Influence of fullerene derivative replacement with TiO2 nanoparticles in organic bulk heterojunction solar cells

In an effort to develop hybrid organic solar cells with improved power conversion efficiency (PCE), devices based on poly (3-hexylthiophene) (P3HT):phenyl C₆₁-butyric acid methyl ester (PCBM) active layer and poly (3,4-ethylenedioxythiophene) (PEDOT):poly (styrenesulfonate) (PSS) buffer layers were prepared. A systematic replacement of PCBM was achieved by introducing nanostructured TiO₂ (∼15 nm particle size), dissolved separately in chlorobenzene (CB) and 1,2 –dichlorobenzene (DCB), to the (P3HT:PCBM) active layer while keeping a fixed amount for P3HT. To understand the effect of fullerene replacement with the inorganic metal oxide nanoparticles on different properties of resulting devices, a variety of techniques such as Current–Voltage (J–V) characteristics, Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), Ultravoilet-Visible (UV–Vis) Spectrophotometry and External Quantum Efficiency (EQE) were employed. The addition of TiO₂ nanoparticles in the active layer improved the power conversion efficiency (PCE) of P3HT:PCBM devices. The addition of TiO₂ nanoparticles using CB as solvent enhanced the absorption in visible region and also introduced a red shift in the absorption spectra. A significant increase in EQE was observed for devices with TiO₂ nanoparticles in the active layer. Mixing TiO₂ also increased the surface roughness of the active layer where TiO₂ nanoparticles were found to agglomerate as their concentration increased relative to fullerene derivative. A complete agglomeration of TiO₂ was observed in the absence of PCBM.     

非晶/结晶共混对聚合物光伏电池性能的影响

以局域规整聚（3-己基噻吩） （P3HT）制备了TiO₂/聚合物型双层结构光伏电池.利用稳态电流-电压测试和动态强度调制光电压谱,结合差热分析、吸收光谱和荧光光谱, 研究了非晶支化聚亚乙基亚胺（BPEI）作为P3HT膜层的添加成分对TiO₂/P3HT双层电池性能的影响.由于P3HT链的高结晶性,使得TiO₂/P3HT界面接触不好,导致电池性能差.当在P3HT中共混重量比W_BPEI/P3HT=1％—5％的BPEI时,电池性能得到显著改善;尤其是当W_BPEI/P3HT= 1％时,电池表现出近0.8V的开路电压和20μA/cm²的短路电流.结果表明BPEI对电池性能的影响不是源于P3HT-BPEI共混体系光学性能的变化,而主要是由于其改变了TiO₂/P3HT界面接触性能.BPEI对TiO₂/P3HT界面接触有两个相互竞争的影响,这取决于P3HT-BPEI共混体系的组成.一方面,通过降低P3HT的结晶度和增强与TiO₂表面的相互作用,改善P3HT链在TiO₂ 表面的附着;另一方面,当BPEI含量过高时,BPEI在TiO₂表面的附着量将增加,反而会阻碍P3HT与TiO₂表面的接触.良好的TiO₂/P3HT界面接触有利于提高激子的界面分离效率、光生电子的寿命和电池效率.本文结果有望为聚合物光伏电池性能的改善提供新的认识和方法. 关键词： 聚（3-己基噻吩） 二氧化钛 共轭聚合物 光伏电池     

Improved exciton dissociation efficiency by a carbon-quantum-dot doped workfunction modifying layer in polymer solar cells

In addition to the phase-separated morphology of donor and acceptor, the internal field created by work-function difference between cathode and anode can also influence the exciton dissociation probability. In this study, we have demonstrated enhanced photovoltaic performance by increasing exciton dissociation efficiency. To improve both work-function modification effect and charge transport properties, we have incorporated novel carbon quantum dots (CQD) having NH₂ ligands into the polyethyleneimine (PEI) work-function modifying layer as a dopant. A study of net photocurrent density as a function of effective voltage showed that devices with a CQD-doped PEI layer had a much higher charge separation probability compared to devices with a pristine PEI layer. A Kelvin-probe force microscopy study demonstrated that a CQD-doped PEI layer induced lower work-function of ITO than that of ITO with a pristine PEI, which induced a stronger internal field. This strengthened internal field induced better exciton dissociation efficiency, thereby improving solar cell performance.     

