Efficient Tin-Based Perovskite Solar Cell with a Cesium Acetate Pre-buried PEDOT:PSS Hole Transport Layer.

The strong Lewis acid tin halide leads to an excessively fast crystallization rate, resulting in more defects in the film and degraded device performance. In this work, a cesium acetate (CsAc) pre-buried poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer acts as nucleation points during the crystallization of tin-based perovskite, which can induce preferential orientation growth of crystals and increase the grain size to improve the quality of crystallization. The addition of CsAc not only can increase the conductivity of PEDOT:PSS but also can improve the wettability of the perovskite precursor solution to enhance the interface contact between the hole transport layer and perovskite layer. Because of the incorporation of CsAc in PEDOT:PSS, the average short-circuit current density increases from 23.80 to 27.60 mA cm-2. Furthermore, a power conversion efficiency of 10.99% is achieved for a tin-based perovskite solar cell with CsAc-doped PEDOT:PSS as the hole transport layer.

Understanding of the Intrinsic Difference between n- and p-Doping Propagation in Polymer Light-Emitting Electrochemical Cells: A Numerical Modeling Approach.

Distinct doping propagation characteristics between p-doping and n-doping in light-emitting electrochemical cells (LECs) have been highlighted by intensive reports. Typically, there are significant differences in the doping speeds between p-doping and n-doping, with the former exhibiting a sawtooth frontier and the latter displaying a more uniform frontier profile. In addition, experimental observations demonstrate a uniform motion instead of the theoretically suggested accelerated electrochemical doping frontier propagation. Therefore, there is an urgent need to establish a quantitative model that delves into the underlying mechanisms responsible for doping propagation in LECs. In this study, four variables were selected to investigate the detailed mechanism of electrochemical doping propagation: temperature, voltage, and concentrations of salt and solid electrolyte. Fluorescence imaging revealed that the n-doping and p-doping propagations behaved contrarily with increasing temperature and voltage. By numerically fitting the doping propagation frontier, equations were derived to describe the relationship between the speed of electrochemical doping propagation and temperature/voltage. The underlying mechanisms were elucidated, indicating that anions undergo motion through the cooperative effects of electric field drift and concentration diffusion, while cation transport strongly relies on poly(ethylene oxide) (PEO) segmental motions. In other words, the movement of anions within the electrolyte is characterized by a greater degree of freedom, whereas the motion of cations is significantly dependent on the segmental motions of PEO. The resulting equations were well-fitted with experimental data, providing a solid foundation for further theoretical investigations into electrochemical doping in various devices.

Low-Operating-Voltage Two-Dimensional Tin Perovskite Field-Effect Transistors with Multilayer Gate Dielectrics Based on a Fluorinated Copolymer.

The fabrication of organic-inorganic perovskite field-effect transistors (FETs) with polymer gate dielectrics is challenging because of the solvent corrosion and wettability issues at interfaces. A few polymers have been integrated into perovskite transistors; however, these devices have high operating voltages due to low dielectric constants. Herein, poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) with a high dielectric constant is introduced into bottom-gate phenylethylammonium tin iodide perovskite [(PEA)2SnI4] FETs. Polytetrafluoroethylene (PTFE) and cross-linked poly(4-vinylphenol) (PVP) (CL-PVP) are used to address the issues of solvent corrosion and wettability. We design the PVDF-TrFE/PTFE and PVDF-TrFE/PTFE/CL-PVP dielectric layers, where the ferroelectric properties of PVDF-TrFE are reduced by PTFE. The (PEA)2SnI4 FETs operate at relatively low gate voltages, exhibiting good overall performance with average hole mobilities of 0.42 and 0.36 cm2 V-1 s-1. Our findings provide a feasible strategy for constructing low-operating-voltage perovskite FETs with large-dielectric-constant ferroelectric polymers as gate dielectrics by a solution processing technique.

Ionic Liquid-Gated Near-Infrared Polymer Phototransistors and Their Persistent Photoconductivity Application in Optical Memory.

