Organic Photovoltaic Cells with Improved Performance Using Bathophenanthroline as a Buffer Layer

采用Bphen作为缓冲层,研究Bphen处在电子受体材料C₆₀和阴极Ag之间对有机薄膜光伏电池(OPV)性能的影响．通过引入2.5nm厚的Bphen,在100 mW/cm²光照下,CuPc/C₆₀结构的器件效率从0.87%提高到2.25%． 对光生电流-电压的分析表明,Bphen缓冲层可以有效的提高电子从C₆₀层向Ag阴极的传输能力和平衡器件中载流子的传输能力．系统研究了Bphen厚度对OPV器件性能的影响,发现随着Bphen厚度的增加,电导率的降低是限制器件性能的主要原因．此外,采用紫外-可见光分光光度计测试了器件的吸收光谱,发现Bphen缓冲层可以增强CuPc/C₆₀的光吸收能力．     

利用不同阴极缓冲层来改善Pentacene/C60太阳能电池的性能

制备了结构为ITO/Pentacene/C₆₀/Al的双层光伏电池器件,在C₆₀/Al界面插入了常用的缓冲层材料bathocuproine(BCP)作为阴极缓冲层,通过优化BCP层的厚度来提高电池的性能并研究了阴极缓冲层的作用机理.实验发现,BCP厚度为10 nm时器件的效率最高,为0.46%.在此基础上,利用bathophenanthroline(Bphen)和3,4,9,10-Perylenetetracarb-oxylicdianhydride(PTCDA 关键词： 有机太阳能电池 Pentacene 60')" href="#">C₆₀ 缓冲层     

Performance improvement in pentacene organic thin film transistors by inserting a C60 ultrathin layer

The contact effect on the performances of organic thin film transistors is studied here. A C₆₀ ultrathin layer is inserted between Al source--drain electrode and pentacene to reduce the contact resistance. By a 3 nm C₆₀ modification, the injection barrier is lowered and the contact resistance is reduced. Thus, the field-effect mobility increases from 0.12 to 0.52 cm²/(V·s). It means that inserting a C₆₀ ultra thin layer is a good method to improve the organic thin film transistor (OTFT) performance. The output curve is simulated by using a charge drift model. Considering the contact effect, the field effect mobility is improved to 1.15 cm²/(V·s). It indicates that further reducing the contact resistance of OTFTs should be carried out.     

4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺掺杂MoOx作为空穴传输层对有机太阳电池性能的影响

通过采用4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺 (m-MTDATA)掺入MoO_x作为器件的空穴传输层来提高酞菁铜(CuPc)/C₆₀小分子 有机太阳电池的效率. 采用真空蒸镀的方法制备了一系列器件, 其中结构为铟锡氧化物 (ITO)/m-MTDATA:MoO_x(3:1)(30 nm)/CuPc(20 nm)/C₆₀(40 nm)/4,7-二苯 基-1,10-菲罗啉 (Bphen)(8 nm)/LiF(0.8 nm)/Al(100 nm)的器件, 在AM1.5 (100 mW/cm²)模拟太阳光的照射条件下, 开路电压V_oc=0.40 V, 短路电流J_sc=6.59 mA/cm², 填充因子为0.55, 光电转换效率达1.46%, 比没有空穴传输层的器件ITO/CuPc(20 nm)/C₆₀(40 nm)/Bphen(8 nm)/LiF(0.8 nm)/Al(100 nm) 光电转换效率提高了38%. 研究表明, 加入m-MTDATA:MoO_x(3:1)(30 nm)空穴传输层减小了有机层和ITO电极之间的接触电阻, 从而减小了整个器件的串联电阻, 提高了器件的光电转换效率.     

使用不同阴极缓冲层提升性能的反型有机太阳能电池

使用含Al_q3、Bphen、BCP、HATNA和Cs₂CO₃作为阴极缓冲层,制备了基于SubPc和C₆₀的反型平面异质结有机太阳能电池. 比较了有机阴极缓冲层的最低未占有分子轨道和电子迁移率对有机太阳能电池性能的影响. 结果显示,Al_q3、Bphen和HATNA能够大幅度提升器件的性能. 使用退火过后的HATNA作为阴极缓冲层获得了最高的效率,比没有阴极缓冲层的器件提升了7倍. 另外,使用空间电荷限制电流理论进行仿真的结果表明,通过加入HATNA作阴极缓冲层,反型有机太阳能电池结构中处于有机/电极界面处的肖特基势垒降低了27%.     