Investigation of polymer/inorganic hybrid photovoltaic device using quantum dots as the interface modifier

CdS quantum dots (QDs) were introduced as an interface modifier in the poly(3-hexylthiophene) (P3HT)/TiO₂ nanorod arrays hybrid photovoltaic device. The presence of CdS QDs interlayer was found to provide enhanced light absorption, increased interfacial recombination resistance at the P3HT/TiO₂ interfaces, thus leading to a lower recombination rate of the electrons due to the stepwise structure of band edge in P3HT/CdS/TiO₂, which accounts for the observed enhanced photocurrent and photovoltage of the hybrid solar cells. The optimized performance was achieved in P3HT/CdS/TiO₂ hybrid solar cells after deposition of CdS QDs for 10 cycles, with a power conversion efficiency of 0.57 %, which is nearly ten times higher than that of P3HT/TiO₂. The findings indicate that inorganic semiconductor quantum dots provide effective means to improve the performance of polymer/TiO₂ hybrid solar cells.     

Role of sweep direction and substrate work function on resistive switching of titanium-di-oxide [TiO2] nanoparticles

The resistive switching mechanism in titanium-dioxide nanoparticles (TiO₂ NP) is studied using the current-voltage (I–V) measurements. The TiO₂ NP are spin-coated on different substrates like FTO, ITO, Gold, and p-Silicon. The I–V measurements are carried out by changing the initial potential of the substrates to either 0 V (sweep1) or −1 V (sweep2). Resistive switching (RS) was observed only for FTO/TiO₂ NP and ITO/TiO₂NP devices in sweep1 direction. Whereas, in sweep2 direction, no such RS was observed in any of the devices. The detailed I–V analysis infers the Ohmic conduction followed by space charge limited conduction (SCLC) during the RS forming process for FTO/TiO₂ NP and ITO/TiO₂NP devices. The Au and p-Si substrates act as blocking contact for TiO₂ and exhibit Schottky/thermionic emission at lower voltages and SCLC at higher voltages. The TiO₂ NP coated on p-Si substrate exhibits rectifying behaviour with a current ratio of 3 orders of magnitude.     

Effect of surface roughness of TiO2 films on short-circuit current density of photoelectrochemical cell of ITO/TiO2/PVC-LiClO4/graphite

This paper reports the effect of surface topography of titanium dioxide films on short-circuit current density of photoelectrochemical solar cell of ITO/TiO₂/PVC-LiCLO₄/graphite. The films were deposited onto ITO-covered glass substrate by screen-printing technique. The films were tempered at 300 °C, 350 °C, 400 °C, 450 °C and 500 °C for 30 min to burn out the organic parts and to achieve the films with porous structure. The surface roughness of the films were studied using scanning electron microscope (SEM). Current–voltage relationship of the devices were characterized in dark at room temperature and under illumination of 100 mW cm⁻² light from tungsten halogen lamp at 50 °C. The device utilising the TiO₂ film annealed at 400 °C produces the highest short-circuit current density and open-circuit voltage as it posses the smoothest surface topography with the electrolyte. The short-circuit current density and open-circuit voltage of the devices increase with the decreasing grain size of the TiO₂ films. The short-circuit current density and open-circuit voltage are 0.6 μA/cm² and 109 mV respectively.     