Organic phototransistors (OPTs) based on polymers have attracted substantial attention due to their excellent signal amplification, significant noise reduction, and solution process. Recently, the near-infrared (NIR) detection becomes urgent for OPTs with the increased demand for biomedicine, medical diagnostics, and health monitoring. To achieve this goal, a low working voltage of the OPTs is highly desirable. Therefore, the traditional dielectric gate can be replaced by an electrolyte gate to form electrolyte-gated organic phototransistors (EGOPTs), which are not only able to work at voltages below 1.0 V but also are biocompatible. PCDTPT, one of the most popular narrow band gap donor-acceptor copolymer, has been rarely studied in EGOPTs. In this work, an organic NIR-sensitive EGOPT based on PCDTPT is demonstrated with the detectivity of 7.08 × 1011 Jones and the photoresponsivity of 3.56 A/W at a low operating voltage. In addition, an existing persistent photoconductivity (PPC) phenomenon was also observed when the device was exposed to air. The PPC characteristic of the EGOPT in air has been used to achieve a phototransistor memory, and the gate bias can directly eliminate the PPC as an erasing operation. This work reveals the underlying mechanism of the electrolyte-gated organic phototransistor memories and broadens the application of the EGOPTs.

Two-Dimensional Layered Simple Aliphatic Monoammonium Tin Perovskite Thin Films and Potential Applications in Field-Effect Transistors.

Two-dimensional (2D) layered organic-inorganic perovskites have great potential for fabricating field-effect transistors due to their unique structure that enables the horizontal transport of charge carriers in metal-halide octahedra, resembling the transport behavior in semiconducting channels. Their electronic band structures are mainly dominated by the metal-halide octahedra, which eventually determine the optical and electrical characteristics, whereas organic cations have no direct contributions but would impact the electronic structures via distorting the octahedra. So far, high performance has been achieved in 2D Sn perovskites compared to their Pb counterparts because the intrinsic differences of Sn promote transport properties. The champion hole mobility has been obtained in single-ring aromatic phenylethylammonium tin iodide perovskite [(PEA)2SnI4]. However, simple aliphatic monoammonium tin perovskites and their device applications have rarely been reported. Herein, 2D layered n-butylammonium tin iodide perovskite [(BA)2SnI4] thin films have been synthesized by a spin-coating approach. A structural phase transition occurs at about 225 K in the films, accompanied by the changes in the photoluminescence peak and exciton binding energy. Longitudinal optical (LO) phonons are found to govern the scattering of charge carriers and excitons via the Fröhlich interactions in the temperature range 77-300 K. The first-principles calculations predict that the perovskite has excellent transport characteristics comparable to those of molybdenum disulfide (MoS2) and methylammonium lead iodide perovskite (MAPbI3). The (BA)2SnI4 thin film field-effect transistors constructed on polymer dielectrics with a maximum hole mobility of 0.03 cm2 V-1 s-1 in ambient conditions have been successfully demonstrated for the first time. Our findings not only offer a deep insight into the physical properties of 2D layered aliphatic monoammonium tin perovskite thin films but also provide important experimental and theoretical guidance for their potential applications in lateral-type flexible optoelectronic devices.

Metal oxide nanoparticle-modified ITO electrode for high-performance solution-processed perovskite photodetectors.

Low dark current density plays a key role in determining the overall performance of perovskite photodetectors (PPDs). To achieve this goal, a hole transport layer (HTL) on the ITO side and a hole blocking layer (HBL) on the metal electrode side are commonly introduced in PPDs. Unlike traditional approaches, we realized a high-performance solution-processed broadband PPD using metal oxide (MO) nanoparticles (NPs) as the HBL on the ITO electrode and PC61BM as another HBL on the metal electrode side to reduce the device dark current. The PPDs based on TiO2 and SnO2 NP-modified layers show similar device performances at -0.5 V: a greater than 105 on/off ratio; over 100 dB linear dynamic range (LDR) under different visible light illumination; around 0.2 A W-1 responsivity (R); greater than 1012 jones detectivity (D*); and ∼20 µs rise time of the device. The MO NP interfacial layer can significantly suppress charge injection in the dark, while the accumulated photogenerated charges at the interface between the MO layer and the perovskite layer introduce band bending, leading to dramatically increased current under illumination. Therefore, the dark current density of the devices is significantly reduced and the optical gain is drastically enhanced. However, after UV illumination, the dark current of the TiO2 device dramatically increases while the dark current of the SnO2 device can stay the same as before since the UV illumination-induced conductivity and barrier height changes in the TiO2 layer cannot recover after removing the UV irradiation. These results indicate that the TiO2 NP layer is suitable for making a vis-NIR photodetector, while the SnO2 NP layer is a good candidate for UV-vis-NIR photodetectors. The facile solution-processed high-performance perovskite photodetector using MO NP-modified ITO is highly compatible with low cost, flexible, and large-area electronics.

Property Modulation of Two-Dimensional Lead-Free Perovskite Thin Films by Aromatic Polymer Additives for Performance Enhancement of Field-Effect Transistors.