Properties of C60 thin film transistor based on polystyrene

This paper reports that the n-type organic thin-film transistors have been fabricated by using C60 as the active layer and polystyrene as the dielectric.The properties of insulator and the growth characteristic of C60 film were carefully investigated.By choosing different source/drain electrodes,a device with good performance can be obtained.The highest electron field effect mobility about 1.15 cm 2 /(V·s) could reach when Barium was introduced as electrodes.Moreover,the C60 transistor shows a negligible 'hysteresis effect' contributed to the hydroxyl-free of insulator.The result suggests that polymer dielectrics are promising in applications among n-type organic transistors.     

A study on Al2O3 passivation in GaN MOS-HEMT by pulsed stress

This paper studies systematically the drain current collapse in AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors （MOS-HEMTs） by applying pulsed stress to the device. Low-temperature layer of Al2O3 ultrathin film used as both gate dielectric and surface passivation layer was deposited by atomic layer deposition （ALD）. For HEMT, gate turn-on pulses induced large current collapse. However, for MOS-HEMT, no significant current collapse was found in the gate turn-on pulsing mode with different pulse widths, indicating the good passivation effect of ALD Al2O3. A small increase in Id in the drain pulsing mode is due to the relieving of self-heating effect. The comparison of synchronously dynamic pulsed Id - Vds characteristics of HEMT and MOS-HEMT further demonstrated the good passivation effect of ALD Al2O3.     

C60 sputtering of organics: A study using TOF-SIMS, XPS and nanoindentation

Sputtering of organic materials using a C₆₀ primary ion beam has been demonstrated to produce significantly less accumulated damage compared to sputtering with monatomic and atomic-cluster ion beams. However, much about the dynamics of C₆₀ sputtering remains to be understood. We introduce data regarding the dynamics of C₆₀ sputtering by evaluating TOF-SIMS depth profiles of bulk poly(methyl methacrylate) (PMMA). Bulk PMMA provides an ideal test matrix with which to probe C₆₀ sputter dynamics because there is a region of steady-state secondary ion yield followed by irreversible signal degradation. C₆₀ sputtering of PMMA is evaluated as a function of incident ion kinetic energy using 10 keV C₆₀⁺, 20 keV C₆₀⁺ and 40 keV C₆₀⁺⁺ primary ions. Changes in PMMA chemistry, carbon accumulation and graphitization, and topography as a function of total C₆₀ ion dose at each accelerating potential is addressed.     

Electrical characteristics of top contact pentacene organic thin film transistors with SiO<Subscript>2</Subscript> and poly(methyl methacrylate) as gate dielectrics

Organic thin film transistors (OTFTs) were fabricated using pentacene as the active layer with two different gate dielectrics, namely SiO₂ and poly(methyl methacrylate) (PMMA), in top contact geometry for comparative studies. OTFTs with SiO₂ as dielectric and gold deposited on the rough side of highly doped silicon (n⁺-Si) as gate electrode exhibited reasonable field effect mobilities. To deal with poor stability and large leakage currents between source/drain and gate electrodes in these devices, isolated OTFTs with reduced source/drain contact area were fabricated by selective deposition of pentacene on SiO₂/PMMA through shadow mask. This led to almost negligible leakage currents and no degradation in electrical performance even after 14 days of storage under ambient conditions. But, the field effect mobilities obtained were lower than 10⁻³ cm² V⁻¹ s⁻¹, whereas by using PMMA as gate dielectric with chromium deposited on the polished side of n⁺-Si as gate electrode, improved field effect mobilities (>0.02 cm² V⁻¹ s⁻¹) were obtained. PMMA-based OTFTs also exhibited lower leakage currents and reproducible output characteristics even after 30 days of storage under ambient conditions.      

