Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes.

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne ) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

On the Ion Coordination and Crystallization of Metal Halide Perovskites by In Situ Dynamic Optical Probing.

Controlling the crystallization to achieve high-quality homogeneous perovskite film is the key strategy in developing perovskite electronic devices. Here, an in situ dynamic optical probing technique is demonstrated that can monitor the fast crystallization of perovskites and effectively minimize the influence of laser excitation during the measurement. This study finds that the typical static probing technique would damage and induce phase segregation in the perovskite films during the excitation. These issues can be effectively resolved with the dynamic probing approach. It also found that the crystallization between MAPbI3 and MAPbI2 Br is strikingly different. In particular, MAPbI2 Br suffers from inefficient nucleation during the spin-coating that strongly affects the uniform crystal growth in the annealing process. The commonly used pre-heating process is found at a lower temperature not only can further promote the nucleation but also to complete the crystallization of MAPbI2 Br. The role of further annealing at a higher temperature is to facilitate ion-dissociation on the crystal surface to form a passivation layer to stabilize the MAPbI2 Br lattices. The device performance is strongly correlated with the film formation mechanism derived from the in situ results. This work demonstrates that the in situ technique can provide deep insight into the crystallization mechanism, and help to understand the growth mechanism of perovskites with different compositions and dimensionalities.

Light Management of Metal Halide Scintillators for High-Resolution X-Ray Imaging.

The ever-growing need to inspect matter with hyperfine structures requires a revolution in current scintillation detectors, and the innovation of scintillators is revived with luminescent metal halides entering the scene. Notably, for any scintillator, two fundamental issues arise: Which kind of material is suitable and in what form should the material exist? The answer to the former question involves the sequence of certain atoms into specific crystal structures that facilitate the conversion of X-ray into light, whereas the answer to the latter involves assembling these crystallites into particular material forms that can guide light propagation toward its corresponding pixel detector. Despite their equal importance, efforts are overwhelmingly devoted to improving the X-ray-to-light conversion, while the material-form-associated light propagation, which determines the optical signal collected for X-ray imaging, is largely overlooked. This perspective critically correlates the reported spatial resolution with the light-propagation behavior in each form of metal halides, combing the designing rules for their future development.

Influence of Component Properties on the Photovoltaic Performance of Monolithic Perovskite/Organic Tandem Solar Cells: Sub-Cell, Interconnecting Layer, and Photovoltaic Parameters.

Wide-bandgap perovskite sub-cells (WPSCs)-based tandem solar cells attract considerable interest because of their capability to surpass the Shockley-Queisser limit. Monolithic perovskite/organic tandem solar cells (POTSCs) integrating WPSCs and small-bandgap organic sub-cells (SOSCs) are famous compositions owing to their simple fabrication method and compatibility with flexible devices. Most studies on POTSCs focus on enhancing device efficiency by modifying one or two of the device components (WPSCs, SOSCs, and interconnecting layers). The characteristics of POTSCs are not extensively investigated so far, especially in terms of the influence of the device structure and component properties on the tandem device photovoltaic performance. In this study, the existing p-i-n type WPSC-based p-i-n POTSCs and n-i-p type WPSC-based n-i-p POTSCs are reviewed and their advantages and limitations are highlighted. Furthermore, the influence of the tandem device component properties (optical, electrical, and photovoltaic properties) on the photovoltaic parameters (open-circuit voltage, short-circuit current density, fill factor, and power conversion efficiency) and the existing device modification methods are discussed to provide comprehensive guidance for the development of POTSCs.

Multidentate Molecule Anchoring Halide Perovskite Surface and Regulating Crystallization Kinetics toward Efficient Light-Emitting Diodes.

Functional passivators are conventionally utilized in modifying the crystallization properties of perovskites to minimize the non-radiative recombination losses in perovskite light-emitting diodes (PeLEDs). However, the weak anchor ability of some commonly adopted molecules has limited passivation ability to perovskites and even may desorb from the passivated defects in a short period of time, which bring about plenty of challenges for further development of high-performance PeLEDs. Here, a multidentate molecule, formamidine sulfinic acid (FSA), is introduced as a novel passivator to perovskites. FSA has multifunctional groups (S≐O, C≐N and NH2 ) where the S≐O and C≐N groups enable coordination with the lead ions and the NH2 interacts with the bromide ions, thus providing the most effective chemical passivation for defects and in turn the formation of highly stable perovskite emitters. Moreover, the interaction between the FSA and octahedral [PbBr6 ]4- can inhibit the formation of unfavorable low-n domains to further minimize the inefficient energy transfer inside the perovskite emitters. Therefore, the FSA passivated green-emitting PeLED exhibits a high external quantum efficiency (EQE) of 26.5% with fourfold enhancement in operating lifetime as compared to the control device, consolidating that the multidentate molecule is a promising strategy to effectively and sustainably passivate the perovskites.

Molecularly Functionalized SnO2 Films by Carboxylic Acids for High-Performance Perovskite Solar Cells.

Metal oxides are commonly employed as electron transport layers (ETLs) for n-i-p perovskite solar cells (PSCs), but the presence of surface traps and their mismatched energy alignment with perovskites limits the corresponding device performance. Therefore, the interfacial modification of ETLs by functional molecules becomes an important strategy for tailoring the interfacial properties and facilitating an efficient charge extraction and transport in PSCs. However, an in-depth understanding of the influences of their molecular structures on the surface chemistry and electronic properties of ETLs is rarely discussed. Herein, three carboxylic acid-based molecules with different chemical structures were employed to modify the SnO2 ETL and their effects on the performance of PSCs were systematically investigated. We found that the alkyl-chain length and carboxyl number in molecular structures can dramatically alter their binding strength to SnO2, providing a good strategy to fine-tune their film quality, electron mobility, and energy offset at the cathode interface. Benefiting from the optimal coordination ability of citric acid (CA) to SnO2, the corresponding PSCs show better charge transport properties and suppressed nonradiative recombination, leading to a champion efficiency of 23.1% with much improved environmental stability, highlighting the potential of rational design of molecular modifiers for high-performance ETLs applied in PSCs.