The inevitable oxidation of Sn2+ and p-type self-doping has plagued the development of two-dimensional (2D) Sn-based perovskite field effect transistors. In this work, we demonstrate the modulation of the properties of phenethylammonium tin iodide ((PEA)2SnI4) perovskite thin films by introducing the aromatic polymer additives of poly(4-vinylphenol) (PVP) and poly(vinyl pyrrolidone) (PVPD) during the crystallization processes, keeping the 2D layered structure of (PEA)2SnI4 unchanged. The proposed formation mechanisms of the polymer-assisted (PEA)2SnI4:PVP and (PEA)2SnI4:PVPD films disclose that the interactions between the polymers and (PEA)2SnI4, such as hydrogen bonds, π-π interactions, and coordination bonds, lead to the improvement of the morphology and crystallization as well as the inhibition of Sn2+ oxidation of the films. However, the field-effect transistors based on the two polymer-assisted (PEA)2SnI4 thin films constructed on the dielectric of poly(vinyl alcohol) (PVA) modified by crosslinking PVP (CL-PVP) exhibit quite a different performance. Compared with the (PEA)2SnI4 transistor, without sacrificing the hole mobility, the on-off current ratio of the (PEA)2SnI4:PVP device increases by one order of magnitude, and the subthreshold slope declines slightly due to the reduced leakage current, which results from the reduction of p-type self-doping of the perovskite film and the improved quality of the perovskite/dielectric interface because of the strong π-π interactions between the benzene rings in CL-PVP and (PEA)2SnI4:PVP. In contrast, the (PEA)2SnI4:PVPD transistor exhibits relatively poor overall performance because of the N-vinylpyrrolidone of PVPD. More importantly, employing PVP and PVPD as additives can effectively enhance the chemical stability of (PEA)2SnI4 as well as the operational stabilities of the corresponding transistors. Our work provides an effective strategy for selecting chemical additives to improve 2D perovskite properties and suppress the oxidation of Sn-based perovskites, and paves a way toward the future applications of Sn-based perovskite optoelectronic devices with high performance and stability.

Strong Triplet-Exciton-LO-Phonon Coupling in Two-Dimensional Layered Organic-Inorganic Hybrid Perovskite Single Crystal Microflakes.

Two-dimensional (2D) layered hybrid perovskites provide an ideal platform for studying the properties of excitons. Here, we report on a strong triplet-exciton and longitudinal-optical (LO) phonon coupling in 2D (C6H5CH2CH2NH3, PEA)2PbBr4 perovskites. The triplet excitons exhibit strong photoluminescence (PL) in thick perovskite microflakes, and the PL is not detectable for monolayer microflakes. The coupling strength of the triplet exciton-LO phonon is approximately two to three times greater than that of the singlet exciton-LO phonon with a LO phonon energy of about 21 meV. This difference might due to the different locations of singlet excitons located in the well and triplet excitons located in the barrier in the 2D layered perovskite. Revealing the strong coupling of triplet exciton-LO phonon provides a fundamental understanding of many-body interaction in hybrid perovskites, which is useful to develop and optimize the optoelectronic devices based on 2D perovskites in the future.

Grain Growth of MAPbI3 via Diethylammonium Bromide Induced Grain Mergence.

Various additives are used to improve the film morphology, crystal quality, and grain size for the sake of enhancing the performance of three-dimensional perovskite solar cells. Although significant enhancement in the performance of devices has been made due to the introduction of additives, an in-depth understanding of the additive-related crystallization kinetics and the growth mechanism is still lacking. Here, the grain growth mechanism of diethylammonium bromide (DABr)-doped MAPbI3 is investigated using in situ dynamic microscopy techniques. The results reveal that the alkyl chains of DABr restrain the growth of grains of MAPbI3 during spin-coating, and DABr-induced grain mergence during the annealing process, achieving large grains on the micrometer scale. Meanwhile, the crystallization of MAPbI3 with DABr is significantly improved and the number of defects is reduced. The solar cell with optimized DABr doping MAPbI3 as the active layer presents a higher power conversion efficiency (PCE) of 19.58% with a fill factor of 79.81%.

Impacts of carrier trapping and ion migration on charge transport of perovskite solar cells with TiO x electron transport layer.

