Functional materials and devices for organic electronics

Resume : Light emitting field effect transistors (LEFETs) based upon organic semiconductors are an emerging new class of optoelectronic devices. LEFETs are advantageous as they can simultaneously execute light-emission and the standard logic functions of a transistor in a single device architecture1-2. However, current LEFET device architecture delivers either high brightness or high efficiency but not both simultaneously, thus limits their use in technological applications. In this presentation we introduce a non-planar electrode device strategy that significantly improves the three key performance parameters, namely brightness, quantum efficiency and switching, in a simultaneous fashion for the RGB (red, green blue) colour gamut. We demonstrate transistor ON/OFF ratios > 105 in full RBG colour with a brightness exceeding 2000 cd/m2 with an External Quantum Efficiency (EQE) of 0.06% for yellow-green; 750 cd/m2 with an EQE of 0.06 % for red; and 800 cd/m2 with an EQE of 0.05% for blue emitting devices3. These metrics are higher by a factor of 20 compared to traditional top contact LEFETs and also exceed the best reported LEFETs References 1.A. J. Heeger, N. S. Sariciftci and E.B. Namdas, Semiconducting and Metallic Polymers. Oxford University Press 2010. 2.Kristen Tandy, et al, Organic Electronics, Volume 14, Issue 11 (2013). 3.Mujeeb Ullah, et al, Adv. Mater. (2013). doi: 10.1002/adma.201302649.

Resume : Recently, organic devices such as organic light emitting diodes (OLED) and organic photovoltaics (OPV) gain intense attentions due to their potential lower fabrication cost, materials tailoring capability, easy adoptability to flexible devices and lightweight. Efficiency of both OPV and OLED can be dramatically enhanced by light scattering engineering. In order to improve the efficiency of OLED and OPV, optical functioning pattern was formed inside the device using nanoimprint lithography. However, the applying the optical pattern layer in organic devices has limitation owing to high sensitivity about surface roughness. Organic devices were easily degraded due to occurred leakage current by roughness of pattern. Especially, pattern with sharp edge and size over hundreds nm sharply aggravate the current and resistance problems of organic devices. Fabrication of patterns with low surface roughness is important for improving the efficiency of organic devices. In this study, we fabricate nano-sized light scattering structures with low surface roughness. The scattering pattern was fabricated by nanoimprint lithography (NIL) using functional materials such as ZnO. After fabrication of optical functioning structure, the pattern was planarized by spin-coating of materials with different refractive index value. By controlled light scattering at the interfaces, the outcoupling efficiency of OLED and conversion efficiency of OPV devices were increased significantly without degradation.

Resume : Ionizing radiation can be detected by directly converting it into an electrical signal. Only few and expensive inorganic semiconductors (e.g. CdTe, SiC) offer the possibility of realizing portable direct detectors that operate at room temperature. Organic semiconductors have been so far mainly proposed as detectors for ionizing radiation in the indirect conversion approach, i.e. as scintillators, which convert ionizing radiation into visible photons, or as photodiodes, which detect visible photons coming from a scintillator and convert them into an electrical signal. We have recently reported on the use of organic semiconductors as intrinsic direct ionizing radiation detectors [1,2]. In particular, organic semiconducting single crystals (OSSCs) show a quite interesting performance, thanks to their stability, good transport properties and large interaction volume [1]. X-ray detectors, based on low-cost solution-grown OSSCs are here shown to operate at room temperature, providing a stable linear response with increasing dose rate in atmosphere and in radiation-hard environments. We report here on the characterization of 4-hydroxycyanobenzine single crystals by both monochromatic synchrotron radiation (10-25keV) and a standard Mo target X-ray tube (35kV), equipped with a series of subsequent slits to focus the beam up to few mm2 area, so limiting the contribution of X-ray interaction with the substrate and the electrodes. The photoresponse to an increasing X-ray dose is always linear in the 10-35keV energy range here investigated, also for very low doses, down to 1 mGy/s. The combination of monochromatic X-rays generated by synchrotron radiation with the broad X-ray spectrum generated by a standard Mo target X-ray tube (35kV), allowed us to better simulate the actual doses employed in medical diagnostic applications (typically mammography), covering and assessing a wide range of dose rates (1 – 180 mGy/s). A dedicated study of the collecting electrodes geometry allowed us to maximize the charge collection efficiency and to take advantage of the transport and charge collection anisotropy of the crystals. The calculated sensitivity is over 100 nC/Gy , thus assessing how OSSCs perform very well al low operating voltages (down to 10 Volts); hundreds of Volts are usually required to operate room temperature inorganic semiconductor detectors) and offer a great potential in the development of novel ionizing radiation sensors. [1] B.Fraboni et al., Adv. Mater., 24, 2289 (2012) [2] A.Intaniwet, C.Mills, M.Shkunow, P.J.Sellin , J.L.Keddie, Nanotechnology, 23, 235502 (2012)

Resume : Organic field effect transistors (OFETs) have been the focus of research for more than two decades and their performance increases steadily. Nevertheless, they have not yet overcome the prototype stage in their application for active matrix backplanes or flexible RFID tags. This is partially related to a lack of performance, flexibility in parameter design, as well as stability of current OFETs. In this contribution, we will demonstrate the realization of novel organic transistor concepts to overcome the current limitation of OFETs. By reducing the molar doping ratio down to 10-4, we demonstrate that doping in pentacene behaves similarly to doping in inorganic semiconductors, and we can identify the impurity saturation and impurity reserve regimes, as well as filling of intrinsic trap distributions. Employing these doped layers in organic inversion and depletion transistors, highly reproducible and tunable device characteristics can be realized. The formation of depletion zones in doped organic semiconductors and their control by doping concentration furthermore opens the path to dielectric-free organic junction field effect transistors. Besides the application of molecular doping, the performance of organic transistors can be further enhanced by changes in device geometry. The transconductance and cut-off frequency of horizontal OFETs are substantially limited by the charge carrier mobility and channel length. Even when employing injection layers or surface treatment procedures, the optimum channel length is in the range of 1 μm. While being hard to realize in planar devices, we demonstrate that these dimensions can be easily achieved in vertical devices like vertical organic field effect transistors and organic triodes.

Resume : Memory devices based on organic field-effect transistors (OFETs) have been intensively studied during the last years. We report a new approach to the design of memory devices based on photoswitchable OFETs with conceptually new architecture, where active photochromic layer is sandwiched between the semiconductor and dielectric layers. It was shown that the proposed device architecture allows for a fast switching of the transistor between two (or even many) stable states which are characterized by considerably different voltages Vth. The IDS currents of the same device in different states differ from each other by a factor of 10-10000. Typically, optical input (laser beam) is required to record the information; reading and erasing is performed while applying different biases to the gate electrode of the transistor. The influence of the chemical nature of the semiconductor materials (both p- and n-type) on the performance of the photoswitchable OFETs will be particularly discussed. It was shown that variation of semiconductors provides additional wide opportunities for designing memory devices with different modes of switching between distinct states. It is noteworthy that optical, electrical or electrooptical programming of OFETs results in the formation of many stable states. Therefore, these devices perform as advanced memory elements capable of recording, nonvolatile storing and erasing of many (at least several) bits of information.

