru(II)- to the Os(II)-based units in the heterometallic C5 and C6 complexes.

Modulation of carrier conduction in CsPbBr3 perovskite quantum dots with band-aligned electron and hole acceptors

The lead halide perovskites have emerged as promising materials with intriguing photo-physical properties and have immense potential for photovoltaic applications. A comprehensive study on the kinetics of charge carrier (electron/hole) generation and transfer across the interface is key to realizing their future scope for efficient device engineering. Herein, we investigate the interfacial charge transfer (CT) dynamics in cesium lead halide (CsPbBr3) perovskite quantum dots (PQDs) with energetically favorable electron acceptors, anthraquinone (AQ) and p-benzoquinone (BQ), and hole acceptors such as pyrene and 4-(dimethylamino)pyridine (DMAP). With various steady-state and time-resolved spectroscopic and microscopic measurements, a faster electron transfer rate is estimated for CsPbBr3 PQDs with BQ compared to that of AQ, while a superior hole transfer for DMAP is divulged compared to pyrene. In concurrence with the spectroscopic measurements, conducting atomic force microscopic studies across the electrode-PQD-electrode junction reveals an increment in the conductance of the PQD in the presence of both the electron and hole acceptors. The variation of the density of states calculation in the presence of the hole acceptors offers strong support and validation for faster CT efficiency. The above findings suggest that a careful selection of simple yet efficient molecular arrangements can facilitate rapid carrier transfer, which can be designed as auxiliary layers for smooth CT and help in the engineering of cost-effective photovoltaic devices.

Colloidal Quantum Dots as an Emerging Vast Platform and Versatile Sensitizer for Singlet Molecular Oxygen Generation

Singlet molecular oxygen (1O2) has been reported in wide arrays of applications ranging from optoelectronic to photooxygenation reactions and therapy in biomedical proposals. It is also considered a major determinant of photodynamic therapy (PDT) efficacy. Since the direct excitation from the triplet ground state (3O2) of oxygen to the singlet excited state 1O2 is spin forbidden; therefore, a rational design and development of heterogeneous sensitizers is remarkably important for the efficient production of 1O2. For this purpose, quantum dots (QDs) have emerged as versatile candidates either by acting individually as sensitizers for 1O2 generation or by working in conjunction with other inorganic materials or organic sensitizers by providing them a vast platform. Thus, conjoining the photophysical properties of QDs with other materials, e.g., coupling/combining with other inorganic materials, doping with the transition metal ions or lanthanide ions, and conjugation with a molecular sensitizer provide the opportunity to achieve high-efficiency quantum yields of 1O2 which is not possible with either component separately. Hence, the current review has been focused on the recent advances made in the semiconductor QDs, perovskite QDs, and transition metal dichalcogenide QD-sensitized 1O2 generation in the context of ongoing and previously published research work (over the past eight years, from 2015 to 2023).

Quantum shells versus quantum dots: suppressing Auger recombination in colloidal semiconductors

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of attention in recent decades. The quantum efficiency of many optoelectronic processes based on these nanomaterials, however, declines with increasing optical or electrical excitation intensity. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, whereby reducing the efficiency and lifespan of NC-based devices, including lasers, photodetectors, X-ray scintillators, and high-brightness LEDs. Recently, semiconductor quantum shells (QSs) have emerged as a viable nanoscale architecture for the suppression of Auger decay. The spherical-shell geometry of these nanostructures leads to a significant reduction of Auger decay rates, while exhibiting a near unity photoluminescence quantum yield. Here, we compare the optoelectronic properties of quantum shells against other low-dimensional semiconductors and discuss their emerging opportunities in solid-state lighting and energy-harvesting applications.

Unraveling the Relevance of Electron and Hole Transfer in Lead Halide Perovskite Nanocrystals on Current Conduction

Optimization of perovskite-based optoelectronic performance demands prudent engineering in the device architecture with facile transport of generated charge carriers. Herein, we explore the charge transfer (CT) kinetics in perovskite nanocrystals (PNCs), CsPbBr3, with two redox-active quinones, menadione (MD) and anthraquinone (AQ), and its alteration in halide exchanged CsPbCl3. With a series of spectroscopic and microscopic measurements, we infer that both electron and hole transfer (ET-HT) prevail in CsPbCl3 with quinones, resulting in a faster CT, while ET predominates for CsPbBr3. Furthermore, current-sensing atomic force microscopy measurements demonstrate that the conductance across a metal-PNC-metal nanojunction is improved in the presence of quinones. The contributions of ET and HT to current conduction across PNCs are well supported and validated by theoretical calculations of the density of states. These outcomes convey a new perspective on the relevance of ET and HT in the optimal current conduction and optoelectronic device engineering of perovskites.

