Energy level alignment of blended organic semiconductors and electrodes at the interface

The energy level alignment of a blended mixture of organic semiconductors is often depicted as having a common vacuum level. However, this is not a universal phenomenon among the vast number of organic semiconductors that currently exist, as in many cases the energy levels align via the Fermi level. In this report, the energy level alignments of the mixtures; poly(9-vinylcarbazole) (PVK) and 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13) and poly(3-hexylthiophene-2,5-diyl) (P3HT) and SPPO13, with varying SPPO13 concentrations, are measured. It was found that the blended systems exhibit two different vacuum levels with the dipole between the PVK and SPPO13 increasing with the SPPO13 concentration, whilst the P3HT and SPPO13 vacuum levels only experience a small change. This is attributed to the decreasing electronic screening with increasing SPPO13 concentration. These new observations have an important implication in our understanding of interfacial behaviour for blended systems commonly used in various organic electronic devices.     

Effects of rubrene mixing on the electronic structures of donor/acceptor interface in organic photovoltaic device

Rubrene mixing has been shown to be an effect mean for enhancing both the open circuit voltage (Voc) and the short-circuit current (Jsc) of copper-phthalocyanine (CuPc)/fullerene (C₆₀) based solar cell. While the increase in Jsc can be readily explained by the additional rubrene absorption and the introduction of a bulk heterojunction; causes for Voc increase are still not clear. The energy offset between the highest occupied molecular orbital (HOMO) level of donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor (HOMO_D-LUMO_A) at the CuPc/C₆₀ interface was found to increase substantially upon rubrene mixing in either side of the interface. As the HOMO_D-LUMO_A is generally considered to limit the Voc, its increase agrees well with the device results. Energy level bending and associated built-in electric fields were also observed and their possible implications to device performance are discussed.     

Natural hybrid organic–inorganic photovoltaic devices

Natural hybrid organic–inorganic photovoltaic devices based on TiO₂ have been realized. Chlorophyll A (from anacystis nidulans algae), chlorophyll B (from spinach), carmic acid (from insect Coccus cacti L.), synthetic trans-β-carotene, natural fresh picked Morus nigra, and their mixtures have been used as an organic photo active layer to fabricate photovoltaic prototypes. In order to reduce the charge’s interfacial recombination, different thicknesses (5–45 nm) of Si layers, subsequently oxidized in air, were inserted between the TiO₂ and chlorophyll B. Scanning electron microscopy of TiO₂ and Si/TiO₂ systems shows the coexistence at least of four classes of nanoparticles of 60, 100, 150 and 250 nm in size. Auger electron spectroscopy of the Si L_2,3V V transition demonstrates the presence of silica and SiO_x suboxides. Photocurrent measurements versus radiation wavelength in the range 300–800 nm exhibit different peaks according to the absorption spectra of the organic molecules.All realized photovoltaic devices are suitable for solar light electric energy conversion. Those made of a blend of all organic molecules achieved higher current and voltage output. The Si/TiO₂-based devices containing chlorophyll B exhibited an enhanced photocurrent response with respect to those with TiO₂ only.     

瞬态光电流对有机薄膜光伏器件中肖特基接触的研究

制备了结构为氧化铟锡(ITO)/有机半导体/金属的有机薄膜光伏器件,电流--电压曲线显示其具有整流特性但有机半导体和电极间肖特基接触的内建电场方向很难判定.为了研究有机半导体和电极的肖特基接触特性,分别制备了结构为ITO/有机绝缘层/有机半导体/金属和ITO/有机半导体/有机绝缘层/金属的器件,通过调制激光照射下器件的瞬态光电流方向可容易判断有机半导体和电极间肖特基接触的内建电场方向,外加偏压下瞬态光电流的强度变化进一步证实了判断的正确性.     

金纳米四面体增强有机太阳电池光吸收及光伏性能研究

为增强有机太阳能电池的光利用率,提高能量转换效率,本文合成了金四面体形状的纳米粒子,并用聚苯乙烯磺酸钠(PSS)包裹形成了核壳结构的金纳米四面体(Au@PSS tetrahedra NPs).将其掺杂到有机太阳能电池空穴提取层与活性层的界面处,利用表面等离子体共振效应来增强活性层对光的吸收,从而提高有机太阳能电池的能量转换效率.研究了掺杂浓度和PSS包裹厚度对电池性能的影响.结果表明:掺杂浓度为6%时,器件性能最佳,能量转换效率达到3.08%; PSS壳层厚度优化为2.5 nm时,转换效率达到3.65%,较标准电池提升了22.9%.电池性能的改善主要源于金四面体纳米粒子的共振吸收峰位于给体材料吸收谱范围内,纳米粒子的共振促进了给体的吸收,同时PSS壳层的引入促进了激子的解离和电荷的转移,上述因素的改善提升了电池的短路电流、填充因子和转换效率.     