Resume : Conventional silicon electronics’ greatest strength has always relied on its ability to scale down the device dimensions and generate low-cost functions regardless the cost/area ratio. On the other hand, organic semiconductors have always held the promise of complementing the need for low-cost production as well as large-area applications by virtue of their characteristic chemical versatility and ease of processability. However, the dream of scaling the device size down is somehow getting to an end for the silicon-based electronics because of the photolithography limitations involved in the production process of the devices. In this regard, organic electronics cannot complement its inorganic counterpart and the relentless need of miniaturization unless new avenues are pursued. Hence, a grand challenge for organic electronics is to integrate multiple functions in a single material by combining organic semiconductors with an additional molecular component, which confers additional functions to the device. Here we report on the realization of a memory device through the engineering of the energy levels in a polymer semiconductor by blending two molecular components, a photochromic diarylethene (DAE) derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT’s hole transport, as previously reported[1]. In this study, the blend between DAE derivatives, which do not exhibit proper semiconductor behavior themselves but feaConventional silicon electronics’ greatest strength has always relied on its ability to scale down the device dimensions and generate low-cost functions regardless the cost/area ratio. On the other hand, organic semiconductors have always held the promise of complementing the need for low-cost production as well as large-area applications by virtue of their characteristic chemical versatility and ease of processability. However, the dream of scaling the device size down is somehow getting to an end for the silicon-based electronics because of the photolithography limitations involved in the production process of the devices. In this regard, organic electronics cannot complement its inorganic counterpart and the relentless need of miniaturization unless new avenues are pursued. Hence, a grand challenge for organic electronics is to integrate multiple functions in a single material by combining organic semiconductors with an additional molecular component, which confers additional functions to the device. Here we report on the realization of a memory device through the engineering of the energy levels in a polymer semiconductor by blending two molecular components, a photochromic diarylethene (DAE) derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT’s hole transport, as previously reported[1]. In this study, the blend between DAE derivatives, which do not exhibit proper semiconductor behavior themselves but feature two different HOMO levels which depends on the irradiation wavelength, with an organic semiconducting polymer such as P3HT is used as a bi-component film forming the electroactive layer of organic thin-film transistors (OTFTs). The device illumination at defined wavelengths enables the erase/write operation of the memory through the switching of the diarylethene’s electronic states in the blend, which modulates the output current. Furthermore, our study sheds light on the device switching mechanism which differs from other models proposed so far in the literature for memories integrating photochromic molecules and organic semiconductors at the same time. This modular blending approach allows for convenient incorporation of various molecular components, and opens up perspectives on multifunctional devices. [1] E. Orgiu et al., Nature Chem., 2012, 4, 675-679.

Resume : Since organic thin-film transistors (TFTs) can be fabricated at temperatures below the glass transition temperature of plastic substrates, they are potentially useful for the realization of electronic functionality with low or medium complexity in flexible electronics systems, such as rollable or foldable information displays, conformable sensor arrays, and plastic circuits. In many cases it is desirable that the organic TFTs can be operated with voltages similar to those of high-efficiency organic light-emitting diodes or state-of-the-art silicon integrated circuits, i.e. about 2 to 3 V. A promising approach to the realization of organic TFTs that can be operated with such low voltages are ultra-thin gate dielectrics based on a plasma-grown aluminum oxide (AlOx) layer in combination with an alkylphosphonic acid self-assembled monolayer (SAM). By utilizing vacuum-deposited small-molecule organic semiconductors with a crystal structure and a thin-film morphology that favor efficient charge transport, low-voltage organic p-channel TFTs with field effect mobilities as large as 4 cm2/Vs and cutoff frequencies above 1 MHz have recently been fabricated, along with excellent long-term stability. Organic n-channel TFTs, which are required for the realization of low-power organic complementary circuits, have also shown promising improvements in terms of performance and stability, with field-effect mobilities exceeding 1 cm2/Vs and cutoff frequencies approaching 100 kHz.

Resume : We present light-induced switching characteristics of porphyrin-coated silicon nanowire field effect transistors (Si NW FETs) and their capability to design efficient hybrid nano devices that combine organic shell of photosensitive molecules and inorganic NWs. Si NW FETs containing Schottky barriers were functionalized by porphyrin which absorbs broad range of visible light and easily transfers electrons due to the conjugated ring structure. The current-switching of the devices is investigated upon violet light illumination. We demonstrate that switching dynamics of the FETs depends on the thickness of the porphyrin shell wrapping the NWs, that is related to the packing density of the molecular layer and thus to electron diffusion through the layer. Moreover, we extract the ratio of the drain current under light and dark condition for different concentrations of porphyrin. Switching of the devices is caused by the additional electrical potential produced by charging of the porphyrin shell due to diffusion of mobile charge carriers excited by light. Therefore, the porphyrin layer acts as an optical gate of the device since the oxide layer between the NW and the porphyrin shell blocks the direct charge transfer from the molecules to the NW. Summarizing, we demonstrate that charging of a light-sensitive organic molecular shell can be used as an effective method to switch the conductance of NW FETs and aids the systematic realization of molecular hybrid devices.

Resume : Magneto-dielectric functions provide an effective mechanism to develop internally coupled electric and magnetic orders towards the development of multiferroic properties. Primarily, magneto-dielectric functions can be obtained by combining magnetic and electric structures through materials design, namely an approach of using ground states. Recently, we observed that optically-generated intermolecular excited states can exhibit a magneto-dielectric phenomenon in an organic semiconducting donor:acceptor system containing poly(N-vinylcarbazole) (PVK) as an electron donor and 1,2,4,5-Tetrachloro-3-nitrobenzene (TCNB) as an electron acceptor. This experimental observation presents a new phenomenon in the family of magnetic field effects. In particular, using optically-generated intermolecular excited states can lead to three major unique properties. First, it allows an optical tuning on magneto-dielectric functions by using photoexcitation. Second, the long-range Coulomb interaction between intermolecular excited states can be used as a convenient tool to influence short-range spin-spin interaction within individual intermolecular excited states. Third, it can enable convenient engineering method of using materials mixing technique to control intermolecular excited states in generating magneto-dielectric functions. This presentation will discuss the fundamental mechanisms of electric-magnetic coupling in intermolecular excited states based on pi-pi and pi-d interactions.

