Organic field-effect transistors (OFETs) are promising candidates for applications, which require medium-speed operation, such as circuits for displays, and sensors. Within OFETs, electrolyte-gated OFETs have attracted much attention lately since thanks to their high capacitance (up to hundreds of F/cm2), they can be operated at very low voltage (below 1 V) and therefore be driven by printed batteries or solar cells. Their integration in logic circuits has been demonstrated by us[1] and others.[2] However, when considering integrated circuits, it is crucial to be able to adjust the threshold voltage. The origin of the threshold voltage lies in a combination of effects such as the presence of charge traps at the organic-dielectric interfaces, residual charges carriers inside the semiconductor and the work function difference between the gate and the semiconductor. We focused our work on the modification of the gate work function by using various metal as the gate electrode. Since the operating voltage is smaller than the metal work function divided by the charge, using different metals as gate electrode allows tuning the threshold voltage over the complete operating potential window. The shift in the threshold voltage, for a poly(3-hexylthiophene) transistor, when switching from gold (high work function) to calcium (low work function) is approximately 1 V (from 0.4 V to -0.55V). The possibility to make the transistor switch on at positive or negative gate voltage is of particular interest in the case of the zero-gate load unipolar inverter where a positive threshold voltage is required for the load and a negative threshold voltage for the driver. We demonstrate that the performance of logic circuits is improved by the appropriate choice of metal gate patterns. [1] Herlogsson, L., Cölle, M., Tierney, S., Crispin, X., Berggren, M., Low-Voltage Ring Oscillators Based on Polyelectrolyte-Gated Polymer Thin-Film Transistors, Adv. Mater. 22, 76 (2010). [2] Xia, Y., Zhang, W., Ha, M., Cho, J.H., Renn, M.J., Kim, C.H., Frisbie, C.D., Printed Sub-2 V Gel-Electrolyte-Gated Polymer Transistors and Circuits, Adv. Func. Mater. 20, 587 (2010).

OTFT-based complementary circuits that combine n-channel and p-channel semiconducting materials lead to lower power dissipation requirements, a higher noise tolerance margin, improved robustness, and easy circuit design. For the construction of complementary circuits, it is more desirable to use single-component ambipolar transistors than a combination of p- and n-channel transistors, which can simplify the design of circuits and minimize the number of device fabrication steps. However, low band gap polymer-based ambipolar semiconductors for this purpose have not been extensively studied. Moreover, to further advance the development of ambipolar polymer transistors, an in-depth investigation of the key factors that affect charge transport characteristics is required. Here, we present the synthesis and characterization of low band gap polymers based on systematically different oligothiophene-alt-DPP polymers with two different oligothiophenes and substituted alkyl side chain lengths. Excellent ambipolar behavior was observed with very high hole and electron mobilities of up to 2.17 and 0.19 cm2Vâ?"1sâ?"1, respectively. The correlation between semiconductor crystallinity and carrier mobility was investigated by synchrotron x-ray diffraction (XRD) studies and atomic force microscopy (AFM). Longer substituted alkyl side chains and longer inter-distances between alkyl chains in the polymer backbone improved the crystalline structures, which improved further after thermal annealing. This work demonstrates that the chemical structure and processing temperature significantly affect the interchain organization and, therefore, the carrier mobilities of the polymer field-effect transistor devices.

The evolution of organic electronics is now poised to enter the commercial phase, with the recent market introduction of the first prototypes based on organic transistors fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Organic solar cell efficiencies are currently increasing rapidly based on organic bulk heterojunction devices fabricated from solution. Central to these device efficiency improvements are the development of new photoactive semiconducting donor and acceptor materials, designed at the molecular level to optimise both absorption of the long wavelength region of the solar spectrum and generation of high cell voltages. This presentation will examine some of the key design strategies to control the molecular orbital energy levels and microstructure of donor polymer semiconductors and illustrate with examples and their characterisation. Specifically, the systematic reduction of the bandgap in a series of bridged ladder type indenofluorene copolymers, in combination with the progressive lowering of the HOMO energy level will be shown. Analogues of these polymers also exhibit high charge carrier mobilities, and we will present transistor data. Reference 1) Zhang, W.; Smith, J.; Watkins, S. E.; Gysel, R.; McGehee, M.; Salleo, A.; Kirkpatrick, J.; Ashraf, S.; Anthopoulos, T.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc., 2010, 132, 11437.

Organic field-effect transistors (OFETs) have attracted significant fundamental and applied attention due to their potential to realize circuitry for low cost portable electronic devices such as active-matrix displays, radio frequency tags and smart cards. However, the low carrier mobility and carrier densities of organic semiconductors compared to conventional inorganic materials such as silicon currently limits the potential of OFETs to expand into a wider range of applications. One of the main challenges in OFETs is to achieve sufficient source-drain current at practically reasonable voltages for lower power operation. For over a decade efforts have been directed towards the quest for higher-performance organic semiconductors â?" one may consider these as â?omaterial-centricâ? strategies. However, typical values of carrier mobility remain comparable to or lower than amorphous inorganic semiconductors and below that required for most electronic control circuitry applications. Therefore, the development of conceptually different OFET architectures must also be of a high priority with in particular a clear goal to increase source-drain current without sacrificing the simplicity of the fabrication process or compromising the other benefits of organic semiconductors We report the fabrication and electrical characteristics of structured gate (3D) organic field effect transistors consisting of a gate electrode patterned with three-dimensional pillars. The pillar gate electrode was over-coated with a gate dielectric (SiO2) and solution processed organic semiconductors producing both unipolar p-type and bipolar behavior. We show that this new 3D gated architecture delivers higher source-drain currents, higher gate capacitance per unit equivalent linear channel area, and enhanced charge injection (both electron and holes) versus the conventional planar structure in all modes of operation. The maximum source-drain current enhancements in p- and n-channel mode were > 600% and 28% respectively. This assists in better matching of carrier mobilities which is advantageous for creating organic logic circuit elements such as inverters and amplifiers. The structuring appears to have no detrimental effect upon the underlying electronic properties of the organic semiconducting channel materials. Hence, the method represents a facile and generic strategy for improving standard OFET performance and also has the potential to assist in engineering channel properties in light emitting or sensing field effect transistor applications.

In this work we discuss the operating mechanisms of the organic electrochemical transistor (OECT): a device with the potential to provide significant contributions to the field of bioelectronics. An OECT comprises a conducting polymer channel in contact with an electrolyte. A gate electrode submersed in the electrolyte controls the level of doping and therefore the conductivity of the transistor channel. The conducting polymer in the active layer has the ability to conduct ionic and electronic carriers, and can be chemically tuned for specific functions or applications. The materials are soft and compatible with flexible substrates. These characteristics make an OECT an ideal platform for integrating the functions of a biological system with electronics. We explore the effect of molecular architecture of the conducting polymer on OECT performance, comparing solution processable poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and vapor deposited PEDOT:tosylate (PEDOT:TOS). We also discuss the effect of blending poly(ethylene glycol) with both PEDOT:PSS and PEDOT:TOS on film microstructure, channel conductivity, surface capacitance and overall OECT performance. We are motivated by the application of the OECT as a biosensor and tool for the development of in vitro cell models, and therefore pay particular attention to characteristics important in sensing applications in biological conditions such as channel conductivity, device to device reproducibility, and long term stability in aqueous environments.

Organic field-effect transistors (OFETs) are currently attracting considerable attention for potential applications in flexible integrated circuits such as radio-frequency identification tags (RFID-tags) and microprocessors [1]. Key parameters for such applications are a sufficiently high mobility (â?¥0.1 cm²/V.s) at short channel length (Lâ?¤ 10 μm) [2], and high yield and uniformity of the electrical parameters. In current state of the art organic electronic circuits [1], these requirements are met for polycrystalline p-type pentacene bottom-gate bottom-contact (BG-BC) OFETs with photolithographical patterned gold source and drain (S/D) electrodes. In this configuration charge injection and favorable pentacene morphology is assured by self-assembled monolayer (SAM) based surface modification of the gold BCs and the dielectric, leading to saturation mobilities (μ_sat) of up to 0.6 cm²/V.s at L= 5 μm [3]. The high gold price however seriously hampers low-cost applications. Alternative S/D materials (e.g. metals, metal oxides, carbon based materials, charge-transfer complexes, â?¦), typically evaluated for long channel (â?¥ 50 μm) pentacene OFETs using shadow masking, lead to μ_sat of 0.1 cm²/V.s. Although such mobilities might be sufficient for organic circuits, materials or processes are mostly incompatible with photolithography, and reports about operation at short channel length are scarce, likely because of contact resistance issues [4]. Recently we presented a first generation of high performance short channel pentacene OFETs with decreased S/D metallization costs consisting in noble metal based metallic bilayer BCs [3]. Here we present a second generation metallic multilayer for BCs of short-channel (Lâ?¤ 10 μm) OFETs. In addition to the fact that no noble metals (Au, Pd, Pt) were used (decreasing the S/D metal costs below 1% of that of 30 nm gold), our process is compatible with photolithography (including harsh cleaning conditions) and SAM treatments. Corresponding short channel pentacene BG-BC OFETs (with 120 nm SiO_x dielectric) achieved a very good μ_sat of 0.3 cm²/V.s and a threshold voltage of -1 V. As a consequence, this new BC metallization that achieved high mobilities at low metallization costs can be considered as a significant contribution towards high-performance low-cost organic electronic circuits. This work was performed in a collaboration between IMEC and TNO in the frame of the HOLST Centre. The authors acknowledge financial support of EC-funded project Polaric (FP7-247978). 1. K. Myny et al., â?An 8-bit, 40 instructions-per-second organic microprocessor on plastic foilâ?, IEEE J. Solid-State Circuits (DOI: 10.1109/JSSC.2011.2170635). 2. D.M. De Leeuw et al., IEDM Tech. Dig. (2002) 293. 3. R. Müller et al., Org. Electr. 12 (2011) 1227. 4. M.J. Panzer and C.D. Frisbie in Z. Bao and J. Locklin â?Organic Field-Effect Transistorsâ?, CRC Press, 2007, p. 139.

For the design of efficient organic integrated circuits, there is an urgent need for complementary technology, where both n-type and p-type transistor operation is realized in a single layer, while maintaining the attractiveness of low-power consumption, wide noise margins, and higher robustness. We have demonstrated the complementary transistor circuits which comprised two ambipolar organic transistors with new active layer structure. In the device, the new ambipolar organic active layer is triple layer structure (hole blocking layer/Ambipolar active layer/electron blocking layer). We can control the hole and electron transport of organic semiconductors by inserting carrier blocking layer into Source/Drain and Ambipolar active layer. In this ambipolar semiconductor, we use N, N'-ditridecylperylene-3, 4,9,10 -Tetracarboxylic diimide (PTCDI-C13) and 2,3:6,7-Dibenzanthracene (Pentacene) to achieve a balance Ambipolar properties. Then we use the 2,9-dimethyl-4 ,7-diphenyl-phenanthrolin (BCP) as a hole blocking layer (HBL), and use the 4 ', 4''-Tris (N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) as the electron blocking layer. In this way, we had verified the unipolar N-type properties on ambipolar organic transistors with Source/Drain electrodes on BCP side. The devices exhibited large electron mobility of more than 0.13 cm²/Vs, on/off ratio of exceeding 10⁵. We also had verified the the unipolar P-type properties on ambipolar organic transistors with Source/Drain electrodes on m-MTDATA side. The devices exhibited large electron mobility of more than 0.11 cm²/Vs, on/off ratio of exceeding 10³. Finally, we have demonstrated the organic complementary inverters with two ambipolar organic transistors with source/drain electrodes on different side of new ambipolar organic active layer. At a small input voltage (V_IN), the output voltage (V_OUT) is almost equal to the supply voltage (V_DD). When the value of V_IN exceeds the threshold voltage of the inverters, the value of V_OUT switches to become almost equal to zero. When the organic complementary inverters operated at a voltage of 30V, it shows a maximum voltage gain of 60.

Most applications of organic electronics require a non-volatile memory function that retains its data when the power is turned off, and that can be programmed, erased and read-out electrically. Ferroelectric polymers are ideal candidates for switching as they exhibit an intrinsic bistable, remnant, polarization that can be switched by an electric field. An overview will be presented on reported ferroelectric memories, viz. capacitors, field-effect transistors and diodes. The simplest ferroelectric memory is a thin film capacitor. Information is stored by aligning the polarization either up or down with applied field. To retrieve the information one applies a switching voltage to obtain a high or a low charge displacement current response depending on the alignment of the polarization with the direction of the applied field. Ferroelectric capacitors therefore have a destructive read-out. If the polarization direction was changed during the read operation then a reset voltage needs to be applied. Ferroelectric field-effect transistors (FeFET) are suitable as memory element due to the non-destructive read-out and low power consumption. The polarization of the ferroelectric gate insulator does (On) or does not (Off) induce an accumulation layer in the semiconducting channel. Here we present an analytical model that describes the charge transport FeFETs. The model combines an empirical expression for the ferroelectric polarization with a density dependent hopping charge transport in organic semiconductors. Transfer curves can be calculated with parameters that are directly linked to the physical properties of both the comprising ferroelectric and semiconductor. We show that a good agreement between experimental and calculated current can be obtained. The method is generic; any other analytical model for the polarization and charge transport can be easily implemented. FEFETs have a three terminal layout that complicates the integration. Ferroelectric diodes alleviate this problem. Blending semiconducting and ferroelectric polymers yields phase separated networks in which the ferroelectric yields the binary information and the semiconductor is used to read the stored information. The operation is based on the modulation of the injection barrier. For a p-type semiconductor, negative ferroelectric polarization at the injecting contact is compensated by accumulated holes in the semiconductor. The resulting band bending then lowers the injection barrier and, hence, the resistance of the diode. Excellent current rectification and high on/off current ratios have been reported. We show that the origin of the resistive switching is the stray field of the polarized ferroelectric. The current voltage characteristics can quantitatively be reproduced. We will argue that the ultimate memory density that can be realized is in the order of 1 Gb/cm2.

Solution deposition of organic semiconductors (OSC) is a leading contender for producing large-area, inexpensive, and flexible organic electronics. We recently developed a solution deposition method for OSCs which resulted in better organic field effect transistor (OFET) performance compared to those fabricated from simple drop casting or spin casting methods. We found that this method can achieved a wide range of reproducible OSC morphologies, which results in differing FET performance. Certain conditions have yielded remarkable FET performance, yielding large area films (> 4 cm²) on which FETs show an average charge carrier mobility around 2 cm² V^-1s^-1 with some transistors showing mobilities as high as 4 cm² V^-1s^-1, a current On/Off ratio of > 10⁶, low hysteresis and a low threshold voltage. We discuss the characteristics of this method and its applicability to a wide range of OSCs. We quantitatively measure the molecular ordering resulting from different deposition conditions and show effective tuning of molecular packing that yields higher charge carrier performance without changing the molecular structure. We investigate the stability of this molecular packing structure and also measure the relationship between the morphological features and its impact on charge carrier transport. We also study the general applicability of this method onto various OSCs.

Recently n-channel organic based thin film transistors (TFTs) transistors based on core-chlorinated naphthalene diimides are emerging as air stable materials with high mobility (1,2). However, towards their used in applications, TFT reliability and operational stability are crucial issues. Many organic TFTs have been found to exhibit undesirable threshold voltage shifts, hysteresis, or mobility degradation as a result of prolonged bias stress witch ultimately limit their technologically development. In this contribution we report the on operational stability of thin-film transistors based on n-type semiconductor the naphthalene bisimide processed by zone casting. Transistors fabricated using parlyne c as top gate exhibit a field effect mobility of 0.4 cm2/(V.s) threshold voltage of 10 V and an on/off current ratio 106. Most impressive they are also insensitive to changes in the threshold voltage when submitted to a prolonged gate-bias stress. This high stability is attributed to the dielectric/semiconductor interface, which is not susceptible to water-related traps. The high operational stability makes then interesting candidates for complementary metal oxide semiconductor (CMOS) circuits. A further advantage of the high stability is that allow us to isolate and study other effects namely the so-called â?ocontact effectsâ? which are visible as non-linear current-voltage (I-V) characteristics near the origin. It is believe that this carrier injection problem is possibly caused by traps. However, detail insight into the signature of these traps is often prevented by the simultaneous presence of trapping caused by the gate-bias stress. The naphthalene bisimide based TFTs (being stable against gate-bias stress) are an ideal tool to study the carrier injection problem. In this contribution it is shown that traps responsible by the â?ocontact effectsâ? can be studied by using thermally detrapping experiments and furthermore they can be made inactive leading to optimization of the carrier injection into the transistor channel and to a high transistor performance. References [1] High-Mobility Air-Stable Solution-Shear-Processed n-Channel Organic Transistors Based on Core-Chlorinated Naphthalene DiimidesWen-Ya Lee, Joon Hak Oh, Sabin Suraru, Wen-Chang Chen, Frank Wurthner and Zhenan Bao, Adv. Funct. Mater. 2011, XX, 1â?"9 [2] Effect of N-Substituents on Redox, Optical, and Electronic Properties of Naphthalene Bisimides Used for Field-Effect Transistors Fabrication, P. Gawrys, D. Djurado, J. Rimarcik, A. Kornet, D. Boudinet, J.M. Verilhac, V. Lukes, I. Wielgus, M. Zagorska and A. Pron, J. Phys. Chem. B,2010, 114, 1803-1809.

The unique property of ambipolar organic light-emitting field-effect transistors (OLETs), that the position of the light-emission zone within the transistor channel can be controlled by the applied biases, opens novel aspects for applications. A straightforward application is an electrically changeable emission colour by taking advantage of the controlled displacement of the light-emission zone. However, most of the envisioned concepts for the realization of a colour tuneable OLET faces one common problem: Provided that the control of the position of emission within the channel is the key for the colour change, the system modification required for the colour variation need to maintain the ambipolar character of the transistor. To retain ambipolarity and the related shift in the emission position a decoupling of the optoelectronic processes of the ambipolar transistor and of the colour conversion processes is advisable. A rather simple approach seems to consist of a colour change by an emission-absorption-reemission process. Here a polychromatic OLET is presented which employs an efficient poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT) based ambipolar field-effect transistor and a colour conversion layer consisting of rubrene deposited on top of the top-gate transistor [1]. It will be shown that a bias controlled change in emission colour from green to yellow can be obtained with little loss in external quantum efficiency. The independent adjustability of the emission intensity and colour will be demonstrated. On basis of a device model it will be shown that the colour conversion relies rather on the filter properties of the rubrene layer than on reemission. The presented model offers a closer insight into the colour conversion process of the polychromatic OLET providing the relation between applied biases and photometric quantities of the transistor. [1] E.J. Feldmeier and C. Melzer, Org. Electron. 12, 1166-1169 (2011).

Self-assembled monolayers (SAMs) of organic molecules such as silanes, carboxylic acids (CA) or phosphonic acids (PA) have been intensively investigated as functional molecular layers in organic thin film transistors due to their excellent properties [1]. They can be easily deposited from solution, tailored to a desired functionality in order to tune the device performance and are applicable to flexible substrates. Although they are often utilized as gate dielectrics their application surpasses pure dielectric functionality: It has been shown that bifunctional molecules consisting of semiconducting moieties, insulating alkyl chains, and an anchor group can be deposited forming self-assembled monolayer field-effect transistors (SAMFETs) [2]. Only recently first n-type SAMFETs have been demonstrated based on fullerene (C₆₀) functionalized alkyl phosphonic acids [2d]. The major issue with SAMFETs is the alignment of the semiconducting moieties to obtain effective Ï?Ï?-stacking and the insulating alkyl chains. Here we report on a novel approach towards the self-alignment of multifunctional, complex molecules in a mixed SAM enhancing the semiconducting performance (channel formation of the SAM) and insulating properties (ordered n-alkyl chains across the SAM) of the layer. We use the difference in binding strengths of carboxylic acids and phosphonic acids, respectively, to an oxidized aluminum gate electrode which enables a (partial) replacement of a CA-SAM by immersion of the substrates into a PA solution. By substituting CA molecules of a dense octadecanoic acid SAM with C₆₀ functionalized PA molecules (C₆₀-C₁₈-PA) an upright alignment of the PA molecules can be enforced. CA molecules remain on the substrate underneath the C₆₀ moieties preventing the fullerenes from touching down onto the gate electrode and pushing them into a confined layer on top of the CA-SAM. This leads to enhanced charge transport and a strongly reduced leakage current since a dense carboxylic acid remains underneath the voluminous C₆₀ head of the functionalized molecules. Compared to pure C₆₀-C₁₈-PA SAMFETs, strongly increased drain currents (factor 10 higher than in previous n-type SAMFETs [2d]) and on/off-ratios (up to 10³) were found. The results of this novel approach towards molecular scale electronics were encouraged by molecular dynamics simulations and various spectroscopic measurements, amongst others XPS and SFG. References [1] a) Halik, M., H. Klauk, et al. (2004), Nature 431(7011): 963. b) Klauk, H., U. Zschieschang, et al. (2007), Nature 445(7129): 745. c) DiBenedetto, S. A., A. Facchetti, et al. (2009), Advanced Materials 21(14-15): 1407. [2] a) Smits, E. C. P., S. G. J. Mathijssen, et al. (2008), Nature 455(7215): 956. b) Mathijssen, S. G. J., E. C. P. Smits, et al. (2009), Nat Nano 4(10): 674. c) Mottaghi, M., P. Lang, et al. (2007), Advanced Functional Materials 17(4): 597. d) Novak, M., A. Ebel, et al. (2011), Nano Letters 11(1): 156.

Organic Light Emitting Diodes (OLED) have developed rapidly in the past years. For small mobile displays, they are already a commercially established technology. For OLED lighting, where white OLED with very high efficiency and stability are needed, a further improvement of parameters is still needed. I this talk, I will discuss some of our recent progress on highly efficient monochrome and white OLED. I will compare experimental results with a theoretical model which reveals how the outcoupling of the emitted photons proceeds and how the efficiency can be increased. For monochrome OLED, quantum efficiencies of well beyond 50% have been reached. Furthermore, I will discuss the difference of bottom- and top-emitting OLED, showing that the latter indeed show the better outcoupling properties as theoretically predicted. Furthermore, approaches to improve outcoupling by introducing hybrid inorganic-organic layers for wave guide mode scattering will be discussed.

In small-molecule OLEDs, vacuum-deposited amorphous organic films are commonly used, even though they generally have much lower carrier mobilities than those in other types of organic semiconductors such as single crystals and polycrystalline films. This is mainly because the amorphous films have many advantages in fabrication of OLEDs with a large area and a high uniformity: (1) nanometer-scale surface smoothness, (2) easy controllability of thickness, (3) no restriction in the choice of underlying layers, and (4) simple high-purity fabrication processes. These advantages make it possible to stack any materials having different properties orderly and also derive full potential and higher-order functionalities of organic materials with high stability and durability. When developing new OLED materials, we usually deduce the physical properties of films only from the chemical characteristics of the single molecules, and have not sufficiently considered how molecules are oriented and ordered in the bulk of the amorphous films. Since the beginning of research on OLEDs using vacuum-deposited thin films in 1987, it has been believed for around 20 years that molecular orientation in small-molecule OLEDs is generally random and isotropic. This simple assumption has been used unconsciously and contributed to the efficient development of new OLED materials since the early stage of OLED research. However, we have recently found that molecules in vacuum-deposited amorphous organic films are generally oriented depending on the anisotropy of the molecular shape. Linear and planar molecules are generally oriented along the substrate surface, whereas bulky and compact molecules are randomly oriented. Since this horizontal orientation of anisotropic molecules occurs on any underlying layer and even in doped films, we can effectively use it for applications in any structures of devices. This presentation will show an overview of recent studies on molecular orientation in small-molecule OLEDs. The molecular orientation of various kinds of OLED materials will be demonstrated, and the general characteristics of the orientation will be discussed. It will be also shown that the horizontal molecular orientation of charge transport materials and emitting materials can contribute to the enhancement of carrier mobilities and light outcoupling efficiency, respectively. Now, research on OLEDs has reached maturity, and the device performance is coming close to the theoretical limit. Thus, we have to understand the device physics and the underlying chemistry beyond the above simple assumption to further improve the performance and reliability of OLEDs. Through this study, we can find that the investigation of higher-order structures such as molecular orientation and ordering are very important to make fundamentals for the next step of research on organic optoelectronic devices. Reference: D. Yokoyama, J. Mater. Chem. (2011), in press, doi: 10.1039/c1jm13417e.

Our research provides a concept for increasing the performance of printable OLEDs. To achieve high efficiency of electrophosphorescence OLEDs (PhOLEDs), it is crucial to develop host materials for PhOLEDs and to develop a method for multilayer architecture based on crosslinking, which are challenges.

Patternable protein polymers from chicken albumen were utilized as dielectrics for organic electronics. The solution of albumen was spin-coated and thermally stamped with a patterned mold, and then developed a patterned dielectric films. The patternability of albumen dielectrics is attributed majorly to the localized disulfide bond formation by which different protein molecules with thiol side chains are crosslinked and netted without adding any crosslinking agent. The patterned albumen dielectrics are water-insoluble and ready for next fabrication process, such as the vacuum- or solution-processed deposition of organic semiconductors, metals, or a second layer dielectric. This work demonstrated that patternable albumen dielectrics were deployed multifunctionally as dielectrics for capacitors or field-effect transistors, as insulators for inductors or multilayer circuits, and as materials for substrates or capsulators. It indicates that albumen is a practical material for the simplified fabrication of organic electronics.

Non-volatile organic memory has emerged as a candidate for light-weight, flexible, and disposable charge storage media. To this end, a variety of approaches has been progressed including capacitor-type bistable devices and organic field-effect transistor (OFET)-based memories. Here we report on a solution-processed organic transistor memory using a n-channel (and potentially ambipolar) polymer semiconductor, poly{[N,Nâ?T-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarbo-ximide)-2,6-diyl]-alt-5,5â?T-(2,2â?T-bithiophene)} (P(NDI2OD-T2)), and bi-layered polymer gate di-electrics. High electron mobility (~0.3 cm²/Vs) and air-stability of the P(NDI2OD-T2) molecule enables to develop high performance organic flash memory element with large memory window (~39 V), high on/off-current ratio (~10⁵), and long retention time more than ~10⁶ s. These memory characteristics were achieved by proper selection of the dielectric materials and their orthogonal solvents, and systematically analyzed by embedding various metal nanoparticles, such as Au, Cu, Al, and Ag, in bi-layered gate dielectrics of the OFET.

Printed electronics have attracted increasing attentions because of its realization of flexible, large-area, and low-cost for organic devices and circuits. Numerous printing techniques have been shown to realize simple devices or circuits such as solar-cell photovoltaic, memory, sensors, and RFID. Reverse offset printing (ROP) is one of candidates for printed electronics. Using ROP method is very attractive owing to a variety of advantages, such as very high resolution, high throughput, no limitation to substrate types or sizes, and ability to print fine and complicate patterns. In this study, Ag electrodes were printed to 15 μm of line width and to 3 μm of space width by ROP using the paste with Ag nanoparticles which were almost monodispersed with a mean particle size of ~ 5 nm confirmed by TEM and easily dispersible with common organic solvent such as octane, toluene, etc. It would be enable facile self-assembly of these particles into close packing when printed, thus greatly facilitating their coalescence into a continuous conductive film. After printing and sintering of Ag paste, microstructure of the Ag film was observed by SEM. From these results, we confirmed that the electrical resistivity was decreased with 1.6 μΩcm corresponding to densification of the printed film and growth of nanoparticles. Printed Ag electrodes were applied to source/drain of organic thin-film transistors (OTFTs) based on semiconducting polymer as poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3,3â?Tâ?Tâ?T-dialkyl-quarterthiophene) (PQT-12). The OTFT with short channel of 3 μm showed reproducible characteristics. Field-effect mobility, threshold voltage, and on/off-current ratio were 0.01 cm2 V-1 s-1, 5 V, and ~104, respectively.

We show that electrical bias-stress effects on pentacene-based organic thin-film transistors (OTFTs) can be used to verify the two different mechanisms of charge trapping in pentacene-gate dielectric interface and irreversible degradation of pentacene film. Generally, Thin-film transistors (TFTs) are suffering from the electrical instability by bias stress which is threshold voltage shift caused by the positive/negative voltage bias-stress through gate dielectric materials. These instabilities were found to be the results of both complex mechanisms in a charge trapping and mobility degradation. To clarify these mechanisms, the operating performance with bias stress of TFTs depended on the environmental conditions such as vacuum, air atmosphere, oxygen, etc. And no significant changes in threshold voltage shift measured in vacuum state. Furthermore, the metastable creation in the pentacene and gate-dielectric interface was not detected with the thermal accelerated degradation test. We attributed the external contents of oxygen gas or moisture to a trap-assisted charge trapping process in a conduction channel formed in the vicinity of the gate dielectric. We conclude that, exposure to environmental active gases such as oxygen, humidity generates trap-assisted charge trapping in channel when charge accumulated with gate-bias. And the charge trapping called reversible/irreversible threshold voltage shift due to the positive/negative bias stress effects are originated from temporally trapped charge in shallow-trap state, and the chemical reactions with gas ions attracted by strongly accumulated positive hole-carriers.

Recent advancements achieved in organic materials greatly promoted the development of devices such as organic light-emitting diodes (OLEDs), solar cells, ion batteries, organic memories, organic field-effect transistors (OFETs) and organic light emitting field effect transistors (OLFETs). In principle, pure organic semiconductors provide both electron and hole equivalent conduction, although organic semiconductors are typically classified as either hole-conducting (p-type) or electron-conducting (n-type) materials. This is known as the ambipolar action in organic semiconductors and is very different from that in inorganic semiconductors. However, in order to be accessible to such ideal ambipolar conduction, accurate control in both the electrode-semiconductor contact (charge injection process) and the semiconductor-dielectric gate interface (charge accumulation process) is crucial. If such ideal ambipolar conduction can be realized, electron and hole can simultaneously be injected into an intrinsic semiconductor layer and highly efficient OLFETs can be constructed without paying any attention to the complex p-n junctions, which are generally indispensable for making inorganic LFETs. We have carefully examined both injection and accumulation processes for the purpose of simultaneous gaining a large amount of electrons and holes. We have tested 2,5-biphenyl bithiphene (BP2T) at the present study because this compound is known to be a good candidate for OLFETs For improving the injection process a various metals with different work functions have been employed as the source/drain contacts. A good methodology has been searched by employing various methods, such as modification by polymer thin films, self assembled monolayers (SAMs) and deposited aliphatic molecular thin films. In this conference, we will demonstrate that a good ambipolar carrier injection can be realized even using gold (Au) electrodes for both source and drain, and therefore an air stable and high performance OLFET can be fabricated when both injection and accumulation conditions are optimized. The role of the injection barriers on the device performance will be dicussed in terms of the Bardeen limit and the Schottkey limit.

Substantial progress has been made in the exploration of organic semiconductors as active elements in electronic devices i.e., field-effect transistors (FETs). Various conjugated polymers containing diketopyrrolopyrrole (DPP) in the repeating group were reported in the literature for showing their superb electronic device applicability. In the study, we successfully synthesized new low bandgap alternating copolymers containing quaterthiophene (QT), di(thiophen-2-yl)-2,2â?T-biselenophene (DTBSe), and dithienothiophene (DTT). Those copolymers were highly crystalline and provide very promising charge transport phenomenon. Thermal analaysis, cyclic voltammetry, absorption spectroscopy and photoluminescence spectroscopy were employed to investigate their intrinsic materials properties. Eventually, we fabricated field effect transistors and photovoltaic devices to study their semiconducting property. According to our past study, these polymers were found to exhibit quite high carrier mobilities of around 0.6~1.5 cm2V-1s-1.

Organic molecular materials and crystals can serve as active layer in a variety of optoelectronic devices, including solar cells, light emitting diodes and transistors. A comprehensive understanding of the opto-electronic properties of crystals is complicated by the occurrence of surface states, defects and disorder in crystals [1]. The problem may be simplified by studying two dimensional layers of organic molecule rather than the three-dimensional crystals. Recently, we reported closely packed, two-dimensional, self-assembled monolayers of quinque thiophene (5T). The layers are characterized by a high degree of order and are electronically active in a transistor. Self-assembled monolayer field effect transistor (SAMFET) can be grown on SiO2 or polymeric gate dielectrics and combined into integrated circuits. The SAMFETs feature bulk-like carrier mobility, large current modulation and high reproducibility. [2,3] Here we investigate the optical properties of the monolayer using fluorescence, excitation and extinction spectroscopy. [4] For partially covered substrates we find that the fluorescence is dominated by isolated molecules. Complete monolayers show optical properties that differ markedly from the isolated molecules. For the complete monolayer, we observed a red-shifted fluorescence spectrum combined with a blue shift of the absorption maximum. Hence the complete monolayer behaves as an H-type aggregate. The optical properties of the monolayers share many characteristics with those reported for oligothiophene crystals. Thus the optical properties of two dimensional, defect free oligothiophene monolayer are influenced by delocalized of the optical excitations over more than one molecule. Because of their fluorescence, the monolayers may use as active medium in light emitting devices. [1] Como, E. e t al. J. Am. Chem. Soc. 2006, 128, 4277. [2] Gholamrezaie, F.et al. in preperation. [3] Smits, E. C. P et al. Nature 2008, 455, 956 [4] Gholamrezaie, F.et al. Nano Letters, 2011, 10, 1998.

A high voltage field effect transistor has been fabricated using a thin-film organic semiconductor technology. The transistors have an offset drain/source structure, which enables operation at drain-to-source voltages in excess of 450V with gate voltages no higher than 20V. Similar FET structures based on amorphous Si have been reported by other investigators. This work exploits the process advantages of organic materials to demonstrate high voltage devices capable of driving MEMS actuators or for applications requiring high voltages on flexible media. We believe this to be the first demonstration of integrated high voltage field effect transistors based on an organic semiconductor technology with a low temperature (� 95°C) and completely lithographic process that is compatible with both flexible and rigid substrates. One way to increase the drain-to-source driving voltage or electric field across an FET is to place a resistive structure between the drain or source electrode and the gated channel. In this work, high driving voltages are achieved by offsetting the drain or source electrode from the gate creating an ungated semiconductor region in series with a gated semiconductor region. The saturation current (IDsat) through the devices is controlled by the gated semiconductor region, limiting the voltage dropped across (and electric field) this region. The excess voltage is dropped across the ungated semiconductor enabling high voltage operation. There are many methods available to tune the threshold voltage (and transconductance) of these FETs as the gate insulator directly affects carrier transport in the device. Further, it is well documented that high-κ insulator materials are effective in reducing operating voltages (gate-to-source) in FETs while minimizing current leakage through the gate. We previously reported a process using a high-κ gate insulator that is compatible with pentacene. Specifically, BZN (Bi1.5Zn1Nb1.5O7) is a paraelectric pyrochlore system that features a high dielectric constant (~40-50), which can be deposited at room temperature through RF sputtering. We show that the threshold voltage of these pentacene-based high voltage FETs can be reduced from ~-9V to ~-0.5V when using a BZN/parylene-C insulator stack as opposed to an all organic gate insulator (parylene-C). Using the offset drain/source device structure and a dual stacked insulator system, these high voltage field effect transistors can be precisely matched for a multitude of MEMS applications requiring high voltages. The devices are fabricated with a low temperature and completely lithographic process allowing them to be readily integrated with high voltage applications. Electrical performance is evaluated through standard current-voltage and quasi-static capacitance-voltage measurements.

