Influence of small-molecule material on performance of polymer solar cells based on MEH-PPV:PCBM blend

In this work,the influence of a small-molecule material,tris(8-hydroxyquinoline) aluminum (Alq 3),on bulk het-erojunction (BHJ) polymer solar cells (PSCs) is investigated in devices based on the blend of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM).By dop-ing Alq 3 into MEH-PPV:PCBM solution,the number of MEH-PPV excitons can be effectively increased due to the energy transfer from Alq 3 to MEH-PPV,which probably induces the increase of photocurrent generated by excitons dissociation.However,the low carrier mobility of Alq 3 is detrimental to the efficient charge transport,thereby blocking the charge collection by the respective electrodes.The balance between photon absorption and charge transport in the active layer plays a key role in the performance of PSCs.For the case of 5 wt.% Alq 3 doping,the device performance is deteriorated rather than improved as compared with that of the undoped device.On the other hand,we adopt Alq 3 as a buffer layer instead of commonly used LiF.All the photovoltaic parameters are improved,yielding an 80% increase in power conversion efficiency (PCE) at the optimum thickness (1 nm) as compared with that of the device without any buffer layer.Even for the 5 wt.% Alq 3 doped device,the PCE has a slight enhancement compared with that of the standard device after modification with 1 nm (or 2 nm) thermally evaporated Alq 3.The performance deterioration of Alq 3-doped devices can be explained by the low solubility of Alq 3,which probably deteriorates the bicontinuous D-A network morphology;while the performance improvement of the devices with Alq 3 as a buffer layer is attributed to the increased light harvesting,as well as blocking the hole leakage from MEH-PPV to the aluminum (Al) electrode due to the lower highest occupied molecular orbital (HOMO) level of Alq 3 compared with that of MEH-PPV.     

一种多层白色磷光有机电致发光器件的制备及性能研究

制备了一种结构为ITO/NPB/NPB:Ir(piq)₂(acac)/CBP:TBPe/BAlq:rubrene/BAlq/Alq₃/Mg:Ag的白色磷光有机电致发光器件.其中空穴传输型主体NPB掺杂磷光染料Ir(piq)₂(acac)作为红色发光层,双载流子传输型主体4,4′-N,N′-dicarbazole-biphenyl (CBP)掺杂TBPe作为蓝色发光层,电子传输型主体材料BAlq掺杂rubrene作为绿色发光层.以上发光层夹于 关键词： 电致发光 磷光染料 异质结 白光     

同时掺杂磷光和荧光染料的白色有机电致发光器件性能研究

以磷光染料Ir(piq)₂(acac)作为发光掺杂剂,掺入空穴传输性主体材料NPB中得到红色发光层,荧光材料TBP掺入到主体CBP中作为蓝色发光层,制备了结构为ITO/NPB/NPB：Ir(piq)₂(acac)/CBP/CBP：TBPe/BCP/ALq/Mg：Ag的双发光层白色有机电致发光器件.其中ALq₃、未掺杂的NPB和CBP及BCP层分别作为电子传输层、空穴传输层和激子阻挡层.实验中通过调节发光层厚度及Ir(piq)_{2关键词： 磷光 激子阻挡层 有机电致发光}     

Organic Light Emitting Diodes Using Doped Alq3 as the Hole-transport Layer

Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane （F4TCNQ） doping on the hole conductivity of Alq3 layer are measured. In the hole-only device of Alq3, the current densities increase in 1-3 orders of magnitude upon doping with F4TCNQ, suggesting that the doping can effectively enhance the hole-injection and hole- transport ability of Alq3. An organic light-emitting device using an F4TCNQ doped Alq3 layer as the hole- injection and hole-transport layer, and pristine Alq3 as the electron-transport and emitting layer is fabricated and characterized. Bright emission is achieved in the simple OLED with p-doped Alq3 as the hole-transport layer and the intrinsic Alq3 as the electron-transport and emitting layer. The emitting efficiency and brightness of the device are further improved by inserting a thin electron block layer to confine the carrier recombination zone in the middle of the organic layers.     

