Ion-Mediated Recombination Dynamics in Perovskite-Based Memory Light-Emitting Diodes for Neuromorphic Control Systems.

Neuromorphic devices can help perform memory-heavy tasks more efficiently due to the co-localization of memory and computing. In biological systems, fast dynamics are necessary for rapid communication, while slow dynamics aid in the amplification of signals over noise and regulatory processes such as adaptation- such dual dynamics are key for neuromorphic control systems. Halide perovskites exhibit much more complex phenomena than conventional semiconductors due to their coupled ionic, electronic, and optical properties which result in modulatable drift, diffusion of ions, carriers, and radiative recombination dynamics. This is exploited to engineer a dual-emitter tandem device with the requisite dual slow-fast dynamics. Here, a perovskite-organic tandem light-emitting diode (LED) capable of modulating its emission spectrum and intensity owing to the ion-mediated recombination zone modulation between the green-emitting quasi-2D perovskite layer and the red-emitting organic layer is introduced. Frequency-dependent response and high dynamic range memory of emission intensity and spectra in a LED are demonstrated. Utilizing the emissive read-out, image contrast enhancement as a neuromorphic pre-processing step to improve pattern recognition capabilities is illustrated. As proof of concept using the device's slow-fast dynamics, an inhibition of the return mechanism is physically emulated.

Amplified Spontaneous Emission Threshold Dependence on Determination Method in Dye-Doped Polymer and Lead Halide Perovskite Waveguides.

Nowadays, the search for novel active materials for laser devices is proceeding faster and faster thanks to the development of innovative materials able to combine excellent stimulated emission properties with low-cost synthesis and processing techniques. In this context, amplified spontaneous emission (ASE) properties are typically investigated to characterize the potentiality of a novel material for lasers, and a low ASE threshold is used as the key parameter to select the best candidate. However, several different methods are currently used to define the ASE threshold, hindering meaningful comparisons among various materials. In this work, we quantitatively investigate the ASE threshold dependence on the method used to determine it in thin films of dye-polymer blends and lead halide perovskites. We observe a systematic ASE threshold dependence on the method for all the different tested materials, and demonstrate that the best method choice depends on the kind of information one wants to extract. In particular, the methods that provide the lowest ASE threshold values are able to detect the excitation regime of early-stage ASE, whereas methods that are mostly spread in the literature return higher thresholds, detecting the excitation regime in which ASE becomes the dominant process in the sample emission. Finally, we propose a standard procedure to properly characterize the ASE threshold, in order to allow comparisons between different materials.

Defect Passivation Using a Phosphonic Acid Surface Modifier for Efficient RP Perovskite Blue-Light-Emitting Diodes.

Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the n-domain distribution of the Ruddlesden-Popper (RP, L2An-1PbnX3n+1, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb2+ ( notorious for nonradiative recombination) and therefore passivates them. Here, with the help of various spectroscopic techniques, such as X-ray photon-spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence quantum yield (PLQY) measurements, we demonstrate the capability of OPA to bind and passivate unpaired Pb2+ defect sites. Modification to crystallization promoting higher n-domain formation is also observed from steady-state and transient absorption (TA) measurements. With OPA treatment, both the PLQY and EQE of the corresponding PeLED showed improvements up to 53% and 3.7% at peak emission wavelength of 485 nm, respectively.

Enhanced Thermal Stability of Planar Perovskite Solar Cells Through Triphenylphosphine Interface Passivation.

While extensive research has driven the rapid efficiency trajectory noted to date for organic-inorganic perovskite solar cells (PSCs), their thermal stability remains one of the key issues hindering their commercialization. Herein, a significant reduction in surface defects (a precursor to perovskite instability) could be attained by introducing triphenylphosphine (TPP), an effective Lewis base passivator, to the vulnerable perovskite/spiro-OMeTAD interface. Not only did TPP passivation enable a high power conversion efficiency (PCE) of 20.22 % to be achieved, these devices also exhibited superior ambient and thermal stability. Unlike the pristine device, which exhibited a sharp descend to 16 % of its initial PCE on storing in relative humidity of 10 %, at 85 °C for more than 720 h, the TPP-passivated devices retained 71 % of its initial PCE. Hence, this study presents a facile yet excellent approach to attain high-performing yet thermally stable PSCs.

