Highly efficient and low turn-on voltage quantum-dot light-emitting diodes using a ZnMgO/ZnO double electron transport layer

A ZnMgO and ZnO double-layered structure was prepared to create a stepwise interfacial electronic structure to improve the electron-injection and electron-transport behaviors in quantum-dot light-emitting diodes (QLEDs). The current density of the electron-only device (EOD) with ZnMgO/ZnO was higher than that of the EOD with only ZnMgO. The detailed QLED interfacial electronic structure was measured using X-ray and ultraviolet photoelectron spectroscopy. A stepwise interfacial electronic structure for electron injection and electron transport was observed connecting the aluminum cathode to the ZnMgO conduction band minimum (CBM) via the ZnO CBM. The QLEDs with the ZnMgO/ZnO double electron transport layer showed an improved performance, with a maximum luminance and current efficiency of 90,892 cd m⁻² and 19.2 cd A⁻¹, respectively. Moreover, the turn-on voltage of the device was significantly reduced to 2.6 V due to the stepwise interfacial electronic structure between the aluminum cathode and ZnMgO CBM. This research provides a useful method for developing highly efficient and low turn-on voltage QLEDs using a ZnMgO/ZnO double ETL for next-generation display.     

Buffer-less Cu(In,Ga)Se2 solar cells by band offset control using novel transparent electrode

Cu(In,Ga)Se₂ (CIGS) solar cells without buffer layers have been demonstrated. Currently, CdS, Zn(O,S,OH), ZnS, or InS buffer layers are used in high efficiency CIGS solar cells to suppress interface recombination. One of the important parameters to reduce the recombination is the conduction band offset (CBO) between the buffer and CIGS layers. In this study, we have proposed the use of a novel transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. The device simulation was used to verify the effect of CBO control theoretically. Then, the novel TCO material of ZnO_1?xS_x:Al prepared by co-sputtering of ZnO:Al₂O₃ and ZnS targets was fabricated to verify the CBO effect experimentally. The efficiency of a CIGS solar cell with a ZnO:Al/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure using a conventional TCO, was significantly low because of severe shunting. In contrast, the use of ZnO_1-xS_x:Al instead of ZnO:Al increased the shunt resistance of the CIGS solar cell, resulting in higher open-circuit voltage and efficiency. The result is the first proof of the concept of the buffer-less CIGS solar cells.     

Dye-sensitized solar cells based on ZnO nanowire array/TiO2 nanoparticle composite photoelectrodes with controllable nanowire aspect ratio

The ZnO nanowire (NW) array/TiO₂ nanoparticle (NP) composite photoelectrode with controllable NW aspect ratio has been grown from aqueous solutions for the fabrication of dye-sensitized solar cells (DSSCs), which combines the advantages of the rapid electron transport in ZnO NW array and the high surface area of TiO₂ NPs. The results indicate that the composite photoelectrode achieves higher overall photoelectrical conversion efficiency (η) than the ZnO NW alone. As a result, DSSCs based on the ZnO NW array/TiO₂ NP composite photoelectrodes get the enhanced photoelectrical conversion efficiency, and the highest η is also achieved by rational tuning the aspect ratio of ZnO NWs. With the proper aspect ratio (ca. 6) of ZnO NW, the ZnO NW array/TiO₂ NP composite DSSC exhibits the highest conversion efficiency (5.5 %). It is elucidated by the dye adsorption amount and interfacial electron transport of DSSCs with the ZnO NW array/TiO₂ NP composite photoelectrode, which is quantitatively characterized using the UV-Vis absorption spectra and electrochemical impedance spectra. It is evident that the DSSC with the proper aspect ratio of ZnO NW displays the high dye adsorption amount and fastest interfacial electron transfer.     

Successive ionic layer adsorption and reaction of ZnSe shells for ZnO nanowire-based dye-sensitized solar cells

Thin ZnSe layers were deposited on ZnO nanowires by a novel successive ionic layer adsorption and reaction technique in order to solve recombination problems in ZnO nanowire-based dye-sensitized solar cells (DSSCs). Cell efficiency increased from 0.1 to 1.3–1.4% with the deposition of a 9- to13-nm-thick ZnSe shell on ZnO nanowires due to a large increase in J_SC. The dramatic increase in J_SC and cell efficiency is due to the facilitation of electron transfer related to ambipolar diffusion by the formation of a type II band alignment and the suppression of recombination in the presence of the ZnSe shell.     

