Exploring Electronic and Excitonic Processes toward Efficient Deep-Red CuInS2/ZnS Quantum-Dot Light-Emitting Diodes

Boosting Cu─In─Zn─S-based Quantum-Dot Light-Emitting Diodes Enabled by Engineering Cu─NiOx/PEDOT:PSS Bilayered Hole-Injection Layer

The imbalance of charge injection is considered to be a major factor that limits the device performance of cadmium-free quantum-dot light-emitting diodes (QLEDs). In this work, high-performance cadmium-free Cu─In─Zn─S(CIZS)-based QLEDs are designed and fabricated through tailoring interfacial energy level alignment and improving the balance of charge injection. This is achieved by introducing a bilayered hole-injection layer (HIL) of Cu-doped NiOx (Cu─NiOx)/Poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS). High-quality Cu─NiOx film is prepared through a novel and straightforward sol-gel procedure. Multiple experimental characterizations and theoretical calculations show that the incorporation of Cu2+ ions can regulate the energy level structure of NiOx and enhance the hole mobility. The state-of-art CIZS-based QLEDs with Cu─NiOx/PEDOT:PSS bilayered HIL exhibit the maximum external quantum efficiency of 6.04% and half-life time of 48 min, which is 1.3 times and four times of the device with only PEDOT:PSS HIL. The work provides a new pathway for developing high-performance cadmium-free QLEDs.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Charging and Charged Species in Quantum Dot Light-Emitting Diodes

Despite recent rapid advances in improving quantum dot light-emitting diodes, many fundamental aspects of the device operating mechanism remain unresolved. Through transient electroluminescence and time-resolved photoluminescence measurements, the effects of offset voltage on charging and charge transport are examined. First, capacitive charging occurs with a time constant of ∼500 ns, followed by electron transport through quantum dots with a mobility of ∼10^-5 cm² V^-1 s^-1. Hole injection then initiates an electroluminescence rise that is independent of offset voltage. The photoluminescence lifetime is also unaffected by the offset voltage, indicating no injection of charges into the quantum dots or on their surfaces prior to the voltage pulse. A slower equilibration to steady-state electroluminescence is dependent on the offset voltage, indicative of another charging process. Elemental mapping shows that ZnO deposition from solution can lead to the diffusion of charged species into the quantum dot layer, which may cause the slower process.

Calibrating the Hole Mobility Measurements Implemented by Transient Electroluminescence Technology

To date, measuring the carrier mobility in semiconductor films, especially for the amorphous organic small-molecule films, is still a big challenge. Here, we demonstrate that transient electroluminescence (TrEL) spectroscopy with quantum-dot light-emitting diodes as the platform is a feasible and reliable method to evaluate the carrier mobility of such amorphous films. The position of the exciton formation zone is precisely determined and controlled by employing a quantum dot monolayer as the emissive layer. The electrical field intensity across the organic layer is evaluated through the charge density at the electrode calculated by the transient current. Then, the charge carrier mobility is obtained by combining the electroluminescence (EL) delay time and the thickness of the organic layer. Additionally, we demonstrate that the large roughness of the organic layer leads to serious charge accumulation and, hence, a high localized electrical field, which provides preferred charge injection paths, reducing the EL delay time and underestimating the EL delay time. Therefore, a thick organic film is the prerequisite for a reliable measurement of charge carrier mobility, which can circumvent the negative effect of film roughness.

Improving the voltage tolerance of perovskite light-emitting diodes via a charge-generation layer

A high electrical field is necessary to achieve a high brightness for halide perovskite light-emitting diodes (PeLEDs). Charge accumulation in the perovskite film becomes more serious under a high electrical field owing to the imbalanced charge injection in PeLEDs. Concomitantly, the perovskite film will suffer from a higher electrical field increased by the accumulated-charge-induced local electrical field, dramatically accelerating the ion migration and degradation of PeLEDs. Here we construct a voltage-dependent hole injection structure consisting of a ZnO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) bilayer, which can properly adjust the hole injection according to the driving electrical field, matching with the injected electrons. As a result, the ZnO/PEDOT:PSS-containing PeLED can be operated under higher driving voltage with a higher peak brightness of 18920 cd/m², which is 84% higher than the reference device based on a PEDOT:PSS single layer. Moreover, the ZnO/PEDOT:PSS-containing PeLED delivers a much higher power efficiency than the reference device under high driving voltages.

Efficient flexible quantum-dot light-emitting diodes with unipolar charge injection

The exfoliation between the electrode film and the adjacent functional layer is still a big challenge for the flexible light emitting diodes, especially for the devices dependent on the direct charge injection from the electrodes. To address this issue, we design a flexible quantum-dot light-emitting diodes (QLEDs) with a charge-generation layer (CGL) on the bottom electrode as the electron supplier. The CGL consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO can provide sufficient electron injection into the QDs, enabling a balanced charge injection. As a result, the CGL-based QLED exhibits a peak external quantum efficiency 18.6%, over 25% enhancement in comparison with the device with ZnO as the electron transport layer. Moreover, the residual electrons in the ZnO can be pulled back to the PEDOT:PSS/ZnO interface by the storage holes in the CGL, which are released and accelerates the electron injection during the next driving voltage pulse, hence improving the electroluminescence response speed of the QLEDs.

