Symposium EL17—Halide Perovskite and Other Low-Dimensional Light Emitters—Synthesis, Defect Passivation and Optoelectronic Applications

I will discuss issues at the materials interface (e.g. between active layer, ETL/HBL and HTL/EBL) especially as these relate to constructing perovskite tandem and triple-junction solar cells.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent—approaching those of commercial organic light-emitting diodes—have been demonstrated in both the infrared and the green emission channels. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to ‘clean’ the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications—such as photovoltaics—for which bandgap stability is required.

Metal halide perovskites are a promising candidate as gain medium for a thin-film electrically-pumped laser. This is owing to their excellent optoelectronic properties combining high carrier mobilities and near unity photoluminescence quantum yield. Furthermore, their high optical gain values lead to low amplified spontaneous emission (ASE) thresholds. Encouragingly, optically-pumped lasing has been routinely observed from perovskite-based active layers in different resonator structures, while continuous-wave lasing has been realized not only under cryogenic conditions, but also at room temperature. Moreover, perovskite light-emitting diodes (PeLEDs) show external quantum efficiencies (EQE) values above 1% at high current densities above 1 kA/cm². Despite this progress, an electrically-pumped perovskite laser has not been demonstrated so far.

In this work, we demonstrate a novel PeLED architecture that yields low ASE thresholds. At the same time, the device maintains excellent electrical injection properties up to kA/cm² current densities. Furthermore, we study the device performance under cryogenic conditions, where we show a 3-fold increase in EQE and more than an order of magnitude lower ASE threshold compared to room temperature operation. These interesting device features allow us to study the response of the PeLED to electrical and optical excitations at different temperatures. Moreover, we study the effect of simultaneous electrical and optical excitation on device performance. Finally, by comparing the device response to optical and electrical excitations, we estimate the threshold current density needed to achieve ASE in our structure. These results pave the way toward a proof-of-concept demonstration of an electrically-pumped perovskite laser.

While light-emitting diodes (LEDs) made from lead halide perovskites have demonstrated external quantum efficiencies (EQEs) well over 20%, their electrical stability must be addressed before they are seriously considered for commercial applications. In an effort to improve the optoelectronic properties of lead halide perovskites for light emission, many researchers have investigated introducing both alkaline-earth metal ions (e.g., Ba²⁺ and Sr²⁺) and transition metal ions (e.g., Mn²⁺, Zn²⁺, Cd²⁺, and Ni²⁺) into the B-site of the perovskite’s ABX₃ structure. Additionally, the factors that limit the electrical stability of perovskite LEDs remain under investigation. In this work, we dope Mn²⁺ ions into an organic-inorganic hybrid quasi-bulk 3D perovskite resulting into (PEABr)_0.2Cs_0.4MA_0.6Pb_0.7Mn_0.3Br₃ thin films with the addition of tris(4-fluorophenyl)phosphine oxide (TFPPO) dissolved in a chloroform antisolvent to achieve an EQE of 13.4% and a peak luminance of 95,400 cd/m². While the inclusion of TFPPO into the chloroform antisolvent dramatically increases the EQE of perovskite LEDs, the electrical stability is severely compromised. At an electrical bias of 5 mA/cm², our perovskite LED fabricated with a pure chloroform antisolvent (2.5% EQE) decays to half of its initial luminance in 90.68 minutes. Alternatively, our perovskite LED fabricated with TFPPO (13.4% EQE) decays to half of its initial luminance in 2.07 min. In order to investigate this trade-off in EQE and electrical stability, we study both photophysical and electronic characteristics before and after electrical degradation of the perovskite LEDs. We find that given identical electrical degradation conditions, the TFPPO-based device’s turn on voltage and overall electrical resistance increases in a much larger fashion as compared to the pure chloroform-based device. While the EQE characteristics of this Mn²⁺ doped perovskite LED show promise for B-site engineered perovskites, there is still large concern to simultaneously achieve both energy-efficient and electrically stable perovskite-enabled lighting. Uncovering the effects from the TFPPO additive on perovskite LEDs will reveal pathways on how to mitigate their negative consequences on electrical stability while retaining their energy-efficiency boosting properties.

As the technology has advanced and the amount of information grows in an information society, display technology needs to show information more vividly and efficiently. In this sense of line, display technology has found a high color purity light emitter, metal halide perovskite, and tried to demonstrate high efficiency light-emitting diodes (LEDs). However, device efficiency of perovskite LEDs is limited by lack of comprehensive materials design strategies both to suppress formation of defects and to enhance charge carrier confinement inside perovskite nanocrystals (PNCs). Here, we report comprehensive strategies that generate smaller, monodisperse colloidal particles (confining electrons and holes and boosting radiative recombination) with fewer surface defects (reducing nonradiative recombination) to demonstrate high efficiency perovskite LEDs.
Firstly, we performed ligand engineerings during the PNC synthesis. We synthesized methylammonium lead bromide (MAPbBr₃) and formamidinium lead bromide (FAPbBr₃) PNCs with a dimension larger than exciton Bohr diameter (regime beyond quantum size), which have emitting light with size-insensitively high color-purity and constant wavelength, by controlling ligand concentration and ligand length, respectively. Ligand-engineered PNCs can increase photoluminescence quantum efficiency (PLQE) upto 70% by preventing non-radiative recombination of charge carriers and also improve the charge injection/transport capability in PNC films, resulting in external quantum efficiency (EQE) of 5.5% in PNC-LEDs.
Secondly, we conducted crystal engineering on the PNCs. We used substitutional doping of guanidinium (GA) into FAPbBr₃ PNCs to incorporate an optimal proportion of GA cations into the structure. The GA cations can reside in the bulk of the PNC in low concentrations (~10%). The surplus GA then accumulates on the surface of the PNC. Guanidinium incorporation provides bulk entropy stabilization, surface stabilization (by additional hydrogen bonding contributed by their extra amino group), and better electron–hole confinement, which results in high PLQE over 90%. A GA to FA ratio of 0.1 was shown to maximize the electroluminescence efficiency. Moreover, a 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (TBTB) overcoat was introduced to heal the residual halide vacancy defects in FA_1–xGA_xPbBr₃ PNCs and to improve the charge balance in PeLEDs. The result is highly efficient PNC-based LEDs that have current efficiency of 108 cd/A (EQE of 23.4%), which rises to 205 cd/A (EQE of 45.5%) with a hemispherical lens.
Furthermore, we report that highly efficient large-area perovskite LEDs with high uniformity can be realized through the use of colloidal PNCs which decouple the crystallization of perovskites from film formation. PNCs are precrystallized and surrounded by organic ligands, and thus they are not affected by the film formation process, in which a simple modified bar-coating method facilitates the evaporation of residual solvent to provide uniform large-area films. PeLEDs incorporating the uniform bar-coated PNC films achieve an external quantum efficiency (EQE) of 23.26% for a pixel size of 4mm² and EQE of 22.5% for a large pixel area of 102mm² with high reproducibility and EQE of 21.46% in 900mm².

Recently, substantial efforts have been made in enhancing the performances of blue perovskite light-emitting diodes (PeLEDs), and efficiencies over 10% have been achieved. However, the performances still lag far behind with the efficient green, red, and near-infrared PeLED analogs. It is highly desirable to develop highly-performed and stable blue PeLEDs.
In this work, we will improve the performance from quasi-two dimensional (quasi-2D) blue PeLEDs by enhancing the carrier injection, transfer and confinement. To improve the carrier injection, we will suppress the trap states and improve the band alignment of device structure[1]. Then, we will modify introduce mixed quasi-2D perovskite structure to enhance the carrier transfer and narrow the n-phase of quasi-2D perovskites for improving the emission efficiency [2,3]. To further optimize the performance of quasi-2D blue PeLEDs, we systematically study the effects of organic passivating agents with different carbon chain lengths. Our results show that the appropriate carbon chain length of the organic agent simultaneously offer good carrier confinement for the massive radiative recombination within the perovskites, and efficient carrier transfer in quasi-2D perovskites for high emission efficiency. Benefitting from the good optical and electrical properties of TBPO incorporated perovskites, we achieve high-efficient blue PeLEDs with an EQE of 11.5 % and prolonged operational stability of 41.1 min [4]. Consequently, this work contributes the effective approaches to design high-quality perovskite films and efficient blue PeLEDs.


[1] Z. Ren, X. Xiao, R. Ma, H. Lin, K. Wang, X.W. Sun, W. C. H. Choy, Adv. Function. Mater., 29, 1905339, 2019.
[2] Z. Ren, L. Li, J. Yu, R. Ma, X. Xiao, R. Chen, K. Wang, X.W. Sun, W.J. Yin, W.C.H. Choy, ACS Energy Lett., 5, 2569, 2020.
[3] Z. Ren, J. Yu, Z. Qin, J. Wang, J. Sun, C.C.S. Chan, S. Ding, K. Wang, R. Chen, K.S. Wong, X. Lu, W.J. Yin, W.C.H. Choy, Adv. Mater., 33, 2005570, 2021.
[4] Z. Ren, J. Sun, J. Yu, X. Xiao, Z. Wang, R. Zhang, K. Wang, R. Chen, Y. Chen, W.C.H. Choy, Nano-Micro Lett, 14:66, 2022.

Quasi-two-dimensional (2D) perovskite material has been recently receiving a lot of attention as a light emitter due to its unique quantum-well properties. However, forming undesired multi-quantum-well, called as phase separation, spontaneously occurs in quasi-2D perovskite films since ions can be migrated during the crystallization process. The phase separation produces unexpected emission wavelength, decreases external quantum efficiency (EQE) and stability in light-emitting diode (LED) devices. Therefore, many research groups try to overcome the phase separation using a new design of organic ligands or crystallization kinetics.
In the LED fields, mixing halide ions such as iodide and bromide has widely been used since it can control energy bandgap of the perovskite films. However, mixing halide ions in quasi-2D perovskite films induces phase separation much more than pure halide ions because smaller halide ions in the mixed-halide system have faster ion migration than larger halide ions. In addition, halide segregation also appears in the quasi-2D perovskite film when mixing halide ions. In this work, we define the phase separation and halide segregation as binary phase separation. And our newly designed ligands can suppress the binary phase separations effectively compared to conventional ligands.
Heterogeneity and the phase separation of the mixed-halide quasi-2D perovskite films can be spatially demonstrated using a confocal-photoluminescence (PL) measurement with different ranges of PL wavelengths. The quasi-2D perovskite films using the conventional ligands are suffered from phase separation having small n phase and high n phase even to three-dimensional phase. Moreover, the halide distribution in mixed-halide quasi-2D perovskite LED devices is analyzed by in-situ X-ray fluorescence (XRF). In the in-situ XRF mapping, the halide ions in the quasi-2D perovskite film using the conventional ligand are observed to be migrated in operating LED devices, and already segregated before applying bias. On the other hand, well-suppressed the binary phase separation using our ligands can manufacture the quasi-2D perovskite LED have controllable emission wavelength by mixing halide ions and high EQE of over 26% and higher operational stability compared to other quasi-2D perovskite LED devices.

The emergence of perovskite quantum dots (QDs) has enabled light-emitting diodes (LEDs) with narrowband emission and high external quantum efficiencies (EQEs). Epitaxially growing a perovskite matrix onto perovskite QDs provides robust QD surface passivation and efficient charge transport, thus further boosting luminescence efficiency and operation stability.

Efficient and stable red LEDs have been developed based on CsPbI₃ QD-in-perovskite matrix solids^[1]. However, the synthesis of this heterostructure has so far remained elusive for wide-bandgap (E_g) perovskite emitters relevant to blue photon sources. The chemical challenge has resided in the paucity of wide-E_g QD:matrix combinations: the identification of the rare pairings that implement type-I band alignment and lattice matching between QDs and matrix.

Here, we report a wide-E_g (>2.6 eV) CsPbBr₃ QD-in-perovskite matrix solid that enables efficient sky-blue LEDs^[2]. We initiated a wide screen of prospective A-site and B-site alloying candidates and found promising for CsPb_1-xSr_xBr₃ as a matrix: it was the sole candidate that minimized lattice mismatch with the blue-emitting perovskite QDs while allowing an E_g of the perovskite matrix larger than that of the QDs.

In light of the hygroscopic nature of Sr²⁺, we then developed an approach to passivate the surface of the CsPb_1-xSr_xBr₃ matrix. By uniting molecular dynamics simulations with experimental efforts, we found that bis(4-fluorophenyl)phenylphosphine oxide (DPPO) strongly coordinates with Sr²⁺ located on the matrix surface and offers sufficient steric hindrance to block H₂O from the atmosphere. As a result, the QD-in-matrix solids exhibit efficient charge transport with enhanced stability. Sky-blue LEDs using this material as the active layer exhibit an EQE of 13.8% and a brightness exceeding 6000 cd m^-2.

Reference:
[1] Y. Liu et al, J. Am. Chem. Soc. 2021, 143 (38), 15606–15615
[2] Y. Liu, Z. Li et al, J. Am. Chem. Soc. 2022, 144 (9), 4009–4016.

Hybrid perovskites are emerging as highly efficient materials for optoelectronic applications. However, their operational lifetime has remained a limiting factor for their continued progress. In thin-film perovskite light-emitting devices, ionic redistribution may distort the perovskite crystal structure, lowering conductivity and light emission due to the formation of vacancies and other traps. Our strategy for enhanced lifetimes involves producing differentiated ion motion within a device with a rational materials blend. The materials selectively move additive ions while restricting the transport of perovskite ions. To accomplish differentiated ion transport with optimal thin-film morphology, we combine a perovskite, a polyelectrolyte such as poly(ethylene oxide), and a salt additive such as LiPF₆. The added mobile Li⁺ and PF₆⁻ ions redistribute more favorably than the intrinsic ionic species and largely preserve the inherent structure of the perovskite film. At 0.5 wt% LiPF₆, CsPbBr₃ devices exhibit 100 h operation at more than 800 cd/m² under constant current driving, achieving a maximum luminance of 3260 cd/m². Incorporating zero-dimensional perovskite nanocrystals into these devices enhances the lifetime further to a luminance half-life of 129 h at 1530 cd/m². We rationalize these findings through optical spectroscopy, impedance spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction.

Luminescent solar concentrators (LSCs) are light-managing devices capable of absorbing and concentrating solar energy to a specific wavelength range and the target region. By coupling with photovoltaic (PV) cells, a PV-LSC system can significantly reduce the total material consumption for solar panel fabrication and simplify the design of large-scale solar harvesting devices for practical utilization. In particular, halide perovskite nanocrystals have shown promising potential for LSC applications due to their excellent optical properties, such as high photoluminescent quantum yield (PLQY), large Stoke’s shift, and tunable optical bandgaps. Here, we report the fabrication of high-performance luminescent solar concentrator devices using copper halide perovskite nanoplatelets (NPLs). The Cs₃Cu₂Cl₅ perovskite NPLs with high photoluminescence quantum yield (PLQY) and large stock shift are synthesized through a hot-injection method. The NPLs are then deposited onto a polydimethylsiloxane (PDMS) substrate through our recently reported ultrasonic nebulization spray method with a low scattering effect and high average visible transmittance. The obtained LSC devices show a high optical efficiency performance and high-power conversion efficiency for the photons in both UVB and UVC regions, making them an ideal LSC device for capturing and energy conversion of high-energy photons.