Hybrid photoanode films based on sparse ZnO rod array-TiO2 nanoparticles in dye-sensitized solar cells

ZnO-TiO2 hybrid photoanodes were fabricated via the doctor-blade method by integrating vertically-grown sparse ZnO arrays with hydrothermal TiO2 nanoparticles. A special surface-coating technique was developed to deposit a thin TiO2 layer on the surface of ZnO rods. Microstructure, optical and photoelectrochemical performance of the hybrid photoanodes were investigated. The denser ZnO array exhibited bad filling behavior of nanoparticles in the interspace of ZnO rods, strong scattering and low conversion efficiency (0.27%). The sparser array showed a much better integrated microstructure, improved transmittance and high conversion efficiency (2.68%). The surface modification of ZnO rods by the TiO2 thin layer was found useful in improving the interfacial microstructure between the ZnO rod and the TiO2 bulk film, and the total conversion efficiency of 3.01% was achieved, higher than that of the pure TiO2 nanoparticle cell (2.93%). The increased scattering effects on the incident light, the enhanced electron transportation at TiO2/dye/electrolyte interface, and the inhabited recombination were responsible for this improvement.     

有机/无机复合双层电子传输层的量子点发光二极管

采用有机/无机复合双层电子传输层（ETL）研制绿色QLEDs,其中有机ETL采用OLED中常见的ETL材料,无机ETL采用ZnO纳米颗粒,并通过调控有机ETL厚度改变电子注入,使电子/空穴达到平衡。制备的器件结构为:ITO/PEDOT:PSS/TFB/QDs/ZnO NPs/TPBI:Liq/Al,其中有机电子传输层TPBI:Liq采用真空蒸镀沉积。与仅采用ZnO电子传输层的器件相比,可以使器件性能得到大幅提升:器件的最大电流效率从11.53 cd/A提升到22.77 cd/A,同时器件的启亮电压、电致发光光谱无明显变化。判断有机ETL的主要作用是抑制了过量电子的注入和传输,在发光亮度变化不大的情况下,降低了器件的无效复合（例如俄歇复合）电流,从而使电流效率明显提升。     

Mg-doped ZnO layer to enhance electron transporting for PbS quantum dot solar cells

In this work, the effect of Mg doping on the performance of PbS quantum dot (QD) solar cells (QDSCs) is investigated. To elucidate that, PbS QDSCs with pristine ZnO and Mg-doped ZnO (ZMO) as electron transporting layers (ETLs) are fabricated, respectively. The current density-voltage (J-V) measurements are performed. The results show that the cell efficiency of the device with ZMO as an ETL is 9.46%, which increases about 75% compared to that of the pristine ZnO based device (5.41%). Enhanced short current density (J_sc) and fill factor (FF) are observed. It is demonstrated that Mg doping could passivate the surface defects and suppress the carrier recombination in ZnO ETL, thus resulting in larger bandgap and higher Fermi level (E_F). The strategy of Mg-doped ZnO ETL provides a promising way for pushing solar cell performance to a high level.     

Increased open‐circuit voltage of ZnO nanowire/PbS quantum dot bulk heterojunction solar cells with solution‐deposited Mg(OH)2 interlayer

An ultrathin Mg(OH)₂ layer was solution‐deposited onto the ZnO nanowires to solve the problem of interfacial charge recombination, caused by the increase of interfacial area in bulk heterojunction (BHJ) PbS colloidal quantum dot solar cells (CQDSCs). This Mg(OH)₂ interlayer efficiently passivated the surface defects of ZnO nanowires and provided tunnel barrier at ZnO/PbS interface. As a result, the charge recombination at ZnO/PbS interface was largely suppressed, proved by the significantly elongated electron lifetime and the increased open‐circuit voltage of the Mg(OH)₂‐involved BHJ CQDSCs. Careful thickness optimization of Mg(OH)₂ interlayer finally brought a ～33% increase in V_oc and ～25% improvement in power conversion efficiency.     

