Distribution control enables efficient reduced-dimensional perovskite LEDs.

Light-emitting diodes (LEDs) based on perovskite quantum dots have shown external quantum efficiencies (EQEs) of over 23% and narrowband emission, but suffer from limited operating stability1. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations show high exciton binding energies and have the potential to increase the stability and the photoluminescence quantum yield2,3. However, until now, RDP-based LEDs have exhibited lower EQEs and inferior colour purities4-6. We posit that the presence of variably confined QWs may contribute to non-radiative recombination losses and broadened emission. Here we report bright RDPs with a more monodispersed QW thickness distribution, achieved through the use of a bifunctional molecular additive that simultaneously controls the RDP polydispersity while passivating the perovskite QW surfaces. We synthesize a fluorinated triphenylphosphine oxide additive that hydrogen bonds with the organic cations, controlling their diffusion during RDP film deposition and suppressing the formation of low-thickness QWs. The phosphine oxide moiety passivates the perovskite grain boundaries via coordination bonding with unsaturated sites, which suppresses defect formation. This results in compact, smooth and uniform RDP thin films with narrowband emission and high photoluminescence quantum yield. This enables LEDs with an EQE of 25.6% with an average of 22.1 ±1.2% over 40 devices, and an operating half-life of two hours at an initial luminance of 7,200 candela per metre squared, indicating tenfold-enhanced operating stability relative to the best-known perovskite LEDs with an EQE exceeding 20%1,4-6.

Efficient Tin-Based Perovskite Solar Cells Enabled by Precisely Synthesized Single-Isomer Fullerene Bisadducts with Regulated Molecular Packing.

Designing and synthesizing fullerene bisadducts with a higher-lying conduction band minimum is promising to further improve the device performance of tin-based perovskite solar cells (TPSCs). However, the commonly obtained fullerene bisadduct products are isomeric mixtures and require complicated separation. Moreover, the isomeric mixtures are prone to resulting in energy alignment disorders, interfacial charge loss, and limited device performance improvement. Herein, we synthesized single-isomer C60- and C70-based diethylmalonate functionalized bisadducts (C60BB and C70BB) by utilizing the steric-hindrance-assisted strategy and determined all molecular structures involved by single crystal diffraction. Meanwhile, we found that the different solvents used for processing the fullerene bisadducts can effectively regulate the molecular packing in their films. The dense and amorphous fullerene bisadduct films prepared by using anisole exhibited the highest electron mobility. Finally, C60BB- and C70BB-based TPSCs showed impressive efficiencies up to 14.51 and 14.28%, respectively. These devices also exhibited excellent long-term stability. This work highlights the importance of developing strategies to synthesize single-isomer fullerene bisadducts and regulate their molecular packing to improve TPSCs' performance.

Phase Engineering Reinforced Energy Transfer for High-Performance Blue Perovskite Light-Emitting Diodes.

Layered metal-halide perovskites, a category of self-assembled quantum wells, are of paramount importance in emerging photonic sources, such as lasers and light-emitting diodes (LEDs). Despite high trap density in two-dimensional (2D) perovskites, efficient non-radiative energy funneling from wide- to narrow-bandgap components, sustained by the Förster resonance energy transfer (FRET) mechanism, contributes to efficient luminescence by light or electrical injection. Herein, it is demonstrated that bandgap extension of layered perovskites to the blue-emitting regime will cause sluggish and inefficient FRET, stemming from the tiny spectral overlap between different phases. Motivated by the importance of blue LEDs and inefficient energy transfer in materials with phase polydispersity, wide-bandgap quasi-2D perovskites with narrow phase distribution, improved crystallinity, and the pure crystal orientation perpendicular to the charge transport layer are developed. Based on this emitter, high-performance blue perovskite LEDs with improved electroluminescence (EL) external quantum efficiency (EQE) of 7.9% at 478 nm, a narrow full width at half-maximum (FWHM) of 22 nm and a more stable EL spectra are achieved. These results provide an important insight into spectrally stable and efficient blue emitters and EL devices based on perovskites.

Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent.