Two-dimensional perovskite quantum wells have emerged as promising materials for optoelectronics applications, including LEDs, nanoscale lasers, and phototransistors. Ruddlesden-Popper (RP) perovskites have monovalent cation spacers and often exhibit a solid-solid phase transition, in which the organic layer undergoes an order-to-disorder transition. This “melting” of the organic layers induces changes in the inorganic layers, impacting the emissive properties of the material. Here, we demonstrate control over the phase transition temperature of RP 2D perovskites by alloying two similar organic cations. Although halide alloying is used regularly to precisely tune electronic properties, cation alloying to control thermodynamic properties has not been explored. By blending hexylammonium (HA) and pentylammonium (PA) cations in different ratios, we can tune the phase transition temperature of 2D perovskites over a large temperature range. We demonstrate this tunability in single crystals and solution-processed thin films. By correlating temperature dependent grazing incidence wide angle X-ray scattering (GIWAXS) and photoluminescence (PL) measurements, we show that the phase transition has a direct impact on the PL intensity. We also directly image this phase transition with temperature-dependent PL imaging to show that the phase transition is highly heterogeneous at the microscale. Overall, our work provides the necessary design principles to control phase transitions in 2D perovskites with the potential to use these materials as dynamically switchable optoelectronic materials.

Even if perovskite nanocrystals (PNCs) have been well known as the defect tolerance in as-synthesized conditions, the unplausible defects are easily formed during the washing procedures. It is because no deep level defect states are generated due to the formation of halide vacancies, while the loss of PbX₂ shell induces the severe degradation of PNCs. Therefore, in the colloidal solution states, only loss of PbX₂ shell visibly deteriorated the performance of PNCs. However, during light-emitting diodes (LEDs) operations, halide vacancies can provide the ion migration pathways, resulting in the rapid degradation of perovskite structures. Simultaneously, Pb⁰ states, generated due to electron injection, have been reported as the deep level defect states. Therefore, the rational strategies toward all possible defects are important to enhance the optical properties of PNCs and the performance of PNCs LEDs.
Recently, diverse defect passivation strategies have been reported to passivate all possible defect states. General approach toward defect passivation of PNCs might be treatment of surface ligands having suitable functional groups such as ammonium, carboxylate, sulfonate, and so on. However, there is another important consideration to apply the colloidal PNCs into thin films: the binding affinity of surface ligands. Since PNCs have ionic nature, the binding of surface ligands usually has ionic nature, leading ease detachment of surface ligands from the surface PNCs. The easier detachment of surface ligands in colloidal states may induce the surface ligands loss during thin film formation of PNCs, resulting in the severe loss of efficiency of PNCs LEDs. Therefore, there are lots of effort to understand the surface binding affinity of PNCs with DOSY. Several alternatives, such as quaternary ammonium bromide and sulfonate, to replace native ligands have been reported, having higher binding affinity than native ligands. Along with the tightly bound ligands, the optical properties of PNCs are maintained adding of PbX₂ into PNCs during washing process. The addition of PbX₂ during washing can mitigate the loss of PbX₂ shell, maintaining the original photoluminance quantum yield (PLQY).
Bodnarchuk et al. reported treatment of PbX₂ and DDAB improved the colloidal stability of PNCs. The PLQY of PbBr₂ and DDAB treated PNCs was maintained in 90% after polar solvent washing process, while PLQY of native ligands was reduced from 60% to 30% after washing. Moreover, perovskite has self-doping properties where the electronic band structure is tuned in the ratio control over the PbX₂ and AX. The band structure tuning is important to the charge injection of LEDs operation. Additionally, the stability of LEDs is one of the important issues in perovskite societies. One of the well-known approaches is B-site metal doping. The formation energy of B-site doped PNCs was enhanced, resulting in the stable PNCs.
In this study, the ratio control over Pb to DDA was conducted to tune the band structure of PNCs. The post-synthetic surface treatment with Pb-DDA was enhanced PLQY from 46% to near unity. Based on the ratio control, highest occupied molecular orbital (HOMO) became shallow with increasing DDA ratio. Further, the charge injection of PNCs with different ratio was measured in hole only devices. The charge injection was best at Pb-DDA2, implying trade-off between energy level and ligand density in charge injection ability. Consequently, the ratio control over Pb to DDA was resulted in highly efficient LEDs, optimizing charge injection. To further enhance the stability of PNCs, additional NiBr₂ was treated in PNCs, called Ni-DDA. There was no significant different in PLQY. Finally, LED devices were fabricated to compared Pb-DDA and Ni-DDA. Even if the device efficiencies were similar with each other, the device stability of Ni-DDA was enhanced compared to Pb-DDA.

Defect passivation, through decreasing nonradiative recombination rate, has long been considered as the key to efficient perovskite light emitting diodes (LEDs). Here, by revisiting the two commonly used defect passivation strategies, stochiometric ratio tuning and additive engineering, in various perovskite systems, we found its limitations in explaining the superior high performance in light emitting. Surprisingly, a different mechanism other than defect passivation is proved to be responsible for efficient perovskite LEDs. Our low-temperature photoluminescence and ultraviolet photoelectron spectroscopy (UPS) results indicate that these strategies lead to increment of shallow traps and thus p-dope the perovskites. Such p-doping largely enhances the photoluminescence quantum efficiency especially at low carrier density region and can even go beyond 90%. Through carrier dynamics analysis, we point out that the dominant factor responsible for high efficiency in perovskites is the enhancement of radiative recombination via doping rather than defect passivation. Furthermore, this p-doping effect is verified by special device design where poor hole injection results in high external quantum efficiency of perovskite light emitting diodes (PeLEDs). Our discoveries provide design rules to fabricate high-efficiency PeLEDs.

Perovskite nanocrystals (NCs) have emerged as promising luminescent materials for full-colour display applications due to their strong optical properties, widely tunable emission, and narrow full-width half maximum. By precise tuning of their emission spectra, high-resolution full-colour light-emitting diode (LED) arrays can be achieved by micro-patterning red, green, and blue sub-pixels with perovskite NCs as emitters. Currently, photolithography is the most widely used patterning technique in the industry due to its capabilities to achieve high resolution, large area production, and low cost. However, due to the ionic nature of perovskite NCs, conventional photolithography techniques are incompatible as the use of polar solvents can damage underlying perovskite NC patterns, resulting in defect formation and luminescence quenching. Herein, we report a series of novel photo-crosslinkable ligands for the microscale patterning of perovskite NCs. We incorporated photosensitive acrylate groups in the ligands, which upon exposure to ultraviolet light, undergo polymerization and changes the solubility of the NC films. This enables the direct photo-patterning of perovskite NCs without the need for additional photoresist. Through a facile solution-phase ligand exchange process, we functionalized cesium lead halide (CsPbX₃) NCs with our photo-crosslinkable ligands. We also demonstrate the photopatterning of 10 µm features on the CsPbX₃ NC films via direct laser writing with a 405 nm laser. Our approach offers a versatile strategy for the patterning of perovskite NC films in a simple and cost-effective manner, thus enabling the integration of perovskite NCs in various display applications.

Metal-halide perovskites have emerged as promising gain media for lasing applications. In the state-of-the-art, perovskite gain media can be deposited atop a pre-patterned substrate. Alternatively, a negative mould pattern can be directly embedded into the perovskite to form desirable photonic nanostructures. However, conventional thin-film deposition techniques such as spin-coating, vacuum evaporation, etc., exhibit poor conformality and are only marginally compatible with the deposition on textured substrates or above high-aspect-ratio features.

In this work, we present an all-solution fabrication approach of external second-order one-dimensional distributed feedback (1D DFB) gratings by means of soft UV-nanoimprint lithography (UV-NIL). This high-throughput fabrication method can be carried out in the ambient environment, and it requires mild temperatures as low as 70 °C, gentle imprint pressure, and the use of compatible UV-NIL resin that does not attack the chemically fragile methylammonium lead iodide (MAPbI₃) perovskite film used in this work. The UV-NIL gratings are isolated from the perovskite layer and are limited to the UV-NIL resin deposited above the perovskite, thus preserving perovskite morphology and low roughness. We show that commercially available UV-NIL products are suitable for a high-fidelity pattern transfer to perovskite-based optical lasers without affecting perovskite optoelectronic properties.

As a result, we reproducibly achieve a single-mode, low-threshold (below 100 μJ/cm²), narrow linewidth (below 0.2 nm), and strongly polarized (extinction ratio above 50) optically pumped lasing. In addition, we explore characteristic lasing far-field and mode patterns of these 1D DFB gratings. We believe that the proposed resonator integration approach can be extended towards the complete electrically active devices, enabling an alternative integration route towards perovskite injection-lasing.

The efficient generation of polarized light, including linear and circular polarization, is essential in multiple application scenarios such as displays, optical sensing and encryption. Although traditional polarizers and wave plates can be used to obtain high-purity polarized light, these optical devices are bulky and over 50% of light intensity is lost through the polarizers. Alternatively, polarized light can be obtained from various optical active molecules, nanoparticles and their assemblies. Despite the diversity of materials and low-cost thanks to solution processability, these materials usually show a very small degree of polarization.
Herein, we develop a new route to full polarization generation using one-dimensional inorganic nanomaterials as the building blocks. By aligning the quantum nanorods (NRs) with ultrathin inorganic nanowires, a high degree of linear polarization of up to 70% is achieved. To obtain circular polarization, aligned ultrathin transparent nanowire composites with a distinct phase retardance are further deposited with a defined relative orientation angle. The fabricated films show visible intensity contrast through circular polarizers, with an anisotropic factor (g_lum) for circularly polarized luminescence up to 0.7. This approach simultaneously exhibits the advantages of efficient light usage, a high degree of polarization, and low cost, making it promising in various applications using the polarization of light.

Two-dimensional Ruddlesden-Popper (2D RP) perovskites have emerged as promising materials in optoelectronics. The 2D RP perovskites possess significantly improved stability against oxygen, moisture, heat, and ion migration while they still have novel properties such as large exciton binding energies and tunable band gaps. However, 2D RP perovskites suffer significantly from photostability. Photodegradation induces various defects in 2D RP perovskites, such as undercoordinated halide or metal ions like 3D perovskites. The photodegradation of 2D RP eventually leads to the metallic lead (Pb⁰), a critical defect for all kinds of lead-based perovskite. The Pb⁰ not only plays a role as a deep trap for carrier and exciton transport but also accelerates the degradation process. We employed a strong organic molecular acceptor 2,2′-(perfluoronaphthalene-2, 6-diylidene) dimalononitrile (F6-TCNNQ), to suppress such Pb⁰ formed on phenylethylammonium lead iodide (PEA₂PbI₄). We systematically derived the Pb⁰ with a blue laser (405 nm) irradiation on PEA₂PbI₄, then deposited the F6-TCNNQ molecules on its surface. In-situ X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements showed the changes in Pb⁰ states with F6-TCNNQ deposition. Pb⁰ on the surface was completely suppressed after the deposition of F6-TCNNQ. It reduced the non-radiative recombination and consequently recovered the photoluminescence slightly. Interestingly, the recovered film showed much-improved photostability by preventing the formation of photoinduced Pb⁰. UPS measurements revealed that the Pb⁰ is passivated into Pb²⁺ due to the electron transfer from Pb⁰ states into F6-TCNNQ, which is the only possible route. Our results provide a new efficient passivation strategy to improve the performance and stability of 2D RP perovskites, suggesting the importance of understanding the charge transfer at the interface.

Low-dimensional quasi-two-dimensional (2D) perovskites – a mixed structure of 2D and three-dimensional (3D) perovskite lattice – are now the leading materials platform for the demonstration of functional optoelectronics. The dimensional confinements of perovskite lattice by molecular spacer cations and the resulting quantum-well structures bestow their unique functionalities including chemical robustness and strong light-emission properties. Despite its versatility, little has been known on the principle of phase control in quasi-2D perovskites. Herein, by using a robot-based high-throughput automated synthesis platform, we explore the phase formation behaviors of quasi-2D phenethylammonium-formamidinium perovskite system in a wide range of 2D (PEA₂PbI₄):3D (FAPbI₃) compositions. We observe that, in contrast to the stoichiometric consideration, incorporation of only 1% of the 3D component provides substantial emergence of n=2 2D phase, which is prominent up to the 3D ratio of 92%; after this composition, 3D-like phases that are more stable than the pristine 3D FAPbI₃ mainly emerge. Coupled with hyperspectral cathodoluminescence microscopy, we uncover that thermal annealing is necessary for thin film fabrication of quasi-2D perovskites, but concurrently involves the emergence of PbI₂. Our work provides practical insights into the fundamental principles in 2D perovskite phase control, further benefitting the functionalities of novel materials systems towards sustainable device applications.

Lead halide perovskites are promising candidates for future lasers as they are proven good gain medium, low-cost solution processable, and exhibit bandgap-tunable luminescence with high color purity and photoluminescence quantum yields. Sandwiching the perovskite layer into transport layers is inevitable during the integration of electrical devices for electrically pumped perovskite lasers. However, the appearance of perovskite/transport layer interfaces is absent in most reported optically pumped perovskite lasers, and is usually considered as detrimental factors to lasing actions. This is why the electrically pumped perovskite lasers are still not realized even with the realization of high injection current densities (> 1 kA/cm²) and achievement of optically pumped continuous wave (CW) perovskite lasers at room temperature. Here we report the treatment of perovskite polycrystal thin films with different transport layers effect of transport layers, and the amplified spontaneous emission (ASE) threshold are studied to investigate the influence of perovskite/transport interfaces on lasing actions. We demonstrate largely reduced ASE threshold (22.0%) and enhanced ASE intensity (18.6%) with introducing an additional hole transport layer poly(triaryl amine) (PTAA) on top of perovskite layer. We show that the key role of PTAA layer is to accelerate the hot carrier cooling process from 488 fs to 422 fs by extracting the excess hot holes of perovskites. With suppressed hot holes, the Auger recombination loss is largely reduced, which in turn reduces the ASE threshold. Our work for the first time exemplifies how to further reduce ASE threshold with transport layer engineering. This is critical to maintain the good gain medium properties of perovskites when integrating perovskites into electrical devices and could also have a broader impact on paving the way for future electrically pumped laser fabrication.

Semiconductor nanorods have often been observed to emit polarized light, which provide great potential for emission applications such as displays, optical communications, and biological labeling. In this study, controlled anisotropic growth of Cs₃Cu₂Br₅ nanorods is realized by a modified low temperature injection (LTI) method and relatively short chain octylammonium bromide ligand precursor. The self-trapped excitons (STEs) generated by strong electron-phonon coupling result in a large Stocks shift and finally a broad-spectrum luminescence centered at 455 nm is observed. The non-toxic nanorods exhibit good uniformity and higher aspect ratio than those previously reported, further providing polarized anisotropy of 0.26 for the sample population and degree of polarization (DOP) up to 0.88 ± 0.04 for single rods, both the highest yet reported for any perovskite-like material. Strong dielectric mismatch is confirmed to dominant the origin of polarized emission by a dielectric ellipsoid model. Further a thin film with polarized blue emission is obtained by photo-alignment method, enables the development of optoelectronic devices with polarized features such as LED and LCD.


Reference:
Ng, M., Geng, P., Shivarudraiah, S. B., Guo, L., Halpert, J. E., Synthesis of Cesium Copper Bromide Nanorods with Strong Linearly Polarized Emission. Adv. Optical Mater. 2022, 2201031.

