Polymorphism and semi-crystalline nanomorphology are two essential structural parameters that control charge transport in thin films of semi-conducting polymers e.g. regioregular poly(3-alkylthiophene)s and more complex macromolecular systems like rylene diimide copolymers. It is therefore essential to understand fundamental growth mechanisms and develop means to control/observe the nanomorphology and polymer structure in thin films. This presentation will give fundamental insight into the structure/morphology of both regioregular poly(3-alkylthiophene) and rylene diimide copolymers obtained by Transmission Electron Microscopy (TEM). In the first part, we will present recent results using electron diffraction and low dose High Resolution TEM on the semi-crystalline nanomorphology and the polymorphism of various P3ATs including P3HT. Structural models obtained from Electron diffraction analysis of form I and form II of P3HT will be presented. The second part of this presentation will be devoted to poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDI2OD-T2). It is demonstrated that controlled growth of highly oriented films by epitaxy on oriented poly(tetrafluoroethylene) substrates and directional epitaxial crystallization lead to two different polymorphs characterized by two distinct stacking modes (segregated and mixed) of naphthalenediimide and bithiophene units that translate into specific spectroscopic signatures.
[1] N. Kayunkid, S. Uttiya and M. Brinkmann, Macromolecules 2010, 43, 4961
[2] K. Rahimi, et al. Angew. Chem. Int. Ed. 2012, 51, 11131
[3] .T. S. Salammal, et al. Macromolecules, 2012, 45, 5575.
[4] Hartmann, L.; Tremel, K.; Uttiya, S.; Crossland, E.; Kayunkid, N.; Ludwigs, S.; Kayunkid, N.; Vergnat, C.; Brinkmann, M. Adv. Funct. Mat. 2011, 21, 4047.
[5] M. Brinkmann, et al. ACS Nano, 2012, in press.

Understanding the microstructure and donor-acceptor interfaces is vital to the development and optimisation of solar cell devices. However, analysis of microstructure and interfaces in cross-sections of organic solar cells using the transmission electron microscope (TEM) has proved to be a challenging research topic due to problems associated with sample preparation and radiation damage. In this contribution we report a new approach that has been optimised for cross-sectional analysis of organic films in solar cells. This process involves the removal and thinning to electron transparency of a TEM foil from the bulk of the sample using a focussed ion beam (FIB) instrument. We have developed a methodology to minimise the damage induced by the Ga ions and demonstrate that this allows the acquisition of high quality imaging and spectroscopy data.
In the TEM samples prepared using this approach, the crystalline nature of both copper phthalocyanine (CuPc) and C₆₀ films can be observed in cross-section. The crystallographic orientation in the films is consistent with the x-ray diffraction (XRD) data and demonstrates that the damage during the sample preparation has been minimised. The high resolution TEM images of the CuPc film exhibit lattice fringes which are a result of diffraction from the Cu-rich (100) plane that is parallel to the substrate surface. The HRTEM images of the C₆₀ film show fringes that result from diffraction from the (111), (220) and (311) planes.

Analysis of the HRTEM images reveals regions where the (100) fringes in the CuPc film are discontinuous. These regions are attributed to grain boundaries within the CuPc film. Since charge transport in CuPc is highly anisotropic, the presence of these boundaries is highly relevant to the charge transport mechanism in the film and is directly correlated to device efficiency.
Using scanning transmission electron microscopy (STEM) combined with high efficiency detector energy dispersive x-ray spectroscopy (EDX), elemental maps of the heterostructure can be obtained. This approach allows the interfaces between donor and acceptor materials to be clearly identified. Estimates of the roughness of the donor-acceptor interface can be obtained and analysis as a function of interfacial area provides insights in chemical or molecular interdiffusion at the interface. Both these parameters can be correlated with device efficiency.
In conclusion, we show for the first time that cross-sections of organic solar cells can be prepared and imaged in the TEM without the loss of their crystalline nature. This enables the crystal structure, local orientation and chemical composition to be examined with high spatial resolution. This information has been used to obtain insights into domain size, interface roughness and interfacial chemistry in bulk heterojunctions.

Recent developments in materials and device fabrication processes are leading to rapid improvements in performance of organic semiconductor devices. In particular, solar conversion efficiencies up to 9-10 % were reported for solution-processed organic photovoltaic (OPV) devices. Despite their significantly improved device performance, a number of scientific challenges remain to more fully understand, quantify, and predict the behaviour of bulk heterojunction OPV devices. To understand the interplay between the organic blend thin films and devices, it is therefore important to identify the structure-property relationships of the organic materials and their impact on device performance.
Here, we report the key advances on our fundamental understanding of the structure-property relationships of bulk heterojunction OPV devices with a particular focus on the effects of molecular order and thin film morphology of conjugated polymer/ fullerene blends. As one of the most valuable structural probe techniques for OPV blends, we have extensively used Raman spectroscopy [1-5]. Raman spectroscopy is a non-destructive technique which can deliver valuable chemical/ structural information together with optical properties of materials with sub-micrometer spatial resolution from surface films as well as from buried layers in the devices. We will present our recent resonant Raman studies performed on bulk heterojunction OPV thin films and devices; (i) to identify the nature of Raman modes of P3HT and their correlation to the degree of molecular order in P3HT:PCBM blend thin films and devices [1], (ii) to clarify the effects of heteroatoms in thiophene rings (P3HT vs P3HS) in terms of quality and quantity of ordered phase and their impact on devices [2] and (iii) to monitor in-situ diffusion and crystallization of P3HT molecules in blends (with/ without additives) during thermal annealing. Our results clearly unveil the important structure-property relationships of bulk heterojunction OPV thin films and devices. Furthermore, it clearly demonstrates resonant Raman spectroscopy as a simple, powerful structural probe which is sensitive not only to ordered phase (as similar to X-ray based methods) but also to disordered phase of molecules. Hence, it provides important information on “fraction/ quantity of ordered phase” which is not easily accessible using traditional XRD techniques.
References
[1] Tsoi et al., J. Am. Chem. Soc., 133, (2011), 9834-9843
[2] Tsoi et al., ACS Nano (2012), http://dx.doi.org/10.1021/nn304024g
[3] James et al., ACS Nano 5 (12), (2011), 9824-9835
[4] Yim et al., Nano Lett., 10, (2010), 385-392
[5] Kim et al., J. Am. Chem. Soc., 130, (2008), 13120-13131

Understanding and controlling carrier transport in conjugated polymer films and composites is critical to the development and application of plastic solar cells. Recent efforts have focused on “bulk heterojunction” (BHJ) structures where a conjugated polymer donor is mixed at the nanoscale with a fullerene acceptor to achieve large interfacial areas for exciton splitting. In these systems, fabrication protocols dramatically affect device efficiency and charge transport is intimately tied to film morphology through local order, domain formation, and compositional heterogeneity. We employ both far-field (absorbance, low-temperature PL, Raman) and confocal/near-field optical spectroscopy to study polymer order (aggregation, π-stacking), photo-oxidation, and local morphology in conjugated polymer (PPV and polythiophene) - fullerene (PCBM) blends. Through quantitative analysis of exciton bandwidths, emission intensity, and vibronic lineshapes, we demonstrate that competition exists between the chemical “disordering” effect of photo-degradation and the physical “ordering” effect of aggregation, each of which dominate under different processing conditions. Large changes in PL and Raman show that PCBM begins to significantly hinder planarization (intra-chain) and π-overlap (inter-chain) of polymer chains at a critical PCBM weight fraction [1]. Furthermore, the critical weight fraction is a function of the polymer regiochemistry (occurring at lower PCBM weight fractions for a more regio-random polymer). Mild thermal annealing of blended films was seen to restore order (i.e., vibronic PL line shapes, indicative of H-like aggregation), which result from PCBM phase segregation (lower dispersion) and growth of polymer aggregates. Spatially resolved spectral analysis of PL was used to map fullerene diffusion and agglomeration as well as detect local changes in interfacial contact between donor and acceptor domains due to thermal annealing.
[1] C. Carach, I. Riisness, and M.J. Gordon, Appl. Phys. Lett. 101, 083302 (2012).

Recently, a new molecular semiconductor dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene (DNTT) has attracted extensive interest due to its high air-stable mobility in transistors and relative ease of synthesis. In this work, thermally deposited DNTT films with thickness of 1-2 monolayers have been continuously monitored ex situ using dynamic mode scanning probe microscopy (SPM) techniques. An unusual recrystallization is observed occurring in two-layer films with close-to-complete first monolayer and low coverage of second layer; however, this process is absent in sub-monolayer films within the time frame of interest. A local strain accumulation within the uncovered first layer originating from the nucleation and growth of the second layer is thus proposed and examined. We demonstrate that the resultant inhomogeneous strain distribution drives the highly strained, dynamically active first layer molecules to seek strain-relieved states by climbing up to their nearest upper layer, which is characterized as a continuous growth of the second layer grains until the depletion of the neighboring first layer. Surprisingly, concurrent surface potential mapping of these two-layer films reveals an insignificant lateral evolution of different domains as opposed to the striking morphological change, suggesting that a distinct interlayer coupling is very likely adopted by the reconstructed domains than the original film. This study provides a direct visualization of the film degradation process, emphasizes the critical role of strain on the morphological integrity of organic molecular films, and therefore has important implications for fabricating organic thin film devices with improved stability and lifetime.

Scanning transmission x-ray spectro-microscopy (STXM) has been demonstrated to be an excellent tool for the study of organic materials due to its high spatial resolution and strong contrast based on a variety of spectroscopic mechanisms. Of particular interest to the study of organic electronics is the use of carbon K-edge spectroscopic features to derive image contrast based on the molecular structure of the sample. This gives STXM a strong advantage over other imaging techniques that are often limited to density or elemental contrast mechanisms. A further advantage of STXM is the ability to utilise spectroscopic information to easily derive quantitative maps of composition and/or molecular orientation.
The PolLux STXM instrument has demonstrated novel analyses of conjugated polymer systems, including quantitative molecular orientation and orientational order mapping of annealed films, and simultaneous surface and bulk composition mapping of blend films. The NanoXAS instrument, on the other hand, is a combination of STXM and a scanning probe microscope (SPM) that aims to combine the strong contrast mechanisms of STXM with the superior spatial resolution of SPM. Together, the PolLux and NanoXAS instruments are striving to reveal the nanostructures that heavily impact the performance of organic electronic devices.

Small molecule organic thin films can be manufactured from domestically available, low toxicity materials, making these materials attractive for portable photovoltaic devices. The properties of devices made using these materials can vary dramatically depending on their preparation procedures. Thin films prepared using different methods are observed to have different structures on the micro scale: small microcrystalline domains are observed in these films, and their size, orientation, shape and arrangement define this microstructure. Little is known about how this microstructure affects local photophysics or how that translates into bulk device performance, since measurements on these materials are usually performed without spatial resolution on the bulk film.
In this work, spatially resolved transient absorption measurements of exciton dynamics were performed on thin films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) prepared using various techniques. Owing to its high hole mobility, stability in air and solution processibility, TIPS-pentacene is a promising material for high-performance organic semiconductor devices. Optical microscopy indicates that thin films of TIPS-pentacene consist of long, thin domains that can reach up to millimeters wide and up to a centimeter long, depending of preparation procedures. However, it is unknown whether photophysical properties are uniform within one domain; the nature of the photophysical variation between domains and at domain boundaries also remains a mystery. By utilizing sub-domain sized excitation volumes in our transient absorption measurements, significant spatial inhomogeneity was revealed in the photophysics of individual domains. Domains which appear to be homogeneous in linear optical microscopy are shown to have defects and variation, and notable differences in behavior are observed at domain boundaries. A number of film characterization techniques, including double polarizer, local absorption and profilometry measurements, were used in concert to provide a physical picture of why these photophysical differences were observed. This deeper understanding of how microstructure affects photophysics will inform the selection and preparation of these materials in the future.

Molecular ordering in organic thin films significantly affects their usefulness as electronic materials. Control of this ordering is a central challenge that requires significant research and will have profound implications for the development of flexible electronics. Direct writing gives us the ability to deposit films from solution with controlled thickness, grain structure and orientation. We have investigated TIPS-Pentacene films deposited from toluene solution at various speeds via a combination of real time synchrotron x-ray scattering and polarized-light video microscopy. Through video microscopy we observe a well-defined crystallization front that becomes less defined as the writing speed is increased. In synchrotron x-ray scattering we observe that the ordering process is an order of magnitude slower than what is seen under the optical microscope. The time resolved study gives an insight that the subsurface regions in the film remain disordered for up to several seconds while the top surface looks completely crystalline when seen under the microscope.
In this present investigation, we have observed the formation of buckles in the film that can also lead to its delamination from the surface. This effect can produce mechanically failed thin films due to presence of cracks and defects in the final stage. This phenomenon occurs due to the development of in-plane stress in the film under defined conditions. Grazing incidence X-ray diffraction measurements suggests a relationship between the evolution of in-plane stress and the lateral crystallization from solution at various speeds, concentrations and substrate temperatures. Finally, the influence of in-plane stress on the charge carrier mobility has also been verified by Field effect Transistor (FET) measurements.

Organic non-volatile rewritable memories are key elements for low-cost organic electronic circuits. A promising realization of such an element combines functional properties of different materials in one device. In the MEMory Organic Light-Emitting Diode (MEM-OLED) an organic ferroelectric component provides the switching functionality whereas the (modulated) conductivity and electroluminescence of a conjugated polymer provide the read-out. A major advantage of this design is the greatly enhanced freedom to independently tune the ferroelectric and semiconducting properties [1]. However, the incomplete understanding of their structural and optoelectronic properties at the nanoscale severely hampers exploiting this freedom.
Here, we combine advanced scanning probe microscopy techniques with selective chemical dissolution experiments for investigation of the 3D structure and the electrical properties of the MEM-OLED at the nanoscale. We show that the blends consist of round, 300 - 750 nm sized semiconducting (F8BT) domains in an homogeneous crystalline ferroelectric (P(VDF-TrFE)) matrix. A 3D reconstruction of the blend reveals three different types of domains: completed (concave), floating (convex) and non-completed (sticking to the bottom electrode). We find that only the completed domains are electrically active. We show that the current is injected at the perimeter of such semiconductor domains and is modulated by the stray field of the nearby ferroelectric material. This supports the concept proposed in Ref. 2. Moreover, we find that the domains can be individually addressed, i.e. be individually switched and be individually read out. They can be used as separate bits. This translates into an ultimate information density of this type of device of ~30 Mb/cm2.
Local switching is achieved by scanning a device (without top contact) with an appropriately biased AFM tip. Subsequent monitoring of the polarization state of the ferroelectric is done by piezo force microscopy. The local conductivity, i.e. the on- or off-state of individual domains is probed by low-current conductive AFM. For the 3D reconstruction additional use is made of scanning Kelvin probe microscopy and selective dissolution of either the ferroelectric or the semiconductor component followed by conventional AFM.
1. K. Asadi, D. M. de Leeuw, B. De Boer and P. Blom, Nat. Mater. 2008, 7, 547.
2. M. Kemerink, K. Asadi, P. W.M. Blom, M. de Leeuw, Org. Electron. 2012, 13, 147-152.