Resume : We reports that OFETs including low-Ci dielectrics can be operated at low voltages if highly conjugated organic semiconducting layers can be introduced to the hydroxyl-free dielectric surfaces. On grafted or cured polymer-assisted SiO2 dielectrics showing Ci = 10 − 11 nFcm−2, solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) crystals were observed easily by the naked eye compared to micron-sized pentacene crystals. The resulting TES-ADT OFETs operated with a low voltage (| V | ≤ 5 V) showed high electrical performance (μFET , Vth, and SS values up to 1.3 cm2V−1s−1 , approximately −0.5 V and ∼ 0.2 Vdecade−1 , respectively). In contrast, polycrystalline pentacene-based OFETs require much higher operating voltages (| V | > 20 V). TES-ADT could be tuned intrinsically with better π-conjugated structures to transfer the charge-carriers, as determined by atomic force microscopy (AFM), X-ray diffraction, and in situ photo-excited charge-collection spectroscopy (PECCS).

Resume : Mechanically durable polymer semiconductor/soft elastomer blends are promising composites for flexible organic field-effect transistor (OFET) applications. Here, nanofibrillar network structures of poly(3-hexyl thiophene) (P3HT) embedded in a cured polydimethylsiloxane (PDMS) elastomer matrix were facilely developed via blending of ultrasound-assisted P3HT mother solutions and PDMS pre-polymer, spin-casting, and curing. Blend film morphologies of P3HT/PDMS (1/9 in w/w) on polymer-coupled SiO2 dielectrics could be controlled reproducibly by elongating the ultrasonic time, which significantly improved the aggregates of P3HT self-assembled directly in the solutions, while simple solution blending of these two immiscible materials yielded the micro-phase separated domains in the cast blend films. The optimization of ultrasound-assisted solution processing could produce optically feature-less blend films. P3HT/PDMS blend OFETs prepared from 10 - 15 min ultrasound-assisted P3HT solutions showed high electrical performance: field-effect mobility of about 0.03 cm2V-1s-1 and on/off current ratio of > 5×105, as well as uniform and smooth cast films, P3HT nanofibrils embedded were well-dispersed and percolated in the elastomer negligible hysteresis and environmental stability by the PDMS-surrounding channel.

Resume : We report the deposition of zinc and magnesium phthalocyanine (ZnPc and MgPc) thin films by spin-coating technique. Metal phthalocyanines are suitable organic materials for photovoltaic structures due to the high absorption in the visible range of the solar spectrum. In our study, different concentrations (1% (w/v), 2 %(w/v) and 3% (w/v)) of ZnPc or MgPc in dimethylsulphoxide (DMSO) were used. ZnPc and MgPc spin-coating films were deposited on ITO, fused silica and silicon substrates. Optical and structural properties of the films were characterized by UV-VIS, Photoluminescence and FTIR spectroscopy. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to investigate the grain size and the morphological features of the obtained layers. We found out that the grain size of the films depends on the concentration of the metal phthalocyanines in DMSO. I-V characteristics of (metal/organic/ITO/glass substrate) structures were recorded in dark and under the illumination with solar simulator (AM1.5). A correlation between the layer grain size and the optical and electrical properties of the obtained structures was made. Also, we observed an increase of the current value and quantum efficiency with the increase of the layer grain size.

Resume : Considering the standard OLED structure define by a sandwich of organic layers between a transparent conductor electrode and a metallic one deposited on a transparent substrate, the purpose of this paper is to realize different types of heterostructures for this application using N,N’-Bis(naphthalene-1-yl)-N,N’-bis(phenyl)-benzidine)–NPB as hole transporting layer (HTL) and Bathophenanthroline-Bphen as emissive and electron transporting layer (ETL). The role of the transparent electrode in the majority of OLED structures is assured by ITO and therefore, one of the reference analyzed structure is Al/Bphen/NPB/ITO. Starting from the above structure we investigate the improvement of hole injection by adding different types of phthalocyanine (ZnPc, CuPc and MgPc) thin films between ITO and HTL to reduce the energetic barrier at the interface. A thin (<10 nm) layer of 8-hidroxyquinoline aluminium salt- was used to protect the Bphen layer during the vacuum deposition of the metallic electrode. All the organic layers are deposited by vacuum evaporation. The effect of the type of transparent conductor electrode on the optical (UV-VIS transmission and PL) and electrical properties of the heterostructures is analysed comparing the properties of the heterostructures realized on ITO with those of the heterostructures realized on the cheaper AZO (Al:ZnO) deposited by pulsed laser deposition on glass substrate and the heterostructure showing the best electrical behavior is evidenced.

Resume : Functionalized acenes have proven to be especially promising compounds in the field of molecular electronics, due to their unique features in terms of stability, performance and ease of processing. Among a wide variety of well-established techniques for the deposition of soluble acenes, spray-coating represents an interesting approach towards the formation of uniform crystalline films over a large area. To this end, electrostatic spray deposition (ESD), was employed as an alternative to the conventional spray-coating processes, for the fabrication of high performance organic field- effect transistors (OFETs). We herein first report the utilization of a specific operation mode of ESD, microdripping, which was applied in order to control the droplet size, transport and evaporation, by properly adjusting the experimental parameters. By the proposed method, high quality crystal domains were obtained, via an optimized one-step process. The fabricated OFETs exhibited excellent electrical characteristics, with high field-effect mobility up to 0.6 cm2/Vs, Ion/Ioff ≈10^5 and near zero threshold voltages. Furthermore, OFETs based on flexible substrates were also realized, showing good potential as a versatile manufacturing process for large-area electronics.

Resume : Rare-earth complexes have become subject of intensive research due to the high quantum efficiency of their emission, very narrow bands, and excellent fluorescence monochromaticity. The chemical design and characterization of Eu complexes based on β-diketone ligands hexafluoroacetylacetate (hfac) and dibenzoylmetanate (dbm) is reported here. K[Eu(dbm)4] and K[Eu(hfac)4] complexes were immobilized as thin films by using the spray technique, a promising methodology for practical applications. The latter provides not only a faster layer deposition but also larger coated areas compared to conventional methods. The growth of the sprayed films was monitored through microbalance (QCM) and ultraviolet–visible (UV-Vis) absorption spectroscopy, which reveal a higher mass and absorbance per deposited layer of K[Eu(dbm)4] film. Micro-Raman images display a more homogeneous spatial distribution of the K[Eu(dbm)4] complex throughout the film, when compared to K[Eu(hfac)4] film. At nanometer scale, atomic force microscopy (AFM) images indicate that the roughness of the K[Eu(hfac)4] film is approximately one order of magnitude higher than that for the K[Eu(dbm)4] film, revealing that the morphological pattern found at micrometer scale by micro-Raman is kept at nanometer scale. The photoluminescence data show that the complexes remain as pure red emitters upon spray immobilization. Besides, the quantum efficiency for the sprayed films are found equivalent to the values achieved for the powders, highlighting the potential of the films for application in light conversion devices.