Achieving high-quality cesium-formamidinium lead iodide (Cs_xFA_1-xPbI₃) perovskites with widely tuneable bandgaps is highly desired for their optoelectronic applications yet remains a challenge because of the different phase transition behaviour of FAPbI₃ and CsPbI₃ perovskite during film depositions.[1]^,[2] Herein, by utilizing an alkaline-interface-assisted cation-exchange method, we fabricate a set of highly emissive Cs_xFA_1-xPbI₃ perovskite films with fine-tuning Cs-FA alloying ratio for emission-tuneable near-infrared light-emitting diodes (NIR-LEDs). We reveal that the deprotonation of FA⁺ cations and the formation of hydrogen-bonded gels consisting of CsI and FA facilitated by zinc oxide underneath effectively removes the Cs-FA ion-exchange barrier, promoting the formation of phase-pure Cs_xFA_1-xPbI₃ films with emission filling the gap between that of pure Cs- and FA-based perovskites.[3] The obtained NIR-LEDs with narrow electroluminescence peaking from 715 to 780 nm simultaneously demonstrate high peak external quantum efficiencies of over 15%, maximum radiances exceeding 300 W sr^-1 m^-2, and high power conversion efficiencies above 10 % at 100 mA cm^-2, representing the best-performing devices based on solution-processed wavelength-tuneable NIR emitters in the similar region.[4]


[1] Li, Z., et.al. Chem. Mater. 28, 284-292 (2016)

[2] Lee, J.-W., et.al. Adv. Energy Mater. 5, 1501310 (2015)

[3] Z, Yuan., et.al. Nat. Commun 10, 2818 (2019)

[4] Z, Yuan., et.al. Joule 2022, https://doi.org/10.1016/j.joule.2022.08.003

Metal halide perovskite (MHP) nanocrystals have been recognized as a promising material class for next-generation realistic displays, owing to their excellent color purity. To adopt MHP nanocrystals to the next-generation displays (e.g., augmented reality displays) with high-resolution, precise patterning method should be developed to fabricate RGB pixel arrays. However, the traditional photolithography is not suitable for patterning colloidal MHP nanocrystals since the solvent in photoresist often results in the degradation of the emissive properties of MHP nanocrystals. Direct optical patterning is a photoresist-fee method to pattern MHP nanocrystals, which uses the change in solubility due to a chemical reaction upon light source irradiation. However, the photo-induced chemical reaction often affects the passivation of MHP nanocrystals which is critical in maintaining the photoluminescence quantum yield throughout the patterning process. Herein, we propose a nondestructive photo-induced in-situ ligand exchange mechanism for direct optical patterning of colloidal MHP nanocrystals. The MHP nanocrystals became photosensitive with thiol additives; high-resolution 2-µm line patterns of MHP nanocrystals were achieved without compensating the emissive properties, which is due to the effective passivation of in-situ-exchanged thiolate ligands. The patterning mechanism was in-depth investigated by analyzing the surface properties of MHP nanocrystals at each patterning step.

Direct optical patterning of colloidal nanomaterials has recently received a lot of attention because of photoresist-free simple patterning procedure. The use of photoacid generator (PAG) additive has been studied as one of the promising approaches for direct optical patterning of colloidal quantum dots (QDs). With PAG, high-resolution QD patterning without complete ligand exchange is enabled. However, direct optical patterning of QDs with PAG may result in decrease in photoluminescence quantum yield (PLQY) in case of insufficient surface passivation of patterned QDs. Furthermore, such PLQY decrease could be enhanced for colloidal InP-based QD, which has high oxophilicity. Therefore, systematic and fundamental studies to achieve high-resolution InP-based QD patterns while minimizing the surface damage are required. Here, we show high-resolution direct optical patterning of InP-based core/shell QDs using a triazine-based photoacid generator. We investigate the changes in optical and structural properties of the core/shell QDs with each patterning step. Also, our study involves ligand post-treatment for the PLQY recovery of patterned InP QDs. The post-treatment passivated the surface defects and thereby increased the PLQY of InP QDs by ~90%. We believe our study suggests a facile way of patterning air-sensitive colloidal nanomaterials for various electronic and optoelectronic applications.

Due to the advantages such as exceptional optoelectronic properties, readily tunable emission, high photoluminescence quantum efficiency (PLQE), narrow emission bandwidth, and solution processability, metal halide perovskites are attractive for applications in light-emitting diodes (LEDs). Despite the halide perovskite LEDs (PeLEDs) have achieved high external quantum efficiency (EQE) of above 20%, there are still many issues to be further investigated in the process of promoting the industrialization of PeLEDs. Unfortunately, the solution-processed polycrystalline perovskite films suffer from severe trap-mediated non-radiative recombination and ion migration. A tremendous amount of effort has been carried out to improve the performance of PeLEDs, including blocking iodide ion migration by strengthening the space restriction on carriers, preparing high-quality crystalline perovskites, and forming interface engineering between the ETL and the perovskite light-emitting layer. Since both ion migration and non-radiative recombination are highly related to the defects in perovskites, defect passivation is essential to achieve efficient and stable PeLEDs. Although ligand-assisted mediation has proven to be a useful method for passivating the imperfections of PeLEDs, the control strategy of alkyl chain length and the electron donating ability of the ligands bring great challenge to rational passivation. Herein, by considering the electron donating ability for the defect passivation efficiency and steric hindrance for the crystallization process, we find that methoxy group can be introduced to phenethylammonium molecular served as an efficient additive, namely 4-methoxy-phenethylammonium (4-MeO-PEA) to facilitate defect passivation process in perovskite light-emitting materials. We have demonstrated that the 4-MeO-PEA agent can substantially increase the crystal orientation, enlarge the crystalline grain size and mitigate the defect centers through strong bonding between the methoxy group with Pb ions. The external quantum efficiency (EQE) value of the PeLEDs with optimized passivation reaches to a maximum of 21.57%, which is one of the best records in the near infrared PeLEDs (~790 nm emission peak) to date. In addition, a threefold increase in the T₅₀ operational time of the devices is observed, compared to the control samples. The findings provide a simple and effective strategy to produce highly efficient perovskite films and to devise the high-performance optoelectronics devices.

The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. N_CUHK438/18, and CUHK Group Research Scheme.

Hot carrier (HC) cooling accounts for the significant energy loss in lead halide perovskite (LHP) solar cells. Here, we study HC relaxation dynamics in Mn-doped LHP CsPbI₃ nanocrystals (NCs), combining transient absorption spectroscopy and density functional theory (DFT) calculations. We demonstrate that Mn²⁺ doping (1) enlarges the longitudinal optical (LO)–acoustic phonon bandgap, (2) enhances the electron–LO phonon coupling strength, and (3) adds HC relaxation pathways via Mn orbitals within the bands. The spectroscopic study shows that the HC cooling process is decelerated after doping under band-edge excitation due to the dominant phonon bandgap enlargement. When the excitation photon energy is larger than the optical bandgap and the Mn²⁺ transition gap, the doping accelerates the cooling rate owing to the dominant effect of enhanced carrier–phonon coupling and relaxation pathways. We demonstrate that such a phenomenon is optimal for the application of hot carrier solar cells. The enhanced electron–LO phonon coupling and accelerated cooling of high-temperature hot carriers efficiently establish a high-temperature thermal quasi-equilibrium where the excessive energy of the hot carriers is transferred to heat the cold carriers. On the other hand, the enlarged phononic band-gap prevents further cooling of such a quasi-equilibrium, which facilitates the energy conversion process. Our results manifest a straightforward methodology to optimize the HC dynamics for hot carrier solar cells by element doping.

Electrically pumped lasers or laser diodes based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. Such devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and colloidal quantum dots (QDs). The latter materials are especially attractive for implementing laser diodes as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, a low optical-gain threshold, and high temperature stability of lasing characteristics stemming from a wide energy separation between their atomic-like discrete energy levels.

Several challenges complicate the realization of colloidal QD laser diodes (QLDs). These include extremely fast nonradiative Auger recombination of optical-gain-active multicarrier states, poor stability of QD solids under high current densities required to achieve lasing, and unfavorable balance between optical gain and optical losses in electroluminescent (EL) devices wherein a gain-active QD medium is a small fraction of the overall device stack comprising multiple optically-lossy charge-transport layers.

Here we resolve these challenges and achieve electrically driven laser action due to amplified spontaneous emission (ASE) in a colloidal-QD optical-gain medium. To demonstrate this effect, we employ compact, continuously graded QDs with strongly suppressed Auger recombination incorporated into a low-loss photonic waveguide integrated into a pulsed, high-current density light-emitting diode. These prototype QLDs exhibit strong, broad-band optical gain and demonstrate low-threshold, room-temperature laser action which leads to intense edge-emitted EL with intensity of more than 100 microwatts.

Many functional nanomaterials used for catalysis, healthcare, solid-state lighting, and displays are synthesized by colloidal techniques. The scope of chemical transformations accessible to colloidal chemists is determined by the stability and compatibility of solvents and surfactants used as a reaction medium. For example, very few traditional solvents can handle temperatures above 400C, while many inorganic phases require even higher temperatures to form. We are developing comprehensive understanding of a novel class of colloidal systems, colloids in molten inorganic salts. Nanoparticles of different transition metals, semiconductors, oxides, and magnetic materials can form true colloids in molten inorganic salts. The colloidal stability of nanoparticles in molten salts could not be explained by traditional electrostatic and steric stabilization mechanisms. Our experimental and computational studies point to the importance of the long-range ion correlations in the molten salt near the nanocrystal interface.
In addition to the fundamental exploration of new colloidal systems, molten salts expand the boundaries for solution synthesis of many nanomaterials that have been out of reach for colloidal chemists. We have used molten salts to synthesize colloidal GaAs, In_xGa_1-xAs, In_xGa_1-xP and GaN quantum dots, which resisted numerous synthetic attempts for over two decades. By further developing colloidal chemistry in molten salts, we hope to enable synthetic routes toward various functional nanomaterials previously considered unsynthesizable by colloidal methods.

The presence of impurities or defects in most quantum dots leads to irreversible carrier trapping. However, high quality InP/ZnSe/ZnS quantum dots (QDs) behave very differently. In this case, excess indium in the ZnSe shell of gives rise to hole traps that are transiently populated following photoexcitation. This leads to a slow (hundreds of picoseconds to nanoseconds) photoluminescence (PL) risetime.^[1] Equilibrium between these traps, which are optically dark, and the emissive valence-band state also results in delayed emission and therefore longer PL lifetimes. The radiative lifetime can be varied from 32 to 48 ns, depending on the density of traps. Transiently trapped holes also affect the biexciton kinetics, allowing for variation of the electron versus hole Auger excitation ratios under high flux. In addition, the amplitude and time scale of the slow PL rise and the delayed emission time can be varied by different treatments of the InP cores prior to shell deposition, as indicated by resonance Raman spectroscopy. The Raman spectra are interpreted in terms of different radial distributions of the indium-based hole traps which can be related to differences in the interfacial lattice strain. Modulating the core/shell interface can also create interfacial dipoles varying the trap depth, in extreme cases leading to 10% of the photoluminescence coming from repopulation of the band edge by deep traps that occurs on the microsecond timescale.^[2]
Several possible chemical and structural assignments of the traps are considered, and a substitutional indium adjacent to a zinc vacancy, In³⁺/VZn^2- , is found to be the most likely.^[3] This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. We also show that addition of alkyl carboxylic acids causes increased trapping, presumably by creation of additional zinc vacancies.
This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Award Number DE-EE0009164 and NSF CHE-1506803.


[1] P. Cavanaugh et al., Radiative Dynamics and Delayed Emission in Pure and Doped InP/ZnSe/ZnS Quantum Dots. J. Chem. Phys. 2021, 155, 224705 - 224715.
[2] H. Sun et al. Reversible Interfacial Charge Transfer and Delayed Emission in InP/ZnSe/ZnS Quantum Dots with Hexadecanethiol. J. Phys. Chem. 2022, in press.
[3] A.M. Kelley et al., Identity of the reversible hole traps in InP/ZnSe core/shell quantum dots. J. Chem. Phys. 2022, in press.

Colloidal ZnS:Cu nanocrystals (NCs) are prototypical colloidal nanophosphors in bio-imaging and optoelectronic applications thanks to their bio-compatibility and broadly tunable luminescence properties. Visible luminescence from ZnS:Cu colloidal NCs is typically dominated by green and blue peaks (denoted G-Cu and B-Cu), with observations of red (R-Cu) luminescence from nanocrystalline ZnS:Cu remaining extremely limited. Here, we present a synthetic method for obtaining colloidal ZnS:Cu NCs that emit primarily R-Cu. We use time- and temperature-resolved luminescence spectroscopies to study the emission mechanism, which provides advanced knowledge of the electronic structure produced by defects in ZnS:Cu that is critical for controlling the luminescence properties of this material. We also discuss the applicability of colloidal nanocrystals containing color centers to quantum information science, where electronically isolated defect states can serve as optically addressable spin qubits.

Lighting and displays are integral parts of human activities and economic development. Semiconductor nanocrystals, now offering a market volume exceeding 1 Billion Euros annually, have attracted great interest in quality lighting and displays in the last decade. Such colloidal semiconductors enable enriched color conversion essential to superior lighting and displays. These colloids span different types and heterostructures of semiconductors, starting in the form of colloidal quantum dots and most recently extending to the latest sub–family of nanocrystals, the colloidal quantum wells. In this talk, we will present most recent examples of photonic structures and device architectures using the colloidal quantum wells for lighting and displays. Also, we will present a powerful, large–area self–assembly technique for orienting these colloidal quantum wells either all face down or all edge up. We will demonstrate three–dimensional constructs of their oriented self–assemblies with monolayer precision. Finally, we will show record high–efficiency colloidal LEDs of these quantum wells employed as the electrically–driven emitter layer. Given their current accelerating progress, these solution–processed quantum wells hold great promise to challenge their epitaxial thin–film counterparts in semiconductor optoelectronics in the near future.

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate size-controlled synthesis of low-phonon-energy potassium lead halide nanoparticles and the ability to tune nanocrystal phonon energies. These nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of these nanoparticles promote upconversion emission from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission when doped with neodymium ions. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Multifunctional skin-interfaced electronics is one of the most anticipated medial Internet of Things (IoT) technologies that enables non-invasive collection of high-quality data on biological activity. This is realized thanks to their wearability and conformal coverage of our body. Building on these data and recent advances in deep learning, artificial intelligence-based bioimaging, diagnosis, and therapy will be performed with higher accuracy and speed than ever before. Furthermore, skin-like multifunctional sensors are revolutionizing the technologies and applications of soft robotics. This talk will first focus on the current knowledge gap in designing intrinsically deformable optoelectronic materials using low dimensioal perovskite. We will then discuss new material design strategies to achieve mechanical deformability in perovskite optoelectronic material and devices.

Lead halide perovskites are solution processable semiconductors, which have shown promise for applications in solar cells and light emitting diodes. Further, the feasibility of device fabrication at low- temperature (100 °C) opens the door to explore new opportunities on integrated lasers on-chip, waveguide lasers on the silicon nitride (Si₃N₄) platform [1] and other electro-optical modulators [2]. However, perovskites have much higher refractive indices (n = 2.3-2.6) than that of a typical Si₃N₄ waveguide (n = 2). Such a contrasting refractive index at the perovskite/Si₃N₄ interface makes it challenging to extract light from the perovskite lasers. In this work, we address this challenge by using the perovskite as the emission medium and the Si₃N₄ as the cavity medium. Upon optical pumping, photons emitted from the perovskite couple to the cavity via the evanescent field for light amplification leading towards lasing on-chip ([1], [2]).

In this study, the cesium lead bromide (CsPbBr₃) thin film (90 nm) is used as a laser gain medium and Si₃N₄ as a distributed feedback (DFB) medium. The DFB constitutes of a 200 nm thick Si₃N₄ rib waveguide with a quarter-wavelength shifted first-order grating structure etched directly into the Si₃N₄ waveguide. Using a finite difference time domain simulation (Lumerical software), the grating dimensions and the cross-section of the Si₃N₄ waveguide was optimised to maximize the effective refractive index of the waveguide mode. The mode overlap between the perovskite film and Si₃N₄ waveguide mode was identified to be ~24%. Using the optimised configuration obtained from the simulation studies, our experiments were designed to realise the perovskite DFB lasers on a 6” Si wafer with 2.3 μm of thermally grown SiO₂. The experimental workflow to achieve a high-quality DFB system will be presented. 90 nm thick CsPbBr₃ film was spin coated on the DFB structure followed by a planar hot process to reduce the grain boundary defects [3]. The perovskite film was patterned as reported previously ([1], [3]) and encapsulated with a 1 µm thick PMMA protective layer. The device was optically pumped using a 300 nm pulsed excitation source with 300 ps pulse width and 1 kHz repetition rate at room temperature. At low excitation fluence, the weak photoluminescence signal showed a broad linewidth (16 nm). With increasing pump fluence, a single peak with ultranarrow linewidth (0.24 nm) was observed. An S-shaped curve for the emission intensity vs pump fluence confirming a clear sign of lasing with a threshold power density of 775 μJ/cm² will be presented. Further, the change in the emission wavelength upon varying the grating periodicity and etch depth will be presented. Our work demonstrates a DFB CsPbBr₃ laser on a silicon nitride waveguide platform operating at room temperature, which opens new avenues for integrated optoelectronics.