Exploring the emission mechanism of dichromatic white-light quantum-dot light-emitting diodes using wavelength-resolved transient electroluminescence analysis

Exploring electroluminescence (EL) processes is extremely vital to fabricate efficient white-light quantum-dot light-emitting diodes (QLEDs). A model white QLED consisting of a bilayer CdSe/ZnSeS quantum-dot (QD)//CuInS₂/ZnS QDs emissive layer has been used to analyze the white-light emission mechanism. In this design, the CdSe/ZnSeS QDs and CuInS₂/ZnS QDs contribute to the blue and yellow emissions, respectively, in the dichromatic white QLED. Wavelength-resolved transient EL (TrEL) results demonstrate that the excitons are mainly formed on the CuInS₂/ZnS QDs in the QLED operated at low biases due to the low barrier to hole injection and energy transfer from the CdSe/ZnSeS QDs to the CuInS₂/ZnS QDs. Further, the TrEL decays of both white and monochromic devices reveal that the emission behavior of the white QLED is closely related to that of the monochromic device, but is minimally affected by the interactions between different emission units. The simulation results performed by the solar cell capacitance simulator model agree well with the experimental data. Our results show an insight into the EL processes in the white device QLED and demonstrate a powerful tool to investigate emission behavior of the white QLEDs.

Red Light-Emitting Diodes with All-Inorganic CsPbI3/TOPO Composite Nanowires Color Conversion Films

This work presents a method for obtaining a color-converted red light source through a combination of a blue GaN light-emitting diode and a red fluorescent color conversion film of a perovskite CsPbI3/TOPO composite. High-quality CsPbI3 quantum dots (QDs) were prepared using the hot-injection method. The colloidal QD solutions were mixed with different ratios of trioctylphosphine oxide (TOPO) to form nanowires. The color conversion films prepared by the mixed ultraviolet resin and colloidal solutions were coated on blue LEDs. The optical and electrical properties of the devices were measured and analyzed at an injection current of 50 mA; it was observed that the strongest red light intensity was 93.1 cd/m2 and the external quantum efficiency was 5.7% at a wavelength of approximately 708 nm when CsPbI3/TOPO was 1:0.35.

Solution-processed double-layered hole transport layers for highly-efficient cadmium-free quantum-dot light-emitting diodes

The search for heavy-metal-free quantum-dot light-emitting diodes (QD-LEDs) has greatly intensified in the past few years because device performance still falls behind that of CdSe-based QD-LEDs. Apart from the effects of nanostructures of the emitting materials, the unbalanced charge injection and transport severely affects the performance of heavy-metal-free QD-LEDs. In this work, we presented solution-processed double hole transport layers (HTLs) for improving the device performance of heavy-metal-free Cu-In-Zn-S(CIZS)/ZnS-based QD-LEDs, in which N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) as an interlayer was incorporated between the emitting layer and the HTL. Through optimizing the thickness of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl-amine (TFB) and TPD layers, a maximum external quantum efficiency (η_EQE) of 3.87% and a current efficiency of 9.20 cd A^-1 were achieved in the solution-processed QD-LEDs with double-layered TFB/TPD as the HTLs, which were higher than those of the devices with pristine TFB, TPD and TFB:TPD blended layers. The performance enhancement could be attributed to the synergistic effects of the reduction of the hole injection barrier, the increase of the hole mobility and suppressed charge transfer between the HTL and the emitting layer. Furthermore, the best η_EQE of 5.61% with a mean η_EQE of 4.44 ± 0.73% was realized in the Cu-In-Zn-S-based QD-LEDs by varying the annealing temperature of TPD layer due to the more balanced charge injection and transport as well as smooth surface of TPD layer.

Spectroscopic and Magneto-Optical Signatures of Cu1+ and Cu2+ Defects in Copper Indium Sulfide Quantum Dots

Colloidal quantum dots (QDs) of I-III-VI ternary compounds such as copper indium sulfide (CIS) and copper indium selenide (CISe) have been under intense investigation due to both their unusual photophysical properties and considerable technological utility. These materials feature a toxic-element-free composition, a tunable bandgap that covers near-infrared and visible spectral energies, and a highly efficient photoluminescence (PL) whose spectrum is located in the reabsorption-free intragap region. These properties make them attractive for light-emission and light-harvesting applications including photovoltaics and luminescent solar concentrators. Despite a large body of literature on device-related studies of CISe(S) QDs, the understanding of their fundamental photophysical properties is surprisingly poor. Two particular subjects that are still heavily debated in the literature include the mechanism(s) for strong intragap emission and the reason(s) for a poorly defined (featureless) absorption edge, which often "tails" below the nominal bandgap. Here, we address these questions by conducting comprehensive spectroscopic studies of CIS QD samples with varied Cu-to-In ratios using resonant PL and PL excitation, femtosecond transient absorption, and magnetic circular dichroism measurements. These studies reveal a strong effect of stoichiometry on the concentration of Cu¹⁺ vs Cu²⁺ defects (occurring as Cu_In^″ and Cu_Cu^• species, respectively), and their effects on QD optical properties. In particular, we demonstrate that the increase in the relative amount of Cu²⁺ vs Cu¹⁺ centers suppresses intragap absorption associated with Cu¹⁺ states and sharpens band-edge absorption. In addition, we show that both Cu¹⁺ and Cu²⁺ centers are emissive but are characterized by distinct activation mechanisms and slightly different emission energies due to different crystal lattice environments. An important overall conclusion of this study is that the relative importance of the Cu²⁺ vs Cu¹⁺ emission/absorption channels can be controlled by tuning the Cu-to-In ratio, suggesting that the control of sample stoichiometry represents a powerful tool for achieving functionalities (e.g., strong intragap emission) that are not accessible with ideal, defect-free materials.