On the origin of exciton formation in dye doped Alq3 OLEDs

Electrically Detected Magnetic Resonance (EDMR) was used to investigate the influence of dye doping on spin-dependent exciton formation in aluminum (III) 8-hydroxyquinoline (Alq₃) based Organic Light Emitting Diodes (OLEDs) with different device structures. 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)-phenyl]ethyl}-4H-pyran (DCM-TPA) and 5,6,11,12-tetraphenylnaphthacene (Rubrene) were used as dopants. Results at room temperature show significant differences on the EDMR spectra (g-factor and linewidth) of doped and undoped devices. Signals from DCM-TPA and Rubrene dye doped OLEDs showed strong temperature dependence, with signal intensity increasing by 2 orders of magnitude below 200 K for DCM-TPA dye doped OLEDs and increasing by ～1 order of magnitude below 225 K for the Rubrene dye doped device, while undoped devices shows almost no temperature dependence. By adding a “spacer” layer of undoped Alq₃ at the recombination zone, changes in bias voltage were used to shift the recombination from doped to undoped region and correlate that with changes in the EDMR spectrum. Our results are indicating that charge trapping on the dopant followed by recombination is the main mechanism of light emission in the investigated devices.     

Li掺杂8-羟基喹啉铝的密度泛函理论研究

采用密度泛函理论研究了Li原子掺杂8-羟基喹啉铝(Alq₃)分子的几何构型、 前线分子轨道及电子转移特性. 研究结果表明, Li原子掺杂Alq₃后, Li原子与Alq₃的O, N原子键合, 形成电子转移复合物. Li原子将部分电子转移到Alq₃的吡啶环上, 在Alq₃的带隙内形成施主能级, 这种n型掺杂结构有效地提高了电子的传输效率; 但过多的Li原子的掺杂会使Alq₃分解, 从而减弱其电子传输能力. 为使Alq₃的电子传输能力达到最高, Li原子的掺杂应保持在2:1左右的比例.     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L2₁结构Co₂FeSi高有序合金之间的Co₅₀Fe_50-xSi_x合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似(GGA+U)方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co₂FeSi合金的分子磁矩和居里温度分别达到5.92μ_B和777 ℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co₂FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内. 关键词： 磁性 Heusler合金 结构相变     

Optimal thickness of hole transport layer in doped OLEDs

Current-voltage (I–V) and electroluminescence (EL) characteristics of organic light-emitting devices with N,N’-Di-[(1-naphthalenyl)-N,N’-diphenyl]-(1,1’-biphenyl)-4,4’-diamine (NPB) of various thicknesses as the hole transport layer, and tris(8-hydroxyquinoline)aluminum (Alq₃) selectively doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) as the electron transport layer, have been investigated. A trapped charge induced band bend model is proposed to explain the I–V characteristics. It is suggested that space charge changes the injection barrier and therefore influences the electron injection process in addition to the carrier transport process. Enhanced external quantum efficiency of the devices due to the electron blocking effect of an inserted NPB layer is observed. The optimal thickness of the NPB layer is experimentally determined to be 12±3 nm in doped devices, a value different from that for undoped devices, which is attributed to the electron trap effect of DCM molecules. This is consistent with the result that the proportion of Alq₃ luminescence in the total electroluminescence (EL) spectra increases with NPB thickness up to 12 nm under a fixed bias. PACS 72.80.Le; 85.60.Jb     

不同主体双发光层白色有机电致发光器件的性能研究

制备了结构为ITO/NPB/CBP:TBPe:rubrene/BAlq:Ir(piq)₂(acac)/BAlq/Alq₃/Mg:Ag的白色磷光有机电致发光器件.利用两种不同的主体材料,即用双载流子传输型主体材料CBP掺杂荧光染料TBPe及rubrene作为蓝光和橙黄光发光层;用电子传输型主体材料BAlq掺杂磷光染料Ir(piq)₂(acac)作为红色发光层.以上双发光层夹于空穴传输层NPB与具有电子传输性的阻挡层BALq之间.讨论了如何控制 关键词： 有机电致发光 磷光染料 掺杂 白光     

富勒烯掺杂NPB空穴传输层的有机电致发光器件

报道了不同掺杂浓度NPB：C₆₀（富勒烯）作为空穴传输层对有机电致发光器件性能的影响.采用真空热蒸镀方法,制作了ITO/ NPB：C₆₀ （x % ）/Alq₃/LiF/Mg:Ag结构的四种有机电致发光器件.当NPB：C₆₀的掺杂浓度是15％时,器件的启亮电压是4 V,最大亮度是11000 cd/m².然而,当NPB：C₆₀的掺杂浓度是20％时,器件的最大亮度降     

Enhanced power efficiency for white OLED with MoO3 as hole injection layer and optimized charge balance