Resume : The limitations of the electrical conduction which characterise the organic devices can be reduced by the use of adequate device structures to overcome the problems related to the high resistivity of the organic materials and difficulty to obtain a good charge carrier injection from electrodes in the organic. This paper presents some investigations on the optical and electrical properties of the monomer, synthesised from maleic anhydride and aniline derivatives, thin films on glass covered by ITO deposited by Pulsed Laser Deposition. We have also analysed the characteristics of the glass/ITO/monomer/Si structures. The influence of this monomeric buffer layer on the electrical properties of the heterostructures based on single/multiple organic layers deposited by vacuum evaporation [p type: pentacene and rubrene; n type: 2-(4-tert-buthylphenyl)-5-(4-biphenylyl)-1,3,4 oxadiazole (buthyl-PBD) and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine synthetic] prepared between glass/ITO and Si electrodes has been emphasised. We have studied the effect of the morphological and structural particularities of the organic compounds, energetic barrier at interfaces and presence of dipolar layer at the contact with ITO and Si electrodes, on the I-V characteristics. A blocking effect has been evidenced for most of heterostructures, exception the heterostructure glass/ITO/monomer prepared from maleic anhydride and 2,4 dinitroaniline /buthyl-PBD/Si[n] showing a good rectifier diode behaviour.

Resume : Pc-AFM is a method with potential for imaging nanoscale features in photo-active thin films. It has the ability to image features that affect optoelectronic properties, even where these features are buried below the film surface. There is interest in applying the technique to the study of bulk heterojunction photovoltaics. Optimal photo-active layers comprise donor and acceptor phases intimately mixed on a < 10 nm scale. This mixing is too fine to be resolved by Pc AFM. However, defects in the morphology frequently occur on length scales of 10 – 1000 nm. These, being on a similar scale to the film thickness, can have a significant impact on the macroscopic device properties, while still being too small to identify with conventional optoelectronic methods. Pc-AFM has been used to image donor nanowires buried within an acceptor film [1]. It has also been used to identify sub-surface acceptor aggregate defects [2]. However, the interpretation of the high-resolution photocurrent images requires deep physical insight. We have developed a tool for simulating the processes of photocurrent generation and conduction under the AFM tip. We report on simulations of model photovoltaic systems and compare them to experiments. The tool is used to help interpret experimental results, and to determine the limitations of Pc-AFM as a instrument for imaging mesoscopic defects. [1] Tsoi et al., Energy & Environ. Sci. 4, 3646 (2011) [2] Coffey and Ginger, Nat. Mater. 5, 735 (2006)

Resume : Semiconducting compounds that can be processed using simple and scalable solution based-manufacturing processes represent an emerging class of electronic materials that could potentially be used in a wide range of emerging optoelectronic applications. However, significant hurdles that prevent exploiting the full potential of this class of electronic materials still remain. In this presentation I will discuss the development of solution-processable organic and inorganic semiconductors in our laboratory focusing on the technological application of these materials in a range of opto/electronic devices including thin-film transistors, solar cells and light-emitting diodes. Particular emphasis will be placed on the use of n-/p-type molecular dopants, in conjunction with simple doping protocols, for improving the performance characteristics of these opto/electronic devices through their utilization in the active/passive layers of the devices. Novel and simple approaches for assessing the doping efficiency of organic molecular dopants will also be presented.

Resume : Polymer-based organic photovoltaic devices (OPVs) have attracted considerable attention as they are low-cost, lightweight, flexible and can deliver clean renewable energy. The so-called inverted architecture is expected as a promising approach from the viewpoint of durability improvement. Despite a dramatic improvement in operational lifetime, inverted OPVs can demonstrate relatively lower performances due to unfavorable energetic and incompatible chemical interfaces. Extensive efforts to improve their efficiency have been done modifying the interface including metal oxides such as TiOx to work as electron-selecting layer and modifying the oxide surface with a self-assembled C60 monolayer. This work consists in the synthesis and characterization of a novel n-type polymer based on fullerenes in the backbone. The system aims to incorporate an azide-derived N atom directly connected to the C60 sphere. Fullerene is well-known as an excellent electron acceptor, ideal for organic solar cells applications due to his ability to stabilize negative charges, and it is expected that these azafulleroids-based materials will give rise to uniform thin films. Azides are targeted due to the ease of their reaction with C60. The macromolecular structures were prepared by a 1,3-dipolar cyclo-addition of the comonomer to the C60 under catalyst-free conditions and was characterized by NMR, FTIR, UV-Vis, GPC and TG-DTA techniques. From theoretical studies that have been performed, favorable changes to opto-electronically active material can be expected and then the material will be investigate as an acceptor in the main active layer of inverted devices.

Resume : It is usually supposed that the cause of low performing bulk-heterojunction solar cells fabricated from blends of poly(9-hexylthiophene), P3HT, and 6,6-phenyl-C61-butyric acid methyl ester, PCBM, presenting an S-shape courant-voltage characteristic results from a vertical phase separation between P3HT and PCBM in the active layer, independently of the nature of the cathode. This hypothesis is based on the low surface energy of the polymer, which tends to accumulate at the air surface while the fullerene molecules tend to diffuse toward the anode region. We have made use of the charge-based Deep Level Transient Spectroscopy (Q-DLTS), a variation of the standard DLTS method, to investigate the trap parameters (density, energy level, and capture cross section) of P3HT:PCBM based devices equipped with different metal cathodes. We show that a strong reduction of trap states in the cells occurred in devices having an Al cathode but not in devices having a CaAl or LiFAl one. We suggest that a phase separation was effectively produced at the interface Al/(P3HT:PCBM) leaving a polymer rich surface, which reduces the charge transport and collection and consequently the cell power conversion efficiency. On the contrary, a direct contact between Ca or LiF and the active layer preserves the blend composition and provides a better stability and performance to the cells.

Resume : Titanium dioxide (TiO2) has been widely used as cathode interlayer in organic photovoltaics due to its high electron extraction rates. However, charge recombination occurring at trap states that are present on its surface is an important factor limiting device performance. We verified through DFT calculations that these trap states are negatively charged oxygen vacancies on the oxide surface, and they induce significant band bending resulting in a large electron extraction barrier. Hitherto, a solution to this problem has been the exposure of the devices to UV light. We suggest alternative strategies to passivate trap states either by using conformal coatings of alumina (Al2O3) or zirconia (ZrO2) films deposited with ångström-level precision by atomic layer deposition on top of the TiO2 layers or by applying a hydrogen annealing step to the TiO2 layers. A significant enhancement in power conversion efficiency has been obtained in both Al2O3/ZrO2-coated and H2-treated TiO2 based bulk heterojunction solar cells with an inverted architecture, as compared to control devices. Our results indicate that a successful passivation of traps located at the surface of TiO2 can be achieved with both methods, even without UV illumination, and the advantages and/or limitations of these two approaches will be explored. This work clearly demonstrates effective alternative routes to address the loss processes occurring at the surface of TiO2 interlayer leading to efficient and stable OPVs.