The ion migration in perovskite materials has been extensively studied by researchers, but the charge dynamic distribution caused by ion migration and carrier trapping is partly unclear. To investigate the impacts of ion migration and defect induced carrier trapping on the carrier transport and the carrier collection, we measured the evolution of the photocurrent response in microseconds, milliseconds and seconds for the perovskite solar cells pretreated at different biases. Our results reveal that the photocurrent of the solar cells pretreated at negative bias decreases with time and achieves its minimum at several milliseconds, then rises and achieves its maximum at tens of seconds. For the device pretreated at positive bias beyond built-in potential, the time to reach maximum photocurrent is much shorter than that of the solar cell pretreated at negative bias. The transient photocurrent responses to the sequence of single-light-pulses also show that there is obvious carrier trapping in a positive bias treated device, which indicates that defect induced carrier trapping is the critical factor for the perovskite solar cells with an n-TiO x electron transport layer. In order to improve the performance of the perovskite solar cells with nano-TiO x ETL, it is very necessary to significantly reduce defects. Our results also demonstrate that cation accumulation at the interface between the perovskite active layer and ETL can enhance the device performance to a certain extent.

Improving ternary blend morphology by adding a conjugated molecule into non-fullerene polymer solar cells.

The use of ternary polymer solar cells (PSCs) is a promising strategy to enhance photovoltaic performance while improving the fill factor (FF) of a device, but is still a challenge due to the complicated morphology. Herein, ternary PSCs are fabricated via adding the conjugated small molecule p-DTS(FBTTh2)2 into a well-known blended film, PTB7-Th:IEICO-4F. The ternary blend morphology and device characterization reveal that the addition of p-DTS(FBTTh2)2 can improve crystallinity and optimize morphology, leading to the FF of the optimized device increasing to 73.69%. In combination with the advantages of an ultra-narrow bandgap material, IEICO-4F, with a broad optical absorption spectrum, the optimized ternary solar cell exhibits a high short-circuit current-density (J SC) of 25.22 mA cm-2. The best power conversion efficiency (PCE) is 12.84% for this optimized ternary device with 10 wt% p-DTS(FBTTh2)2 in the donors. This work indicates that incorporating a small molecule with high crystallinity into host binary non-fullerene PSCs would give an active layer with high crystallinity, thus greatly enhancing the FFs and PCEs of PSCs.

Efficient Quasi-Two-Dimensional Perovskite Light-Emitting Diodes with Improved Multiple Quantum Well Structure.

Quasi-two-dimensional (quasi-2D) perovskites with a multiple quantum well structure can enhance the exciton binding energy and controllable quantum confine effect, which are attractive materials for efficient perovskite light-emitting diodes (PeLEDs). However, the multiphase mixtures contained in these materials would cause nonradiative recombination at the perovskite film surface. Here, a facile solution surface treatment is adopted to improve the multiple quantum well structure of the quasi-2D perovskite emitting layer, which can reduce the influence of defectinduced nonradiative recombination and the electric-field-induced dissociation of excitons for the PeLEDs. The improved multiple quantum well structure is verified by UV absorption spectra and temperature-dependent photoluminescence spectra measurements. The photoluminescence quantum yield of the quasi-2D perovskite film with surface treatment has been approximately increased by 200%. Meanwhile, the electroluminescence device achieves a current efficiency of 45.9 cd/A.

Two-dimensional additive diethylammonium iodide promoting crystal growth for efficient and stable perovskite solar cells.

Methylammonium lead iodide perovskite photovoltaics have attracted remarkable attention due to their exceptional power conversion efficiencies (PCEs). The film morphology of organometallic halide perovskite plays a very important role in the performance of planar perovskite solar cells (PVSCs). Previous methods have been explored to control the crystal growth for getting a compact and smooth perovskite film. Here, we report an effective and reproducible approach for enhancing the stability and the efficiency of PVSCs by incorporating a small quantity of two-dimensional (2D) material diethylammonium iodide (DAI) in three-dimensional (3D) MAPbI3, which can facilitate the perovskite crystallization processes and improve the resulting film crystal quality. The fabricated (DA2PbI4)0.05MAPbI3 perovskite hybrid films exhibit good morphology with larger grains and uniform morphology. Simultaneously, reduced defects and enhanced carrier lifetime within a full device indicate enhanced crystallization effects as a result of the DAI inclusion. The photovoltaic device attains a high photocurrent of 22.95 mA cm-2 and a high fill factor of 79.04%, resulting in an overall PCE of 19.05%. Moreover, the stability of the 10% DAI doped perovskite solar cell is also improved. These results offer a promising stable and efficient light-absorbing material for solid-state photovoltaics and other applications.

Investigation on the Overshoot of Transient Open-Circuit Voltage in Methylammonium Lead Iodide Perovskite Solar Cells.