Recent research and progress in organic photovoltaic (OPV) repeatedly insist on the importance of the molecular organization of the compounds forming the active bulk-heterojunction (BHJ) blends. The morphology of the blend has been to tremendously affect both the charge transfer at the donor-acceptor interface and the carrier transport to the electrodes. And still, for each material combination, much remains to be understood to fully assess its specific and ultimate morphology. For this purpose, high resolution characterization methods are of primary interest to locally depict the different electrical mechanisms ruling the photovoltaic process. Conductive Atomic Force Microscopy (C-AFM) and Kelvin Probe Force Microscopy (KPFM) have already proven to be of significant help to yield nanoscale two-dimensional mapping of electrical properties. C-AFM and related PeakForce TUNA emerged as powerful technique to electrically evidence phase separation in blends. An additional key feature lies in local I-V curve providing useful information about the charge transport mechanisms within the materials forming the blends. Quantitative measurements leading to local determination of hole mobility have already been reported. It appears that upon illumination the technique has shown to be sensitive to photocurrent. With photoconductive-AFM (pc-AFM), a dedicated external calibrated module has been recently introduced allowing full quantitative mapping of photovoltaic mechanisms. Here, we are carefully analyzing BHJ made of poly(3-hexylthiophene) (P3HT) and fullerene derivative (PCBM) as a case study. While photocurrent is determined at 0 V DC bias, additional parameters including the open-voltage, the fill factor and the resistances can be obtained spanning the DC bias between the probe and the sample back electrode. The measured current is attributed to hole transport in P3HT. The zero signal across the PCBM cluster yields from large injection barriers at the contacts with electrode. Furthermore, opposite current values while varying DC sample bias from 0 to -1 V allows clear differentiation between photo-generated from injected current. Kelvin Probe Force Microscopy (KPFM) is also used in this work to delineate phase separation and potential variations at interfaces. Upon illumination, photovoltage can also be evidenced. Yet, in organic electronics, KPFM still suffers from harsh operating environment (ultra-high vacuum and low temperature) to reach satisfactory spatial resolution and lacks for modeling for quantitative measurements. Augmenting KPFM with the PeakForce Tapping technology allows to sufficiently improve the spatial resolution for KPFM measurements in ambient conditions. With the additional external calibrated illumination module, mapping of photovoltage in BHJ blends can be obtained, opening the doors of local characterization of charge transfer at donor-acceptor interfaces, where crucial processes are occurring in photovoltaic devices.

The electrical properties of solution-processed conducting polymer/nanocrystal composite films have been studied as a function of nanocrystal loading and chemical doping. In the hybrid Polyanaline and nanoparticle-Titania films fabricated for this study, the electrical conductivity exhibits a peak at an intermediate nanoparticle loading fraction. This peak in conductivity scales with chemical doping of the polyanaline, and can be explained with a model that includes a high conductivity volume of polyanaline at the nanoparticle-polymer interface. Our studies suggest that this increase in conductivity results from a structural modification of the polymer, and elucidates a novel route for optimizing transport in nanoscale organic/inorganic composites.

Recently, we have synthesized and characterized potassium 3,3'-(pentacene-6,13-diylbis(sulfanediyl))dipropanoate, the first water-soluble pentacene (WSP) derivative. Photodegradation studies indicate that WSP is the longest lived pentacene derivative known. With full exposure to light and air, the solution phase half-life of WSP is approximately 4 days, making it a promising candidate for thin-film devices prepared via solution processing. Thin-film devices including organic photovoltaics and organic light emitting diodes require multi-layer constructions in which successive thin-films are deposited from solvents of different polarity. Thus, a water soluble organic semiconductor should be deposited atop a layer of organic soluble semiconductor and vice versa. Likewise, a bilayer solar cell was constructed using WSP as a donor and C₆₀ as an acceptor. WSP ink formulations suitable for ink-jet printing have been prepared and printed onto a variety of substrates including paper and flexible plastic. WSP has also been patterned onto a positively biased damascene [Au-(modified)SiO₂] template using an electrophoretic method. Fabrication of flexible transistors using this method is now under way. Based on cyclic voltammetry studies, WSP is expected to show molecular-switching behavior. Finally, the synthesis and characterization of several different carboxylate salts (e.g., potassium, sodium, cesium) of WSP will be discussed.

Pentacene and C₆₀ are well-known donor and acceptor organic materials used in thin-film organic photovoltaics. Whether they are utilized in bi-layer or bulk heterojunction solar cells, it is important to understand their chemistry with one another in order to predict molecular structure at the donor-acceptor interfaces. Although the Diels-Alder chemistry between C₆₀ and pentacene (and also aryl substituted pentacenes) has been thoroughly studied, there have been no reports concerning the chemistry between C₆₀ and 6,13-bis(organothio) substituted pentacenes. Yet these pentacene derivatives are the most promising for solar cell applications due to their unusual resistance to photooxidation and their excellent solubility, both of which enable solution processing. We now report Diels-Alder cycloaddition reactions between C₆₀ and organothio substituted pentacenes. Both mono and bis [60]fullerene adducts of substituted pentacenes have been detected and characterized. The solution state and solid state reactions between [60]fullerene and 6,13-bis(decylthio)pentacene (DTP) were studied and compared. Based on morphological studies using SEM and electronic property determinations using UV-vis spectroscopy and electrochemical methods, we expect that the soluble and super hydrophobic bis([60]fullerene) adduct of DTP (BC₆₀DTP) can be used as an acceptor for solution processed organic solar cells. Fabrication of bulk heterojunction solar cells using a BC₆₀DTP:P3HT blend is ongoing.

Simulation of hopping events of excitons and charge carriers is important to understand and predict the nanoscale charge transport processes and device efficiency in organic photovoltaics. Markov chains can be used to predict the execution of exciton hopping with significant reduction in the computational time. Forster method provides basis for the exciton hopping, where, the excitons hop in the active layer until they either recombine or reach donor - acceptor interface. Monte-Carlo methods can simulate nanoscale variations in the blend morphology and associated energetic disorder in the active layer. Markov chains, or more specifically, absorbing Markov chains can be created to theoretically determine characteristics of exciton behavior with respect to device morphology and the energetic landscape. Absorbing Markov chains can determine the number of hops from a location until absorption, the absorption probability at a given interface, and the probability that a given exciton will fail to dissociate. In order to begin charge transport based on Marcus theory, charge injection probabilities must be determined from absorption probabilities. Using Monte Carlo simulations to generate these injection probabilities can be computationally exhausting. However, Markov chains quickly use simple mathematical formulas to generate these injection probabilities in substantially less computational time. Different morphologies can also be rapidly evaluated to determine the most efficient morphologies to generate optimum exciton hopping. Results agree with Monte Carlo simulations of the success rate of exciton dissociation and frequency of dissociation at interfacial sites with respect to different morphologies and energetic landscapes. Markov chains represent a quick and efficient way to model multiple aspects of exciton hopping. A fast evaluation of morphology performance with respect to exciton dynamics can be generated using Markov Chains.

Modeling of nanoscale carrier transport processes in bulk heterojunction solar cell is challenging due to its complex microstructure, morphology and energetic disorder. Several recent reports discussed the diffusion of excitons in simple morphologies. Present study explains the results of detailed Monte-Carlo (MC) simulations of exciton dynamics using Förster model in 3D morphologies of bulk heterojunction solar cells. The study details the influence of donor and acceptor domain sizes and their macroscale ordering in various geometries (such as checkerboard, columnar, bilayer, random blend) and energetic disorder on statistical quantities such as exciton diffusion length, dissociation time and diffusion coefficient. Excitons are generated in both donor and acceptor materials, following Beers Lambert&’s law and exponential decay lifetimes. Site energies are assigned using Gaussian distribution function and energetic disorder is varied from 0 meV to 400 meV. Calculated exciton diffusion coefficient is 3x10^-3cm²/s at 100 meV energetic disorder, which is similar to the reported experimental values [1, 2]. Simulation results show that exciton success efficiency and diffusion coefficient decreases exponential with increasing energetic disorder. Moreover exciton diffusion coefficient decreases with increasing domain size and saturates for energetic disorder greater than 200 meV. The strength of geminate recombination is calculated by taking into account of exciton binding energy, bound polaron pair binding energy and the driving force. The driving force is LUMO_A-LUMO_D for the exciton coming from donor side while it is HOMOA_A-HOMO_D for the exciton entering from acceptor side. It is found that exciton diffusion is a function of morphology and energetic distributions while charge transfer state recombination probability increases with local energetic disorder. Simulation results show an averaged exciton diffusion length of 10 nm at 100 meV energetic disorder and successful excitons are diffusing within 200 ps to Donor/Acceptor interface.
[1] A. Lewis, et al., "Singlet exciton diffusion in MEH-PPV films studied by exciton-exciton annihilation," Organic electronics, vol. 7, pp. 452-456, 2006.
[2] P. E. Shaw, et al., "Exciton diffusion measurements in poly (3-hexylthiophene)," Advanced Materials, vol. 20, pp. 3516-3520, 2008.

The active region of solution processed organic photovoltaic films is typically a blend of electron donating and electron accepting molecules with pronounced concentration gradients on length scales of 20 nm. The rationale for this basic morphology is to increase the probability that a diffusing exciton reaches a molecular heterojunction where it can undergo charge separation before it recombines. We observe that ultrafast carrier generation accounts for 70% while exciton diffusion accounts for only 30% of the initially photogenerated carriers. This ratio is expressed in a variety of systems, indicating that detailed sample morphology is irrelevant in fixing this ratio. The generality of this behavior can be explained by pointing out that the position of the initial photoexcitation is subject to the Heisenberg Uncertianty Principle, and as such, is delocalized on length scales comparable to those in a bulk heterojunction solar cell.

The performance of the light-harvesting active layer of several photovoltaic (PV) technologies (e.g. amorphous silicon and organic photovoltaics) is limited by the overlap of the absorption spectrum with the solar spectrum. Over the last decade the phenomenon of triplet-triplet annihilation-assisted photon upconversion (UC) has been demonstrated for a wide variety of molecular triplet sensitizer/annihilator systems. More recently these efforts have focused on efficient UC of photons from the near-infrared to visible regions of the solar spectrum. These observations suggest that photons below the bandgap of amorphous silicon (a-Si) can be converted to photons with higher energies, which can then be absorbed in the active layer of a-Si devices and converted into carriers, thereby enhancing the photocurrent.
We will outline recent efforts to synthesize novel light-harvesting chromophores that (i) possess large near-infrared extinction coefficients, (ii) manifest ultrafast intersystem crossing to generate the triplet state at unit quantum yield, and (iii) possess triplet lifetimes exceeding several microseconds. We will subsequently discuss factors affecting the optimization of the photon UC efficiency of these sensitizers in combination with various triplet annihilators, the incorporation of the best-performing systems into device architectures employing transparent a-Si devices, and the observed performance of the coupled UC-PV system.

Extensive research activities have been focused on developing new fullerene bis-adducts in order to increase the open-circuit voltage (VOC) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Fullerene tris-adducts have a potential to further increase the VOC in PSC due to their higher LUMO level than that of fullerene bis-adducts. However, no successful example for the use of fullerene trisadduct in the PSCs has been reported because of their low electron mobility, causing much decreased short circuit current (JSC) and fill factor (FF). In this work, we have exploited fullerene tris-adducts as third component in the active layer consisting of P3HT and fullerene mono-adducts. We observed that when small amount of fullerene tris-adducts was incorporated into P3HT:fullerene mono-adducts, fullerene tris-adducts can enhance the VOC and JSC by facilitating the charge separation at the P3HT:acceptor interface, because they can mediate the energy levels between P3HT and fullerene mono-adducts. Therefore, the PSC devices consisting of ternary blend of P3HT, fullerene mono- and tris-adducts exhibited higher power conversion efficiency than those of P3HT:fullerene mono-adducts. The electrical and optical properties of the PSCs as a function of the weight fraction of fullerene tris-adducts were carefully investigated to elucidate the role of fullerene tris-adducts in the devices, which will be discussed in our presentation.

Structural control of solubilizing side groups in fullerene-based electron acceptors is critically important to optimize their performance in bulk heterojunction (BHJ)-type polymer solar cell (PSC) devices. The structural changes of fullerene derivatives affect not only their optical and electrochemical properties but also their solubility and miscibility with electron donor polymers. Herein, we synthesized a series of o-xylenyl C60 bis-adduct (OXCBA) derivatives with different solubilizing side groups to systematically investigate the effects of fullerene derivative structures on the photovoltaic properties of PSCs. The xylenyl side groups on the OXCBA were modified to produce several different OXCBA derivatives in which the xylenyl groups were functionalized with fluorine (FCBA), nitro (NXCBA), methoxy and bromine (BMXCBA), and phenyl groups (ACBA). End group modifications of OXCBA dramatically affect photovoltaic performance in blend films with poly(3-hexylthiophene) (P3HT), resulting in power conversion efficiencies (PCEs) ranging from 1.7 to 5.3%. We found that this large range in PCE values is mainly due to differences in the blend morphology and interfacial area of the P3HT:OXCBA derivative films caused by changes in the hydrophobicity of the OXCBA derivatives and their interfacial interaction with P3HT. The trend in photovoltaic performance of the different OXCBA derivatives agrees well with those of the interfacial tension, PL quenching, and exciton dissociation probability, which suggests that changes in the interfacial interaction with P3HT are largely responsible for their photovoltaic performances. Finally, the OXCBA derivatives were applied in inverted type PSC devices. We note that P3HT:OXCBA blend devices exhibited over 5% PCE with excellent air stability, which is one of the best inverted type devices based on the P3HT polymer in a simple device architecture without any extra interlayers.

We describe synthesis of a series of semi-conducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in the carbon, silicon and germanium bridge promote solubility. All the three type of copolymers were fractionated by preparative gel permeation chromatography. We investigated the effects of this additional purification step to the conventional methods used for purification of conjugated polymers. The device performances of these materials in solar cells were compared to non-purified polymers. The performance of fractionated polymers was enhanced by 30% in comparison to the non-purified parent polymers, reaching the power conversion efficiencies of about 6.5%. Nano- to milli-second Transient absorption spectroscopy (TAS) was employed to analyze the charge generation dynamics in polymer/fullerene blend thin films.