Resume : We have demonstrated the EHD (Electrohydrodynamic) printing of Ag-grid transparent electrodes. The width of the patterned grid was less than 10 um, which cannot be identified by the naked eyes. The electrical and optical properties of the EHD-printed Ag-grid electrodes are investigated. Ag-grid is carefully designed in order to maximize both conductivity and transparency of the EHD-printed transparent electrodes. Depending upon the grid pitch (distance between adjacent Ag lines), the resistivity varies accordingly. We introduce the concept of “figure of merit (FM)” which defines the condition for the maximum conductivity with the maximum transmittance (90%). The calculated FM is used to find out the optimized grid pitch for their future application to optoelectronic devices. The optimized Ag-grid transparent electrode is employed to the fabrication of optoelectronic devices.

Resume : We introduce a new strategy for control of the in-plane orientation of pentacene molecules using nanometer-scale periodic groove patterns as an alignment layer. The process is based on the integration of scratch lithography for OTFT of the amorphous silicon oxide substrates and growth of the pentacene film by high-vacuum sublimation on the fabricated pattern. With this technique, we can control the distribution and size of the nanometer-scale substrate relief to control the morphology and orientation of the pentacene. The nanometer-scale groove patterns have been shown to control the in-plane orientation of the pentacene molecules. Compared to pentacene films that are not aligned, enhanced field effect mobility of 0.302 cm2 V-1 s-1 has been achieved when the optimal p-orbital overlap direction is parallel to the direction of the current flow. The ability to control the molecular ordering at a local scale opens up important perspectives for high-performance OTFTs.

Resume : Organic photovoltaics (OPVs) have attracted significant scientific and commercial interest due to their potential use for low cost flexible solar cells. Transition metal oxides (TMOs) have been successfully employed as anode and/or as cathode interfacial layers in OPVs to improve charge extraction and to enhance device stability. However, their relatively low conductivity and poor transport properties are limiting the device efficiency. Herein, molybdenum and tungsten oxide films that are subjected to microwave (MW) irradiation are introduced as low resistance anode interfacial layers in OPVs. More specifically, substoichiometric MoO3-x and WO3-x layers were deposited using a hot-wire technique and then were subjected to MW irradiation. The crystal structure of these films is of great interest, since the MW irradiation enhances the crystallinity of the initially amorphous MoO3-x and WO3-x films. A significant improvement in the Jsc and the cell power conversion efficiency (PCE) has been demonstrated. We suggest that the oxide hole extraction layer improves the device efficiency due to the enhanced crystallinity of MoO3-x or WO3-x films, which facilitates hole transfer to/from organic molecules. Acknowledgement: This research has been co-financed by the European Union (European Social Fund-ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF)-Research Funding Program: Archimedes III.

Resume : We describe our recent theoretical work on the modeling of magnetic, transport and optical properties of organic devices based on a combination of density-functional theory (DFT) quantum-chemical calculations and stochastic simulations. We focus on two problems: (1) The theoretical modeling of molecular hyperfine fields in organic magnetoresistance (OMAR) devices. In this work, we establish a protocol for the accurate determination of the average hyper?ne field in organic molecules and and apply it to selected molecular ions: NPB, TPD, and Alq3. Contrary to common belief, we find that molecular hyper?ne fields are not only caused by hydrogen nuclei. We also find that dipolar contributions to the hyper?ne fields can be comparable to the Fermi contact contributions. However, such contributions are restricted to nuclei located in the same molecular ion as the charge carrier (intramolecular), as extramolecular contributions are negligible. (2) Emission redshift due DCM2-doping in Alq3. DCM2-doped Alq3 displays a redshift in light-emission frequency which is extremely sensitive to the dopant concentration. This effect can be used to tune the emission frequency in organic light-emission devices (OLEDs). We show that the large permanent dipole moments of the DCM2 molecules generate random electric fields that are large enough to cause a non-linear Stark shift in the band gap of neighboring molecules. As a consequence of these non-linear shifts, a non-Gaussian probability distribution of band gaps for the DCM2 molecules in the Alq3 matrix is developed, with long exponential tails to the low-energy side. We discuss the importance of this distribution, together with the conditions of thermal equilibrium, to explain the experimentally observed emission redshift. (*) work done in collaboration with R. Giro, F. P. Rosselli, M. A. de Cicco, R. S. Carvalho, M. Cremona and C. A. Achete

Resume : Processing conditions for the growth of organic thin films are crucial for device performance, but the simulation of the complicated growth of organics is lacking compared to the predictive simulations possible for some inorganic materials. Here we study the structural order and morphology of the prototypical organic semiconductor C60 in real-time and in-situ experiments. By the unique combination of X-ray scattering methods at PETRA III (DESY) and kinetic Monte-Carlo simulations (KMC) we are able to quantify the nucleation and growth processes of C60 on C60. From the 2D diffuse x-ray scattering pattern we extract the island density and island diameter as a function of temperature, deposition rate and film thickness. Simultaneously we measure the layer coverages during the vertical film growth by detecting anti-Bragg growth oscillations. We compare the island formation and successive layer filling as observed in real time x-ray scattering with a particle resolved kinetic Monte Carlo simulation of the growth process, and from this are able to determine the diffusion energy (0.54eV), step-edge barrier (0.11 eV) and binding energy (0.13 eV). This experiment and theoretical model represents a first example of a molecular system for which the growth can be modelled in the multilayer regime, enabling a detailed understanding and predictive simulation of C60 growth.

Resume : Graphene is a material endowed with a distinctive Dirac like energy dispersion presenting exotic quantum behavior. Shortly, it was soon found out that its electronic properties are strongly sensitive to substrates and its environment, resulting in pronounced changes on charge carriers migration. Curiously, the pursue for the understanding of electron scattering mechanisms has been mainly focused on monolayer graphene, even though bilayer graphene and few layer graphene also present interesting tunable quantum properties and potential for electronic applications. In this talk, I will present how we expose bilayer graphene to oxygen molecules and were able to asymmetrically tune the hole and electron mobilities. These effects reveal that two different sources of carrier scattering are being observed: hole mobility is mainly affected by the long range scattering process (mainly induced from the underlying substrate) and increases due to the screening promoted by oxygen molecules trapped between the graphene and the substrate. Our theoretical calculations demonstrate that the electron coupling with such resonant states is the main responsible for suppression of the electron mobility, resulting in an asymmetry between electron and hole charge transport. These understandings are fundamental for a better control over graphene charge scattering mechanisms and may lead to novel manipulations of its electrical properties.