Metal halide perovskite materials are an emerging class of solution-processable semiconductors with considerable potential for use in optoelectronic devices1-3. For example, light-emitting diodes (LEDs) based on these materials could see application in flat-panel displays and solid-state lighting, owing to their potential to be made at low cost via facile solution processing, and could provide tunable colours and narrow emission line widths at high photoluminescence quantum yields4-8. However, the highest reported external quantum efficiencies of green- and red-light-emitting perovskite LEDs are around 14 per cent7,9 and 12 per cent8, respectively-still well behind the performance of organic LEDs10-12 and inorganic quantum dot LEDs13. Here we describe visible-light-emitting perovskite LEDs that surpass the quantum efficiency milestone of 20 per cent. This achievement stems from a new strategy for managing the compositional distribution in the device-an approach that simultaneously provides high luminescence and balanced charge injection. Specifically, we mixed a presynthesized CsPbBr3 perovskite with a MABr additive (where MA is CH3NH3), the differing solubilities of which yield sequential crystallization into a CsPbBr3/MABr quasi-core/shell structure. The MABr shell passivates the nonradiative defects that would otherwise be present in CsPbBr3 crystals, boosting the photoluminescence quantum efficiency, while the MABr capping layer enables balanced charge injection. The resulting 20.3 per cent external quantum efficiency represents a substantial step towards the practical application of perovskite LEDs in lighting and display.

Efficient Perovskite Light-Emitting Diodes with Chemically Bonded Contact and Regulated Charge Behavior.

Efficient charge injection and radiative recombination are essential to achieving high-performance perovskite light-emitting diodes (Pero-LEDs). However, the perovskite emission layer (EML) and the electron transport layer (ETL) form a poor physically interfacial contact and non-negligible charge injection barrier, limiting the device performance. Herein, we utilize a phosphine oxide, 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), to treat the perovskite/ETL interface and form a chemically bonded contact. Specifically, PO-T2T firmly bonds on the perovskite's surface and grain boundaries through a dative bond, effectively passivating the uncoordinated lead defects. Additionally, PO-T2T has high electron mobility and establishes an electron transport highway to bridge the ETL and EML. As a result, a maximum external quantum efficiency (EQEmax) of 22.06% (average EQEmax of 20.02 ± 1.00%) and maximum luminance (Lmax) of 103286 cd m-2 have been achieved for the champion device. Our results indicate that EML/ETL interface modifications are crucial for the fabrication of highly efficient Pero-LEDs.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis.

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

Cross-Linkable Fullerene Enables Elastic and Conductive Grain Boundaries for Efficient and Wearable Tin-Based Perovskite Solar Cells.

Tin-based perovskite solar cells (TPSCs) have received increasing attention due to their low toxicity, high theoretical efficiency, and potential applications as wearable devices. However, the inherent fast and uncontrollable crystallization process of tin-based perovskites results in high defect density in the film. Meanwhile, when fabricated into flexible devices, the prepared perovskite film exhibits inevitable brittleness and high Young's modulus, seriously weakening the mechanical stability. In this work, we design and synthesize a cross-linkable fullerene, thioctic acid functionalized C60 fulleropyrrolidinium iodide (FTAI), which has multiple interactions with perovskite components and can finely regulate the crystallization quality of perovskite film. The obtained perovskite film shows an increased grain size and a more matched energy level with the electron transport material, effectively improving the carrier extraction efficiency. The FTAI-based rigid device achieves a champion efficiency of 14.91 % with enhanced stability. More importantly, the FTAI located at the perovskite grain boundaries could spontaneously cross-link during the perovskite annealing process, which effectively improves the conductivity and elasticity of grain boundaries, thereby giving the film excellent bending resistance. Finally, the FTAI-based wearable device yields a record efficiency of 12.35 % and displays robust bending durability, retaining about 90 % of the initial efficiency after 10,000 bending times.

Conductive Phosphine Oxide Passivator Enables Efficient Perovskite Light-Emitting Diodes.

Recently, surface passivation has been proved to be an essential approach for obtaining efficient and stable perovskite light-emitting diodes (Pero-LEDs). Phosphine oxides performed well as passivators in many reports. However, the most commonly used phosphine oxides are insulators, which may inhibit carrier transport between the perovskite emitter and charge-transporter layers, limiting the corresponding device performance. Here, 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13), a conductive molecule with two phosphine oxide functional groups, is introduced to modify the perovskite emitting layer. The bifunctional SPPO13 can passivate the nonradiative defects of perovskite and promote electron injection at the interface of perovskite emitter and electron-transporter layers. As a result, the corresponding Pero-LEDs obtain a maximum external quantum efficiency (EQE) of 22.3%. In addition, the Pero-LEDs achieve extremely high brightness with a maximum of around 190 000 cd/m2.