Fabrication of 6,13‐bis(triisopropyl‐silylethynyl)–pentacene thin‐film transistors with the silver ink transfer method using a polymer stamp

We fabricated 6,13‐bis(triisopropylsilylethynyl)–pentacene (TIPS–pentacene) thin film transistors using a direct metal transfer method. Using different metals, such as Au and Ag ink, electrode patterns are formed from the relief region of the polymer mold. TIPS–pentacene TFTs using the Ag ink transfer method show a similar performance to those using the Au metal transfer method. This method has advantages over the Au metal transfer method because it does not require vacuum equipment and a dry etching process. The self‐assembled monolayer (SAM) treated device exhibits a carrier mobility of 9.5 × 10^–2 cm²/V · s, and an on/off ratio of 4.6 × 10⁴. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

对以并五苯和酞菁铜为不同有源层的有机薄膜晶体管特性研究

通过扫描电镜和X射线衍射对SiO₂衬底上生长并五苯和酞菁铜薄膜的表面形貌进行表征,并得到在SiO₂衬底上生长的并五苯薄膜是以岛状结构生长,其大小约为100nm,且薄膜有较好的结晶取向,呈多晶态存在. 酞菁铜薄膜则没有表现出明显的生长机理,其呈非晶态存在. 还对通过掩膜的方法制作得以酞菁铜和并五苯为有源层的顶栅极有机薄膜晶体管的特性进行了研究. 有源层的厚度为40nm,绝缘层SiO₂的厚度为250nm,器件的沟道宽长比(W/关键词： 有机薄膜晶体管 并五苯薄膜 酞菁铜薄膜 μ_EF)')" href="#">场效应迁移率(μ_EF)     

Enhanced charge carrier injection in heterojunction organic field-effect transistor by inserting an MoO3 buffer layer

A top-contact organic field-effect transistor(OFET) is fabricated by adopting a pentacene/1,1’-bis(di-4tolylaminophenyl) cyclohexane(TAPC) heterojunction structure and inserting an MoO3 buffer layer between the TAPC organic semiconductor layer and the source/drain electrode.The performances of the heterojunction OFET,including output current,field-effect mobility,and threshed voltage,are all significantly improved by introducing the MoO3 thin buffer layer.The performance improvement of the modified heterojunction OFET is attributed to a better contact formed at the Au/TAPC interface due to the MoO3 thin buffer layer,thereby leading to a remarkable reduction of the contact resistance at the metal/organic interface.     

Air-stable ambipolar organic field effect transistors with heterojunction of pentacene and N,N'-bis(4-trifluoromethylben-zyl)perylene-3,4,9,10-tetracarboxylic diimide

Fabrication of ambipolar organic field-effect transistors (OFETs) is essential for the achievement of an organic complementary logic circuit. Ambipolar transports in OFETs with heterojunction structures are realized. We select pentacene as a P-type material and N,N'-bis(4-trifluoromethylben-zyl)perylene-3,4,9,10-tetracarboxylic diimide (PTCDI-TFB) as a n-type material in the active layer of the OFETs. The field-effect transistor shows highly air-stable ambipolar characteristics with a field-effect hole mobility of 0.18~cm²/(V.s) and field-effect electron mobility of 0.031~cm²/(V.s). Furthermore the mobility only slightly decreases after being exposed to air and remains stable even for exposure to air for more than 60 days. The high electron affinity of PTCDI-TFB and the octadecyltrichlorosilane (OTS) self-assembly monolayer between the SiO₂ gate dielectric and the organic active layer result in the observed air-stable characteristics of OFETs with high mobility. The results demonstrate that using the OTS as a modified gate insulator layer and using high electron affinity semiconductor materials are two effective methods to fabricate OFETs with air-stable characteristics and high mobility.     

Experimental observation of negative group velocity in C60 PMMA

The authors experimentally observed the superluminal propagation at negative group velocity in C₆₀ polymethyl methacrylate (PMMA) firstly. The largest time advancement of 7.61 ms was obtained at 1 mg/cm³ of sample concentration, the corresponding group velocity was −0.657 m/s. Fast light in C₆₀ PMMA offers several advantages over liquids or vapors for a variety of possible applications: (i) easy for experimental measurement and practical applications (ii) easy for manufacture, (iii) sample concentration is another convenient parameter to vary the signal delay.     