The performance of both polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs) via cathode modification using solution-processed tetraoctylammonium bromide (TOAB) has been studied. However, it was reported that the TOAB crystals undergo a packing adjustment at 50°C and form isotropic melt at 101°C. To understand the temperature effect of TOAB on the electron extraction/injection in PSCs/PLEDs, the films of TOAB atop the active layers in PSCs and PLEDs were annealed at different temperatures. The active layers in PSCs and PLEDs were the blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) and green emissive poly(9,9-dialkylfluorene) derivative (G-PF), respectively. The ethanol solution of TOAB (2mg ml-1) was spin-coated atop P3HT:PCBM and G-PF to prepare the films of TOAB and then annealed at 30°C, 60°C, or 100°C (denoted as TOAB-30, TOAB-60, and TOAB-100). The annealed films of TOAB were characterized by atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD). The different surface morphologies, explored by AFM, indicates that the TOAB molecules atop P3HT:PCBM and G-PF may re-assembled during annealing processes. The re-assembly of TOAB molecules, being evidenced by synchrotron XRD, showed the ordered lamellar structure of TOAB stacked upright atop P3HT:PCBM and G-PF after being annealed at 30°C and 60°C. However, it became amorphous after being annealed at 100°C. PSCs and PLEDs with TOAB-30/Al and TOAB-60/Al cathodes showed much better performance than those with TOAB-100/Al and Al cathodes. The power conversion efficiency was 2.14%, 3.97%, 3.10% and 0.84%, respectively, for PSCs with TOAB-30/Al, TOAB-60/Al, TOAB-100/Al and Al cathodes. Meanwhile, The brightness at 7 V was 35289 cd m-2, 25341 cd m-2, 16 cd m-2, and 130 cd m-2, respectively, for PLEDs with TOAB-30/Al, TOAB-60/Al, TOAB-100/Al and Al cathodes. Our results demonstrated that the ordered lamellar structure of TOAB play an important role of the introduced interfacial dipoles for enhancing the electron injection/extraction in PSCs and PLEDs.

The affect of impurities on semiconductor device function and lifetime is well known in the more mature, inorganic semiconductor technologies. Less is known about what role impurities play in organic electronics, especially in device lifetime. In a lifetime study of bulk heterojunction solar cells, fabricated from the semiconducting polymer poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-bprime;]dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD), we find that introducing more stringent purification procedures, specifically aimed at removing low molecular weight organic impurities, leads to greatly improved device lifetimes.
The molecular weight of PBDTTPD affects the device performance of PBDTTPD:fullerene solar cells. The power conversion efficiencies with higher molecular weight PBDTTPD can reach 8.1%, even without the use of solvent additives. However, devices fabricated from higher molecular weight PBDTTPD degrade significantly in only a few days after storage under inert conditions and in the dark. The symptom of degradation presents itself as a loss in short circuit current, caused by S-shapes in the reverse bias region of current-voltage curves that grow with time. This degradation symptom is correlated to low molecular weight impurities, possibly including monomers, trapped within the higher molecular weight polymer during polymerization procedures. Further purification of higher molecular weight PBDTTPD, aimed at reducing the content of low molecular weight organics, eliminates the S-shape degradation symptom and improves device lifetime, without decreasing initial device efficiency. We conclude that by improving materials purity, organic electronic device lifetimes can be dramatically improved.

Organic photovoltaics (OPV) have attracted increasing interest over the last decade resulting in efficiencies reaching more than 10 %. [1] This promising and rapid development is driving organic photovoltaic technologies towards commercialization. To fully exploit the touted potential of this plastic electronics platform, however, other prerequisites need to be fulfilled: for example, good mechanical stability and ease of processing. So far, these relevant attributes have, however been challenging to realise in OPV architectures. One reason for this is the fact that generally OPV active layers are of less than 100 to 150 nm thickness. This is required to optimise device performance. [2] Here we present a range of optoelectronic and structural properties of ternary blends for photovoltaic applications comprising an inert, insulating component that provides for some of these features. Indeed, such systems previously have been demonstrated to display favourable rheological and mechanical properties compared to the common donor:acceptor binaries. [3] We will focus on blends of the prototypical donor:acceptor combination of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PC61BM), with the electrically insulating high-density polyethylene (HDPE) as a third component. We will illustrate that various morphological, and thus optoelectronic properties, strongly depend on the active layers&’ thickness and provide evidence that films comprising the ternary blend display sufficiently high photovoltaic properties at increased thicknesses compared to the systems that contain no HDPE. These observations are supported by data obtained on other OPV systems such as polydiketopyrrolopyrrole derivatives and PC71BM.
1. Green, M. A., Emery, K., Hishikawa, Y., Warta, W. & Dunlop, E. D. Solar cell efficiency tables (version 40). 606-614 (2012).doi:10.1002/pip
2. van Bavel, S., Sourty, E., de With, G., Frolic, K. & Loos, J. Relation between Photoactive Layer Thickness, 3D Morphology, and Device Performance in P3HT/PCBM Bulk-Heterojunction Solar Cells. Macromolecules 42, 7396-7403 (2009).
3. Müller, C. et al. Tough, Semiconducting Polyethylene-poly(3-hexylthiophene) Diblock Copolymers. Advanced Functional Materials 17, 2674-2679 (2007).

Efficient organic photovoltaics depend on a high conversion of incident photons to charge carriers, and thus heavily rely on the ability for excitons to migrate to a heterojunction interface. Typically this is achieved through thorough mixing of the electron donor and acceptor materials, which results in a disordered, hard-to-control morphology. Here we present a novel self-assembling ordered heterojunction system for potential use in organic photovoltaics. In our system, the liquid-crystalline electron donor forms ~25x25nm helical nanofilaments (HNF), a nanorope with a diameter on the order of a typical exciton diffusion length. HNFs can be aligned with a thermal gradient and nano-phase separate when mixed with other small molecules. This leads to an ordered heterojunction architecture. We have studied the HNF-forming P-9-OPIMB in mixtures with PCBM, a soluble fullerene. When mixed 50:50 by weight, but under different conditions, P-9-OPIMB and PCBM form two unique morphologies: domains of HNFs, and domains of a lamellar structure, as characterized by TEM images and X-ray diffraction. Using flash-photolysis time-resolved microwave conductivity (TRMC) we have quantified the exciton-to-carrier efficiency of both of these morphologies. The helical nanofilament phase shows a promising improvement in over the lamellar morphology composed of the same materials.

We theoretically investigate the relation between the current and the photodissociation rate of excitons at the bilayer interface by taking into account Coulombic interaction among holes and electrons and recombination between them at the interface. We obtain exact analytical results on the relation between the current and the photodissociation rate and on the electrostatic internal potential in each layer. These results are useful to interpret experimental results between the current and the excitation light intensity in bilayer organic photovoltaic cells.

We observed a shift in the photoluminescence of molecular organic thin films due to mechanical compression. We demonstrate that the mechanical pressure causes film deformation and decreases intermolecular distance, creating different dielectric solvent environments around the luminophore molecules without changing the overall film composition. In our experiments we excited a 100nm thick Alq3:DCM thin film doped with 1% of DCM molecules, using a CW laser emitting lambda; = 500nm wavelength light. We observe a repeatable and reversible bathochromic shift in peak PL emission of more than 10nm with increasing pressure. The magnitude of the shift is equivalent to the previously observed spectral shift with increased doping concentration of DCM in Alq3, which is quantifiably attributable to the solid state solvation effect (SSSE) in doped molecular thin films. For increasing compression, the shift in the peak wavelength is consistent with “continuum” dielectric model, where increasing pressure corresponds to decreasing the size of the spherical cavity in the approximation of the guest molecule in the host material. Controlled opto-mechanical probing experiments and modeling of the effect of intermolecular distance on excitonic energy levels, self consistently explain the observed phenomenon. Consequences of the spectral shift of molecular thin films under mechanical pressure have a profound implication on the energy level structure of molecular optoelectronic devices, as will be discussed.

Small molecule acceptors in organic solar cells (OSC) have displayed promise lately as potential alternatives to the fullerene PCBM (phenyl-C61-Butyric acid-Methyl ester). Acenes seem to be a suitable building block for such acceptors, due to their strong absorption and tunable LUMO energies. However, although our acene molecules show a very high mobility when evaporated or drop cast, they typically exhibit poor morphology when spin cast from solution. Even so, out CN TCPS pentacene has shown promising power conversion efficiency of 1.3% [1] when spin casted with P3HT in spite of the disadvantage listed previously. In this presentation we reveal a new acene generation that merges the advantages of small molecules (easy characterization and purification) with the solution processability of polymers. By increasing the molecular weight of our small molecules we changed its physical properties in solution, leading to better films from spin cast solution. Different silylethynyl substituted thienyltetracenes and anthraditiophenes were used to create oligomers via homocoupling reactions resulting in materials with high intermolecular order, beneficial for good charge transport mobility. Linking units such as benzothiadiazole (acceptor) with thienyltetracene (donor) were also used to construct donor-acceptor-donor (D-A-D) and (D-A-A) framework oligomers that allow us to manipulate the electronic properties of these molecules leading to Eg as low as 1.48 eV and exhibiting efficient absorption throughout the solar spectrum. The capability to manipulate the crystal packing of these molecules by using different trialkylsilylethynyl substituents is another plus proving the promising future of these molecules as n-type organic semiconductors for BHJ solar cells.

Conjugated polymer blend solar cells are devices where the active layers are composed of polymer donor and polymer acceptor pairs. These devices suffer from macrophase separation in the active layer, limiting efficiency. The self-assembly properties of block copolymers have the potential to overcome the thermodynamic incompatibility between different polymers and form unique nanoscale structures for efficient photovoltaic operation. Using a poly(3-hexylthiophene) - poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2&’&’-diyl) conjugated block copolymer (P3HT-PFOTBT), we demonstrate for the first time that devices composed of donor-acceptor block copolymers can work as solar cells with efficiencies around 3%. Lamellar morphologies formed in block copolymer thin films have been characterized using resonant soft X-ray scattering.

In this work, we demonstrate that ultrasound induced supramolecular ordering of poly(3-hexylthiophene) (P3HT) is significantly improved by adding small quantities of a poor solvent. Ultrasonication of P3HT conjugated polymer solutions has been introduced as a successful and successful and simple route for creating nano-sized P3HT aggregates. The extent of aggregation can be easily tuned by controlling ultrasonication time. Increased ultrasocniation time, hence increased aggregate formation, leads to enhanced charge carrier transport properties, but only up to the percolation limit. However, introduction of small quantities of a poor solvent into the P3HT/good solvent solution leads to dramatic development of nanostructure in terms of size, shape, and quantity. Poor solvent induced aggregation of P3HT, a well-known phenomenon, is incorporated with ultrasound induced aggregation in synergic manner. Additionally, we report the correlation between morphology and charge carrier transport through varying the boiling points of the poor solubility co-solvents. Shapes, sizes, and overall quantity of the nanostructures obtained through this methodology are characterized with respect to solvent boiling point and coupled with the charge carrier transport properties of the thin polymer film.

Organic photovoltaics (OPVs) are promising energy conversion devices that offer flexibility, lightweight, and low-cost renewable power. Despite recent advancement in power conversion efficiency, further progress in improving charge collection at the electrode-semiconductor interface is necessary for the continued development of OPVs. Contact doping at the interface of the metal electrode and organic semiconductor has the potential to enhance charge extraction by reducing the interfacial barrier width and promoting charge tunneling through the barrier. We have demonstrated that polymer acids can act as p-type dopants for poly(3-hexylthiophene) and poly[N-9&’&’-hepta-decanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole)] through UV-vis spectroscopy and field effect transistor measurements. The performance of contact-doped OPVs is near that of devices which utilize PEDOT:PSS. We also investigated the effect of polymer acids with different backbones and cationic dopants on polythiophene/fullerene systems.

Small molecule organic semiconductors are non-toxic, readily available materials, most of which can be solution processed to form polycrystalline thin films, as is the case for 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). These properties make them extremely promising materials for inexpensive semiconducting devices like solar cells. However, such devices are currently limited by relatively low light harvesting efficiencies and a predisposition toward damage. Both of these effects are strongly modulated by the morphology of the film and could be improved upon by understanding how the electronic structure and dynamics are correlated to the local morphology. While electronic structure and dynamics can be studied with ultrafast transient absorption spectroscopy, making the connection to spatial heterogeneity requires spatially resolving such measurements and being able to compare them to linear optical measurements and structural characterizations in order to interpret all data consistently.
Since we have observed significant heterogeneities within spatially resolved maps of transient absorption measurements on TIPS-pentacene films, we have undertaken a series of additional measurements in order to elucidate how specific features in the underlying morphology are responsible for the variation in the photophysics. Several techniques have been implemented that facilitate pre, post, and in situ structural determinations. Since TIPS-pentacene forms anisotropic crystals, the grain orientation may be determined in polarized optical microscopy in combination with modeling of the light matter interactions that we have developed. In addition, localized spectrally-resolved linear absorption measurements are performed in order to examine variations in the band edge. These techniques are conducted in the same microscope used for our transient absorption microscopy without moving the sample, allowing direct correlation to the ultrafast photophysics. Furthermore, to account for variations in the thickness of the sample, both AFM and optical profilometry are performed and then correlated with the region of interest of our transient absorption measurements. The development and implementation of these characterization techniques has facilitated the identification of the morphological features that correlate with the heterogeneity of the photophysics, aiding our understanding of how local structure impacts material performance, and should be generally applicable to the characterization of polycrystalline thin films of anisotropic crystals.

Highly efficient organic devices based on the blend of an electron-donating and an electron-accepting polymer require fine control of these materials phase separation in order to improve the charge transport by creating electron acceptor-donor interfaces in the nano-scale. In this context, we present organic photovoltaic devices (PVDs) and light emitting diodes (LEDs) based on polymer nanoparticles obtained in aqueous phase via the miniemulsion process which is an efficient approach to impose a fixed length scale of phase separation. This method also offers the feasibility of coating several layers on top of each other without leading to interdiffusion and undefined interfaces. Solutions of electron-donating and electron-accepting polyfluorene derivatives, Poly(9,9prime;-dioctylfluorene-co-bis-N,Nprime;-(4-butylphenyl)-bis-N,Nprime;-phenyl-1,4-phenylenediamine (PFB) and Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) for PVDs and PFB and Poly((9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole)-2',2''- diyl) (TBT) for LEDs, in chloroform (at 2.5 wt%) were successfully miniemulsified in water by the method previously reported by Kietzke at al [1]. We have prepared dispersions with several concentrations of the PFB:F8BT and PFB:TBT blend ratios and tested them as the active layer in the PVDs and LEDs, respectively. The nanoparticles were spin-coated on top of ITO/PEDOT:PSS substrates from the aqueous dispersions forming layers of particles with thickness equivalent to the particles diameter (50 - 100 nm). PVDs devices were fabricated with an ITO/PEDOT:PSS/PFB:F8BT/Ca/Al structure and LEDs with an ITO/PEDOT:PSS/PFB:TBT/LiF/Al structure. The topography of the nanoparticle films was characterized by Atomic Force Microscopy and Scanning Electron Microscopy. The photovoltaic devices were characterized by the External Quantum Efficiency (EQE) and by J-V curves measured under solar simulation using air mass (AM1.5) filter with a power illumination of 100 mWcm-2. The LEDs were characterized by their external quantum efficiency, irradiance, and current voltage characteristics under electrical excitation from 0-10V. For both type of devices the performance of the nanoparticles obtained by the aqueous dispersions are comparable to those devices prepared from solutions based on organic solvents and can be applied as active layers in large area devices made by printing technicques.
[1] T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Güntner, U. Scherf, Nature Materials, 2, 2003, 408.