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.     

Effect of size and morphology of Au nanostructures on boosting performance of organic photovoltaic devices: Plasmonic and non-plasmonic effects

We fabricated organic photovoltaic (OPV) devices containing various Au nanostructures mixed with hole-collecting buffer layer. The presence of the Au nanostructures results in enhancement of the external quantum efficiencies (EQE) at dissimilar wavelengths of visible light, which can be attributed to the modulated plasmonic absorption frequency of the Au nanostructures. In addition to this plasmonic effect induced by visible light absorption, an increase in the EQE was also found upon UV excitation, which can be attributed to scattering effects induced by Au particles. The optical response pattern of organic photovoltaic devices can be modulated in a wide range of visible and UV wavelengths, by controlling sizes and shapes of the Au nanostructures.     

Modulation spectroscopy study on additive-induced efficiency enhancement of PTB7:PC71BM organic photovoltaic devices

Additives have been known to play an important role for improving the photovoltaic performance of organic solar cells. However, the reasons why additives improve the performance have not been clearly known yet. We employed an electrical modulated optical spectroscopy to investigate the relationship between the photovoltaic performance and additive concentration in organic photovoltaic devices. Our measured modulation spectra of a sample without additive showed both first- (-α’) and second-derivative (α”) components of the absorption spectrum. The second-derivative (α”) component in the modulation spectrum increased with the additive concentration. These results indicate that the sample without additive contains both localized (or Frenkel type) and relatively delocalized excitons and the added additive results in a significant increase of the relatively delocalized excitons. We conclude that the increased delocalized excitons lead to a significant additive-induced improvement of the photovoltaic-device performance.     

The solid–liquid interface energy of organic crystals

A simple thermodynamic model, originally developed for metals based on the Gibbs–Thomson equation and related considerations for homogeneous nucleation, has been extended to predict the solid–liquid interface energy γ_sl of organic crystals. The model predictions correspond to available experimental and other theoretical results for 38 organic crystals. Copyright © 2007 John Wiley & Sons, Ltd.     

Measuring the energy level repulsion in quantum dots

Energy level repulsion is one of the remnants of classical chaos in quantum mechanics. Measurements of the distribution of nearest neighbor spacings in quantum dots reveal, in contrast to other classically chaotic systems, deviations from the predictions made by random matrix theory. Here, we survey possible contributions to these deviations from experimental peculiarities present in measurements on quantum dots, and discuss the methods to eliminate or reduce such distortions.     

Effect of sorbitol doping in PEDOT:PSS on the electrical performance of organic photovoltaic devices

Organic photovoltaic cells have important advantages, such as low cost and mechanical flexibility. The conducting polymer poly(3,4 ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as an interfacial layer or a polymer electrode in polymer electronic devices, such as photovoltaic devices and light-emitting diodes. In this report, we discuss the direct current (DC) conductivity of PEDOT:PSS films containing various weight ratios of sorbitol dopant. The work function is shown to steadily decrease with increasing dopant content. With different dopant contents, illuminated current–voltage photovoltaic characteristics were observed. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed that the work function of the PEDOT:PSS was affected by its sorbitol content. The morphologies of the doped PEDOT:PSS films were characterized by atomic force microscopy (AFM). For the device fabrication, we made organic photovoltaic cells by a spin-coating process and Al deposition by thermal evaporation. The sorbitol dopant is able to improve the efficiency of the device.     

Influence of intramolecular vibrations in charge redistribution at the pentacene-graphite interface

We have investigated the relation between the intramolecular vibrational modes of pentacene and the charge redistribution at the pentacene-graphite interface by using high-resolution electron-energy-loss-spectroscopy. The three main vibrational peaks shift to lower energies as the pentacene film thickness decreases. In order to discuss this energy shift, we have calculated the vibrational energies of a free pentacene molecule by changing its charge state. We have also calculated the vibrational energies of a pentacene molecule adsorbed on a graphite sheet by changing the pentacene-graphite distance. Taking the experimental and calculation results into account, we conclude that the observed energy shifts result from an intramolecular charge redistribution. The present results indicate that the effect of an intramolecular charge redistribution is essential to discuss the origin of an energy shift observed in a vibrational study of an organic molecule/substrate interface.     