Interfaces of high-efficiency kesterite Cu_2ZnSnS(e)_4 thin film solar cells

Cu_2ZnSnS(e)_4(CZTS(e)) solar cells have attracted much attention due to the elemental abundance and the nontoxicity.However,the record efficiency of 12.6% for Cu_2ZnSn(S,Se)_4(CZTSSe)solar cells is much lower than that of Cu(In,Ga)Se_2(CIGS)solar cells.One crucial reason is the recombination at interfaces.In recent years,large amount investigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods.This paper gives a review of progresses on interfaces of CZTS(e)solar cells,including:(i)the band alignment optimization at buffer/CZTS(e)interface,(ii)tailoring the thickness of MoS(e)_2 interfacial layers between CZTS(e)absorber and Mo back contact,(iii)the passivation of rear interface,(iv)the passivation of front interface,and(v)the etching of secondary phases.     

微波吸收法研究ZnO光电子衰减过程

微波吸收无接触测量技术可以用于半导体粉体材料、微晶材料等研究光生载流子衰减过程。本文采用微波吸收法在室温下分别测量了ZnO纳米材料和微晶材料的光电子衰减过程。发现在紫外激光短脉冲激发下,两种材料的导带光电子寿命有很大的差异,ZnO微晶粉体材料的光电子寿命为50ns,而ZnO纳米材料的光电子寿命仅为10ns。分析认为纳米ZnO的光电子寿命缩短是由于纳米ZnO晶体的表面积远远大于体材料的表面积,纳米材料的表面形成了大量的缺陷能级,加速了光电子的表面复合,缩短了光电子的寿命。纳米材料内部缺陷增多和量子限域效应同样会缩短光电子的寿命。     

Laser-Induced Growth Device and Optical Properties of ZnO MicrocrystalsEI北大核心CSCD

ZnO is third-generation semiconductors which can be used as the carrier of ultraviolet photoluminescence and multiresonance mode laser. In recent years, ZnO microcrystals prepared by optical vapor supersaturation precipitation (OVSP) have shown important advantages in photocatalysis, efficient multi-color light source and efficient electroluminescence. However, the high preparation cost and low production efficiency hinder the development of the large-scale device. In this work, we designed and built a set of growth devices with a working wavelength of 1 080 nm and a power of 18% (@2 500 W) laser heating. The height of the raw material rod was 6. 5 mm, and the diameter was 8 mm. The results show that the morphology, structure, and luminescence properties of the products prepared by this device are very close to those prepared by the OVSP method, and the production efficiency is greatly improved (similar to 500 %). The growth device successfully prepared acceptor-rich ZnO single crystal micro rods with complete hexagonal cross-section morphology. The diameter and length of ZnO micro rods are about 3. 8 and 10 similar to 20 mu m, respectively. Raman spectra show that the Raman peaks of ZnO micro rods are sharp, and the Raman mode at 437 cm(-1) indicates that the ZnO micro rods are hexagonal wurtzite structures with good crystallinity. By analysing the PL spectra of ZnO micro rods, it was found that the ZnO microtubes prepared by the OVSP method had a similar ultraviolet bimodal structure, indicating that there exists an abundant zinc-vacancies acceptor. In the 80 similar to 280 K range, with the increase of temperature, the fluorescence intensity of ZnO microrods appears "thermal quenching-negative thermal quenching-thermal quenching" behavior. The negative thermal quenching behavior in the range of 166 similar to 200 K is related to the intermediate state energy level (trap center) at 477 meV below the conduction band bottom, and the thermal quenching phenomenon in the range of 200 similar to 280 K is related to the non-radiative recombination center at 600 meV below the conduction band bottom. The appearance of both is related to the prepared ZnO microrod oxygen vacancy (V-o) defect. The laser growth device developed in this paper has high feasibility and practicability. This preparation method lays a technical foundation for the rapid batch growth of ZnO single crystal micro rods with rich acceptors and is also of great significance for its application in optoelectronic devices.     