Achieving a more complete understanding of the thin film morphology, and subsequently obtaining higher degrees of control over the microstructure in thin-film polymer blends has become a focal point for many groups interested in organic electronics. This steady growth in interest is largely due to recent studies identifying a strong relationship between the blend microstructure and inherent film properties. As a result, and in conjunction with recent developments in designing new device architectures and synthesizing new organic semiconductors, improvements in device efficiency have come from optimizing material processing to improve morphology. However, this approach is not without major challenges to overcome. Currently, complete morphology control remains elusive; and while many factors contribute to this lack of success, a clear drawback is the inability to experimentally capture the intricate details of morphology evolution during device fabrication; which may be attributed to the extreme multi-scale nature of the process. This limitation, among others, prevents the development of a well-defined blueprint for morphology control and also hinders efforts to decipher the complex relationship between blend microstructure and device performance.
In this work we attempt to address these shortcomings by exploring the nanoscale morphology evolution during phase separation and investigate potential venues for morphology control. In particular, we use a validated computational framework, developed to predict morphology evolution during solvent-based fabrication of organic thin film devices. This framework models evaporation-induced and substrate-induced phase separation for a ternary system in two and three-dimensions. We explicitly include material specific parameters to observe the evolution of a well-studied PS:PMMA blend, in which we reproduce the experimental works of Ginger et al. in 2008 (JACS) and Composto et al. in 2006 (Macromolecules). Additionally, we explore the phase space of generated morphologies and readily identify distinguishable classes of structures; including the 'columnar' and 'checkered' modes observed in experiments and simulations. This insight allows us to effectively predict when certain morphologies will occur. Finally, we systematically investigate substrate geometries (using a 1D striped and 2D checkered patterns on the substrate) and evaporation rates, and construct a comprehensive phase-diagram of morphologies. This allows us to search for specific combinations which allow morphology tuning; and thus open new venues for improving morphology in organic electronic devices.

Molecular doping enhances the performance of state of the art organic solar cells and light emitting diodes. Furthermore, the threshold voltage of organic field effect transistors as well as the breakdown voltage of organic Zener diodes can be precisely tuned by applying molecular doping. However, the physical mechanism of the doping process in organic semiconductors is still controversially discussed, in particular with respect to the doping efficiency, which is the ratio of free charge carriers per dopant molecules.
In this contribution, we systematically measure the Fermi level position of p- and n-type doped Pentacene thin films by ultraviolet photoelectron spectroscopy (UPS). Varying the doping ratio of different p- and n-dopants by 5 orders of magnitude, we can show that the Fermi level shifts from its intrinsic value close to mid-gap position towards the highest occupied molecular orbital (HOMO) for p-type and towards the lowest unoccupied molecular orbital (LUMO) for n-type doping. Furthermore, at very high concentrations the Fermi level is pinned close to the respective transport states.
These results are discussed in terms of a statistical model. In particular, we observe a transition from impurity saturation to impurity reserve regime, which is characterized by the Fermi level crossing of the dopant acceptor level at a doping molar ratio of MR=9.0×10^{minus;4}. For lower concentrations, doping efficiencies close to 100% are predicted by the model, which will be compared to experimentally determined doping efficiencies. The model shows that at even lower concentrations (MR<4.0×10^{minus;5}) molecular doping is hindered by deep gap states and that for a dopant molar ratio above MR>1.0×10^{minus;2}, the doping efficiency is as well reduced to only a few percent (impurity reserve).
Furthermore, we show by UPS that intentionally introduced electron traps in Pentacene by a well defined density of the buckminster fullerene C60 can be filled up by n-type doping and thus control the Fermi level position until completely occupied.
The results of the present investigation represent a step forward in the understanding of the physical mechanism of molecular doping and are thus helpful for further optimization of molecular matrix:dopant systems.

Here we demonstrate the electroluminescence of highly ordered self-assembled para-sexiphenyl (PSP) nanofibers grown by hot wall epitaxy (HWE). These crystalline organic nanostructures have been investigated intensively and show an outstanding material combination which provides polarized emission and lasing action on random optical modes along the fiber axis [Quochi2010]. Moreover, the spectral emission of the nanofibers can be tuned from blue via white to green by organic-organic-organic heteroepitaxy of sexithiophene (6T) and PSP [Simbrunner2010, Simbrunner2012].
The growth of self-assembled fibers is only possible on highly crystalline substrates like muscovite-, phlogopite-mica and KCl [Simbrunner2011a, Simbrunner2011b, Haber2010]. Therefore, a non-contact electrode deposition technique is desirable for the device manufacturing without disrupting the active material. The aerosol jet technique is a unique tool that allows the deposition of co-planar asymmetric electrodes by spraying the mist of a metal ink or polymer solution onto the substrate. Moreover, it provides the possibility to deposit a large variety of organic materials and metallic inks with a resolution down to 10 µm line width and almost no restrictions in path design.
For the fabrication of the electroluminescent devices, microstructures of 20 µm line gap between the anode and cathode have been deposited. A complex based silver ink (PR-010 by InkTec) and highly conductive PEDOT:PSS (PH1000 by Hereaus), have been used as anode and cathode materials respectively. The electrical and spectral characterization of the devices is presented in this work.

We report the formation of well-defined monolayer coverage onto regular spin-coated conjugated polymers by a conjugated polymer with fluorinated alkyl side-chains. This layer is formed by coating and then washing with a fluorinated solvent. We show how this layer can be arranged as the charge capture layer in a range of multilayer polymer LEDs, to produce red and green emitting devices.

Our objective in this presentation is two-fold. First, based on molecular mechanics / molecular dynamics simulations, we provide a molecular picture of the packing configurations (“local” morphology) at the interface between the donor (polymer or small molecule) and acceptor (fullerene derivative or n-type oligoacene derivative) components [1-3]. In a second part, we discuss the impact that these local packing configurations have on the exciton-dissociation and charge-separation processes [4].
References
(1) N. Cates Miller, E. Cho, et al., Advanced Materials, 2012 (DOI: 10.1002/adma.201202293).
(2) N. Cates Miller, E. Cho, et al., Advanced Energy Materials, 2012 (DOI: 10.1002/aenm.201200392).
(3) Y.T. Fu, C. Risko, and J.L. Bredas, Advanced Materials, 2012 (DOI: 10.1002/adma.201203412)
(4) J.L. Bredas, J. Norton, J. Cornil, and V. Coropceanu, Accounts of Chemical Research 42, 1691 (2009).

The charge transfer at organic/semiconductor interfaces largely affects the properties of the hybrid devices, e.g., transistor structures used for chemical sensing, because of the formation of chemical bonding. However, it is hard to study this charge transfer in a quantitative manner largely due to the complexity of the surface structure of most surfaces commonly prepared. We have recently shown that well-ordered nanopatterned Si surfaces could be prepared with one-third ML surface coverage of methoxy groups, each surrounded by a hydrogen ring [1]. Using this surface as a template, we have grafted a variety of molecules and studied the resulting interface charge transfer. We find that, with atomically controlled nanopatterning, the surface band bending extracted from current voltage and capacitance voltage measurements can be well correlated with the interfacial charge transfer and thus chemical bonds at the surface. Although an ohmic IV is found to be characteristic of H-terminated n-type Si surfaces, a large Schottky barrier (0.63 eV) is measured at methoxylated n-Si surfaces, even though most of the surface still hydrogen terminated (1/3 methoxy + 2/3 hydrogen). The large surface band bending can be explained by the electronegativity difference between atomic silicon and oxygen. We further find that, while the hydrogen ring has minimal effects on the charge transfer in most cases, it strongly impedes the formation of interfacial dipole when fluorine is attached to Si surfaces, and use DFT calculations to account for this unexpected finding.
[1] D. Michalak, S. Rivillon, D. Aureau, M. Dai, A. Esteve, Y.J. Chabal. Nature Materials 9 (3), 266 (2010)

Donor-acceptor copolymers employing the high electron affinity Naphathalene Diimide (NDI) unit represent promising semiconductors for applications in organic electronics. However, the mechanisms leading to the good transport properties of this class of materials are still being debated. In this work we report a comparative study about charge injection and transport properties of NDI-based copolymers newly synthesized starting from the well known P(NDI2OD-T2). The thiophene donor moiety has been modified according to different strategies, e.g. thienyl rings condensation and/or increase in the total number of rings, to unveil the effect of its rigidity and conjugation length on the overall chemical-physical properties of the co-polymer. To this extent, a thorough theoretical and experimental approach was adopted. DFT quantum-chemical calculations (molecular scale), solutions and thin films optical and morphological investigation (supramolecular scale) and Field-Effect Transistors (FETs) Room Temperature and Variable Temperature electrical characterization (device scale) have been carried out.
The modification of the donor unit is indeed effective in modulating both the energy- and optical-gap of the copolymers, and from an intra-molecular point of view DFT calculations show that at the polymeric limit condensed thienyl rings can significantly increase both holes and electrons reorganization energies. This result suggests that molecular design aiming at balancing ambipolar transport needs to carefully address charge (i.e. polaron) relaxation effects in terms of structural and electronic reorganization. The implications on charge carriers transport is then tested in FETs devices, where a much more complicated picture arises due to solid-state effects and strong variations in charge injection efficiencies from the metal electrodes. Our findings evidence that inter-molecular effects are strongly affected by morphological modifications induced by small changes in the thiophene unit, and that the resulting charges mobility directly influences the charge injection efficiency.
Our experimental and computational complementary multi-scale approach offers an effective method to rationalize such effects, to understand and control charge injection and transport in donor-acceptor copolymers and in providing further evidences towards predictive molecular design of high-mobility polymers.

Current models for the electronic properties of semiconducting conjugated polymers do not include the hierarchical connectivity between charge transport units that results from the physical makeup of the materials. Concepts like on-chain vs. interchain mobility anisotropy have been known for a long time, yet they must be artifically incorporated into simulations. Models that can achieve remarkable predictive power but provide limited physical insight when applied to this new class of materials are of limited use for the rational design of new conjugated polymers. Here we present a new model in which the morphology of individual polymer chains is determined by well-known statistical models and the electronic coupling between units along a polymer chain and on different molecules is described using Marcus theory. Combining knowledge from polymer physics and semiconducting materials into an analytical and computational model that realistically incorporates the structural and electronic properties of conjugated polymers, it is possible to explain observations that previously relied on phenomenological models. The multi-scale behavior of charges in these materials (high mobility at short scales, low mobility at long scales) can be naturally described with our framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational fluctuations and correlations.

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 called the solution shearing method (SSM) which resulted in better organic field effect transistor (OFET) performance compared to those fabricated from simple drop casting or spin casting methods. We measure the molecular ordering resulting from a variety of solution shearing conditions, and we show that we can controllably tune the molecular packing of OSCs by generating lattice strain in the growing crystalline thin film. We discuss the characteristics of this method and its applicability to a wide range of OSCs. Using the OSC TIPS-pentacene as a model, we show that certain conditions yield remarkable FET performance, yielding large area films, with some transistors showing mobilities as high as 4 cm² V^-1s^-1, high current On/Off ratios, low hysteresis and low threshold voltage. We discuss the applicability of the SSM to change the molecular packing for a wide variety of both small molecular and polymeric OSCs, and we show effective tuning of molecular packing that can yield higher charge carrier performance without changing the underlying chemical structure of the OSC. This method is the first time a solution processing method has been used to finely tune the molecular packing and the electronic properties of OSCs. We investigate the stability of the metastable molecular packing structures of the various OSCs obtained through shearing and also measure the relationship between the morphological features and its impact on charge carrier transport. Finally, we also explore whether other solution processing methods can be utilized to tune the molecular packing of various OSCs.

Understanding the impact of 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.

An all-polymer organic photovoltaic (OPV) can potentially address many limitations of polymer-fullerene blends: absorbance is broadened through the use of polymers with complementary optical properties, and it has a typically higher open circuit voltage compared with polymer-fullerene blends. However, micron-scale phase separation in polymer blends is detrimental to charge separation and transport, resulting in low power conversion efficiencies (PCEs) in the all-polymer devices. All-conjugated block copolymers that combine electron donor and electron acceptor materials can potentially resolve this limitation.
In this study, an improved synthetic approach towards P3HT-based all-conjugated block copolymers is manifested using combination of Grignard metathesis polymerization with LiCl as additive and Suzuki polycondensation. Three series of electron acceptor materials are demonstrated: poly(9prime;,9prime;-dioctylfluorene) (PF), poly (9prime;,9prime;-dioctylfluorene-alt-benzothiadiazole) (PFBT) and poly(2,7-(9prime;,9prime;-dioctylfluorene)-alt-5,5-(4prime;,7prime;-di-2-thienyl-2prime;,1prime;,3prime;,benzothiadiazole) (PFTBT). This improved method gives versatile block copolymers (Mw > 50 kg/mol) with little or no homopolymer impurities measured by size-exclusion chromatography with refractive index, UV-VIS absorbance and nuclear magnetic resonance.
In contrast to previous studies of P3HT-based block copolymer where P3HT typically dominates the final morphology through crystallization, here we find that P3HT crystallization is reduced or completely suppressed by the attachment of different sizes of second conjugated blocks. This discovery is revealed through differential scanning calorimetry (DSC) and grazing-incidence X-ray scattering (GIXS) measurements. This finding indicates that competitive crystallization does occur in all-conjugated block copolymers and that proper balance of block ratios is important for the development of all-conjugated block copolymers for use in OPVs.
Processing also plays an important role in tuning nanostructure in thin film. P3HT-b-PF films that are thermally annealed show a morphology dominated by crystallization of P3HT or PF, depending on the block ratios. However, solvent annealed films show primarily P3HT characteristic crystallites oriented out-of-plane on the substrate surface but with expanded spacing. Subsequent thermal annealing at high temperatures results in loss of the P3HT expanded spacing, indicating that the observed orientation and structure of P3HT is in non-equilibrium status so that proper processing condition can be critical in determining final nanostructure.
This research presents a straightforward, versatile, and scalable synthetic route to prepare P3HT-based all-conjugated block copolymers. It also reveals the structure-property relationships in all-conjugated block copolymer in thin film and suggests that proper tuning of mass ratios and processing conditions can potentially yield enhanced PCE in all-polymer OPVs.