Efficient and Stable Low-Bandgap Perovskite Solar Cells Enabled by a CsPbBr3-Cluster Assisted Bottom-up Crystallization Approach.

Recently, low-bandgap formamidinium lead iodide FAPbI3-based perovskites are of particular interest for high-performance perovskite solar cells (PSCs) due to their broad spectral response and high photocurrent output. However, to inhibit the spontaneous α-to-δ phase transition, 15-17% (molar ratio) of bromide and cesium or methylammonium incorporated into the FAPbI3 are indispensable to achieve efficient PSCs. In return, the high bromide content will increase bandgap and narrow the spectral response region. If simply reducing the bromide content, the corresponding PSCs exhibit inferior operational stability due to α-to-δ phase transition, interface degradation, and halide migration. Herein, we report a CsPbBr3-cluster assisted vertically bottom-up crystallization approach to fabricate low-bromide (1% ∼ 6%), α-phase pure, and MA-free FAPbI3-based PSCs. The clusters, in the size of several nanometers, could act as nuclei to facilitate vertical growth of high quality α-FAPbI3 perovskite crystals. Moreover, these clusters can show further intake by perovskite after thermal annealing, which improves the phase homogeneity of the as-prepared perovskite films. As a result, the corresponding mesoporous PSCs deliver a champion efficiency of 21.78% with photoresponse extended to 830 nm. Moreover, these devices show remarkably improved operational stability, retaining ∼82% of the initial efficiency after 1,000 h of maximum power point tracking under 1 sun condition.

The Difference Se Makes: A Bio-Inspired Dppf-Supported Nickel Selenolate Complex Boosts Dihydrogen Evolution with High Oxygen Tolerance.

Inspired by the metal active sites of [NiFeSe]-hydrogenases, a dppf-supported nickel(II) selenolate complex (dppf=1,1'-bis(diphenylphosphino)ferrocene) shows high catalytic activity for electrochemical proton reduction with a remarkable enzyme-like H2 evolution turnover frequency (TOF) of 7838 s-1 under an Ar atmosphere, which markedly surpasses the activity of a dppf-supported nickel(II) thiolate analogue with a low TOF of 600 s-1 . A combined study of electrochemical experiments and DFT calculations shed light on the catalytic process, suggesting that selenium atom as a bio-inspired proton relay plays a key role in proton exchange and enhancing catalytic activity of H2 production. For the first time, this type of Ni selenolate-containing electrocatalyst displays a high degree of O2 and H2 tolerance. Our results should encourage the development of the design of highly efficient oxygen-tolerant Ni selenolate molecular catalysts.

Boron Doping of Multiwalled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-Based Perovskite Solar Cells.

Compared to the conventional perovskite solar cells (PSCs) containing hole-transport materials (HTM), carbon materials based HTM-free PSCs (C-PSCs) have often suffered from inferior power conversion efficiencies (PCEs) arising at least partially from the inefficient hole extraction at the perovskite-carbon interface. Here, we show that boron (B) doping of multiwalled carbon nanotubes (B-MWNTs) electrodes are superior in enabling enhanced hole extraction and transport by increasing work function, carrier concentration, and conductivity of MWNTs. The C-PSCs prepared using the B-MWNTs as the counter electrodes to extract and transport hole carriers have achieved remarkably higher performances than that with the undoped MWNTs, with the resulting PCE being considerably improved from 10.70% (average of 9.58%) to 14.60% (average of 13.70%). Significantly, these cells show negligible hysteretic behavior. Moreover, by coating a thin layer of insulating aluminum oxide (Al2O3) on the mesoporous TiO2 film as a physical barrier to substantially reduce the charge losses, the PCE has been further pushed to 15.23% (average 14.20%). Finally, the impressive durability and stability of the prepared C-PSCs were also testified under various conditions, including long-term air exposure, heat treatment, and high humidity.