Resume : Conjugated polymer nanoparticles (CPNs) are attracting considerable interest in a wide variety of fields owing to their potential for use in applications such as biological labels, chemical sensors, and optoelectronic devices. In this work, we have demonstrated the preparation of white-emissive conjugated polymer nanoparticles wrapped with graphene oxide (GO) nanosheets. GO is fascinating because its oxygen-containing functional groups enable decoration of the material surface with other molecules, producing multi-functional hybrids and composites. Highly stable, GO wrapped, poly(9,9-di-n-octylfluorenyl-2,7-diyl) nanoparticles (GO-PFO NPs) with diameters in the range of 30 - 150 nm were successfully obtained by utilizing the GO nanosheets as an interface stabilizer in an emulsification process. Interestingly, the synthesized GO-PFO NPs exhibited unique white-emitting photoluminescence with a characteristic green-emissive broad band above 500 nm. We verified that the green emission was deduced to originate from the presence of GO nanosheet shell surrounding the PFO NPs, rather than from luminescence of GO itself or formation of keto-defects in the PFO chain. PL decay analysis confirmed that highly efficient energy transfer to lower energy state induced by the GO occurred. This work provides a new strategy for the production of highly useful conjugated polymer / graphene hybrid materials, as well as a facile synthesis for graphene-wrapped nanomaterials.

Resume : Electronic detection of biologically relevant species performed by means of disposable organic devices has the potential to revolutionize the current approach to strip testing. Bio-systems interfaced to an electronic device is presently one of the most challenging research activity that has relevance not only for fundamental studies but also for the development of highly performing bio-sensors. Completely novel approaches either involving OFET devices comprising a Functional Biological Interlayer (FBI-OFET) or Electrolyte gated-OFET (EGOFET) integrating bio-recognition elements were recently proposed by our group. Specifically, in the FBI-OFET device configuration a biological layer, acting as biosensor recognition element, is fully integrated into the device structure, right at the interface were the OFET two-dimensional transport occurs. While in the EGOFET structure the bio-recognition layer is deposited directly on the organic semiconductor using a strategy that allows a well-oriented immobilization of the biological molecules. Both the structures have been successfully employed for the detection of biological molecules reaching very low detection limits. The specific features of each configuration as well as their performances in terms of device operation, selectivity and sensitivity will be presented. The proposed bio-electronic FBI-OFET platform, besides resulting in extremely performing biosensors, can open to gather insights into biological relevant phenomena involving interfacial modifications that can be electronically detected. The proposed device configurations are also implementable with organic electronic printing processes using paper and other flexible substrates. References M.D. Angione et al. PNAS 109 (17), 2012, pp 6429-6434. M. Magliulo et al. Adv. Mater. 2013, 25, 14, pp 2090-2094. Luisa Torsi et al. Chem. Soc. Rev., 2013, 42, 8612—8628.

Resume : Melanin is an organic biopolymer that has interesting physical and chemical properties, as for example, a featureless strong absorption in the UV-Vis spectral range and good electronic and ionic conductivity. Detailed chemical studies demonstrate that eumelanin is a heterogeneous macromolecule derived from copolymerization of two molecular precursors, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The proportion of DHI:DHICA is very important for photoprotection and quelant action in the organism, and can be altered by the synthesis conditions. In this work we study the influence of oxygen pressure in the melanin synthesis and compare with the traditional synthesis describe in the literature. Synthetic melanin was obtained by oxidation of L-DOPA in a basic pH aqueous medium and with 4 atm of oxygen pressure in a reactor. Through 13C CP/MAS NMR and UV-Vis spectroscopy, it was possible to follow the process of polymerization and the optical properties of D-Melanin under different syntheses conditions. Oxygen pressure enhances the reaction kinetics and also influences the proportion of DHI:DHICA present in the monomers, changing the polymerization of synthetic melanin. With pressure the synthesis of eumelanin was accomplished in 15 hours, in comparison with the traditional method which takes typically 21 days. The obtained material has an increase proportion of DHICA/DHI, much higher than in the traditional synthesis 10%, and close to 50% very similar to natural melanin.

Resume : Paper electronics is rapidly evolving and the possibility of extending the available information beyond the printed graphic is very interesting. However, for paper electronics to be truly recyclable and/or disposable the components need be environmentally friendly and bio-compatible. Here we propose an environmentally safe approach to such active circuits and electronic functionalities, by making ion-modulated transistors on paper using a novel approach of a bio-polymer blend as the semiconducting layer and environmentally safe ionic liquids as the ion modulation layer. By blending the semiconductor poly(3-hexyl thiophene) (P3HT) with a biodegradable poly-lactic acid (PLLA), we have increased the operating speed of ring oscillators manufactured on rough and absorptive paper substrates with four orders of magnitude. The reason is the spontaneous phase-separation between the P3HT and PLLA leading to thin but continuous semiconductor films. Furthermore, we have used environmentally friendly ionic liquids as the ion-dielectric allowing for operation at ~1V. Comparing the propagation delay time of ring-oscillators made out of transistors with and without the vertical phase separation, we see a decrease in the delay time from 870 s to 35 ms, leading to ring oscillators operating at 5 Hz, manufactured on recyclable paper substrates.

Resume : In polymeric semiconducting materials, both molecular and crystalline orientations determine optical, electronic and opto-electronic properties in thin films since these properties are by essence highly anisotropic. [1] Therefore, it is of interest to control the contact plane of crystalline domains in the active layers as it determines, for instance, the direction of facile charge transport in the cases of OPV and OFET. Mechanical rubbing of polymer films has been widely used in the liquid crystal display industry to prepare oriented alignment layers of polyimides. In this work, this fast orientation method is successfully applied to a large palette of thiophene and fluorene based pi-conjugated systems i.e. semi-conducting homopolymers, donor-acceptor alternated copolymers and block co-oligomers. Highly oriented films are obtained without the use of alignment layer. Both, the temperature of the films during rubbing (Trub) and the molecular weight distribution of the polymer determine the level of orientation. All investigated semi-conducting materials show a strong increase of alignment with Trub. Interestingly, as visualized by HR-TEM, the in-plane orientation, crystallinity and polymorphism of the rubbed films can be substantially modified by post-deposition annealing. Preliminary results show that high-T rubbing of donor-acceptor block co-oligomers is also particularly efficient to create continuous and oriented domains of each block. These high levels of orientations result in highly anisotropic electronic properties (UV-vis absorption, fluorescence and charge transport). 2 1 Brinkmann M. et al. Macromol. Rapid. Comm. 2014, 35, 9. 2 Biniek L. et al. Macromolecules 2013, 46, 4014.