Fractality in the neuron axonal topography of the human brain based on 3-D diffusion MRI

We report our results on the effect of incorporation of inorganic fullerene like nanoparticles (IF) and inorganic nanotubes (INT) of WS₂ into hybrid LED device structures. To disperse into a uniform fashion, the semiconducting INT/IF WS₂ NTs were functionalized with SDS (sodium dodecylsulphate). The IF/INT WS₂ nanotubes were used in combination with PEDOT:PSS and P3HT to realize the following LED device structures: ITO/(PEDOT:PSS):(WS₂:SDS)/P3HT/LiF-Al; ITO/PEDOT:PSS/P3HT/WS₂:SDS/LiF-Al. Morphological, optical and electrochemical analysis were performed to obtain the HOMO and the LUMO energy levels to hypothesize the most efficient device structure. The spectral positions of the electroluminescent bands were found out to be device-dependent and exhibits blue shift when the proposed nanostructure is dip coated on top of P3HT. Electro-optical analysis indicate that the WS₂:SDS based P3HT/semiconductor film can improve the charge recombination probability owing to its dual functionality as hole blocking layer and electron injection moiety.     

Enhanced resistance switching stability of transparent ITO/TiO2/ITO sandwiches

We report that fully transparent resistive random access memory(TRRAM) devices based on ITO/TiO2/ITO sandwich structure,which are prepared by the method of RF magnetron sputtering,exhibit excellent switching stability.In the visible region(400-800 nm in wavelength) the TRRAM device has a transmittance of more than 80%.The fabricated TRRAM device shows a bipolar resistance switching behaviour at low voltage,while the retention test and rewrite cycles of more than 300,000 indicate the enhancement of switching capability.The mechanism of resistance switching is further explained by the forming and rupture processes of the filament in the TiO 2 layer with the help of more oxygen vacancies which are provided by the transparent ITO electrodes.     

Fabrication and characterization of CdS-sensitized TiO<Subscript>2</Subscript> nanotube photoelectrode

CdS quantum dot (Qd)-sensitized TiO₂ nanotube array photoelectrode is synthesised via a two-step method on tin-doped In₂O₃-coated (ITO) glass substrate. TiO₂ nanotube arrays are prepared in the ethylene glycol electrolyte solution by anodizing titanium films which are deposited on ITO glass substrate by radio frequency sputtering. Then, the CdS Qds are deposited on the nanotubes by successive ionic layer adsorption and reaction technique. The resulting nanotube arrays are characterized by scanning electron microscopy, X-ray diffraction (XRD) and UV–visible absorption spectroscopy. The length of the obtained nanotubes reaches 1.60 μm and their inner diameter and wall thickness are around 90 and 20 nm, respectively. The XRD results show that the as-prepared TiO₂ nanotubes array is amorphous, which are converted to anatase TiO₂ after annealed at 450 °C for 2 h. The CdS Qds deposited on the TiO₂ nanotubes shift the absorption edge of TiO₂ from 388 to 494 nm. The results show that the CdS-sensitized TiO₂ nanotubes array film can be used as the photoelectrode for solar cells.     

Mechanisms controlling the efficiency of polymer solar cells

To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters are very informative for analyzing losses. We report on the current–voltage characteristics and the external photogeneration quantum yield of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al as well as of ITO/PEDOT:PSS/P3HT:PCBM/Al devices investigated in the broad temperature range 120–325 K under variable illumination, between 0.02 and 100 mW/cm². We discuss the recombination on traps and the low mobility of charge carriers caused by poor morphology of active layers as possible mechanisms limiting the efficiency of these devices. PACS 73.50.P; 73.61.P; 72.80.R     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Redox driven conductance changes for resistive memory

The relationship between bias-induced redox reactions and resistance switching is considered for memory devices containing TiO₂ or a conducting polymer in “molecular heterojunctions” consisting of thin (2–25 nm) films of covalently bonded molecules, polymers, and oxides. Raman spectroscopy was used to monitor changes in the oxidation state of polythiophene in Au/P3HT/SiO₂/Au devices, and it was possible to directly determine the formation and stability of the conducting polaron state of P3HT by applied bias pulses [P3HT = poly(3-hexyl thiophene)]. Polaron formation was strongly dependent on junction composition, particularly on the interfaces between the polymer, oxide, and electrodes. In all cases, trace water was required for polaron formation, leading to the proposal that water reduction acts as a redox counter-reaction to polymer oxidation. Polaron stability was longest for the case of a direct contact between Au and SiO₂, implying that catalytic water reduction at the Au surface generated hydroxide ions which stabilized the cationic polaron. The spectroscopic information about the dependence of polaron stability on device composition will be useful for designing and monitoring resistive switching memory based on conducting polymers, with or without TiO₂ present.     