Acknowledgments: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 956270 and NRW project PEROVSKET funding code No EFRE-0801508.

References

[1] P. J. Cegielski et al., ‘Monolithically Integrated Perovskite Semiconductor Lasers on Silicon Photonic Chips by Scalable Top-Down Fabrication’, Nano Lett., vol. 18, no. 11, pp. 6915–6923, Nov. 2018.
[2] P. J. Cegielski et al., ‘Integrated perovskite lasers on a silicon nitride waveguide platform by cost-effective high throughput fabrication’, Opt. Express, vol. 25, no. 12, pp. 13199–13206, Jun. 2017.
[3] N. Pourdavoud et al., ‘Room-Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films’, Adv. Mater., vol. 31, no. 39, p. 1903717, 2019.

The threshold carrier density, conventionally evaluated from optical pumping, is a key reference parameter towards electrically pumped lasers, with the widely acknowledged assumption that optically excited charge carriers relax to the band edge through an ultrafast process. However, characteristically slow carrier cooling in perovskites challenges this assumption. Here, we investigate the optical pumping of state-of-the-art bromide- and iodine-based perovskites. We find that the threshold decreases by one order of magnitude with decreasing the excitation energy from 3.10 eV to 2.48 eV for methylammonium lead bromide perovskite (MAPbBr₃), indicating that the low-energy photon excitation facilitates faster cooling and hence enables efficient carrier accumulation for population inversion. Our results are then interpreted due to coupling of phonon scattering in connection to the band structure of perovskites. This effect is further verified in two-photon pumping process, where the carriers relax to the band edge with a smaller difference in phonon momentum that speeds up the carrier cooling process. Furthermore, by extrapolating the optical pumping threshold to the band edge excitation as an analog of electrical carrier injection to the perovskite, we obtain a critical threshold carrier density of ~1.9 X 10¹⁷ cm^-3, which is one order of magnitude lower than that estimated from the conventional approach. Our work thus highlights the feasibility of metal halide perovskites for electrically pumped lasers.

Metal halide perovskites have emerged as promising gain media with the potential to realize wavelength-tunable, non-epitaxial laser diodes. This talk will focus on recent progress toward this goal, including continuous-wave optically-pumped lasing from methylammonium lead iodide (MAPbI₃), the operation of MAPbI₃ LEDs at ~kA/cm² current densities, distributed feedback MAPbI₃ LEDs that lase under optical pumping, and combined electrical+optical pumping experiments that demonstrate a reduction in the optical threshold for amplified spontaneous emission when the optical pulse overlaps in time with a 3 ns electrical pulse injecting >1 kA/cm² at low temperature.

Layered two-dimensional organic-inorganic hybrid perovskites (OIHPs) are promising materials for light emitting purposes owing to their higher exciton binding energies compared to the three-dimensional lattices. Stacking of the 2D layers into ordered phases such as Ruddlesden-Popper phase with well-formed external morphologies is expected to create nanostructures that will bring in nano-confined optical properties while maintaining the quantum-confined electronic nature of layered perovskite sheets. Coupling band excitons with optical cavity modes, for example, has been exploited in nanowires (NWs) and nanoplatelets of 3D perovskites to enable ultra-small, low-threshold lasers. The cavity-coupled light emission is expected to experience a substantial enhancement if the nanostructures are made of layered 2D perovskite sheets. In this presentation, I will show the morphology-controlled synthesis of NWs of monolayered butylammonium lead bromide, (C₄H₉NH₃)PbBr₄, via chemical vapor deposition (CVD). We have systematically investigated the morphological outcomes with independent growth parameters including source temperatures, reactor pressure, and gas flow rates, which is well-summarized in a morphology phase diagram. Structural, elemental, crystallographic analyses will be presented to validate the quality of our NWs. We have observed a sharp, quantized absorption around 400 nm and strong photoluminescence at 405 nm, an indication of the presence of layered (C₄H₉NH₃)PbBr₄ sheets. I will present time-resolved photoluminescence and variable-temperature measurements to show stronger exciton binding characteristics in NWs of 2D (C₄H₉NH₃)PbBr₄ than of 3D CsPbBr3. Lastly, I will share the amplified light emission behaviors of the NWs upon pulsed excitation. Our NWs of layered OIHPs present potential to develop ultra-compact, coherent ultraviolet and visible light sources.

During carrier multiplication (CM) a high-energy, ‘hot’ electron or hole relaxes within the same band by creating new electron-hole (e-h) pairs.¹ This occurs via one or more impact ionization events whereby a pre-existing valence-band electron is excited to the conduction band after an Auger-type collision with a high-energy carrier. In principle, CM could improve the performance of a variety of optoelectronic, photovoltaic (PV) and photocatalytic devices because due to this process, the quantum efficiency of photon to e-h pair conversion (Qeh) becomes greater than one.^2,3 However, practically realized CM efficiencies are still not sufficiently high to achieve an appreciable boost in device performance, suffering from the high CM threshold (hv_th) and the high e-h pair creation energy (e_eh).
The use of engineered colloidal quantum dots (QDs) allows one to reduce hv_th down to the energy-conservation-defined value of 2E_g.⁴ However, e_eh is not appreciably decreased compared to bulk solids. To reduce e_eh, one needs to increase the energy-gain rate associated with Coulomb collisions versus the phonon-related energy-loss rate. Here, to accomplish this objective, we exploit not ‘direct’ but ultrafast ‘spin-exchange’ Coulomb interactions in magnetically doped QDs.^5,6 For this purpose, we synthesize Mn-doped core/shell PbSe/CdSe QDs wherein the dopants exhibit strong spin-exchange coupling to both CdSe and PbSe QD components. By applying transient photoluminescence measurements, we observe a highly efficient excitation transfer from the light-harvesting CdSe shell to the Mn dopants, which is followed by a CM-like spin-exchange process which leads to generation of two core excitons. Due to spin conservation, the biexciton produced via relaxation of the excited Mn ion is a combination of a dark and a bright exciton (spins 1 and 0, respectively) in two different L-valleys of PbSe. The corresponding quantum efficiency measured at 2.5E_g is ~140%, almost three-fold enhancement versus undoped QDs, implying that the e-h pair creation energy is less than 1.25E_g. This is near the fundamental one-bandgap limit and is also considerably smaller (a factor of >2.5) than for the reference undoped QDs. These results suggest that the use of spin-exchange interactions represents a viable approach for realizing highly efficient CM-enhanced solar-photoconversion schemes.
1. Klimov, V. I. Ann. Rev. Condens. Matter Phys. 5, 285-316 (2014).
2. Semonin, O. E. et al. Science, 334, 1530-1533 (2011).
3. Yan, Y. et al. Nat. Energy 2, 17052 (2017).
4. Cirloganu, C. M. et al. Nat. Commun. 5, 4148 (2014).
5. Singh, R., Liu, W., Lim, J., Robel, I. & Klimov, V. I. Nat. Nanotech. 14, 1035-1041, (2019).
6. Livache, C, et al. Nat. Photon. 16, 433-440 (2022).

Bandgap tuning is a crucial characteristic of metal-halide perovskite, with benchmark lead-iodide compounds having a bandgap of 1.6 eV. To increase the bandgap up to 2.0 eV a straightforward strategy is to partially substitute iodine with bromine in so-called mixed-halide lead perovskites. Such compounds are prone, however, to defect-induced halide segregation resulting in bandgap instabilities, which limits their application in tandem solar cells and a variety of optoelectronic devices. The optimization of the perovskite composition and surface passivating agents can effectively slow down, but not completely stop, such light-induced instabilities. Here we identify the defect and the intra-gap electronic state that triggers the material transformation and bandgap shift. It allows us to engineer the perovskite band edge energetics to radically deactivate the photo-activity of such defects and stabilize the perovskite bandgap over the entire spectral range above 1.6 eV. Then, I will discuss how the generation of I2 in mixed-halide perovskites under illumination has a crucial role in halide segregation. In fact, I2 reacts with bromide (Br−) in the lattice to form a trihalide ion I2Br-, which mediates a Br−/I− ion exchange reaction and the formation of an I-rich phase. Importantly, we observe that the efficiency of the process is highly dependent on the binding strength of the bromide within the crystalline structure. Based on this, we propose a general paradigm where an iodine redox based model rationalizes the initiation and growth of halide photo-segregation. Thus, we show how the design of the chemical composition of the metal-halide perovskite crystalline unit can stabilize the semiconductor bandgap, and eventually the performance of related solar cells, not only acting on the reduction of the density of native defects, but stabilizing the elemental composition of the perovskite crystalline unit. This is demonstrated by the fabrication of photo-stable purely inorganic CsPbI1.5Br1.5 solar cells.

Circularly polarized photoluminescence (CPL) have been observed in organic-inorganic hybrid perovskites. In general, CPL require the orbital momentum in light-emitting states that can be circularly polarized by a photoexcitation and substantially conserved within PL lifetime. Both chiral structures and Rashba band structures can fulfill this requirement: generating circularly polarizable orbitals with sufficient conservation time constant to develop CPL. In chiral structures, our experimental studies found that light-emitting states establish chirality dynamics with ultrafast response (< picoseconds) and then conserve their circular polarization up to nanoseconds, enabling CPL. This provides a circular polarizer functionality in light emission shown in chiral structures. Furthermore, spin scattering between circularly polarized light-emitting states can shorten the conservation time constant of circularly polarized orbitals, decreasing CPL. This indicates that spin interaction between circularly polarized orbitals plays an important role in developing CPL. In Rashba band structures, we found that left-hand and right-hand circularly polarized orbitals can be optically operated between spin-up and spin-down bands to generate selective CPL by using circularly polarized photoexcitation. Moreover, we found that spin polarizations can directly interact with circularly polarized orbitals in Rashba band structures through spin-exchange coupling, generating spin-switchable phenomena in 2D-superlattice perovskites. This demonstrates switchable CPL between left-hand and right-hand circular polarizations when changing spin polarizations between positive and negative magnetic field directions. This indicates that the spin-exchange coupling functions as the key parameter towards developing CPL in Rashba band structures in hybrid perovskites.

Periodicity in long range-ordered nanocrystal superlattices (NC SLs) can be exploited to alter or even tailor the properties of the individual NC building blocks while generating new collective electronic and optical phenomena as a result of the enhanced interactions between NCs. Perovskite NCs have emerged as highly attractive building blocks for SLs, based on their facile fabrication as sharp, monodisperse cubes as well as due to the observation of superfluorescence from single and multi-component perovskite NC superstructures.
Binary superlattices of the ABO₆ type, produced via self-assemblies of CsPbBr₃ NCs with sizes in the strong (~5 nm) and weak (~18 nm) confinement regime are being demonstrated for the first time. We discuss the optical properties of such binary SLs, focusing on the electronic interactions between the strongly and weakly-confined NC excitons. Transient photoluminescence and absorption measurements indicate efficient depletion of the small NC exciton population at the few to tens of ps scale; a concomitant delay of the transient absorption rise time signal of the large NC exciton bleaching is also observed. Such results are consistent with a highly efficient energy transfer process from the small to the large NCs within the binary superstructure. On-going experiments are investigating whether the fast energy funneling into the weakly confined NC excitons can compete and influence the characteristics and dynamics of the superfluorescence observed in such binary perovskite SLs.

Colloidal nanocrystals (NCs) have emerged as a diverse class of materials with tunable composition, size, shape, and surface chemistry. From their facile syntheses to their unique optoelectronic properties, these solution-processed NCs are a promising alternative to materials grown as bulk crystals or by vapor-phase methods. However, the use of NCs in real-world devices is often held back by challenges in depositing them as a patterned two-dimensional layer. Alternative approaches to patterning colloidal NCs need to be established to aid the transition from individual devices to integrated optoelectronic circuits, multi-color NC displays, and nanophotonic devices. This talk will focus on the development of photo-chemical methods that enable direct optical lithography of colloidal nanocrystals. We will discuss various light-sensitive ligands and their suitability for patterning various NCs including lead halide perovskite NCs, semiconducting quantum dots, metal oxide nanoparticles, and lanthanide-doped upconverting NCs. We will show that this patterning approach preserves many of the desirable properties of the NCs, such as high photoluminescence, good charge transport, and high refractive index. Ultimately, this work aims to expediate the implementation of colloidal NCs in a broader range of functional devices.

Metal halide perovskites (MHPs) have been considered next-generation light-emitting materials due to their excellent emission property of narrowband emission. Still, even after various material types of perovskites including 3D, low-dimensional quasi-2D, and colloidal nanocrystal structures were studied, perovskite light-emitting diodes (PeLEDs) have not realized high luminance, high efficiency, and long lifetime simultaneously, because of the intrinsic limitations related to the trade-off properties between charge transport and confinement in each type of perovskite materials. Here, we report an ultra-bright, efficient, and stable PeLEDs made of core/shell perovskite nanocrystals with a size of ~10 nm obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with 3D polycrystalline perovskite films without separate synthesis process. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement while maintaining good charge-transport properties of 3D perovskites. We demonstrate simultaneously efficient, bright, and stable PeLEDs that have maximum brightness of ~470,000 cd m^-2, maximum external quantum efficiency of 28.9% (maximum current efficiency of 151 cd A^-1), and half-lifetime of 520 h at 1,000 cd m^-2 (estimated half-lifetime >30,000 h at 100 cd m^-2). These results suggest that PeLEDs are not only laboratory-level high-efficiency devices but also promising candidates for commercial self-emissive displays and lighting applications.

Understanding the excited state and transport dynamics of self-trapped excitons (STEs) in lead-free double perovskites (DPs) is essential for popularizing their optoelectronic applications. Herein, we apply a series of spectroscopies and microscopies techniques to systematically investigate the formation, transport, and relaxation dynamics of SETs in Bi-doped double perovskites (Bi-DPs). The PLQY, consistent with the STEs activation energy of Bi-DPs shows an increasing and then decreasing trend as a function of Bi³⁺ content, suggesting tiny Bi³⁺ can passive the defects and more Bi could generate more non-radiative decay channels. Moreover, we find the Bi-DPs exhibit dual white emission, originating from the intrinsic STEs (i-STE) and Bi³⁺ induced STEs (Bi-STEs), respectively. The STEs formation and relaxation timescale will be detailed discussed in as well. Finally, we will present the temperature-active transport behavior. The fundamental mechanism understanding of the doping engineering established in this work will provide helpful guidance about how to improve the performance of DPs optoelectronic devices.

Perovskite nanocrystals functionalized with chiral molecular ligands exhibit unique optical activity, making them promising materials for optoelectronic applications. However, the ligand type has been limited to cationic chiral amines in majority of the studies. In order to expand the ligand selection, chiral anionic binaphthyl molecules with phosphate functional groups have been introduced to synthesize CsPbBr<span style="font-size:10.8333px">₃ </span>nanocrystals. Tramission electron microscopy, X-ray diffraction, together with UV-vis spectroscopy revealed that the nanocrystals are plateletes, with a thickness of five unit cells. The nanoplates show distinct circular dichroism signal and maintain their chiral properties after purification using anti-solvent. They also exhibited linearly polarized emission in solution due to the anistropy in their geometry.