FA-Assistant Iodide Coordination in Organic-Inorganic Wide-Bandgap Perovskite with Mixed Halides.

Mixed-halide wide-bandgap perovskites are key components for the development of high-efficiency tandem structured devices. However, mixed-halide perovskites usually suffer from phase-impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA+ )-based mixed-halide perovskites, MAPb(I0.6 Br0.4 )3 , the halide composition of the spin-coated perovskite films is preferentially dominated by the bromide ions (Br- ). Additional thermal energy is required to initiate the insertion of iodide ions (I- ) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA+ ) in the precursor solution, it can effectively facilitate the I- coordination in the perovskite framework during the spin-coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high-crystallinity perovskite film with high Br- content. As a result, high-quality MA0.9 FA0.1 Pb(I0.6 Br0.4 )3 perovskite film with a bandgap (Eg ) of 1.81 eV is achieved, along with an encouraging power-conversion-efficiency of 17.1% and open-circuit voltage (Voc ) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.

Synergistic Effect of Pseudo-Halide Thiocyanate Anion and Cesium Cation on Realizing High-Performance Pinhole-Free MA-Based Wide-Band Gap Perovskites.

The performance of wide-band gap perovskite solar cells has a profound impact on the multijunction tandem device efficiency. However, once bromide (Br-) has been adopted to substitute the iodide (I-) in the MAPbI3 framework, it becomes very challenging to achieve uniform and high crystalline perovskite films. Here, a synergistic effect of pseudo-halide anion thiocyanate (SCN-) and inorganic cation cesium (Cs+) on the crystallization and film formation of MA-based wide-band gap perovskite is reported. It is found that the intrinsic ability of SCN- for increasing the perovskite crystal size can make the crystallization process more tolerable to the different affinity of the initial inhomogeneous small particles. However, the introduction of SCN- usually comes along with undesired large PbI2 aggregates. By further incorporating Cs+ in the precursor solution to improve the solubility of the halide/pseudo-halide coordination to Pb2+, the formation of the aggregated PbI2 particles is successfully inhibited. As a result, uniform pinhole-free MA0.9Cs0.1PbI2Br(SCN)0.08 perovskites with a wide band gap of 1.77 eV can be achieved. The corresponding photovoltaic device exhibits a record-high fill-factor over 80% and a promising power conversion efficiency of 16.3%.

Charge transfer-induced photoluminescence in ZnO nanoparticles.

The quality of solution-processed zinc oxide (ZnO) nanoparticles (NPs) is often correlated with their photoluminescence (PL) spectral characteristics. However, the reported PL spectral characteristics lack consistency and remain controversial. Here we report that "defect-emission free" PL spectra can even be obtained in thin films composed of as-synthesized ZnO NPs. It is found that both the PL spectral line-shape and intensity are extremely sensitive to nitrogen and oxygen. By conducting time-dependent PL (t-PL) and photothermal deflection spectroscopy (PDS) measurements under vacuum and different gases, it is proposed that both inert (N2) and reactive (O2) molecules can be absorbed on the ZnO NP surface and induce charge transfer (CT). The CT states induced by N2 are non-radiative which significantly reduces the band emission. Whereas the CT states induced by O2 are radiative at the visible region, and the exciton transfer is efficient which increases the overall PL quantum yield. Owing to such effects, the previously reported correlation between defects and PL emission becomes questionable and needs to be revisited. Particularly, the visible emission from the ZnO NPs is proved to be facilitated by external effects, instead of direct recombination from defect states.

Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells.

Depth-dependent growth of perovskite crystals remains challenging for high-performance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential accompanying changes are urgently required. Here, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the underlying mechanism is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA0.85Cs0.15PbI3 is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA0.85Cs0.15PbI3 perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photostability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI2-(CsI)0.15 nanorods scaffold to achieve high-quality perovskite films in a two-step method but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.

The Control of Conjugation Lengths and Steric Hindrance to Modulate Aggregation-Induced Emission with High Electroluminescence Properties and Interesting Optical Properties.

A series of novel AIE-active (aggregation-induced emission) molecules, named SAF-2-TriPE, SAF-3-TriPE, and SAF-4-TriPE, were designed and synthesized through facile reaction procedures. We found that incorporation of the spiro-acridine-fluorene (SAF) group, which is famous for its excellent hole-transporting ability and rigid structure, at different substitution positions on the phenyl ring affected the conjugation lengths of these compounds. Consequently, we have obtained molecules with different emission colors and properties without sacrificing good EL (electroluminescence) characteristics. Accordingly, a device that was based on compound SAF-2-TriPE displayed superior EL characteristics: it emitted green light with ηc, max =10.5 cd A(-1) and ηext, max =4.22 %, whereas a device that was based on compound SAF-3-TriPE emitted blue-green light with ηc, max =3.9 cd A(-1) and ηext, max = 1.71 %. These compounds also displayed different AIE performances: when the fraction of water in the THF solutions of these compounds was increased, we observed a significant improvement in the ΦF of compounds SAF-2-TriPE and SAF-3-TriPE; in contrast, compound SAF-4-TriPE showed an abnormal phenomenon, in that it emitted a strong fluorescence in both pure THF solution and in the aggregated state without a significant change in ΦF . Overall, this systematic study confirmed a relationship between the regioisomerism of the luminophore structure and its AIE activity and the resulting electroluminescent performance in non-doped devices.