6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is a promising active channel material for organic thin-film transistors (OTFTs) due to its solubility, air stability, and high mobility. However, the growth of TIPS pentacene crystal suffers from intrinsic crystal anisotropy, grain boundary limitation and â?ocoffee-ring effectâ?, which leads to poor performance and significant performance variation of OTFTs. In this work, air flow is utilized to effectively navigate crystal growth. Air flow facilitates uniform solvent evaporation by reducing the contact angle between solvent and silicon substrate, and pushes contact line and solute in solution forward along the crystal growth direction. Collectively these effects prevent solute diffusion from center to edge, resulting in uniform TIPS pentacene seed deposition for crystal growth. The crystallization of TIPS pentacene starts from front contact line, and moves in the same direction as air flow. As a result, air flow effectively navigates TIPS pentacene crystallization and reduces crystal anisotropy over the whole substrate. Under air flow navigation (AFN), TIPS pentacene has been observed to form thin films with improved crystal orientation and uniform coverage on substrate. In addition, the field-effect mobilities of OTFTs based on TIPS pentacene without AFN guidance vary from 7.2Ã-10^-5 cm²/Vâ-s to 8.8Ã-10^-1 cm²/Vâ-s, while AFN OTFTs consistently show mobilities around 0.1 cm²/Vâ-s, which indicates a great reduction in performance variation. Therefore, we conclude that AFN is an effective approach to reduce TIPS pentacene crystal anisotropy and improve performance of OTFTs. Key words: Air Flow Navigation, TIPS Pentacene, Crystal Anisotropy, Organic Thin-Film Transistors

As Organic Field-Effect Transistors (OFET) are now close to applications questions about their reliability under realistic atmospheric conditions become more and more important[1]. Electrical instabilities are evidenced, for instance, in the bias stress effect where under a prolonged application of a gate potential the device characteristics, such as the threshold voltage, evolve with time. Several mechanisms were proposed to understand these degradations of the device properties. An example of such mechanism was proposed recently based on proton production by electrochemical reactions involving water molecules adsorbed at the organic/oxide interface, followed by proton diffusion through the oxide[2,3] . In this work, we have considered the Si/SiO2/Pentacene/Au junctions that are the two terminal pendants of thin film transistors with the same layer structure. We study the dynamic electrical response of these junctions under ambient conditions over a large frequency window as function of a superimposed dc voltage. The complex admittance, Y(Ï?), was measured using a frequency response analyzer Solartron 1260/1296 that provides directly in general case, the parallel conductance and capacitance, G and C: Y(Ï?)=G(Ï?)+jÏ?C(Ï?). Compared to previous works, our frequency range is more extended, going down to 10-1Hz, and the analysis of our data insist on the crucial role played by the interactions of the defects with their surroundings which determine the shape of the measured response functions. At lower frequencies (100Hz) we observe anomalous Low Frequency Dispersion (LFD) that is consistent with the ionic diffusion current proposed in Refs. [2,3] but instead of normal diffusion considered in these works, our results evidence fractional diffusion to occur. The dynamical responses of the polarizable species (dipoles or ions) are strongly influenced by their surroundings. If they are markovian (without memory) we get a Debye response for the dipoles and a normal diffusion for the ions as shown in Si/SiO2/Au â?oreferenceâ? junctions (without the organic semiconductor layer). If they are non-markovian (with memory) the responses are more complex; in their simplest forms we get a Cole-Cole response for the dipoles and a fractional diffusion for the ions. Our data show anomalous behavior in presence of the organic layer. These observations could be the signature of particular structural organizations which can be located either, in the organic layer itself or, in the organic/oxide interface. [1]H. Sirringhaus, Adv. Mater. 2009, 21, 3859. [2]A. Sharma, S.G.J. Mathijssen, M. Kemerink, D.M. de Leeuw, and P.A. Bobbert, Appl. Phys. Lett. 2009, 95, 253305. [3]A. Sharma, S.G.J. Mathijssen, E.C.P. Smits, M. Kemerink, D.M. de Leeuw, and P.A. Bobbert, Phys. Rev. B. 2010, 82, 075322.

Hole blocking effect of the interfaces of aluminum (Al) and pentacene in organic field-effect transistors (OFETs) was reported. Originally ambipolar pentacene-based OFETs of aluminum (Al) source-drain electrodes performed in n-type conduction only by inserting a coevaporated Al-pentacene layer. The highest occupied molecular orbital (HOMO) of the coevaporated Al-pentacene layer was moved downward 1.3 eV measured by ultraviolet-visible absorption spectroscopy and ultraviolet photoemission spectroscopy. The hole blocking effect was also observed in Al pentacene-based OFETs without the coevaporated layer for 24 hoursâ?T storage and verified by thermoionic emission model. This study highlights that the interaction of active metals and organic semiconductors impacts the performance of OFETs.

In this work, we describe the realization and the optimization of field effect transistors based on Ï?-conjugated PE monomer and OPE oligomers as active layers. The 1,4â?"dipropynyl benzene monomer PE was first prepared by Sonogashira coupling between 1.4-diodobenzene and propyne catalyzed by a Pd/PR3/basis/Cu system. The OPE was synthesized by an original route: the triple bond metathesis reaction of a para disubstituted alkyne. Metathesis polymerization of PE was done using the tungsten carbyne [(tBuO)3Wâ?¡CtBu] as catalyst.The obtained OPE was characterized by 1H and 13C NMR in solid and liquid states, IR and AFM. The average degree of polymerization is low (n = 4). The TFT devices were constructed on a highly doped n-type silicon substrate covered with 220nm-thick silicon oxide layer. The organic semiconductors were vacuum evaporated on patterned gold source and drain electrodes. The thickness of the active layers was ranging from 15 to 30nm and the channel length L from 1 to 50 μm. The TFT characteristics were measured both in Nitrogen atmosphere (Glove Box) and in air at room temperature. Both monomer PE and oligomer OPE based devices exhibited a field effect behaviour with motilities of around 10E-5cm2/V.s. These performances were improved in the case of OPE oligomer based devices. The mobility enhancements were obtained, either via functionnalization of the SiO2 substrate with Self Assembled Monolayers of perfluorodecyltrichlorosilane (µ= 10E-4 cm2/V.s), or via the control of the substrate temperature at 40°C during deposition (µ ~10E-3cm2/V.s). These results will be discussed according to the morphology and the structural modifications of the organic semiconductors induced by temperature and SAM treatments. Other conjugated oligomers synthesised by the metathesis route will also be presented in field effect transistor applications.

We have successfully developed the transfer technology of a few nanometer thick self-assembled monolayers (SAMs) using polydimethylsiloxane (PDMS) stamping, and fabricated organic thin-film transistors and circuits where two different SAMs were spatially transferred onto a same silicon substrate. Utilizing the SAM spatial-transfer method to the fabrication of gate dielectric layers, threshold voltages in organic transistors and inversion (switching) voltages in organic unipolar inverter have been systematically controlled. Furthermore, using transmission electron microscope (TEM), the formation of uniform SAMs was clearly observed, indicating the feasibility of the stamping that can transfer a single nanometer structure and that different SAMs can be spatially allocated on same substrates. The transfer method and transistor fabrication using two different SAMs (tetradecylphosphonic acid (HC14-PA) and pentadecylfluoro-octadecylphosphonic acid (FC18-PA)) have been reported in our previous work [1]; however, in this paper, we will report nanostructure analysis of transferred SAMs using TEM, and its relation to electrical performances on organic circuits. An organic semiconducting layer, DNTT [2], and electrodes of the organic circuit were fabricated using vacuum evaporation, while a SAM gate dielectric was formed using the stamping method. The stamped SAMs showed the equivalent insulating characteristics to conventional dipped SAMs [3], where the leakage current density was the order of 10^-6 A/cm². To improve transistor performances using stamping process, stamping time was systematically changed from 15 s to 10 min. As a result, the mobility exceeding 2.0 cm²/Vs was obtained when the time was more than 1 min. The uniformity of the stamped SAMs was observed using TEM, which was comparable to SAMs formed using conventional dipping process. This study was partially supported by JST/CREST and the Special Coordination Funds for Promoting and Technology. [1] I. Hirata, U. Zschieschang, F. Ante, T. Yokota, K. Kuribara, T. Yamamoto, K. Takimiya, M. Ikeda, H. Kuwabara, H. Klauk, T. Sekitani, T. Someya, MRS Communications (2011) 1-4. [2] T. Yamamoto, K. Takimiya, Journal of the American Chemical Society 129 (2007) 2224-5. [3] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 445 (2007) 745-8.

Solution processible inorganic semiconductors such as metal oxide precursors and nano-structured inorganic materials were widely investigated for high performance display backplanes, 3D-LCD display, smart-TV, and AM-OLED display. However, there have been some limitations for the utilization of organic gate dielectrics for flexible display with solution processible inorganic semiconductors such as high processing temperature and weak chemical resistance in basic precursor solution. In this study we designed and synthesized polybenzoxazole as organic gate dielectric for solution processible inorganic semiconductor TFT. The prepared polybenzoxazole thin-film showed remarkable chemical resistance and thermal stability. The insulating properties and chemical resistance results after treatment with inorganic semiconductor solution of prepared polybenzoxazole dielectric films will be discussed.

A model was developed that describes the voltage dependence of the contact resistances of organic thin film transistors in staggered and coplanar contact configuration. The contact resistance of pentacene transistors in coplanar (bottom drain/source contacts) is approximately one order of magnitude higher than resistance of transistors in staggered configuration (top drain/source contacts). The contact resistance of transistors in the bottom drain/source contact configuration is higher due to the reduced lateral conductivity of the pentacene film in the contact area. Treating the gold drain and source contacts with a thiol based self assembled monolayer (SAMs) leads to an improved growth of the pentacene molecules on the electrodes and an increased lateral charge transport. As a consequence the contact resistance drops to values comparable to transistors with top drain/source contacts. The presented analytical model describes the influence of gate and drain voltages on the contact resistance of the thin film transistors. The model is in a good agreement with obtained experimental results. Both staggered and coplanar transistor configurations can be described by the model. Furthermore, the model can be used for explaining non-linear contact behavior (S-shape) which is very often observed in transistors with bottom drain/source electrical contacts.

Recently, memory devices fabricated as organic thin film transistors (OTFTs) have been widely developed to facilitate organic electronics era on flexible substrates. To have an efficient monolayer of charging element in the memory OTFT, metallic or semiconductor nanoparticles (NPs) and surface of dielectric layers have been functionalized to accommodate peculiar binding mechanisms. In this study, using a strong streptavidin-biotin binding system, Au NPs were monolayered between pentacene and polyvinylalcohol (PVA) dielectric layer in OTFT device. The Au NPs were functionalized with streptavidin and the PVA surface was modified by 3-amino-propyltriethoxysilane(APTES) and N-hydroxysuccinimide (NHS)-biotin. Characteristics of memory charging effect were examined with threshold voltage shift (â-³Vth) of the pentacene OTFTs. The memory OTFT can be applicable to develop a new conceptual biosensor device.

Flexible pentacene organic transistors are rapidly developing and have now reached mobility of 0.5-2.0 cm²V^-1s^-1, which is comparable with that of a â?" Si:H TFTs. Metals are often used for the gate or source/drain electrodes on a flexible substrate, but it has limited flexibility due to its plastic deformation and poor adhesion to pentacene semiconductor, and thus leading to the degraded electrical performance especially under a tensile strain. In this study, we have developed high conductive polymers, PEDOT:PTS(conductivity = 650 S/cm) which were used as gate electrode and source/drain electrodes in pentacene transistors. The use of poly(3,4-ethylenedioxythiophene)(PEDOT) electrodes combined with Al₂O₃/poly(4-vinyl phenol)(PVP) multilayer gate insulators and a polyethersulfone(PES) substrate allowed for the excellent mechanical stability and transparency. In addition, the addition of thin Al₂O₃ layer between PVP and PEDOT layers effectively suppressed the interaction of PVP with PEDOT film, thus leading to the high electrical performance of flexible organic thin film transitors(threshold voltage of -3V, threshold voltage shift of 1.12V, I_on/I_off ratio of 1.5E+7, mobility of 0.41 cm²V^-1s^-1). Furthermore, the encapsulation of flexible organic transistors with Tetratetracontane(TTC) showed the long term stability of the device in terms of mobility, I_on/I_off ratio, and threshold voltage upon exposure to air.

We report here the synthesis and characterization of cyclohexylethynyl end-capped quaterthiophenes. Additionally, we describe an investigation of the performance of organic field-effect transistors (OFETs) based on these quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and the electronic performance. UVâ?"vis absorption measurements revealed that the asymmetrically substituted cyclohexylethynyl end-group, CHE4T, induced formation of H-type aggregates whereas the symmetrically substituted cyclohexylethynyl end-groups, BCHE4T, favored formation of J-type aggregation. Two-dimensional grazing-incidence wide-angle X-ray scattering studies were performed to support the molecular structure-dependent packing (H or J) of the CHE4T and BCHE4T films. Solution-processed CHE4T and BCHE4T were tested as the active layers of p-type organic field-effect transistors (OTFT) with a bottom gate/top contact geometry. The field-effect mobility of devices that incorporated H-aggregated molecules, CHE4T, was quite high, exceeding 0.02 cm2/Vs, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of devices that incorporated J-aggregated molecules, BCHE4T, was low, above 5*10â?"4 cm2/(Vs), due to the formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asymmetrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing, determined by the cyclohexylethynyl end-groups, was responsible for influencing the OTFT performance.

In this work, we present bias-dependent degradation characteristics of pentacene Organic Field Effect Transistors (OFETs) in Hermetic Condition. It has been reported that exposure to atmosphere led to performance degradation of OFETs. OFETs were affected by environmental factors such as humidity and air, etc. OFETs also degraded under electric bias stress test performed in atmosphere. However, electric-field induced degradation should be distinguished from the atmosphere-induced degradation to evaluate stability and reliability of OFETs properly. In order to investigate degradation characteristics varying electric field distributions preventing ambient-related degradation, we performed bias-stress tests with different biasing schemes in vacuum chamber. All measurements before and after stress tests were performed in vacuum condition and compared. First, we performed gate step stress test in the VDS = 0 state. In this test, VGS was stepped from 0V to -100V in -1V steps, and the devices were stressed for 1 minute at each bias. We monitored gate leakage current (IG) during the stress test. We found that there is a critical gate voltage which triggered sudden increase of IG. During the step stress test, IG started increasing significantly at near VG = -50V. IG of stressed OFETs was increased. The stress test with positive VGS did not increase IG. Second, OFETs were stressed under high channel current condition. VGS and VDS were simultaneously stepped from 0V to -100V in -1V steps, and the devices were stressed for 1 minute at each bias. Vertical Electric field between gate and source increased with gate bias stepping, with electric field between gate and drain maintained at zero. During the stress test, IG started increasing significantly at near VGS = VDS = -50V. IG of stressed OFETs was increased. The measurement with source and drain swapped revealed that the degradation mainly occurred at the source and gate overlap region. Third, OFETs were stressed under low channel current condition with stepping VDS from 0V to -100V in -1V steps at VGS = -10V. The devices were stressed for 1 minute at each bias. Sudden increase of IG was not observed during the stress test. Stressed OFETs did not show any symptom of significant degradation, accordingly. Vertical electric field across the gate is a dominant degradation agent compared to lateral field along the channel. Detailed analysis on the electric field-induced degradation characteristics of OFETs, with ambient effect eliminated, will be presented. Temperature dependence of field-induced degradation will also be discussed.

Organic thin-film transistors (OTFTs) with hybrid dielectrics composed of thin oxide layers modified with a self-assembled monolayer (SAM) bear high potential for application in low-power integrated circuits (ICs). For a reliable operation of these integrated circuits it is crucial to have full control over the threshold voltage (V_TH) of individual transistors which form the ICs.[1] Several groups have addressed this topic by showing that the turnâ?"on characteristics of transistors are strongly related to the dipole moment of the SAM-forming molecules, with the SAMs providing an additional electric field that either generates or withdraws mobile charge carriers in the semiconductor (SC) channel, depending on the direction of the dipole.[2-4] However, a quantitative correlation between V_TH and dipole moment has not yet been established. We have investigated a set of functionalized n-alkane phosphonic acid molecules with different dipole moments (calculated by DFT method) that were used to form self-assembled monolayers on an aluminum oxide dielectric. We show that the shift of V_TH correlates linearly with the dipole moment along the molecular axis for these transistors. This quantitative examination was validated for the semiconductors α,Ï?-dihexylsexithiophene, pentacene and C₆₀. Our results enable a general approach to tune V_TH for several organic semiconductors with dipolar monolayers and even predict V_TH-values by knowledge of the z-component of the molecular dipole of the SAM molecules. A smart approach to fine-tune the threshold voltage of OTFTs is the use of Mixed-SAMs.[1] When redox active species, e.g. C₆₀, are then introduced as one of the components, this additionally offers the possibility to realize non-volatile memory.[5] Such transistors show significant hysteresis during operation due to charging of the redox active species. The correlation between hysteresis and amount of redox active species in the SAM was investigated for Mixed-SAMs containing C₆₀ modified phosphonic acids.[6] [1] U. Zschieschang , F. Ante , M. Schlörholz , M. Schmidt , K. Kern, and H. Klauk, Advanced Materials, 2010, 22, 4489â?"4493. [2] S. Kobayashi , T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa and Y. Iwasa, Nature Materials, 2004, 3, 317-322. [3] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, and B. Batlogg, A. N. Rashid, G. Schitter, Journal of Applied Physics, 2004, 96(11), 6431-6438. [4] F.D. Fleischli, S. Suárez, M. Schaer and L. Zuppiroli, Langmuir, 2010, 26(18), 15044-15049. [5] M. Burkhardt, A. Jedaa, M. Novak, A. Ebel, K. Voitchovsky, F. Stellacci, A. Hirsch, and M. Halik, Advanced Materials, 2010, 22, 2525â?"2528. [6] A Jedaa, M. Salinas, C. M. Jäger, T. Clark, Alexander Ebel, Andreas Hirsch and Marcus Halik, Submitted to Applied Physics Letters.

We developed high-performance stable n-type organic field-effect transistors (OFETs) using indenofluorenediones with different numbers of fluorine substituents (MonoFIF-dione, DiF-IF-dione, and TriF-IF-dione). Top-contact OFETs were fabricated via the vacuum deposition of indenofluorenediones as the semiconducting channel material on polystyrenetreated SiO2/Si substrates. TriF-IF-dione FETs with Au source/drain contacts exhibited good device performances, with a field effect mobility of 0.16 cm2/(V s), an on/off current ratio of 106, and a threshold voltage of 9.2 V. We found that the electrical stability for OFETs based on indenofluorenedione improved with the number of fluorine substituents, which was attributed to higher activation energies for charge trap creation. Moreover, the TriF-IF-dione FETs yielded excellent environmental stability properties, because the LUMO energy levels were relatively low, compared with those of the MonoF-IF-dione FETs.

The field of organic field-effect transistors (FETs) has seen an increasing usage in polymer dielectric materials and a progressively lower dependence on traditional oxide dielectrics. Appropriate gate dielectrics that are free of electron-trapping groups have allowed the demonstration of both p-channel and n-channel FETs. In order to achieve stable and low-operating voltage FETs, the dielectric surface should be hydrophobic and must provide a relatively high capacitance, which is often a challenge with many polymer dielectrics that have an inherently low dielectric constant (k). Our recent work demonstrates that FET characteristics significantly improve when the difference in the solubility parameters of the organic dielectric and solvent is high. [1] In this work we demonstrate low-operating voltage and stable pentacene field-effect transistors using low-k gate dielectric layers of poly (methyl methacrylate) (PMMA) and poly (4-vinyl phenol) (PVP). These devices exhibit field-effect (FET) hole motilities in the range of 0.1 â?" 1 cm2V-1s-1, threshold voltages of less than -1 V, and little off-regime current hysteresis. The smallest sub-threshold slope of 192 mV/dec and highest FET mobility of 1 cm2V-1s-1 were observed at a gate voltage of -2 V. Simultaneous capacitance-voltage and conductance-voltage measurements from accompanying metal-insulator-semiconductor diodes exhibit very low hysteresis and interface trap densities of the order of 1011 â?" to low 1012 eV-1cm-2. Solvents of varying dipole moments used to dissolve the polymer dielectrics were observed to play a significant role in improving the FET characteristics. Furthermore, the devices exhibited excellent operational and environmental stability with no shift and a small shift in threshold voltage after bias stress and days of air exposure, respectively. [1] N. B. Ukah, D. Adil, J. Granstrom, R. K. Gupta, K. Ghosh, and S. Guha, Org. Electron 12, 1580 (2011).

Improved performance of p-type organic light emitting transistors (OLETs) is demonstrated by introducing a conjugated polyelectrolyte (CPE) layer and symmetric high work function (WF) source and drain metal electrodes. The OLET is compose of a tri-layer film consisting of a hole transporting layer, an emissive layer and a CPE layer as an electron injection layer. The thickness of the CPE layer is critical for achieving good performance and provides an important structural handle for consideration in future optimization studies. We have also demonstrated for the first time, high performance solution-processed blue OLETs. Overall, these results show that there is no need to evaporate two different metals as source and drain electrodes in order to inject holes and electrons into OLETs. With the benefits of the simplicity in device fabrication, the use of a CPE layer is beneficial for use in a variety of organic optoelectronics devices.

In this project, thermal gradient method was employed to produce well-orientated, high-quality 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) crystals. TIPS pentacene, a solution processable organic semiconductor, has been widely studied as the active channel material for organic thin-film transistors (OTFTs) due to its excellent carrier transport and stability in air. In spite of all these advantages, TIPS-pentacene thin films are acutely anisotropic when they are grown from simple solution drop casting. This in turn gives rise to irregularities in OTFT device performance. The application of a thermal gradient to guide the TIPS-pentacene crystal growth alleviates the intrinsic anisotropy, allowing for a more controlled film morphology, uniformity and crystal orientation. A solution with specific concentrations of TIPS-pentacene in toluene and dimethyl formamide (DMF) was drop cast onto a substrate subjected to a thermal gradient. This leads to a difference in the solubility of the solute along the substrate and drives crystallization from the lower temperature end to the hotter end. Evaporation was controlled through the addition of a high boiling point solvent (DMF), which greatly improved the areal coverage on the substrate as well as the quality of the crystals. The resulting TIPS pentacene crystals were large with uniform morphology. To analyze the TIPS pentacene films, optical microscopy and X-ray diffraction were utilized. The successful improvement of the crystal orientation and concurrent increase in crystal coverage on the substrate, as seen from the results of the analysis, proved the efficiency of the thermal gradient approach.

Organic pentacene thin film transistors have been of great interest because of their potential use as a low-cost alternative to the conventional amorphous silicon based transistors. Pentacene transistors show good carrier mobility. The OTFTs performance mainly depends on the structural properties of pentacene and the contact resistance. In addition, as the device dimensions decreases, the contact resistance as a part of the total device resistance dominates over the channel resistance, and therefore the speed of organic integrated circuits may be limited by the contact resistance, not by the intrinsic carrier mobility of the pentacene. As a result, the reduction of contact resistance is the central part to enhance the electrical performance. Iodine is an efficient acceptor for doping of pentacene, increasing the conductivity as well as hole mobility in this material. Iodine doping of pentacene can also modify the interface properties of source/drain and pentacene, thus resulting low contact resistance. We have studied on the iodine doping of pentacene film, which was confirmed by Raman spectroscopy, revealing the characteristic peaks of I₃^- and I₅^-. We also investigated the Iodine doping effect on the surface morphology, grain size, and conductivity of the pentacene films with doping time from 30 seconds to several hours using atomic force microscopy and four point probe respectively. Pentacene films grain sizes drastically increased with the doping time from an average value of 200 nm to 640 nm as a result of the facile movement of PEN⁺ cation radicals, generated by iodine anions. In addition, the XRD analysis showed both the bulk and thin film crystallinity increased with the low doping time while bulk peak disappears with high doping time. We also fabricated iodine doped source/drain contact pentacene TFTs, in which doping of iodine was limited to source/drain contacts using novel structures. This doping of contact area decreased the contact resistance to half of the devices without doping as well as increased the field effect mobility to 1.03 cm²/Vs, which is two times that of the devices without doping. We have fabricated all devices using octadecyltrichlorosilane (OTS) SAMs on the insulating oxide layer (100 nm SiO₂). The effectively contact area iodine doped pentacene based OTFT showed mobility of 1.03 cm²/Vs with the on/off ratio of 3.86Ã-10⁵ and threshold voltage of -2.13 V whereas the without doped pentacene based OTFT showed the mobility of 0.51 cm²/Vs with the on/off ratio of 3.55Ã-10⁵ and threshold voltage of -4.10 V.

Balancing electron and hole transport lies at the heart of organic optoelectronics. It is very important in organic light emitting diodes (OLEDs), solar cells, and light emitting field effect transistors (LEFETs). LEFETs are of interest because they offer the combined advantage of an active channel from a thin film transistor (TFT) that controls the light emission from an OLED created in situ in the same device. In LEFETs, however, achieving balanced electron and hole transport and hence efficient electron-hole recombination is particularly challenging. Here we report an innovative strategy to actively and independently control the injection, transport and recombination of holes and electrons in the transistor channel by using the split-gate LEFET (SG-LEFET) architecture. A SG-LEFET is a 4-terminal device with Gate 1, Gate 2, Source, and Drain electrodes. Based on the operating voltages, (i.e. the bias polarities on Gate 1 and Gate 2), a split-gate field effect transistor (SG-FET) can be operated as a unipolar transistor, a bipolar transistor or a diode in different I-V quadrants. Because of the independent control of injection at Gate 1 and Gate 2, transport of specific carrier species can be enhanced or switched-off. Using the SG-LEFET, we have demonstrated that the largest injection current at the center of the transistor channel can be efficiently converted into light emission with maximum brightness of 609 cd/m2 at 1.27% EQE.

Enhanced hole mobility in pentacene-based field-effect transistor is investigated experimentally as a function of composition of polystyrene mixture. First, we present measurements of the electrical performance with monotonously increased molecular weight of polystyrene as gate dielectric including the following: the discovery of high performance with higher molecular weight as gate dielectric, the morphological image of polystyrene surface, XRD diffraction of pentacene grain on the polystyrene, adhesiveness on glass substrate and aluminum electrode, and simple electrical characteristics such as capacitance, current density. Next, variable composition of polystyrene film fabricated that low (Mw 35,000) and high (Mw 980,000) molecular weights in the polystyrene film was mixed in the proportion range of 10:1 - 1:10, and largest mobility of 0.73 cm2/V-s is observed with composition of 1:10, which is higher when used single component of molecular weight. Furthermore, a mobility behavior is proposed based on the proportion of molecular weight materials to estimate the highest performance of fabricated devices. We attributed higher performance of device with less trap site in the vicinity of the polystyrene surface to the strongly conjugated polystyrene chains and enclosed packing by relatively short length of the polymer which of small molecular weight of polystyrene. Further details will be discussed.

In this paper, we figured out that the performance of OTFTs using TIPS-pentacene strongly depended on the grain boundary direction relative to the current flow between source and drain electrode. The OTFTs with the grain boundary direction parallel to the current flow exhibited the mobility (0.44±0.08 cm^2/ V.s) 2 times larger than those with the grain boundary direction angled to the current flow (0.21±0.03 cm^2/ V.s). It is meaning that the grain boundary seriously affected the carrier transport. We also found that when the center area of crystal, where the different grains met together, located in the channel, the mobility was seriously degraded to 0.09 cm^2/ V.s. Therefore, it was important to make the grain boundary direction parallel to the current flow and make the center area of crystal be out of channel for high performance. * This research was supported by a grant(F0004021-2011-34) from Information Display R&D Center, one of the Knowledge Economy Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean government.

In general, most of the organic thin-film transistors (OTFTs) operate in p-type accumulation mode, meaning that the semiconducting layer play a role as hole-transporting material. However, for development and application in many areas, such as CMOS-like logic element, not only high-performing but stable electron-transporting (n-type) materials are required as well. Recently n-type small molecule material, N,Nâ?T-ditridecyl-3,4,9,10- perylenetetracarboxylic diimide (PTCDI-C13), has attracted attentions for last few years, due to its air-stability better than C₆₀-based device , high-performance comparable to p-type organic materials. For real-world application, it is someoneâ?Ts mission to research on device reliability and in-depth understanding of instability under ambient atmosphere. We fabricated three-types of PTCDI-C13 based OTFTs employing pristine, hexamethyldisilazane (HMDS)-treated, and fluoropolymer CYTOP^TM -treated Al₂O_3+x dielectric operating at 7 V. We report on the behavior of hysteresis evolution (in transfer curve) in above-mentioned three types of TFTs. In case of pristine and HMDS-treated TFT, the hysteresis reduces exponentially (from 3.5 V to 0.02 V), meanwhile, CYTOP-interfacial TFT exhibits almost remained identical hysteresis (~0.07 V) consecutive measurement. We also discuss the evolution of threshold voltage (V_th) of the samples undergone positive or negative gate bias stress along with time-dependent analysis of sub-threshold swing. Under positive gate bias stress, in case of pristine and HMDS-treated TFT reveals that V_th shift occurs rapidly in early phase (500 s), meanwhile, for CYTOP-interfacial TFT, the evolution of V_th is described by the stretched exponential equation. These results lead us to conclude that semiconductor/dielectric interface difference leads to different behavior in stability (hysteresis, V_th shift). In order to understand in depth the trap behavior and its distribution in our TFTs, we performed bias stress test along with photo-excited charge collection spectroscopy (PECCS) measurement to elucidate interfacial trap charge density of state (DOS).

Due to the distinct advantages and functionalities, organic devices on glass or flexible substrates have extensively been studied in such basic forms as thin-film transistors (TFTs) and light emitting diodes (LEDs). Organic pentacene TFTs are the most representative among many other organic devices. Since the pentacene-based and other organic TFTs have bottom gate device architecture opening their back channel surface, ambient protecting or passivation layers on the organic devices are necessary. Such passivation layers have thus been studied to keep those devices from damages or degradation under various ambiences involved with air, humidity, and ultraviolet light (UV). The adopted protection layers were inorganic oxide or polymers in general, however small molecule crystalline layer has rarely been chosen. Here, we chose a small molecule crystalline layer comprised of 6,13 pentacenequinone (PQ: oxidized pentacene) with the following two reasons. First of all, crystalline PQ/crystalline pentacene interface is quite analogous to that of SiO₂/Si structure where SiO₂ plays as natural protection layer over Si devices making a natural interface with Si. Because the hydrophobic PQ layer is an already-oxidized pentacene layer, diffusion of water and oxygen molecules into channel pentacene may not be easy when the PQ layer is well grown on pentacene making a good PQ/pentacene interface. Next, the PQ protects the pentacene from even high intensity UV light, since it converts or transduces the UV into green light as known. In the present work, we have fabricated a pentacene-based complementary photo-gating inverter devices to demonstrate the effects of the PQ layer, which was deposited on top of our pentacene channel. Our complementary inverter has a form of vertically-stacked TFT devices: bottom gated p-channel pentacene TFT on top-gated n-channel GaSnZn oxide (GTZO)-TFT with atomic layer deposited (ALD) Al₂O₃ and Al as common dielectric and gate. Our inverters with the crystalline-grown PQ on pentacene channel demonstrated strong ambient-endurance preserving good inverter characteristics for 60 days under quite a humid air ambience, and they also demonstrated dynamic photo-gating at a low voltage of 5 V without any photo-induced degradation.

Polymer-based nanocomposites filled with inorganic fillers have been intensively studied due to their excellent mechanical, electrical, thermal and dielectric properties. Especially, the dielectric properties of polymer/inorganic nanocomposites have been examined for not only embedded-capacitor technology but also electroactive polymers (EAPs) that respond to the external electric field by changing their shape. Recently, a considerable amount of studies have focused on reducing the operating voltages of dielectric elastomer actuators (DEAs) in order to enhance their commercial feasibility and remove the dangers associated with high voltage. The basic method for reducing the operating voltage is to increase the dielectric constant of the elastomer films. A number of approaches have been explored for increasing the dielectric constant of elastomers introducing high-k ceramic fillers or conductive fillers such as metal nanoparticles and nanocarbons. In this study, we fabricated a dielectric elastomer films using single-walled carbon nanotubes (SWNTs) as a dielectric filler and polydimethylsiloxane (PDMS) as a polymer matrix. To achieve a high dielectric constant and low loss factor of SWNTs/PDMS nanocomposites, the SWNTs were highly exfoliated by using an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. We demonstrate that SWNTs/PDMS nanocomposites with ionic liquids show the higher dielectric constant and lower loss factor than nanocomposites without ionic liquids. We also characterize the dispersity of SWNTs in the nanocomposites by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Single molecules provide ideal systems for the investigation of charge transport on the molecular scale, which is of great importance for both practical applications and fundamental understanding of molecular electronics. For example, in electronic devices based on organic semiconducting materials, intramolecular electron transport over distance of up to 100 nm between intermolecular transfers could significantly increase bulk charge mobilities. In order to study intramolecular charge transport on the length scale of tens to hundreds of nanometers, we designed a high molecular weight conductive polymer as a model. This polymer should (1) have a Contour length higher than 100 nm so that a single polymer chain is long enough to be placed between two electrodes for the investigation; (2) be rigid enough so that a single molecule does not fold to itself on the surface and (3) have a relatively large feature height when deposited onto surface so that it can be easily detected by atomic force microscopy. In order to achieve these goals, ruthenium catalyzed ring opening metathesis polymerization of norbornene units was chosen for the synthesis of such high polymer, because of the feasible and robust nature of this reaction. Polystyrene was employed as the solublizing group in the monomer on account of its compatibility to C60 unit and ease of synthesis. C60 unit was attached to the backbone as methanofullerene by mean of Bingel reaction. The ring opening metathesis polymerization reactions were carried out at a monomer/catalyst ration of >800/1 to give C60-containing polymers with molecular weights larger than one million. With these polymers in hand, we investigated the polymer chain distribution on surfaces using atomic force microscopy. Thin-film and single polymer chain field effect transistor devices, based on these polymers, have also been fabricated and investigated.