Organic electronics has a tremendous potential for the development of electronic products that are air-stable, non-toxic, and environmentally friendly. In their quest to identify novel materials featuring all the above characteristics, scientists are often inspired both by the apparent simplicity and by the true complexity of nature. We took up the inspiration by natural chromophores and investigated the yet unexplored and uncharted class of H-bonded natural and nature-inspired semiconductors. In nature, many pigments are hydrogen-bonded small molecules as opposed to larger, Van der Waals bonded synthetic molecules like pentacene, phthalocyanines or oligothiophenes. Such natural pigments demonstrate H-bonding as well as pi-stacking. When processed in thin films, they generate extensive long-range ordered structures with crystalline textures showing a single preferential orientation, unlike the typical Herringbone pattern found in many van der Waals bonded semiconductors. We implemented air-stable unipolar and ambipolar H-bonded, natural or nature-inspired semiconductor materials in organic field effect transistors and various types of integrated circuits. We recorded field-effect mobilities in excess of 1 cm2/Vs for many investigated molecules (e.g. indigoids, anthraquinones, acridones, etc.) in OFET device configuration. We found excellent stability to degradation of those devices and excellent charge transport of both electrons and holes in air during testing periods of several months. Sub monolayer growth experiments were started and combined with an electrical in-situ study of the ambipolar charge transport occurrence during semiconductor layer deposition. We correlated also the variation of the semiconductor field effect mobility in OFET devices relative to the length and resistance of contact electrodes. Nature inspires us to choose among a wide range of cheap and extensively available materials for creating new electronic functionalities, coming closer to a vision of a sustainable electronics world.

In this paper we will discuss the energy levels of materials used in thin film electronic devices. The paper will focus on the characterisation of energy levels of semiconductors through the use of Photo Electron Spectroscopy in Air (PESA). This technique enables the rapid analysis of films prepared under real fabrication conditions. We will discuss the design of new n-type organic small molecules and correlate the calculated and measured energy levels with the performance of these materials in organic photovoltaics. We will also discuss the relationships between deposition conditions, molecular packing, energy levels and device characteristics through the study of small molecule p-type materials, also in organic photovoltaic devices. Finally we will discuss the use of PESA to investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS and PbSe semiconductor quantum dots. We will compare the results to those of previous studies, based on differing experimental methods, and to theoretical calculations.

We describe a strategy to fabricate doped injection-layers with extreme work functions which can provide ohmic carrier injections of the desired sign into poly(dialkylfluorene), an important model of a wide bandgap organic semiconductor. This has not previously been possible. The attainment of ohmic carrier injection has been confirmed here by measurement of space-charge limited currents. We discuss new aspects in device contact energetics and in work-function tuning which have been revealed by these experiments. This opens new possibilities for improving energy efficiency and performance across light-emitting diodes, solar cells and field-effect transistors based on organic semiconductors, quantum dots and carbon nanotubes.

Over the last decade organic semiconductors (OSC) have been heavily investigated owing to their potential for flexible electronic applications. The next major step for organic electronics is the realization of stable and controllable doped transport layers for both p- and n-type materials. Such doped layers would be indispensable for a variety of applications including organic light-emitting diodes (OLED), organic photovoltaics (OPV) and transparent conducting electrodes.
For several years many have reported the efficient doping of OSCs using solution processable small molecules. In this work we focus on the p-type doping of a prototypical, high performance semiconducting polymer poly(3-hexylthiophene) (P3HT) using F4TCNQ (7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane) and TCNQ (7,7,8,8-Tetracyanoquinodimethane). Although several groups have successfully doped semiconducting polymers using F4TCNQ, there still lacks a general understanding of the doping process and how physical factors such as dopant dissolution, complex formation and crystalline structure affect doping. The manner in which dopant molecules are arranged in thin film is crucial to the efficacy of free-carrier formation and as such has a large impact on the overall conductivity of the doped system.
In this report we elucidate how dopant complex formation in solution modulates the doping efficiency in P3HT films blended with F4TCNQ. Using a combination of optical absorption and electrical measurements, we show that doping occurs efficiently when P3HT:F4TCNQ charged complexes form in solution. Better dissolution of the dopant in solution hinders charged complex formation and thereby reduces film conductivity. We also reveal the formation of a previously unreported crystalline phase and present experimental evidence for improved charge carrier dissociation via the formation of densely packed dopant-polymer crystallites. Furthermore, the concentration-dependent conductivity shows that charge generation in doped OSCs is limited by the physical size of the dopant: smaller molecular acceptors are expected to yield higher charge density. Our results clearly elucidate the formation of doped OSC thin films and, by revealing the role of solvation properties and molecular structures in modulating doping efficiency, we provide key general insights for the future design and processing of organic semiconducting dopants.

The controllable doping of inorganic semiconductors has been critical to the development of modern electronics. Similarly, it is envisioned that the development of efficient doping methods for organic materials will aid the design of new organic based electronic devices and materials. Organic p-type dopants have been extensively developed, but the discovery of air-stable n-dopants has been hindered by the necessity of high energy HOMO&’s and the air sensitivity of compounds that satisfy this requirement. One strategy for circumventing this problem is to utilize stable precursor molecules that form the active doping complex in situ during the doping process or in a post-deposition thermal- or photo-activation step. Our group has already reported on the use of 1H-benzimidazole (DMBI) and benzimidazolium (DMBI-I) salts as solution and vacuum processable n-type dopant precursors respectively.
The doping of fullerenes by either 1-H-benzimidazoles or benzimidazolium salts can increase the conductivity by as much as 8 orders of magnitude with a maximum achievable conductivity of 5.5 S/cm. The drastic doping of fullerenes by DMBI derivatives has already been applied in inverted solar cells to improve electron extraction at the cathode. Likewise, the solution doping of PCBM by DMBI derivatives not only improved the conductivity, but also enabled air-stable high electron mobility OTFT&’s. Significantly, the mobility of 2 wt.% doped PCBM OTFT&’s remained higher than 10^(-2) cm2/(Vs) over as much as 20 days. Imidazole dopants have also been used in the doping of sorted and unsorted single-walled carbon nanotubes. The doping of semi-conducting nanotubes results in electron saturation mobilities as high as 10 cm2/(Vs) while retaining on/off ratios on the order of 10^4.
It was hypothesized that the DMBI dopants function as single electron radical donors. Electron spin resonance, UV-Vis-NIR and 1H-NMR spectroscopic studies of n-doped organic solutions and thin-films suggest a complex mechanism for this doping system. Specifically, a multi-step reaction sequence between the dopant and host which ultimately leads to the formation of host radical anions is responsible for the doping effect. The results of this research outline the applications of current organic n-doping technology and will drive the design of next generation n-type dopants that are air-stable and capable of doping low electron affinity host materials in organic devices.

Controlling the work function of semiconductor crystals is of critical importance for industrial products such as varistors, and more recently inorganic/organic hybrid devices. There, the relative offset of the work function and the electronic levels of the active organic material determines charge-injection and -extraction barriers, and thus important properties like, the driving voltage in light emitting devices. The adsorption of dipolar molecules forming self-assembled monolayers (SAMs) is a common pathway to reduce the work function of electrode materials. However, the limits of this approach remain largely unexplored. The selection of appropriate molecular dipoles has been studied, but few structure-to-property relationships have been established to guide further development.
In our contribution, we demonstrate - based on simple level alignment considerations - that molecules with a negative electron affinity do not limit the achievable work-function modification, unlike conventional SAMs. The use of such molecules should in principle facilitate a work function reduction all the way down to zero. As a proof of concept, we study the interaction between the ZnO(10-10) -surface and pyridine using density functional theory (including van der Waals corrections in a recently developed scheme that accounts for the screening of long-range dispersion forces (Zhang et al., Phys. Rev. Lett., 2011)), as well as thermal desorption and one- and two-photon photoelectron spectroscopy. We observe an extraordinarily large pyridine induced work function change of up to -2.9 eV under ultrahigh vacuum conditions. The combination of experimental and theoretical methods allows us to validate the predictive power of our calculations, which then gives us microscopic insight into the interface properties and the mechanism that leads to the large work function change.

All-solution processed organic field effect transistors (OFETs) are expected to play a key role in the mass production of organic electronic devices via high throughput printing techniques.
In this study we focus on solution processing of self-assembled monolayers (SAMs) for enhancement of charge carrier injection at the metal-semiconductor interface. The common way of SAM preparation include many hours of dip coating in very dilute solutions, a procedure which is not suited for any pursued wet printing process. Hence one necessity in order to make SAMs printable is an understanding of the accumulation process of molecules in dependency to time. In literature it has been shown, that the work function shift and the injection barrier are not linearly correlated. Therefore, it proves worthwhile to investigate directly the correlation of the OFET performance to the SAM preparation parameters, instead of the bare shift in metal work function.
We used well established benchmark materials, Polytriarylamines/Cytop to investigate the assembly and functionality of different benzyl mercaptan and alkanethiol SAMs in OFETs, in dependency to the process parameters, accumulation time and concentration. Threshold voltage, mobility, on/off ratio, hysteresis and sub threshold slope have been evaluated in devices containing differently processed SAMs and correlated to Kelvin probe measured work function shifts of metal surfaces treated with SAMs accordingly.
This methodic evaluation of OFET devices, in which the transistor effectively serves as a characterization tool, revealed an expected correlation between metal work function shift and device threshold voltage. Most interestingly, an optimum ratio of accumulation time/SAM concentration concerning device performance has been identified. This seems to contradict numerous investigations of SAM isle accumulation on crystalline Au 111 surfaces, which show that the achieved shift of the work function saturates in time, with the formation of a closed SAM. To our understanding this apparent contradiction likely originates from the nonlinear relation between the dipole strength of a charge-injection layer and the induced shift of the injection barrier. OFET devices with SAMs processed by inkjet printing have been prepared using the optimized processing parameters, and their performance has been compared to the devices containing dip coated SAMs.

It is well known that conjugated polymers are difficult to study because the optical and electronic properties of the polymer are highly dependent on the processing history, and thereby the structural order. We report on the fabrication and characterization of crystalline nanofibers of regio-regular poly-3-hexylthiophene (rr-P3HT). Nanofibers that are formed slowly display highly aligned domains of crystalline polymer and display absorption and emission spectra that are consistent with a J-aggregate. By comparison, nanofibers that are formed quickly display smaller and less well aligned crystalline domains and display absorption and emission spectra consistent with an H-aggregate. We detail spectroscopy that verifies our assignments and analysis of the how control of fabrication steps leads to increased structural order. This culminates in predictive molecular dynamics modeling of polymer nanoparticle formation in solution.

Mesostructure formation in donor/acceptor mixtures of conjugated organic molecules is governed by various complex processes. For example, the miscibility of the components can strongly affect the final morphology of mixtures. In an effort to quantify these effects, we have estimated the Flory-Huggins interaction parameter of polythiophene/fullerene blends. Detailed knowledge of the thermodynamic driving forces enables us to identify the intricate role that miscibility plays on the structure formation process. We find that polymer crystallization dominates the mesostructure in many polythiophene/fullerene systems. Furthermore, charge transport measurements reveal that the miscibility of the components strongly affects electron transport within blends. Immiscibility promotes efficient electron transport by promoting percolating pathways. Our results suggest that an optimum degree of miscibility exists to promote both large amounts of interfacial area and efficient charge transport in polythiophene/fullerene solar cells. As a consequence, the domain compositions (obtained from energy-filtered TEM experiments) in fullerene-rich phases of the active layer and short-circuit currents of low band gap poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]/fullerene solar cells are correlated. Our results suggests that the miscibility can modulate the effect of phase mixing on charge transport and consequently recombination.

The recent demonstration of molecular miscibility/solubility between polymers and fullerenes [1] has revealed a much more complex picture of nanostructure, charge dynamics, and device stability - aspects that are all entangled. Here we show that miscibility is important in several ways that depends on the particular material blend. For example, recent absolute measurements on domain size and composition [2] have revealed nanostructure in PTB7:PC71BM blends that is controlled by miscibility and that well-mixed regions likely hinder charge separation in this system. On the other hand, PBDTTPD:PC61BM blends rely on high levels of mixing for electron percolation.[3] Such evidence leads to a complex interplay between charge separation, electron trapping, and percolation. Miscibility, a thermodynamic parameter, can, furthermore, determine the thermal stability of device active layers, which we show varies widely between materials systems. This suggests tailoring of the molecular interactions between donor and acceptor materials in solar cells may be the key to high-performing, highly stable and, therefore, economically viable organic electronics technologies.
[1] B. A. Collins et al., J Phys. Chem. Lett. 1, 3160, (2010).
[2] B. A. Collins et al., Adv. Energy Materials DOI: 10.1002/aenm.201200377
[3] J. A. Bartelt et al., Adv. Energy Materials DOI: 10.1002/aenm.201200637

The performance of organic bulk heterojunction solar cells is critically dependent on the solid-state structure - from the nano- to the micro-scale - of the photoactive layer. One promising route to manipulate this structure is the use of additives.[1] In our presentation we will focus on a new strategy of using commercially available, low-cost polar ester molecules to modify the supra-molecular arrangement of photovoltaic polymer:fullerene systems. We will concentrate on poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM[60]) blends as these binaries offer two advantages: (i) they display high photoluminescence quenching (>99%; analogous to the high efficiency polymers such as poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithio-phene -2,6-diyl] ], PCPDTBT) indicative of intermit mixing of essentially all the polymer in the film with PCBM, (ii) In pBTTT:PCBM[60], the intermixed phase comprises of a well-defined co-crystal, thereby facilitating structural analyses of such blend films.[2]
We will demonstrate based on structural, physico-chemical and spectroscopy data that the additives successfully prevent intercalation of the fullerene between the polymer side chains - i.e. prevents formation of the pBTTT: PCBM[60] co-crystal. This promises that the fullerene content possibly can be reduced in solar cells based on such systems as crystalline PCBM[60] percolating pathways can be reached without the need to go to pBTTT:PCBM[60] weight ratios of 1:4. In addition, the microstructure can be manipulated such that intercalated regions co-exist with relatively phase pure pBTTT and PCBM domains, allowing for efficient exciton dissociation and possibly, efficient charge collection through the phase pure regions.
[1] T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner, U. Scherf, Nat Mater 2003, 2, 408.
[2] a) N. C. Miller, C. E. Miller, V. Verploegen, Z. Beiley, M. Heeney, I. McCulloch, Z. Bao, M. F. Toney, M. D. McGehee, Journal of Polymer Science Part B: Polymer Physics 2011, 49, 499; b) F. C. Jamieson, E. Buchaca-Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin, J. R. Durrant, Chem. Sci., 2012, 3, 485.