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells.

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs+ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs0.05 (MA0.17 FA0.83 )0.95 PbBr3 (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Regulating Vertical Domain Distribution in Ruddlesden-Popper Perovskites for Electroluminescence Devices.

Efficient perovskite light-emitting diodes are designed by employing an ordered vertical domain distribution in quasi-two-dimensional (quasi-2D) perovskites to induce better electron flow to the emitting domains. Dimethyl sulfoxide is added to the precursor solution to tune the crystallization rate and promote the formation of high-m domains near the substrate surface via the one-step deposition method. Optimized deposition conditions yielding a film with favorable energetic landscape for both carrier injection and confinement results in a 4-fold external quantum efficiency (EQE) enhancement with maximum EQE of 5.79%. Better carrier injection is further supported by a turn-on voltage value that is comparable to the band gap of the emitter material (∼2.25 eV).

Hot carrier extraction in CH3NH3PbI3 unveiled by pump-push-probe spectroscopy.

Halide perovskites are promising materials for development in hot carrier (HC) solar cells, where the excess energy of above-bandgap photons is harvested before being wasted as heat to enhance device efficiency. Presently, HC separation and transfer processes at higher-energy states remain poorly understood. Here, we investigate the excited state dynamics in CH3NH3PbI3 using pump-push-probe spectroscopy. It has its intrinsic advantages for studying these dynamics over conventional transient spectroscopy, albeit complementary to one another. By exploiting the broad excited-state absorption characteristics, our findings reveal the transfer of HCs from these higher-energy states into bathophenanthroline (bphen), an energy selective organic acceptor far above perovskite's band edges. Complete HC extraction is realized only after overcoming the interfacial barrier formed at the heterojunction, estimated to be between 1.01 and 1.08 eV above bphen's lowest unoccupied molecular orbital level. The insights gained here are essential for the development of a new class of optoelectronics.

Cesium Oleate Passivation for Stable Perovskite Photovoltaics.

Despite their emergence as promising materials for low-cost and efficient energy power generation technology, the instability of hybrid organic-inorganic lead-halide perovskites toward moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material interface. Passivation using cesium oleate does not induce the formation of any low-dimensional perovskites, suggesting that the organic species only passivate the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of the perovskite film is realized upon passivation, evidenced by a large water contact angle of 107.4° and improved stability at ambient conditions (a relative humidity of ∼70%, room temperature). Concomitantly, the proposed passivation strategy leads to an increased amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85 °C. By maintaining the three-dimensional (3D) structure of the solar absorber while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (Voc) and unsacrificed short-circuit current density (Jsc) were obtained from the treated devices, leading to power conversion efficiencies of more than 18%. When stored in a humid environment (a relative humidity of ∼55%), devices with cesium oleate passivation maintain 88% of their initial power conversion efficiency after 720 h, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with a unique combination of inorganic cations and long-chain organics to provide hydrophobicity and moisture stability to the solar absorber layer while maintaining good device performances.

Perturbation-Induced Seeding and Crystallization of Hybrid Perovskites over Surface-Modified Substrates for Optoelectronic Devices.

Growing a monocrystalline layer of lead halide perovskites directly over substrates is necessary to completely harness their stellar properties in optoelectronic devices, as the single crystals of these materials are extremely brittle. We study the crystallization mechanism of perovskites by antisolvent vapor diffusion to its precursor solution and find that heterogeneous nucleation prevails in the process, with the crystallization dish walls providing the energy to overcome the nucleation barrier. By perturbing the system using sonication, we are able to introduce homogeneously nucleated seed crystals in the precursor solution. These seeds lead to the growth of closely packed crystals over surface-modified substrates kept in the precursor solution. This crystallization process is substrate independent and scalable and can be utilized to fabricate planar optoelectronic devices. We demonstrate a methylammonium lead iodide planar crystal photoconductor with a colossal detectivity of 1.48 × 1013 Jones.