CsPbI₃ quantum dots (QDs) are promising light-emitting materials for pure-red display applications. Several methods have been tried for perovskite to obtain the red-light emission in the range 620-650 nm, including mixed-halide perovskite CsPbI_xBr_3-x, quazi-2D perovskite, and quantum-confined CsPbI₃. Among those methods, quantum-confined CsPbI₃ is the most exciting material as it is not subject to ion migration effects or contains organic cations that lead to spectral instability. Unfortunately, small colloidal quantum dots contain excess resistive ligands, negatively affecting the efficiency and stability of the resulting light-emitting diodes (LEDs). Here we developed a facile ligand-exchange method using amino acids to reduce native long-chain ligands on CsPbI₃ quantum dots. As a molecule with amine and carboxylate groups, amino acids can mildly exchange the oleate ligands and oleylammonium ligands on the surface of quantum dots. We also assessed a variety of related amino acids on their side-chain effects on LED performance. We found that in cysteine-exchanged QDs, both thiol and amino acid groups passivated the surface of quantum dots and led to the best LED performance. The optimized LEDs achieved an external quantum efficiency (EQE) of 18.0% and a T₅₀ of 87 minutes, comparable with many of the best-reported perovskite pure red LEDs.

Halide perovskites have attracted great attentions for optoelectronic elements with excellent quantum efficiency. Recent studies have focused on lead-free, tin halide perovskites with low-toxicity, but the low oxidation barrier of tin induces substantial in-gap states, which are responsible for non-radiative recombination. Previous reports show that addition of reducing agents during the material syntheses can enhance their photoluminescence (PL) quantum efficiency, however it is still unclear if the PL intensities are simply enhanced by reducing the oxidation states or changes in the electrochemical potential of the material also play a key role. Here, we show that van der Waals (vdW) contacts of aluminum (Al) onto two-dimensional (PEA)₂SnI₄ (PEA = phenylethylammonium) films after the material synthesis can still enhance its PL intensity by 4 times, reaching a record high quantum yield of 10 % for (PEA)₂SnI₄. The PL intensities do not increase in the cases of vdW contacts of copper and gold. Ultraviolet photoelectron spectroscopy measurements show that the energy level of electronic bands for (PEA)₂SnI₄ are shifted by 0.6 eV with the Al contact, compared to that of the as-grown sample, indicating a significant n-type doping by Al. Our results suggest that electron doping can effectively suppress the PL quenching in tin-based perovskites, mediated by defect-states.

So far, perovskite light-emitting diodes (PeLEDs) research topics highlight that perovskite emission intensity and charge carrier lifetime can be significantly optimized through fine-tuning charge carriers' transport and recombination properties. Among explored approaches for charge carrier recombination optimization, we find that exploiting dipole-dipole interaction is highly promising. We propose blending poly (4-vinylpyridine)-P4VP with Cs_xPbBr_y allow the formation of dipole-dipole interaction between pyridine-N and Pb²⁺. Simultaneously, the pyridine-N act as a "counterion" for the Cs_xPbBr_y precursor to improving precursors solubility by promoting Pb²⁺ cation dispersion, thereby assisting in suppressing the hexagonal-Cs₄PbBr₆ phase growth. As a result, the growth of an orthorhombic-CsPbBr₃ unitary crystalline phase is achieved. Interestingly, dipole-dipole interaction induces the polaron region formation between P4VP and CsPbBr₃ to order the distribution of excess carriers under an electric field, thus preventing disordered surface charges from structurally damaging the crystal. To optimize charge carrier transport, the hole transport layer was mixed with graphene quantum dots to optimize HOMO alignment between hole transport and emissive layer, significantly improving and achieving high-efficiency interlayer hole transmission. Accordingly, we conclude that radiation recombination of carriers substantially enhances the stability and operation lifetime of PeLEDs. Furthermore, the transport properties of carriers play an essential role in enhancing PeLEDs luminance. Ultimately, we fabricated PeLEDs with a peak luminance of 20420 cd m⁻² and a fatigue half-life four times higher than the original PeLEDs. Our PeLEDs stable performance highlight charge carrier engineering importance and open a path for future commercial development of PeLEDs.

Although metal halide perovskites have moderate mechanical flexibility compared with conventional inorganic semiconductors, it is still higher than those of organic semiconductors or conjugated polymers used in flexible/stretchable electronic devices.[1] Therefore, it is a good approach to combine perovskite nanocrystals (PeNCs) with organic/polymeric materials for stretchable photoluminescence-type color-conversion displays. PeNCs are promising emitters due to their high color purity, solution processability, and easily tunable bandgap.[2] However, the synthesis of PeNCs needs complicated synthetic processes that require multiple purification steps including antisolvent reprecipitation, which also can bring about issues related on the aggregation of PeNCs.[3]
Here, we present stretchable blue-emitting color-conversion films composed of in-situ formed CsPbBr₃-based PeNCs by incorporating bulky aromatic ammonium ligands in perovskite precursor solution. The phase-controllable in-situ fabrication of inorganic PeNC film emits primary blue (460 nm), which can achieve near 100% of Rec.2020 color gamut. By the method, PeNCs can be directly fabricated on any types of substrates through phase-controlled in-situ PeNC growth. The in-situ fabricated PeNC films showed improved photoluminescence, mechanical stability, and ambient stability compared to those of conventional bulk perovskite film. Furthermore, a multilayer fabrication of stretchable color-conversion-layer enables simple adjustment of blue photoluminescence and color temperature of white light.

1. Han, T.-H. et al. Spontaneous hybrid cross-linked network induced by multifunctional copolymer toward mechanically resilient perovskite solar cells. Advanced Functional Materials 32, 2207142 (2022).
2. Han, T.-H. et al. A roadmap for the commercialization of perovskite light emitters. Nature Reviews Materials 7, 757–777 (2022).
3. Cheng, Y.-H. et al. Gel permeation chromatography process for highly oriented Cs3Cu2I5 nanocrystal film. Scientific Reports 12, 4620 (2022).

Organo-metallic Halide Perovskite (OMHP) thin films have been of great interest in the field of photovoltaics due to the rate at which their power conversion efficiency (PCE) has increased while maintaining low production costs. Perovskite quantum dots (PQDs) have also garnered interest for similar applications. Despite this, both PQDs and OMHP thin films have their drawbacks. Thin films have chemical stability issues, particularly in the presence of moisture and oxygen, while PQDs have not demonstrated efficiencies as high as their thin film counterparts. Some of these issues could be addressed by combining the two to form heterostructures of PQDs layered atop OMHP thin films. We have investigated charge and energy transfer mechanisms between spin-coated methylammonium lead iodide (MAPI) thin films and PQDs, where we varied both the PQD composition and the ligand molecules used for functionalizing their surfaces. We have used organic-inorganic PQDs (MAPbBr₃) with insulating and conducting ligands, and inorganic PQDs (CsPbBr₃) with insulating ligands, for a systematic study. All the selected PQDs have band gaps larger than that of MAPI, allowing for multiple routes of energy and charge transfer, not just between the PQDs and the MAPI film but also between the quantum dots themselves. Using photoluminescence (PL) spectroscopy, we have found that the emission from both MAPbBr₃ and CsPbBr₃ dots was quenched, while the MAPI film emission intensity was increased. Further, the charge carrier recombination lifetimes of the MAPI films with PQDs were significantly longer, nearly twice as long, compared to bare films, indicating that the PQDs were passivating the surface trap states in the films. However, all these changes in the thin films’ optical properties depended on the types of ligands on the PQD surfaces, the excitation energy used on the heterostructures, and even the density of PQDs on the film. In particular, we discovered that a high density of close-packed PQDs reduced both emission intensity and recombination lifetimes of the MAPI films. Our results demonstrate an approach to modulating thin film behavior that could have applications not only in the field of traditional optoelectronics but also in novel areas, such as environmental sensing and detection.

The authors acknowledge funding from NASA grant no. NNH18ZHA008CMIROG6R and NSF grant no. NSFDGE-2125510.

Solution-processed organo-lead halide perovskite has emerged as a promising next-generation light-emitting devices owing to the low process cost, wavelength tunability from visible to near-infrared light, and narrow full width half maximum (FWHM) characteristics [1]. Unlike perovskite materials used in photovoltaic devices, the materials used in light-emitting devices often require small grain sizes to spatially confine excitons at nano scale, thus ensure high luminous efficiencies by inhibiting exciton dissociation and increasing the radiative recombination rate. However, it is difficult to achieve high emission intensity with small grain perovskites at a large thickness, because the large framework may aggravate defects such as pinholes and grain boundary gaps [2]. Also, a thin film with small grain perovskites may generate high density of non-radiative recombination centers at the surface defects via deformation and decomposition [3].
In this work, we propose to use 4-fluorobenzylammonium iodide (FPMAI), a bulky organo-ammonium halide additive, for surface treatment through recrystallization of methylammonium lead iodide (MAPbI₃) perovskite thin film, thereby decreasing the grain sizes and suppressing defects in the perovskite film at relatively large thickness. The FPMAI molecules are well attached to MAPbI₃ nanocrystals and has high surface coverage due to its short alkyl chain length and high binding energy to perovskite. Also, we realized it is an appropriate additive to form perovskite nanocrystals with reduced defect sites in a thin film with relatively large thickness. We have fabricated FPMAI-treated perovskite thin films and demonstrated an enhanced optical gain of the perovskite films by showing the amplified spontaneous emission (ASE) under picosecond pulse pumping. As a result, the ASE threshold are reduced approximately 6 μJ/cm², and the optical gain coefficient are increased from 316.3 cm^-1 to 507.1 cm^-1. Furthermore, we have measured significant increases in photoluminescence (PL) intensity and PL lifetime, thus show that non-radiative recombination processes are suppressed in the perovskite films with FPMAI surface treatment. Our result shows that the optical gain is improved through the surface treatment method with organo-ammonium halide additives, and it can be applied to highly efficient light emitting devices in the future organic optoelectronic systems.

[1] FU, Yongping, et al. Metal halide perovskite nanostructures for optoelectronic applications and the study of physical properties. Nature Reviews Materials, 2019, 4.3: 169-188.
[2] CHENG, Huiyuan, et al. Understanding and minimizing non-radiative recombination losses in perovskite light emitting diodes. Journal of Materials Chemistry C, 2022.
[3] KIM, Young-Hoon; CHO, Himchan; LEE, Tae-Woo. Metal halide perovskite light emitters. Proceedings of the National Academy of Sciences, 2016, 113.42: 11694-11702.

Metal halide perovskites (MHPs) have been considered as a promising light-emitting materials due to their high color purity, tunable bandgap energy, low-cost synthesis, and solution processability, which is appropriate for low cost high-color-purity display applications.¹ However, solution processed polycrystalline perovskite thin films inevitably contains many charged defects at the grain boundaries and film surfaces.²The charged defects function as an easy ion migration paths, which accelerate degradation of materials and devices during the operation.
In this work, we present synergetic strategy that includes A-site cation engineering and introduction of polymeric capping ligands to form an in-situ fabricated highly oriented cube-shaped polycrystalline perovskite thin films. The crystal growth is effectively controlled to have a low-charged defect density, well-oriented nanosized cubic crystals by introducing polymeric ligands and A-site cation with different ionic radius during the crystal growth of perovskite thin film. The light-emitting diodes that use the in-situ fabricated perovskite nanocube thin films as an emitting layer shows higher luminous efficiencies, and much improved operational stability compared with those of conventionally used MHP light emitter thin film.

1. Han, T.-H. et al. A roadmap for the commercialization of perovskite light emitters. Nature Reviews Materials 7, 757–777 (2022).
2. Han, T.-H. et al. Interface and defect engineering for metal halide perovskite optoelectronic devices. Advanced Materials 31, 1803515 (2019).

Perovskite-based light emitting diodes (PeLEDs) are promising candidates for next-generation display technologies due to their potentials such as solution processing, excellent color purity, and color tunability. However, the blue-emitting PeLEDs lag far behind those of green or red-emitting perovskites in terms of optoelectronic properties and long-term operative stability, strongly restricting the commercialization of the display technology. In this study, we first introduced CsCl in a typical sky-blue PeLEDs (PEA-CsFAPbBr₃) to enlarge the optical band gap and to enforce pure blue color emitting properties. Yttrium chloride (YCl₃) is then added in the perovskite emitter as an additional inorganic chloride dopant to hamper severe phase segregation and ion migration in the perovskite emitters. As a result, the multiple inorganic chloride salts simultaneously enforce pure blue electroluminescence and phase stabilization, leading to improved quantum efficiency and long-term operative stability of the pure blue PeLEDs, achieving a stable electroluminescence device at 473 nm with a full-width half-maximum (FWHM) of 17 nm. The optimized blue-light PeLEDs exhibited a maximum external quantum efficiency (EQE) of 1.06% and excellent operational stability (LT50 > 250 min), which suggests the efficacy of the multiple inorganic chloride salts for improving optoelectronic properties of pure-blue perovskite LEDs.

Recently, perovskite light-emitting diodes (PeLEDs) have promoted considerable research interest as a next-generation LED that can replace the existing OLED and QLED due to the attractive properties of metal halide perovskite (MLH) materials such as outstanding carrier mobility, bandgap tunablity, great chemical flexibility, and remarkable color purity. However, MLH has yet met the critical requirement for viable LED devices such as inferior moisture stability, stable blue emitter, and cost-effective large area fabrication. To remedy the disadvantage, quasi-2D perovskites, e.g. (PEA)₂(MA)_n-1Pb_nBr_3n+1 have been proposed that evidenced improved humid-stability and tunable optoelectronic properties. In this respect, here, we show a generalized strategy to develop Br-based blue quasi-2D perovskites with enhanced stability and spectral tunability. By controlling the ratio of cation spacers, we could establish a protocol to fine-tune the spectral range of the blue emissive perovskites which may open a pathway toward commercialized PeLEDs.

Metal halide perovskites (MHPs) have been intensively studied in the fields of solar cells and light-emitting diodes (LEDs) due to its excellent optical and electrical properties.^[1] Despite the development of MHP emitters and LEDs, the toxicity of Pb is one of the biggest problems for industrial use of MHP-based light emitters for display or lighting applications.^[2] Also, a poor operational stability of blue MHP LEDs remain a big challenge for achieving efficient blue MHP LEDs although the blue LED is a core component for high-color-purity display applications. A zero-dimension (0D) lead-free Cu-based halide perovskite, Cs₃Cu₂I₅, shows strong deep-blue emission (~440 nm), higher environmental stability, and less charged defect migration than those of APbCl₃ blue emitting MHPs.^[3,4] However, poor surface morphology and a large bandgap (~ 4.0 eV) of Cs₃Cu₂I₅ act as hurdle for its deep blue LED applications. Here, we control intermolecular interaction of precursor molecules with organic additive molecules to amend the activation energy for crystal growth of Cs₃Cu₂I₅ in the thin film, resulting in enhanced crystallinity and uniform surface morphology of emitting layer. To overcome a large energy barrier formation between hole injection layer and large bandgap Cs₃Cu₂I₅, hole injection interface engineering is conducted using molecular strategies to induce self-organization from the hole injection layer to the emitting layer, which results in improved hole injection in LEDs. By using our synergetic strategies, the eco-friendly lead-free deep-blue emitting electroluminescence devices are achieved with higher efficiency and operational stability.

1. Han, T.-H. et al. Interface and defect entgineering for metal halide perovskite optoelectronic devices. Advanced Materials 31, 1803515 (2019).
2. Han, T.-H. et al. A roadmap for the commercialization of perovskite light emitters. Nature Reviews Materials 7, 757-777 (2022).
3. H. Chen. et al. Multiple self-trapped emissions in the lead-free halide Cs₃Cu₂I₅. The Journal of Physical Chemistry Letters 11, 4326-4330 (2020).
4. L. Wang. et al. Colloidal synthesis of ternary copper halide nanocrystals for high-efficiency deep-blue light-emitting diodes with a half-lifetime above 100h. Nano Letters 20, 3568-3576 (2020).