One of the least understood features of bulk heterojunction (BHJ) polymer/fullerene organic solar cells are the structural properties at donor/acceptor interfaces, yet these interfaces are critical to fundamental device processes, since photoexcited charge transfer and bimolecular recombination occurs at these material boundaries. Using soft x-rays [1], we characterize and demonstrate control of polymer molecular orientation at the distributed network of donor/acceptor interfaces in BHJ thin films and show that it is critically important to device performance. Fluorine substitution on the polymer backbone is found to be a general method to control molecular orientation and enhance performance for a variety of polymers. In particular, for a fluorinated polymer based on benzodithiophene and benzotriazole moieties, we explore the influence of polymer molecular weight and side chain placement on molecular orientation at donor/acceptor interfaces and the resulting impact on device performance.
[1] B. A. Collins, J. E. Cochran, H. Yan, E. Gann, C. Hub, R. Fink, C. Wang, T. Schuettfort, C. R. McNeill, M. L. Chabinyc, H. Ade, Nat. Mater. 2012, 11, 536-543.

We have long studied the relationship between crystal packing and device performance in acenes, demonstrating that two-dimensional pi-stacked arrays function best in transistor materials, while more one-dimensional arrays exhibit better performance in photovoltaic applications. We are now beginning to investigate the relationship of other structural parameters to device performance, requiring significant changes to our typical functionalization strategies to explore the broadest possible structure space and develop enhanced design rules. We have prepared soluble materials with significantly reduced symmetry, unusual variations in solubilizing substituents, dramatically increased conjugation length, and varieties of groups attaching the chromophore to the solubilizing moiety. We have studied the structure of these materials crystallographically, and determined their performance in simple devices. We will report how parameters such as crystallographic layering, unit cell anisotropy, and inter-layer steric interactions impact charge transport and device performance.

Organic semiconductor materials are interesting alternatives to inorganic semiconductors in applications where low cost, flexible or transparent substrates, and large area format is required. Currently they have been incorporated into organic thin-film transistors, integrated display driver circuits, photovoltaics artificial electronic skin, and radio frequency identification tags. One of our fundamental interests is to understand how we can ultimately perform rational design of organic semiconductors. In this talk, I will present recent results on understanding of molecular design rules for achieving efficient charge carrier transport and controlled growth of organic semiconductors.

Semiconducting polymers have a semicrystalline microstructure, where they are neither completely crystalline nor completely amorphous. Using electroluminescence, I will show that in such a microstructure transport occurs in a network of percolating crystallites, hence disorder in these crystallites limits transport. Using synchrotron radiation we measure disorder in a number of high-performance polymers and note that it depends on molecular weight. At high molecular weight, disorder saturates to approximately the same point for all high-performance polymers we investigated. Hence we hypothesize that in high molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for sustaining efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, strongly disordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials.

Device measurements in transistor based on semiconductor polymers are based on phenomenological models of transport that make a number of assumption on the energy distribution of states, their localization and their coupling. We show in this contribution how it is possible to describe from atomistic simulation the same parameters and to define a more rigorous connection between the polymer structure and electron transport properties. The difference between semicrystalline and amorphous polymers is discusses together with the effect of regioregularity with exemplifications from the most common polymers currently under investigation (P3HT, PPV derivatives, PBTTT). The results also provide insight on the chemical structure of low energy states.

As solution-processing of organic solar cells emerges as a viable manufacturing process, supported by recent reports of high efficiency organic photovoltaic (OPV) devices prepared using laboratory-based methods, such as spin-casting, there is increasing urgency to speed up the development of scalable, robust and reproducible manufacturing processes capable of producing . Complementing traditional trial-and-error methods with rigorous in situ investigations should help to transfer lessons over to industrially relevant scalable processes and achieve rapid optimization and performance improvements. Fundamental understanding of the underlying mechanisms of model systems and processes is therefore a crucial and informative step. Here, we report on recent investigations of the formation of the bulk heterojunction (BHJ) photoactive layer (P3HT:PCBM) during solution-processing by deploying multiple, powerful probes, often used simultaneously. In the first part of this talk, we investigate the BHJ layer formation via spin-coating of a series of blends with composition changing by steps of 12.5 wt.%; this series provides new insights into the formation process of the BHJ layer in terms of crystallization and phase separation of the components and their synergistic relationship. In the second part, we report on the role of a process additive (octanedithiol) on the dynamics of formation of the bulk heterojunction layer, providing useful hints into the inner workings of the additive and the reasons for an optimal loading of process additives in the solution. In situ techniques were used extensively, including grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS), spectroscopic ellipsometry (SE), spectroscopic reflectometry, and fast polarized videomicroscopy. OPV device testing and space-charge limited current (SCLC) measurements were also complemented by plan-view and cross-sectional energy filtered transmission electron microscopy (EF-TEM).

Solution coating of organic semiconductors has a great potential for achieving low cost manufacturing of large area and flexible electronics. During the coating process, crystalline thin films of organic semiconductors are usually formed under kinetic conditions driven by the rapid solvent evaporation needed for high-throughput industrial-scale production. Yet, highly kinetic crystallization poses challenges to the control of thin film morphology. Two commonly encountered problems are mass-transport-limited crystal growth and uncontrolled nucleation. Both phenomena severely limit the electrical performance of organic semiconductors by introducing randomly distributed grain boundaries and structural defects. To address these challenges, we introduce a new approach for controlling morphology of solution printed thin films, wherein we developed micro-swirl assisted crystallization (MAX) for enhancing crystal growth and contact line engineering for anchored nucleation (CLEAN) to control nucleus formation. We demonstrate for the first time a fast direct coating of patterned, millimeter-wide, centimeter-long, highly-aligned single-crystalline organic semiconductor thin films. Such a film morphology enabled an unprecedented average mobility of 8.1 ± 1.2 cm2 V-1 s-1 from lattice-strained single-crystalline domain thin films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), with the maximum mobility reaching 11 cm2 V-1 s-1. This is also the first report of lattice-strained single-crystalline domain organic semiconductors achieved via solution processing.

Patterning organic thin-film transistors (OTFTs) is a critical step towards achieving high electronic performance and low power consumption. We report on a high-yield, low-complexity patterning method based on exploiting the strong tendency of halogen-substituted organic semiconductors to crystallize along interfaces chemically tailored with halogenated self-assembled monolayers (SAMs). This method allows fabrication of self-patterned devices with channel lengths up to 50 µm, characterized by on/off current ratios as high as 10⁷. Smaller device size can be obtained by using conventional photolithography to define the source-drain separation prior to deposition of the organic semiconductor layer. Particularly noteworthy is that this patterning is self-established, requires a reduced number of processing steps during fabrication, all of which are at moderate temperatures. Using microbeam grazing incidence wide-angle X-ray scattering (µ-GIWAXS) with a spatial resolution of 10 µm, we simultaneously map film microstructure, texture, grain size and molecular orientation for the first time in a device structure. We show that the organic semiconductor molecules are highly ordered on and in the vicinity of the OTFT treated contacts as a result of halogen-halogen interactions between the fluorinated SAM and the fluorinated organic semiconductor. The ordered films exhibit mobilities as high as 0.3 cm²V^-1s^-1 and constrain the current paths to within the device. The areas surrounding the devices, where the interaction is inhibited, consist of a high-resistivity film of mixed molecular orientations with mobilities as low as 10^-3 cm²V^-1s^-1, and electrically insulate neighboring devices. To identify the role of F-F interactions in the development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated benzene thiol treated contacts, with the F atom in positions 2, 3, and 4 on the benzene ring of the SAM molecule. Combining the results obtained from quantitative grazing incidence X-Ray diffraction, Kelvin probe, and field-effect transistor measurements, we show that the surface treatments not only induce structural changes in the films, but in doing so alter the injection picture as a result of work function shifts that they introduce. By investigating several pentacene and di-thiophene derivatives and their interactions at interfaces with fluorinated SAMs, we uncover the mechanism of self-patterning by differential microstructure.

The ductility of polymer semiconductor films, which in part makes them very attractive for flexible device applications, also provides a unique opportunity to elucidate a number of structure-property relationships. In this talk, we discuss the mechanical behavior of polymer semiconductors and the application of strain on film morphology and electronic properties. More specifically, we will discuss how strain can be used to control molecular packing in poly(3-hexylthiophene) P3HT films, and how this in turn alters charge transport behavior in the material. For example, biaxially strained films are shown to result in highly face-on packing (conjugated ring plane parallel to the substrate) while maintaining high field effect mobility similar to that typically seen in edge-on packing films. In addition, we will discuss the application of strain on P3HT:PCBM blend films. We find that processing methods used to optimize film morphology for photovoltaic device applications also significantly impacts the stiffness and ductility of the films. Furthermore, straining ductile P3HT:PCBM films provides a unique opportunity to isolate morphological features in the film enabling detailed studies of photovoltaic energy conversion processes.

Non-radiative deactivation of neutral excited states in highly ordered organic semiconductors such as single crystals or aggregated polymers can determine the overall efficiency of devices such as solar cells or lasers. For energy harvesting in solar cells, for example, ultrafast non-radiative multiexciton generation through ‘singlet fission&’ could prove fundamental for future increases in efficiency. On the other hand, in organic semiconductor lasers, all non-radiative processes should be avoided and high photoluminescence (PL) quantum efficiencies are required. Here we present a spectroscopic study of a series of highly ordered organic materials including naturally occurring carotenoid aggregates, regioregular polymer aggregates (polythiophenes and polythienylene-vinylenes) and small molecule single crystals (thiophenes and oligoacenes). Using steady-state and time-resolved spectroscopy, we demonstrate the interplay between radiative and non-radiative processes (including singlet fission, intersystem crossing by spin-orbit coupling and internal conversion) and how it depends on the molecular and crystal structure.

Understanding the nature of the electronic coupling at all-organic donor/acceptor heterojunctions is important for improving the future generation of organic solar cells. Yet progress in this area has been rather slow, with important results on interfacial charge-transfer states emerging only recently. In order to extend our knowledge of interfacial electron dynamics, we have measured rates of ultrafast photoexcited electron transfer at model bilayer donor/acceptor heterojunctions using copper (II) phthalocyanine (CuPc) as the model donor with C60 and F16CuPc as the archetypal electron acceptors. By varying the relative donor/acceptor molecular orientation, we are able to study both the weak-coupling and the (moderately) strong-coupling limits of interfacial electronic coupling The oriented bilayer structure is confirmed using grazing incidence X-ray diffraction and angle-dependent X-ray absorption spectroscopy. Having characterized the oriented heterojunctions, we probe the ultrafast interfacial electron transfer dynamics in energy space using the core-hole-clock technique of resonant Auger electron decay spectroscopy. Our results indicate that in the weakly-interacting, edge-on CuPc/ C60 interface, charge transfer times are of order 50 fs. In the case of the lying down, i.e. strongly interacting interface case, the proximity of C60 and the lying down configuration result in a more localized photoexcited electron than in pure CuPc, which we rationalize in terms of interfacial dipoles. In order to transition from energy space into the time-domain X-ray spectroscopy, we have also studied X-ray absorption spectra of doped organic semiconductors in order to isolate the spectral signatures of ionic vs. neutral electronic states of organic semiconductors. We find an appearance of some oscillator strength at the red edge of the absorption edge as well as a blue shift of the first π* resonance.

The dynamics of energy and charge transfer in hybrid photoconversion systems based on combinations of molecular semiconductors and inorganic nanocrystals are poorly understood. Here we probe these processes using time resolved absorption and luminescence techniques in a model system based on blends of colloidal cadmium-selenide (CdSe) nanoparticles with Poly[2-methoxy-5-(3prime;,7prime;-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). The use of various capping ligands, oleic acid, pyridine and butylamine as well as thermal annealing allowed for tuning of the polymer-nanoparticle interaction. We demonstrate that energy transfer from the PPV to CdSe occurs on ultrafast time scales (<300fs), independent of the choice of ligand. Upon thermal annealing electron transfer becomes competitive with energy transfer. Hole transfer from CdSe to MDMO-PPV occurs on slower time scales (>10ps) and is strongly ligand dependent. This is in contrast to all-organic systems where hole transfer occurs on ultrafast time scales. Charge transfer states were studied using a newly developed pump-push technique. Crucially we find that the charge transfer binding energy is within 4kT at room temperature. This leads to trap-free charge separation. Our results emphasise the importance of tuning the organic-nanoparticle interaction to achieve efficient charge transfer and highlight the potential of trap-free charge separation in polymer-nanoparticle blends.

Photovoltaic cells made from conjugated polymers offer the promise of cheap flexible devices for solar energy conversion. However, improvements must be made before widespread deployment will become feasible, and our incomplete understanding of how these devices operate hampers progress. Transient absorption studies of photogenerated polaron dynamics in conjugated polymers have played a crucial role in shaping our understanding to date; these measurements include a wide array of timescales and probe frequencies: from ps to ms, and near-infrared to microwave. However, all of these methods share the common limitation that only qualitative information can be easily obtained; the absorption coefficient of the transient polaron is notoriously difficult to measure, and in the case of the microwave and terahertz probes, it may change with sample composition, microstructure, and/or time. A second common feature of all these methods is that the apparent quantum yield of photo-generated charges declines with increasing pump fluence, a phenomenon that has been attributed to efficient quenching of excitons by charges. Herein, we show that the shape of the relative yield-fluence curve can be used to quantitatively determine the yield and absorption coefficient of the polymer polaron. We employ capacitor structures to modulate the charge-density in neat polymer films, and measure the change in photoluminescence (PL) lifetime and yield that results. The PL data is fit with a kinetic model to ascertain the exciton-charge quenching rate constant and dopant density. These model parameters are used to fit the yield-fluence curves obtained from time resolved microwave conductivity measurements, and thus determine the yield and GHz absorption coefficient (mobility) of polymer polarons. We present results using this procedure on films of poly[3-hexylthiophene] (P3HT), Poly[ [9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), and poly[2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT). Our results are consistent with previous measurements of doping density, quenching rate constant, yield, and absorption coefficient where such values exist in the literature.