TiO2 Nanoparticles Produced by Electric-Discharge-Nanofluid-Process as Photoelectrode of DSSC

利用自制TiO₂纳米粒子研究敏化染敏太阳能电池. 使用自制的旋转涂布加热平台装置将产出的TiO₂粒子均匀的涂布在ITO导电玻璃上形成薄膜,浸泡于N-719 染料中12小时以上作为DSSCs的光电极 元件,最后完成染料敏化太阳能电池的系统组装并进行光电转换效率测量. 实验结果表明,放电过程产出的TiO₂纳米粒子具有锐钛矿晶相,粒径尺寸可控制在20～70 nm,粒子表面电位约为-30 mV,是稳定的纳米悬浮夜. 添加0.5 mL 的Triton X-100在导电玻璃表面上,利用的旋转涂布加热到22 oC可以制得厚度均匀缜密的薄膜结构,不但粒子不受到热处理效应与介面活性剂的影响而发生晶相改变,并且薄膜也有良好的染料吸附效果. 较厚二氧化钛薄膜的光电极会提升敏化染敏太阳能电池的效率. 实验结果得知,以15 μm的二氧化钛薄膜组装DSSCs测得最高效率2.15%,但是当薄膜厚度超过15 μm 则会导致开路电压与充填因子逐渐下降,光电转换效率变差.     

Surface‐enhanced Raman scattering investigations on silver nanoparticles deposited on alumina and titania nanotubes: influence of the substrate material on surface‐enhanced Raman scattering activity of Ag nanoparticles

Silver nanoparticles deposited on various ‘inert’ porous materials (mainly Al₂O₃ and TiO₂) are often used as substrates for surface‐enhanced Raman scattering (SERS) measurements. In this study, we used the sputter deposition technique to cover tubular arrays of Al₂O₃ and TiO₂ with Ag nanoparticles. Raman spectra of pyridine (as a probe molecule) and of two selected dyes (5‐(4‐dimethylaminobenzylidene)rhodanine and 5‐(4‐(dimethylamino)benzylidene)‐3‐(3‐methoxypropyl)rhodanine) adsorbed on fabricated Ag/TiO₂‐n/Ti and Ag/Al₂O₃‐n/Al substrates were measured. We found that the SERS spectra of pyridine adsorbed on Ag nanoparticles deposited on an Al₂O₃‐n/Al substrate are distinctly different from those measured for an Ag/TiO₂‐n/Ti composite. Similar effects were observed for dyes adsorbed on the surface of both composites. The spectral differences between two kinds of composites (Ag/TiO₂‐n/Ti and Ag/Al₂O₃‐n/Al) are discussed in terms of (1) the modified electronic structure of the Ag nanoparticles due to their interaction with different substrate materials and (2) the different atomic topology of the metal particles thus deposited on the surfaces of the substrates. Composite samples were also studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N,N′-bis-(1-naphthyl)-N,N′-1-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq₃) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq₃/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq₃ increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq₃ benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).     

Photosensitization of nanocrystalline TiO2 film electrode with cadmium sulphoselenide

Nanocrystalline titanium dioxide (TiO₂) thin films composed of densely packed nanometer-sized grains have been successfully deposited onto an indium-doped-tin oxide (ITO) substrate. Then cadmium sulphoselenide (CdSSe) thin film was deposited onto pre-deposited TiO₂ to form a TiO₂/CdSSe film, at low temperature using a simple and inexpensive chemical method. The X-ray diffraction, selected area electron diffraction, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and water contact angle techniques were used for film characterization. Purely rutile phase of TiO₂ with super-hydrophilic and densely packed nanometer-sized spherical grains of approximate diameter 30-40 (±2) nm was observed. The increase in optical absorption was observed after CdSSe film deposition. Nest like surface morphology of CdSSe on TiO₂ surface results in air trapping in the crevices which prevents water from adhering to the film with increase in water contact angle. Photosensitization of TiO₂ with CdSSe was confirmed with light illumination intensity of 80 mW/cm².     