Electrically conductive materials capable of substantial elastic stretching and bending are required in foldable displays, robotic skins, stretchable solar cells, implantable devices, and dielectric elastomer actuators. A wide variety of conductive polymer composites have been studied based on carbon nanotubes and grapheme, however, high stretchability and high conductivity cannot be simultaneously obtained. Here we present highly conductive, stretchable, and transparent hybrid composites comprised of silver nanowire (AgNW) network embedded in polymer matrix. The interpenetrating networks of AgNWs and the polymer matrix in the surface layer of the composites lead to high surface conductivity, high transparency and high mechanical compliance. A composite with 7.5 Ω/sq sheet resistance and 80% transmittance only experiences a 8 times increase in sheet resistance when it is stretched to 80% strain. Composites with higher conductivity exhibit better stretchability. Results of morphological studies will be presented to expound the conductivity changes during reversible stretching and relaxation.

Poly(methyl methacrylate) (PMMA) is ubiquitously used in optics and solar technologies, for example, as condenser lenses due to its high optical and UV transmission and is thought to have superior resistance against degradation. PMMA is also an attractive candidate as a polymer layer for ultra barrier coating systems for solar cells and display technologies. In these applications, PMMA layer or lens components may be exposed to high-fluence solar UV radiation. Little is known, however, about the coupled effects of mechanical stress, moisture, thermal cycling and particularly UV light on the degradation of PMMA in such environments and the role of environmental parameters on accelerating the growth of defects. Understanding those effects is critical for solar technologies that use PMMA components. In the present study, we measured the cohesive fracture energy of PMMA as well as investigated the adhesion of a PMMA layer with adjacent materials including oxides and organics. Bulk PMMA interfaces were fabricated by thermal bonding PMMA substrates at the PMMA glass transition temperature. The cohesive fracture energy was characterized and related to processing condition and the testing environment. In addition, samples were prepared by spin-coating a PMMA precursor solution onto a quartz substrate. Two substrates were subsequently bonded together to form a PMMA to quartz interface that is characteristic of solar application. For both samples, experiments to quantify the UV radiation effect on PMMA were conducted in an environmental chamber. This included both pre and in-situ UV exposure where the effect of UV light on significantly degrading cohesion and adhesion properties was quantified. Implication for the reliable use of PMMA in solar applications is discussed.

In this article, we investigate the magnetoconductance (MC) responses under specific conditions which including photocurrent, unipolar injection, and bipolar injection in pentacene-based diodes. Both photocurrent and bipolar injection contributed MC responses are resulted from triplet-polaron interaction (TPI). At unipolar injection regime the MC response is resulted from the charge transport through the bipolaron states. These results suggest that both single carrier and electron-hole pair contributed MC responses could be observed in pentacene-based devices. In addition, by blending 2,3,5,6-tetrafluoro-7,7,8,8 -tetracyanoquinodimethane (F4-TCNQ) into pentacene, the MC response at unipolar injection regime would be enhanced owing to the induced charge transfer complex states (CT complex states).

Research of the new composite nanomaterials and structures for electronics and photoelectronics based on organic acceptors and inorganic donors is relevant and important in case of the technical applications, as well as in terms of new knowledge in the field of organic semiconductors. Moreover, inorganic semiconductors have been investigated in search of a significant improvement of the lifetime for the organic devices [1, 2]. The properties of bulk heterostructures obtained by the different methods were investigated: 1. By the vacuum evaporation in quasi-equilibrium conditions 2. By the vacuum evaporation in non equilibrium conditions 3. By spin- coating method This types of samples have different structures of the composite film. In the first case polycrystalline films with about 300nm crystals were obtained. The samples obtained by the second method also shows a polycrystalline structure but with about 30nm crystals, and the third method of deposition leads to amorphous films with an inclusion of inorganic particles. The structures with composition uniformity through the thickness and with composition gradient through the thickness with a C60 - C60(1-x)CdS(x) â?" CdS structure were investigated. Morphology, structure and IVCs measured in the 'sandwich' geometry were analyzed. Structures with the composition gradient have IVCs with a rectification ratio up to 3 orders of magnitude. Raman spectroscopy of the C60- CdS structures was investigated, and the fullerene polymerization was shown, which is due to a charge transfer from the donor (CdS) to the acceptor component (C60) of the bulk heterojunction. References: 1. A. Briseno, T. Holcombe, A. Boukai et al., Oligo- and Polythiophene/ZnO Hybrid Nanowire Solar Cells, J. Nano Letters, Vol 10 (2010) p. 334-340 2. A. Takeda, T. Oku, A. Suzuki, K. Kikuchi, S.Kikuchi, Fabrication and characterization of inorganic-organic hybrid solar cells based on CuInS2, Journal of the Ceramic Society of Japan, Vol. 117 (2009) p.967-969

As information technology (IT) has been made scientific progress, organic memory has attracted great attention due to potential application for next generation information storage. There are many suggested switching mechanisms in organic memory, e.g. conformational change, charge transfer, space charge limited current (SCLC), and filamentary conduction. However, the switching mechanisms are still controversial issue. For this reason, exact understanding of switching mechanism is important for further improving memory performances. Especially, the investigation of relationships between chemical structure of the polymer and switching performances is required. Thus, the systemic studies by changing side chains of the polymer will contribute to verifying the performance-structure relationships in polyfluorene based organic memory devices. In our previous work, non-volatile memory devices fabricated using single component water-soluble polyfluorene-derivative showed good performance characteristics such as large on/off ratio, a long retention time, and thermally stable switching behavior. As the reasons for these good performances, we speculated that the SCLC with filamentary conduction contributed to the switching behavior of the memory device; in addition, side groups of polyfluorene derivatives also played crucial roles in device performances. Based on this motivation, we synthesized four kinds of polyfluorene derivatives incorporating different side chains and investigated organic resistor-type memory devices.

The optoelectronic properties of organic devices depend largely on the density of states (DOS) in the band gap of the active semiconductor layers. The DOS distribution determines the electrical transport, photosensitivity, doping efficiency and, at the end, impairs the device performance. The study of defects with states within the band gap is of great interest, since they act as traps and recombination centres with a strong influence in the electrical behaviour of the fabricated devices. Experimental information on the distribution of defects cannot be obtained in a straightforward way. Different techniques have been used for extracting information about the energy distribution of the DOS in thin-film semiconductors. Some of them are based on thermal emission measurements involving, in most cases, the effects of trapped charge in electronic states. On the other hand, the DOS can be also studied by measuring the weak subgap optical absorption related to electronic transitions involving defect states . In this paper we study the density of states in n-type N,Nâ?T-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. PTCDI-C13 thin-film transistors were fabricated on n-type crystalline silicon wafers with a 150 nm thick thermally grown silicon dioxide. A field-effect mobility of 0.12 cm²/Vs and a threshold voltage of 46 V were calculated. The transfer current-voltage characteristics was measured over a range of temperature (30 â?" 90°C) and the activation energy (E_A) of the channel conductance was calculated as a function of the gate-source bias (Vgs). The variation of the activation energy of the conductance channel could be described in terms of a standard semiconductor model. The dependence of E_A on Vgs corresponds to a gradual shift of the Fermi energy level (E_F) toward the conduction band as more empty traps become filled due to the injected electrons from contacts. This dependence allows us to determine the DOS of the organic semiconductor. The density of localized states was around 10¹⁸ cm^-3 eV^-1 in the midgap and increases one order of magnitude towards the conduction band. DOS distribution could be described by the sum of two exponential contributions. We attribute the exponential regions in the DOS distribution to the presence of deep and band tail states. In addition, PDS measurements were performed to get information about the spectral distribution of the subgap optical absorption. PDS is a very sensitive technique that can measure very weak optical absorptions involving sub gap electronic transitions. In the case of PTCDI-C13, PDS measurements also revealed tail and deep distributions of localized states.

A series of novel organic-inorganic hybrid materials by using diol and alkoxysilane were through a non-hydrolytic sol-gel reaction. The diol compounds were reacted with inorganic precursors in the presence of a catalyst of barium hydroxide monohydrate. The amount of diol compounds was changed to 30 mol% and 60 mol% and that of catalyst was fixed at 0.1 mol% of the total silane compound. The mixed solution was transferred into a vacuum to remove the alcohol, a byproduct of the condensation reaction. The thin films of the organic-inorganic hybrids were obtained from spin-coating on a Si wafer. The spin-coated films were exposed to a UV lamp for crosslinking and annealed for additional hours at high temperature. The films exhibited excellent adhesion with the substrate. The surfaces of the films were flat and smooth. The films contained no crack and displayed uniform thickness. The film thickness increased with the relative amount of diol compounds. UV embossing was used in the fabrication of various optical waveguides and devices of diverse patterns with a PDMS stamp.

In this presentation, we will demonstrate a simple patterning method for the deposition of the conducting polymer such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) by using the difference in wettability between hydrophobic surfaces and hydrophilic surfaces on the substrate. And we will also demonstrate the influence of inner flow (such as convection flow and Marangoni flow) in the PEDOT/PSS droplet to achieve homogenous conducting polymer films. Finally, we will confirm the potential of our patterning method for polymer electrodes such as the PEDOT/PSS by fabricating pentacene thin-film transistors (TFTs) and measuring the electrical properties of the pentacene TFTs. **This study was supported by a grant (B551179-08-03-00/ B551179-10-01-00/ NK162D/ NK162H) from the cooperative R&D Program funded by the Korea Research Council Industrial Science and Technology, Republic of Korea.

Thin films of carbon nanotubes are generating enormous interest as a potential new material for use in modern electronics [1]. Applications include thin film conductors and as the active layer in field effect transistors. Nanotube films can be deposited by direct growth methods and other high temperature techniques. However, solution-based processing methods are favoured as these lend themselves to coating a wide range of flexible substrates. This research builds on previous work using the Langmuir-Blodgett (LB) deposition technique as a method of forming thin networks of carbon nanotubes, in particular single-walled carbon nanotubes (SWCNTs). This technique allows precise control over the thickness of deposited networks. Furthermore, it is undertaken at ambient temperatures so is ideal for deposition onto flexible substrates. Generally, SWCNTs are a mixture of both semiconducting and metallic types. Both have been found to exhibit condensed isotherms at the air/water interface, and both can be transferred to solid supports using the LB technique and show anisotropy in the electrical conductivity, suggesting alignment during transfer [2]. This work now focuses on sorted semiconducting and metallic single-walled carbon nanotubes for improved and more predictable electrical properties. Electrical measurements have been taken for both types of nanotube network on a selection of electrode dimensions ranging from sub micron (~ 130 nm) to 1 mm separations and with several different metals including palladium, gold, silver and aluminium. The effect of temperature on the conductivity has been investigated between 77 K â?" 300 K and various electrical conductivity models have been used to fit to the resulting current-temperature data. Field effect transistor architectures have been built up on sub-micron and larger (50 micron) channel lengths using semiconducting nanotubes. Field effects have been observed in some of these devices and will be reported on in more detail. [1] L. Hu, D. S. Hecht, G. Grüner, Chem. Rev., 110(10), 2010, 5790-5844. [2] M. K. Massey, C. Pearson, D. A. Zeze, B. G. Mendis, M. C. Petty, Carbon, 49, 2011, 2424-2430.

Solution-processable molecular switches are an intriguing option for implementation in nonvolatile memory elements due to the possibility for inexpensive fabrication over large areas on flexible substrates. Of the available molecular candidates, dithienylethenes (DTEs) are rivaled by few others in regard to their thermal stability (i.e. nonvolatile) and reversibility (re-writable). DTEs represent a class of photochromic molecular switches that undergo a change in both their UV-visible absorption and energy level position due to a photo- and/or electrically-induced ring-opening/-closing reaction. Here, we present a light-emitting organic memory (LE-OMEM) device platform employing a DTE transduction layer that offers the ability for both optical and electrical writing and readout. The memory effect in these diodes results from the significant difference in the highest occupied molecular orbital (HOMO) between the open and closed isomer of the DTE layer, which subsequently changes the hole injection barrier in the diode. Optimized LE-OMEM devices show impressive ON/OFF ratios (OORs) for both electroluminescence (EL) and current readout of ca. 10000 for optical switching and ca. 1000 for electrical switching. Both optical and electrical writing protocols demonstrate the ability to write an extraordinary number of grey levels (greater than 20 levels), which is promising for higher storage density memory. We elucidate the difference in the molecular-scale mechanisms that are responsible for the optically- and electrically-driven memory effect in these devices by in-situ monitoring of the fraction of closed molecules as a function of the external stimulus. The overall design and measurement principles discussed here provide a road map for the rational optimization of similar devices incorporating a range of photochromic compounds that may be used in nonvolatile memory devices.

In recent years, conjugated polymers are emerging candidates for the construction of the active layers of organic thin film transistors (OTFTs) and organic photovoltaics (OPVs) because of comparable or even higher mobility with benchmark performance of amorphous silicon and their various flexible, large-area electronic applications through cost-effective graphic arts printing techniques. This class of conjugated polymer is considered on the basic of following design rules to obtain high performance. i) structural good planar Ï?-conjugated unit such as fused aromatic units for efficient electron and hole transport Ï?-electron delocalization and solid-state packing though favor intermolecular Ï?-Ï? stacking, ii) structural regularity, iii) alkyl side chains for solubility and processibility without steric hindrance of backbone, iv) decrease of lowest unoccupied molecular orbital (LUMO) energies by adopting an electron deficient unit or an electron-withdrawing substituent for achieving electron transport and air-stability. In general, newly synthesized conjugated polymers by aforementioned design rules are made with fused aromatic units. However, these fused aromatic units have some limitations such as poor solubility by rigid structure and complex synthesis. Therefore, the appearance of core unit which is different from fused aromatic unit is required to solve these limitations. In this presentation, we introduce thienylenevinylene unit for replacement of fused aromatic groups. Thienylenevinylene (TV) based materials can show a high charge-carrier mobility as a result of a high planarity by introduction of vinylene with double bond character between thiophene units and a large intermolecular attractive force by the thiophene. Therefore, the introduction of a vinylene moiety between the thiophene units resulting in a TV unit is an attractive building block for the active layer of OTFTs and OPVs. Previously mentioned design rules were selectively applied to TV-based conjugated polymers, and we synthesized two types of conjugated polymers which are predominant hole transport polymers (maximum hole mobility is exceeding 1 cm²/Vs) and ambipolar transport polymers (maximum hole and electron mobility are 0.75 and 0.06 cm²/Vs, respectively). These polymers were synthesized via Grignard, Kumada, and Stille coupling reactions. The basic material characterizations using NMR, DSC, TGA and GPC were investigated. In addition, optical and structural properties were analyzed using optical spectroscopy tools such as UV/Vis spectroscopy and X-ray diffraction. In addition, we investigated a significant morphological change by annealing process.

One of the biggest advantages of organic electronics is their solution-processability, which allows for easy and inexpensive manufacturing of devices (e.g. organic memory, organic light-emitting diodes, organic photovoltaics, organic field-effect transistors, etc.) over large areas and even on temperature-sensitive/flexible substrates. Optimized device layouts commonly employ multi-layer stacks, which requires either the use of orthogonal solvents for alternating layers or crosslinkable precursors in order to prevent dissolution of the previous layer. Here, we describe multi-layer solution-processed light-emitting organic memory (LE-OMEM) devices that employ crosslikable dithienylethene (DTE) molecules, which are intriguing for switch and memory applications due to their extraordinarily high thermal and fatigue stability. This class of photochromic molecules can be triggered optically (and/or electrically) between an open and closed form by irradiation with different wavelengths. The change in the highest occupied molecular orbital (HOMO) of the DTE layer upon isomerization corresponds to a change in charge injection barrier for holes, which is responsible for the modulation of the electroluminescence (EL) and current of our devices. LE-OMEM devices utilizing these DTE molecules have shown impressive ON/OFF ratios of 10³-10⁴ for both EL and current. We focus on the implementation of a variety of molecular switching molecules as transduction layers in LE-OMEM devices. An assortment of unique molecules is made possible by a simplified materials scheme where the absorption/transmission window (from blue to red color) and frontier orbital energies of the transduction layer can be systematically and predictably modified by variation of the chromophores attached to a central/crosslinkable DTE core. This enormous simplification (i.e. modification of a central crosslinkable core) makes this materials approach a convenient way to screen a variety of materials in solution-processable LE-OMEM devices.

Semiconducting polymers have garnered significant interest for use in large-area, low-cost, and flexible electronic applications in which the thickness of the active semiconducting layer is on the order of tens to hundreds of nanometers. As polymer films approach these small dimensions, polymer-interface interactions become more significant and the film properties may begin to deviate significantly from those of the bulk. Therefore, as semiconducting polymers continue to increase in commercial significance, it becomes imperative to have a good understanding of how the material properties change in the confined geometry of ultrathin films. We study the effects of interfaces on microstructure and charge transport in thin films of the high performance, semicrystalline polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), by systematically varying film thickness. We find that the interfaces disrupt the formation of crystallites but improve film texture and that the interplay of these two effects regulates the charge carrier mobility. Using x-ray diffraction (XRD) we quantify overall crystallinity and crystalline texture within the film and show that these vary significantly with film thickness and thermal processing. Crystallinity increases by an order of magnitude from the thinnest to thickest films and by a factor of 2-3 upon thermal annealing. Field effect transistor (FET) measurements are used to characterize the electrical properties of these films and charge carrier mobility was found to vary with film thickness, maximizing at values of 0.13 cm² V^-1 s^-1 and 0.22 cm² V^-1 s^-1 for as cast and annealed films respectively. Changes in device performance can be explained by the thickness induced variations in film morphology. We demonstrate how the observed trends in charge carrier mobility are controlled by overall crystallinity in the thinnest films and dominated by the effects of crystalline texture as the film is made thicker.

Recently, many high performance organic electronic devices, especially transistors, have been fabricated from blends of polymers with organic small molecules, such as acenes. The incorporation of acene within polymers has generated interest as a means to combine the functional properties of the small molecules with the processibility and stability of polymers. Whereas charge transport is the key requirement for transistors, this current research considers 9,10-bis(triethylsilylethynyl) anthracene (TES-anthracene)/polymer blends as candidate materials for lasers. Specifically, the research correlates the photoluminescence (PL) and quantum efficiency (QE) of the blends with the TES-anthracene crystal structure as it is influenced by important factors: the amount of TES-anthracene, the viscosity of the polymer, and film processing method. TES-anthracene and poly(styrene) were blended in a mutual organic solvent, 1,2,3,4-tetrahydronaphthalene (tetralin). Films were deposited by drop-casting (solvent-processed) or heated and cooled at controlled rates and times (melt-processed). The maximum temperature was above the polymerâ?Ts glass transition temperature and the aceneâ?Ts melting point. The blend films show a sharp absorption edge and exhibit high intensity PL peaks in the blue wavelength region. Notably, the materialâ?Ts QE is comparable to that of F8BT, a highly luminescent polymer. Differential scanning calorimetry revealed that acene crystallisation occurred in the solvent-processed films. The blend films with higher polymer viscosity re-crystallised when cooled from the melted state. On the other hand, films with lower polymer viscosity do not re-crystallise during cooling. However, re-crystallisation occurs slowly over 24 hours at room temperature. It was discovered that the polymer viscosity and the thermal processing conditions both have a pronounced impact on the TES-anthracene crystalline structure and the light emission of TES-anthracene/poly(styrene) blends. Melt-processing of blends with higher molecular weight (high viscosity) polymers results in needle-like acene crystals, and the QE is approximately the same value as in solvent-processed films. However, melt-processing of blends with lower molecular weight polymers results in a higher QE than in solvent-cast films. Melt-processing leads to more densely-packed TES-anthracene crystals, as was determined with emission microscopy. There is a strong correlation between the blendâ?Ts crystalline structure and the QE. The highest QE (ca. 20%) was obtained when the TES-anthracene was blended with a lower molecular weight polymer, and the resulting film was melt-processed. Thus, by increasing the light emission from the novel organic materials through control of processing and crystal morphology, our work is an important first step in the development of a new candidate for lasing applications.

Phthalocyanines are extensively studied as molecular semiconductor materials for chemical sensors, dye-sensitized solar cells, and other applications. Pthalocyanines offer high tunability through the choice of metal center atom; nearly all transition metals and many other heavier elements can reside at the relatively stable square planar center of the phthalocyanine. H₂-, Fe-, Co-, and Cu-phthalocyanine molecules in films deposited on gold substrates show prostrate orientation, as opposed sapphire substrates, where phthalocyanines stand in a more upright conformation under deposition conditions used. Angular dependence in Near-Edge X-ray Absorption Fine Structure (NEXAFS) resonances, commonly attributable to pi* and sigma* resonances, in both carbon and nitrogen K-edges, quantify the orientational order. H₂-phthalocyanine shows the cleanest angular dependence, with nearly no intensity in the pi* regime with normal beam incidence. Metal L-edges in prostrate films, on the other hand, have dramatic variation in angular dependence of resonances into empty states, particularly Cu-phthalocyanine. The combination of NEXAFS and StoBe DFT is used to explain and illuminate this unique electronic structure of Cu- compared to H₂-, Fe-, and Co-phthalocyanines.

Recently, self assembly monolayer (SAM) of nanoparticles (NPs) for memory effect has drawn tremendous attentions for organic device fabrication. Meanwhile, various binding mechanisms for the formation of the SAM can be derived from specific intermolecular interactions, such as bio-inspired mechanisms. In this study, organic memory device structure using monolayer formation between aptamer conjugated gold nanoparticles (Au NPs) with prostate-specific membrane antigen (PSMA) protein hybridization was introduced and characterized. The capacitor type memory effect was examined by using pentacene and dielectric SiO2 layer intervened by the Au NPs layer. PSMA protein was immobilized onto the SiO2 layer combined by formation of amide bond. Capacitance-voltage(C-V) hysteresis loops and retention measurement of the device was characterized and discussed.

Recently, ferroelectric organic field-effect transistors (OFETs) have attracted much attention for non-volatile memory applications such as a ferroelectric random-access-memory (FeRAM) due to simple fabrication and low cost. For FeRAM, two different types of the OFETs, one for the data selection by a conventional OFET and the other for the data storage in a ferroelectric OFET, should be integrated in an individual ferroelectric memory cell [1]. For the integration of two different OFETs, the patterns of two different insulators need to be prepared. A FeRAM with ferroelectric OFETs was demonstrated using inkjet printing [2]. However, the inkjet process has the difficulty in integrating memory cells into an array because of limited resolution. In this work, we present ferroelectric memory cells for the FeRAM in which a conventional OFET and a ferroelectric OFET are integrated by transfer-printing of a buffer layer on a ferroelectric insulator. The region with a pattern of the transfer-printed buffer layer acts as a dielectric surface for the conventional OFET while the region without a buffer layer provides a ferroelectric surface for a ferroelectric OFET. It was found that the transfer curves of the ferroelectric OFETs show hysteresis characteristics needed for memory applications but those of the conventional OFETs exhibit no hysteresis. The transfer-printing method of a buffer layer, in principle, provides high-resolution and thus enables to easily integrate two types of the OFETs into a unit memory cell for the FeRAM. References: [1] R. C. G. Naber, K. Asadi, P. W. M. Blom, D. M. de Leeuw, and B. de Boer, Adv. Mater. 22 (2010) 933. [2] T. Sekitani, K. Zaitsu, Y. Noguchi, K. Ishibe, M. Takamiya, T. Sakurai, and T. Someya, IEEE Trans. Electron. Dev. 56 (2009) 1027. Acknowledgement: This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the grant 2011-0028422.

Low dielectric materials have been used as interlayer dielectrics for system IC over the years but there has not been much progress in the development of ultralow dielectrics required for less than 25 nm devices. Our group developed ultralow dielectrics (ULK) which had modulus of 9.1 GPa at dielectric constant of 2.12 by designing reactive porogens. We also treated ULK by either UV or ozone to optimize the structures of nanoporous ULK for higher mechanical strengths and better pore morphology. Recently we used ozone treatment which resulted in smaller pore size (less than 2 nm) and narrower pore distribution. Ozone treatment converted the alkoxy groups in the matrix and the reactive porogen to silanol groups and resulted in increased reactivity between the reactive porogen and the matrix. In addition, ozone treatment above 150 °C converted T structures of both the reactive porogen (right??) and the matrix to Q or SiO2-like structures, which are responsible for high mechanical strength as well as high dielectric constant. The structural changes partially came from the decrease in methyl groups of the copolymer of methyl trimethoxysilane (MTMS) and 1,2-bis(triethoxysilyl) ethane (BTESE) as the matrix. Therefore, in order to optimize ozone treatment condition, we studied the structural changes of reactive porogen and organosilicate matrix induced by ozone treatment as a function of time. FTIR and Si NMR were used to characterize their chemical and structural structures as a function of time and temperature.

The systematic study of the thermal treatment has been carried out on the solution processed organic semiconductor (OSC)/organic gate insulator (OGI) interface system and its application of soluble anthradithiophene deriative based organic field effect transistors (FETs). Here we show that the property of solution processed device is strongly attributed to the thermal treatment effect with resulting in the fundamental aspect of migration phenomenon at the OSC/OGI organic interfacial as well as the significant change of the crystalline polymorph and the film morphology. After the post-annealing of organic semiconductor close to a solid-isotropic (K-I) transition temperature at 140 °C, the solution processed device exhibits the noticeable performance improvement up to the carrier mobility of ~ 0.5 cm2/Vs, subthreshold swing of 1.1V/decade, and Ion/Ioff current ratio 9.6Ã-106 with the highly ordered moleculeâ?Ts structure in the film. It is also found that the polymorph of crystalline grains is significantly deteriorated after the post-annealing over a K-I temperature with suppressing the structural order of molecules. The depth profile of elemental mapping in the interfacial region clearly turns out that the decomposition of OSC and its migration toward the organic gate insulator are occurred during the post-annealing over a K-I temperature, resulting in the low electrical performance of the device.

The preparation of uniform large-area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field-effect transistors. Quantitative control over the drying speed during dip-coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the airâ?"solutionâ?"substrate contact line are still not well understood. Here, we report the facile one-step growth of self-aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field-effect mobilities (up to 1.5 cm2Vâ?"1sâ?"1) via an optimized dip-coating process. We discover that optimized acene crystals grew at a particular substrate lifting-rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0°C) or chloroform (b.p. of 60.4°C). Variable-temperature dip-coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom-up study of soluble acene crystal growth during dip-coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder-shaped substrates).

Study of interfacial properties of hybrid junctions is scientifically challenging due to large dissimilarities in the structural and electronic nature of the two materials. Unique properties of hybrid interfaces can be utilized for modifying the characteristics of devices based on electron transport across the junction. In this work, bilayer junctions of Hexa-peri-hexabenzocoronene (HBC) or Graphene with copper oxide thin films have been studied for resistive memory applications. X-ray photoelectron spectroscopy and spectroscopy ellipsometry studies were carried out for studying the interfacial modification at the hybrid junction. In case of CuO-HBC based hybrid structure, it was observed that interfacial region has a number of distinct features in the absorption spectra of the interfacial layer obtained from optical modelling of the spectroscopic ellipsometric data. It was observed that Ti-CuO-HBC-Cu structure has resistive switching properties entirely different from the electrical properties of individual layers. In case of CuO-MLG bilayer structure, resistive switching characteristics are related to the electronic interaction at the interface as revealed by XPS studies. The resistive switching parameters are observed to be sensitive to the ambient conditions. Electrical characterization of the junction based on CuO, MLG and CuO-MLG layers shows that MLG plays an important role of oxygen ion storage and blocking layers and resistive switching is due to the two way migration of oxygen ions across the junction. Resistive switching properties of bilayer structures studied in the present study provide useful information about the nature of electrical and structural interaction at the hybrid interface. Keywords: Hybrid interface, resistive switching, Copper oxide, Hexa-peri-hexabenzocoronene, Graphene.

In the last few years, the successful growth of organic semiconducting thin films and 1D structures in the form of wires, fibers and ribbons based on small conjugated molecules has been reported and their potential applications in nano scale organic field effect transistors (OTFTs) with improved charge carrier mobility has been demonstrated. We present here some of the recent results on the growth of organic nanowires grown by organic molecular beam deposition technique. Most of the cases, planer Ï?-conjugated molecules are used to grow wires where molecules are stacked by week Ï?â^'Ï? interactions. We have synthesized a Ï?-conjugated donor-acceptor molecule where the donor and the acceptor units are connected via a planar spacer unit and thus giving rise to a charge transfer character. These molecules form stacked layer structure held both by aromatic CH-Ï?-hydrogen bonding as well as Ï?-stacking between the donor unit in one layer and the acceptor aromatic unit of other in solid crystal. However, H-bonding is playing the prime role over Ï?â^'Ï? stacking for giving the layer structure and thus leading to the unique properties for 1D growth. It has been shown that hydrogen bonding along the perpendicular direction to the molecular plane is stronger than bonding within plane. Therefore, these molecules prefer to grow unidirectional with higher air stability. We have used this property of the molecules to grow nano wires with higher aspect ratio (typically 10 μm long and 100 nm width) on top of SiO2 surface. Detail growth mechanism of these wires has been studied. We have found very high persistent current that shows promises to be a very good candidate for designing optoelectronic memory units.

A large effort is now devoted to the research of new computing paradigms associated to innovative nanotechnologies that should complement the classical Von Neumann/CMOS association. Among various propositions, spiking neural networks (SNN) seem a valid candidate. We demonstrated a memory transistor made of conjugated molecules and metal nanoparticles, which behaves as a biological spiking synapse [1], with a dynamic in the frequency 0.01-10 Hz depending on the NP size (5-20 nm) and the channel length (200 nm to 20 μm). This behavior is obtained by coupling the charging/discharging property of the nanoparticules when the gate voltage of the device is biased and the interaction into the gap of the transistor between the charges stored in the nanoparticules and the free carriers of the pentacen thin film. Here we demonstrate the implementation of the Spike Timing Dependent Plasticity (STDP) learning algorithm. First, we demonstrate that the NOMFET is a memristive device. Based on the formalism of Chua [2], we propose a physical model that describes the NOMFET behavior. Secondly, based on a theoretical proposition of STDP implementation with a memristor [3], we demonstrate a direct implementation of STDP using NOMFETs. The STDP allows tuning the weight of a synaptic connection depending on the timing between the input synapse spike (i.e the activity of the pre-synapse neuron) and the feedback pulse signal (due to the post-synapse neuron activity) [4]. As a demonstrator, we fabricated an original implementation of an associative learning neural network - inspired by the famous Pavlov's dogs experiment - using a single NOMFET to implement each synapse. We show how the physical properties of this dynamic memristive device can be used to perform zero-power write operations for the learning and implement short-term association using temporal coding. An electronic circuit was build to validate the proposed learning scheme with packaged devices, with very good reproducibility despite the complex synaptic-like dynamic of the NOMFET in pulse regime. [1] F. Alibart et al. Adv. Func. Mater. 20, 330 (2010). [2] L.O. Chua, IEEE Trans on Circuit Theory 18, 507 (1971). [3] B. Linares-Barranco et al.IEEE NANO 2009, pp. 601-604. [4] H. Markram et al., Science 275, 213 (1997); G. Bi and M. Poo, J. Neurosci. 18, 10464 (1998).

Recently, organic thin films have been attracting attention due to their large area, low cost, and flexibility for display applications and flexible electronics. Alternatively, inorganic thin films have several benefits over organic thin films such as high functionality (e.g. high conductance, high dielectric constant and polarization, etc.) Organic-inorganic hybridization is interesting because it widened the range of applications by not only offering new functions but also manipulating the surface properties. A novel technique has been reported using a modified atomic layer deposition (ALD) method, called molecular layer deposition (MLD), for building an organic and inorganic hybrid stack as designed by a manufacturer. MLD hybrid thin films minimized the formation of defects during the growth of the organic and inorganic layers because they are deposited by sequential, self-limiting, surface reactions similar to ALD process. In this study, we investigated the growth characteristics of organic-inorganic laminates, i.e. octenytrichlorosilane with 7 alkyl chains (7-OTS) and zinc-oxide (ZnO) hybrid thin film, using ozone based MLD at temperatures between 100°C and 150°C. 7-OTS is deposited by an exchanging reaction between functional group and water with growth rate of ~0.6 nm per cycle. The terminal methyl group (-CH3) of 7-OTS is converted into a carboxylic group (-COOH) through in-situ ozone (O3) activation. ZnO is deposited as a linker layer in-between each OTS layers using diethyl-zinc (DEZ) and DI water (H2O) as precursors with deposition rate of ~0.2 nm per cycle. Reference study using aluminum-oxide (Al2O3) and titanium-oxide (TiO2) as a linker layer will be also discussed. Materials characterization of organic-inorganic hybrid thin films have been extensively investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Ellipsometer and Fourier transform infrared spectroscopy (FTIR). Electrical characteristics, including I-V and C-V, of the laminate structure will be analyzed. This research is partially funded through Korea-US collaboration R/D program by MKE-COSAR-KETI.

Abstract: It appeared in the past decades that semi-conducting organic liquid crystals could be used as the active layer in organic thin film transistors (OTFTs). They can be processed by simple methods such as inkjet printing, which paves the way to new applications for plastic electronics such as electronic tags, biosensors, flexible screens, â?¦ However, the measured field-effect mobility in OTFTs is relatively low compared to inorganic devices. Generally, such low field-effect mobility values result from extrinsic effects such as grain boundaries or imperfect interfaces with source and drain electrodes. It has been shown that reducing the number of grain boundaries between the source and drain electrodes improves the field effect mobility [1-3]. Therefore, it is important to understand the transport mechanisms by studying the structure of organic thin films and local electrical properties within the channel and interfaces with metal electrodes in order to improve the field-effect mobility in OTFTs. Kelvin probe force microscopy (KPFM) is an ideal tool to simultaneously investigate the local structure and the electrical potential distribution in electronic devices for the above mentioned purposes. In this work, we studied the structure and the electronic properties of OTFTs based on dioctylterthiophene (DOTT). The transistors were fabricated by spin-coating DOTT on the transistor structures with treated (silanized) and untreated channel oxide. The potential profiles across the channel and at the metal-electrode interfaces was measured by KPFM. The effect of surface treatment on electrical properties, charge trapping phenomenons and hysteresis effects was also studied. Keywords: Organic thin film transistor (OTFT), Oligothiophene, Kelvin probe force microscopy (KPFM) [1] G. Horowitz, M.E. Hajlaoui Synth. Met.122 (2001), 185. [2] T.W. Kelley, C.D. Frisbie J. Phys. Chem. B 105 (2001), 4538. [3] T. Minari, T. Nemoto, S. Isoda J. Appl.Phys. 96 (2004) 769.