Charge transfer states at the interface of the donor and acceptor play a crucial role in photocurrent generation and recombination in organic solar cells. Precise external and internal quantum efficiency measurements of organic solar cells at low energies allow us to measure the location, intensity and width of the charge transfer peak, as well as the slope of the exponential tail which correlates with the disorder present in the system and the density-of-states tail. These measurements along with Marcus theory are used to calculate the energy and density of charge transfer states, as well as the reorganization energies of the charge transfer states. In particular, we have studied organic solar cells with polymeric and molecular donors and fullerene-dye adducts to examine the impact of multiple donor-acceptor states on optoelectronic performance. We have varied the interfacial area between the donor and the acceptor through processing methods in order to vary the density of CT states present. These methods have allowed us to quantify the impact of processing methods on interfacial states in organic photovoltaics.

We have studied the charge generation and recombination dynamics in conjugated polymer films blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) by transient absorption spectroscopy. Immediately after the laser excitation, polymer singlet excitons are generated first. Subsequently, polymer polarons are generated from singlet excitons efficiently. In amorphous blend films such as RRa-P3HT/PCBM and PCPDTBT/PCBM, polymer polarons are promptly generated from singlet excitons in a picosecond and therefore the exciton diffusion and the charge separation efficiency is almost 100%. In RR-P3HT/PCBM crystalline blend films, on the other hand, there are two pathways for the polaron generation: one is the prompt generation at a P3HT/PCBM interface and the other is the delayed generation after the exciton diffusion to the interface on a time scale of tens picoseconds. As a result, the exciton diffusion efficiency is less than 90% although the charge separation efficiency is almost 100%. The charge dissociation efficiency is as high as >90% for RR-P3HT/PCBM crystalline films but 60-70% at most for amorphous polymer/PCBM blends. The rest of them recombine geminately to the ground state. The relatively low charge dissociation efficiency in amorphous polymer/PCBM blends is due to the formation of interfacial CT state, which is coulombically bound pairs of polymer polaron and PCBM anion at the heterojunction. The bimolecular recombination rate in RR-P3HT/PCBM is evaluated to be 10^minus;12 cm³ s^minus;1, which is at least two orders of magnitude smaller than the Langevin recombination rate. This is partly because the recombination is not diffusion-limit reaction due to the efficient charge dissociation. Interestingly, there are two types of polarons in RR-P3HT/PCBM blends: one is trap-free delocalized polarons and the other is trap-limited localized polarons. Under the illumination at 1 sun, both polarons are in thermal equilibrium, have a carrier lifetime as long as ~10 mu;s, and hence contribute to the efficient charge collection. On the other hand, the bimolecular recombination rate in amorphous polymer/PCBM blends is comparable to the Langevin recombination rate. This suggests that the recombination is diffusion-limited. As a result, the charge carrier lifetime is as short as a few mu;s, which is comparable to the charge collection time in thin devices. Thus, the bimolecular recombination is not negligible in thick devices.

Photoexcitation in blends of organic semiconductors leads to the formation of strongly bound excitons. These excitons can be dissociated at the heterojunction between donor and acceptor via charge transfer leading to the formation of charge transfer (CT) states. How these CT states separate to form free charges is a question of fundamental physical interest as well as being technologically relevant to organic photovoltaics. Here we explore the role of morphology and electronic structure on this process using four model polymer-fullerene systems; PCDTBT:PCBM, PPV:PCBM, P3HT:PCBM and PTB7:PCBM. Using ultrafast transient absorption spectroscopy with time resolution below 30fs we monitor charge transfer, while ultrafast anisotropy measurements allow for the motion of charges to be tracked, as they move away from the heterojunction. In addition, newly developed pump-push photocurrent spectroscopy measurements are used to track the yield and dynamics of bound CT states at the heterojunction. We find that increasing the concentration of fullerenes in the system (which leads to the formation of crystalline domains of fullerene) reduces the yield of bound CT states by up to ten fold. This allows for charges to move away from the heterojunction on sub 20ps time scales. Our results suggest that fullerene crystallisation is one of the keys to efficient charge separation in organic systems, allowing for electrons to be moved away from the heterojunction into the bulk of the fullerene domain. Furthermore we contrast charge separation efficiency in the new generation of push-pull polymers against non push-pull systems. We find that the yield of bound charge pairs is much lower for push-pull systems. The results are supported by quantum chemical calculations suggesting that in these systems, the hole wavefunction delocalisation for low energy CT states is very similar to that for high energy CT states. This allows push-pull systems to separate charges more efficiently that conventional polymers as the electronic structure allows for greater levels of charge delocalisation. These results add to our understanding of charge separation in molecular systems and also help to frame design rules for future materials.

Interpenetrating phases of organic polymer (donor) and fullerene (acceptor) bulkheterojunctions (BHJ), which are investigated as active material for thin film photovoltaics[1], have complex material properties strongly dependent on the processing conditions.[2] Importantly, BHJ properties such as volume fraction of donor to acceptor[3], nanomorphology and composition of the donor(acceptor) domains[2] and, the molecular orientation within the domains[4][5] influence the optical absorption, exciton dissociation, charge transport and bimolecular recombination crucial to their photovoltaic performance. Moreover, it has been known that these material properties vary spatially throughout the photoactive layer in the out-of-plane direction to the substrate[3][5]. However, the effective nanoscale optoelectronic properties such as the internal quantum efficiency, mobility and lifetime which also could vary spatially in the out-of-plane direction have not been thoroughly investigated.[6] To this end, we utilize substrate (superstrate) illumination at normal incidence and evanescently coupled guided optical mode excitations to selectively excite specific regions within the BHJ active layer and directly measure the optoelectronic properties to obtain spatially resolved nanoscale information. As a first step, we obtained the nanoscale internal quantum efficiency, by combining the wavelength dependent, spatially resolved photocurrent with detailed spectroscopic ellipsometry and finite difference time domain calculations in BHJ active layers with the thickness varied from 0.1 - 1 mu;m. We report a direct correlation between the increase of the generated exciton fraction, localized within 10 nm from the electron extraction interface, and an increase in internal quantum efficiency corrected for parasitic absorption, suggesting an electron transport limited behavior of these inverted devices. Also, we will the present nanoscale BHJ characteristics such as carrier mobility and life time using transient photocurrent measurements carried out under spatially resolved optical excitations and correlate these findings with the BHJ phase segregation data obtained from spectroscopic ellipsometry. We believe that the technique presented could be useful for identifying the out-of-plane spatial regions which critically limit the overall photovoltaic conversion efficiency and would also assist targeted nanoscale optimizations to gain performance improvements in BHJ devices.
REFERENCES
[1] G. Yu et al., Science 270, 1789 (1995).
[2] G. Li et al., Nature Mater. 4, 864 (2005).
[3] D. S. Germack et al., Macromolecules 43, 3828 (2010).
[4] S. S. van Bavel, E. Sourty, G. de With, and J. Loos, Nano Lett. 9, 507 (2009).
[5] B. A. Collins, J. R. Tumbleston, and H. Ade, J. Phys. Chem. Lett. 2, 3135 (2011).
[6] J. R. Tumbleston, Y. Liu, E. T. Samulski, and R. Lopez, Adv. Energy Mater. 2, 477 (2012).

We use spectroscopic methods to build a complete picture of charge dynamics in efficient, solution-processed small-molecule OPV devices. We experimentally measure the electron-hole distance with a 300 fs precision and show that, as the charge-transfer states are being pulled apart, the electron-hole separation reaches 9 nm in 30 ps without any influence of the applied bias. This implies that the energetic landscape at the interface is dominant in determining the efficiency of the charge-separation process. Moreover, we show that the majority of donor-acceptor interfaces have this energy-cascade structure that pushes charges away, efficiently reducing bimolecular recombination. We then turn to charge dynamics and show that no recombination occurs for the first nanosecond and that geminate recombination is negligible. We also show that bimolecular annihilation of charges result in measurable amounts of PC70BM emission, which shows that recombination events occur predominantly within 1 mu;s. Finally, we were able to detangle the emission arising from both compound and show that once charges have being swept out, emission occurs predominantly on the small-molecule donor. This suggests that the dominant source of traps is the donor. Combining these observation results in an exhaustive description of charge dynamics in these devices.

Increasing the open-circuit voltage (Voc) in organic photovoltaics remains one of the key challenges to achieving higher power conversion efficiencies; however, routes to achieving this increase are not well understood. Previously it has been proposed that the Voc can be increased by decreasing the electronic coupling between donor and acceptor molecules, with support for this claim coming from current-voltage modeling utilizing forms of the ideal diode equation. Steric effects, i.e. bulkier side groups that introduce more spatial separation between the electroactive parts of the donor and acceptor molecules, have been proposed as a promising route to decrease electronic coupling and increase the Voc. In our work, tetracene/C60 and rubrene/C60 bilayer systems were analyzed and the electronic coupling parameters measured directly from sub-bandgap external quantum efficiency measurements. Analysis of these systems reveals that electronic coupling displays the opposite trend as would be predicted based on sterics, and that the Voc differences correspond directly with the energy differences of the charge-transfer states. Furthermore, based on near-edge X-ray absorbance fine structure spectroscopy and grazing incidence X-ray scattering measurements to probe molecular orientation, the electronic coupling differences can be ascribed to the edge-on and more face-on orientations adopted by tetracene and rubrene respectively.

The generation of free charge carriers and their extraction to the electrodes are key processes in organic solar cells. There is an ongoing debate on how the electric field across the active layer of such devices affects the efficiency of these processes in competition to geminate and non-geminate recombination.
We have used time-delayed extraction experiments to quantify generation and extraction of charge in various polymer-based solar cells. We find that for P3HT:PCBM, the efficiency for free carrier generation is not affected by the electric field in both poorly-performing as-prepared blends as well as in efficient annealed blends, meaning that the solar cell properties of this material combination are entirely determined by the field-driven sweep-out of carriers in competition with non-geminate recombination [1]. Similar conclusions are drawn for polymer-polymer blends based on P3HT with N2200. These blends were shown to exhibit exceptionally high fill factors [2]. In contrast, blends of the low bandgap polymer PCPDTBT with PCBM exhibit a pronounced field-dependence of charge generation, suggesting efficient geminate recombination [3]. We show that the efficiency of geminate and non-geminate recombination in these blends is correlated to the blend morphology, and that both decay channels are strongly reduced in blends with extensive interchain order [4].
[1] J. Kniepert, M. Schubert, J.C. Blakesley, D. Neher, Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments, J. Phys. Chem. Lett. 2011, 2, 700.
[2] M. Schubert, D. Dolfen, J. Frisch, S. Roland, R. Steyrleuthner, B. Stiller, Z. Chen, U. Scherf, N. Koch, A. Facchetti, D. Neher, Influence of aggregation on the performance of all-polymer solar cells containing low-bandgap naphthalenediimide-copolymers, Adv. Energy Mater. 2012, 2, 369.
[3] S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J.C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf., D. Neher, On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives, J. Phys. Chem. Lett. 2012, 3, 640.
[4] S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kuipiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, D. Neher, Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells, J. Am. Chem. Soc. 2012, 134, 14932.

Poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS) is a well known highly conductive polymer. It is widely used as a hole transport layer and also as a transparent electrode in organic electronic devices because of its high visible light transmission combined with elevated conductivity. 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 diffuses through the layer and weakens the hydrogen bonds formed between PEDOT:PSS grains resulting in an accelerated decohesion rate. Additionally, water uptake on the crack surfaces creates a compressive layer, which increases the effective driving force for crack propagation and thereby further accelerates decohesion. We use a thin-film cohesion technique to precisely measure the kinetics of the decohesion process. We characterize the kinetics under systematically varied environmental conditions with changing temperature, relative humidity and in-situ UV exposure. To elucidate the degradation processes leading to environmental assisted debonding, the kinetics of crack propagation are interpreted using atomistic and reaction-rate kinetic models. 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. Wafer-curvature measurements are used to characterize the changes in thin film stress state under the different environmental conditions. This quantitative analysis provides the contribution of the different environmental variables and most importantly their synergistic effect, leading us to an in-depth understanding of the mechanisms of decohesion and delamination and further yields guidelines for the design of more reliable organic devices containing PEDOT:PSS layers.

We have developed a highly-stretchable transistor active matrix, which exhibits stretchability without any electrical and mechanical degradation. Organic transistors were manufactured on rigid plastic substrates and we realized a modulus gradient from rigid islands to a stretchy PDMS region. The modulus from the rigid transistor islands to the stretchable PDMS region is reduced gradually over four orders of magnitude from 9.1 GPa to 140 kPa. Elastic conductors with conductivity greater than 100 S/cm were stencil printed to create interconnects between the rigid transistor islands after RIE activation to increase surface adhesion. The gradual transition from the electrically-active rigid components to the soft interconnects realizes simultaneous achievement of excellent stretchability, electrical performance, and reliability during stretching, thus enabling dependable, stretchable electronics.
First, we fabricated an organic transistor array on a 25µm-thick polyimide (PI) substrate, with a dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)[1] semiconductor layer, a 4nm-thick aluminum oxide gate dielectric and a 2nm-thick self-assembled monolayer [2]. The surface was encapsulated with a 750nm-thick parylene layer and laminated onto a PEN film using a 5µm-thick PDMS layer. Rigid PI material in between the transistors was then removed and the transistor islands were coated in a stiff PDMS layer. A soft PDMS substrate was then spin coated to create the bulk substrate and to enable stretchability of the matrix. To access the transistors, stretchable interconnects [3] were stencil printed through laser-cut vias to form word and bit lines.
The transistors in this stretchable matrix exhibit a mobility of more than 1.2 cm^2/Vs and on/off ratio exceeding 10^5, and there was no deformation-induced degradation to the electrical or mechanical properties during or after strain.
[1] T. Yamamoto and K. Takimiya, Journal of the American Chemical Society, 129, 2224-2225 (2007).
[2] H. Klauk et al., Nature, 445, 745-8 (2007).
[3] T. Sekitani et al., Nature Materials, 8, 494-499 (2009).

