A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence.

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Hybrid FeNiOOH/α-Fe2O3/Graphene Photoelectrodes with Advanced Water Oxidation Performance.

In this study, the photoelectrochemical behavior of electrodeposited FeNiOOH/Fe2O3/graphene nanohybrid electrodes is investigated, which has precisely controlled structure and composition. The photoelectrode assembly is designed in a bioinspired manner where each component has its own function: Fe2O3 is responsible for the absorption of light, the graphene framework for proper charge carrier transport, while the FeNiOOH overlayer for facile water oxidation. The effect of each component on the photoelectrochemical behavior is studied by linear sweep photovoltammetry, incident photon-to-charge carrier conversion efficiency measurements, and long-term photoelectrolysis. 2.6 times higher photocurrents are obtained for the best-performing FeNiOOH/Fe2O3/graphene system compared to its pristine Fe2O3 counterpart. Transient absorption spectroscopy measurements reveal an increased hole-lifetime in the case of the Fe2O3/graphene samples. Long-term photoelectrolysis measurements in combination with Raman spectroscopy, however, prove that the underlying nanocarbon framework is corroded by the photogenerated holes. This issue is tackled by the electrodeposition of a thin FeNiOOH overlayer, which rapidly accepts the photogenerated holes from Fe2O3, thus eliminating the pathway leading to the corrosion of graphene.

Observing the Structural Evolution in the Photodissociation of Diiodomethane with Femtosecond Solution X-Ray Scattering.

Resolving the structural dynamics of the initial steps of chemical reactions is challenging. We report the femtosecond time-resolved wide-angle x-ray scattering of the photodissociation of diiodomethane in cyclohexane. The data reveal with structural detail how the molecule dissociates into radicals, how the radicals collide with the solvent, and how they form the photoisomer. We extract how translational and rotational kinetic energy is dispersed into the solvent. We also find that 85% of the primary radical pairs are confined to their original solvent cage and discuss how this influences the downstream recombination reactions.

14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials with a Large Electrostatic Potential Difference.

Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices still show power conversion efficiencies far behind those of commercialized solar cells. One of the main reasons is the large driving force required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer donor PTO2 and a nonfullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large, and the induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement by judicious modulation of ESP.

Photodetector Based on Spontaneously Grown Strongly Coupled MAPbBr3/N-rGO Hybrids Showing Enhanced Performance.

Recently, metal-halide perovskites have emerged as a candidate for optoelectronic applications such as photodetectors. However, the poor device performance and instability have limited their future commercialization. Herein, we report the spontaneous growth of perovskite/N-rGO hybrid structures using a facile solution method and their applications for photodetectors. In the hybrid structures, perovskites were homogeneously wrapped by N-rGO sheets through strong hydrogen bonding. The strongly coupled N-rGOs facilitate the charge carrier transportation across the perovskite crystals but also distort the surface lattice of the perovskite creating a potential barrier for charge transfer. We optimize the addition of N-rGO in the hybrid structures to balance interfacial structural distortion and the intercrystal conductivity. High-performance photodetection up to 3 × 104 A/W, external quantum efficiency exceeding 105%, and detectivity up to 1012 Jones were achieved in the optimal device with the weight ratio between perovskites and N-rGO to be 8:1.5. The underlying mechanism behind the optimal N-rGO addition ratio in the hybrids has also been rationalized via time-resolved spectroscopic studies as a reference for future applications.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon-neutral solar fuels in a low-cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co-catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2 S2 (CO)6 ] or [Fe3 Te2 (CO)9 ], using an interface-directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 µmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time-resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

Unveiling Excitonic Dynamics in High-Efficiency Nonfullerene Organic Solar Cells to Direct Morphological Optimization for Suppressing Charge Recombination.

Nonfullerene acceptors (NFAs)-based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors-based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady-state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC-based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC71BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC71BM yield a better PCE of 11.4% without any additive or annealing treatment.

Benefiting from Spontaneously Generated 2D/3D Bulk-Heterojunctions in Ruddlesden-Popper Perovskite by Incorporation of S-Bearing Spacer Cation.