Lead halides perovskite nanocrystals (NCs) have gained a tremendous attention on optoelectronic device recently such as light-emitting diode (LED), solar cell. It is because of its remarkable optical properties such as high color purity, tunable color, high photoluminescent quantum yield (PLQYs), solution processability and flexibility. In recent years, green and red perovskite NCs LED has achieved EQE over 20%,^{1, 2} while blue perovskite NCs LED still lag behind because large bandgap on blue perovskite NCs make it difficult to inject charge on NCs and reduce the radiative combination. Therefore, high efficiency blue NCs LED are needed for red, green, and blue perovskite LED technology. Here we report a room temperature and open-air synthesis of 0% to 48% Rb alloyed sky-blue perovskite Cs_xRb_1-xPbBr₃ NCs. We synthesized the NCs on the average size of 13.0 nm to 15.6 nm with tunable emission from 515 nm to 496 nm with a narrow bandwidth of 19 nm to 26 nm and high PLQYs of 94.5% to 60.9%. LEDs were fabricated using these NCs as the active layer. These devices exhibited with maximum EQE of 2.40% and maximum luminescence of 1239 cd/m² at 496 nm and stable spectra have been achieved.


References
1. Fang, T. et al Science Bulletin 2021, 66, 36-43.
2. Chiba, T. et al Nature Photonics 2018, 12, 681-687.

Quasi-two-dimensional (2D) perovskites have recently emerged as emitters in blue perovskite light-emitting diodes (PeLEDs). The cascading energy-transfer process between different 2D phases plays an essential role in the high performance of this class of PeLEDs. Herein, we propose an interfacial engineering strategy by incorporating a zwitterionic additive, L-phenylalanine, into the hole-injection layer (HIL), enabling suppression of trap-assisted deactivation channels by virtue of the coordination interactions between the additive and Pb2+ defects in the perovskite phase. In addition, the introduction of L-phenylalanine reduces the release of metallic indium species from indium tin oxide substrates initiated by acidic HILs, resulting in the suppression of luminescence quenching in the perovskite layer. The synergetic benefits create an ideal energy landscape, blocking energy losses and boosting PeLED performance with a peak external quantum efficiency of 10.98% at 480 nm and extended device lifetimes. Our approach provides a versatile strategy to achieve high-performance blue PeLEDs.

GaP has an innate crystal structure of zin blende and an indirect bandgap. Although its light conversion efficiency is not high due to intrinsic limitations resulting from the energy band structure, it is a material used for solid-state light-emitting diodes (LEDs) due to economic advantages. For these reasons, several computational studies have been conducted to search conditions under which GaP can have a direct bandgap. Simulation results expected a direct bandgap of GaP can be obtained when it has i) a wurtzite structure or ii) a zinc blende structure with a radius of 1 nm or less. Direct bandgap wurtzite GaP was proven through artificially grown GaP nanowire on a (111) oriented zinc blende GaP substrate in 2013. However, unlike the typical direct bandgap material, it forms a pseudo-direct bandgap between the dark conduction band and the heavy hole band, limiting its optical properties. In addition, there has been no experimental study that confirmed direct bandgap formation of zinc blende GaP via quantum confinement because synthesis if
Here, we experimentally demonstrated the direct bandgap of zinc blende GaP based on the high photoluminescence (PL) intensity and quantum yield. This was realized through the reverse type I quantum dot, which has an opposite order of band gap size between the core and the shell of general QD. In ZnS/GaP reverse type I quantum dots, electrons and holes are confined to the GaP shell rather than the ZnS core, and PL is exhibited by the GaP shell. In addition, ZnS has the advantage of stable stacking of GaP shell because of the similar lattice parameter (ZnS: 5.40A, GaP: 5.45A). We confirmed that ZnS/GaP actually has a zinc blende structure via XRD analysis, and the radius difference of 0.48 nm between ZnS and ZnS/GaP was confirmed through STEM image. These results suggest that GaP is stacked on the ZnS core surface with a one-unit cell thickness (monolayer), achieving a direct bandgap. As a result, it showed strong PL intensity even after two times purification processes and was able to achieve a high absolute PL quantum yield of 29.6%. In addition, it was confirmed that PL quantum yield hardly decreased even after 55 days of stability test. This study demonstrates a new pathway to switch many indirect-gap semiconductors to light-emitting materials with a high efficiency and good stability.

Solvents play a crucial role in developing high quality hybrid organic−inorganic perovskite films using roll-to-roll slot die coating methods which is an efficient process for the fabrication of large area optoelectronic devices. γ-butyrolactone (GBL) is a well explored solvent for direct growth of lead halide-based perovskite single crystals. GBL coordinates weakly with Pb²⁺ center in precursor solution compared to other solvents (N, N-dimethylformamide, dimethyl sulfoxide, etc.) due to its lower Gutmann’s donor number, D_N. Thus, GBL favors the formation of Pb-I-Pb bonds necessary for the octahedral framework and subsequent rapid single crystal growth. Methylammonium lead iodide (MAPbI₃) in GBL follows retrograde solubility, wherein there exists a threshold temperature (~60 °C) beyond which solubility decreases as temperature increases. This behavior provides a mechanism to drive supersaturation (and subsequently nucleation and bulk crystal growth) by increasing temperature, without the need for evaporation. However, the strong tendency towards single crystal growth at elevated temperatures poses a challenge for the deposition of pin-hole free perovskite thin films from GBL-based inks because elevation of processing temperature can push the precursor solution towards supersaturation rapidly. To achieve interconnected, compact and uniform morphology, the drying rate must be carefully tuned to control the nucleation and growth rates.
In this work, we describe a facile slot die coating method using a hot air knife and preheated substrate to control the perovskite crystallization and produce better quality thin films from GBL. To mitigate the dominating impact of retrograde solubility and have better control of morphology, a low concentration MAPbI₃ precursor solution was utilized to delay supersaturation. The crystallization of MAPbI₃ in GBL as function of processing temperature and drying rate was studied by microscopy. The phase and morphology of resulting films were characterized using XRD, SEM and AFM. MAPbI₃ crystallization at low temperatures was found to be thermodynamically controlled, which favors solvent-perovskite adduct formation. The perovskite films deposited at temperatures below 60 °C and without any air knife had poor morphology dominated by sparse, 10-100s µm sized needle-shaped crystals with large voids between them. This crystal phase can be attributed to solvent-perovskite adducts like (MA)₂(GBL)₂Pb₃I₈ that do not easily decompose into the perovskite crystalline structure. Slow evaporation of a thick layer of GBL perovskite solution can trigger this kind of GBL-perovskite adduct formation. High temperature processing mitigated the adduct formation, but crystal growth kinetics of the perovskite phase produced relatively poor film coverage. In contrast, the combination of hot air knife and heated substrate produced a smooth film with better coverage due to faster drying rate and better control over crystallization kinetics. In this work, we thoroughly studied crystallization of MAPbI₃ during large area coating and its correlation with solvent and different drying conditions. This provides insight about the processing condition which is crucial to control morphology during fabrication of large area optoelectronic devices.

Recently, chiral nanomaterials have attracted significant attention for potential applications in diverse fields such as ferroelectrics, chiro-spintronics and chiroptoelectronics. Among them, hybrid organic-inorganic chiral perovskites are considered critical candidates as alternative active layers owing to their superior optoelectronic properties such as circulary polarized light (CPL) detection and effective light absorption.² In addition, in the ABX₃ structure of perovskite, it is possible to easily introduce chiral properties by adjusting the A site cations. Until now, chiral perovskites using (R, S)-α-methylbenzylamine, (R, S)-phenylethylamine, (R, S)-1-(1-naphthyl)ethylamine etc. have been studied as chiral organic cations.^{1, 2} However, the development of perovskite-based CPL photodetectors with appreciable function is still limited. So, it is necessary to increase the performance of the CPL photodetector by developing chiral perovskites with a high g-factor.
In this work, we suggest a new class of chiral perovskites by integrating aromatic organic cations. Record-high level g-factor value is obtained compared with the existing materials reported so far. We confirmed the outstanding optical properties of 1D and 2D chiral perovskites, respectively, through dimensional control of chiral perovskites. Further, the excellent crystallinity of chiral perovskite using new chiral organic cations increases the charge transfer, resulting in enhanced efficiency of the CPL photodetector. These new chiral perovskites are expected to open the pathway for multifunctional chiroptic applications.

Limited understanding exists as to the response of highly ionic metal halide lattices upon photoexcitation. Structural deformations such as polaron formation influence local energetics and photophysical properties, which can heavily influence utility of these materials in optoelectronics. This talk will discuss structural response of 0D and 2D lead halide perovskites from direct investigations using ultrafast transient X-ray diffraction as a function of optical excitation fluence. 0D structures show slow initial evolution of structure that recovers with a timescale that depends dramatically on cation identity. For 2D structures both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. Observations are attributed to transient relaxation as well as ordering of distorted lead iodide octahedra stimulated chiefly by electron–hole pair creation. In the case of (BA)2PbI4, appearance of the (110) reflection in following excitation would suggest a transient crystal phase transition. However, through high energy single crystal XRD we find reflections that violate glide plane conditions in the reported Pbca structure, conveying that the literature assignment of the space group is incorrect and is instead P212121. In cases of both 0D and 2D perovskites, lattice relaxation times exceed electronic carrier lifetimes which can be understood in terms of their poor thermal dissipation, but also suggests shallow potentials wells.

The performance of quantum dot (QD) devices in optoelectronic and quantum information applications relies on not only the properties of the individual QDs but also the collective properties arising from interactions between QDs within a solid. The interactions between QDs determine whether long-range coherence and transport can be achieved. Due to their large oscillator strengths, perovskite QD superlattices (SLs) have the potential for exciton superradiance. However, the ultrafast dephasing processes can limit their coherence lengths. We provide a detailed study on the key factors that control exciton delocalization and transport in perovskite QD SLs. Through temperature-dependent and time-resolved ultrafast microscopy measurements, we demonstrate coherent exciton propagation in highly ordered SLs. We unravel the effect of disorder by comparing SLs with varying degrees of inhomogeneity. Our results point to the exciting opportunities in engineering exciton coherence in perovskite solids.

The continued innovation of on-demand sources of non-classical light is critical to fully realizing the technological potential of quantum photonics. In particular, single, indistinguishable photons from quantum emitters are the fundamental building blocks of many quantum optical applications, such as quantum networks, linear optical quantum computing, and quantum teleportation, Hong-Ou-Mandel (HOM) interference is the cornerstone photon-photon interaction that governs these quantum light technologies. In perfect HOM interference, two indistinguishable photons incident on a beam splitter leave coalesced, due to the overlap of their identical wavefunctions. Pure-dephasing processes, interactions which collapse and obfuscate the phase information of the photon-generating exciton wavefunction in a quantum emitter, impact the indistinguishability of sequentially emitted single photons. As such, HOM interference serves not only as an integral tool to quantum computing, but also an important heuristic for the pure-dephasing rate in a quantum emitter.
In our work, we show sizeable HOM interference in colloidal cesium lead bromide perovskite nanocrystals (PNCs), a first for colloidal, solution-processed material. Unlike other quantum emitters that require Purcell enhancements and device architectures to increase the radiative rate and suppress dephasing, these PNCs have yet to undergo the optimizations seen for other emitters. By comparing correlation functions for indistinguishable and distinguishable photons, the visibility of HOM interference can be calculated. We demonstrate that PNCs exhibit HOM visibilities which are factors higher than expected from indirect measurements. Our work showcases the potential of PNCs to become the first colloidal transform-limited quantum light source.

Lead halide perovskite semiconductors are recently attracting much attention as a new class of optoelectronic device materials, because of their outstanding optical and electronic properties at room temperature. It is considered that strong electron–phonon interactions play a key role in unique optical and transport responses of halide perovskites [1]. They determine the effective carrier mass, the carrier mobility, the photoluminescence (PL) linewidth, the Urbach tail of optical absorption, and so on. The efficient anti-Stokes PL and the slow hot-carrier cooling are also closely related to strong electron–phonon interactions [2–5]. Since individual nanocrystals (NCs) show extremely high PL quantum yields and narrow PL linewidths, longitudinal-optical (LO) phonon replicas reflecting electron–phonon interactions can be observed in the PL spectra at cryogenic temperatures. Therefore, single dot spectroscopy is a powerful tool to investigate the exciton–phonon interactions of perovskite NCs.
Here, we prepared CsPbBr₃, FAPbBr₃, and MAPbBr₃ perovskite NC samples and measured their PL spectra of single NCs [6–8]. At low temperatures, CsPbBr₃ NCs show very sharp PL lines including fine-structure splitting in the PL peaks of excitons, biexcitons, and the LO-phonon replicas of excitons. We discuss the exciton–phonon and trion–phonon interactions in perovskites from the PL fine structures in CsPbBr₃ NCs. We found that the LO-phonon energies are independent of the NC size. However, the Huang–Rhys (HR) factors of excitons, i.e., the strength of the exciton–phonon coupling, depend on the LO-phonon energy. Both HR factors for excitons and trions increase as the NC size decreases. The HR factors of trions are larger than those of excitons in larger NCs. We conclude that the size-dependent HR factors for excitons and trions are closely related to the spatial distributions of the electron and hole wavefunctions in the NC.
Part of this work was supported by JST-CREST (JPMJCR21B4) and JSPS KAKENHI (JP19H05465, JP20K03798, JP21K03424, and JP22H01990).

[1] Y. Yamada and Y. Kanemitsu, NPG Asia Materials 14, 48 (2022).
[2] T. Yamada et al., Phys. Rev. Materials 3, 024601 (2019).
[3] Y. Yamada et al., Phys. Rev. Lett. 126, 237401 (2021).
[4] F. Sekiguchi et al., Phys. Rev. Lett. 126, 077401 (2021).
[5] Y. Kajino et al., Phys. Rev. Materials 6, L043001 (2022).
[6] S. Masada et al., Nano Lett. 20, 4022–4028 (2020).
[7] K. Cho et al., Nano Lett. 21, 7206–7212 (2021).
[8] K. Cho et al., Nano Lett. 22, 7674–7681 (2022).

Photon upconversion describes a process which effectively shortens the wavelength light emitted upon irradiation, yielding photons with a higher energy than the incident photons. To ensure compliance with energy conservation laws, this process occurs by combining the energy of two low energy photons. In the triplet-triplet annihilation upconversion process of interest here, the photon energy is combined by forming a high energy singlet state from two lower energy triplet states. However, since direct optical excitation of triplet states is ‘spin-forbidden’, sensitizers are used to indirectly populate the triplet state.

Successful triplet sensitizers span a broad range of material classes including metal-organic complexes, inorganic nanomaterials such as quantum dots or nanoplatelets, and bulk perovskite thin films. Understanding the fundamental energy transfer mechanism across the hybrid organic/inorganic interface is crucial for the advancement of optoelectronic devices based on this process. The exact energy transfer mechanism at the interface of inorganic and organic materials remains obscure, particularly where there is a large amount of structural inhomogeneity or high defect density within the material.

To date, rubrene has been the most investigated annihilator. However, it is not the ideal annihilator for use with lead halide perovskites. Here, I discuss the recent developments of perovskite-sensitized upconversion, including new annihilators such as 1-chloro-9,10-bis(phenylethynyl)anthracene (1-CBPEA), which enables solid-state near-infrared-to-green upconversion.

Recently, hybrid perovskites have gained attention as sensitizers for molecular triplet generation. Layered, two-dimensional (2D) perovskites are especially well-suited for this purpose because the triplet donor (inorganic framework) and triplet acceptor (organic layer) are self-assembled into adjacent sheets, so that with the appropriate energetics, triplets can be driven across the interface. By leveraging interlayer energy transfer, this emerging class of 2D perovskites has potential to impact several energy-related fields and processes, including solar photochemistry, photonics, and optoelectronics. In this talk I will discuss how several 2D perovskite design parameters affect photophysical outcomes. In particular, we will (1) examine how exciton energy offsets and frontier orbital alignments between the metal-halide and molecular layers affect the dynamics of ultrafast energy transfer and (2) discuss how organic layer morphology and composition can be engineered to generate multi-colored triplet-based molecular emission. Ultimately, 2D hybrid perovskites, with their vast compositional and structural tunability, offer an exciting platform for manipulating energy flow for light-emitting applications.