Author Correction: Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites.

The original version of this article incorrectly listed the present address of Bo Wu as 'Present address: Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province 510006, China'. This is the author's primary affiliation. This has been corrected in both the PDF and HTML versions of the article.

Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites.

Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.

Designing Efficient Energy Funneling Kinetics in Ruddlesden-Popper Perovskites for High-Performance Light-Emitting Diodes.

Mixed Ruddlesden-Popper (RP) perovskites are of great interest in light-emitting diodes (LEDs), due to the efficient energy transfer (funneling) from high-bandgap (donor) domains to low-bandgap (acceptor) domains, which leads to enhanced photoluminescence (PL) intensity, long PL lifetime, and high-efficiency LEDs. However, the influence of reduced effective emitter centers in the active emissive film, as well as the implications of electrical injection into the larger bandgap donor material, have not been addressed in the context of an active device. The electrical and optical signatures of the energy cascading mechanisms are critically assessed and modulated in a model RP perovskite series ((C8 H17 NH3 )2 (CH(NH2 )2 )m-1 Pbm Br3m+1 ). Optimized devices demonstrate a current efficiency of 22.9 cd A-1 and 5% external quantum efficiency, more than five times higher than systems where funneling is absent. The signature of nonideal funneling in RP perovskites is revealed by the appearance of donor electroluminescence from the device, followed by a reduction in the LED performance.

Grain Size Modulation and Interfacial Engineering of CH3 NH3 PbBr3 Emitter Films through Incorporation of Tetraethylammonium Bromide.

Metal halide perovskites have demonstrated breakthrough performances as absorber and emitter materials for photovoltaic and display applications respectively. However, despite the low manufacturing cost associated with solution-based processing, the propensity for defect formation with this technique has led to an increasing need for defect passivation. Here, we present an inexpensive and facile method to remedy surface defects through a postdeposition treatment process using branched alkylammonium cation species. The simultaneous realignment of interfacial energy levels upon incorporation of tetraethylammonium bromide onto the surface of CH3 NH3 PbBr3 films contributes favorably toward the enhancement in overall light-emitting diode characteristics, achieving maximum luminance, current efficiency, and external quantum efficiency values of 11 000 cd m-2 , 0.68 cd A-1 , and 0.16 %, respectively.

Highly efficient Cs-based perovskite light-emitting diodes enabled by energy funnelling.

The incorporation of phenylethylammonium bromide (PEABr) into a fully inorganic CsPbBr3 perovskite framework led to the formation of mixed-dimensional perovskites, which enhanced the photoluminescence due to efficient energy funnelling and morphological improvements. With a PEABr : CsPbBr3 ratio of 0.8 : 1, PeLEDs with a current efficiency of 6.16 cd A-1 and an EQE value of 1.97% have been achieved.

Rapid Crystallization of All-Inorganic CsPbBr3 Perovskite for High-Brightness Light-Emitting Diodes.

Research into perovskite-based light-emitting diodes (PeLEDs) has been rapidly gaining momentum since the initial reports of green-emitting methylammonium lead bromide (CH3NH3PbBr3)-based devices were published. However, issues pertaining to its stability and morphological control still hamper progress toward high performing devices. Solvent engineering, a technique typically employed to modulate film crystallization, offers little opportunity for scale-up due to the tendency for inhomogeneous film growth and low degree of reproducibility. Here, we propose and show a simple gas-facilitated process to deposit a stable, all-inorganic perovskite CsPbBr3 film. The formation of smaller and less percolated grains, which gives rise to enhanced optical properties, highlights the importance of spatial charge confinement in the film. Consequently, the performance of our PeLEDs shows great improvement, with luminance as high as 8218 cd m-2 and turn-on voltage as low as 2.4 V. Concomitantly, the current efficiency and EQE of our device were increased to 0.72 cd A-1 and 0.088%, respectively. High reproducibility in the performance of PeLEDs fabricated using this process opens the path for large-area devices.