Material properties controlling photocurrent on TiO2 aggregates with plane orientation for dye-sensitized solar cells

Effective design of a dye-sensitized solar cell (DSSC) requires a clear understanding of the reaction mechanisms of the constituent cell materials. The relationship between structural and photo-electrochemical properties of the photo-anodic materials is of the first priority for such investigations. Highly oriented aggregates of anatase phase TiO₂ nanoparticles were deposited on Indium Tin oxide (ITO) glass substrates; over which N719 dyes were adsorbed through electrophoretic deposition under a strong magnetic field. The properties were evaluated by electrochemical measurements, UV–VIS spectroscopy, and electrical resistance measurements. The results showed that the absorbed photon number in the TiO₂ aggregates with adsorbed dye and their resistivity showed different dependences on the orientation of the crystal plane in the TiO₂ particle. The dependence of the photocurrent on the plane orientation of aggregates of dye-sensitized TiO₂ nanoparticles has been determined from a combination of the electrical conductivity of TiO₂ aggregate and the amount of dye adsorbed on the surface of TiO₂.     

Structural properties of single and multilayer ITO and TiO2 films deposited by reactive pulsed laser ablation deposition technique

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser. The films were deposited on SiO₂ substrates heated at 200 and 400 °C. ITO and TiO₂ films with uniform thicknesses of about 400 and 800 nm, respectively, over large areas were prepared. X-ray diffraction (XRD) analysis revealed that the ITO films are formed of highly orientated nanocrystals with an average particle size of 10-15 nm. Atomic force microscopy (AFM) observations indicate rough ITO films surfaces with average roughness of 26-30 nm. Pores were also observed. TiO₂ films deposited on the prepared ITO films result less crystalline. Annealing at 300 and 500 °C for three consecutive hours promoted formation of TiO₂ anatase phase, with crystal size of ∼6-7 nm. From the scanning transmission electron microscope (STEM) images, it can be seen that the TiO₂ films deposited onto the prepared ITO films present a relatively high pore sizes with an average pore diameter of ∼40 nm and excellent uniformity. In addition, STEM cross-sectional analysis of our films showed a columnar structure but no evidence of voids in the structure. Therefore, films exhibited large surface area, well suited for dye-sensitized solar cells (DSSC) applications.     

基于光学与光—电转换模型对聚合物电池功能层厚度与性能相关性分析

本文基于Forouhi-Bloomer 模型得到了这种功能层的光学常数.根据菲涅耳系数矩阵法计算了这种器件内的光电场分布,并计算了不同厚度的聚合物功能层的光子吸收数.同时,通过Onsager-Braun理论,分析了在无外加电场下聚合物功能层厚度对激子分离概率的影响.理论分析和实验结果证明:在特定的薄膜制备工艺下,器件结构为ITO/PEDOT/ P3HT:PC₆₀BM /LiF/Al时,聚合物功能层厚度在100 nm左右时,可以使器件的光子吸收数最大化,同时避免了激子分离概率的降低. 关键词： 光学常数 激子 聚合物太阳能电池     

Enhanced performance in organic photovoltaic devices with a KMnO4 solution treated indium tin oxide anode modification

<正>The properties of poly（3-hexylthiophene）:（6,6）-phenyl C₆₁ butyric acid methyl ester（P3HT:PCBM） organic photovoltaic devices（OPVs） with an indium tin oxide（ITO） anode treated by a KMnO₄ solution are investigated.The optimized KMnO₄ solution has a concentration of 50 mg/L,and ITO is treated for 15 min.The modification of ITO anode results in an enhancement of the power conversion efficiency（PCE） of the device,which is responsible for the increase of the photocurrent.The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO₄,and then the charge collection efficiency is improved.