Resume : Phthalocyanines are promising candidates for several optoelectric devices, small molecular organic solar cells or chemiresistive gas sensors. Phthalocyanine thin films are most commonly fabricated by vacuum evaporation technique. Pulsed Laser Deposition (PLD) was recently also shown as successful PVD technique to fabricate these films. We chose Zinc Phthalocyanine (ZnPc) as representative example to compare optical and electrical properties. ZnPc thin films were fabricated by organic molecular evaporation (OME) and PLD in vacuum on fused silica substrates at room temperature. PLD used a KrF laser (λ=248 nm, τ=5 ns), where laser fluence was varied in the region from 10 mJ.cm-2 to 100 mJ.cm-2 and frequency in the interval from 50 Hz to 200 Hz. Optical properties were characterized by optical spectroscopy, spectrophotometric measurement and spectral ellipsometry. The electrical properties were analyzed by van der Pauw method. Electronic structure was analyzed by XPS. Morphology was studied by AFM and SEM. FTIR was used to examine deterioration of the deposited ZnPc films. We show that PLD could produce ZnPc thin films of competing quality with respect to evaporation technique, even further better values of Hall mobility were obtained for PLD films. Moreover PLD profits from its simplicity, modesty and flexibility that could provide also important results toward development of functional materials and devices for organic electronics.

Resume : The possibility of using naturally occurring or “man-made” organic and organic-inorganic hybrid materials for applications in the electronics and the semiconductor industry has been of great scientific and technological interest for several decades. As the total size of electronic components scales down to few nanometres and the fabrication of electronic circuits moves towards low-cost flexible substrates, the development of new high dielectric constant (so-called high-k) dielectrics that can be economically processed in ambient conditions and possess good mechanical strength is highly desirable. Organic and organic-inorganic hybrid nanocomposites are one very promising class of materials for future generations of bendable low-power electronic components and circuits. The aim of this work is to develop novel, high capacitance dielectrics based on alternative organic-inorganic nanocomposite materials that combine very high dielectric constant values intrinsic to ferroelectric ceramic materials with mechanical flexibility, low-cost and easy processing of polymers. Such nanocomposite materials will pave the way towards low-cost fabrication and integration of high performance, low-voltage electronic components and circuits on flexible substrates.

Resume : Silver nanowires are especially good candidates for transparent conducting electrode, to replace indium tin oxide (ITO) for flexible optoelectronics such as organic solar cells, organic light emitting diodes, and display devices. To produce a low resistant and highly transmittive electrode, the silver nanowire needs to have longer length and thinner diameter. In this study, the transparent conducting electrodes (TCE) were fabricated and characterized. Silver nanowires with the length of 35 μm and the diameter of 60nm were used for TCE. In order to improve the coating properties to produce the high performance electrodes, organic binders and dispersants were studied. The morphologies of coated films were characterized with an optical microscopy. According to the binders and dispersants, the properties of TCE were varied. The silver nanowire electrode on polyethylene terephthalate substrate exhibited sheet resistance of less than 100 Ω/□ and transmittance of higher than 85%.

Resume : The density of states (DOS) in the band gap of the active semiconductor layer determines the device performance. Then, it is rational to assume that material with lower DOS will improve the device performance; however, little work has been presented on the evolution of the DOS and its influence on the photovoltaic effect. In this work we correlate the evolution of the hole mobility and the density-of-states (in the donor semiconductor deposited at different substrate temperature) with the performance of organic solar cells. To evaluate charge carrier mobility a series of Thin-Film Transistors (TFTs) were fabricated at different substrate temperature (30, 60, 90 and 120 C) using tetraphenyldibenzoperiflanthene (DBP) as donor semiconductor. TFTs electrical measurement at different temperature allows evaluating the DOS distributionin the band gap, i.e., the trap densities as a function of energy. Finally, a series of solar cells with the structure glass/ITO/MoO3 (3nm)/DBP(10nm)/C70(40nm)/BCP(8nm)/Al were fabricated, with the DBP layer deposited at different substrate temperature. Best performance was obtained for solar cells in which the DBP was deposited at 60oC (efficiency 2.5%, Voc:0.89, Jsc:4.8mA/cm2). A correlation was observed with electrical performance of the solar cells and the density of localized states. Finally, a bilayer solar cell was fabricated at the optimum substrate temperature of 60C, yielding a solar cell efficiency of 4%.

Resume : For the first time we designed the synthetic pathway by which we obtained a new donor-acceptor pi-conjugated oligothiophene-fullerene C60 compound in which the two counterparts are linked covalently thanks to two ethynyl bridges. [1] As our preliminary theoretical calculations show, the HOMO→LUMO transition could experience an electron-transfer process. [1] Further preliminary photophysical steady-state (absorption and emission) and ultrafast time-resolved characterizations highlight the same kind of electron transfer process from the electron donor DTBT moiety to the electron acceptor C60 counterparts. Additional measurements are on their way to elucidate the exact mechanisms involved. In parallel with the experimental data, TD-DFT theoretical calculations were conducted on the donor-acceptor molecule and on its oligothienyl donor precursor with the purpose to study more deeply the electronic structure of the system under analysis. Following our preliminary results, further experimental evaluations are in progress in order to pinpoint the parameters affecting the occurrence of such electron transfer phenomena. Our hypothesis indicate that the donor-acceptor molecule designed and synthesized may be a good candidate for organic solar cell applications with respect of the results shown in this work. [1]D’Auria, M.; Guarnaccio, A.; Racioppi, R.; Santagata, A.; Teghil, R. Synlett 2013, 24(8), 943-946.

Resume : Metal phthalocyanines are organic compounds exhibiting interesting electrical and semiconductive properties. Metallic nanoparticles can significantly influence the electrical properties and the electronic band structure. In this work we study interaction of zinc phthalocyanine (ZnPc) with silver nanoparticles. The ultrathin silver layer (<5nm) was deposited by RF magnetron sputtering on fused silica or silicon substrates. The prepared silver layer was thermally annealed exploiting effect of Rayleigh instability for transformation of the continuous layer into nano-particles (np-Ag). The lateral size of nanoparticles was between 20 to 30 nm. The obtained layer of Ag nanoparticles was coated by thin ZnPc layer by means of organic molecular evaporation method. The thickness of this layer was around 20 nm. The resulting ZnPc/np-Ag structure was analyzed by spectral ellipsometer in the spectral range 245 to 1000 nm. The absorption bands of ZnPc were simulated by a dispersion model consisting of four Lorenz or Gaussian oscillators. The plasmon resonance feature of the Ag nanoparticles was described by Lorentz oscillator in the dispersion function. The ellipsometric analyses were combined with spectrophotometric measurements of transmittance and reflectance. The resulting band structure of ZnPc and ZnPc/np-Ag layers was compared. The surface morphology of the completed layer was analyzed by scanning electron microscopy and atomic force microscopy.