Since the early researches in organic thin film transistors (TFTs), new organic semiconductors and materials optimization have pushed the field close to marked applications^[1].The tuning of interfaces and thin film morphology has been highlighted during the last years due to the optimization of promising materials^[2,3], and to possibility to adapt materials properties to electrical device properties as needed for application. A set of symmetric C₁₃-BTBT-C₁₃, and asymmetric C₁₃-BTBT organic semiconductors^[4] was synthesized and applied in thin film transistors (TFTs). The asymmetric substitution pattern is favourable, because preserving large charge carrier mobilities (up to 3.4 cm²/Vs) and easy processing but strongly reduced injection barrier as investigated in detailed studies of thin film morphologies, contact- and channel resistance. The top-contact bottom-gate structure of TFT allows us to study the interface characteristics related to the surface energy and interface functionalization. The formation of the first monolayer of BTBTs on different hybrid dielectrics was investigated with AFM. The high threshold/operation voltage motivated us to apply ultra thin hybrid dielectrics (AlO_x/SAM) providing high capacitance, low leakage current, and low operation voltage while having the charge carrier mobility up to 3.6 cm²/Vs. Varying the SAM in hybrid dielectric, the threshold voltage can actively be tuned. For instance, using fluorinated SAM (FC₁₈-PA) and C₁₃-BTBT lead to devices with good charge charier mobility of 1.84 cm²/Vs and a Vth shift closer to zero ( -0.9 V), providing excellent characteristics for further integration. References: ^[1] G. Gelinck, P. Heremans, K. Nomoto and T. D. Anthopoulos, Advanced Materials, 2010, 22, 3778 ^[2] A. Virkar, S. Mannsfeld, Z. Bao and N. Stingelin, Advanced Materials, 2010, 22, 3857. ^[3] H. Ma,H. L. Yip, F. Huang and A. K. Jen, Advanced Functional Materials, 2010, 20, 1371 ^[4] H. Ebata, I. Takafumi, M. Eigo, T. Kazuo, I. Masaaki, K. Hirokazu and Y. Tatsuto, Journal of the American Chemical Society, 2007, 129 (51), 15732.

Many research efforts have been focused on new organic compounds in n-Type materials due to the various applications such as OTFT and OPV. New n-type polymer of Poly-6,6,12,12-Tetrahexyl-2,8-bis-(5-methyl-thiophen-2-yl)-6,12-dihydro-diindeno[1,2-b;1',2'-e]pyrazine (PTIDP) was recently synthesized. UV-Visible absorption maximum values of the synthesized polymer were 499 and 497 nm in solution and film state. PL spectrum showed maximum values of 525 and 530 nm in solution and film state, respectively. In this presentation, the related physical property as well as the device data using PTIDP will be discussed.

The multiplicity of fabrication process variables (solvent type, solvent blends, evaporation profile, annealing, spinning rate, solvent atmosphere, blend ratio, substrate effect, additives) makes tailoring (solvent based) fabrication processes a challenging task. It is of particular interest to be able to comprehensively estimate the morphological features depending on particular combinations of various process and system variables. A phase-diagram that maps the phase space of processing conditions to various classes of morphology formed is an attractive method to understand the interplay between multiple thermodynamic and kinetics parameters involved in morphology evolution. Such phase-diagrams will be very useful as we seek to tailor fabrication conditions to achieve designer morphologies. We utilize a predictive computational framework to model morphology evolution during solvent-based fabrication of thin films. This method provides the flexibility to search the phase space of process and system variables in a very efficient way. A systematic study of the various processing conditions reveals disparate modes of morphology evolution. We concentrate on three main variables: evaporation rate, blend ratio and Flory Huggins interaction parameters. We construct a phase diagram and quantify the interplay between kinetic, thermodynamic and system variables (for model systems) that typically leads to various morphological features. Finally, we illustrate the utility of such approach by focusing on a specific polymeric system of interest to the OPV community.

To turn organic electronic circuits into viable technology platform, transistors require high field-effect mobility and operating voltages below 5 V. In addition, technology that can be adapted to roll-to-roll processing is desirable. Here, dry vacuum technologies provide an alternative path to wet printing of organic devices.

Our bottom-gate and top-contact transistor fabrication process employs dry fabrication procedures only. The gate dielectric consists of aluminium oxide (AlO_x) prepared by UV/ozone exposure of thermally evaporated Al and a vapour-deposited monolayer of n-octyl phosphonic acid. The total thickness of the dielectric is ~ 20 nm, leading to capacitance value of 2.76 x 10^-7 F/cm², breakdown voltage in excess of 10 V, and the gate leakage current less than 4 x 10^-7 A/cm². To increase the field-effect mobility of the transistors, the interface between the gate dielectric and organic semiconductor must be optimized. We improved the transistor performance and device uniformity by using pentacene of higher purity and by choosing moderate pentacene evaporation rates. Transistors with one-time (TCI), three-time (Sigma-Aldrich), and four-time purified (in-house) pentacene were fabricated while the evaporation rate was varied from 0.12 to 4.2 nm/min. Transistors with four-time purified pentacene and moderate evaporation rate, achieved higher field-effect mobility and lower inverse subthreshold slope as compared to one-time and three-time purified pentacene. To date the best transistors have field-effect mobility of 0.09 cm²/Vs, threshold voltage of -1.1 V, and inverse subthreshold slope of 93 mV/decade. The increase in the field-effect mobility correlates with the decrease in the channel resistance (determined by the transmission line method) achieved for moderate evaporation rates of pentacene. Studies of surface roughness are underway to provide further optimization of the dielectric/semiconductor interface.

We have previously studied the transport properties of rubrene electrical double layer transistors (EDLTs) as a function of ionic liquid composition and temperature. We have successfully achieved effective carrier mobility up to 3.2 cm²V^-1s^-1 at charge densities larger than 10¹³ cm^-2. At lower temperatures when a larger gate bias can be applied, the maximum attainable charge density reaches 6.5Ã-10¹³ cm^-2 (0.34 holes per rubrene molecule), and more remarkably, two pronounced maxima in channel conductivity have been reproducibly and stably observed. This feature, which has not been reported for any EDLTs gated with electrolytes, is independent of ionic liquid compositions, current-voltage sweep rate, and crystallographic directions of rubrene crystals. We have identified that the first and second conductivity peaks occur at charge densities of 2.1Ã-10¹³ cm^-2 and 5.2Ã-10¹³ cm^-2, respectively. Capacitance-voltage (C-V) measurements have also shown two maxima at the same gate voltages as in current-voltage measurements. These observations imply that the double conductivity maxima are very likely related to the electronic structure on the surface of rubrene crystals.

Polyimide is one of the most promising candidate materials for gate dielectric applications due to its high chemical resistance as well as good insulating properties. Rigid polyimides with high packing density are known to have high resistance to chemicals currently used in electronics industry such as echant and photoresist stripper. Because thermally cured polyimide films cannot be patterned by facile methods in most cases, photo-patternable polyimide gate dielectrics have been proposed recently. We have developed photo-patternable polyimide gate dielectric with a relatively high-dielectric constant and low leakage current density. The polyimide gate dielectric could be easily patterned by selective UV light exposure and immersion into gamma-butyrolactone to dissolve-out the uncrosslinked part. To investigate the potential of the polyimide as the gate dielectric, we fabricated pentacene TFT devices. Interestingly, hole mobility of pentacene TFT device was increased after the solvent immersion process for the patterning. Hole mobility of the device with the polyimide, UV-exposed, but not immersed in the patterning solvent, was in the range of 0.039 ~ 0.067 cm2/Vs. When using the patterned polyimide, UV-exposed and immersed in the patterning solvent, hole mobility was in the range of 0.10 ~ 0.21 cm2/Vs. There are two possible reasons of the above result. In most cases, gate dielectrics after the patterning process were damaged, but the crosslinked polyimide is robust against the patterning solvent. And the solvent-treated dielectric surface may affect the molecular ordering and morphology of pentacene film. After the solvent immersion process, the grain size of pentacene molecules was decreased. This might affect the device property. In summary, we have developed photo-patternable polyimide gate dielectric which is robust against the patterning process. The pentacene TFTs with the patterned polyimide were obtained with a carrier mobility of 0.10 ~ 0.21 cm2/Vs and I(on)/I(off) of ~ 10^6.

We have demonstrated efficient solution-processed small-molecule solar cells based on a novel molecular donor, DTS(PTTh₂)₂ A record power conversion efficiency (PCE) is achieved for small-molecule organic photovoltaics: PCE=7% under AM 1.5 G irradiation (l00 mW cm^-2) from bulk heterojunction (BHJ) composites of DTS(PITh₂)₂:PC₇₀BM (donor to acceptor ratio of 7:3) with short circuit current (J_sc) of 14.4 rnA cm^-2, open circuit voltage (V_oc) of 0.78 V and fill factor (FF) of 59%. These high values were obtained by using remarkably small percentages of solvent additive (0.25% v/v of diiodooctane, DIO) during the film forming process. Transmission electron microscopy was used to directly image crystalline DTS(PTTh₂)₂ domains in BHJ films and investigate changes with the varying concentrations of the solvent additive. These results provide innovative guidelines for the realization of high performance solution-processed organic solar cells fabricated using molecular materials. The final step in the preparation of p-DTS(PTTh₂)₂ involves end capping of the PT-DTS-PT core with hexyl bithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of hexylbithiophene transfer) can occur leading to the formation of (MePT)DTS(PTTh₂) . Trace impurities of (MePT)DTS(PTTh₂) in BHJ solar cells fabricated from synthesis batches of p-DTS(PTTh₂)₂ significantly influence the photovoltaic properties, causing a ~50% reduction in efficiency and affecting all of the relevant device parameters J_sc, V_oc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination. The mechanism of charge transfer in blended films of organic molecules relevant to solar energy production is a central problem in the field. In particular, the structure property relationship regarding the process is of wide interest. We show that the timescale of ultrafast charge transfer between a small molecule donor and a fullerene acceptor is affected by the use of processing additives during film formation, decreasing to less than 50 fs. In light of these results, a model is put forth which describes ultrafast and high yield charge transfer as the result of excited state delocalization.

Organic field effect transistors (OFETs) are promising candidates for flexible, transparent, and low cost electronics. Hence, they attract increasing scientific and economic interest and first products containing OFETs are ready for market entry. OFETs, however, are currently limited in their performance and accordingly in their usability for electronic applications. This is partially related to the rather low mobility of organic semiconductor in comparison to e.g. silicon. Moreover, there is a lack in appropriate structuring techniques for OFETs, which might allow to compensate for the low mobility of organic semiconductors. In this contribution, we present high performance vertical organic field effect transistors (VOFETs) with a vertical channel length down to 50nm. These devices are manufactured by a direct photo-lithography method, which is compatible to organic semiconductor materials. By this technique we can vertically stack source and drain electrodes and the organic semiconductor material in between. If a source-gate voltage is applied, a conductive channel is created in the semiconductor film and its resistance is controllable by the applied gate field. This vertical conductive channel of the VOFET, however, is a complex and 3-dimensional structure and thus OFET theory relying in the gradual channel approximation is not adoptable to such devices. Nevertheless, VOFETs presented in this study exhibit very high on/off ratios of 10^6 as well as high transconductance values. As a consequence of the extremely short vertical channel, which is likely governed by the film thickness, we clearly obtain short channel device characteristics as e.g. threshold voltage dependence on the source-drain field, and disappearance of the saturation region. The photo-lithography technique used for fabrication of these vertical transistors is based on fluorinated photo-resist compounds and thus it can be adopted for structuring of different organic materials under ambient conditions. Owing to this universality of the process, we present p-type VOFETs comprising Pentacene as well as n-type VOFETs comprising C60. Thus, the combination of high performance complementary VOFETs on the one hand and the advantages of the photo-lithographic patterning technique on the other hand, represents a promising way for high density integration in low-power electronic devices made of organic semiconductor materials.

Recently, carbon nanotube enabled vertical field effect transistors (CN-VFETs) demonstrated high on-off ratios with record on-currents (for given device area) using an organic semiconductor for the channel material.^1,2 Transconductance in these devices arises from a gate field modulation of the contact barrier at the organic semiconductor/nanotube interface. The porosity of the nanotube network allows unscreened access to the interface, while the low density of electronic states allows for gate field induced Fermi level shifts that boost the transconductance. The low density of states in the vicinity of the Dirac-point of graphene also makes it a good candidate source electrode for VFETs with some additional advantages. Here we demonstrate a novel transfer technique, which reduces disorder and improves areal yields, while eliminating the polymeric residue that typically plagues transferred films. The resulting planar graphene sheets do not suffer from the particulates and associated shorting pathways that necessitate thicker channel layers in nanotube-based devices. Thinner organic channel layers in these graphene based devices further boost the currents attained at low drain voltage. We also report a facile technique for fabricating perforated graphene films with tunable hole size and density, allowing for a direct assessment of the role of self-screening effects in VFETs. These initial graphene enabled VFETs achieve on/off ratios and on-current densities exceeding 10⁶ and 250mA/cm², respectively, with drive voltages below 4V. 1. M. A. McCarthy, B. Liu, A. G. Rinzler, Nano Lett. 10, 3467â?"3472 (2010). 2. M. A. McCarthy, B. Liu, R. Jayaraman, S. M. Gilbert, D. Y. Kim, F. So, A.G. Rinzler, ACS Nano, 5, pp 291â?"298 (2011).

We present two easy methods to reversibly switch OTFTs from enhancement to depletion mode and back by either a photochemical reaction using photo-acid generators as interfacial layers, or by the usage of pH-sensitive acidic SAMs. The latter allow the fabrication of chemo-responsive OTFTs detecting the presence of bases like ammonia that compensate the acid doping. Based on these methods good quality depletion-load inverters with tuneable characteristics [1] were manufactured. In case of the photo-acid generator SAM a significant refinement of the concept of chemical channel doping is achieved by replacing the covalently bonded silane layers bearing sulfonic acid groups with photo-acid generator polymers. The goal hereby is to use them as an interface-modification layer, whose properties can be patterned photochemically, as the acid group is formed only upon illumination. Thus an accurate local control of VTh through the illumination dose is achievable by a method that is fully compatible with lithographic techniques omnipresent in conventional semiconductor industry. The inverters with acid/base-sensitive interfacial layers on the other hand are shown to be highly responsive switches to the presence of HCL and NH3 gases. [1] Marco Marchl , Matthias Edler , Anja Haase , Alexander Fian , Gregor Trimmel , Thomas Griesser , Barbara Stadlober , and Egbert Zojer., Adv. Mat. 22, 5361 (2010).

In this work we propose a novel flexible structure for the fabrication of ultra-low voltage Organic Thin Film Transistors (OTFTs) working at 1V. All devices have been fabricated on flexible Polyethylenetherephtalate film (PET) and are based on a unique combination of different, ultra-thin, insulating films. The core of the structure is a Al2O3 thin film grown by UV-Ozone treatment at the top of an aluminium film that acts as the gate electrode. At the top of this structure an ultrathin Parylene C film has been deposited by chemical vapour deposition. Assisted by Atomic Force and Transmission Electron Microscopy we have investigated on the structural and morphological properties of the deposited thin insulating films. Very interestingly it was found that the UV-Ozone grown Al2O3 films have an average thickness of 6nm and are characterized by a very high surface corrugation which usually gives rise to high conductive paths through the oxide film. On the contrary, the deposition of a continuous Paylene C film (average thickness 20nm) led to a proper smoothing of the final surface roughness (reduced by almost one order of magnitude) and also contributed to an impressive lowering, by more than 5 orders of magnitude, of the leakage current. We have fabricated OTFTs using this approach employing TIPS-pentacene as p-type active organic semiconductor. Devices are characterized by a remarkable very small leakage current, around 20-50pA, charge carrier mobility up to 0.3 cm2/Vs, threshold voltages around -0.1V and very low substhreshold swings, close to 100-150mV/dec. Moreover, thanks to a proper shielding of electron trap sites, usually present on the Al2O3 layer, due to the presence of the continuous and smooth Parylene C film, we have succeeded in fabricating n-type OTFT using N1400 as semiconductor, obtaining a maximum mobility of 1x10-2 cm2/Vs. Interestingly enough, using this approach full swing complementary inverters operating at 1V have been fabricated reaching a maximum gain of 50. Finally, we have also demonstrated that, by simply reducing the thickness of the plastic substrate, it is possible to obtain highly mechanically stable transistors. In particular, we have fabricated ultra-low voltage transistors on very thin PET films (1.5 µm thick) that, thanks to the very small surface strain induced by the substrate can be bent down to very small bending radii (below 1mm) without getting damaged and most importantly without giving evidence of change of their electrical performances. These results represent a step forward for the fabrication of low-voltage flexible electronic devices to be employed for the fabrication of innovative applications in the wearable electronics field.

Metal oxide semiconductors have recently emerged as an important class of materials for the development of next-generation opto/electronic devices. Although many of the proposed applications aim to supersede other transpiring technologies such as organic semiconductors, the two classes of materials can be combined to produce devices with enhanced properties and potentially new functionalities. One example is the dye-sensitized solar cell (DSSC) which is based on a combination of metal oxide nanoparticles and an organic or metal complex dye sensitizer. The same material combination could be extended to thin-film transistors (TFTs) by functionalizing the oxide semiconductor surface with the organic dye to produce bifunctional devices such as phototransistors. Here, we report the electrical characteristics and photoresponse of dye-sensitized ZnO TFTs which, despite their high transparency (> 80%), are found to respond to green light with extremely high responsivity and large optical-to-electrical gain. TFT fabrication and dye functionalization were all processed from solution using spray pyrolysis and dip coating techniques. A green inorganic light-emitting diode with λ_max = 522 nm was used to selectively excite the organic dye in the photoresponse study. With the increasing light intensity from dark condition to the maximum irradiance of â^¼1.53 mW/cm², the threshold voltage ( V_TH) of the device is strongly shifted in the negative direction, from -55 V to -116 V, while the drain current ( I_D) increases by an amount that directly depends on the applied gate bias ( V_G). Specifically, I_D increases by six orders of magnitude at V_G = -90 V and by only one order of magnitude at V_G = 50 V. The analysis shows that the responsivity (R), which is the ratio between photocurrent and incident optical power, can be as high as 10,000 A/W, and the sensitivity of the device is also controllable through the application of V_G. The present work can be viewed as a significant step towards the development of a new generation of high performance optical sensors that are highly transparent and can be processed onto large-area substrates using low-cost processing methods.

In 2001 Nantero was incorporated with the goal of developing a universal memory, NRAMâ"¢, using carbon nanotubes (CNTs) as the switching element. NRAM incorporates all the positive attributes of currently available memory chips such as DRAM, SRAM and Flash and allows scaling to higher memory densities than any other technology. To enable development and production of the NRAM, Nantero had to envision and implement a process which used the existing CMOS fabrication tools and introduce a new nano-material such as CNTs into semiconductor fabs. Towards this goal, Nantero was successful in developing a CNT-based liquid formulation that meets CMOS process requirements such as high purity, stability and consistency. The formulation was qualified for use in several Fabs worldwide, and was integrated with other materials and tools to develop NRAM memory chips. This presentation will present our recent efforts to create high purity CNT formulations that are compatible with commercial, leading edge semiconductor manufacture technology nodes. Key characterization and application results related to specialized CMOS grade carbon nanotube formulations and their resulting CNT fabrics will be presented.

Molecular electronics describes charge transport processes whereby molecules serve as active elements (e.g., rectifiers, switches, sensors) or passive ones (resistors or surface passivating agents) in electronic devices. Despite the diversity of investigated systems, many fundamental questions regarding the mechanisms for electrical current passing through molecules between two electrodes and the possibility of gaining predictive power and control over the electrical properties of molecular junctions remain unsolved. In short, even for one of the simplest systems, i.e., that of alkyl chains between Au electrodes, there is a large discrepancy between the average tunnel barrier height, extracted from current â?" voltage (I-V) measurements (~1.2 eV), and the barrier that is expected from the experimentally determined electrode work function and alkyl monolayer ionization potential, as found by Ultraviolet Photoelectron Spectroscopy, UPS (~5 eV). Thus, studying the alignment between the molecular orbitals and the Fermi level (EF) of the electrodes, is a crucial step towards understanding electrical transport through molecules. It would seem that having a semiconductor (SC) instead of a metal as one of the electrodes in a molecular junction increases the difficulty in understanding the mechanisms that govern charge transport through the junction. As we shall see, though, the intrinsic asymmetry of a junction with a SC, with respect to charge carrier type, provides unique information on the energy levels involved in transport, in ways that are impossible when using only metal electrodes. To find how charge is transported through a monolayer of saturated alkyl chains we measure and analyze electrical transport across heavy-doped semiconductor â?" molecular insulator / metal junctions. Our results provide unequivocal evidence that charge transport in these junctions is controlled by the energy difference between the metal Fermi level and the lowest unoccupied molecular orbital of the molecule, |EF â?" LUMO| (LUMO: lowest unoccupied molecular orbital), rather than by the energy difference with the highest occupied molecular orbital, |EF â?" HOMO| (highest occupied molecular orbital). This finding agrees with complementary spectroscopic measurements of the samples without Hg contact, but challenges the prevailing concept of HOMO-dominated transport.

Conducting polymers are excellent candidates for a variety of applications that involve mixed electronic/ionic conductivity, including light-emitting electrochemical cells, dye-sensitized solar cells, batteries, electrochromic devices and biosensors. Yet, while the transport of electronic carriers is understood at a reasonable level of sophistication (Gaussian disorder, correlated disorder, etc.), ionic transport in these materials is not understood in any real detail. We developed a new experimental technique that allows us to directly probe ion injection and transport in conducting polymers. It consists of simultaneous electrical and optical measurements that can decouple ion mobility and concentration. We applied this technique to the state-of-the-art conducting polymer PEDOT:PSS poly(3,4-ethylenedioxythiophene) :poly(styrene sulfonate), and measured the mobility of several cations, including potassium and choline. Numerical simulations based on forward time iteration of drift-diffusion equations validated the analysis of the experimental results.

The highly conductive polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is widely used as a hole transport layer or electrode in organic electronic devices because of its good film forming properties and high visible light transmissivity. In this presentation we demonstrate how the cohesion of the PEDOT:PSS layer is significantly influenced by moisture along with temperature and mechanical loads. Due to its hygroscopic nature the PEDOT:PSS absorbs water from the environment, weakening the layer. The water easily diffuses through the layer and breaks the hydrogen bonds formed between PEDOT:PSS grains resulting in an accelerated decohesion rate. We use a thin-film adhesion technique that enables us to precisely measure the energy required to separate layers. We determine this energy under systematically varied environmental conditions with changing temperature, relative humidity and pre and in-situ UV exposure. This extensive series of quantitative analysis provides the impact of the different environmental variables and most importantly their synergies, leading us to an in-depth understanding of the mechanisms of delamination. We complement this investigation by surface characterization with XPS and AFM of the debonded surfaces to investigate the physics and chemistry of this degradation process. Additionally, we will compare the effects of a range of adjacent layers such as oxides, metals, glass and ITO. This insight into the mechanism of decohesion yields guidelines for the design of more reliable organic devices containing PEDOT:PSS layers.

We report on the n (electron) and p (hole) doping of graphene by modifying the graphene surface by various polymers. Modification of graphene with polymers decreases the graphene sheet resistance by 40-70%. Through such modification, we report sheet resistance values as low as 129 Ω, thus attaining values comparable to those of indiumâ?"tin oxide (ITO), while displaying superior environmental stability and preserving electrical properties over extended time scales. Electrical transport measurements reveal that, after doping, (1) the carrier density of holes increases, consistent with the acceptor nature of some of the polymers used, (2) after n-doping graphene, the carriers changes the sign. We find that the mobility of the graphene decreases due to enhanced short-range scattering and charged impurity scattering. The Drude formula predicts that competition between these two effects yields an overall increase in conductivity. We confirm changes in the carrier density and Fermi level of graphene through changes in the Raman G and 2D peak positions. Doped graphene samples display high transmittance in the visible and near-infrared spectrum, preserving graphene's optical properties without any significant reduction in transparency, and are therefore superior to ITO films in the near infrared. The presented results allow integration of doped graphene sheets into optoelectronics, solar cells, and thermoelectric solar cells as well as engineering of the electrical characteristics of various devices by tuning the Fermi level of graphene.

Organic materials offer the opportunity to process device active layers from solution precursors, which is highly desirable over more cumbersome vacuum processing methods, as it permits the utilization of high throughput, low temperature and inexpensive printing techniques for device processing. Here, we report on a prototype of a nonvolatile light-emitting organic memory (LE-OMEM) device composed of multiple solution-processed layers, which is made possible by crosslinkable molecular materials. This presentation will focus on a LE-OMEM device with a 12x12 passive matrix crossbar geometry that demonstrates both optical and electrical multilevel recording, which is made possible by the incorporation of both electrically and optically susceptible dithienylethene (DTE) photochromes that may be toggled between two energetically distinct and thermally stable states. The DTE material is incorporated into an organic light-emitting diode (OLED) structure that enables readout capability on both optical (electroluminescence, EL) as well as electrical (voltage dependent current density) channels, where optimized unstructured devices have displayed ON/OFF ratios for both current and EL of greater than 1000. We discuss the design considerations when moving from larger unstructured devices (pixel size ca. 0.1 cm^2) to significantly smaller pixilated structures (ca. 0.003 cm^2). On the basis of this passive matrix device, we highlight the statistically viable number of available grey scale levels as a function of both external optical and electrical writing stimulus.

Light-emitting electrochemical cells (LECs) are one of the simplest type of molecular electroluminescent devices. LECs have a simple architecture, are prepared from solution and operate with air-stable electrodes allowing for less rigorous encapsulation, which make them suitable for low cost/large area efficient lighting and signing applications.[1-4] In its simplest form they consist of a single active layer composed of an ionic transition-metal complex (iTMC) which supports all three processes of charge injection, charge transport and emissive recombination.[2-4] We showed that by using iridium(III) complexes capable of forming a supramolecularly-caged structure the lifetime of LECs can be increased to more than 3000 hours.[5,6] Additionally, using fast J-L-V scans during their operation the occurrence of dynamical doping was confirmed.[7] We will present our latest developments on iTMC based LECs, subsecond turn-on combined with a half lifetime in excess of 4000 hours at luminances above 600 cd/m2 and show additional proof of the occurrence of dynamical doping. Also we will present results on large area deposition on flexible substrates. (1) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (2) Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 3525. (3) Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426. (4) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abruña, H. D.; Malliaras, G. G. J. Mat. Chem. 2007, 17, 2976. (5) Bolink, H. J.; Coronado, E.; Costa, R. D.; Ortí, E.; Sessolo, M.; Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.; Constable, E. C. Adv. Mater. 2008, 20, 3910. (6) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Housecroft, C. E.; Constable, E. C. J. Am. Chem. Soc. 2010, 132, 5978. (7) Lenes, M.; Garcia-Belmonte, G.; Tordera, D.; Pertegas, A.; Bisquert, J.; Bolink, H. J. Adv. Funct. Mater. 2011, 21, 1581.

Highly conductive transport layers are one of the technical prerequisites to manufacture organic light emitting diodes (OLEDs) with high luminous efficacies above 50 lm/W. The conductivity is usually enhanced by doping of the organic hole or electron conductor host material. In the class of inorganic and organometallic compounds, the electron withdrawing or donating properties can be varied in a very large range. In the presentation, we show a structure-property relationship within a series of p-dopants based on organometallic Lewis-acids. The conductivity enhancement of the hole conductor was balanced to other important OLED requirements, such as cost, transparency and off-state-appearance. The fundamental properties of the doped layers were characterized by current-voltage measurements, impedance, UV/Vis and photoluminescence spectroscopy. The maximum conductivity was found to be in the range of 10^-4 to 10^-3 Scm^-1 without a significant additional absorption owing to a charge transfer band or the radical cation of the host material. The selected dopant was incorporated into a white OLED stack. Luminous efficacy and lifetime data will be presented. The synthetic depth of the preparation of the dopants determines its cost and thus, its contribution to an OLED product. The compounds were prepared in large quantities from readily available basic chemicals in only two steps. The compounds were characterized by standard chemical analytics and X-ray crystallography. Molecular modeling supported the course of Lewis acidity within the series of compounds.

Controllable molecular doping of organic semiconductors has important advantages for organic light-emitting diodes (OLEDs), organic solar cells (OSCs) and organic thin-film transistors (OTFTs). Previous studies on doping of organic semiconductors showed that the addition of strong electron donors or acceptors as dopants can generate extra electrons or holes, respectively. This leads to a shift in Fermi level and significantly improves film conductivity. However, the design of n-type dopants is considerable challenging due to the requirement of a high HOMO level of the dopant, making these dopants unstable in ambient condition. We developed and synthesized a new benzoimidazole derivative as a strong n-type dopant for fullerene C60, a well-known n-channel semiconductor. The co-evaporated thin film showed a maximum conductivity of 5.6 S/cm at a 8.0wt% doping concentration, which is the highest reported value for molecular n-type conductors to our knowledge. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation, thus it is promising for various organic electronic devices.

The polarization control of light is of particular importance for optical information processing devices, such as displays, optical storage, and stereoscopic imaging systems etc. For instance, linearly polarized electroluminescence (EL) devices would be useful for backlights of liquid-crystal displays (LCDs). With polarized backlights, the displays can be made â?ogreenerâ? (i.e. more power-efficient) and simplified by removing polarizers needed for conventional non-polarized backlights and the optical losses it induced. As such, there had been substantial efforts in developing polarized organic light-emitting devices (OLEDs). Among them, linearly polarized OLEDs adopting mesogenic polyfluorenes/oligofluorenes together with appropriate molecular alignment techniques are some distinctive examples. S.H. Chen et al. also reported the use of liquid-crystalline oligofluorenes as hosts to guide molecular orientation of guest emitters, forming the polarized host-guest emitting systems permitting wide tuning of emission colors, including white emission. The previous efforts in polarized OLEDs were mainly on fluorescent OLEDs. Yet, the development of OLEDs in recent years has largely shifted toward phosphorescent OLEDs, since phosphorescent OLEDs could effectively utilize both singlet and triplet excitons and realize ideally 100% internal quantum efficiencies. As such, it is of both scientific and technical importance to explore the possibility of achieving polarized phosphorescent OLEDs. In this paper, we will present an initial attempt to realize workable and functional polarized phosphorescent OLEDs by using a recently developed mesogenic phosphorescent metal (Pt(II)) complex, which is combined with mesogenic oligomer hosts to form the corresponding mesogenic host-guest emitting systems. Both the host and guest molecules in the host-guest films were successfully aligned by using the rubbed conducting polymer as the alignment layer. With such aligned host-guest films, polarized red-emitting phosphorescent OLEDs were successfully implemented, showing a maximal brightness exceeding 2000 cd/m².

Quantum dot (QD) semiconductor nanocrystals have been considered in a broad range of applications, from biological tagging to LEDs, lasers, displays, lighting and solar cells. In their photoluminescent mode of operation, QDs are currently in lighting products, and have the promise to be in liquid crystal display products in the near future. In electroluminescent mode, quantum dot light emitting devices (QLEDs) are an emerging class of thin-film hybrid organic-inorganic structures that can potentially achieve best-in-class performance amongst large-area emissive light sources. This paper will describe recent advancements in device design and materials performance that has enabled QLEDs to reach near-unity internal quantum efficiency. In addition, we will discuss printing methods that have enabled the creation of active matrix QLED displays that demonstrate the beneficial properties of QLED pixels.

Successful commercialization of emerging display technologies is increasing dependant on reduced material costs and processing. Polymer based light emitting diodes are considered low cost alternative to small molecule counterparts. Polymer solutions can be made into thin films over relatively large areas without need for heating or high vacuum. Displays require creation of addressable subpixels consisting of at least one of each primary colour (red, green, blue) and the resolution of the ensemble must be high enough to render an image amicable to the human eye (< 50 µm). The work presented in this talk details our methods of making polychromic pixelated polymer displays with minimal steps. Our process began with the invention of a photopatternable polysilafluorene (PSF) electroluminescent polymers synthesized via a low cost route with narrow solid state emission in the deep blue. PSF is a suitable host for numerous higher wavelength emitters. Solutions of PSF with epoxidized vinyl side groups are loaded with photoacid and spun cast to make transparent, smooth films varying in thickness from 10 nm to several microns. Photopatterning occurs with single photon absorption of UV light through a shadow mask or the more versatile two photon absorption of 545 nm laser light via a direct laser writing (DLW) system. The latter method is used to produce three dimensional light emitting structures with submicron resolution. PSF shows stimulated emission in both solution and thin films. Observation of amplified spontaneous emission and lasing from photocrosslinked films on a diffraction grating demonstrate PSF can be used in combination with DLW to produce resonator cavities with sufficient gain for lasing. Our initial DLW efforts have focused on log pile and whispering gallery mode microtoroid resonators; the latter have extremely high Q factor (> 10^7) which could potentially reduce lasing thresholds to the limits of electrical excitation.

Flexible display is believed as an innovative device which can change our living mode in the future. Therefore many researchers focus many attentions on the key technology for realization of flexible displays. Recent technology advances show the possibility of building with real flexible display which has small package, low weight and design freedom and make active matrix organic light emitting diode (AMOLED) and electrophoretic display attractive in replacing existing glass display. To make real flexible displays, all display components should be replaced by the new flexible parts and processes to make real flexible display. Recently, soluble polyimide (PI) is introduced as a flexible substrate due to its several advantages such as simple processes for coating and curing, good chemical resistance for processing. Additionally, this new flexible substrate can be easily adopted on the current mass production factory. In this paper, we will discuss on flexible display as an emerging display, and review the recent technical approaches and highlight results of flexible display using soluble polyimide material as a new flexible substrate.

Electrochromic materials are potentially useful for color-changing displays and dynamic windows with the active materials varying from transition metal oxides, ionic metal complexes, conjugated organic molecules, and conjugated polymers. With their relatively fast switching rates, mechanical flexibility, low redox potentials, and highly transmissive oxidized states, conjugated polymers provide a suite of properties desirable for both application types. Our group has created a set of conjugated polymers with a complete color palette that spans the visible spectrum with neutral state colors that range from yellow, orange, red, magenta, green, cyan, blue, and black. Their high level of solubility allows for processing using a variety of methods including roll-to-roll coating, inkjet printing, and spray-coating. This presentation will outline our efforts to develop advanced characterization methods for measuring the electrochemical, spectroscopic, and colorimetric properties of these polymers, including monitoring color properties during dynamic switching, along with rapid spectral recording for definition of switching speeds. Using electrochromic devices (absorptive/transmissive and absorptive/reflective types) as a characterization platform, we demonstrate how these colored-to-transmissive switching polymers behave in regards to color, spectra, switching speed, lifetime, durability, and bistability.

Organic light-emitting diodes (OLEDs) have the potential to revolutionize the lighting market. For lighting applications OLEDs made on flexible foils have a number of beneficial properties, such as free form factor, tunable color, being difficult to break compared to glass, having a large area emitting surface, and being potentially very efficient and low cost. This presentation will give an overview of the key technological developments that are required in order to realize such devices. Highlights from our work will be presented, including devices on plastic and metal foil substrates with solution processed layers deposited using our R2R line, inclusion of metal grids and high conductivity PEDOT for ITO-free OLED devices, and state-of-the-art thin-film flexible barrier performance.