Electronic non-equilibrium conditions at C60–pentacene heterostructures

The electronic structure of vacuum-sublimed layered organic heterostructures of pentacene (PEN) and fullerene (C₆₀) on conducting polymer substrates was investigated using ultraviolet photoelectron spectroscopy (UPS). The conditions at the PEN/C₆₀interface changed from thermodynamic non-equilibrium (i.e. the onset of the PEN highest occupied molecular orbital above the substrate Fermi-energy) for thin PEN coverages on C₆₀ to thermodynamic equilibrium for thicker PEN coverages (i.e. Fermi-level pinning of PEN). This finding is attributed to a coverage-dependent pinhole connection of PEN through the C₆₀ layer with the substrate. The experiments demonstrate the importance of organic thin film morphology for UPS measurements to assess the energy level alignment at organic/organic heterointerfaces.     

Suppression of bias stress-induced degradation of pentacene-TFT using MoOx interlayer

The bias stress effect in pentacene thin-film transistors (TFTs) with and without MoO_x interlayer was characterized. The device without MoO_x interlayer showed a large threshold voltage shift of 5.1 V after stressing with a constant gate-source voltage of −40 V for 10000 s, while at the same condition, the device with MoO_x interlayer showed a low threshold voltage shift of 1.9 V. The results can be attributed to the stable interface between MoO_x/pentacene and small contact resistance change for the device with MoO_x/Cu electrode. Pentacene-TFTs with MoO_x interlayer showed a high field-effect mobility of 0.61 cm²/V s and excellent bias stability, which could be a significant step toward the commercialization of OTFT technology.     

Surface morphology characterization of pentacene thin film and its substrate with under-layers by power spectral density using fast Fourier transform algorithms

Surface morphology of pentacene thin films and their substrates with under-layers is characterized by using atomic force microscopy (AFM). The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD plots of pentacene films on glass substrate are successfully approximated by the k-correlation model. The pentacene film growth is interpreted the intermediation of the bulk and surface diffusion by parameter C of k-correlation model. The PSD plots of pentacene film on Au under-layer is approximated by using the linear continuum model (LCM) instead of the combination model of the k-correlation model and Gaussian function. The PSD plots of SiO₂ layer on Au under-layer as a gate insulator on a gate electrode of organic thin film transistors (OTFTs) have three power values of PSD. It is interpreted that the specific three PSD power values are caused by the planarization of the smooth SiO₂ layer to rough Au under-layer.     

The effects of exciton-diffusion blocking layers on pentacene/C60 bulk heterojunction solar cells

Organic thin film solar cells, with structures based on pentacene/C₆₀ bulk heterojunctions, have been fabricated and characterized. The effect of introducing an exciton-diffusion blocking layer (EBL) of 3,4,9,10-perylenetetracarboxylic dianhydride, between the active layer and the metal layer in the solar cell, was investigated. It was shown that optimized structures containing EBLs resulted in an improvement in solar cell conversion efficiencies. The energy levels of the molecules were calculated and are subsequently discussed.     

Sputtering yields of PMMA films bombarded by keV C60 ions

A quartz crystal microbalance (QCM) has been used to determine total-mass sputtering yields of PMMA films by 1-16 keV C₆₀^+,2+ ion beams. Quantitative sputtering yields for PMMA are presented as mass loss per incident ion Y_m. Mass-lost rate QCM data show that a 13 keV C₆₀ cluster leads to emission equivalent to 800 PMMA molecules per ion. The power law obtained for the increase in sputtering yield with primary ion energy is in good agreement those predicted by “thermal spike” regime and MD models, when crater sizes are used to estimate sputtering.     

Detailed analysis of ultrathin fluorescent red dye interlayer for organic photovoltaic cells

The influence of an ultrathin 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) fluorescent dye layer at donor/acceptor heterojunction on the performance of small-molecule organic photovoltaic (OPV) cell is studied. The structure of OPV cell is of indium-tin oxide (ITO)/copper phthalocyanine (CuPc)/DCJTB/fullerene (C₆₀)/bathophenanthroline (Bphen)/Ag. The results show that open circuit voltage (V_OC) increases to 0.57 V as the film thickness of DCJTB layer increases from 0.2 to 2.0 nm. By using an equivalent circuit model, the enhancement of V_OC is found to be attributed to the reduced reverse saturation current density (J_S) which is due to the lower highest occupied molecular orbital (HOMO) level in DCJTB than that in CuPc. Also, the short circuit current density (J_SC) is affected when the DCJTB layer becomes thicker, resulting from the high series resistance R_SA due to the low charge carrier mobility of fluorescent red dye.