硝酸铟诱导的电沉积氧化锌纳米柱光学性质裁剪

为在新型太阳能电池等光电器件中应用ZnO纳米结构,需要对ZnO纳米结构阵列的几何形貌及光电物理性质进行裁剪与操控。采用电化学沉积路线制备ZnO纳米柱阵列,In(NO3)3与NH4NO3两种盐类被溶入在传统Zn(NO3)2主电解液中。对ZnO纳米柱阵列进行扫描电子显微镜、透射反射光谱、光致发光光谱测试,分析其形貌与光电物理性质。随着引入的In(NO3)3浓度的增加,ZnO纳米柱阵列的平均直径随之由57 nm减小至30 nm。同时ZnO纳米柱的阵列密度也可降低,进而增大纳米柱间距至41 nm。由于新的盐类的引入,ZnO纳米柱的光学带隙由3.46 eV蓝移至3.55 eV。随着电解液中In(NO3)3的增加,ZnO纳米柱的斯托克斯位移由198 meV减小至154 meV,ZnO纳米柱中的非辐射复合可以得到一定程度的抑制。通过在主电解液中引入In(NO3)3与NH4NO3两种盐类,可对ZnO纳米柱的直径、密度、间距、透射反射率、光学带隙、近带边发射与非辐射复合进行操控与裁剪。     

White light electroluminescence from poly(9-vinylcarbazole) (PVK)/ZnO NRs/poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV) dual heterojunctions

In this paper, we fabricated a p-PVK/n-ZnO nanorods (NRs)/p-MEH-PPV dual heterojunctions white light-emitting diode. Relative to previously reported p–n heterojunction structure including ZnO NRs and polymer, the device exhibits a low turn-on voltage of 7 V. An obviously broad electroluminescence emission band, originated from the overlap of PVK emission and ZnO defects emissions, was observed extending from 360 up to 700 nm. The influence of the two introduced p-type polymer layers on the device characteristic is discussed. With its hole conductivity, the p-PVK layer cannot only improve the holes tunnel into ZnO NRs layer, but also lower the barrier between ITO and the valance band of ZnO NRs. On the other hand, p-MEH-PPV could be regarded as block layer for the injection of electrons from the Al electrode. Both of two p-type polymers dramatically improve the injection balance of carriers, leading to a low turn-on voltage. Meanwhile, the carrier transport mechanism of the device under different forward bias region was discussed on the basis of current–voltage curve.     

Non-vacuum, single-step conductive transparent ZnO patterning by ultra-short pulsed laser annealing of solution-deposited nanoparticles

A solution-processable, high-concentration transparent ZnO nanoparticle (NP) solution was successfully synthesized in a new process. A highly transparent ZnO thin film was fabricated by spin coating without vacuum deposition. Subsequent ultra-short-pulsed laser annealing at room temperature was performed to change the film properties without using a blanket high temperature heating process. Although the as-deposited NP thin film was not electrically conductive, laser annealing imparted a large conductivity increase and furthermore enabled selective annealing to write conductive patterns directly on the NP thin film without a photolithographic process. Conductivity enhancement could be obtained by altering the laser annealing parameters. Parametric studies including the sheet resistance and optical transmittance of the annealed ZnO NP thin film were conducted for various laser powers, scanning speeds and background gas conditions. The lowest resistivity from laser-annealed ZnO thin film was about 4.75×10⁻² Ω cm, exhibiting a factor of 10⁵ higher conductivity than the previously reported furnace-annealed ZnO NP film and is even comparable to that of vacuum-deposited, impurity-doped ZnO films within a factor of 10. The process developed in this work was applied to the fabrication of a thin film transistor (TFT) device that showed enhanced performance compared with furnace-annealed devices. A ZnO TFT performance test revealed that by just changing the laser parameters, the solution-deposited ZnO thin film can also perform as a semiconductor, demonstrating that laser annealing offers tunability of ZnO thin film properties for both transparent conductors and semiconductors.     

Performance enhancement of ZnO nanowires/PbS quantum dot depleted bulk heterojunction solar cells with an ultrathin Al_2O_3 interlayer

Depleted bulk heterojunction(DBH)PbS quantum dot solar cells(QDSCs),appearing with boosted short-circuit current density(J_(sc)),represent the great potential of solar radiation utilization,but suffer from the problem of increased interfacial charge recombination and reduced open-circuit voltage(V_(oc)).Herein,we report that an insertion of ultrathin Al_2O_3 layer(ca.1.2 A thickness)at the interface of ZnO nanowires(NWs)and Pb S quantum dots(QDs)could remarkably improve the performance of DBH-QDSCs fabricated from them,i.e.,an increase of V_(oc) from 449 mV to 572 mV,Jsc from21.90 mA/cm~2 to 23.98 mA/cm~2,and power conversion efficiency(PCE)from 4.29% to 6.11%.Such an improvement of device performance is ascribed to the significant reduction of the interfacial charge recombination rate,as evidenced by the light intensity dependence on Jsc and Voc,the prolonged electron lifetime,the lowered trap density,and the enlarged recombination activation energy.The present research therefore provides an effective interfacial engineering means to improving the overall performance of DBH-QDSCs,which might also be effective to other types of optoelectronic devices with large interface area.     