We have studied a range of organic photovoltaic devices (OPVs) based on a thin-film blend of P3HT and PCBM, and show that by comparing device studies with a characterisation of the thermal transitions of the blend, we can provide a mechanistic description of the minimum temperature required to improve device efficiency. For as-cast P3HT:PCBM blend thin-films we evidence two glass transition temperatures corresponding to the existence of two compositionally different amorphous states. We demonstrate that an improvement in device efficiency only occurs once the film has been heated above the upper apparent glass transition temperature of the blend. If annealing is performed above the optimum temperature, excessive phase-separation and a partial reduction in film optical density leads to a general decrease in device efficiency. Both of these characteristic temperatures are dependent upon the composition of the blend. The temperature-dependent competition between such processes therefore opens a â?~windowâ?T within which device efficiency can be optimised and provides an opportunity to design effective annealing protocols for future polymer:fullerene blend OPVs.

We present a comparative study of two different ZnO porous film morphologies for DSC fabrications. Nanoparticulate ZnO were prepared by the classical doctor-blade technique starting from a paste containing ZnO nanoparticles. Nanoporous ZnO films were grown by a dye-assisted electrochemical growth technique. The film thicknesses were adjusted at similar roughness of about 300 in order to permit a worthy comparison. The effects on the cell performances of sensitization by dyes belonging to three different families, namely, xanthene (eosin Y) and indoline (D102, D131, D149 and D205) organic dyes as well as a ruthenium polypyridine complex (N719), have been investigated. It is shown that the mesoporous electrodeposited matrix exhibits significant morphological changes upon the photoanode preparation that yield to a dramatic increase in the layer internal specific surface area. In the case of indoline dyes, better efficiencies were found with the electrodeposited ZnO porous matrices compared to the nanoparticulate ones in spite of significantly shorter electron lifetimes measured by impedance spectroscopy. The observation is interpreted in terms of much shorter transfer time in the oxide in the case of the electrodeposited ZnO films. Among the tested dyes, the D149 and D205 indoline organic dyes were found the most interesting.

Donor-acceptor interface plays a crucial role in the electron-hole charge separation process of organic photovoltaic cells (OPCs). It has been demonstrated in the last decade that efficient conversion of photo-generated excitons to the geminate electron-hole pairs becomes possible with donor-acceptor bulk heterojunction which is a percolation structure of the donor and acceptor phases. On the other hand, it is known that the ultimately-ordered donor-acceptor interface is realized in a series of molecular donor-acceptor compounds, or the charge-transfer (CT) complexes. For example, an alternating mixed stack of donor and acceptor molecules is formed to afford a rich variety of narrow-gap organic molecular semiconductors whose gap energy can be tuned between 0.5 and 2 eV, depending on the combination of ionization energy of donors and electron affinity of acceptors. The materials should provide a model system for investigating the microscopic photovoltaic processes in OPCs which have not been disclosed yet. Here we report fabrication and photovoltaic (PV) characteristics of prototypical single crystal PV cells based on a typical mixed-stack CT complex of dibenzotetrathiafulvalene (DBTTF) with tetracyanoquinodimethane (TCNQ). We used functional organic metal electrodes for selective hole- and electron-ejections to fabricate metal-insulator-metal (MIM)-type single-crystal PV diode. It was found that the cell presents clear rectification and photovoltaic characteristics above hν > 0.8 eV [1]. High-spatial-resolution laser beam-induced current (LBIC) technique was used to investigate the interfacial charge-separation and diffusion characteristics of the CT excitons in the cell. The measurement allowed selective detection of hole and electron photocurrents through the metalâ?"semiconductor interfaces. We observed an exceptionally long diffusion length in the LBIC photocurrent profiles; this length was as much as 20 μm at excitation photon energy of above 1.3 eV, which can be ascribed to direct generation of photocarriers. Note that the exciton diffusion length of usual organic semiconductors is in a range of a few tens of nm. We discuss that the readiness to undergo CT exciton dissociation and the long diffusion characteristics of photocarriers, which should illuminate fundamental aspects of photoelectric conversion in the OPCs. [1] J. Tsutsumi et al., Phys. Rev. Lett. 105, 226601 (2010).

Hybrid photovoltaic devices composed of semiconducting nanocrystal acceptors and organic polymer donors are an intriguing alternative to traditional organic photovoltaic devices based on polymers and C61-butyric acid methyl ester (PCBM). Semiconducting nanocrystals such as CdSe, TiOx, and ZnO have several potential advantages including relatively high electron mobility, tunable band gap, and various morphologies. In particular, ZnO is an attractive possibility due to its environmentally friendly solution processing and non-toxicity. However, to date, hybrid photovoltaic device performance lags far behind that of devices using PCBM as the electron acceptor due to morphology. Furthermore, devices using ZnO nanoparticles as an acceptor and poly[2-methoxy-5-(3�,7�-dimethyloctyloxy)-p-phenylene vinylene](MDMO-PPV), a relatively large band-gap polymer as a donor have significantly better performance than those using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) as an acceptor despite P3HT having a lower band gap and better overlap with the solar spectrum.(1-3) In this presentation, I will discuss the reason for this performance discrepancy and include recent results using additional low band gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b�]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT),(4) poly[N-9�-heptadecanyl-2,7-carbazole-alt-5,5-(4�,7�-di-2-thienyl-2�,1�,3�-benzothiadiazole)] (PCDTBT),(5) Poly[2,7-(9,9-di-octyl-fluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT),(6) Poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazol] (PSiF-DBT),(7) Silaindacenodithiophenes,(8) and A,B-Alternating Poly(arylenevinylene) Copolymers.(9) (1) Park, I.; Lim, Y.; Noh, S.; Lee, D.; Meister, M.; Amsden, J. J.; Laquai, F.; Lee, C.; Yoon, D. Y. Org. Electron. 2011, 12, 424. (2) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2006, 16, 1112. (3) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2004, 16, 1009. (4) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884. (5) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295. (6) Zhou, Q. Appl. Phys. Lett. 2004, 84, 1653. (7) Wang, E. Appl. Phys. Lett. 2008, 92, 033307. (8) Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang, W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopoulos, T. D.; Sirringhaus, H.; de Mello, J. C.; Heeney, M.; McCulloch, I. Chemistry of Materials 2010, 23, 768. (9) Choi, T.-L.; Han, K.-M.; Park, J.-I.; Kim, D. H.; Park, J.-M.; Lee, S. Macromolecules 2010, 43, 6045.

Conjugated polymer-based photovoltaic (PV) devices have attracted increasing attention in the past decade due to solution processability and easy fabrication. The fabrication and photovoltaic performance of the TiO2 nanodendrite (ND) array â?" poly(3-hexylthiophene) (P3HT) heterojunction solar cells will be reported in the presentation. The aligned rutile TiO2 nanorod (NR) array was first synthesized on the FTO substrate by hydrothermal method and the branches of the TiO2 nanodendrites (ND) were then formed using an aqueous chemical bath deposition (CBD). TEM characterization reveals that the successful growth of TiO2 ND is attributed to the presence of rutile {101} twinning structure. Significant enhancement of the cell efficiency is achieved by interfacial modification using metal-free organic dye, D149. With the TiO2 film thickness of 1.5 μm, the D149-modified TiO2 ND array/P3HT solar cell possesses an efficiency of 3.12%. The charge separation and recombination in the cells are investigated by time-resolved photoluminescence (TRPL) and impedance spectroscopy. The mechanism for photocurrent generation in the D149-midified TiO2 ND/P3HT solar cells will be discussed in the presentation.

We use graphene nanoflakes (GNFs) to greatly enhance the charge extraction out of photoactive blend in inverted hybrid poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM)/ZnO-nanorod photovoltaic cells. Instead of a continuous film, solution processed GNFs with dimensions less than 200 nm Ã- 200 nm are homogeneously scattered on top of the well-aligned ZnO-nanorods. Those GNFs play key roles, including to serve as an electron drain to collect electron flow out to ZnO-nanorods, enhance the carrier mobility of the device and promote holes to drift toward the surface in contact with the anode. As a result, there is a large enhancement in photocurrent and photovoltage of 35% and 27%, respectively, leading to an improved cell efficiency by up to about 100%.

Bulk heterojunction polymer solar cells are one of the very promising candidates for photovoltaic devices due to the efficient charge transfer from conjugated polymers to fullerene derivatives. Compared with inorganic solar cells, however, one important hindrance for the efficiency improvement of PSCs is the limited light absorption due to the thin active layer limited by the short exciton diffusion length and low carrier mobility. In this work, we will demonstrate the impact of the incorporation of monofunctional poly(ethylene glycol) (PEG)-capped Au NPs into the active layer of polymer blend. A newly synthesized polymer poly[2,7-(9,9-dioctylfluorene)-alt-2-((4-(diphenylamino) phenyl)thiophen-2-yl)malononitrile] (PFSDCN) is used as the donor of the active layer. The improvement of open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF) are all obtained with an appropriate amount of Au NPs incorporated into the active layer. In order to understand the improvement, we experimentally and theoretically investigate the effects of LSPR introduced by Au NPs on the optical properties of PSCs, particularly the PSC active layer. We also experimentally and theoretically study the effects of Au NPs on PSC electrical properties. Our results show that due to the interesting feature of the strong lateral distribution of LSPR near field along the active layer, light absorption is enhanced by incorporating Au NPs into the active layer. The understanding can be applied to other metallic NPs incorporated organic solar cells. Meanwhile, our results show that electrical properties can counter-diminish the optical enhancement from LSPR which reduces the overall performance improvement. It is important that both optical and electrical properties need to be studied and optimized simultaneously. Our results show that after optimization, power conversion efficiency can be improved by ~32%. The study therefore contributes to better understanding the uses of Au NPs for enhancing PSC performances.

Novel absorber molecules are investigated for high efficient organic solar cells. Especially merocyanines provide high photovoltages as donor materials in photoactive organic hetero junction cells. The electronic alignment and the growing behavior of a merocyanine (donor) / fullerene (acceptor) hetero junction is presented. Fullerene was stepwise evaporated onto a 30nm thick merocyanine layer. After each deposition step synchrotron induced photoelectron spectra were taken in situ using the the TGM7 dipole beamline and the SoLiAS (Solid Liquid Analysis System) end station at the electron storage ring Bessy II in Berlin. During the first deposition steps the fullerene molecules diffuse into the merocyanine layer. After some steps intercalation pauses and the growth of a pristine fullerene layer starts. The band diagram of the heterojunction was determined considering the two different growth regimes. The merocyanine / fullerene HOMO offset is determined to be 850meV leading to a high LUMO offset, the driving force for exciton splitting. In situ bias illumination of the interface with 150W/m2 of a halogen lamp leads to a reversible shift of the C60 emission features, indicating a high photovoltage. The height of the photovoltage depends on the fullerene layer thickness. It starts at 200mV for submonolayer coverage and increases up to approximately 1V for coverages of some ten nanometers. Reversing the experiment by stepwise deposition of merocyanine on a 30nm thick fullerene layer leads to no intercalation and no photovoltage showing that the sequence of the layer deposition has a high impact on the functionality of the solar cell device.

Recent progress in silicon nanocrystals (Si-ncs) research and in the understanding of carrier multiplication phenomena has highlighted significant potential implications for the development of next generation solar cells. In this context, the control of surface characteristics and the capability of tuning Si-ncs surface properties are crucial research aspects. The combination of quantum effects due to the small size with the enhancement of the surface area offers the possibility of tuning Si-ncs properties not only through the core of the material but also through accurate engineering of the surface. Furthermore, an accurate three dimensional (3D) control of Si-ncs surface characteristics is fundamental to achieve high-performance devices. Indeed, Si-ncs that were stabilized by organic ligands have shown promising results, however â?obulkyâ? and long organic molecules cannot provide adequate surface coverage (~ 60 %) of the small Si-ncs and as a result the carrier transport within the device is compromised. In this contribution we discuss how an alternative approach that uses an atmospheric-pressure DC microplasma in liquid medium can be efficiently used for 3D-enginering and stabilization of Si-ncs optoelectronic properties. Induced non-equilibrium liquid/Si-ncs surface chemistry and efficient replacement of hydrogen terminations with oxygen-/organic-based terminations is the key factor. In particular, the microplasma treatment drastically enhances (x 10) the silicon nanocrystals photoluminescence (PL) properties. Enhanced PL properties are stable after several days of storage in either ethanol or water. Furthermore, we show that 3D surface engineering of Si-ncs directly in liquid media significantly impacts in the overall solar cell performance. Our investigation shows a very promising approach to achieve enhanced hybrid solar cells performance without using any surfactants. In particular, we demonstrate that 3D surface engineering of Si-ncs by microplasma processing in liquid medium can be used to enhance the electronic interactions with conjugated polymers such as with polythieno[3,4-b]thiophenebenzodithiophene (PTB7). Our results suggest that surface engineering and the PTB7/Si-ncs coupling is electronically favorable for exciton dissociation and carrier generation/transport. We illustrate that bulk-heterojunction cells consisting of Si-ncs embedded in PTB7 show clear improvement after Si-ncs surface engineering. Hybrid bulk heterojunction solar cells show relatively good rectification behavior, improved short circuit current (ISC) and a very high open circuit voltage (VOC) exceeding 1 V. The results confirm that the PTB7 and 3D surface engineered Si-ncs with quantum confinement effects via atmospheric-pressure DC microplasma in liquid medium could be a promising low-cost and environmental friendly technology for preparation of new class of energy materials and for their integration in high-performance solar cell devices.

We investigated spray deposition of the absorbing layer of bulk heterojunction solar cells comprised of poly[4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-bâ?~]dithiophene-2,6-diyl-alt-2,1,3-benzoxadiazole-4,7-diyl and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Specifically we examined, host solvents and substrate temperatures. Interestingly, we find that spray deposition leads to higher efficiency solar cells than is observed for spin-coating using the same host solvent. Spraying the donor and acceptor blend from 1,2-dichlorobenzene in on to a 80 °C substrate results in an average power conversion efficiency of 5.4% (max.=5.8%) compared with 3.5 % achieved spin-coating from the same solvent. This high efficiency is observed despite AFM topography images revealing a very rough surface indicative of spray coating. We observe a low fill factor consistent with previous reports of spray deposition, however, high short circuit current and open circuit voltage compensate for this.

A detailed study of the photocurrent spectra of organic bulk heterojunction solar cells is presented, providing insight on the energy dependent factors limiting photo-charge capture. Increasing the thickness of the active layer in an MDMO-PPV / PCBM solar cell reduces the magnitude of the photocurrent due to the low mobility of the photogenerated holes. Measurements show that the photocurrent reduction is predominately due to loss in carriers generated at the polymer absorption maximum, while the low energy response is relatively unaffected. In a thick enough sample, the low energy response (1.5-2 eV) dominates, and a photocurrent peak is no longer observed at the main absorption maximum (2.6 eV). The normal photocurrent spectral response can be made to return by applying bias to the sample. The main photocurrent peak increases by a factor of 5 under a bias of -1V compared to the low energy response. A similar effect is observed for samples with thinner active layers, although the change is less pronounced. The results imply that hole transport is blocked for carriers generated in the polymer compared to those generated in the PCBM. The difference can be attributed to the relatively low energy of the holes generated in the polymer, caused by the PCBM / polymer band offset. This work was funded by Dept. of Energy (Grant No. DEFG02-07ER46375) and National Science Foundation (GrantNo.NSF-DMR-0906961).

We have developed an improved small-angle X-ray scattering (SAXS) modeling and analysis methodology to quantitatively evaluate the nanostructures of a blending bulk heterojunction (BHJ). This method was first applied to resolve the various structures of self-organized poly(3-hexylthiophene) /C61-butyric acid methylester (P3HT/PCBM) thin active layer in solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by different annealing process is a typical processing which help for better understanding the enhanced power conversion of P3HT/PCBM solar cell (from 1.2% to 3.8%). The quantitative structural characterized by this method includes (1) the mean size, volume fraction and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration,(4) the correlation length of PCBM moleculesâ?T network within P3HT phase and (5) the specific surface area of PCBM cluster which can all be well-correlated to the device performance with different thermal history. The in-situ GISAXS measurements were also conducted to provide informative details of the temporal evolution of PCBM-related structures during phase separation. This work established a useful SAXS approach to present an insight into the modeling of the morphology of P3HT/PCBM film. Based on the developed model, simultaneous GISAXS and GIWAXD technique was performed to characterize a distinctive nanostructure induced by the newly developed strategy: embedding optimized amount (2.8x10-3wt%) of inorganic nanoparticles into the P3HT:PCBM BHJ film. Beside the typical process of thermal annealing, it is demonstrated for the first time that the P3HT:PCBM BHJ nanostructure can be manipulated by including inorganic nanoparticle copper disulfide (Cu2S) or Cadmium selenide (CdSe), leading to the promising improved power conversion efficiency from 3.5% to 4.3% (23% improvement). To sum up, the results of this investigation significantly extend the current knowledge of relationship of BHJ morphology to device performance. Benefited from the established characterization technique, the effect of nanoparticle-tuned self-organization of BHJ can be tailored which successfully provides an alternative strategy to optimize the nanostructure that is beneficial for roll-to-roll large area processing.

Metal-phthalocyanines (MPcs) and fullerene (C60) are widely used as donor and acceptor molecules, respectively, for small molecular planar heterojunction organic photovoltaic cells (OPVs). However, MPc/C60 based small molecular planar heterojunction photovoltaic cells show low power conversion efficiencies (ηp) of 1~2% and short lifetimes. Moreover, MPc/C60 planar heterojunction solar cells showed the characteristics of photo-degradation under the illumination of light even for a short time range in a nitrogen environment. Here, we present that the use of MoO3 and CuI as a double interfacial layer improves the ηp value and the photo-stability in ZnPc based solar cells at the same time. The device without the interfacial layer resulted in low efficiency and low photo-stability. We used CuI as a template layer to enhance the absorption of the ZnPc film. The absorption coefficient of the ZnPc film increased 1.6 times when thermally deposited on a CuI layer. Resultingly, the OPV with a CuI interfacial layer increased the efficiency significantly to 3.3%. However, the photo-stability was lowered even further. In order to improve the photo-stability, a MoO3/CuI double interfacial layer was used. The double interfacial layer retains the role as a template layer and acts a buffer layer between ITO and organic layer. The device with the double interfacial layer shows high efficiency and very much improved photo-stability at the same time. The TOF-SIMS analysis revealed that the decomposition of CuI and the diffusion of Cu is the origin of the photo-degradation of the device. Insertion of the MoO3 layer between the ITO and CuI prevents the decomposition of CuI to Cu and the diffusion of Cu under UV illumination. Our results indicate that the selection of materials for anode engineering and the combination of interfacial layers are important to improve the efficiency and stability of the planar heterojunction solar cells.

Proper light trapping schemes are crucial in increasing the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells, which typically have low absorption efficiency due to their thin active layers. Although many studies on nanostructured photoactive layers have been done for efficient light trapping, the enhancements have not been sufficient. Furthermore, structured active layers are often subject to electrical defects and thus can complicate the manufacturing process. On the other hand, structuring performed on the external surface of the substrates do not affect the electrical properties of the devices, yet can significantly enhance the PCE of the devices by improving the chance of photon absorption. Our study indicates that ideal Lambertian scattering on the external surface of a glass substrate can result in 28.2% absorption enhancement in the archetypal P3HT:PCBM60-based OPV cells. In this work, we investigate the influence of various micron-scale substrate-texturing methods and geometries on the performance of OPV cells. In particular, simulation based on both geometrical ray tracing and wave-optical transfer-matrix formalism is employed in combination for quantitative analysis of OPV cells with the textured substrates. Among the various structures we studied, a one-dimensional periodic array of V-groove with a vertex angle of 90 degree is predicted to yield approximately 14% absorption enhancement in P3HT:PCBM60-based OPV cells. This agrees relatively well with our experimental results showing ca. 8% improvement in short-circuit current density with both open circuit voltage and fill factor unaffected. Experimental details and the strategies for further optimization will also be provided.

Inorganic-organic nanocomposite or hybrid solar cells have attracted researchers due to the unique possibilities of tuning inorganic semiconductor nanoparticles in size and shape. One requisite for high photovoltaic efficiencies is the purity of the inorganic semiconductor. That means that in the active layer no stabilizer or capping layer should be present. To solve this problem, research was carried out on the development of in-situ preparation methods of inorganic metal sulfides directly in the organic semiconductor matrix. In this contribution we present novel copper and indium xanthates as precursors which are soluble in apolar organic solvents and are thus compatible with common conjugated polymers used in organic photovoltaics. However, the major advantage of this procedure is that the conversion of the precursor into CdS /PbS takes place below 140°C resulting in a dense network of the inorganic particles directly in the conjugated polymer matrix. By using P3HT as polymer and CdS:polymer and CdPbS/polymer composite was prepared in situ. The synthesis temperatures and compositions were optimized for high efficieny. The formation process as well as the morphology of the nanocomposite layer was analyzed by complementary spectroscopic and microscopic methods.

A conjugated polymer as a donor material, poly(3,4â?Tâ?Tâ?T-di(decylthiophenyl) quaterthiophene) (PDTQT), containing of decylthiophenyl side chains on the polymer backbone, was synthesized for use in organic photovoltaic cells. The crystallinity of poly(quaterthiophenes) (PQTs) was remarkably reduced by substituted-decylthiophenyl side chain of quaterthiophene backbone units. A well-interpenetrated nano-scale morphology of PDTQT:PC71BM blended active layer arose from this modification, which increased the power conversion efficiency relative to devices based on highly crystalline PQTs. Bulk heterojunction solar cells based on PDTQT:PC71BM blends with a 1:4 weight ratio annealed at 120°C presented the best photovoltaic performances, with a high short-circuit current density (Jsc) of 9.8 mAcm2, an high open-circuit voltage (Voc) of 0.91 V, a fill factor (FF) of 0.36 and a high power conversion efficiency (PCE) of 3.2 % under illumination of AM 1.5 with light intensity of 100 mWcm2. PDTQT:PC71BM are characterized by UV-visible absorption and Photoluminescence spectra, AFM and TEM images, and GIXD patterns. The charge transport characteristics and morphology studies indicated that an optimized interpenetration network composed of PDTQT: PC71BM could be achieved in a blend ratio of 1:4 annealed at 120°C. Decylthiophenyl side chain substitution appears to be an effective strategy for high device performance of organic photovoltaic cells.

The interface between photo-active layer and charge extraction layer is critically important for a better design of the polymer-fullerene solar cell with improved power conversion efficiency and device stability. Significant progress has been made in the performance of organic bulk heterojunction solar cells through the development of active layer materials, while detailed experimental studies on how to minimize interfacial charge collection losses are not systematically performed yet. Comparing the devices with thin layer of PEDOT:PSS and semiconducting p-type tungsten oxide (WO3) inserted between the active organic layer (poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 - P3HT:PCBM) and tin-doped indium oxide (ITO) anode, 3.2% and 2.8% of power conversion efficiency was obtained, respectively. Such a limited performance of charge collection at WO3 interface might be explained by the relatively low conductivity behavior and interfacial composition mismatch. However, in case of the inverted devices (ITO/titanium oxide (TiOx)/P3HT:PCBM/PEDOT:PSS or WO3/Ag), efficiencies were almost comparable each other (~3.0%). Although PEDOT:PSS was slightly modified with small amount of additive hydrophobic solvents, inferior interfacial adhesion of PEDOT:PSS on the active layer limits the efficient hole extraction. Upon using of the CW3 as hole collection layer, operational stability under AM 1.5 illumination was significantly improved both for normal and inverted devices. For a further improvement of efficiency and stability, we have designed various condition of bulk heterojunction by the surface-mediated self assembly and contact transfer process. Vertical phase separation of P3HT:PCBM blends were adjusted by the templating method on top of silicon oxide with methacryloxypropyltrimethoxysilane (MPS) and octadecyltrichlorosilane (OTS) modified surface, followed by the transfer process. Relationship between the surface functionality of metal oxides (TiOx, WO3) and the interfacial composition of bulk heterojunction transfer layer was investigated in detail.

We have synthesized poly[(4,4-didodecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P1), a class of narrow bandgap conjugated polymers currently under investigation for use in polymer solar cells. Herein, we present a systematic study of the simple physical method to facilely eliminate low MW part, and simultaneously to improve the molecular structure and electrical properties in a spin-coated P1 thin-film by immersing it into a poor solvent for a few minutes. The molecular structures of conjugated polymer films thus prepared can be translated to different levels of order and the low MW component from pristine P1 thin-film can be removed facilely by dipping method. FETs with improved performance therefore can be easily fabricated. Correlation of nanoscalar structural features and electrical property allowed us to determine the appropriate dipping time and how small MW influences electronic properties. This method is also straightforward and obviously useful in practical polymeric device fabrication.

Semiconducting organic/inorganic nanocomposites are appealing optoelectronic materials due to the possibility of combining desired properties from both components in one, thus creating new properties that may not be present in each individual material. In particular, blends of conjugated polymers (CPs) and semiconductor quantum dots (QDs) have attracted interests for applications such as light-emitting diodes (LEDs), photovoltaics (PV), and biosensing. The interaction between the excited states of a CP and a QD, which determines the optoelectronic properties of the resulting hybrid material, needs to be fully understood. Photoinduced hole transfer (HT) from QD to CP which is an essential process in achieving high power conversion efficiency in PV cells, but a detrimental process for applications such as LEDs and biosensing, remains little understood. Here, we investigate photoinduced HT from a CdSe/ZnS core/shell QD with varying shell thickness to a water-soluble conjugated polymer poly(9,9â?²-bis(6-N,N,N-trimethylammoniumhexyl)fluorene-alt-phenylene (FHQ). For this donor/acceptor hybrid, the emission peaks of FHQ (450nm) and QD (634-638nm) are well separated, enabling to selectively excite only the QD in the hybrid, so that HT from photoexcited QD to CP can be probed exclusively by monitoring the steady-state and transient photoluminescence (PL) decay from QD. In this work, HT in FHQ/QD composite is investigated both in solution and solid thin films by ensemble-based PL measurement. Furthermore, single particle spectroscopy is used to resolve the dynamics and heterogeneity of HT at single CP/QD interface and to probe the effect of this process on the PL blinking dynamics of QDs.

Organic solar cells have gained in the last years a growing interest within the research community. Among all organic solar cells, small molecule and polymer based technology are very promising. In particular, polymer bulk-heterojunction based technology has shown wide margins of improvement in terms of energy conversion efficiency, reaching values above 8% (certified by NREL), and uncertified 9%, actually doubling the performance in the last two years [1-4]. Despite the low efficiency, when compared with the conventional inorganic solar cells, the potential of a Roll-to-Roll (R2R) process and the low-cost large-area definition on flexible substrates, make the BHJ-SCs technology an interesting and economic solution for the production of energy. The solar cell is built up in a multilayer structure in which the deposition of the layers can be performed with different techniques like for example casting, spin-coating, screen and ink-jet printing [5]. Recently conventional spray-coating was introduced as a large area deposition method for the layers of organic solar cells (OSCs) [6]. In this context, we present polymer solar cells that use as active layer P3HT:PCBM, in which we replaced the standard spin-coating deposition of the active layer and of the hole transporting layer (PEDOT:PSS) with the spray-coating technique, reaching power conversion efficiency above 4% [6]. In these cells a fine tuning of the realization parameters such as the solvent used, the substrate temperature, the film thickness, time of spray, distance between sample and airbrush, substrate temperature has been performed. Furthermore different cells architectures are presented as function of the required characteristics of the substrate and of the electrode deposition. These results point out the spray-coating as a powerful printing technique to be embedded in a R2R process for the realization of flexible organic photovoltaic devices. References [1] C.J. Brabec et al. Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies, Wiley-VCH, Weinheim (2008). [2] Konarka Power Plastic (www.konarka.com) [3] Solarmer (www.solarmer.com) [4] J.Nelson, Materials Today, 14 (2011), pp. 462-470 [5] F.C. Krebs et al. Sol. Energy Mater. Sol. Cells 93 (2009), pp. 1968â?"1977. [6] G.Susanna et al. Sol. Energy Mater.Sol Cells, 93 (2011), pp. 1775-1778.

The nanoscale morphology of bulk heterojunction solar cells is very crucial for their performance. Hence, a lot of effort has been invested to study and control it. In order to elucidate the nanoscale morphology, many different complex techniques have been applied. Recently, TEM-tomography has been used to portray the structure of the polymer-fullerene active layer in all three dimensions. In this work we used another non-destructive method, in order to gain information about the predominant morphology and the donor-acceptor distribution within P3HT:PCBM blend films. We demonstrate that spectroscopic ellipsometry can be used to investigate the spatial composition over the film thickness and provides detailed information about the shape and extension of phase separation. The shape of nano-inclusions of the fullerene phase within the polymer matrix, as well as the concentration profile of the both components was determined. Morphological aging was investigated to trace the change in the concentration profile initiated by surface-directed spinodal decomposition of the film.

Polymer/fullerene bulk heterojunction (BHJ) photovoltaic cells are continually proving themselves to be an attractive route to potentially inexpensive solar energy conversion. Molecular weight is among the most important properties governing polymer behavior, and in the rapidly developing field of polymer based solar cells, understanding molecular weight effects is necessary to maximizing device efficiency and realizing commercialization. The trend of increased performance with increasing molecular weight is generally accepted, yet not rigorously understood. Here, semicrystalline poly(3-hexadecyl-2,5-thienylene vinylene) (P3HDTV), with a low band gap of 1.65 eV, was synthesized by acyclic diene metathesis polymerization with tuned chain length (Mn = 6 kg/mol to Mn = 33 kg/mol). Neat P3HDTV samples with systematically varied molecular weight were characterized by size-exclusion chromatography, UV-Vis spectroscopy, differential scanning calorimetry, and wide-angle X-ray scattering. This characterization revealed dramatic increases in polymer ordering and aggregation leading to the appearance of liquid crystal behavior at high molecular weight. The molecular weight dependent hole mobility of neat P3HDTV was investigated in field effect structures. Hole mobility was observed to increase over two orders of magnitude, from the 3.1 ± 0.5 Ã- 10-5 to 8.0 ± 0.3 Ã-10-3 cm²/V s, with increasing molecular weight. P3HDTV was blended with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) to form the BHJ photovoltaic cell active layer. Completed cells were characterized with diode measurements in the dark and under simulated solar illumination, and with monochromatic external quantum efficiency (EQE) measurements. BHJ cells were optimized for active layer thickness and P3HDTV/PCBM composition. Cells incorporating high molecular weight P3HDTV readily showed PCE of ~1%. In a series of 100 nm thick, constant composition devices, increasing molecular weight produced large increases in cell short circuit current (Jsc) and power conversion efficiency (PCE), with both increasing by a factor of two. The improved hole transport and solar cell performance observed with increasing molecular weight was attributed to improved level of polymer ordering, which using DSC and UV-Vis was shown to be persistent in the nano-segregated P3HDTV/PCBM BHJ blends even at extreme PCBM loading. Finally, combining the aforementioned characterization techniques with atomic force microscopy imaging, the thorough temperature-composition phase diagram was mapped in order to relate solar cell performance with composition, phase behavior, and morphology.

Although one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. We find that solar cells incorporating a diketopyrrolopyrrole (DPP) as a donor and a dicyanoimidazole (VinazeneTM) acceptor function as an active layer in BHJ solar cells, producing a relatively high open circuit voltage of up to 1.23 V and power conversion efficiencies (PCEs) of 1%.

The n-type wide-gap semiconductors, such as titanium oxide and zinc oxide are used preferably because they are deposited by solution based technique, and their electronic and optical features suitable for the electron-extracting/injection buffer layer due to their excellent electron withdrawing and transport properties and the exciton and hole blocking natures. Recently, Sasaki et al. developed a set of transition metal oxides with 2 dimensional structure (called â?onanosheetâ?) including titania. The nanosheet has several important features: (i) ultimate two-dimensionality with a thickness of ~1nm and a lateral size ranging from sub-micrometers to 10 micrometers, (ii) high crystallinity and well-defined composition, and (iii) novel and enhanced physical properties due to the quantum size effect. For example, the band gap of titania nanosheet (TN) (~3.8eV) is much larger than that of conventional anatase titania (~3.2eV) which is favorable for solar cell application. Moreover, the exfoliated TN can be deposited by low temperature process, such as electrophoresis, Langmuir-Blodgett technique, and the layer-by-layer (LBL) deposition of the diluted dispersion of aniodic nanosheet in pure water and polymer based polycations(PDDA) which is suitable for fabricating uniform, nano-controlled TN ultra-thin film. In this study, we have fabricated the hybrid bulk-heterojunction solar cells with ultra-thin TN as an electron buffer layers. We have investigated both "normal" and "inverted" cell consisting of ITO/NiO (or MoO3)/P3HT:PCBM/TN/Al and ITO/TN/P3HT:PCBM/MoO3/Ag structures. Only one or two nanosheet (1.5~3nm thick) layer markedly suppress the leakage current and improved both VOC and FF in inverted cells. That is we could obtain the pin-hole free ultra-thin nanosheet film by the insertion of ultra-thin TN film deposited by layer-by-layer technique. The reduction in FF and the current density above VOC for the device with thicker TN layers are ascribed to the interfacial potential barrier at ITO/TN interface and the series resistance across the multilayers of TN and PDDA. The FF is markedly improved by the insertion of anatase TiO2 layer between ITO and TN resulting in the improved efficiency of 2.7%. In normal cell, on the other hand, the contact barrier is essentially low compared with inverted cell. The hydrophobic organic active layer was covered by the repeated TN deposition and the improved efficiency was successfully obtained by the insertion of TN layer as an electron buffer layers. The detail will be discussed in the symposium.

Organic photovoltaic (OPV) devices are amenable to low-cost roll-to-roll production on plastic substrates using solution processing at low temperature. The photoactive layer is in form of a blend of acceptor polymers and donor fullerene with nanometer scale domains that promote efficient exciton dissociation and charge transport to respective electrodes. However, the power conversion efficiency (PCE) and stability of the device are a direct function of segregation of acceptor and donor polymer domains during operation of OPV. In addition, most OPV devices are based on costly and brittle indium tin oxide (ITO) coated glass, which leads to a high cost and incompatibility to flexible plastic substrates. This work presents our latest results on OPV devices based on blends of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) fabricated on clear plastic substrates, with composite nanomesh transparent electrodes, having high-performance nanostructured ZnO hole blocking layers. In comparison to conventional ITO, the nanomesh electrode (deposited by electrospinning process) provides the required mechanical flexibility for plastic substrate, low sheet resistance (<50 Ohm/sq) and high transparency (>90%). A nanowire ZnO hole-blocking layer is deposited by a combination of electrochemical solution deposition and transfer of chemical vapour deposition grown nanowires, which provides the selectivity for electrons in an inverted OPV structure. The nanostructure ZnO layer also provides a vertically penetrating contact layer in contact to the photoactive layer, acting as electron selective branches for improved electron transport and quantum efficiency. In addition, we believe the nanostructured morphology of the ZnO layer serve as an inorganic mechanical grid for stabilizing the nanoscale morphology of the photoactive layer, hence delaying the phase segregation and improving stability. The device is finished by hole collecting electrode based on higher work function metals (e.g., silver), providing improved stability in comparison to Al based layers. Preliminary results display a PCE in excess of 2% for this ITO-free hybrid organic-inorganic PV device on plastic substrate. We have systematically investigated the effect of thickness, roughness, doping and crystal quality of ZnO layer on the performance of OPV devices. Thicker ZnO films initially improve the performance due to increased surface area, but when the thickness increases beyond 50 nm, it suppresses the performance due to the low conductivity of the contact layer, increased loss of high energy photons, and reduced open circuit voltage possibly due to the parasitic role of ZnO:P3HT heterojunctions.

