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.

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.

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.

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.

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.

Dendritic CsSnI3 for Efficient and Flexible Near-Infrared Perovskite Light-Emitting Diodes.

All-inorganic and lead-free CsSnI3 is emerging as one of the most promising candidates for near-infrared perovskite light-emitting diodes (NIR Pero-LEDs), which find practical applications including facial recognition, biomedical apparatus, night vision camera, and Light Fidelity. However, in the CsSnI3 -based Pero-LEDs, the holes injection is significantly higher than that of electrons, resulting in unbalanced charge injection, undesired exciton dissipation, and poor device performance. Herein, it is proposed to manage charge injection and recombination behavior by tuning the interface area of perovskite and charge-transporter. A dendritic CsSnI3 structure is prepared on the hole-transporter, only making a bottom contact with the hole-transporter and exposing all other available crystal surfaces to the electron-transporter. In other words, the interface area of perovskite/electron-transporter is significantly higher than that of perovskite/hole-transporter. Moreover, the embedding interface of perovskite/electron-transporter can spatially confine holes and electrons, increasing the radiation recombination. By taking advantage of the dendritic structure, efficient lead-free NIR Pero-LEDs are achieved with a record external quantum efficiency (EQE) of 5.4%. More importantly, the dendritic structure shows great superiorities in flexible devices, for there is almost no morphology change after 2000-cycles of bends, and the fabricated Pero-LEDs can keep 93.4% of initial EQEs after 50-cycles of bends.

Efficient and stable inverted perovskite solar cells enabled by inhibition of self-aggregation of fullerene electron-transporting compounds.

Fullerene-based electron-transporting layers (ETLs) significantly influence the defect passivation and device performance of inverted perovskite solar cells (PSCs). However, the π-cage structures of fullerenes lead to a strong tendency to self-aggregate, which affects the long-term stability of the corresponding PSCs. Experimental results revealed that [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based ETLs exhibit a certain degree of self-aggregation that affects the stability of the device, particularly under continuous irradiation stress. To modulate the aggregation behavior, we replaced a methyl hydrogen of PCBM with a phenyl group to yield [6,6]-phenyl-C61-butyric acid benzyl ester (PCBB). As verified through X-ray crystallography, this minor structural modification results in more non-covalent intermolecular interactions, which effectively enhanced the electron-transporting ability of the PCBB-based ETL and led to an efficiency approaching 20%. Notably, the enhanced intermolecular forces of PCBB suppressed its self-aggregation, and the corresponding device showed significantly improved stability, retaining approximately 90% of its initial efficiency after 600 h under one-sun irradiation with maximum power point tracking. These findings provide a viable approach for the design of new fullerene derivatives to tune their intermolecular interactions to suppress self-aggregation within the ETL for high-performance PSCs.

Ultrathin PEDOT:PSS Enables Colorful and Efficient Perovskite Light-Emitting Diodes.

Recently, metal halide perovskite light-emitting diodes (Pero-LEDs) have achieved significant improvement in device performance, especially for external quantum efficiency (EQE). And EQE is mostly determined by internal quantum efficiency of the emitting material, charge injection balancing factor (ηc), and light extraction efficiency (LEE) of the device. Herein, an ultrathin poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (UT-PEDOT:PSS) hole transporter layer is prepared by a water stripping method, and the UT-PEDOT:PSS can enhance ηc and LEE simultaneously in Pero-LEDs, mostly due to the improved carrier mobility, more matched energy level alignment, and reduced photon loss. More importantly, the performance enhancement from UT-PEDOT:PSS is quite universal and applicable in different kinds of Pero-LEDs. As a result, the EQEs of Pero-LEDs based on 3D, quasi-3D, and quasi-2D perovskites obtain enhancements of 42%, 87%, and 111%, and the corresponding maximum EQE reaches 17.6%, 15.0%, and 6.8%, respectively.

Composition engineering to obtain efficient hybrid perovskite light-emitting diodes.

Metal halide perovskites have received considerable attention in the field of electroluminescence, and the external quantum efficiency of perovskite light-emitting diodes has exceeded 20%. CH3NH3PbBr3 has been intensely investigated as an emitting layer in perovskite light-emitting diodes. However, perovskite films comprising CH3NH3PbBr3 often exhibit low surface coverage and poor crystallinity, leading to high current leakage, severe nonradiative recombination, and limited device performance. Herein, we demonstrate a rationale for composition engineering to obtain high-quality perovskite films. We first reduce pinholes by adding excess CH3NH3Br to the actual CH3NH3PbBr3 films, and we then add CsBr to improve the crystalline quality and to passivate nonradiative defects. As a result, the (CH3NH3)1-xCsxPbBr3 based perovskite light-emitting diodes exhibit significantly improved external quantum and power efficiencies of 6.97% and 25.18 lm/W, respectively, representing an improvement in performance dozens of times greater than that of pristine CH3NH3PbBr3-based perovskite light-emitting diodes. Our study demonstrates that composition engineering is an effective strategy for enhancing the device performance of perovskite light-emitting diodes.

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.

Exciton-phonon interaction in quasi-two dimensional layered (PEA)2(CsPbBr3)n-1PbBr4 perovskite.

Two-dimensional (2D) Ruddlesden-Popper perovskites with bulky organic cations have attracted extensive attention in light-emitting devices and photovoltaics due to their robust environment stability, tunable luminescent color, strong exciton binding and promising efficiency. A quantum well (QW) structure is spontaneously formed by sandwiching PbBr4 layers into bulky organic cations. However, some intrinsic excitonic mechanisms in these materials still need to be elucidated. In this study, the exciton-phonon interaction of quasi-2D (PEA)2(CsPbBr3)n-1PbBr4 with different PbBr4 layer numbers (n) was analyzed by temperature-varied photoluminescence (PL), scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD). The mechanism of bandgap shifting with temperature was found to be dominated by the thermal expansion effect in the large-n 2D and bulk perovskite, and gradually switched to exciton-phonon interaction in the n = 1 (PEA)2PbBr4 phase, indicating enhanced exciton-phonon interaction in the thinner quantum well structure. Further analysis showed that the enhanced exciton-phonon interaction originated from the longitudinal optical phonon-exciton Fröhlich interaction rather than acoustic phonon-exciton coupling. We believe that our results will benefit the further optimization of light-emitting devices based on 2D 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.