Hybrid halide perovskite solar cell precursors: colloidal chemistry and coordination engineering behind device processing for high efficiency.

The precursor of solution-processed perovskite thin films is one of the most central components for high-efficiency perovskite solar cells. We first present the crucial colloidal chemistry visualization of the perovskite precursor solution based on analytical spectra and reveal that perovskite precursor solutions for solar cells are generally colloidal dispersions in a mother solution, with a colloidal size up to the mesoscale, rather than real solutions. The colloid is made of a soft coordination complex in the form of a lead polyhalide framework between organic and inorganic components and can be structurally tuned by the coordination degree, thereby primarily determining the basic film coverage and morphology of deposited thin films. By utilizing coordination engineering, particularly through employing additional methylammonium halide over the stoichiometric ratio for tuning the coordination degree and mode in the initial colloidal solution, along with a thermal leaching for the selective release of excess methylammonium halides, we achieved full and even coverage, the preferential orientation, and high purity of planar perovskite thin films. We have also identified that excess organic component can reduce the colloidal size of and tune the morphology of the coordination framework in relation to final perovskite grains and partial chlorine substitution can accelerate the crystalline nucleation process of perovskite. This work demonstrates the important fundamental chemistry of perovskite precursors and provides genuine guidelines for accurately controlling the high quality of hybrid perovskite thin films without any impurity, thereby delivering efficient planar perovskite solar cells with a power conversion efficiency as high as 17% without distinct hysteresis owing to the high quality of perovskite thin films.

High-performance graphene-based hole conductor-free perovskite solar cells: Schottky junction enhanced hole extraction and electron blocking.

Multilayered graphene and single-layered graphene are assembled onto perovskite films in the form of Schottky junctions and ohmic contacts, respectively, for the production of a graphene-based hole transporting material-free perovskite solar cell. Multilayered graphene extracts charge selectively and efficiently, delivering a higher efficiency of 11.5% than single-layered graphene (6.7%).

Efficient photoelectrochemical water splitting with ultrathin films of hematite on three-dimensional nanophotonic structures.

Photoelectrochemical (PEC) solar water splitting represents a clean and sustainable approach for hydrogen (H2) production and substantial research are being performed to improve the conversion efficiency. Hematite (α-Fe2O3) is considered as a promising candidate for PEC water splitting due to its chemical stability, appropriate band structure, and abundance. However, PEC performance based on hematite is hindered by the short hole diffusion length that put a constraint on the active layer thickness and its light absorption capability. In this work, we have designed and fabricated novel PEC device structure with ultrathin hematite film deposited on three-dimensional nanophotonic structure. In this fashion, the nanophotonic structures can largely improve the light absorption in the ultrathin active materials. In addition, they also provide large surface area to accommodate the slow surface water oxidation process. As the result, high current density of 3.05 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode (RHE) has been achieved on such nanophotonic structure, which is about three times of that for a planar photoelectrode. More importantly, our systematic analysis with experiments and modeling revealed that the design of high performance PEC devices needs to consider not only total optical absorption, but also the absorption profile in the active material, in addition to electrode surface area and carrier collection.

High-rate, ultralong cycle-life lithium/sulfur batteries enabled by nitrogen-doped graphene.

Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.

Epitaxial growth of ZnO Nanodisks with large exposed polar facets on nanowire arrays for promoting photoelectrochemical water splitting.

Single-crystalline and branched 1D arrays, ZnO nanowires/nanodisks (NWs/NDs) arrays, are fabricated to significantly enhance the performance of photoelectrochemical (PEC) water splitting. The epitaxial growth of the ZnO NDs with large exposed polar facets on ZnO NWs exhibits a laminated structure, which dramatically increases the light scattering capacity of the NWs arrays, especially in the wavelength region around 400 nm. The ND branching of the 1D arrays in the epitaxial fashion not only increase surface area and light utilization, but also support fast charge transport, leading to the considerable increase of photocurrent. Moreover, the tiny size NDs can facilitate charge separation and reduce charge recombination, while the large exposed polar facets of NDs reduce the external potential bias needed for water splitting. These advantages land the ZnO NWs/NDs arrays a four times higher power conversion efficiency than the ZnO NWs arrays. By sensitizing the ZnO NWs/NDs with CdS and CdSe quantum dots, the PEC performance can be further improved. This work advocates a trunk/leaf in forest concept for the single-crystalline NWs/NDs in array with enlarged exposure of polar facets, which opens the way for optimizing light harvesting and charge separation and transport, and thus the PEC water splitting.