Tuning the Driving Force for Charge Transfer in Perovskite-Chromophore Systems

Understanding the interplay between the kinetics and energetics of photophysical processes in perovskite-chromophore hybrid systems is crucial for realizing their potential in optoelectronics, photocatalysis, and light-harvesting applications. By combining steady-state optical characterizations and transient absorption spectroscopy, we have investigated the mechanism of interfacial charge transfer (CT) between colloidal CsPbBr3 nanoplatelets (NPLs) and surface-anchored perylene derivatives and have explored the possibility of controlling the CT rate by tuning the driving force. The CT driving force was tuned systematically by attaching acceptors with different electron affinities and by varying the bandgap of NPLs via thickness-controlled quantum confinement. Our data show that the charge-separated state is formed by selectively exciting either the electron donors or acceptors in the same system. Upon exciting attached acceptors, hole transfer from perylene derivatives to CsPbBr3 NPLs takes place on a picosecond time scale, showing an energetic behavior in line with the Marcus normal regime. Interestingly, such energetic behavior is absent upon exciting the electron donor, suggesting that the dominant CT mechanism is energy transfer followed by ultrafast hole transfer. Our findings not only elucidate the photophysics of perovskite-molecule systems but also provide guidelines for tailoring such hybrid systems for specific applications.

Colloidal-ALD-Grown Metal Oxide Shells Enable the Synthesis of Photoactive Ligand/Nanocrystal Composite Materials

Colloidal nanocrystals (NCs) are ideal materials for a variety of applications and devices, which span from catalysis and optoelectronics to biological imaging. Organic chromophores are often combined with NCs as photoactive ligands to expand the functionality of NCs or to achieve optimal device performance. The most common methodology to introduce these chromophores involves ligand exchange procedures. Despite their ubiquitous nature, ligand exchanges suffer from a few limitations, which include reversible binding, restricted access to binding sites, and the need for purification of the samples, which can result in loss of colloidal stability. Herein, we propose a methodology to bypass these inherent issues of ligand exchange through the growth of an amorphous alumina shell by colloidal atomic layer deposition (c-ALD). We demonstrate that c-ALD creates colloidally stable composite materials, which comprise NCs and organic chromophores as photoactive ligands, by trapping the chromophores around the NC core. As representative examples, we functionalize semiconductor NCs, which include PbS, CsPbBr₃, CuInS₂, Cu_2-xX, and lanthanide-based upconverting NCs, with polyaromatic hydrocarbons (PAH) ligands. Finally, we prove that triplet energy transfer occurs through the shell and we realize the assembly of a triplet exciton funnel structure, which cannot be obtained via conventional ligand exchange procedures. The formation of these organic/inorganic hybrid shells promises to synergistically boost catalytic and multiexcitonic processes while endowing enhanced stability to the NC core.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite-Rhodamine Light Harvesting Assemblies

Energy and electron transfer processes allow for efficient manipulation of excited states within light harvesting assemblies for photocatalytic and optoelectronic applications. We have now successfully probed the influence of acceptor pendant group functionalization on the energy and electron transfer between CsPbBr₃ perovskite nanocrystals and three rhodamine-based acceptor molecules. The three acceptors─rhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)─contain an increasing degree of pendant group functionalization that affects their native excited state properties. When interacting with CsPbBr₃ as an energy donor, photoluminescence excitation spectroscopy reveals that singlet energy transfer occurs with all three acceptors. However, the acceptor functionalization directly influences several key parameters that dictate the excited state interactions. For example, RoseB binds to the nanocrystal surface with an apparent association constant (K_app = 9.4 × 10⁶ M^-1) 200 times greater than RhB (K_app = 0.05 × 10⁶ M^-1), thus influencing the rate of energy transfer. Femtosecond transient absorption reveals the observed rate constant of singlet energy transfer (k_EnT) is an order-of-magnitude greater for RoseB (k_EnT = 1 × 10¹¹ s^-1) than for RhB and RhB-NCS. In addition to energy transfer, each acceptor had a subpopulation of molecules (∼30%) that underwent electron transfer as a competing pathway. Thus, the structural influence of acceptor moieties must be considered for both excited state energy and electron transfer in nanocrystal-molecular hybrids. The competition between electron and energy transfer further highlights the complexity of excited state interactions in nanocrystal-molecular complexes and the need for careful spectroscopic analysis to elucidate competitive pathways.