Resume : Zinc phthalocyanine (ZnPc) is an aromatic organic compound that exhibit semiconducting properties. This material is characterized by high thermal and chemical stability and optical absorption in the UV-VIS region. The ZnPc have a wide range of potential applications in the different fields such as sensors, organic thin film transistor, photovoltaic cells and others. In this study, we investigate the effect of the thin film thickness on the optical properties of the ZnPC. The knowledge of the optical properties is very important for better understanding the band structure and the energy band gap of this system. The ZnPc layers of different thickness ranging from 10 – 160 nm were prepared in the vacuum chamber by organic molecular evaporation technique. The pressure during the deposition process was about 3•10-4 Pa, the deposition rate was kept around 0,5 Å/s. The temperature of the evaporating ZnPc source was accurately controlled around 400 0C in order to conserve stable evaporation rate, but avoiding the excessive decomposition of the ZnPc molecule. Thus the high quality thin ZnPc layer can be prepared using this deposition technique. The ZnPc layers were grown on silicon and fused silica substrates. The layer thickness and the deposition rate were monitored in-situ by means of the quartz crystal microbalance (QCM). The optical properties of the prepared samples were characterized by means of spectrophotometry and spectral ellipsometry. The surface morphology was analyzed by atomic force microscopy.

Resume : Composite films consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) and graphene oxide (GO) were electrochemically synthesized using EDOT monomer in the presence of different concentrations of GO. Electropolymerization of the composites were performed in different solvent systems using ionic liquids, organic and aqueous media. PEDOT/reduced graphene oxide (PEDOT/r-GO) composite films were prepared by inducing electrochemical reduction of graphene oxide (GO) incorporated into PEDOT as the dopant. A comparative study was performed for PEDOT/r-GO composite films to that of pure PEDOT focusing on redox behaviour and structural changes.

Resume : Systems featuring large magnetoresistance (MR) at room temperature and in small magnetic fields are strongly sought-after due to their potential for magnetic field sensing and data storage. Usually, the magnetic properties of materials are exploited to achieve large MR. Recently, we have discovered an exceptionally large, room-temperature, small-field MR effect in 1D, non-magnetic systems of molecular wires self-assembled in a zeolite host crystal [1]. This ultrahigh MR effect is ascribed to the dramatic consequence of spin blockade in 1D electron transport. Reference [1] R.N. Mahato, H. Lülf, M.H. Siekman, S.P. Kersten, P.A. Bobbert, M.P. de Jong, L. De Cola and W.G. van der Wiel, Science 341, 257 (2013)

Resume : Organic materials show high potentials in the development of so-called plastic electronics. Although considerable progress was made these last years, investigations still have to be pursued in order to solve bottlenecks and boost new developments. Nowadays, the control of the active layer morphology represents one major objective to improve the stability and performances of the devices. In this communication, a series of monodisperse pi-conjugated donor-acceptor block co-oligomers made of perylenediimide unit as electron acceptor (A) and a thienofluorene-based moiety as electron-donor (D) will be presented. These materials are designed to self-assemble in a lamellar structure with alternated D/A layers, which constitute promising nanostructured organization for the preparation of ambipolar transistors and photovoltaic devices. By exploring different molecular architectures of the block co-oligomers (i.e. DA, ADA, DAD) we could rationalize the stabilization of the lamellar organization at very long range. In addition, the orientation of the lamellae (flat-on versus edge-on) onto the substrate could be controlled by using different deposition techniques and post-treatments. Such nanostructures films show ambipolar charge transport, with mobility values that depend on the length of the D block. Finally, engineering the wavefunction of the D block, turns out to be operative in stabilizing the charge transfer state lifetime up to 2-3 ns. PCCP 14, 273, (2012)

Resume : Recent literature reports revealed that natural pigments indigo and 6,6'-dibromoindigo behave as ambipolar organic semiconductors. Here we report the synthesis and investigation of nine different indigo derivatives comprising electron withdrawing groups: F, Cl, Br, CN and CF3. It was shown that molecular design can be used to tune optoelectronic properties, thin film morphology and crystal packing of indigo derivatives. Introducing electron withdrawing groups switches the semiconductor performance of the indigo derivatives from ambipolar type to unipolar n-type and improves ambient stability of the OFETs. Stable n-type OFETs were demonstrated using 5,5',6,6' tetrafluoroindigo as a semiconductor. Particular important was the observed influence of the dielectric material on the electrical performance of semiconductors. Aliphatic hydrocarbons (paraffin, tetracontane) performed better than conventional benzocyclobutene-type dielectric BCB. This effect becomes more pronounced with increase in the size of the substitutents in the indigo core: F

Resume : Functionalized molecules which tend to organize to self-assembled monolayers (SAMs) are gaining importance in organic electronic devices. They are fully compatible with flexible substrates, amenable to low-cost processing, show reliable film-forming behavior and act as key enabler to tune organic-organic or organic-inorganic hybrid interfaces. The talk provides an overview from simple auxiliary SAM-layers, which improve and modify surfaces and interfaces in thin-film devices to highly functionalized molecules creating SAMs for molecular scale electronics. - Hybrid dielectrics composed of tiny oxide layers and self-assembled molecules provide low-voltage device operation and control the morphology of subsequently deposited organic semiconductors. - By adjusting the molecular dipole of SAM molecules, the threshold voltage in organic transistors can be tuned. - Multifunctional molecules, in which several layer functions of a device are implemented, enabled self-assembled monolayer field effect transistors (SAMFETs) with p- and n-type transport and even on flexible substrates. - A combined theoretical and experimental approach describes the transport in SAMFET depending on the SAM morphology. - Examples of SAMs in organic solar cells, core-shell nanoparticles (ZnO-SAM), and “carbon-electronics” will illustrate the general potential of the molecular self-assembly approach.

Resume : The self-organization of pi-conjugated organic materials forming highly ordered supramolecular architectures has been extensively investigated in the last two decades in view of optoelectronic applications. Indeed, the control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement of charge transport properties and achievement of high charge carrier mobilities. These well-ordered materials are currently either self-organized semiconducting polymers or liquid crystals. In this context, we endeavored to investigate the self-organization of side-chain liquid crystal (SCLC) semiconducting polymers where (i) the backbone is a pi-conjugated polymer and (ii) the side groups are pi-conjugated discotic mesogens. Here we describe the design and synthesis of columnar side-chain liquid crystal homo and alternating (co)polymers with triphenylene mesogens as side groups, and well-defined regioregular polythiophene as backbone. These different kinds of architectures prepared following the Grignard methathesis (GRIM), allow the control of the triphenylene side group ratio along of the polymer chains, and lead to tunable electronic properties and nanostructures. In this work, we will give the details on the synthesis, structural characterization and morphology studied by Polarized-light Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), Temperature-dependent small-angle X-ray diffraction, Grazing-incidence X-ray scattering (GIXS) and Atomic Force Microscopy (AFM). Moreover, their charge transport properties studied in OFET configuration will also be depicted.