All-inorganic lead-halide perovskite (CsPbX₃, X = Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser)^[1], as well as for devices exploiting strong light-matter interaction and operated at room-temperature.^[2] Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, e.g., by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity^{[3, 4]} in quantum light sources. To date, a thorough account of many-body interactions in these newly developed soft perovskite QD compounds is still missing. Here, by combining single-QD optical spectroscopy performed at cryogenic temperatures in combination with configuration interaction (CI) calculations, we address the trion and biexciton binding energies and unveil their peculiar size dependence. We find that trion binding energies increase from 7 meV to 17 meV for QD sizes decreasing from 30 nm to 9 nm, while the biexciton binding energies increase from 15 meV to 30 meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates (5%) from the one of the single excitons, as a result of lattice distortions in the multiexciton regime. Our findings provide a deep insight into the multiexciton properties in all-inorganic lead-halide perovskite (CsPbX₃) QDs, essential for classical and quantum optoelectronic devices.

References
[1] M. V. Kovalenko, L. Protesescu, M. I. Bodnarchuk, Science 2017, 358, 745.
[2] R. Su, A. Fieramosca, Q. Zhang, H. S. Nguyen, E. Deleporte, Z. Chen, D. Sanvitto, T. C. Liew, Q. Xiong, Nat. Mater. 2021, 20, 1315.
[3] C. Zhu, M. Marczak, L. Feld, S. C. Boehme, C. Bernasconi, A. Moskalenko, I. Cherniukh, D. Dirin, M. I. Bodnarchuk, M. V. Kovalenko, Nano Lett. 2022, 22, 3751.
[4] G. Nair, J. Zhao, M. G. Bawendi, Nano Lett. 2011, 11, 1136.

One of the key enabling properties of low-bandgap lead-halide perovskite nanocrystals (CsPbX₃ NCs) for optoelectronic applications is their tolerance to point defects. Thus far, the discussion around defect tolerance has mostly focussed on recombination of charge-carriers from the band-edge. But understanding influence of these defects on hot carrier (HC) cooling will also be critical for achieving more efficient NC-based devices, but has not been thoroughly explored thus far. Here, we address this knowledge gap by elucidating the influence of intentionally-introduced traps (primarily halide vacancies) on HC relaxation above the band-edge in CsPbX₃ NCs with different halide compositions (CsPbBr₃, CsPbBr_xI_3-x, and CsPbI₃). The HC cooling kinetics in these different NC systems are probed by energy-dependent photoluminescence quantum yield measurements, along with pump-probe (two-pulse) and pump-push-probe (three-pulse) transient absorption measurements. We show that the presence of defect states increases the chances of HC trapping, which speeds up the cooling process. However, the change in HC cooling lifetime due to the trap densities becomes smaller as the composition moves from Br-based NCs to I-based NCs. Based on density function theory (DFT) calculation and hot carrier kinetic modelling, the cooling speed is governed by both defect densities and defect position relative to the conduction band edge. Shallower defects would allow faster trapping and de-trapping process for the carriers, which it helps to maintain the free carrier densities and allow free carriers to cool slowly with the effect of Auger reheating. This work firstly demonstrated experimental observations of the defect-tolerance nature of perovskite NCs to hot carriers, which provides insights into designing high-efficiency photovoltaic devices by managing defect densities for different bandgap perovskites.

In 2D perovskites, the choice of the A-site perovskitizer cations is not entirely limited by the so-called Goldschmidt tolerance factor. The stabilizing effect of the spacer cations makes it possible to tolerate larger A-site cations as perovskitizers. By solving the single-crystal structures of (BA)₂(A)Pb₂I₇ where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), we have proven that the relatively large DMA and GA can serve as the A-site cation of 2D perovskites despite the fact that they are too large to formally satisfy the Goldschmidt rule. As the size of the A-site cations increases, the Pb−I bonds are elongated, which reduces the orbital overlap of Pb s- and I p-orbitals, thus decreasing the electronic bandwidths and increasing the bandgaps, as confirmed by DFT calculations. This increases the local distortions of the PbI₆ coordination environment and leads to a decrease of PL intensity and shortening of PL lifetime. The expanded cages also result in soft and anharmonic lattices, as supported by Raman spectra. Exciton-exciton annihilation processes occur in these systems with the rates being slower in the structures with larger A cations. Similar trends are also observed in lead bromide perovskites as well as tin and germanium iodide systems. Understanding the structure-property relationships of these compounds will help illustrate how the large A-site cations may influence physical properties of both 2D and 3D perovskites and facilitate their future optoelectronic applications.

Materials engineering of metal halide perovskites (MHPs) uncovered a number of unique materials with multiferroic properties and tunable visible regime bandgaps, which has been a main limiter of integration for classic perovskite ferroelectrics (BiFeO₃, PbTiO₃, etc.) in multifunctional optoelectronic applications. Chemical variations of multidimensional (2D/3D) heterostructure perovskites have been fabricated into record breaking photovoltaics by benefit from the charge carrier properties of the 3D phase and environmental stability of the 2D phase. Until now, MHP materials exhibiting 2D/3D phase mixing from a single medium-sized cation has not been observed. Here, we uncover textured 2D/3D phase-mixed structures formed by a modified MHP single crystal growth using a medium-sized A-site cation. We use advanced scanning probe microscopy methods, namely band excitation piezoresponse force microscopy (BE-PFM), coupled with other nanometric spectroscopic characterization techniques, such as micro-Raman and scanning electron microcopy cathodoluminescence, to reveal and spatially resolve the ferroelectric, luminescence, and structural characteristics of mixed 2D/3D phases. The textured 2D/3D phase-mixed regions exhibit 2D/3D-heterostructure luminescence and the variability in phase-mixing shows an off-axis tunable ferroelectricity. It is anticipated that the discovery of a ferroelectric mixed 2D/3D phase hybrid perovskite with a single cation may soon debut higher photovoltaic performance of long stability lifetime and high conversion efficiency.

Mixed halide perovskites consisting of multiple ions at the same lattice site (ABX₃) are an emerging class of semiconductors, with potential as high-performance optoelectronic materials. The promise of this class of materials derives from the fact that incorporation of multiple ions is a facile means to tune material properties such as band structure, emission and absorption efficiency, exciton binding energy, and carrier lifetime. Indeed, it has been well established that mixed-cation and mixed-halide composition perovskites lead to enhanced device performance and stability. However, simply using multiple ions during synthesis does not always lead to structures with desirable properties, due to the various degrees of ionic mixing and types of nanoscale arrangements possible in the complex perovskite. In addition, the source of mixing or segregation should be experimentally studied on the scale of an individual particle, enabling ones to deconvolve the effects of ion size, grain boundary, and ion migration on the final crystal morphology. Herein, an emerging method termed evaporation-crystallization polymer pen lithography (EC-PPL) is used to synthesize and systematically study the degree of ionic mixing of Cs_0.5FA_0.5PbX₃ (FA = formamidinium; X = halide anion, ABX₃) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A-site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs-FA ion segregation. This preference for segregation can be overcome to form a solid-solution by decreasing crystal size (< 60 nm) or increasing temperature. Finally, we utilized these tools to identify and synthesize solid-solution nanocrystals of Cs_0.5FA_0.5Pb(Br/I)₃ that significantly suppress photo-induced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.

Despite notable developments in halide perovskite thin-film devices, their integration into on-chip nano-optoelectronics has been limited by a lack of control over nanoscale patterning. Prone to degradation, these materials are often not compatible with the conventional top-down fabrication processes. Bottom-up assembly, on the other hand, can avoid such damage yet current strategies often lack the versatility to accomodate deterministic spatial control at the individual nanocrystal level for integration with other functional device components. Here, we introduce two techniques to enable on-site synthesis of nanocrystals down to a quantum dot-level in a scalable manner and compatible with device integration. In one, halide perovskite nanocrystals with sub-10 nm spatial accuracy and tunable sizes down to 20 nm are synthetized on-site guided using topographical templates with chemical texturing that localize growth precursors to the nanoscale. Using this platform, we have demonstrated arrays of on-chip nanoscale light emitting diodes (nano-LEDs) utilizing single nanocrystals with applications in optical communications, lasers, augmented and virtual reality displays, and quantum technologies. We will then discuss an approach that we have developed using a scanning probe technique for on-site fabrication of deterministically positioned halide perovskites quantum dots on-chip. We have used this technique to demonstrate deterministic arrays of perovskite single-photon sources with performance that can be engineered by direct integration with photonic cavities, enabling applications in quantum technologies.

Metal halide perovskites have been widely implemented in various optoelectronic devices, but their poor stability under light illumination remains a big challenge. While the intrinsic photostability of isolated perovskite samples has been widely discussed, it is important to explore how charge transport layers, which are necessary for a functional device, impact photostability. In this study, we investigate the effect of organic hole transport layers (HTLs) on the spectral stability, reflecting light-induced halide segregation, and temporal stability, reflecting light-induced PL quenching, of the perovskite photoluminescence (PL) spectrum. By employing a series of HTLs with different highest occupied molecular orbital (HOMO) levels, we observe and rationalize a nonmonotonic relation between the rate of light-induced halide segregation for mixed-halide perovskite and HOMO energy of HTLs. Then, combining PL and cross-sectional scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM–EDX), we assert a connection between halogen diffusion into HTLs and enhanced non-radiative recombination at perovskite/organic HTL interfaces. Finally, we conclude that both instabilities in PL spectrum are related to and affected by halogen loss from the perovskite layer into the HTL, with the HTL HOMO energy ultimately determining the PL behavior at perovskite/organic HTL interfaces under light illumination. These findings highlight the detrimental effect of halogen oxidation and diffusion within perovskite devices on stability, and also provides guidelines for the design of HTLs with suitable HOMO levels to mitigate halogen diffusion.

In the past few years, phosphor converted white LEDs have been widely adopted for lighting owing to their remarkable energy efficiency. However, the low color rendering of such white illumination is the main limitation of the commercialized LEDs and, for this reason, white LEDs have limited use in different areas such as medicine. A bright white emission replicating the solar spectrum (i.e., with ultra-high color rendering and tunable color temperature) is necessary to reach such high quality lighting but discovering a single-phase white phosphor with such characteristics remains challenging as it requires to navigate in complex multidimensional spaces (composition of the host matrix/ dopants, experimental conditions…). Thus, such high quality single phase white phosphor has never been discovered.
To navigate in a complex multidimensional space with the aim of designing sun like white phosphors, we decided to substitute the human decision component of the materials discovery process by machine learning (ML) models. ^[1] The Mn doped solid-solution (TDMP)Pb(Br_xCl_1-x)₄:yMn (0 ≤ x ≤ 1) (TDMP = trans-2,5-dimethylpiperazine) was investigated. With the guidance of such ML tools, a series of single-phase sun-like white phosphors with ultra-high color rendering indexes (above 92) and tunable correlated color temperature (from 3000K to 7000 K) were precisely designed. This is the first report of such ultra-high color rendering and broad range of tunable color temperatures for a phosphor.
[1] H. Yuan, L. Qi, M. Paris, F. Chen, Q. Shen, E. Faulques, F. Massuyeau, R. Gautier, Advanced Science 2021, 8, 2101407.

Quasi-2D perovskites are important materials for perovskite light-emitting diodes (PeLEDs) as the multi-quantum well (MQW) structure of the quasi-2D perovskites enhances performance of PeLEDs by efficient carrier funnelling. However, PeLEDs suffer from efficiency roll-off which is the large efficiency drop under high current density. One of the factors which causes efficiency roll-off is Auger recombination. Auger recombination is affected by the excitonic binding energy, exciton-phonon coupling, and the density of trap states. Different organic spacer cations could change these parameters by causing different distortion of the octahedral structure. Thus, studying the effect of different spacer cations on the Auger recombination behaviour in 2D perovskite is crucial to design PeLEDs with reduced roll-off. In this talk, I will discuss the use of ultrafast spectroscopy to investigate the effect of spacer in 2D perovskite on Auger recombination. Using photoluminescence and transient absorption spectroscopy, the modulation of the Auger process by the spacer will be elucidated. We will also discuss the effect of crystal structure change induced by different spacer cations on Auger recombination process, which provide insights into optimising the performance of PeLEDs based on 2D perovskites.

Metal organochalcogenolates (MOCs) are an emerging class of low-dimensional hybrid organic-inorganic semiconductors. Silver phenylselenolate (AgSePh), a prototypical 2D MOC, is a natural quantum well structure exhibiting narrow blue emission, large exciton binding energy, and in-plane anisotropy. Like transition metal dichalcogenides and 2D lead halide perovskites, MOCs can be synthesized in the form of three-dimensional crystals consisting of 2D layers held together by van der Waals forces. However, MOCs are distinguished from perovskites and other vdW crystals by the presence of organic ligands covalently bound to each 2D inorganic layer, which imparts chemical robustness and a unique handle for tuning the electronic properties.

In this talk, I will discuss efforts from my lab to synthesize new AgSePh derivatives with varying dimensionality, and characterize the excitonic nature of light emission and electron-phonon interactions in this new hybrid organic-inorganic materials family.

Organic-inorganic hybrid halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, etc. However, the poor stability is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of organic-inorganic hybrid material - Organic Semiconductor-incorporated Perovskite (OSiP). Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. In addition, I will show that the energy alignment between organic and inorganic components and the assocaited optical properties can be modulated via “chemical tuning” or “pressure-gating”, opening up many new opportunities for next-generation light-emitting applications. Using these novel hybrid materials, I will demonstrate the fabrication of high quality polycrystalline thin films and highly stable and efficient LED devices with suppressed ion migration and improved external quantum efficiency, color purity, and operational lifetime.

In this work, novel two-dimensional lead-free double perovskites ((R)₄M_aM_bM_cX₈, a+b+c=2) were synthesized to achieve novel compositional and dimensional tunability. Herein, (PA)₄AgBiBr₈ was alloyed with combinations of Na/In as well as Ag/In with the goal of forming PL active materials. Moreover, the samples were tuned to be PL-emissive under room temperature via a manganese alloying strategy. Bandgap and PL emission turnability were explored through halogen substitution (chlorine and iodine). An enhanced PL emission was observed for mixed halogen phases. Three emission pathways are investigated including band edge emission, emission stemmed from self-trapped excitons, and the unique energy transfer process of alloyed Mn²⁺ ions, which will be discussed.

Metal halide perovskites and perovskite-related organic metal halide hybrids have recently emerged as new generation light emitting materials with exceptional structure and property tunability. By controlling the morphological dimensionality of metal halide perovskites with ABX₃ structure, low dimensional perovskite nanocrystals, e.g. 2D nanoplatelets, 1D nanowires, and 0D quantum dots, have been developed to exhibit enhanced optical properties and stability as compared to 3D counterparts. Besides 3D ABX₃ perovskites, organic metal halide hybrids with low dimensional structures at the molecular level can be prepared by using appropriate organics and metal halides, in which metal halide building blocks form 2D, 1D, and 0D structures that are completely isolated and surrounded by organic cations. These organic metal halide hybrids exhibit unique properties that are different from those of 3D ABX₃ perovskites. For instance, tunable broadband emissions with near-unity photoluminescence quantum efficiencies have been achieved in a series of 0D organic metal halide hybrids. In this talk, I will present our recent efforts on the development and study of highly luminescent and stable low dimensional metal halide perovskites and organic metal halide hybrids. Their applications in electrically driven light emitting diodes (LEDs) will be discussed.