Light trapping is a promising approach to improve light absorption in organic solar cells by scattering the incident light to oblique angle and increasing the dwell time of the light inside the devices. In this work, the absorption enhancement in organic solar cells is realized using a poly(methyl methacrylate) (PMMA)/indium-tin-oxide (ITO) grating electrode. Photovoltaic properties such as external quantum efficiency and power conversion efficiency of a set of structural identical cells made with a planar ITO anode (control device) and a PMMA/ITO grating electrode were analyzed. The grating incorporated organic solar cells resulted in a 13% increase in short circuit current density (Jsc) as compared to the planar device. The results reveal that over 60% of enhancement in Jsc is due to an enhanced spectral responsivity via the light scattering effect. Our experiment results agree with FDTD simulation in showing that the light coupled into the waveguide mode in the active layer is mainly responsible for the absorption enhancement in the devices. This was contributed by the PMMA/ITO grating electrode.

This paper studied the effect of different Surface Velocity Recombination rates (SVR) at both back and front contacts and electron, hole mobilities in the bulk of material on the IV curve of bilayer organic solar cells. A comprehensive charge transport model was developed to quantitatively study the role of each parameters in the organic and hybric organic-inorganic devices. SVRs and mobilities are used as sweep parameters for simulation to investigate their effect on the shape of illuminated IV in bilayer organic solar cell. These simulation results along with the experimental results from the relevant literatures will be utilized to understand the physics behind the illuminated-IV shape in the bilayer organic solar cells.

Organic semiconducting materials based on Ï?-extended linear-conjugated systems have been very intriguing and significant development has been achieved in these materials over the one decade. Accordingly, a number of researchers have attempted to synthesize Ï?-conjugated small molecules, dendrimers, oligomers, and polymers because of their strong potential applications to electronics and optoelectronics. Organic semiconductors have recently attracted much attention for applications in organic electronics such as organic field-effect transistors (OFETs), organic photovoltaic (OPV) cells, and nano-/meso-/microscopic object devices. In this work, we synthesized a new anthracene based Ï?-conjugated X-shaped molecules for use as soluble p-type organic semiconductors. The UV-visible absorption spectroscopy, PL spectroscopy, cyclic voltammetry, and thermal analyses were performed for understanding their fundamental physical properties. Eventually, the molecules were employed to fabricate thin film transistors, single-crystal FETs, and OPV. The device properties were investigated in detail.

Polymer solar cells (PSCs) are regarded as the next-generation solar cells, but their poor stability has been severely impeded their application. The stability can be significantly improved by adopting an inverted architecture. The inverted structure can give rise to the improvement in the photovoltaic efficiency as well. The major challenge in developing inverted PSCs is to effectively lower the work function of ITO. The methods to lower the work function of ITO strongly affect the photovoltaic performance. Here, we report a novel method to lower the work function of ITO by coating a thin layer of zwitterions. Inverted PSCs with zwitterion-modified ITO as cathode exhibit photovoltaic efficiency comparable to that with normal device architecture.

In recent years, research progress in organic photovoltaic (OPV) devices has been advanced tremendously, driven by the potential for low cost, large area and flexible devices. As an important member in the OPV family, the polymer:fullerene solar cells draw the most research interest due to the relatively high power conversion efficiency (PCE) achieved when compared to others. The nanoscale morphology in polymer:fullerene based photovoltaic devices is a key factor determining the overall device performance. The short exciton diffusion length in combination with the discrete and randomly distributed phases have caused significant charge recombination and thus lowered the efficiency. In our work, we demonstrate a nanoimprint process which allows the formation of ordered and continuously interdigitized morphology in active layers for both efficient charge separation and collection. Through the fabrication of imprinted poly(3-hexylthiophene) (P3HT):fullerene solar cells with different feature sizes, we comprehensively study the geometric effect of nanostructure on device performance which demonstrates that the PCE is highly dependent on the density and aspect ratio of nanostructures. Besides P3HT, similar work is extended to a low band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-bâ?²]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):fullerene solar cells to study the process window of nanoimprint for polymer solar cells.

Conjugated polymers have drawn lots of research interest as cost effective functional materials for organic electronic devices such as solar cells, light emitting diode and field effect transistors. In organic electronics, not only the overall crystallinity of the conjugated polymers is important, but also the conductivity for each direction is equally critical in determining the charge transport. It has been found that crystallization and chain alignment with controlled orientations can be realized in imprinted poly(3-hexylthiophene) (P3HT) nanostructures which lead to higher hole mobilities. Although this nanoiprint lithography induced chain alignment in P3HT has been widely observed, there are contradictory results reported in literature for the exact alignment direction within the imprinted P3HT nanostructures, which make it difficult for device structure design. One possible answer to these conflicting results might come from the different nanostructure geometries used in these reported works. Therefore to solve this issue, we have characterized imprinted P3HT nanostructures with different feature sizes using grazing incidence X-ray diffraction (GIXRD) and found that chain orientation is determined by the aspect ratio. The effect of residual layer thickness on chain alignment direction is also discussed.

The ability of metal oxides to selectively collect and block charge makes them ideal candidates for recombination layers in organic tandem solar cells. The tunnel junction layer, which electrically connects cells in the multijunction stack, fulfills multiple design parameters: (1) optical transparency, (2) â?ometallicâ? electrical contact of the quasi-Fermi levels of one cell to the next, and (3) efficient charge recombination of cell current to prevent voltage and power loss. Leakage between the mechanically connected front and back cells (i.e. inefficient recombination at the tunnel junction) results in a reduction in potential, and hence, power conversion efficiency. This study utilizes doped metal oxides (ZnO, NiO, MoO3) to tune the work function of tunnel junction layers for charge selectivity and efficient recombination at the interlayer, toward optimized multijunction organic photovoltaics.

The electronic properties of wet processed absorber molecules for organic solar cells are investigated in situ by synchrotron induced photoemission spectroscopy (SXPS). As a model system for a typical bulk heterojunction we used α,Ï?-Dihexylsexithiophene (α,Ï?-DH6T) as soluble and evaporable donor and Phenyl-C61-butyric acid methyl ester (PCBM) as typical acceptor system. The aim of the work is the comparison of the solution processed and the evaporated materials. The experiments were conducted at the SoLIAS (Solid Liquid Interface Analysis System) end station at the electron storage ring BESSY II in Berlin. This system offers the possibility to deposit thin films from solution under inert atmosphere and directly transfer the sample into the UHV analysis system. The films investigated were deposited from chlorobenzene solution on sputter-cleaned ITO substrates via drop-casting under clean Argon atmosphere. Firstly, both materials were characterized as pure films from solutions with a concentration of 2 mol/l. The influence of the solvent on the electronic properties was explored in more detail by comparison of α,Ï?-DH6T films prepared by both thermal evaporation and drop casting. In a second step a blend of PCBM and α,Ï?-DH6T was prepared. Therefore the solutions were mixed in a 1:1 ratio by volume prior to the drop casting. However, the obtained spectra of the composite film show a dominance of α,Ï?-DH6T. This could be caused by a segregation of the two materials during the drying process and thus leading to a α,Ï?-DH6T-rich surface layer similar to the thiophene-rich surface layer observed in P3HT-PCBM bulk heterojunctions.

The recent development of high performance OLEDs will be discussed. High quantum efficiencies can be obtained by using phosphorescent emitters such as iridium complexes. We synthesized a variety of wide gap, or high T1 level, materials for host and charge-transporting layers and succeeded to fabricate extremely high efficiency OLEDs. In particular, electron-transporting materials with pyridine side groups exhibited high electron mobility due to hydrogen bonding that controls the orientation of the molecules. High external quantum efficiency (QE) of 25â?"30 percent was achieved for blue, green and red OLEDs, which correspond to the internal QE of nearly 100 percent. By using multiphoton-emission (MPE) structures, a green phosphorescent OLED exhibited an internal QE of 220 percent. Recentry, we developed the MPE devices with two emitting units by spin-coating. The charge generation layers were composed of MoO3 which were vacuum-evaporated. The MPE device exhibited doubly higher EQE than that of conventional unstacked solution-processed single OLED device.

We present current status of OLED lighting devices developed at LG Chem. All of the devices were fabricated using combination of blue fluorescent and yellow phosphorescent materials to emit white color at high brightness level, 3,000 nit. We applied an internal light extraction structure, having a thin form factors, to the devices. High power efficiency up to 60 lm/W, T70 device lifetime over 10,000 hours at 3,000 nit were achieved using 10x10 cm2 one-pixel panels. Only commercially viable processed and cell structures were applied to these panels.

Reducing manufacturing costs of organic light-emitting diodes (OLEDs) is a challenging issue. A key to cost reduction is solution processing instead of vacuum techniques. To realize highly efficient multi-layered OLEDs from solution we introduce oxetane-moieties to our small molecule precursors. This allows for crosslinking e.g. via photo-initiated cationic ring-opening polymerization (CROP) yielding an insoluble network. Consequently, we are able to deposit complex multi-layer stacks to produce highly efficient OLEDs. Here, we introduce a new series of crosslinkable bipolar host materials for triplet emission based on well-known spirobifluorene derivatives. The spiro-configured carbon atom decouples the two pi-systems responsible for hole- or electron conduction both, spatially and electronically. This decreases intramolecular donor-acceptor interactions, yielding high triplet energy levels and high conductivity. Further, we establish novel crosslinkable Iridium-(III)-complexes for highly efficient green emitting OLEDs. Incorporating this host-guest system into a well balanced injection and charge-transporting layer stack consisting of hole- and electron-transporting layers, we are able to fabricate state-of-the-art OLEDs entirely from solution.

Organic electronics is receiving much attention as an alternative cost-effective technology compared to the traditional inorganic ones. Especially research activities concerning organic light-emitting diodes (OLEDs) are enforced in university groups as well as industry since they are promising candidates for next generation displays and solid-state lighting. To achieve high brightness and efficiency at low current density so-called Tandem-OLEDs (vertically stacked devices with two or more electroluminescent units connected in series via a charge generation layer (CGL)) have been introduced about ten years ago. Mostly these Tandem-OLEDs have been realised by evaporation of organic materials. Only few publications report on solution-processed electroluminescent units since depositing further materials can lead to the washing-up of already deposited ones. However, though various device structures have been investigated, only current efficiency enhancements could be shown since the operating voltages of the Tandem OLEDs are still high than their respective single unit references. Here, we present solution-processed polymeric Tandem-OLEDs (Tandem-PLEDs) realized by thermal "crosslinking" of the OLED materials. By optimizing the CGL, the current of the Tandem-PLEDs could be fully recovered with respect to the single-unit device. Therefore, the two-unit devices show high current and power efficiencies at the same time. Compared to the single-unit device an enhancement of almost 90% in current efficiency and 70% in power efficiency was obtained for singlet-emitting Tandem-PLEDs with yellow emission (max. efficiencies: 17 Cd/A and 10 lm/W). Currently, we are extending this approach to white-emitting PLEDs.

The application of graphene sheets as transparent anode is limited due to its relatively low work function (WF) (~ 4.4eV) and conductivity compared with that of conventionally used ITO anode (~4.7 â?¤ WF â?¤ 4.9 eV and 10 ohm/sq) in optoelectronic devices. The relatively low work function of graphene makes the hole injection unfavorable because of high injection barrier at interface between anode and organic layers in organic light-emitting diodes (OLEDs). We modified graphene anode via p-doping with HNO3 and AuCl3 to improve conductivity of graphene sheets and our polymeric hole injection layer (GraHIL) which has gradient work function to makes holes be injected easily to the organic layer despite high injection barrier for holes at interface between graphene anode and organic layer. In this paper, we fabricated flexible fluorescent and phosphorescent OLEDs by modifying the graphene anode to have a high work function and high conductivity, and thus achieved extremely high luminous efficiency (37.2 lm/W in fluorescent OLEDs, 102 lm/W in phodphorescent OLEDs), which is significantly higher than that of the conventionally optimized device using an ITO anode (24.1 lm/W in fluorescent OLEDs, 85.6 lm/W in phosphorescent OLEDs). We also demonstrated that when our GraHIL is introduced on top of graphene anode, the nearly ohmic contact is formed between graphene anode and organic layers by using Dark Injection Space Charge Limited Current (DI SCLC) transient measurement. Futhermore, DI SCLC measurement exhibhits that the hole injection efficiency of GraHIL with graphene anode is higher than that with ITO anode by avoidance of metal diffusion from ITO into the organic layers. Finally, we fabricated flexible white OLEDs lighting devices using the modified graphene anode.

Introducing DBR microcavity into OLEDs is an effective way to enhance light out-coupling and color purity. OLEDs show better efficiency with double emitting layers. By combining the DBR and double emitting structure, we achieved very high efficient green organic light emitting diodes (OLEDs). The noncavity devices containing double emitting layers with a green phosphorescent emitter, fac-tris(2-phenylpyridine) iridium [Ir(ppy)3], doped in 4,4â?T,4â?-tris(N-carbazolyl)-triphenyl triphenyl amine (TCTA) and 4,4â?T-(N-carbazolyl)-biphenyl (CBP), respectively, show a high efficiency as 93cd/A. An enhancement by a factor of 2.4 is achieved with DBR microcavity, resulting in 225 cd/A in current efficiency. We found that such a high enhancement of the current efficiency in the microcavity device comes from two parts. While the number of photon outcoupled into air was partially increased by 30% with the microcavity effect, the major enhancement was caused by the better match of the emission wavelength to the highest eyeâ?Ts sensitivity in the photopic vision function corresponding to 550 nm. Finally, with a hemisphere lens, additional improvement by a factor of 2.4 in the normal direction was achieved due to extraction of substrate mode. The total enhancement of our devices in the normal direction is calculated by a factor of 5.7.

The development of optoelectronic materials satisfying high electrochemical and thermal stability is urgent for commercial application of blue phosphorescent organic light emitting devices (PHOLEDs). We have developed a series of new types inorganic/organic hybrid materials which meet the specification for the realization of blue PHOLEDs. As part of our ongoing research effort, we have turned our attention to three-dimensional aromatic molecules, namely, cluster carboranes and found that depending on geometry of carboranes highly desirable optophysical properties for blue PHOLEDs were identified. Thus, we present our recent results in which carborane clusters not only function as organic linker but also facilitate otherwise unattainable only with organic entities, thermal and electrochemical stabilities. The first rationale development of 3-D network molecules interweaving carboranes with functional organic units has been demonstrated, and their application to blue PHOLEDs has also been achieved. In addition, we found that carborane clusters effectively localized triplet wave-function in each functional organic unit attached and delocalized charges across entire molecular network, resulting in high triplet energy and high charge mobility. Most of all, when new carboranes were engaged as HTL and EML materials, they reached to high T1 of 2.9, 3.0 eV with enhanced hole mobility of around 10-3 cm2/Vs bearing high Tg of 120 °C, 140 °C, respectively. Optimized devices showed EQE of 19% with deep blue color coordinates of (0.15, 0.23) and enhanced device lifetime. Detailed analysis on the photophysical property of each 3-D network structure produced by carboranes (o-, m-, and p-carboranes) will be elaborated by DFT calculations.

In recent years, researches for tandem organic light-emitting diodes (OLEDs) have been widely performed for providing high luminance with enhanced current efficiency or for controlling emission spectra with units emitting different colors [1,2]. Despite the importance of charge generation units (CGUs) for operating tandem OLEDs, there are only a few in-depth studies on the CGUs in tandem OLEDs [3-5]. In this presentation, we report a series of studies on charge generation in CGUs and demonstrate efficient green phosphorescent tandem OLEDs with reduced efficiency roll-off. For the charge generation study, we have fabricated inverted diodes with CGUs and a control inverted diode without any CGU, and analyzed their diode I-V characteristics. The inverted diodes had the following structure: ITO/tris[3-(3-pyridyl)-mesityl]borane (3TPYMB)/CGU/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)/Al. The CGU consisted of Cs2CO3 doped bathophenanthroline (BPhen)/Al/MoO3 or 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN). Drastic increases in current densities of around 10 mA/cm2 at a forward bias of 10 V for the devices with CGUs were observed, while that for the control device remained less than 10-4 mA/cm2 at the same voltage. The device with HAT-CN electron accepting layer (EAL) showed a sharp increase in current density than that with MoO3. The mechanism of current in the devices with CGUs is proposed by the charge generation at the interface between an EAL and a hole transporting layer (HTL), followed by the tunneling of electrons through the depletion region in the n-doped BPhen layer. Based on the charge generation study, we have fabricated Ir(ppy)3-based green phosphorescent tandem OLEDs with the CGUs. We employed TAPC as the main HTL for obtaining high efficiency and N,Nâ?T-bis(naphthalen-1-yl)-N,Nâ?T-bis(phenyl)-benzidine (NPB) as the charge generating HTL for making well operating charge generating interface. We achieved a highly efficient green tandem OLED with the current density of 128 cd/A at 0.1 mA/cm2 (or at 128 cd/m2) and 120 cd/A at 1 mA/cm2 (or at 1200 cd/m2). References [1] T. Matsumoto, T. Nakada, J. Endo, K. Mori, N. Kavamura, A. Yokoi and J. Kido, SID Symp. Digest Tech. Papers 34 (2003) 979. [2] L. S. Liao, K. P. Klubek and C. W. Tang, Appl. Phys. Lett. 84 (2004) 167. [3] M. K. Fung, K. M. Lau, S. L. Lai, C. W. Law, M. Y. Chan, C. S. Lee and S. T. Lee, J. Appl. Phys. 104 (2008) 034509. [4] X. D. Gao, J. Zhou, Z. T. Xie, B. F. Ding, Y. C. Qian, X. M. Ding and X. Y. Hou, Appl. Phys. Lett. 93 (2008) 083304. [5] X. Qi, N. Li and S. R. Forrest, J. Appl. Phys. 107 (2010) 014514.

Polymer light emitting electrochemical cells (LECs) have attracted much attention due to their low voltage operation and insensitivity to electrode materials. Based on the planar LEC structure, we have developed a new device configuration, the light emitting electrochemical transistor (LECT), in which the location of the light emitting p-n junction and the current level can be controlled via a gate terminal. Specifically, an electrochemically active conducting polymer is employed as gate material to modulate the doping profile within the light emitting polymer, and thereby control the position of the light emitting p-n junction. In this configuration, we are able to move the light emitting p-n junction back and forth within an interelectrode gap of 500 µm under a voltage bias lower than 5 volt. Additionally, the LECT exhibits current modulation capabilities, as evidenced by measured transfer curves. The channel current on/off ratio, estimated for gate voltages of 0 V and 4 V, ranges from 10 to 100 and the transconductance value beyond the gate threshold voltage (-2.3 V) is found to be in the range from 1.5 to 3 μS. This device provides a new method to study the fundamental physics of the LECs and opens a new avenue towards tuneable light sources and addressable circuits.

The development of emissive materials for use in organic light emitting diode displays has been the focus of significant research in recent years. As the market for portable, battery driven devices and green products has matured, increasing emphasis has been placed on energy efficiency. The development of phosphorescent emitters with narrow emission spectra is vital to this goal. Narrowly emitting phosphors have the ability to achieve 100% internal quantum efficiency while delivering maximum light output in the desired range. While efficient phosphors based on platinum and iridium have been developed, their emission spectra are often broad, which results in reduced power conversion efficiency as only a portion of the emitted light is useful for display purposes. In this presentation, we will discuss our recent work developing a new class of efficient platinum based emitters with full width half max values at room temperature in the range of 30 nm. The photophysical, electrochemical, and electroluminescent properties of this novel series of compounds will be discussed.

We will present a few methods to improve the efficiency and stability in small molecule based organic solar cells, including the formation of bulk heterojunctions (BHJs) through alternative thermal deposition (ATD), the use of a micro-cavity structure and interface modifications. By ATD which is a simple modification of conventional thermal evaporation, the thicknesses of alternative donor and acceptor layers were precisely controlled down to 0.1 nm, which is critical to form BHJs. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C60) systems was confirmed by AFM, GISAXS and absorption measurements. From analysis of the data, we found that the CuPc|C60 films fabricated by ATD were composed of the nanometer sized disk shaped CuPc nano grains and aggregated C60, which explains the phase separation of CuPc and C60. On the other hand, the co-deposited CuPc:C60 films did not show the existence of separated CuPc nano grains in the CuPc:C60 matrix. The OPV cells fabricated using the ATD method showed significantly enhanced power conversion efficiency compared to the co-deposited OPV cells under a same composition. We will also present by numerical simulation that adoption of microcavity structure in the planar heterojunction can improve the short circuit current in single and tandem OSCs. Interface modifications also allowed us to achieve high efficiency and high stability OSCs.

Window integrated photovoltaics for automotive and building applications are a promising market segment for organic solar modules. Besides semi-transparency, window integrated applications require a reasonable transparency color perception. These boundary conditions are well matched by a blend of PSBTBT:[70]PCBM with an extended absorption to the infrared. We therefore fabricated and investigated highly efficient (η â?^ 3%), semi-transparent PSBTBT:[70]PCBM solar cells under different illumination scenarios such as bright sunlight or cloudy skies. However, a reasonable transparency perception does not necessarily imply good color rendering. The color rendering of items under illumination of transmitted light is of utmost importance when integrating semi-transparent photovoltaic devices in windows or overhead glazing. For example, (transmitted) white light that mainly contains blue and yellow parts of the visible spectrum will render white surfaces acceptably but is incapable of rendering reddish objects. The transmitted light through PSBTBT:[70]PCBM solar cells exhibits remarkable color rendering properties making this polymer/fullerene blend and the respective solar cells suitable for real-life window applications. Unfortunately, most solar cells with efficient polymer/fullerene blends such as the reddish PCDTBT:[70]PCBM, neither appear color neutral nor exhibit good color rendering properties. In this work we successfully balanced the transparency color perception and color rendering properties by incorporating complementary absorbing dyes into the device. In light of roll-to-roll mass production processes for solution deposited solar cells we investigated two concepts: integration of dye doped PMMA layers into the device architecture and blending of the dyes into a highly conductive PEDOT:PSS anode. As a result, the transparency color perception of the modified devices equaled daylight and the color rendering index (CRI) approached unity while we observed only very little effect on the overall device power conversion efficiency. Another way of affecting the solar cellâ?Ts color is the systematic fine tuning of thin film interferences within the device. We deliberately simulated the influence of the cathode TCO and other layers thicknesses on the light interference pattern and hence the light absorption and reflection utilizing optical parameters that were obtained in-house. By carefully selecting the electrode thickness, outstanding color perception and rendering properties can be realized for a variety of absorber materials. These techniques allow for choosing the absorber blend independently of its natural color perception so that the entire range of highly efficient organic absorber materials is available for the fabrication of color neutral, semi-transparent photovoltaic devices. [Ref.: A. Colsmann et al., Adv. Energy Mater, 1 (2011) 599]

It is generally considered that photoinduced charge transfer in hybrid photovoltaic devices immediately results in a pair of free charge carriers due to the increased screening of electron-hole interaction by the higher dielectric constant of inorganic materials. This idea inspired extensive research and development of hybrid organic-inorganic devices where, upon illumination, excitons are created in the highly-absorbing organic material and efficiently dissociated at the interface with the inorganic counterpart of the device. However, these promising hybrid photovoltaic devices generally show very moderate performance. One possible reason for this could be the existence of bound charge pairs (BCP) at the organic-inorganic interfaces. A BCP state comprises of a positive charge localized on an organic molecule (polaron) Coulombically-attracted to an electron in the inorganic material. We employed a novel interface-selective ultrafast optoelectronic technique and observe the existence of BCP states at the organic-inorganic interface formed between electron-accepting zinc oxide and conjugated polymers such as poly(3-hexylthiophene) (P3HT) or poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2â?T,2â?Tâ?T-diyl) (F8TBT). We have developed a state model and utilized a genetic algorithm to quantify the yield of BCP states. In the case of the P3HT/ZnO and F8TBT/ZnO devices, we find that 52% and 54% of photo-induced charges remain in BCP states, respectively, which substantially hinders the device efficiency.[1] We have previously reported that organic interface modification can result in a three-fold increase the external quantum efficiency of hybrid photovoltaic devices.[2] Using the newly developed ultrafast technique and state model allows us to elucidate the physical processes responsible for this remarkable improvement. We find that the interfacial modification improves the light harvesting and decreases the number of bound charges to 28% and 24% in P3HT and F8TBT devices respectively, accounting for the observed 300% enhancement of the device efficiency. To summarize, the efficiency of exciton harvesting and disassociation to free charge carriers strongly depends on the properties of the interface, and can be dramatically enhanced by an organic monolayer modification of the inorganic surface. Our results are of fundamental importance for understanding of the photo-physical processes taking place in organic-inorganic hybrid photovoltaic cells. References: [1] Y. Vaynzof, A. A. Bakulin, S. Gelinas, H. Sirringhaus and R. H. Friend, â?oDirect Observation of Photoinduced Bound Charge-Pair States at Organic-Inorganic Semiconductor Interfaceâ?o, submitted to Phys. Rev. Lett. [2] Y. Vaynzof, D. Kabra, L. Zhao, P. K. H. Ho, A. T-S. Wee and R. H. Friend, â?oImproved Photoinduced Charge Carriers Separation in Organic-Inorganic Hybrid Photovoltaic Devicesâ?, Appl. Phys. Lett. 97, 033309 (2010).

Despite significant study by many research groups, the efficiency-limiting processes that govern the performance of bulk heterojunction photovoltaic devices still remain ambiguous. In particular the role of interfacial charge-transfer (CT) states in determining the bias dependence of the photocurrent is debated. Understanding the bias dependence of the photocurrent is necessary to optimize device fill factors, and therefore resolution of this debate is important. In this contribution we directly determine the field dependence of bimolecular recombination at the charge densities relevant to photovoltaic devices using a novel optical pump electrical probe experiment that enables the ultra fast time resolution of optical spectroscopy to be combined with high signal to noise achievable in current measurements.[1] We relate these results to temperature dependent transient absorption (pump-probe) spectroscopy over the entire relevant timescale from femtoseconds to milliseconds and in a broad spectral range covering 500 to 2000 nm. [2,3] In general the results demonstrate the importance of i.) ultrafast free carrier generation, ii.) suppression of interfacial CT state formation, and especially iii.) high and balanced charge carrier mobilities to achieve good fill factors and high power conversion efficiencies. [1] R. Mauer, I.A. Howard, F. Laquai, J. Phys. Chem. Lett. 2011, 2 (14), 1736-1741. [2] I.A. Howard, R. Mauer, M. Meister, F. Laquai, J. Am. Chem. Soc. 2010, 132 (42), 14866-14876. [3] R. Mauer, I.A. Howard, F. Laquai, J. Phys. Chem. Lett. 2010, 1 (24), 3500-3505.

To enhance photoconversion efficiency (PCE) of organic photovoltaic (OPV) cells it is important to optimize the photon harvesting provided the absorption band of the materials used is rather narrow. In addition, OPV devices suffer from a short exciton diffusion length and a low mobility of charged carriers which prevent the use of thick materials for a more effective photon absorption [1]. From an optical perspective, such organic solar cells can be seen as a stack of several layers with different refractive index. At each interface between any of these layers there is a significant reflection that prevents the most effective photon harvesting. If such reflection could be reduced minimized, the external quantum efficiency (EQE), defined as the ratio between current and incoming photons, would approach the internal one, which is the quotient between current and absorbed photons. In the current work we show experimentally that for an inverted P3HT:PCBM cell it is possible to find a combination of layer thicknesses such that optical interference leads to an EQE that amounts to 91% of the internal quantum efficiency (IQE). We observe that the total reflectivity is close to its minimum possible value in a wavelength range of more than 100 nm. In that range the EQE closely matches the IQE. In a model developed to determine the role played by optical interference in the light harvesting efficiency we confirmed that the EQE can closely match the IQE for that wavelength range. Additionally we observed that a similar cell with an active material 1.7 times thicker exhibited a lower PCE. The poor photon harvesting in the later cell configuration is confirmed by an EQE that amounts only to 72% of the IQE. Such optical interference control of the EQE may enable fabrication of devices with active layers thinner than the absorption length, which has been considered as the optimal solution to prevent problems in the charge transport pathways or cul-de-sacs in OPV devices [1]. [1]. K.S. Nalwa, J.-M. Park, K.-M. Ho, S. Chaudhary. Adv. Mater. 2011, 23, 112â?"116. [2]. B.V. Andersson, D.M. Huang, A.J. Moulé, O. Inganäs. Appl. Phys. Lett. 2009, 94, 043302.

Dye-sensitized solar cells consist of transparent conductive oxide glasses (TCO-glass). TCO-glasses are expensive now. We have focused on structures and fabrication process of transparent conductive oxide less DSC (TCO-less DSC). We prepared nano-composite films consisting of a protected metal mesh/porous titania/dye. By employing the nano-composite films, TCO-less flexible DSCs with 6.1% efficiency (F-1), TCO-less cylinder DSC with 5.6% efficiency (F-2), TCO-less fiber DSC with 2.4 % efficiency (F-3) were fabricated. A new flexible TCO-less cylinder DSC with 4.1 % efficiency (F-4) is also reported. In addition, TCO-less iodine free DSCs consisting of Co2+/Co3+ redox species with 3.5 % efficiency are discussed from the view point of high open circuit voltage (Voc) and low damage toward metal electrodes. Hybrid and tandem cells are candidates for high efficiency solar cells since they can expect to harvest light in near infrared (NIR) and infrared (IR) regions. Hybrid cells (F-5) were fabricated by pressing two nano-composite films (top and bottom electrodes). Tandem cells (F-6) consisting of porous titania/dye top electrode and porous SnO2/dye bottom electrode are expected to harvest light in infrared (IR) region, where developments of high efficiency bottom SnO2/dye electrodes are essential. A problem is low ff values for DSCs consisting of SnO2 nanoporous electrodes. The reason of the low efficiency of SnO2 based solar cell was discussed by using hybrid cell structures. It was concluded that serious charge recombination occurs through dyes, not SnO2 itself. F-5 and F-6 were proved to work properly as tandem and hybrid cells by using model dyes. Near IR and IR dyes for titania and SnO2 electrodes are needed for completing the development of these tandem and hybrid solar cells.

We present polymer solar cells with a photoactive layer based on PCDTBT:PCBM exhibiting lifetimes of more than 1000 hours under continuous illumination and periodic characterization. The accelerated aging was performed under approximately AM1.5 conditions using a metal halide lamp exhibiting considerable amount of UV-radiation. The samples were kept at about 50°C and were characterized by IV-measurements every 30 minutes. Furthermore luminescence imaging and lock-in thermography were applied to yield conclusive information about the degradation mechanisms. For improved understanding of the optimal design of the layer architecture, the top contact electrode material was varied, and some solar cells were kept unsealed.

We use photoluminescence quenching measurements to examine the exciton diffusion length (LD) of the electron donor boron suphthalocyanine chloride (SubPc) and its dependence on dilution in a wide-energy gap host matrix. By diluting SubPc, both the exciton lifetime and photoluminescence efficiency are increased, leading to an increase in the self-Forster radius for energy transfer. This increase in the self-Forster radius leads to a doubling in the exciton diffusion length of SubPc from a value of LD=7.7 nm in neat film to a value of LD=15.6 nm in dilute films containing 25 wt.% SubPc. The increased diffusion length leads to a significant improvement in the short-circuit current density and power conversion efficiency (ηP) of planar heterojunction organic photovoltaic cells (OPVs) based on the donor-acceptor pairing of SubPc and C60. Since LD is often much shorter than the optical absorption length, the performance of planar heterojunction OPVs can be limited by inefficient exciton diffusion. Here, we overcome this limitation and demonstrate a high power conversion efficiency of ηP=(4.2±0.1)% for planar heterojunction OPVs containing a dilute SubPc donor layer, compared to ηP=(3.0±0.2)% for devices with a neat donor layer. Interestingly, the performance of the dilute-donor planar OPV rivals that of optimized mixed and graded heterojunction OPVs. This suggests that with improved control over exciton diffusion in the donor, high ηP can be realized in architectures that are often considered exciton-diffusion limited.

The best polymer-fullerene solar cells have open circuit voltages which are ~0.9V less than the optical bandgap of the polymer. In comparison, inorganic solar cells can produce voltages within 0.4V of their bandgap. Much of the voltage loss in organic solar cells can be attributed to the offset in band energy levels between the donor and acceptor materials required for efficient exciton splitting into free charges. However, it is not clear what minimum offset in energy levels is required for efficient charge generation. It has been proposed that recombination to triplet states can compete with charge generation when the energy of the triplet states lies below that of the charge separated state, which occurs when the offset in band energy levels is made too small. Triplet recombination has been shown to be significant in several polymer-polymer blends and blends of large bandgap polyfluorene copolymers with PCBM. We perform a thorough electronic and spectroscopic study of several high efficiency copolymer blends with bis-indene C60 (ICBA) which produce voltages ~0.7V less than the polymer bandgap. We find from light intensity dependent internal quantum efficiency measurements that geminate recombination limits the performance of these devices. From photoinduced absorption measurements, we conclude that the yield of triplets is not sufficient to explain the loss in photocurrent and that there is a kinetic competition between charge formation and other recombination channels.

Organic electronic devices have been attracting much attention due to several advantages such as cost-efficient large-area device fabrication, limitless materials variety, flexibility, and wide range of device applications.[1] For such device applications, choosing appropriate contact electrode material is significantly important.[2,3] In particular, finding a suitable electrode material for efficient charge injection to organic active layer and also tuning the interface in-between them for better injection have been considered as key issues for improved device performance. Recently, graphene has received great attention as a promising transparent and flexible conducting electrode for organic and inorganic devices. In this presentation, we report on patterned graphene electrode for electrical and optical devices applications.[4-6] First, an efficient charge injection in pentacene organic transistors induced by the use of graphene electrodes was investigated. Through channel length-dependent, temperature-variable electrical characterizations, and Kelvin probe measurements, we found that favorable orientation of interface dipole layer occurred between graphene electrodes and pentacene organic semiconductor for consequential efficient injection of charges.[4] Secondly, the compatibility of graphene electrodes with the well-known substrate treatment techniques for improving the performance of pentacene transistors were systemically investigated. Reduction of channel resistance and charge injection barrier height were observed in terms of various surface treatments such as self-assembled monolayer coating and thermal annealing.[5] Lastly, doping graphene electrodes for high performance organic solar cells was researched. By doping graphene films which possess mechanical flexibility, we fabricated efficient flexible organic solar cells with graphene electrodes.[6] The results of these studies suggest that the use of flexible graphene electrodes will provide significant enhancement for various types of organic devices. References [1] S. R. Forrest., Nature 2004, 428, 911. [2] C.-a. Di et al., Adv. Mater. 2008, 20, 3289. [3] S. Pang et al., Adv. Mater. 2009, 21, 3488. [4] Lee et al., Adv. Mater. 2011, 23, 100. [5] Lee et al., Appl. Phys. Lett. 2011, 99, 083306. [6] Lee et al., submitted. Acknowledgements: The National Research Laboratory Program, the National Core Research Center grant, and the World Class University program by the Korean Ministry of Education, Science and Technology.