Liquid phase deposition of TiO2 nanolayer affords CH3NH3PbI3/nanocarbon solar cells with high open-circuit voltage.

Hybrid organic/inorganic perovskite solar cells are attracting intense attention and further developments largely hinge on understanding the fundamental issues involved in the cell operation. In this paper, a liquid phase deposition (LPD) method is developed to design and grow a TiO(2) nanolayer at room temperature for carbon-based perovskite solar cells. The TiO(2) nanolayer grown on FTO glass is compact but polycrystalline consisting of tiny anatase TiO(2) nanocrystals intimately stacked together. By directly exploiting this TiO(2) nanolayer in a solar cell of TiO(2) nanolayer/CH(3)NH(3)PbI(3)/nanocarbon, we have achieved a Voc as high as 1.07 V, the highest value reported so far for hole transporter-free CH(3)NH(3)PbI(3) solar cells. This is rationalized by the slower electron injection and longer electron lifetime due to the TiO(2) nanolayer, which enhances the electron accumulation in CH(3)NH(3)PbI(3) and consequently the Voc. By employing a rutile TiO(2) nanorod (NR) array as a base structure for the LPD-TiO(2) nanolayer to support the CH(3)NH(3)PbI(3) layer, the photocurrent density is considerably increased without obviously compromising the Voc (1.01 V). As a result, the power conversion efficiency is boosted from 3.67% to 8.61%. More elaborate engineering of the TiO(2) nanolayer by LPD in conjunction with judicious interfacing with other components has the potential to achieve higher performances for this type of solar cell.

Inkjet printing and instant chemical transformation of a CH3NH3PbI3/nanocarbon electrode and interface for planar perovskite solar cells.

A planar perovskite solar cell that incorporates a nanocarbon hole-extraction layer is demonstrated for the first time by an inkjet printing technique with a precisely controlled pattern and interface. By designing the carbon plus CH3NH3I ink to transform PbI2 in situ to CH3NH3PbI3, an interpenetrating seamless interface between the CH3NH3PbI3 active layer and the carbon hole-extraction electrode was instantly constructed, with a markedly reduced charge recombination compared to that with the carbon ink alone. As a result, a considerably higher power conversion efficiency up to 11.60% was delivered by the corresponding solar cell. This method provides a major step towards the fabrication of low-cost, large-scale, metal-electrode-free but still highly efficient perovskite solar cells.

High-performance hole-extraction layer of sol-gel-processed NiO nanocrystals for inverted planar perovskite solar cells.

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH3 NH3 PbI3 in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH3 NH3 PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer.

Surface Treatment with Tailored π-Conjugated Fluorene Derivatives Significantly Enhances the Performance of Perovskite Light-Emitting Diodes.

Surface defect passivation and carrier injection regulation have emerged as effective strategies for enhancing the performance of perovskite light-emitting diodes (Pero-LEDs). It usually requires two functional molecules to realize defect passivation and carrier injection regulation separately. In other words, developing one single molecule possessing these capabilities remains challenging. Herein, we utilized π-conjugated fluorene derivatives as surface treatment materials, 9,9-Spirobi[fluorene] (SBF), 9,9-Spirobifluoren-2-yl-diphenylphosphine oxide (SPPO1), and 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13), to investigate the influence of their chemical structure on device optoelectronic performance, especially for defect passivation and carrier injection regulation. Consequently, the passivation capability of double-bonded SPPO13 surpassed single-bonded SPPO1 and nonbonded SBF, which all showed excellent electron transport properties, enhancing electron injection. The maximum external quantum efficiencies (EQE) for Pero-LEDs treated with SBF, SPPO1, and SPPO13 were 8.13, 17.48, and 22.10%, respectively, exceeding that of the derivative-free device (6.55%). Notably, SPPO13-treated devices exhibited exceptional reproducibility, yielding an average EQE of 20.00 ± 1.10% based on 30 devices. This result emphasizes the potential of tailored fluorene derivatives for enhancing the device performance of Pero-LEDs.