纳米ZnO薄膜对有机电致发光器件性能的影响

由于有机电致发光器件(Organic light-emitting devices,OLEDs)的主动发光、高亮度等优点,在显示和照明领域有极大的应用前景。报道了纳米ZnO薄膜对这种发光器件性能的影响。在普通有机电致发光器件空穴传输层和发光层之间直接蒸镀一层纳米ZnO薄膜,当纳米ZnO薄膜的厚度为1nm时,器件的电流效率可达3.26cd/A,是没有纳米ZnO薄膜同类器件的1.24倍。适当厚度的纳米ZnO薄膜降低了发光层空穴的浓度,提高了电子和空穴的平衡,从而提高了器件的效率。     

等价阴-阳离子共掺杂调节ZnO的能带结构及其光催化活性

采用等价阴-阳离子共掺的方法调节ZnO的能带结构,提高其光催化分解水制氢的效率。计算结果表明：等价阴-阳离子共掺不仅减小了ZnO的带隙,使其在可见光区域的光吸收增强,而且能有效抑制电子-空穴的复合,提高载流子迁移率。（Cd+Te）共掺杂的ZnO是较理想的水分解的光催化剂,因为它具有合适的带隙、较少的电子-空穴复合中心、增强的可见光区域的光吸收和与水氧化还原势相匹配的带边位置。等价阴-阳离子共掺的方法也可运用到其它宽带隙的半导体中以提高光催化活性。     

Reveal the large open-circuit voltage deficit of all-inorganicCsPbIBr2 perovskite solar cells

Due to excellent thermal stability and optoelectronic properties, all-inorganic perovskite is one of the promising candidates to solve the thermal decomposition problem of conventional organic—inorganic hybrid perovskite solar cells (PSCs), but the larger voltage loss (V_loss) cannot be ignored, especially CsPbIBr₂, which limits the improvement of efficiency. To reduce V_loss, one promising solution is the modification of the energy level alignment between the perovskite layer and adjacent charge transport layer (CTL), which can facilitate charge extraction and reduce carrier recombination rate at the perovskite/CTL interface. Therefore, the key issues of minimum V_loss and high efficiency of CsPbIBr₂-based PSCs were studied in terms of the perovskite layer thickness, the effects of band offset of the CTL/perovskite layer, the doping concentration of the CTL, and the electrode work function in this study based on device simulations. The open-circuit voltage (V_oc) is increased from 1.37 V to 1.52 V by replacing SnO₂ with ZnO as the electron transport layer (ETL) due to more matching conduction band with the CsPbIBr₂ layer.     

The effect of etching interfacial layers on the absolute photonic band gap in two-dimensional photonic crystals

The photonic band structures of two-dimensional (2D) photonic crystals with etched interfacial layers between air rods and the background dielectric is studied theoretically. The effect of etching interfacial layers on absolute photonic band gap (PBG) is analyzed quantitatively. Numerical calculations are carried out based on Maxwell's equations and the plane-wave expansion method. It is shown that the physical property of interfacial layers influence the absolute PBG, and the existence of interfacial layers cannot enlarge the largest absolute PBG of an ideal case without interfacial layers.     

Metal oxide buffer layer for improving performance of polymer solar cells

We report the application of aluminum doped ZnO (ZnO:Al) layer as a buffer on ITO glass for fabrication of non-inverted polymer solar cells. The ZnO:Al thin film was deposited using DC magnetron sputtering, with the thickness being varied from 23 to 100 nm. The devices showed most discernible improvements in their efficiencies when a thin layer of ZnO:Al film of thickness ∼40 nm was introduced. The observed enhancement in short circuit current density and open circuit voltage is likely attributed to the role of the ZnO:Al film as an optical tuner and an interfacial diffusion barrier. The result suggests that a metal oxide layer inserted between ITO and polymer layers can be a route for improving both efficiency and stability of polymer solar cells.