Co-host phosphorescent OLEDs are made when the hole transport layer (HTL) and electron transport layer (ETL) are blended together with the phosphorescent emitter to form the emissive layer (EML). The advantage of the co-host EML are that there are no real interface at the start of the EML because it is made from the same materials as the HTL and ETL and the charge balance in the EML can be tuned by varying the co-host EML content. Exciton formation in a co-host EML should form via charge trapping on the phosphorescent emitter. In this work, we present a study that explores the diffusion of triplet excitons in co-host EMLs. We show that the degree to which a triplet exciton is quenched by a low triplet energy transport material is dependent upon the location of the emission zone in the channel. We compare co-host EML results to results for a hole dominated, high triplet energy host material. Finally we show that the distance required to minimize quenching by a low triplet energy transport layer is between 50 and 100 Ã..

With the surge of Organic LEDs in displays and lighting, there has been a need for an ultrathin encapsulating chemical and electrical barrier layer for their employment in flexible substrates. The encapsulating layer needs to be free of pinhole defects and must be able to encapsulate particles. Current OLED encapsulation techniques are either rigid or too thick for flexible substrates, or are made of costly multilayers. A hybrid of SiO2 and silicone polymer that is made by plasma-enhanced chemical vapor deposition in the parallel-plate geometry overcomes these difficulties. Films are deposited on substrates held at room temperature from the source gases hexamethyl disiloxane (HMDSO) and oxygen. The resulting hybrid is an excellent barrier against the permeation of atmospheric gases and is an equally excellent electrical insulator. We have studied how the deposition conditions affect the growth of the hybrid layer on substrates with trenches, or on substrates mounted at an angle to or an offset from the substrate holder. Depositions were conducted on silicon wafers with microtrenches or with exposed undersides. First, microtrenches were fabricated with an inductively coupled plasma deep etcher; hybrid films were deposited on this structure. The thickness of the film on the side wall of the trenches in function of distance from the top edge was measured for different trench widths and depths. Second, a silicon wafer was mounted perpendicularly to the substrate electrode. The thickness of the film deposited on the face of the silicon wafer was measured in function of the distance from the edge of the wafer. Third, two silicon wafers were sandwiched back to back and mounted at an offset to the substrate electrode to expose the underside. The thickness of the film deposited on the bottom face, which is in the shadow of the plasma, was measured as a function of the distance from the edge. These three thickness measurements enable us to characterize film growth as a function of the plasma parameters. We will report these dependences and our interpretation of the film growth mechanism.

A simple printing method was developed to replace indium-tin oxide (ITO) for the fabrication of inverted organic light emitting diodes (OLED). Using metal grid structure, which was formed by the surface-mediated fine line inkjet printing process, hybrid transparent electrodes with tunable work function and transparency were fabricated. On the glass substrate with hydrophobic surface treatment, inkjet printing of silver metal grid with a scale of 8-20um width was performed using Dimatix piezoelectric printhead or electrohydrodynamic (EHD) meniscus. Jetting profiles and surface energy condition of substrates were precisely controlled for the optimum smooth-line patterning and desired pitch-to-pitch separation for optimum transparency and conductivity. By the combination of silver grid geometry, thin hole-transporting polymer (Clevios PH500), and semiconducting tungsten oxide (WO3), transparency in the range of 78-83% at 550nm was achieved (with 93% glass transmittance), resulting in the comparable i-V-L characteristics of OLED (22-23 cd/A) to those with ITO-based one (25 cd/A for green device). Silver grid/solution-processible zinc oxide (ZnO) hybrid electrode also yielded the efficient inverted bottom emission device, while the inverted top emitting structure with this hybrid electrode (Ag/ZnO and n-doped layer/phosphorescent emitters/p-doped and WO3/Ag grid/ capping layer) provided a feasible way for the fabrication of flexible and large area OLED compatible with n-type oxide transistor array. Unlike the imprinting, transfer patterning, or selective surface modification, inkjet printing on the substrate with controlled surface energy is advantageous since the additional processes such as mask or mold patterning are not required. The value of parameters for the interfacial properties of hole- and electron transport layers (energy level, roughness, and conductivity) were clearly demonstrated and analyzed in comparison with the device performance of inverted bottom and top-emitting OLEDs.

The recombination of electrically generated excitons with trapped interfacial charges is a well known phenomenon that occurs in organic light emitting devices (OLEDs).^1-3 Here we demonstrate that this effect can be used to externally influence the current density by varying exciton lifetime in an archetypal bi-layer OLED. Specifically, by using an external capping layer to modify the optical microcavity modes present in the OLED, we show that the exciton lifetime can be modulated, causing the emitter decay rate and current density to vary proportionately and measurably. We ascribe this effect to interfacial exciton-polaron quenching that generates a back-diffusing hole current through the hole transport layer. These findings have potential applications in a new class of electro-optical sensors for, e.g., monitoring nanoscale refractive index variation or molecular detection.

¹M. Pope and C. Swenberg, Electronic Processes in Organic Crystals (Oxford University Press, Oxford, 1982).

²M.A. Baldo et al., Physical Review B.66, 035321 (2002).

³Ichikawa et al., Japanese Journal of Applied Physics40, L1068 (2001).

Multicolor microcavity (microC) organic light-emitting diode (OLED) arrays were fabricated simply by controlling the hole injection and spacer MoO3 layer thickness. The normal emission was tunable from ~490 to 640 nm and can be further expanded. A compact, integrated spectrometer with two-dimensional combinatorial arrays of microC OLEDs was realized. Introducing MoO3 yields more efficient and stable devices, and reduces device breakdown. The pixel current density reaches ~4 A/cm2, resulting in a maximal normal brightness Lnorm,max ~ 140,000 Cd/m^2, which is important for photoluminescence-based sensing and absorption measurements. The results also illustrate a new breakdown mechanism in OLEDs.

We demonstrate very high efficiency (up to 60 lm/W and 69 Cd/A without any outcoupling enhancement, which can double with a high-quality microlens array) spin-coated small molecule OLEDs based on green-emitting tris[2-(p-tolyl)pyridine] iridium(III) (Ir(mppy)3)-doped 4,4'-bis(9-carbazolyl)-biphenyl (CBP) blended with electron- and hole-transporting molecules. An additional electron-transporting/hole-blocking 4,7-diphenyl-1,10-phenanthroline (BPhen) layer is thermally evaporated on the active layer, followed by the LiF/Al cathode.

Efficient ITO-free small molecule organic light-emitting diodes (SMOLEDs) with multilayered highly conductive poly(3,4-ethylenedioxy thiophene): poly(styrenesulfonate) (PEDOT:PSS) as the anode are demonstrated. SMOLEDs with the structure PEDOT:PSS/MoO3/N,Nâ?T-diphenyl- N,Nâ?T-bis(1-naphthylphenyl)-1,1â?T-biphenyl-4,4â?T-diamine (NPD)/tris(8- hydroxyquinoline) Al (Alq3)/ 4,7-diphenyl-1,10-phenanthroline (BPhen)/LiF/Al exhibited a peak power efficiency of 3.82 lm/W, which is 81% higher than that of similar ITO-based SMOLEDs (2.11 lm/W). The increase in the device performance is believed to be due to the advantageous higher workfunction, lower refractive index and decreased surface roughness of PEDOT:PSS as compared to ITO, as well as to Ohmic hole injection from the PEDOT:PSS to the NPD layer via the MoO3 interlayer. The results demonstrate that a polymeric anode has the potential to substitute ITO in OLEDs with strongly improved device performance.

Organic optoelectronics such as organic light emitting diodes (OLEDs), organic sensors, and organic photovoltaics (OPVs) cells have advantages such as low cost, flexible structures, and potential for easier processability. Many of these devices incorporate several components; this is especially the case in OLEDs and OPVs in which a transparent electrode is used as a bottom or a top electrode. Thus, a highly-transparent and conducting electrode materials are required to reduce operation voltage and to increase internal/external quantum efficiencies. Expecially, in order to maximize the performance of devices, the optical properties of transparent electrode should be adjusted and optimized for each system. In this work, we demonstrate a novel way of tuning the optical transmittance of transparent electrode using dielectric/metal/dielectric (DMD) structure. DMD multilayer has an advantage in that it can fulfill the optimum â?~zero-reflectionâ?T condition with metal film, realizing improved transparency characteristic. We found that the refractive index of outer dielectric can tune the zero-reflection condition for specific wavelength, and the wavelength for maximum transmittance of DMD becomes longer as the refractive index of outer dielectric is increased. We measured and simulated how optical transmittance of DMD films is affected by the refractive indices of several kinds of metal oxide materials varies from 1.9 to 2.5. (WO3, ZnS and TiO2). Ag is used as metal layer because it has good electrical conductivity and the lowest refractive index among metals resulting low absorption. WO3 is employed for inner dielectric due to large work function (~ 6.2 eV) enhancing hole injection properties. It is calculated that the wavelength for maximum transmittance of D/Ag/WO3 can be tuned from 480 nm to 620nm by increasing refractive index of D from 1.9 to 2.5. It is well matched with measured transmittance of DMD using WO3, ZnS and TiO2.We fabricated the Ag films sandwiched with outer dielectric layer with different refractive index and WO3, then fabricated and characterized three kinds of OLEDs emitting red(Ir(piq)3 doped CBP), green(C545T doped Alq3) and blue (Firpic doped TCTA) on these materials.

Organic light emitting diodes (OLEDs) are promising for flexible displays because they offer several advantages such as the ease processing, light weight, bright self-emission, and wide viewing angle over liquid crystal displays (LCDs). Indium tin oxide (ITO) is the most extensively used metallization in OLEDs because it offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity. But ITO is hard to adapting the flexible display because ITO has a rigid and brittle property. Therefore, we developed Au single layer for the anode in OLEDs to replace ITO electrodes, and used MoO₃ thin layer to replace hole injection layer (HIL, 2-TNATA). This resulted in Au/MoO₃/NPB/Alq₃:C545T/Alq₃/LIF/Al structures, which yielded the lower turn-on voltage (3V) and higher luminescence intensity (14600 cd/m² with 150 mA/cm² at 7V) than those of ITO-based OLEDs (ITO/2-TNATA/NPB/Alq₃:C545T/Alq₃/LIF/Al) showing turn-on voltage (3.5V) and luminance intensity (3413 cd/m² with 23 mA/cm² at 7V). Transparency and sheet resistance of gold thin films varied with the film thickness; for example, â^¼60% transparency and 8â^¼10 â"¦/sq of sheet resistance for 15nm-thick Au film. The high work-function (5.1â^¼5.3 eV) and low sheet resistance (8â^¼10 â"¦/sq) of gold film allowed for low turn-on voltage and low driving voltage in OLEDs, leading the higher current at the same voltage compared with that of ITO electrode OLEDs. However Au anodes have difficulties with hole-transferring from HIL to emitting layer (EML) possibly due to the metal-polymer interface effects. The replacement of HIL with MoO₃ significantly improved the electrical and optical performances due to the band alignment at the interface. *4,4',4"-Tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine(2-TNATA) *N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB) *Tris(8-hydroxy-quinolinato)aluminium( Alq₃) *2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino [9,9a,1gh]coumarin(C545T)

Very recently many research groups have reported electrical performance changes of organic field effect transistors (OFETs) exposed to various chemical compounds such as humidity, polar solvents, and explosives. Most OFET devices showed decreased current output and mobility after exposure the chemical compounds. The degradation of the electrical performance by exposure to those compounds was affected by reduced charge transport which was caused by charge trapping at grain boundaries. Here we fabricated polythiophene OFETs with or without electron rich material. When the pure polythiophene devices were exposed to nitroaromatic solutions, the current of them was significantly decreased. In contrast, the OFET devices with blend polythiophene film with small amount of electron rich material showed clearly increased current after exposure to the nitroaromatic solutions, which was caused by changed energy level of the complex of electron donor and acceptor. Especially, as the devices were exposed to more dilute nitroaromatic solutions, the relative current increase was much bigger.

We have synthesized high triplet energy and high glass transition temperature hole transport materials derived from fused indole core structure and investigated the device performances of deep blue phosphorescent organic light-emitting diodes with the hole transport materials. The fused indole based hole transport materials showed a high triplet energy over 2.9 eV and high glass transition temperature over 120 oC. The high quantum efficiency over 20% and color coordinate of (0.137, 0.182) could be achieved in deep blue phosphorescent organic light-emitting diodes.

Organic photodetectors (OPDs) have high potential especially for signal processing and optical sensing systems in flexible devices. Phosphorescent materials have heavy atom effects and induce intersystem crossing (ISC), which transforms excitons from singlet into triplet. In general, triplet excitons have a longer lifetime than singlet excitons. This long lifetime can be expected to reduce exciton recombination and improve conversion efficiency. Solution processed organic devices utilizing conjugated polymers have attracted considerable interest because of their advantages in large-area device fabrication. Fluorene-type polymers have emerged as an important class of conjugated polymers due to their efficient emission, high stability and relatively high mobility. To enhance the efficiency of the fluorene-type polymers based OPDs, we investigated the properties of OPDs with Ir or Pt complexes doped in poly(9,9-dioctylfluorene) [F8] or poly(9,9-dioctylfluorene-co-benzothiadiazole) [F8BT], of which the device structures consisted of (+)ITO/F8 or F8BT :dopant (1 wt%) (100 nm)/Al/Ag (-). In order to compare the difference between singlet and triplet excitons in OPDs, the green emissive tris(8-hydroxyquinoline)aluminum [Alq3] and tris(2-phenylpyridine)iridium(III)[Ir(ppy)3] are used as the fluorescent and phosphorescent materials, respectively. Under illumination, the devices show good photoresponses. Then, we find that Ir(ppy)3-doped F8 and F8BT devices show more efficient conversion than both Alq3-doped and non-doped devices. This result suggested that longer exciton lifetime helps the exciton diffusion to organic/metal interface, where excitons are dissociated into carrier pairs. We also investigate the characteristics of the devices utilizing phosphorescent dyes of Ir and Pt complexes with different triplet levels. The photoresponse properties of bilayer devices utilizing solution-processable starburst molecular, methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB) as a hole transporting layer were investigated because F8BT with an electron-withdrawing group was used as an electron-transporting material. The solution-processed bilayer device structures consisted of (+)ITO/F8BT:Ir(ppy)3 (1wt%) (50nm)/TDAPB (30nm)/MoOx(5nm)/Au(10nm)/Ag. Owing to the PL quenching and long exciton lifetime related with the ISC between F8BT and Ir(ppy)3, Ir(ppy)3 doped devices showed larger photocurrent than non-doped device under high applied voltage and Ir(ppy)3 doped bilayer devices exhibited the IPCE of approximately 30 % at 7 V. A blue LD was used to generate optical pulses as an example of opto-electrical processing. The cut-off frequency was estimated as more than 10 MHz under a bias voltage. We demonstrated that these OPDs can be used as opto-electrical conversion devices for transmitting moving picture signals.

Chemiresistors consisting of gold nanoparticle/organic hybrid films are highly sensitive and inexpensive devices for monitoring organic analytes. Previous studies using chemiresistors have been limited to gas-phase operation, although more recently, operation in the liquid phase has been realized through control of the electrode geometry and spacing [1]. The chemiresistors consist of films of gold nanoparticles individually coated with an organothiol self-assembled monolayer and rely on a change in electrical resistance of the film when an analyte partitions into the organothiol layer. The response of a chemiresistor is governed by the partition coefficient of the analyte between the organothiol layer and the solution, the thickness of the gold nanoparticle/organic hybrid film, the analyte concentration and the analyte-water flow velocity. As our model system, we have studied the partitioning of the analytes hexane, heptane and octane into the hexanethiol layer of gold nanoparticle films [2, 3]. By varying the organothiol coating of these nanoparticles and using these to form a chemiresistor array, it is anticipated that we can broaden the range of potential applications. References: [1] B. Raguse, E. Chow, C. S. Barton and L. Wieczorek, Gold nanoparticle chemiresistor sensor: Direct sensing of organics in aqueous electrolyte solution, Anal. Chem., 2007, 79, 7333. [2] E. Chow, K.-H. Müller, E. Davies, B. Raguse, L. Wieczorek, J. S. Cooper and L. J. Hubble, Characterization of the sensor response of gold nanoparticle chemiresistors, J. Phys. Chem. C, 2010, 114, 17529. [3] K.-H. Müller, E. Chow, L. Wieczorek, B. Raguse, J. S. Cooper and L. J. Hubble, Dynamic response of gold nanoparticle chemiresistors to organic analytes in aqueous solution, Phys. Chem. Chem. Phys., 2011, 13, 18208 -18216.

Harnessing the complementary properties of both inexpensive functional organic molecules and inorganic nanocrystalline components presents a unique opportunity for the generation of new classes of hybrid materials.
New sensing technologies could be an ideal way to exploit the benefits of such hybrid species, with the potential to provide rapid, efficient and reliable detection of trace quantities of a number of analytes in aqueous solution. The presence of the metal containing nanoparticles can allow this to be achieved via a variety of optical and electrochemical detection methods including luminescence and photoconductivity changes amongst others.
With this potential in mind, we have recently begun to develop new molecules featuring an analyte receptor unit at one end, and anchoring thiol moieties at the other. The molecules have been designed to allow for both easy adsorption onto nanocrystalline surfaces and for strong and specific binding to a variety of anions and cations including Na⁺, Ca²⁺, PO₄^3-, and F^-. The molecules have been designed along robust synthetic principles which allow for straightforward derivatisation or further functionalisation, serving to demonstrate the flexibility that this approach towards new technologies could allow.
The current synthetic progress towards the receptor molecules under examination will be discussed, alongside relevant physical data and planned future work.
We acknowledge the support of the European Commission of HYSENS (FP7-NMP-2010-SMALL-4, Project 263091)

Organic thin film transistor (TFT) has been demonstrated for various sensing applications including thermal information. Combining organic temperature sensor with nonvolatile memory device for data storage would be ideal for electronic artificial skin or food chain health monitor applications. In this work, we fabricated a two-transistor (2T) thermal memory device that can sense the temperature from 293K to 363K and store the data for later readout. The first transistor is working as the sensing component, in which the drain-source current at subthreshold region shows temperature dependent property and its value in pentacene TFT is 600 pA at 293K and increases 50 times from 293K to 363K. The second transistor is working as the memory component, by inserting a thin layer of silver nanoparticles into pentacene TFT, the transistor shows a memory window of 90V in transfer I-V curve. The sensing component is connected to the gate electrode of the second transistor for the programming. The output current of memory component is read at room temperature, and its value is only proportional to the temperature while the device is being programmed. The gate connection structure enable it to achieve low output current at room temperature (off state), hence we achieved a dynamic range around 8 bit with an off current of 1 nA. The retention property of memory sensor at various temperature and dielectric materials will also be discussed.

Volatile organic compounds (VOCs) are compounds that have high vapor pressure and low water solubility, and it is also the common pollutant found in the indoor air that impact human health. Some of these VOCs are carcinogenic and have been associated with sick building syndrome, fatigue and headache. To improve the sensing performance of VOCs sensor is an important issue in recently years. In this study, we adopt a polymer blend of P3HT/PCBM to as the sensor materials. In the fabrication process of our VOCs sensor, the P3HT/PCBM blend solution was prepared from 10 mg of P3HT (Mw~60000, PDI~1.6, RR>95%) and 8mg of PCBM (Nano-C Inc.) dissolved in 1 ml chlorobenzene. Then, the solution was deposited on glass slide by spin coating at 700 rpm for 1 min, yielding the film thickness of ~ 80 nm. After drying the P3HT/PCBM thin film by vacuum pump for 2 hrs, we can obtain our P3HT/PCBM VOCs sensor successfully. The absorbance of P3HT/PCBM VOCs sensor was measured by UV-Vis absorption spectroscopy after the sensor was exposed to various VOCs for different time. From the data of absorbance spectra, we can observe the intensity at 560 nm and at 600 nm showing an obvious variation. Table 1 is data analysis for P3HT/PCBM VOCs sensor exposed to various volatile organic compounds for 24 hrs. The results of our study display that P3HT/PCBM thin film is suitable for highly sensitivity volatile organic compounds sensor application.

Diagnosis of disease, personalized medicine and drug discovery requires a low-cost and fast detection technique. Most of the DNA sensors involve labeling step which is unnecessary and requires large optical systems and specialized analysis procedure. One of the best candidates for disposable tool is pentacene thin film transistor due to their low-cost fabrication process as well as fast detection. In this study, we integrated pentacene TFTs and PDMS micro-channel for the use as biosensor for flow-detection of single-strand DNA and hybridization on pentacene layer. At first, we flowed double-distilled deionized water through PDMS microchannel on pentacene channel. Sequentially, we flowed ss-DNA (25 mer ployA) dissolved in DDI water on pentacene channel. When ss-DNA flows through PDMS micro-channel, it gets immobilized on pentacene surface. After that, when we flow complementary ss-DNA, the two ss-DNA hybridizes to form a ds-DNA. Immobilization of ss-DNA and ds-DNA dramatically differentiates the electrical response of OTFT. The process produced a dramatic change in the channel current (IDS) and field-effect mobility (μFET) of the devices. We demonstrated the feasibility of OTFT as a disposable sensor for DNA immobilization and hybridization.

The synthesis and application of new organic materials, nanostructured, for the development of technology based in organic devices have received much interest in the scientific community. In recent years entered in the market the first polymeric electronics products (organic semiconductor materials), among them there are the electrochromic devices, which are called smart windows, by controlling the passage of light or heat through a closed environment, like an ordinary window. The main functional aspects of electrochromic devices for use in architectural and automotive industry are to control the passage of light and temperature for thermal and visual comfort. These devices can be flexible and very thin, and do not contain heavy metals. The devices are formed by layers of organic material deposited in various architectures. In this work, the electrodeposition from organic materials in the case Polyaniline, which provides stability in the optical and electrical parameters, were used to develop prototypes of organic electrochromic devices. These materials were characterizated by: absorption spectroscopy ultraviolet-visible (UV-Vis), scanning electron microscopy (SEM), measurement of thickness and electrical measurements. The work objective was to the establishment of the relationship between thickness of the active layer and the value of the electrical resistivity of the layer deposited by electrodeposition technique. The experimental results enabled the equationing of the electrical resistivity in function to the thickness of the deposited layer. The linear fit of these results expresses the thickness of the conducting layer, α, and the lowest value of the electrical resistivity, B, associated with the gap between the valence band and conduction band. The results demonstrate that, when the layer of organic material is completely conductive, we obtain the thickness of the organic material deposited on the substrate.

Recently organic light emitting diodes (OLEDs) have been successfully launched in small sized mobile displays, and a large effort has been made to develop larger sized and flexible OLEDs. Flexible OLEDs on plastic substrates require flexible driving circuits fabricated at low temperature. Oxide thin film transistors (TFTs) are considered as a candidate for this purpose because the materials satisfy the requirements along with high electron mobilities. Moreover, the transistors can be fabricated using a solution process. Since the transition metal oxides are n-type semiconductors, inverted OLEDs are preferred to conventional OLEDs to utilize the oxide TFTs in active matrix OLEDs. The salient feature of inverted OLEDs compared with conventional OLEDs is the electron injection from the bottom electrode to an electron transporting layer (ETL). Most commoly used transparent electrodes such as indium tin oxide (ITO) and indium zinc oxide (IZO) or thin aluminium (Al) and silver (Ag) have work functions larger than 4.3 eV, while the lowest unoccupied molecular orbital (LUMO) level of organic materials used for the ETL is in the region of 2.8-3.3 eV, resulting in a large electron injection barrier from the electrode to the ETL. Moreover, solution processible electrodes such as graphene and PEDOT:PSS have been proposed as transparent electrodes for displays and solar cells. It would be very valuable to develop an electron injection layer operating efficiently independent of the work function of the cathode. In this study, we report an organic p-n junction as an efficient and cathode independent electron injection layer (EIL) for inverted bottom emission OLEDs (IBOLEDs). Forward bias in the OLEDs to inject electrons and holes into the device corresponds to the reverse bias in the p-n junction to generate electrons and holes at the junction. The charge carriers created by the external field at the junction encounter a lower barrier height than the injecting carriers from the electrode to the organic layer when they are extracted from the organic layer to the cathode by an external bias voltage, resulting in an efficient injection of electrons. An overpotential to generate electrons and holes was minimized by using a p-doped copper phthalocyanine (CuPc)/n-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) layer as the charge generation layer, and this was the first successful application of an organic p-n junction as an EIL for organic electronics to the best of our knowledge. The organic p-n junction composed of a p-CuPc/n-Bphen layer shows almost the same electron injection characterisitics for the cathodes with different work functions whereas the injection characterisitics of the n-Bphen EIL significantly depends on the work function of the cathode. These facts indicate that the new pn-i-p structure with the organic p-n junction can be efficiently applied for high performance flexible organic electronics, regardless of the electrodes.

Poly(3-hexylthiophene-alt-9,9-dioctylfluorene) (PTAF), has been synthesized and used to fabricate a White polymeric light-emitting diode (WPLED). This conjugated alternating copolymer, PTAF, was synthesized by Suzuki cross-coupling reaction and comprises 50 mol% of 3-hexylthiophene which is an orange-red color chromophore and 50 mol% 9,9-dioctylfluorene which is a bluish-green color chromophore. Nanocomposite consisting PTAF and graphene nanosheets enhances the optoelectronic properties and the device ITO/PEDOT:PSS/(PTAF + 1% graphene)/Ca/Al shows two-color white electroluminescence. The white luminescence from a single polymer affords the WPLED device a simple structure and low fabrication cost.

The molecular design and synthesis as well as the device performance of novel organic molecules for highly efficient blue emission in OLEDs will be discussed. One of the synthesized blue emitting materials exhibited excellent color coordinates of (0.156, 0.088) and external quantum efficiency of 7 % with three times longer life time than MADN, commercial compound, for deep-blue OLED emitters without doping. Several other compounds were designed in order to control the emitting efficiency based on the model compounds. According to the size of the side group, optimization of chemical structures in organic emitter will be proposed. Furthermore, low work function metal such as aluminum had not been available for solution process in OLED device, but novel solution process was recently achieved to get OLED emission. The related process method and data will be discussed.

Multilayer polymer light emitting diodes (PLEDs) have been fabricated with SuperYellow (PDY-132) purchased from Merck as an emitting layer (EML) and solution-processed small molecular electron transport layers (ETLs). Both EML and ETL are formed by using spin-coating method. Three kinds of ETLs such as 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 4,7-diphenyl-1,10-phenanthroline (BPhen), and 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) are utilized and dissolved in methanol which does not dissolve the underlying EML. By employing the ETL, the efficiency of the devices are improved compared to the device without ETL. The maximum external quantum efficiency (EQE) of the devices with TPBi, BPhen, and TAZ are 2.9 %, 1.7 %, and 1.6 %, respectively, while the device without the ETL shows EQE of 1.5 %. The ETL prevents quenching of radiative excitons at the cathode interface by acting as an efficient exciton blocking layer. The better EQE of the device with TPBi is partly attributed to the better film morphology compared to BPhen and TAZ, as evidenced from atomic force microscopy images.

Charge transport and charge recombination are recognized as key ingredients in the performance of polymer light emitting diodes (PLEDs). In the last two decades a large effort has been put on the characterization of the transport of the dominant charge carrier, holes. It has been demonstrated that the hole transport is governed by hopping between localized states, characterized by a mobility that depends on density, electric field and temperature. The strongly reduced electron currents are generally attributed to the immobile trapping of electrons. We observe that in conjugated polymers the electron transport is limited by traps that are Gaussianly distributed in energy within the band gap. Remarkably, we show that the electron trap distribution is identical for a large variety of polymers, hinting at a common origin for electron traps. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin pre-factor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate. As a result, we show that the ideality factor in the diffusion regime of a bipolar diode is governed by the recombination of trapped electrons with free holes.

Thin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene: polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.

We have designed and synthesized a fused aromatic amine based host material, 10-phenyl-9,9-bis(4-(pyridin-3-yl)phenyl)-9,10-dihydroacridine(TPDAN2). A fused amine was a core structure and pyridine was introduced in the core structure to have bipolar charge transport properties. The TPDAN2 showed a highest occupied molecular orbital 5.97 eV and lowest unoccupied molecular orbital of 2.37 eV with a triplet energy of 2.76eV. It also showed high glass transition temperature of 125 degree Celcius due to the rigid core structure. Green phosphorescent organic light-emitting diodes were fabricated using the TPDAN2 as the host material and a quantum efficiency of 13.5% was achieved at luminance of 1000 cd/m2.

Light extraction from organic light-emitting diodes (OLEDs) is one of the most important issues for highly efficient OLED because, in conventionally structured OLED, less than 20% of the light generated from the emissive layer can escape from the device as useful radiation. In order to extract the light confined inside the devices for increasing external quantum efficiency, defined as the ratio of the number of emitted photons into air to the number of injected electrons, rapid progress have been made in the research about out-coupling techniques for the light extraction from OLEDs, however, those methods have required complex and high cost equipment needing vacuum systems or additional steps for preparing a template. Considering practical application, where it is essential that fabrication techniques are applicable to large area and cost effective, it is very desirable to develop a simple fabrication technique for out-coupling structure. In this study, we developed a very simple fabrication technique for out-coupling structure without use of a mold and a vacuum process to enhance the light extraction efficiency of OLEDs. Corrugated structure can be prepared by only spin-coating process with a small amount of a mixture of SiO₂ and TiO_x sol solution in a short time. The width and the height of the corrugation were characterized by varying the spin-coating speed and the mixing ratio of the two sol solutions. OLEDs were fabricated with and without the corrugated structure used as an out-coupling structure and then their device performance were compared. The OLEDs with the corrugated structure introduced at the backside glass substrate shows enhanced light extraction efficiency by about 10% without a change of electroluminescence spectrum. The enhancement attributes to decreased incidence angle of the light at the interface between the glass substrate and air, which reduces the trapped light by total internal reflection. This mold and vacuum process free approach could provide a simple and cost effective way to fabricate out-coupling structure over large area for practical applications.

We report the characteristics of flexible Al-doped zinc oxide (AZO) films prepared by a plasma damage free linear facing target sputtering (LFTS) system on PET substrate for use as a flexible transparent conducting electrode in flexible organic light emitting diodes (OLEDs). The electrical, optical, and structural properties of LFTS grown flexible AZO electrodes were investigated as a function of DC power. We obtained a flexible AZO films with a sheet resistance of 39 Ohm/sqaure and an average transmittance of 84.86 % in the visible range although it was sputtered at room temperature without activation of the Al dopant. Due to effective confinement of the high-density plasma between the facing AZO targets, the AZO film was deposited on the PET substrate without plasma damage and substrate heating caused by bombardment of energy particles. Moreover, the flexible OLED fabricated on the AZO/PET substrate showed performance similar to the OLED fabricated on ITO/PET substrate despite of a lower work function. This indicates that LFTS is a promising plasma damage-free and low temperature sputtering technique for deposition of flexible and indium free AZO electrode for use in cost-efficient flexible OLEDs.

Organic light-emitting diodes (OLEDs) have been accepted as a technology for mobile and television (TV) displays and for potential future use in the lighting industry. In particular, phosphorescent OLEDs (PHOLEDs) have been of considerable interest in recent years as singlet and triplet excitons contribute to the emission of photons. Highly efficient OLEDs with over 20% external quantum efficiency (EQE), an almost ideal value, have been reported using this electrophosphorescent technology. During the last couple of years, blue PHOLEDs with over 20% EQE have also been reported by several groups. However, the driving voltage is relatively high compared with those of red and green PHOLEDs. Therefore, high driving voltage and short lifetime issues have to be addressed for meaningful application of blue PHOLEDs. In order to make highly efficient PHOLEDs, the triplet energy of the host should be higher than that of the phosphorescent guest to facilitate the exothermic energy transfer from the host to the guest. Thereby, reverse energy transfer from the guest back to the host is prohibited effectively and triplet excitons are confined on guest molecules. However, the host materials with wide band-gaps often cause an increase in driving voltages of the PHOLEDs. Therefore, the selection of suitable host materials for PHOLEDs is very imperative to achieve the high efficiency. In this paper, we report high efficiency and low driving voltage blue PHOLEDs with a simple organic triple stacked structure. Narrow band-gap characteristics of our bipolar transporting host materials are a key parameter to give such results. These new host materials were synthesized based on very well-known hole transporting material, 4,4�,4�-tri(N-carbazolyl)triphenylamine (TCTA), which is consisted of one triphenylamine and three carbazoles. We substitute α-carbolin for three carbazoles in TCTA one by one. As the results, three new materials, KHU-BH-1, KHU-BH-2 and KHU-BH-3 were synthesized. Using a high-triplet-energy-hole transporting material of TAPC and a high-triplet-energy-electron transporting material of TmPyPB, the organic three stacked structure has been realized with three new narrow band-gap blue host materials. These host materials have bipolar characteristics and high triplet energy of >2.8 eV. Very low onset voltages of 2.8~3.0 V and driving voltages of 4.2~4.6 V to obtain a brightness of 1000 cd/m2 are achieved in this three stacked device configuration. Maximum external quantum efficiency above 20% is reported.

Molybdenum oxide has recently been used in multiple papers for both OLED and OPV devices with promising results, however relatively little work has been done on the study of the energy level structure of molybdenum oxide and how this relates to device performance. OLED (F8:F8BT) and OPV (PCDTBT:PCBM) devices have been fabricated using reactively sputtered molybdenum oxide as a hole injection/extraction layer for these devices. Molybdenum oxide layers were deposited from a molybdenum target using an oxygen argon blend for the plasma, the oxygen concentration during deposition was varied from 0-70% in order to vary the stoichiometry and the energy level structure of the layer. Results show that for OLED devices the performance increases until around 20-25% when a plateau is reached and the performance remains constant, this is attributed to the film itself becoming almost fully oxidised and mainly consisting of Mo 6+ with a small amount of 5+, the lower oxidation states appear to severely reduce device performance. Hole injection is believed to be due to transfer of holes from the conduction band of the molybdenum oxide into the HOMO level of the polymer. As for OPV devices they initially begin to work at 15% oxygen concentration, as the oxygen during deposition is increased a sharp rise in device performance is observed where at 40% a peak efficiency of 3.7% is reached. Upon further increase of the oxygen content device performance drops rapidly and at 50% the efficiency was 1.9%, a drop in total efficiency of around 1.8%. The observed changes in the efficiency of these devices are believed to be due to small amounts of dopant states lying just below the Fermi level of molybdenum oxide. These dopant states are attributed to a mixture of impurities and also small amounts of metallic molybdenum oxidation states that still remain, these occupied states allow for the extraction of holes from the HOMO level of the polymer.