2D Ruddlesden-Popper (RP) perovskite solar cells have manifested superior operation durability yet inferior charge transport compared to their 3D counterparts. Integrating 3D phases with 2D RP perovskites presents a compromise to maintain respective advantages of both components. Here, the spontaneous generation of 3D phases embedded in 2D perovskite matrix is demonstrated at room temperature via introducing S-bearing thiophene-2-ethylamine (TEA) as both spacer and stabilizer of inorganic lattices. The resulting 2D/3D bulk heterojunction structures are believed to arise from the compression-induced epitaxial growth of the 3D phase at the grain boundaries of the 2D phase through the Pb-S interaction. The as-prepared 2D TEA perovskites exhibit longer exciton diffusion length and extended charge carrier lifetime than the paradigm 2D phenylethylamine (PEA)-based analogues and hence demonstrate an outstanding power conversion efficiency of 7.20% with significantly increased photocurrent. Dual treatments by NH4Cl and dimethyl sulfoxide are further applied to ameliorate the crystallinity and crystal orientation of 2D perovskites. Consequently, TEA-based devices exhibit a stabilized efficiency over 11% with negligible hysteresis and display excellent ambient stability without encapsulation by preserving 80% efficiency after 270 h storage in air with 60 ± 5% relative humidity at 25 °C.

Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime.

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}-, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the 2LMCT state of [Fe(phtmeimb)2]+ in bimolecular quenching studies with methylviologen and diphenylamine.

Composition Engineering in Two-Dimensional Pb-Sn-Alloyed Perovskites for Efficient and Stable Solar Cells.

Environmentally friendly tin (Sn)-based metallic halide perovskites suffer from oxidation and morphological issues. Here, we demonstrate the composition engineering of Pb-Sn-alloyed two-dimensional (2D) Ruddlesden-Popper perovskites, (BA)2(MA)3Pb4- xSn xI13, for efficient and stable solar cell applications. Smooth thin films with high surface coverage are readily formed without using any additive owing to the self-assembly characteristic of 2D perovskites. It is found that Sn plays a significant role in improving the crystallization and crystal orientation while narrowing the bandgap of Pb-Sn 2D perovskites. Photophysical studies further reveal that the optimal Sn ratio (25 mol %) based sample exhibits both minimized trap density and weakened quantum confinement for efficient charge separation. Consequently, the optimized (BA)2(MA)3Pb3SnI13-based solar cells yield the best power conversion efficiency close to 6% with suppressed hysteresis.

Inorganic Ions Assisted the Anisotropic Growth of CsPbCl3 Nanowires with Surface Passivation Effect.

All-inorganic halide perovskite nanowires (NWs) exhibit improved thermal and hydrolysis stability and could thus play a vital role in nanoscale optoelectronics. Among them, blue-light-based devices are extremely limited because of the lack of a facile method to obtain high-purity CsPbCl3 NWs. Herein, we report a direct and facile method for the synthesis of CsPbCl3 NWs assisted by inorganic ions that served both as a morphology controlling agent for the anisotropic growth of nanomaterials and a surface passivation species modulating the surface of nanomaterials. This new approach allows us to obtain high-purity and size-uniform NWs as long as 500 nm in length and 20 nm in diameter with high reproducibility. X-ray photoelectron spectroscopy and ultrafast spectroscopic measurements confirmed that a reduced band gap caused by the surface species of NWs relative to nanocubes (NCs) was achieved at the photon energy of 160 eV because of the hybrid surface passivation contributed by adsorbed inorganic ions. The resulting NWs demonstrate significantly enhanced photoelectrochemical performances, 3.5-fold increase in the photocurrent generation, and notably improved stability compared to their NC counterparts. Our results suggest that the newly designed NWs could be a promising material for the development of nanoscale optoelectronic devices.

FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime.

The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.

Photostability and Photodegradation Processes in Colloidal CsPbI3 Perovskite Quantum Dots.

All-inorganic CsPbI3 perovskite quantum dots (QDs) have attracted intense attention for their successful application in photovoltaics (PVs) and optoelectronics that are enabled by their superior absorption capability and great photoluminescence (PL) properties. However, their photostability remains a practical bottleneck and further optimization is highly desirable. Here, we studied the photostability of as-obtained colloidal CsPbI3 QDs suspended in hexane. We found that light illumination does induce photodegradation of CsPbI3 QDs. Steady-state spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and transient absorption spectroscopy verified that light illumination leads to detachment of the capping agent, collapse of the CsPbI3 QD surface, and finally aggregation of surface Pb0. Both dangling bonds containing surface and Pb0 serve as trap states causing PL quenching with a dramatic decrease of PL quantum yield. Our work provides a detailed insight about the correlation between the structural and photophysical consequences of the photodegradation process in CsPbI3 QDs and may lead to the optimization of such QDs toward device applications.

Beating Darwin-Bragg losses in lab-based ultrafast x-ray experiments.

The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.