Development of organic light‐emitting diodes for electro‐optical integrated devices

Organic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq₃), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.     

Synthesis of copper monolayer and particles at aqueous–organic interface

Using aqueous–organic interface (water–oleic acid) reduction of Cu²⁺ by ascorbic acid, hydrophobic copper monolayer and copper particles have been prepared and characterized. The resultant monolayer could be transferred from the interface onto solid substrate or be dissolved to yield an organosol and copper nanoparticles.     

Tuning of the emission color of organic electroluminescent devices by exciplex formation at the organic solid interface

′ ,4^′′-tris(3-methylphenylphenylamino)triphenylamine, 1,3,5-tris[(4-diphenylaminophenyl)phenylamino]benzene, N, N^′-bis(3-methylphenyl)-N, N^′-diphenyl-[1,1^′-biphenyl]-4,4^′-diamine, and 4,4^′,4^′′-tri(N-carbazolyl)triphenylamine, emitted bright light resulting from the exciplex formed at the solid interface between TPOB and the hole-transporting material. The exciplex formation was evidenced by the measurements of the photoluminescence spectra and lifetimes of the mixture of an equimolar amount of TPOB and each of the hole-transporting materials. Tuning of the emission color from greenish blue to orange was attained by varying the ionization potential of the hole-transporting material for the fixed electron-transporting material of TPOB. Received: 27 July 1998/Accepted: 28 July 1998     

Evolution of step-terrace structure at Si–SiO2 interface in SIMOX substrate during annealing

The step-terrace structures at the interface between the Si layer and the buried SiO₂ layer of a Separation by IMplanted OXygen substrate has been observed by using atomic force microscopy (AFM) after removing the SiO₂ and Si layers. The time evolution of the Si–SiO₂ interface roughness during high-temperature annealing was analyzed by the scaling analysis of AFM data. The correlation length exhibited a nice correspondence to the size of square domain structures. Decreasing in the index of the length scale indicates that the growth mechanism changes as the annealing proceeds.     

Evaluating effect of surface state density at the interfaces in degraded bulk heterojunction organic solar cell

Degradation and short shelf life have been observed experimentally in poly(3-hexylthiophene) (P3HT): 6,6-phenyl C61-butyric acid methyl ester (PCBM) based blend solar cells. Both dark and illuminated current–voltage characteristics could be explained quantitatively with a proposed single model for a typical degraded organic solar cell-glass/ITO/PEDOT:PSS/P3HT:PCBM/Al. It has been found that surface state density, interface thickness, tunneling coefficient and occupation probabilities of the interface states becomes important with the passage of time. To look into the problem the activity at ITO/PEDOT:PSS and P3HT:PCBM/Al interfaces are studied using realistic values of the interfaces. The experimental J–V characteristics is well explained with the inclusion of tunneling current through these surface states and becomes the dominant current component for the degraded cell. It is also found that surface state density increases to 10¹²–10¹³ cm⁻² eV⁻¹, which has been verified with C–V measurements and also is in agreement with our proposed model for BHJ solar cell after 150 h of fabrication.     

Temperature dependence of the effective anchoring energy for a nematic-ferroelectric interface

Specific features of the anisotropic interaction between a nematic mixture and a polar surface of a ferroelectric triglycine sulfate crystal have been studied over a wide temperature range including the substrate's Curie point T_c. The mixture was composed of two nematic liquid crystals, 60% of p-methoxybenzylidene-p-n-butylaniline (MBBA) and 40% of p-ethoxybenzylidene-p-n-butylaniline (EBBA), and doped with a small amount of a dichroic dye. The temperature dependence of the polarized components of optical density D_j of the dye absorption band for the nematic and isotropic phases of the MBBA+EBBA mixture has been obtained using polarization optic techniques. The temperature-induced structural changes in the nematic layer near T_c were found to be related to the changes in the orientational part of the tensor order parameter Q_ik. The experimental data have been interpreted using the model, in which the dispersive van der Waals forces of the substrate stabilize the planar orientation of the nematic in the bulk competing with the short-range anchoring forces in the vicinity of T_c. At the same time, the anisotropic part of the surface energy has two terms with the orthogonal easy axes. The nature of the surface electric field and its effect on the director alignment at the interface have been clarified. Taking into account the known relation between anchoring strength and the nematic order parameter, the effective anchoring energy w_eff for the studied system has been determined as a function of temperature.