White OLEDs with a different hole injection layer (MoO₃ or m-MTDATA), and a different electron transport layer (Alq₃ or Bphen) have been investigated. With 9,10-bis (2-naphthyl)-2-t-butylanthracene (TBADN) doped with 3% P-bis (P-N, N-diphenyl-aminostyryl) benzene (DSA-ph) and 0.05% 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyl-julolidy-9-enyl)-4H-pyran (DCJTB) as white emitting layer, the MoO₃/ /Bphen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.43 V and 4.54 lm/W respectively, which is independently reduced 40% and improved 57% compared with those of the m-MTDATA/ / Alq₃ based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons for the improvement. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Two peaks observed in the electroluminescence spectra of Alq3-based OLEDs

Tris(8-hydroxyquinolato) aluminum (Alq₃)-based organic light-emitting diodes were fabricated with or without using a hole transport layer (HTL). As a conventional device, the ITO/Alq₃/Mg-Ag device yielded a green-light emission with a single peak at 525 nm in the electroluminescence (EL) spectrum. In contrast, two sub-peaks were observed in the EL spectrum of some ITO/HTL/Alq₃/Mg-Ag devices. This difference was tentatively explained by comparing EL with the photoluminescence (PL) spectrum reported in the literature.     

Bright electroluminescent devices with tunable spectra obtained by strictly controlling the doping concentration of electron injection sensitizer

In this paper, we report an efficient strategy to design bright blue and blue-green electroluminescent (EL) devices by slightly doping tris(8-hydroxyquinoline) aluminum (Alq₃) into N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as the light-emitting layer (EML). Bright EL devices with tunable spectra were obtained by strictly controlling the doping concentration of Alq₃. With increasing current density, EL efficiencies of these devices increase first and then decrease gradually after reaching the maximum. Analyzing the current density-voltage (J−V) characteristics of hole-only and electron-only devices, we found the presence of Alq₃ molecules in EML not only facilitates the injection of electrons from hole block layer (HBL) into EML but also stays the transport of holes in EML, thus causing significant enhancement of EL efficiency and brightness due to improved carriers balance and broadening of recombination zone. More interestingly, the doping concentration of Alq₃ strongly influences the injection and transport processes of electrons, thus determining the distribution of holes and electrons on NPB and Alq₃ molecules.     

利用p型掺杂技术制备的高效有机电致发光器件

利用氧化钼(MoO_x)作为p型掺杂剂,以掺杂层4,4'-bis(carbazol-9-yl)biphenyl(CBP):MoO_x作为空穴注入层,制备了一种结构为ITO/MoO_x/CBP:MoO_x/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)₃)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al的有机电致发光器件.器件中CBP同时作为空穴注入层、空穴传输层以及发光层母体材料,这种结构具有结构简单同时能有效降低空穴注入势垒等优点.研究发现,随着空穴注入层厚度的增加,器件的电流密度增加,表明p型掺杂层的引入能够有效增强空穴的注入;通过优化器件空穴注入层与空穴传输层厚度,器件性能有所提高,最大电流效率为29.8 cd/A,可以认为合理的优化空穴注入层和空穴传输层的厚度,使载流子在发光层中的分布更加平衡是提高器件发光效率的主要原因.值得指出的是,从电流效率最大值到亮度为 20 000 cd/m²时,优化后器件的效率衰减仅为17.7%,而常规器件的效率衰减则为62.1%,优化后器件效率衰减现象得到了明显的改善.分析认为优化后的器件中未掺杂的CBP有助于展宽激子形成区宽度,进而减弱了三线态-三线态湮灭、三线态-极化子淬灭现象,激子形成区的展宽是改善效率衰减的主要原因.     

Blue organic light-emitting diodes with 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as hole transport and emitting layer and the impedance spectroscopy analysis

2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) based fluorescent blue organic light-emitting diodes (OLEDs) are demonstrated. With MADN as emitting layer, experiments indicate that thick MADN (40–60 nm) is preferable for constructing efficient blue OLED. With MADN as hole-transport and emitting layer and tris(8-hydroxy-quinolinato)aluminium (Alq₃) as electron-transport layer, the OLED electroluminescent characteristics show a mixture emission of MADN and Alq₃ with Commission Internationale d'Eclairage (CIE) color coordinates of (0.25, 0.34), indicating feasible hole transporting in MADN. Using 4,7-diphenyl-1,10-phenanthroline (BPhen) replacing Alq₃ as electron-transport layer, the OLED shows deep blue emission with a maximum luminous efficiency of 4.8 cd/A and CIE color coordinates of (0.16, 0.09). The hole transport characteristics of MADN are further clarified by constructing hole-only device and performing impedance spectroscopy analysis. The results indicate that MADN shows superior hole-transport ability which is almost comparable to typical hole-transport material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), suggesting a promising application for constructing efficient blue OLED with integrated hole-transport layer and emitting layer.     