Finding ways of building low cost and energy efficient lighting devices is a very important present-day issue. Therefore zinc(II) bis-terpyridine complexes have been investigated for their emissive properties. On the one hand, Zn(II) has an advantage of being a d¹⁰ transition metal and so is coordinatively flexible and suffers from no unwanted redox processes. On the other hand, 2,2':6',2''-terpyridine is a well-studied chelating ligand which allows for both facile functionalisation and straightforward coordination to a metal centre, forcing Zn(II) into an {M(tpy)₂} domain with a coordination number of 6. A series of homoleptic Zn(II) complexes has been synthesised using 4'-(4-RC₆H₄)-2,2':6',2''-terpyridine ligands, with different R substituents (R = Me, OMe, OH, SMe, NO₂ and NPh₂). The emissive behaviour of the complexes has been studied and a red-shift in the emission has been observed upon altering the substituents from poorly to highly electron donating. Electronic absorption, electrochemical and lifetime properties of the complexes will be presented, along with computational studies.

In this work, the width of the exciton recombination zone in conventional double-emissive layer organic light-emitting devices (OLEDs) is compared to that of devices containing a single-layer graded-composition emissive layer (G-EML). The G-EML consists of nearly 100% hole-transporting material (HTM) at the anode and nearly 100% electron-transport material (ETM) at the cathode, having a continuously varying HTM:ETM composition across the active layer. Electroluminescence originates from a phosphorescent guest that is uniformly doped throughout the G-EML. Conventional OLED emissive layer architectures suffer from a reduction in efficiency at high current density. This roll-off is often attributed to bimolecular exciton quenching and it has been previously suggested that a wide exciton recombination zone may reduce its severity. Here, the exciton recombination zone of each emissive layer architecture is probed through the use of a fluorescent sensitizer capable of quenching excitons formed on the emissive guest. The red-emitting fluorescent material, tetraphenyltetrabenzoporphyrin (TPTBP) is used to quench excitons formed on the archetypical green phosphor tris(2-phenylpyridine)iridium(III) (Ir(ppy)3). Triplet excitons formed on Ir(ppy)3 may undergo a Dexter-type energy transfer to TPTBP and recombine nonradiatively, leading to a decrease in phosphorescence from Ir(ppy)3. It is found that thin, lightly-doped sensitizer strips may be inserted into both double- and graded-emissive layer OLEDs without impacting electronic transport. By comparing the overall emission intensity of sensitized devices to control structures containing no sensitizer, a â?~mapâ?T of the local exciton density may be determined. Here, it is found that the single-layer G-EML architecture exhibits a broad exciton recombination zone, while analogous double-emissive layer devices show a significantly narrower width centered on the HTM:ETM interface. The wider recombination zone of the G-EML comes as a result of the intrinsically broad overlap between the electron and hole densities, which also leads to high efficiency in these structures. The ability to engineer the exciton recombination zone by varying the emissive layer architecture may ultimately permit devices with a reduction in the efficiency roll-off to be realized.

Organic Vapor Jet Printing (OVJP) is a small molecule organic thin film deposition technique that combines growth and device patterning into a single process. It eliminates the need for shadow masking steps in the fabrication of multicolor organic light emitting diode (OLED) arrays.[1] The OVJP process uses an inert carrier gas to drive the vapors of host and dopant materials into a heated microfluidic print head. The evaporants are mixed in microchannels and collimated into jets by an array of micronozzles. The jets produce stripes of doped organic thin film on a chilled substrate as it translates below the print head. Multicolor printing can be achieved in a single pass by incorporating multiple mixing channels into a print head. Alternating printed stripes of different electroluminescent mixtures can form the sub-pixels of a full color OLED display. In this work we discuss a print head fabricated using a double anodic bond that seals the silicon nozzle arrays, to the microfluidic channels etched in glass, to a metal print head support. This assembly technique eliminates sealants that can outgas at the high temperatures required for the evaporation and transport of the organic molecules. OVJP has demonstrated the ability to grow phosphorescent OLEDs (PHOLEDs) with an external quantum efficiency of 8.0%, which is comparable to analogous devices grown entirely by vacuum thermal evaporation. Analytical and computational models for organic vapor and carrier gas transport within the print head have been developed to help understand the OVJP process and have been experimentally confirmed. An array of 30 µm wide, alternating stripes of red and green PHOLED emissive layers separated by 10 µm are patterned by a two color print head. The capability to print features with size and spacing comparable to those found in a high definition color display is evaluated. Cross contamination of electroluminescent materials between stripes can reduce the color rendering ability of the OLED array, and must be minimized. Methods of mitigating cross contamination, and for registering and addressing individual pixels will be discussed. [1] G. J. McGraw, D. L. Peters, and S. R. Forrest, Appl. Phys. Lett. 98, 013302 (2011).

Electrochromic materials have been the current research focus for the application in the electrochromic displays for Color e-Paper due to their vivid color, good visibility in bright sunlight, low power consumption, and wide viewing angle among various candidates such as LCD, OLED, and particle-type display (electrophoretic) with color filter. We will discuss the multielectrochromic materials and devices comprising small organic molecules and viologens demonstrating RGB along with black (K) colors in the oxidized and reduced states. The compounds responsible for the oxidative colored states are small organic molecules with an anchor to help adsorbtion on TiO2, whereas the reductive colors are produced by small molecules as well as viologens. Our devices made by ink-jet printing show good reflectivity, contrast ratio, color gamut, and bistability, important for the reflective displays. The structural modulation for color tuning, color enhancement, fast coloring and bleaching strategy will also be emphasized.

Carbon nanotube enabled vertical organic light emitting transistors (CN-VOLETs) show promise to solve some of the major problems limiting widespread adoption of AMOLED technology. The novel device architecture results in three important benefits over the traditional side-by-side TFT and OLED active matrix design. Firstly, the vertical architecture permits sub-micron transistor channel lengths without high resolution patterning. Such short channel lengths permit comparatively low mobility organic semiconductors to compete against polycrystalline silicon in providing the high drive currents required by OLEDs in AMOLED displays, at a low power burden. Secondly, the high transparency of the dilute nanotube source electrode yields an aperture ratio (excluding the switching TFT and address lines) that approaches unity. A higher aperture ratio allows the OLED to be driven at a lower intensity for a given display brightness, prolonging the lifetime of the OLEDâ?"a major problem still faced by OLEDs today. Thirdly, the CN-VOLET architecture simplifies the fabrication process because it combines the driving transistor, storage capacitor and OLED into a single device, eliminating two of the four components in the most basic conventional AMOLED design.

We demonstrate a photodetector structure that employs metal-oxide charge transport layers and that is sensitized at near-infrared wavelengths by a thin film of a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of 16.1 ± 0.1% (λ = 756 nm) to be achieved at zero-bias in a device consisting of an 8.1 ± 0.3 nm-thick dye film. The specific detectivity (D*) and response speed (f_3dB) of the fully-optimized device are measured to be (4.3 ± 0.1)Ã-10¹¹ cm Hz^1/2 W^-1 and 91.5 kHz, respectively. Modeling of our structure reveals that the photocurrent is limited by the diffusion of photo-generated excitons to the ZnO/J-aggregate hetero-interface and we determine the exciton diffusion length in the J-aggregate film to be L_D = 2.0 ± 0.4 nm. This work provides insights relevant to the use of J-aggregating cyanine dyes in photodetector and photovoltaic applications and highlights the importance of engineering the optical field profile within such structures in order to maximize performance.

Colloidal quantum dots are, like semiconducting organics and polymers, synthesized in and processed from the solution phase. They offer wide and highly-controlled tunability of their bandgap via the quantum size effect, including in the range 400 nm â?" 3000 nm. I will review progress in understanding electron and hole transport and recombination in solution-cast CQD films and at interfaces with metals and transparent conductive oxides. I will discuss how these advances have been applied in achieving photodetectors and image sensors having high sensitivity and excellent response × and in building solar cells that harvest both the sunâ?Ts visible and infrared spectral regions.

Organic semiconductors are attractive materials as active elements for a variety of optoelectronic devices, such as light emitting diodes, solar cells, and photodiodes. In particular the high charge separation efficiency obtained with an interpenetrating donor-acceptor heterojunction [1,2] was key for cost-efficient and large scale fabrication of organic solar cells and organic photodiodes (OPDs).Here, we will discuss the performance of OPDs with spectral sensitivities that can be tailored to cover the range from the visible to the infrared in combination with amorphous Silicon (a-Si) thin film transistor (TFT) backplane technology to create a pixelated photodetector for X-ray or short-wavelength infrared (SWIR) imaging. The detector consists of 256x256 squared pixels with 154 μm pitch. On top of the backplane a photoactive bulk heterojunction (BHJ) layer is solution-processed [3] consisting of a blend of semiconducting polymers as absorber and electron donor material for the visible, fullerenes as electron acceptor material, and additional inorganic absorber for SWIR. The most interesting processing technique of OPDs is effortless sprayâ?"coating, which is a high as well as a low-throughput fabrication method. Despite the surface roughness of spray-coated BHJ layers, the OPDs show superior current-voltage characteristics [4]. Furthermore, no lithographic steps are necessary for the photoactive layer and the top electrode to achieve very low crosstalk levels between neighbouring pixels. Pixelated OPDs show high external quantum efficiency up to 60% in the visible spectrum, low dark currents densities of 5x10^-6 mA/cm² at -5 V reverse bias, as well as high linearity and dynamic range. With a 20 μm thin film encapsulation the detector life time exceeded 1000 h at accelerated aging conditions of 85°C and 85% relative humidity. For applications beyond the silicon bandgap limit hybrid BHJ photodiodes containing PbS colloidal quantum dots as tunable absorber have been applied to extend the spectral sensitivity up to 1.8 μm. Rectification ratios of ~6,000, minimum lifetimes of one year and external quantum efficiencies of up to 51% have been demonstrated [5]. By integration of the solution-processed devices on the a-Si TFT backplane, it has been even possible to show for the first time near-infrared imaging and videos at 1300 nm with hybrid photodiodes. References [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (1995). [2] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474, (1992). [3] C. J. Brabec, S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater., 11, No.1, (2001). [4] S. Tedde, J. Kern, J. Fürst, P. Lugli, O. Hayden, Nano Letters 9, 980-983 (2009). [5] T. Rauch et al, Nature Photonics 3, 332-336 (2009).

The authors report on the fabrication and performance of a silicon/organic heterojunction based IR photodetector. Perylene derivative N,N'-Dimethyl-3,4,9,10-Perylenetetracarboxylic diimide (DiMe-PTCDI) thin film (thickness around 25nm) is grown by the Hot-wall epitaxy technique on boron doped p-Si substrate. As a result, recorded with scanning electron microscope, a closed layer followed by crystallites with lateral dimensions of several 100nm is formed. The ohmic contacts to both silicon and organic layer are formed by thermal deposition of 100nm Al contact. The back Al/Si contact is initially alloyed to provide ohmicity. Such a device exhibits for a bias variation from -1V to +1V a rectification ratio of 10⁴ and 10¹¹ at 300K and at 77K respectively. The presence of barrier for charge carriers at the interface is additionally confirmed by the thermal activation measurement of reverse dark current, resulting in the value of around 0.5eV. It is demonstrated that although both materials composing the device do not absorb light in the NIR range the Al/p-Si/DiMe-PTCDI/Al heterostructure exhibits a room temperature photovoltaic effect up to 2.3μm (0.54eV). It will be outlined that the short circuit photocurrent in the sub-band gap range originates from an absorption mechanism at the organic/inorganic interface, similarly to fullerene based devices reported in [1]. Nevertheless the achieved responsivity of 0.2mA/W at 1.55μm and 300K [2] is two orders of magnitude higher than in our previously presented report [1], in spite of the fact that the device has not yet been fully optimized. A detailed study of the performance of photodetector presented in [2] will be shown, including the response time analysis and the measurement of specific detectivity. The influence of surface treatment, organic layer thickness and electrode area will be presented with the scope of possible improvement of the device performance. The room temperature operation, powerless operation mode and most of all the non-invasive deposition of the organic interlayer onto the Si that results in compatibility with the CMOS process, make the presented approach a potential alternative to all inorganic device concepts. [1] Fullerene Sensitized Silicon for near- to Mid-Infrared Light Detection G. Matt, T. Fromherz, M. Bednorz, S. Zamiri, G. Goncalves, C. Lungenschmied, D. Meissner, H. Sitter, N.S. Sariciftci, C. Brabec, G. Bauer Adv. Mat. 22 (2010), 647 [2] Silicon/organic hybrid heterojunction IR photodetector operating in the telecom regime M.Bednorz, G. Matt, E.Glowacki, T.Fromherz, C.J.Brabec, M.Scharber, H.Sitter, submitted to Advanced Functional Materials

Ultraviolet photodetectors (UV-PDs) are under investigation for many applications in the medical, industrial, communication, defense, and research fields. Today, semiconductor solid UV-PDs are generally made of single crystalline silicon, silicon carbide (SiC) or gallium nitride (GaN) p-n junction photodiodes. However, these single crystalline PDs have limitations that make them unsuitable for many applications due to their high cost, low quantum efficiency of less than 40% (responsivity of less than 0.2 ampere per watt (A/W)), and demand of cooling for very weak light sensing. Solution processed PDs based on organic- and/or nano-materials represent a direction to significantly reduce the cost but always leading to a lower performance than inorganic counterpart. Here, we report a solution processed UV-PD with a nanocomposite active layer composed of zinc oxide (ZnO) nanoparticles (NPs) blended with semiconducting polymers which significantly outperforms the inorganic PDs. This class of PDs shows a self-adaptive transition from a Schottky contact in the dark to ohmic contact under irradiation, and a huge internal gain of 4,080 enabled by the trap-controlled charge injection. The combination of low dark current of the Schottky diode and high responsivity of 1,165 A/W under a relatively low bias below ten volts leads to a very high detectivity of 1.9 Ã- 10^15 Jones at 340 nm at room temperature. It is one to two orders of magnitude higher than that of the existing inorganic semiconductor UV-PDs. The gain-bandwidth product of these PDs is over 10^5 larger than the colloidal quantum dot PDs. These low cost thin-film photodetectors have great potential for the existing applications of very weak UV and visible light detection and can potentially open new application opportunities because of their flexibility, light weight and printability.

Detection of nitrogen dioxide, NO₂, is required to monitor the air-quality for human health and safety. Commercial sensors are typically chemiresistors, however field-effect transistors are being explored for their reported sensitivity gain.^[1] Although numerous investigations have been reported using conjugated monomers and polymers, carbon nanotubes and inorganic nanowires, the operating mechanism of NO₂ sensing is not clear. Here, we show that a positive threshold voltage shift is observed upon exposure to NO₂ using semiconductors as polytriarylamine, a quinquethiophene self-assembled monolayer, perylene and zinc oxide. The generic response is further investigated using zinc oxide as a model semiconductor.^[2]

Upon NO₂ exposure and application of a positive gate bias, the current through the semiconductor gradually decreases. The gate induced electrons are immobilized in traps formed by NO₂. The fixed interface charges yield a shift of the threshold voltage towards the applied gate bias. With time, mobile electrons get trapped until the gate bias is completely compensated by immobile trapped charges. We show that the temporal behavior of the threshold voltage shift can phenomenologically be described by a stretched-exponential function. This description yields two parameters: a characteristic relaxation time (Ï") and a dispersion parameter β equal to T/T₀, where T₀ is a temperature describing the exponential distribution of trap sites. The obtained characteristic relaxation time is inversely proportional to the partial NO₂ pressure. A shift of the threshold voltage is observed for extremely low NO₂ concentrations down to 10 ppb.

Crucial for the sensor application is the temperature dependence of charge trapping and recovery. At room temperature no recovery is observed, however charge detrapping can be achieved at elevated temperatures. We present the activation energy for trapping and detrapping extracted from temperature dependent stress and recovery measurements. The energy level of the trap species is determined by means of thermally stimulated current measurements. A tentative interpretation of the microscopic mechanism will be presented and the consequences for NO₂ TFT sensors will be discussed.

The organic magnetoresistance (OMAR) in organic light emitting diodes (OLED) made of poly(2-methoxy,5-(2'-ethylheloxy)-1, 4-phenylene vinylene) (MEH-PPV) was investigated by means of DC transport and the admittance spectroscopy in the range of 1Hz to 10MHz at room temperature. The measurements were carried out on unipolar and bipolar OLEDs made of pristine MEH-PPV as well as MEH-PPV with traps introduced by the UV light irradiation. We found that in bipolar UV-exposed OLEDs, the magnitude of the magnetoresistance effect in real part of the admittance increases with DC bias, reaching a very high value of 35% (in the field of 30mT) at bias of 4.8V and decreases at higher bias voltages. Also, we observed that the cutoff frequency of OMAR effect monotonically increases with DC bias voltage. The cutoff has extrinsic origin and is likely caused by a dissipative process related to the reorientation of permanent dipoles. At the highest tested bias voltage of 6.7V, we were able to detect the OMAR effect at the highest frequency of our system, 10MHz. We have found that the imaginary part of the admittance is also affected by the the magnetic field. The effect of the magnetic field on dynamical capacitance of the device at low frequencies is very strong and opens up a possibility of using these devices as magnetic field sensors.

The past 24 months has seen dramatic increases in the performance of solution processed organic solar cells (OSCs) with multiple groups reporting in excess of 7% and externally verified but unpublished efficiencies > 8% on â?olaboratory-scaleâ? model devices [1]. The key challenge for OSC is to translate these mini-cell results to the sub-module and full-module scale. In this regard IMEC have recently reported an impressive 5% on a 25cm2 strip sub-module with inverted architecture [2]. One of the key factors suppressing conversion efficiencies in large area cells and sub-modules is the relatively high sheet resistance of the transparent conducting anode - typically indium tin oxide with an appropriate hole transport layer such as poly(3,4-ethylenedioxythiophene)polystyrene sulfonic acid (PEDOT:PSS). ITO is also one of the most expensive components of the cell and the use of PEDOT:PSS brings potential stability issues (plus extra cost). Hence, replacement of both materials with alternative transparent conducting anodes is a high priority on the pathway to viable module-level OSCs. In our paper we will describe efficient, large area organic solar cells fabricated without (ITO) or PEDOT:PSS. We employ a transparent top anode consisting of molybdenum oxide (MoOx) and silver (Ag) in an â?oinverted-illuminationâ? geometry in conjunction with a thick aluminium bottom contact. The optimised MoOx-Ag-MoOx stack has an optical transmission of 80% at 520 nm, sheet resistance of ~5 ohm/sq and yields conversion efficiencies in monolithic (not strip) 25 cm2 â?osub-modulesâ? of 3.2% - significantly outperforming the equivalent conventional ITO/PEDOT:PSS devices. The new stack acts as both a transparent, conducting window and hole-transporting / collecting electrode. Tuning of the stack layer thicknesses allows one to manipulate and optimise the optical field density in the active layer to enhance photocurrent. The proposed architecture and anode materials are well suited to high throughput, low cost all-solution processing. A large area, monolithic architecture reduces device complexity versus serially connected strips and is a more efficient use of substrate area. [1] Prog. Photovoltaics: Research and Applications 19, 84, (2011) [2] http://www.sciencedaily.com/releases/2011/09/110905074316.htm

Charge separation at polymer:fullerene interfaces is the primary process for organic bulk-heterojunction solar cells. Following the ultrafast photoinduced charge transfer, electrons and holes are still bound because of Coulomb interactions, resulting in the formation of charge transfer excitons (CTEs). Those are known to play a crucial role in solar cells being responsible for the open circuit voltage and the short circuit current. In this contribution, we propose a novel strategy to overcome some of the recombination channels due to charge transfer excitons. This new approach considers the modification of the electronic properties of the conjugated polymer by doping with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules. The free holes induced by F4TCNQ on the low-bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] PCPDTBT fill the tail of states in the highest occupied molecular orbital (HOMO) and in addition are expected to screen Coulomb interactions of CTEs at the interface between PCPDTBT and the fullerene derivative PCBM. By the unique combination of time resolved photoluminescence and photoinduced absorption spectroscopy, we demonstrate how doping results in a decreased population of CTEs and enhanced formation of free carriers. The spectroscopy experiments are complemented by measurements on solar cells showing an overall increase of more than 30% in the power conversion efficiency. Deschler et al., Phys. Rev. Lett. 107, 127402 (2011)

An integrated approach combining materials design and synthesis, interfacial modifications, patterning and optical engineering, and device engineering have been adapted to develop low-cost, efficient and stable polymer solar cells. In this talk, new synthetic methods to develop low-bandgap polymers and novel fullerene derivatives with tunable energy levels and crosslinkable functionalities for efficient and thermally stable polymer solar cells will be presented. These new materials will be further studied based on our integrated engineering approaches to fully harness their best potentials as PV materials. We will also discuss our recent development on inverted polymer solar cells and process modifications that enables roll-to-roll processing. Graphene oxide and self-assembled monolayer modified metal oxides were employed to improve both the efficiency and long-term stability of the devices. Through the process optimization and interfacial modifications, inverted cells with power conversion efficiency of ~8% and improved ambient stability can be achieved.

The development of appropriate electron and hole charge blocking and transport layers is paramount to the fabrication of more efficient solution processed organic optoelectronic devices. Transition metal oxides, such as molybdenum trioxide (MoO3), are a class of material that offers the necessary tunability in their electronic properties and intrinsic stability towards oxidation to be considered highly attractive for use as such layers. In this presentation, we present a novel solution processed route to the development of MoOx thin-films using oxomolybdate precursors in methanol. The simple and air-based chemistry of these precursors permits for the reproducible formation of ultra-smooth (RMS <0.4 nm) thin-films directly on ITO. XPS and UPS analysis show that under specific conditions, sub-gap states in the MoOx layer are observed. These states are found to have a profound impact on the performance of P3HT:PC61BM solar cells. We show that when the density of such states is minimized, these MoOx layers can be effectively used as anode modifying layers for a wide variety of organic solar cells.

Organic based PV relies on the absorption and semiconducting properties of organic materials. It offers the charming possibility to built-up organic solar cells with tiny amounts of material by simple low-T coating processes onto nearly any surface. These perspectives make ObPV appear as the cheapest possible PV technology with unprecedented market opportunities. The progress made in laboratory record-cell efficiencies shows that materialâ?Ts design principles and device physics are increasingly better understood. On the other hand first real existing ObPV products have entered consumer markets. However, despite all the progress made, there are still substantial hurdles to take until broader mass markets can be addressed. Specifically, a reasonable operating lifetime of ObPV systems and cost-effective production process technology is required to generate competitive products. To advance ObPV further, integrating of all research disciplines involved is a must. Effective flow of relevant information must be ensured to recognize and reduce complexity at the same time. We use open innovation models to implement multidisciplinary teams working along the value chain of ObPV. These innovation models will be introduced along with the technical and materials development in the respective projects which serve as examples for the industrial development of organic bulk-hetero-junction and solid dye-sensitized solar cells.

We report high-performance inverted polymer solar cells (I-PSCs) introducing novel functional materials, such as nonconjugated/conjugated polyelectrolytes and n- or p-doped sol-gel metal oxides, as efficient interfacial layers. Our functional layers are inserted between the photoactive layer and electrodes via solution processing and low annealing temperature below 80°C, and induce significant functions in the I-PSCs as follows: (i) work function tunning of the electrodes, (ii) selective contact for charge carriers, (iii) transparent optical spacer, (iv) protection of the active layer, and (v) detemination of the device polarity. In addition, we also successfully incorporate our novel functional layers into all-printed PSC modules fabricated using various printing techniques, such as slot-die, ink-jet, and electro-spray printing methods.

There is a global demand for economical and eco-friendly renewable energy sources. While many technologies such as hydroelectric, wind and biomass have been adopted as replacements of fossil fuels, several factors ranging from materials availability to infrastructure have to be considered prior to deployment. Solar power based on photovoltaic (PV) technologies can address significant fraction of the energy demand, provided it can be made cost-competitive1. Plextronics has developed printable ink systems to allow for high efficiency solar performance, with 6.9% verified performance recently achieved with PV3000 ink system. Indoor energy harvesting is well suited to OPV technology due to potential for low cost, matching of OPV spectral response to indoor fluorescent lighting spectrum, and superior performance vs. crystalline silicon at very low light irradiance. Plexcore XC-2010-based inks can be used to achieve competitive power densities under office lighting levels (200 â?" 1000 LUX) to drive low power demand applications such as e-readers, motion and light sensors, and next gen displays. New material developments are also targeted at organic photodector (OPD) applications which seek to minimize leakage current while maintaining high QE at low reverse bias currents. Commercial development of these three applications of printed OPV will be presented along with device and stability data for a variety of ink offerings. 1. K. Zweibel, NREL Technical Report, NREL/TP-520-38350, (2005), pp. 1-49.

In a society concerned with environmental, economic, and geopolitical consequences of energy consumption, organic photovoltaics (OPV) are emerging as viable alternatives to traditional energy sources. In typical OPV devices, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as the hole transport layer. However, the use of PEDOT:PSS invites numerous problems; among them is its extremely low pH, insufficient electron blocking capability, and tendency to absorb atmospheric water; all factors which contribute to significant degradation or performance reduction of the device. An emerging alternative to PEDOT:PSS is nickel (II) oxide (NiO). Non-stoichiometric NiO is a wide bandgap semiconductor with natural p-type conduction from the formation of Ni2+ vacancies and accompanying charge-balance holes. Recently, solution-processed NiO (sNiO) from a protected chemical recipe was employed in high efficiency polymer solar cells (PSCs), but needed post-deposition oxygen plasma treatment to achieve good device performance. In this work, we develop sNiO-based photovoltaic cells without the need for any post-deposition surface treatments â?" amenable to facile processing. We then compare devices fabricated from this approach to ones which incorporate PEDOT:PSS. When we combined our knowledge of our sNiO with a dithieonogermole-based donor-acceptor copolymer, we fabricated PSCs with average PCE of 7.3% - a 13% improvement over PEDOT:PSS devices. Unencapsulated NiO-based devices show a 300% improvement over PEDOT:PSS-based devices in the maintenance of >50% of the original PCE. This represents the highest performing PV cells that incorporate NiO in the literature. Detailed studies of the sNiO will be presented with special emphasis on the chemical composition of the precursor solution, and electronic and physical properties of the NiO thin film as they relate to the overall device performance.

The efficiency of organic solar cells was impressively increased during the last three years to above 8% under AM 1.5G illumination. This was achieved by simultaneously decreasing the bandgap of the donor material and reducing the LUMO-LUMO offset between donor and acceptor, the first leading to a higher current and the second yielding a higher or unchanged open circuit voltage. The optimum bandgap for a single bandgap semiconductor solar cell illuminated with the AM1.5G spectrum lies between 1-1.5 eV, according to the well known Shockley-Queisser limit. From a fundamental point of view, the energy loss due to charge transfer between donor and acceptor leads to a lower open circuit voltage, reducing the maximum obtainable efficiency and as a second effect increasing the optimum bandgap for the donor material. These considerations will be revisited and additionally different illumination spectra will be considered like modern light sources as fluorescent lamps or LEDs. Interestingly the theoretical maximum efficiency is significantly higher, i.e. in the range of 50%, for these narrow spectrum light sources, as there is a less critical tradeoff between losses due to thermalisation of charge carriers and non-absorption losses. Besides theoretical calculations experimental results will be presented, showing the interrelated effects of the light source and the bandgap on the efficiency of organic solar cells. From a technological perspective these results imply that for a consumer product like a battery charger the optimum bandgap might be a different choice than for a rooftop application. Furthermore the upscaling of the solar cells sets additional boundary conditions. As the conductivity especially of transparent contacts like ITO, other TCOs or even organic materials like PEDOT:PSS are limited, on length scales in the range of millimeters or latest centimeters a series circuitry is needed. This is achieved by structuring of the different layers into individual cell stripes, where the electron and hole contacts of neighboring cell stripes are connected, forming the series circuitry. This interconnection involves an area loss. The module dimensions and efficiency result from the tradeoff between area losses for narrow cell stripes and resistive losses for wide cell stripes. The main driver for the resistive losses is the current of the solar cell, making this circuitry issue especially demanding for high current solar cells. As the transparent ITO electrode is a major cost factor, there is urgent need to replace it with cheaper materials. In most cases the alternatives have a lower conductivity, making the series circuitry issue even more critical. Numerical calculations of the obtainable module efficiencies for different case scenarios will be presented, showing that specific technological boundary conditions can influence the optimum choice for the bandgap of organic solar cells toward higher band gaps.

Inverted organic solar cells have been actively studied due to their superior stability compared to solar cells with the conventional structure. Despite great success with inverted organic solar cells having the bulk heterojunction structure, there are only few reports on inverted devices with the lamellar structure. Therefore, study on inverted organic solar cells with the bilayer structure using ITO coated with ZnO as substrate, CuPc as donor, C60 as acceptor and MoO3 as hole transporting materials has been conducted. We carefully analyzed important characteristics of the inverted devices in comparison with a device with the conventional structure on the ITO substrate. Our study reveals the importance of optical field distribution in the devices with the lamellar structure. Inappropriate arrangement of organic layers in the devices with the lamellar structure results in a significantly lower photocurrent. Our results serve as guidance for design of future materials used in inverted solar cells with the lamellar structure.

We discuss the application of impedance spectroscopy to organic semiconductor devices. Electronic and optoelectronic devices composed by this class of materials show important differences with respect to the classical inorganic-semiconductors. We focus on two important aspects that are observed in organic devices: trap states and negative capacitance effects. We show how to interpret and characterize the impedance spectra starting from the general semiconductor theory. The arguments are then applied to the measured spectra of different devices, from organic solar cells to homo- and hetero-junction diodes. Trap states can modify the electric field inside the device, considerably affecting charge generation and transport and consequently the solar cell efficiency [1]. Specific variations in the device structure of solar cells based on a p-i-n structure [2] allow us to identify the trap positions and nature. To completely characterize these states we have implemented previously reported theory [3] and calculated the energy diagram and the impedance spectrum of the solar cell for different traps distributions. The calculated results are compared to measured impedance spectra for different traps distributions and are used to confirm the validity of our discussion. The specific trap states distribution is then evaluated by a fitting procedure and a gaussian distribution of electron trap states with a with around 65meV and a concentration of 6e10cm-3. Negative capacitance is often observed in the impedance spectra of different organic devices. The physical origin of this effect is debated in literature and no clear explanation as been proposed up to now [4]. We show here that negative capacitance effects can be explained starting from the relaxation semiconductor theory when applied to organic semiconductors [5]. The semi-insulating nature of these materials cannot be described by standard semiconductor theory simply assuming low mobilities, since they exhibit electronic transport phenomena which are generically different. We first present a short review of the organic devices in which negative capacitance has been observed and then explain them with a unified approach, based on relaxation semiconductor theory. The implications in device performance coming from our findings are also discussed. Small-molecule based devices are fabricated and characterized in order to confirm the proposed arguments. Starting from a simple metal/organic/metal structure we can design devices which deliberately present negative capacitance effects or not. The results obtained from these simple structures are then generalized and observed in homo- and hetero-junction diodes as well as in complete p-i-n organic solar cells. [1] D.Ray PRB 82 125204 [2] M.Riede Nanotech 19 424001 [3] J.Bisquert PRB 77 235203 [4] C.Lungenschmied OrgEle 10 115; J.Bisquert PCCP 13 4679 [5] W.vanRoosbroeck PRB 5 2154

Photogeneration of charge carriers in organic semiconductors requires the dissociation of strongly bound exctions. Despite the central and efficiency-determining role of this process, the precise mechanism and its dependence on material parameters is still not well understood and hotly debated. Here we have investigated the photodissocation of excitons in the widely used and highly efficient organic semiconductor poly(3-hexylthiophene) (P3HT).(1) By combining it with silicon as electron acceptor and by employing a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup with a 40 fs resolution, we are able to directly monitor the evolution from the instant (<40fs) photogeneration of excitons to the dissociation into polarons. This process occurs with a time-constant of 140fs. By analysing the decay of the polaron absorption signals on a longer timescale, and by considering the transient signals obtained for different excitation wavelength, we show that free charge carriers are generated by exciton dissociation in highly-ordered, regioregular P3HT, where the excitation is well delocalized. In contrast, geminately bound polaron pairs prevail in the more disordered regiorandom P3HT. Thus, we infer that polymer structural order rather than excess energy is the key requirement for free charge generation in hybrid P3HT/silicon solar cells. The general validity of this conclusion for other semiconducting polymers is then probed for a second class of materials.(2) We use a series of polymers based on a poly(p-phenylene) backbone where the degree of conformational disorder is modified continuously. By comparing field-dependent photocurrent measurements of solar cells made with C60 as acceptor against several established models for exciton dissociation we show that the photogeneration of free carriers tails with the intra-chain structural order, consistent with the results obtained for the hybrid P3HT/silicon solar cells. (1) Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann, Eberhard Riedle* â?oRole of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics probed in real time by NIR Broadband Transient Absorptionâ? J. Am. Chem. Soc. 2011, in press, DOI: 10.1021/ja207887q (2) Christian Schwarz, Heinz Bässler, Irene Bauer, Jan-Moritz Koenen, Eduard Preis, Ullrich Scherf and Anna Köhler* â?oDoes conjugation help exciton dissocation ? A study on poly(p-phenylene)s in planar heterojunctions with C60 or TNFâ? Adv. Mater. 2011 submitted

In organic bulk heterojunction solar cells, a deviation of the ideality factor of the dark current from unity is commonly put forward as evidence for the presence of trap-assisted recombination. We demonstrate that for a variety of polymeric donors the non-ideality of the dark characteristics is determined by deeply trapped carriers in the transport-dominating constituent of the donor:acceptor blend, rather than a trap-assisted recombination mechanism. The light-intensity dependence of the open-circuit voltage confirms the absence of trap-assisted recombination and demonstrates that the dominant recombination mechanism in all the investigated polymer:fullerene solar cells is bimolecular. In the case of poly(3-hexylthiophene (P3HT) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), however, the light-intensity dependence of the open-circuit voltage appears to be affected by the presence of trap-assisted recombination, while charge transport appears to be trap free. It is demonstrated that the contribution of recombination from a small amount of trapping sites becomes significant when a reduced bimolecular recombination rate is taken into account.

Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more solar cells with different absorption ranges. In addition, it also reduces the thermal loss of high-energy photons by increasing the open circuit voltage (Voc). The key of tandem solar cell performance is the interfacial layer, which connects the front and rear cells together. In addition, the impedance match (or current match) of the front and rear cells are also critical, I will also discuss the strategy of achieving the current match of the tandem solar cell. Finally, we have also successfully introduced the plasmonic effect into the tandem solar cell to enhance its efficiency.