Although the performance of hybrid organic-inorganic perovskite solar cells (PSCs) is encouraging, the detailed working principles and mechanisms of PSCs remain to be further studied. In this work, an overshoot phenomenon of open-circuit voltage (Voc) was observed when the illumination light pulse was switched off. The evolution of the Voc overshoot was systematically investigated along with the intensity and the width of the light pulse, the background illumination, and pretreatment by different bias. Based on the experimental results, we could conclude that the Voc overshoot originated from carrier motion against carrier collection direction, which happened at the ionic-accumulation-induced band bending areas near the interfaces between the perovskite active layer and the two carrier transport layers. The investigation on the Voc overshoot can help us to better understand ionic migration, carrier accumulation, and recombination of PSCs under open-circuit conditions.

High-Performance Photodiode-Type Photodetectors Based on Polycrystalline Formamidinium Lead Iodide Perovskite Thin Films.

Photodetectors based on three dimensional organic-inorganic lead halide perovskites have recently received significant attention. As a new type of light-harvesting materials, formamidinium lead iodide (FAPbI3) is known to possess excellent optoelectronic properties even exceeding those of methylammonium lead iodide (MAPbI3). To date, only a few photoconductor-type photodetectors based on FAPbI3 single crystals and polycrystalline thin films in a lateral structure have been reported. Here, we demonstrate low-voltage, high-overall-performance photodiode-type photodetectors in a sandwiched geometry based on polycrystalline α-FAPbI3 thin films synthesized by a one-step solution processing method and post-annealing treatment. The photodetectors exhibit a broadband response from the near-ultraviolet to the near-infrared (330-800 nm), achieving a high on/off current ratio of 8.6 × 104 and fast response times of 7.2/19.5 µs. The devices yield a photoresponsivity of 0.95 AW-1 and a high specific detectivity of 2.8 × 1012 Jones with an external quantum efficiency (EQE) approaching 182% at -1.0 V under 650 nm illumination. The photodiode-type photodetectors based on polycrystalline α-FAPbI3 thin films with superior performance consequently show great promise for future optoelectronic device applications.

Enhanced performance of tin halide perovskite solar cell by addition of lead thiocyanate.

Organic-inorganic hybrid halide perovskites have attracted great attention as a new type of photovoltaic materials. However, lead (Pb) perovskite solar cells (PSCs) would cause environmental pollution in future large-scale applications. Therefore, it is imperative to find environmentally-friendly metals to replace lead. Although tin (Sn) halide perovskites can be regarded as a valid alternative to lead perovskites, their poor stability and lower conversion efficiency hinder the substitution of Sn for Pb. In this work, highly uniform and pinhole-free perovskite films were prepared by the introduction of a small amount of lead thiocyanate in precursor solutions. The CH3NH3SnI3 (MASnI3) films with Pb additive show an absorption edge of 950 nm. Besides, lead ions can depress the LUMO energy level of Sn-based perovskite materials, which is a benefit to an increase in the opencircuit voltages of PSCs. Consequently, the enhanced performance was achieved in the PSCs based on MASnI3 with a fill factor of 66%, open circuit voltage of 0.54 V and maximum power conversion efficiency of 6.03%.

Interface studies of well-controlled polymer bilayers and field-effect transistors prepared by a mixed-solvent method.

The properties of semiconductor/dielectric interfaces are crucial to the performance of polymer field-effect transistors. The key to fabricating high-performance polymer transistors by spin-coating is solving solvent corrosion issues, wherein the solvent of the top polymer produces a rough interface or damage on the underlying polymer layer during deposition. Herein, we propose a mixed-solvent method that employs a mixture of an orthogonal solvent of the underlying polymer and a good solvent of the top polymer as the solvent of the top polymer to prepare polymer bilayers and produce a comparative study of the trap density at the semiconductor/dielectric interface of the corresponding transistor. By changing the ratio of orthogonal solvent to good solvent, namely the degree of orthogonality of the mixed solvent with respect to the underlying polymer, the interface and film qualities of polymer bilayers can be well controlled. We applied this method to spin-coat poly(3-hexylthiophene) (P3HT) on poly(methylmethacrylate) (PMMA) with a mixture of cyclohexane (orthogonal solvent) and chloroform (good solvent). The results of morphology characterizations and electrical property studies indicate the optimal ratio of cyclohexane to chloroform for preparing high-quality P3HT/PMMA bilayers for field-effect conduction is 7 : 3. Transistors based on the optimal bilayers with a bottom-gate/top-contact configuration and a long channel length show good performance. The trap density at the P3HT/PMMA interface is evaluated to be 3.6 × 1012 cm-2 eV-1 from the subthreshold swing, characterizing the distribution of the interface trap levels across the bandgap in P3HT. Furthermore, based on deviations from ideality in the capacitance-voltage characteristics of the metal-insulator-semiconductor capacitor in the device, the traps at the interface are found to be acceptor-type, with the trap density determined to be 2.3 × 1011 cm-2. This value is in a good agreement with that estimated from the subthreshold swing.