Improvement in performance of organic light-emitting devices by inclusion of multi-wall carbon nanotubes

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was modified by different concentrations of multi-wall carbon nanotubes (MWNTs), and the nanocomposites of PEDOT:PSS and MWNTs were firstly used as hole-injection layer in fabrication of organic light-emitting devices (OLEDs) by using a double-layer structure with hole-injection layer of doped PEDOT:PSS and emitting/electron transport layer of tris(8-hydroxyquinolinato) aluminum (Alq₃). PEDOT:PSS solution doped with MWNTs was spin-coated on clean polyethylene terephthalate (PET) substrate with indium tin oxide (ITO). It was found that the electroluminescence (EL) intensity of the OLEDs were greatly improved by using PEDOT:PSS doped with MWNTs as hole-injection layer which might have resulted from the hole-injection ability improvement of the nanocomposites. Higher luminescence intensity and lower turn-on voltage were obtained by these devices and the luminance intensity obtained from the device with the hole-injection layer of PEDOT:PSS doped by 0.4 wt.% MWNTs was almost threefolds of that without doping.     

Organic Light-Emitting Devices with a LiF Hole Blocking Layer

We introduce a thin LiF layer into tris-8-hydroxyquinoline aluminium （Alq3 ） based bilayer organic light-emitting devices to block hole transport. By varying the thickness and position of this LiF layer in Alq3, we obtain an electroluminescent efficiency increase by a factor of two with respect to the control devices without a LiF blocking layer. By using a 10nm dye doped Alq3 sensor layer, we prove that LiF can block holes and excitons effectively. Experimental results suggest that the thin LiF layer may be a good hole and exciton blocking layer.     

Improved performances of the organic light-emitting devices by doping in the mixed layer

The brightness, efficiency and lifetime of organic light-emitting diodes (OLEDs) were remarkably improved by doping in a mixed layer. In this device, the emitting layer consists of a mixture of -naphthylphenybiphenyl amine (NPB), tris (8-hydroxyquinolinolate) aluminum (Alq₃) (referred to as the mixed layer) and an emissive dopant 5,6,11,12-petraphenylnaphthacene (rubrene), where the concentration of NPB declined while the concentration of Alq₃ was increased gradually in the deposition process. The device exhibited a maximum emission of 49,300 cd/m² at a destroy voltage of 35 V and a maximum efficiency of 7.96 cd/A at 10 V, respectively. The efficiency improves twofold in comparison with the conventional doped devices. Meanwhile, the device exhibited superior operational stability with a half-life time of 1000 h at a starting luminance of 1000 cd/m² by a constant current driver.     

Effect of polar dopant on energetic and positional disorders in tris(8-hydroxyquinolinato) aluminum (Alq3)

The influence of polar dopant on the charge carrier transport in amorphous tris (8-hydroxyquinolinato) aluminum (Alq₃) was studied by time-of-flight measurement. The 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was doped into Alq₃ with various concentration from 0.5 to 24 wt. %. The electron mobility was reduced by about one order by DCM doping in Alq₃. The electric-field dependence electron mobility in Alq₃:DCM films separated into two discrete regions of critical fields E_c ^1/2. The value of E_c ^1/2ranged from 360 to 405 (V/cm)^1/2 depending on the DCM concentration in Alq₃ films. The energetic disorder in Alq₃:DCM films increased from 0.01 eV to 0.09 eV with DCM doping concentration. The positional disorder in Alq₃:DCM films also increased from 0.3 to 6.5 with DCM doping concentration up to 24 wt. %. These results indicated the strong Coulombic and dipole–dipole interactions between DCM and Alq₃ molecules. The interactions between randomly located DCM molecules, Alq₃ dipoles and oriented dipoles are the major caused of positional disorder. PACS 73.50.-h; 73.61.Ph; 71.20.Rv     

Low driving voltage in organic light-emitting diode using MoO3/NPB multiple quantum well structure in hole transport layer

Driving voltage of organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO₃)/N, N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in hole transport layer. For the device with double quantum well (DQW) structure of ITO/ [MoO₃ (2.5 nm)/NPB (20 nm)]₂/Alq₃(50 nm)/LiF (0.8 nm)/Al (120 nm)], the turn-on voltage is reduced to 2.8 V, which is lowered by 0.4 V compared with that of the control device (without MQW structures), the driving voltage is 5.6 V, which is reduced by 1 V compared with that of the control device at the 1000 cd/m². In this work, the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure, which is attributed not only to the reducing energy barrier between ITO and NPB, but also to the forming charge transfer complex between MoO₃ and NPB induced by the interfacial doping effect of MoO₃.