Charge Transport Dynamics of Quasi-Type II Perovskite Janus Nanocrystals in High-Performance Photoconductors

CsPbBr₃-Pb₄S₃Br₂ Janus nanocrystals (NCs) are the only nanomaterial where the epitaxial structure of perovskite and chalcogenide materials has been realized at the nanoscale, but their exciton dynamics mechanism has not yet been thoroughly investigated or applied in photodetection applications. This work reports an attractive device performance of perovskite photoconductors based on epitaxial CsPbBr₃-Pb₄S₃Br₂ Janus NCs, as well as the carrier relaxation and transfer mechanism of the heterojunction. By a combination of transient optical absorption and quantum dynamics simulation, it is demonstrated that the photogenerated holes on CsPbBr₃ can be successfully extracted by Pb₄S₃Br₂, while the hole transfer proceeds about three times faster than energy loss and remains "hot" for about 300 fs. This feature has favorable effects on long-range charge separation and transport; therefore, the Janus NCs photoconductors exhibit an exceptional responsivity of 34.0 A W^-1 and specific detectivity of 1.26 × 10¹⁴ Jones.

Effect of electronic structure of energy transfer in bimetallic Ru(II)/Os(II) complexes

Novel monometallic (μ-LL')Ru, Ru(μ-LL'), homobimetallic Ru(μ-LL')Ru, and heterodimetallic Ru(μ-LL')Os and Os(μ-LL')Ru complexes based on two asymmetrical ligands LL' (where LL' = L¹L^1', L²L^2') have been synthesized and characterized. Spectroscopic analysis indicates that all complexes exhibit intense spin-allowed ligand-centered (LC) transitions at 288 nm and Ru-based moderate spin-allowed MLCT absorption between 440-450 nm. The Ru(μ-LL')Os and Os(μ-LL')Ru dinuclear complexes show Os-based unit absorption in the range of 565-583 nm. The Ru-based units of the complexes present different emission intensities due to differing steric hindrance at the coordination sites of the two bridging ligands. The Os(μ-LL')Ru dinuclear complexes present weaker emission intensity than their parent monometallic complexes (μ-LL')Ru. These results indicate that the emission of Os(μ-LL')Ru dinuclear complexes is quenched by the Os(II)-based units.

Excited State and Transient Chemistry of a Perylene Derivative (DBP). An Untold Story

Transient chemistry of sensitizing dyes is important to obtain insights into the photochemical conversion processes of light harvesting assemblies. We have now employed transient absorption spectroscopy (pulsed laser and pulse radiolysis) to characterize the excited state and radical intermediates of a perylene derivative, (5,10,15,20-Tetraphenylbisbenz[5,6]indeno[1,2,3-cd:1',2',3'-lm]perylene (DBP). The distinguishable transient absorption features for the singlet and triplet excited states and radical anion and radical cation provide spectral fingerprints to identify the reaction intermediates in photochemical energy and electron transfer processes of composite systems involving DBP. For example, identifying these transients in the energy transfer processes of the rubrene-DBP system would aid in establishing their role as annihilator-emitter for triplet-triplet annihilation up-conversion (TTA-UC). The transient characterization thus serves as an important mechanistic fingerprint for elucidating mechanistic details of systems employing DBP in optoelectronic applications.