Chiral perovskites have been extensively studied as a promising candidate for spintronic- and polarization-based optoelectronic devices due to their unusual spin-related properties such as strong spin-orbit coupling, long spin-life time exceeding 1ns and large Rashba splitting. However, the origin of chiroptical activity in chiral perovskites is still unknown, as the chirality transfer mechanism has been rarely studied. Since the chiral perovskite with the Sohncke chiral space group of P2₁2₁2₁ was reported in 2003, their chiroptical phenomena have been interpreted based on their crystallinity. Although (i) crystallization into a chiral crystal structure induced by chiral organic molecules, (ii) chiral distortion on the surface of chiral perovskite nanoparticles, and (iii) chiral dislocations can provide the explanation of their optical and electronic properties, the electronic interactions between the chiral organic molecules and achiral inorganic framework, should also be considered. Here, we discuss the possibility of the chirality transfer via electronic interaction between the chiral organic molecules and achiral inorganic framework related to the strength and direction of hydrogen bonding. We systematically consider the effect of asymmetric hydrogen bonding and related change in the chiroptical activity of chiral perovskite by inducing the conformational change of stacking order between the two chiral organic molecules. First, through the spatial confined growth of chiral perovskite in anodized aluminum oxide template, we can successfully impose the micro-strain into the lattice of chiral perovskite, resulting in rearrangement of chiral organic molecules. As a results, remarkable chiroptical behavior with different absorption of 2.0 10^-3 and distinct photoluminescence of 6.4 10^-2 for left- and right-handed circularly polarized light was observed in nanoconfined chiral perovskites even at room temperature. Theoretical calculations verify that the chirality transfer phenomena mediated by the asymmetric hydrogen bonding between the chiral organic molecules and achiral inorganic frameworks is critical factor for promoting the chiroptical activity of chiral perovskites. Furthermore, the incorporation of guest molecules into the lattice of chiral perovskite evoke the rearrangement of chiral organic molecules, which also can lead to change in chiroptical activity as well as electrical properties. Through solid-state nuclear magnetic resonance spectroscopic measurements and theoretical calculations, we verify the change in the degree of chiral lattice distortion associated with different electronic interaction between the inorganic framework and guest molecules. Finally, as a preliminary proof-of-concept, a vertical-type circularly-polarized light photodetector based on chiral perovskites was developed, exhibiting an outstanding performance with a distinguishability of 0.27 and responsivity of 0.43 A W^-1. Our findings suggest that electronic interactions between the building blocks should be considered when interpreting the chirality transfer phenomena and designing hybrid materials for future spintronic and polarization-based devices.

Chiral materials are important tools for interconverting the spin angular momentum of circularly polarized light with electronic spin to help realize a wide variety of emerging spin-based technologies. Here we demonstrate that thin films of a Bismuth-based chiral 0D hybrid organic-inorganic semiconductor (HOIS) exhibit large anisotropy values for circularly polarized light emission (CPLE) that approach 50% circular polarization. The observed anisotropy is strongly correlated with the crystallographic orientation of the thin film and is also strongly temperature-dependent, with a marked anti-correlation with exciton transition linewidth. Detailed analysis of the CPLE anisotropy indicates large contributions from structure-dependent scattering that are analogous to the so-called “LDLB” effect observed for circular dichroism, caused by interactions between linear dichroism and linear birefringence. Although this effect has been observed for organic thin-film composites, this first demonstration in a HOIS system provides a unique route for enhancing carrier spin polarization and polarization-dependent emission in self-assembled hybrid semiconductors. These results provide a fundamental framework for understanding and harnessing the properties of low dimensional and low symmetry chiral HOIS materials for circularly polarized light applications.

InP quantum dots (QDs) are the material of choice for QD display applications. They have been used as active layers in QD light-emitting diodes (QDLEDs) with high efficiency and color purity. Optimizing the color purity of QDs requires understanding mechanisms of spectral broadening. While ensemble-level broadening can be minimized by synthetic tuning to yield monodisperse QD sizes, single QD linewidths are broadened by elastic/inelastic exciton-phonon scattering and fine-structure splitting. Here, using photon correlation Fourier spectroscopy, we extract average single QD linewidths of 50 meV at 293K for red emitting InP/ZnSe/ZnS QDs, among the narrowest for colloidal QDs. By measuring InP/ZnSe/ZnS single QD emission lineshapes at temperatures between 4K – 293K, and modelling the spectra using a modified independent boson model, we find that acoustic phonon coupling and fine-structure splitting, which contribute similarly to the homogeneous linewidth, are most prominent at low temperature, whereas pure dephasing is the primary broadening mechanism at elevated temperatures, and optical phonon coupling contributes minimally across all temperatures. By measuring CdSe/CdS/ZnS QD emission lineshapes across the same temperature range, we find that optical phonon coupling is a larger contributor to the lineshape at elevated temperatures likely as a result of the more polar nature of CdSe/CdS/ZnS. For the first time, we are able to reconcile narrow low-temperature linewidths and broad room temperature linewidths within a self-consistent model that enables parameterization of linewidth broadening, for different material classes. Our findings reveal that red-emitting InP/ZnSe/ZnS QDs have intrinsically narrower linewidths than typically synthesized CdSe QDs, suggesting that these materials could be used to realize QDLEDs with unprecedented color purity.

Halide perovskites show excellent optoelectronic properties including bandgap tunability, high radiative recombination rates and narrow emission lines that make them promising candidates for the next generation LEDs. However, their thin film character is yet to be exploited to enable full control over the emission properties, opening avenues to surpass the luminous efficacies of conventional LEDs and facilitating their widespread adoption.
In this talk, we present a novel green perovskite LED architecture where enhanced emission and directionality on demand is achieved by means of a hybrid photonic-plasmonic structure. We show how a code based on the transfer matrix model boosted by a genetic algorithm identifies the best combination of materials and thin film thicknesses to maximise outcoupled light with very narrow and controllable angular dispersion; all in a realistic fashion compatible with the fabrication of efficient LEDs. This strategy leads to hybrid photonic-plasmonic modes highly confined throughout the whole cross-section of the perovskite emitter while ensuring a good band alignment of the building blocks for efficient charge carrier injection and radiative recombination.
The experimental realization of the optimum designs allows us to demonstrate devices with amplified green emission selectively enhanced at different angles. Our low temperature process can tune the perovskite thickness on a nanometric scale to enhanced electroluminescence on demand from forward direction (0°) to up to 40°. This approach expands the role of the perovskite film from a mere emitter to an active photonic layer participating in the strong interference phenomena arising from the designed photonic-plasmonic nanostructures. This strategy is truly unique as opposed to standard methods where emitters are just embedded into the optical cavities. Our methodology is versatile and easily integrable into cost-effective perovskite LEDs with emission lines covering the entire visible spectrum. Thereby opening avenues for their application in displays and light sources where control over angular dispersion of light is crucial.

Many emerging technologies rely on near-infrared emitters for covert illumination and sensing. Examples include the facial recognition devices found on our smart-phones, sensing devices on fitness tracking watches, gaming consoles, robotic cleaners and self-driving cars. Colloidal quantum dots are excellent emitters that could be conveniently solution-processed, and could potentially find useful applications in these devices. Unfortunately, there are very few known colloidal quantum dots that operate in the near-infrared spectral range. Some examples include PbS and and CsPbI3, which contain restricted heavy metals. In this talk, I will discuss new strategies to prepare low-toxicity heavy-metal-free core-shell colloidal quantum dots with high luminescence efficiencies and with long emission wavelengths of above 900 nm. I will also present some of their applications in optoelectronic devices, such as in light-emitting diodes and in luminescent solar concentrators.

The external quantum efficiency (EQE) of perovskite LEDs (PeLEDs) has dramatically improved to above 20% in the past few years. Operation stability becomes the most challenging issue for the application of perovskite LEDs. Ion migration was acknowledged to be one of the main causes of degradation of perovskite LEDs.

In this talk, we will present our efforts to improve the stability of perovskite LEDs, including 1) suppression of ion migration with incorporation of bulky organoammonium ligands, 2) enhanced operation stability in all-inorganic perovskite LEDs due to the suppressed ion migration, and 3) absence of ion migration in tin halide perovskite LEDs under device working conditions. and 4) Suppressed ion migration in perovskite single-crystal LEDs with enhanced operation stability.

Performance of blue perovskite light-emitting diodes (PeLEDs) is still lagging far behind green and red emissions. Inferior carrier injections, non-radiative recombination and random phase distribution mainly account for limited advances in quasi-two-dimensional blue PeLEDs. Here, we modulated of hole injection layer by addition alkali metal salt CsCl, a better band alignment and improved charge injection have been achieved. More interestingly, the modified substrate induced the following growth of perovskite layer with better phase distribution, which lead to the less non-radiative recombination and enhanced charge transport. Accordingly, a maximum external quantum efficiency of 16.07% had been demonstrated in the sky-blue PeLEDs, with a stable emission peak located at 486 nm and improved operational stability. This strategy contributes an effective way for the fabrication of high-performance and stable blue PeLEDs toward practical applications.

Three-dimensional (3D)/low-dimensional (LD) perovskite solar cells (PSCs) offer an effective strategy to overcome the trade-off between perovskite solar cells device performance and stability. Improving the device longevity, while concomitantly simultaneously enhancing increasing the solar cell open circuit voltage and fill factor of the solar cell is the current challenge. Despite being one of the most popular and effective way processing techniques, whether the presence of this surface LDP layer would be - or not-represents the winning crucial path for the future of this technology remains elusive. In particular, atomic layer combined surface and bulk passivation using surface modifiers such as organic dopants, casts the doubts on the effective need for of having an homogeneous LDP cover capping layer. In this talk, I will discuss the interfacial passivation with different cations such as M-PEAI, M-PEABr, and M-PEACl, showing the role of the passivation and how different halides affect the device performance of the devices. In addition, I will compare recent results obtained in the 3D/LDP configuration with the surface cation passivation strategy, which can still pushes produce the performances to values comparable to the LDP/3D bilayers. I will providing a comprehensive perspective on the benefits from of the two different strategies, but also presenting how LDP interfaces can play a role in alternative new configurations, such as tandem solar cells [1,2].

Acknowledgements I acknowledge the “HY-NANO” project that has received funding from the European Research Council (ERC) Starting Grant 2018 under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802862).
Reference
[1] Degani et al, Sci Advances 7, 49, 2021
[2] Degani et al, submitted

Colloidal quantum dots (QDs) are considered the next-generation self-emissive display emitter materials because of their high color purity, and large-scale solution processability. The InP-based QDs have been synthesized with near-unity photoluminescence quantum yield (PL QY), which can meet the RoHS requirements by removing toxic Cd from QD emitters. Despite recent advances in QD light-emitting diodes (QD-LEDs), multicolored and pixelated QD-LED devices still need to be developed. When QDs are incorporated into a device structure and sandwiched into a multilayered thin film, solvent orthogonality should be considered because the pre-coated QD layer can be damaged. This can be a serious issue when the QD film is patterned as a pixelated display, where conventional photolithography cannot be used. When photoresists are coated on top of the pre-existing QD film, the QD film is easily removed, or the two layers intermix. Thus, alternative patterning methods, such as inkjet and transfer printing, have been employed to form multicolor QD patterns. However, inkjet printing has a limited tact time and suffers from nozzle clogging issues for industrial applications. In addition, transfer printing can yield high-resolution patterns, but the process requires precise control of kinetic parameters which limits the reproducibility of the process.
Therefore, it is highly desirable to use conventional photolithography for patterning QD films. Two methods have been suggested for employing photolithography in QD film patterning. One is the addition of light-sensitive cross-linking molecules to the QD solution. The molecule can either replace the original ligands or can be included as an additional substance. Upon light exposure, the molecules form a crosslinked structure between the QDs, and the patterns can be formed by solvent dipping. The other method directly uses a conventional photoresist because it is a well-designed chemical that provides high resolution and etch-resistance. Owing to the absence of solvent resistance in QD films, the lift-off method has been attempted. However, the lift-off process is not compatible with high-resolution patterns and also requires an additional sacrificial layer owing to solvent resistance. We previously reported that atomic layer deposition (ALD) of ZnO can significantly enhance the solvent resistance of QD films, and photolithography can be used to successfully form QD patterns. The green-light emitting Cd-based QD-LED exhibited a luminance of 100,000 cd/m² after the patterning process, but entire photolithography inevitably requires photoresist development and removal processes, which employ polar solvents, such as acetone, alcohol, and tetramethylammonium hydroxide. Compared with Cd-based QDs, InP-based QDs exhibit significant PL quenching during the lithography process, and the performance of patterned QD electroluminescence devices is degraded.
Herein, we report a nondestructive and direct photolithography-based patterning process for extremely bright InP-based QD film patterning. The photoactive compound (PAC) used for the positive photoresist can deteriorate the emission properties of QDs. Diazonaphtoquinone (DNQ) is one of the most popular PACs that reduces the PL of InP-based QDs, according to the present study. To prevent damage to QD during the patterning process, DNQ-free photoresists were employed in this study. After depositing the InP-based QD film, the photoresist was coated, and the following lithography process was carried out. High-resolution patterns of Cd- and InP-based QD films with a maximum of 2,000 PPI were obtained, and systematic PL spectrum studies were performed to understand the effect of the photoresist on the QD film. When the same photoresist was used for the Cd-based QD film, enhanced PL behavior was observed compared to that in our previous study. Additionally, a patterned Cd- and InP-based QD EL device was fabricated with a standard device structure.

Semiconductor nanocrystals or colloidal quantum dots (QDs) are one of the potential emissive display materials owing to their excellent optoelectronic properties and large-scale synthesis. QDs’ Emission wavelengths can cover the whole visible range by changing their composition and size. Photoluminescence quantum yield (PL QY) of QDs has already reached unity for all primary red, green, and blue colors due to state-of-the-art synthetic parameters control and optimization of device architecture. QDs are generally incorporated in multilayer thin-film device structures for optoelectronic applications such as light-emitting diodes (LEDs) and photodetectors. QD-LED can be simply fabricated into a single-color device for self-emissive display applications, but white light emission has also been tried. In specific, high-brightness white QD-LED can be used not only for backlight units but outdoor lighting applications.
White light emission by QD emitter has been successfully done by simply mixing QD solutions with different colors and spin coating them in a single step. Furthermore, mixing a wide range of visible spectra can provide a sunlike lighting device, but Forster resonance energy transfer (FRET) between different QDs results in a severe red shift of the overall emission colors. QDs with two or three different colors are randomly mixed in condensed film, and red QDs typically absorb the light emission from the shorter wavelength of QDs. This issue can be solved by sequentially depositing layers and by separating them from each other, but QDs are easily dissolved by organic solvents due to the absence of solvent resistance. Forming a tandem structure by inserting a charge generation layer may provide high-efficiency white emission QD electroluminescence (EL) devices, but the driving voltage significantly increases and the device structure can be complicated significantly.
In this aspect, depositing each layer of red (R), green (G), and blue (B) QD and separating them to suppress the severe FRET effect is one of the realistic solutions for white emission QD EL devices. Direct spin-casting QD solution on top of pre-coated QD film is not usually preferred due to interfacial mixing between two QDs. Ultrathin ZnO nanoparticles (NPs) can be deposited to provide solvent resistance during the spin-casting of another layer on top of pre-existing QD. Atomic layer deposition (ALD) of ZnO can form an extremely thin passivation layer on top of the QD surface and even a single pulse of diethylzinc (DEZ) precursors can effectively crosslink the surface of the QD film according to our previous studies.
Herein, the ALD ZnO layer is inserted between two or more layers of QD film to suppress the FRET effect and realize white emission at broader operation voltages without affecting charge transport significantly. Different device architectures including randomly mixed and sequentially stacked QD films were compared in terms of color balance and device efficiency. Blue(B)/Green(G)/Red(G) stacking sequence was chosen for efficient white light emission. The device structure suppressed energy transfer compared to randomly mixed film and the exciton recombination zone is located across the whole emission layer.