High sensitivity and fast response solution processed polymer photodetectors with polyethylenimine ethoxylated (PEIE) modified ITO electrode.

Most organic photodetectors utilize a bulk heterojunction (BHJ) photo-active film due to its high exciton dissociation efficiency. However, the low dark current density, a key role in determining the overall performance of photodetectors, is hardly achieved in the BHJ structure since both the donor and acceptor domains are in contact with the same electrode. The most popular strategy to overcome this problem is by fabricating bilayer or multilayer devices. However, the complicated fabrication process is a challenge for printing electronics. In this work, we demonstrate a solution processed polymer photodetector based on a poly (3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM) blend film with polyethylenimine ethoxylated (PEIE) modified ITO electrode. The transparent PEIE efficiently blocks the unnecessary electronic charge injection between the active film and the electrode, which dramatically decrease the dark current. Under illumination, the photoexcited charges accumulated in the PEIE modified ITO region finally can tunnel through the barrier with the help of the applied reverse bias, leading to a large photocurrent. Therefore, the resulting polymer photodetector shows a 2.48 × 104 signal-to-noise ratio (SNR) under -0.3 V bias and an 11.4 MHz bandwidth across the visible spectra under a small reverse bias of 0.5 V. The maximum EQE of 3250% in the visible wavelength is obtained for the polymer photodetector at -1 V under 370 nm (3.07 µW/cm2) illumination. This solution processed polymer photodetector manufacturing is highly compatible with the flexible, low-cost, and large area organic electronic technologies.

[The role of BCP in electroluminescence of multilayer organic light-emitting devices].

As a hole-blocking layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is usually used in blue and white light electroluminescent devices. The ability of blocking holes of BCP layer depends on its thickness, and basically holes can tunnel through thin BCP layer. In order to know the role of BCP layer in electroluminescence (EL) of multilayer organic light-emitting diodes (OLEDs), in the present paper, the authors designed a multilayer OLED ITO/NPB/BCP/Alq3 : DCJTB/Alq3/Al and investigated the influence of thickness of BCP on the EL spectra of multilayer OLEDs at different applied voltages. The experimental data show that thin BCP layer can block holes partially and tune the energy transfer between different emissive layers, and in this way, it is easy to obtain white emission, but its EL spectra will change with the applied voltages. The EL spectra of multilayer device will remain relatively stable when BCP layer is thick enough, and the holes can hardly tunnel through when the thickness of BCP layer is more than 15 nm. Furthermore, the stability of EL spectra of the multilayer OLED at different applied voltages was discussed.

[Cathodoluminescent characteristics of green-emitting ZnAl2O4:Mn thin film phosphors].

Green electroluminescence was obtained from thin films of ZnAl2O4: Mn prepared by rf magnetron sputtering onto thick insulating ceramic sheets. Photoluminescence and stress-stimulated luminescence was obtained for Mn-doped ZnAl2O4 powder synthesized by the solid phase reaction. Since it is extremely stable chemically and thermally, ZnAl2O4 may emerge as an alternative choice to sulphide-based phosphors. In the present paper, thin films of ZnAl2O4: Mn were grown on aluminosilicate ceramic plates using spray pyrolysis of aqueous solutions. The cathodoluminescence (CL) properties of the films under low to medium excitation voltage (<5 kV) were investigated. The films exhibited green CL after being annealed at temperatures above 550 degrees C, which corresponds to the transition between 4 T1 and (6)A1 of Mn2+ ions located in the tetra coordination of the Zn2+ site in the spinel structure. The chromaticity coordinates were x = 0.150 and y = 0.734 with a dominant wavelength of 525 nm and an 82% color purity. The CL luminance and efficiency depended on the excitation voltage and current density. Saturation effects were observed as the current density increased. A luminance of 540 cd x m(-2) and an efficiency of 4.5 lm x W(-1) were obtained at an excitation voltage of 4 kV with a current density of 38 microA x cm(-2).