Layer-by-layer assembly of CsPbX3 nanocrystals into large-scale homostructures

Advances in surface chemistry of CsPbX₃ (where X = Cl, Br or I) nanocrystals (NCs) enabled the replacement of native chain ligands in solution. However, there are few reports on ligand exchange carried out on CsPbX₃ NC thin films. Solid-state ligand exchange can improve the photoluminescence quantum yield (PLQY) of the film and promote a change in solubility of the solid surface, thus enabling multiple depositions of subsequent nanocrystal layers. Fine control of nanocrystal film thickness is of importance for light-emitting diodes (LEDs), solar cells and lasers alike. The thickness of the emissive material film is crucial to assure the copious recombination of charges injected into a LED, resulting in bright electroluminescence. Similarly, solar cell performance is determined by the amount of absorbed light, and hence the light absorber content in the device. In this study, we demonstrate a layer-by-layer (LbL) assembly method that results in high quality films, whose thicknesses can be finely controlled. In the solid state, we replaced oleic acid and oleylamine ligands with didodecyldimethylammonium bromide or ammonium thiocyanate that enhance the PLQY of the film. The exchange is carried out through a spin-coating technique, using solvents with strategic polarity to avoid NC dissolution or damage. Exploiting this technique, the deposition of various layers results in considerable thickening of films as proven by atomic force microscope measurements. The ease of handling of our combined process (i.e. ligand exchange and layer-by-layer deposition) enables thickness control over CsPbX₃ NC films with applicability to other perovskite nanomaterials paving the way for a large variety of layer permutations.

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr₃ with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.

Triplet Energy Transfer from Lead Halide Perovskite for Highly Selective Photocatalytic 2 + 2 Cycloaddition

Triplet excitons are generally confined within a semiconductor. Hence, solar energy utilization via direct triplet energy transfer (TET) from semiconductors is challenging. TET from lead halide perovskite semiconductors to nearby organic molecules has been illustrated with ultrafast spectroscopy. Direct utilization of solar energy, i.e., visible light, via TET for photocatalysis is an important route but has not yet been demonstrated with lead halide perovskite semiconductors. Here, we show that a photocatalytic reaction, focusing on a 2 + 2 cycloaddition reaction, can been successfully demonstrated via TET from lead halide perovskite nanocrystals (PNCs). The triplet excitons are shown to induce a highly diastereomeric syn-selective 2 + 2 cycloaddition starting from olefins. Such photocatalytic reactions probe the TET process previously only observed spectroscopically. Moreover, our observation demonstrates that bulk-like PNCs (size, >10 nm; PL = 530 nm), in addition to quantum-confined smaller PNCs, are also effective for TET. Our findings may render a new energy conversion pathway to employ PNCs via direct TET for photocatalytic organic synthesis.

Förster Resonance Energy Transfer Assisted Enhancement in Optoelectronic Properties of Metal Halide Perovskite Nanocrystals

Regulated excited state energy and charge transfer play a pivotal role in nanoscale semiconductor device performance for efficient energy harvesting and optoelectronic applications. Herein, we report the influence of Förster resonance energy transfer (FRET) on the excited-state dynamics and charge transport properties of metal halide perovskite nanocrystals (PNCs), CsPbBr₃, and its anion-exchanged counterpart CsPbCl₃ with CdSe/ZnS quantum dots (QDs). We report a drop in the FRET efficiency from ∼85% (CsPbBr₃) to ∼5% (CsPbCl₃) with QDs, inviting significant alteration in their charge transport properties. Using two-probe measurements we report substantial enhancement in the current for the blend structure of PNCs with QDs, originating from the reduced trap sites, compared to that of the pristine PNCs. The FRET-based upshot in the conduction mechanism with features of negative differential resistance and negligible hysteresis for CsPbBr₃ PNCs can add new directions to high performance-based photovoltaics and optoelectronics.

Scratching the Surface: Passivating Perovskite Nanocrystals for Future Device Integration

Metal halide perovskite materials have recently upended the field of photovoltaics and are aiming to make waves across a multitude of other fields and applications. Recently, perovskite nanocrystals have been synthesized and are rapidly outpacing traditional semiconductor nanocrystals in application driven fields due to their inherent defect tolerance and facile tunability, resulting in high photoluminescent quantum yields and efficient devices. Future improvements to perovskite nanocrystals toward device driven applications must come at the perovskite surface. The last half decade has resulted in considerable progress in tailoring the perovskite nanocrystal/ligand surface toward maximizing the optoelectronic performance. Here, we review the current progress and discuss how further improvements could be made to further improve this bright class of materials.