Recently, transparent conducting polymers have been widely used for the flexible and stretchable electronics which are exposed to large strains. In contrast to elastic strain region, extrinsic contributions such as defects generation, microstructural and morphological change, and neighboring materials can influence the change of electrical properties in concerted ways under the plastic deformation. Therefore, for the realization of stretchable or flexible systems, understanding the mechanism of change in electrical resistivity subject to mechanical deformation becomes increasingly important, in particular for stretching up to large strains. However, the mechanism is still not fully elucidated because the systematic analysis has not been conducted on the deformation-induced resistivity change based on the morphological changes. In this study, we investigated the direct response of resistivity change of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films to the applied tensile strain and identified the change in the morphology which controls the strain-responsive behaviors. By matching the mechanical properties (Poisson&’s ratio and elastic modulus) between the PEDOT:PSS and underlying substrates which are polyimide (PI), the change in resistance can be successfully observed up to 60 % tensile strain without any generation of defects (buckles or cracks). It is because the films and the substrates are uniformly deformed and experience the same Poisson&’s compression perpendicular to the stretching direction. When we observed the resistivity change, interestingly, the resistivity decreased up to 80 % by the strain for the pristine PEDOT:PSS films. Because we excluded the effect of the crack generation, the resistivity change are directly induced by morphological change of the PEDOT:PSS. To elucidate the mechanisms of the resistivity change, the morphological change with tensile strain was investigated using AFM phase images. The average size of the conductive PEDOT-rich cores increased to 0.88, 1.27, 2.00, and 2.50 × 10^-2 mu;m² for strains of 0, 16, 28, and 42 %, respectively. Moreover, the growth occurs isotropically and the enlarged cores permanently maintained after the release of the films is complete. We find that the coalescence between the cores is the primary mechanism to be enlarging the cores size, and it can be only induced by applied tensile strain. This equivalent growth of conductive parts leads to significant enhancement of charge conductivity, and it provides possibility to modulate the resistivity by this practical way.

Flexible pentacene organic transistors have been rapidly developed to show mobility of 0.5-2.0 cm²V^-1s^-1, comparable with that of a - Si:H TFTs. Metals have been often used for the gate or source/drain electrodes on a flexible substrate, but they have a 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 Al₂ O₃ /poly(4-vinyl phenol)(PVP) multilayer gate insulators and a polyethersulfone(PES) substrate allowed for the excellent mechanical stability(strain 6%, bending cycle 3000) and transparency. In addition, we have conducted adhesion test using 90° - peel test. It showed excellent adhesion to pentacene-PEDOT film. This device exhibited high electrical performance of flexible organic thin film transistors(threshold voltage of -3V, threshold voltage shift of 1.12V, Ion/Ioff ratio of 1.5E+7, mobility of 0.41 cm²V^-1s^-1, ). have importantly, after 6% strain, organic thin film transistors showed stable electrical performance(threshold voltage of -3.5V, threshold voltage shift of 1.45V, Ion/Ioff ratio of 5.6E+4, mobility of 0.279 cm²V^-1s^-1, ).

3,12-Diphenyl-7,8-dicyano-5,6,9,10-tetrahydro[5]helicene (M107), was synthesized in good yield by a convenient method and was successfully demonstrated as a novel emissive material for organic light-emitting diode (OLED). The thermal, electrochemical and optical, properties of this compound were systematically characterized. The compound showed excellent thermal properties with a glass transition temperature at 155 °C and a melting temperature above 350 °C. The energy level of the lowest unoccupied molecular orbital (LUMO, -3.5 eV), the highest occupied molecular orbital (HOMO, -6.2 eV) and the energy band gap (2.7 eV) of this compound were determined using a cyclic voltammetric technique. The photoluminescence (PL) spectrum of a dilute solution in chloroform showed a peak at 455 nm with a full-width at half-maximum (FWHM) of 63 nm and the fluorescence quantum yield was 0.84. The compound was employed as an emitter for OLED with a configuration of ITO/PEDOT:PSS (35nm)/M107 (60nm)/Ca (10nm)/Al (100nm). The OLED exhibited a turn-on voltage of 6.8 V and the maximum brightness of 3,090 cd/m2 at 10.0 V. The device produced a greenish blue electroluminescence (EL) emission with CIE coordinates of (x = 0.17, y = 0.29). The EL spectrum of the device was observed at 477 nm with FWHM of 70 nm. Since M107 showed excellent thermal properties and exhibited very promising properties for emitter in OLED, it opens new windows for molecular structural design and synthesis of other new [5]helicene derivatives as a new family for organic emissive materials.

Organic light emitting diodes (OLEDs) have attracted significant attention because of their great potential for making next-generation flat panel displays, including flexible displays. Indium tin oxide (ITO) has been widely used as transparent electrode in OLEDs, thanks to its excellent transparency and high conductivity. However, it has several issues including its brittleness, the scarcity of indium, and high temperature processing which have to be addressed for its proper application in flexible electronics.
Therefore, we have developed MoO₃/Au/MoO₃ triple layer structure to replace ITO electrodes with transmittances of ~87% at the wavelength of 525nm and lower sheet resistance of ~9 Omega;/sq. with 10 nm-thick Au layer. The results revealed the effect of three variables such as top MoO₃ thickness, Au thickness, surfaces modification of top MoO₃ by self-assembled monolayers (SAMs) on the electrical and optical properties of OLEDs. Top MoO₃ thickness determined the transmittance and the contact resistance; The Au thickness determined the sheet resistance and the transmittance. SAMs coated on MoO₃ changed the barrier height, achieving the low turn-on voltage. Optimization of the thickness of top MoO₃ and Au allowed for the removal of hole transport layer, 2-TNATA, leading of the simpler active layer structure, MoO₃/Au/MoO₃/NPB/Alq₃:C545T/Alq₃/LIF/Al, with excellent electrical, optical performance.

-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)

We analyze electrical response of two- and three-terminal electromechanical switches that operate via modulation of tunneling current through nanometer-scale-thick organic films. The switching action is initiated by the electrostatic compression of thin organic films, sandwiched between conductive contacts, reducing the charge tunneling distance and resulting in an exponential increase in the tunneling current. In our electromechanical structures the extent of compression, and in turn the electrical conduction through the thin film, can be controlled by adjusting the applied voltage. Through numerical simulation we determine the feasibility of the electromechanical modulation of the tunneling current for different organic materials and device structures. For example, we find that the tunneling current increases 9 orders of magnitude as a 4nm-thick film with a Young&’s modulus of 1MPa, placed in between gold contacts, undergoes a 50% compression under a minimum applied voltage of approximately 0.5V. In this geometry, switching response times as fast as 1ns, and switching energies corresponding to only a few eV are feasible. As expected, the compression decreases as the Young&’s modulus of the material increases. Thus, to maximize the range of electrostatic modulation of electrical conduction a softer material is desired. To fabricate thin films of low Young&’s modulus we consider sparse self-assembled molecular layers such as those comprised of polyethylene glycol dithiol, and engineered organic molecules with low packing density such as 1,5-dibenzocyclooctatetraene. Electrical and optical characterization of devices based on such materials show mechanical compression of thin organic films due to an applied electrostatic force. The results show that the development of a reliable electromechanical modulation of tunneling current through organic nano-scale thin-films can enable a multitude of novel low power switching and sensing applications, which we will discuss.

We report the determination of the highest occupied molecular orbital (HOMO) level alignment for the widely studied organic semiconductor, Poly(3-hexylthiophene) (P3HT), by using internal photoemission (IPE) measurements. Specifically, we use IPE to extract the band off-set between the P3HT HOMO and the conduction band (CB) of silicon dioxide (SiO2) from which we determine the P3HT HOMO position relative to the vacuum level. P3HT solution (15 mg/ml in 1,2-dichlorobenzene) was spin coated (10 × 2π radian/second) onto 280 nm thick SiO2 on heavily doped P-type silicon. A 10 nm thick semitransparent aluminum (Al) electrode with adjoining 70 nm thick Al contact pad were deposited onto the P3HT film through aligned shadow masks. All processing was done in an argon filled glove box. IPE measurements were made in air. Spectroscopic Ellipsometry was used to determine the SiO2 bandgap and thickness, and the average P3HT film thickness.
Photocurrent in the IPE measurement was generated by using a monochromator with photon energy ranging from 1.5eV to 6.0eV (0.05 eV steps) and with a DC voltage which ranged from 20V to -20V (-2V steps) applied between the silicon backside and the thick Al contact. Both positive photocurrent (electrons excited from the P3HT HOMO to the SiO2 CB and collected by the Si substrate contact) and negative photocurrent (electrons excited from the Si valence band to the SiO2 CB) were observed. The latter provides greater accuracy in determining the relative position of the SiO2 CB. For the IPE measurement, the yield (Y) is defined as the ratio of the carriers contributing to the photocurrent to the incident photon flux, and the threshold at each applied voltage is obtained by extrapolating Y1/3(hnu;) to zero according to Fowler coordinates [1]. The barrier height is determined from Schottky plots extrapolated to zero field. By using this established method we extract a barrier height of 4.2 eV ± 0.1 eV for the Si:SiO2 interface and 4.0 eV ± 0.1 eV for the P3HT:SiO2 interface, respectively. No photocurrent was observed at photon energy near 3.3 eV, indicating photoexcitation from the Al contact did not contribute to the measured positive photocurrent. The CB of SiO2 is known to reside 0.9 eV below the vacuum level [2], thus yielding a P3HT HOMO position of 4.9 eV ± 0.1 eV relative to the vacuum level. Reported values from Ultraviolet Photoelectron Spectroscopy studies are closer to 4.6 eV for ordered P3HT [3,4]. Work is in progress to extend this study to other donor and acceptor-like organic semiconductors.
[1] R. H. Fowler, Phys. Rev. 38,45 (1931)
[2] J. Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000)
[3] Z.-L. Guan, et al., Org. Electr. 11 1779 (2010)
[4] W. C. Tsoi, et al., Macromolec. 44, 2944 (2011)

π-conjugated metalloles are attracted renewed interest because of their unique electronic structure and resulting fascinating optoelectronic properties. In this study, we synthesized hetero-annulated π-conjugated dithieno[3,2-b:2',3'-d]metallole derivatives¹ incorporating Ge, Si, P, and S atoms as a bridging center and investigated the influence of the hetero-annulated structures on their photophysical properties.
These compounds showed intense fluorescence emission both in solution and in doped thin films. The 3wt%-dithienometallole:2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN) codeposited films showed high photoluminescence quantum efficiencies of 70-94%. Multilayer OLEDs employing the dithienometallole emitters were fabricated with the following device configuration: ITO/4,4&’-bis[N-(1-naphthyl)-N-phenyl]biphenyl diamine (α-NPD, 40 nm)/3wt% dithienometallole:TBADN (30 nm)/4,7-diphenyl-1,10-phenanthroline (BPhen, 50 nm)/LiF (0.8 nm)/Al (80 nm). Among the devices, the sulfur-annulated compound based device exhibited the highest external EL quantum efficiency (EQE) of 6.1%. The high EQE should be attributed to the enhancement of the light out-coupling efficiency by the horizontal orientation of the emitting molecules in the device. We measured polarized PL spectra from the edge of the 3wt%-dithienometallole:TBADN film. The intensity of the transverse electric emission from the edge of doped thin film was found to be larger than that of the transverse magnetic emission, indicating that the transition dipoles for emission lie preferentially parallel in the host matrix on the substrate.
1. R. Kondo, T. Yasuda, Y.-S. Yang, J.-Y. Kim, and C. Adachi, J. Mater. Chem., 2012, 22, 16810.

The study on horizontally oriented amorphous thin films with a large optical anisotropy continues to be of great interest because they can realize not only low driving voltage, but also high light out-coupling efficiency (eta;OCE) in oganic light-emitting diodes (OLEDs).[1,2] Recently, a novel photophysical phenomenon, that is aggregation induced emission has been reported:[3] some conjugated organic molecules are non-luminescent in their solutions, but once aggregated they are strongly fluorescent with a remarkably high photoluminescence quantum yield. In this study, we have developed novel arylamine-based materials, TPA-TPE and PDA-TPE, aiming for the enhancement of horizontal orientation and solid state emission. The resulting molecules function as a bright solid-state emitter and an effective hole-transport amorphous material in OLEDs.
The absolute fluorescence quantum yields (Phi;f) of TPA-TPE and PDA-TPE measured using an integrating sphere are as low as 2.0 % in 1.0 x 10-4 M THF solution. However, Phi;f of vacuum deposited thin films of TPA-TPE and PDA-TPE are increased to 55±1 % and 72±1 %, respectively, hence the emission of these molecules should be enhanced by aggregation. The orientation order parameter S (S = minus;0.5: completely parallel, S = 0: randomly oriented, S = 1: completely perpendicular onto the surface) was evaluated for the thin films by means of the wide-range variable angle spectroscopic ellipsometry (WVASE). The TPA-TPE and PDA-TPE films showed large optical anisotropies (S = minus;0.27 for TPA-TPE and S = minus;0.27 for PDA-TPE), confirming the preferential horizontal molecular orientation onto substrates. The hole mobility of the TPA-TPE and PDA-TPE films was evaluated by space-charge-limited currents (SCLCs) method. It has been found that TPA-TPE and PDA-TPE are effective hole-transport amorphous materials exhibiting a high mobility of approximately 0.01 cm2/Vs. The OLEDs based on TPA-TPE and PDA-TPE as an emitter exhibited a turn-on voltage of 2.6 V and 2.4 V, a power efficiency of 12.4 lm/W and 17.6 lm/W, and an external quantum efficiency (eta;ext) of 4.49 % and 5.89 % at 10 mA/cm2, respectively. The high eta;ext should be attributed to the enhancement of eta;OCE in OLEDs by horizontal orientation of the transition dipole moment of emitting molecules. Since the light is emitted mainly in the direction vertical to the transition dipole moment, the horizontal molecular orientation would be crucial for achieving high eta;OCE.
[1] D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Org. Electron., 10, 127 (2009).
[2] J. Y. Kim, D. Yokoyama, C. Adachi, J. Phys. Chem. C, 116, 8699 (2012).
[3] J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun. 1740, (2001).

One of the open issues in organic electronics is the long-term stability of devices based on organic materials. The focus of this contribution is on the investigation of the effects of low energy ion implantation in the reduction and control of the degradation of organic devices due to the exposure to atmosphere (i.e. oxygen and water). We have studied the effects of N and Ne irradiation on pentacene organic thin film transistors (OTFTs). Ion implantation induces strong molecular structure modifications that affect the organic active layer, but we have observed that a controlled damage-depth distribution preserves the functionality of the device and its major transport parameters, i.e. carrier mobility and threshold voltage. The electrical properties of the pentacene layer and of the OTFT have been investigated by means of current-voltage and photocurrent spectroscopy analyses. We have characterized the structural modification induced by ion implantation and we have monitored the effectiveness of this process in stabilizing the device carrier mobility and threshold voltage over a long time (over 2000 hours), proving how ion implantation can be safely carried out on fully operational OTFTs. By selecting appropriate ion implantation energies and doses we could locally modify the pentacene film conductivity up to 10^6 orders of magnitude. In particular, we have assessed by depth resolved X-ray Photoemission Spectroscopy analyses that, by selectively implanting with ions that can react with the hydrocarbon matrix (e.g. N+), it is possible to locally modify the charge distribution within the organic layer.

To set up a true ubiquitous environment, the optoelectronic devices, such as solar cell, touch screens, and displays, should be not only be portable; they must be able to change their shapes freely. In other word, these optoelectronic devices should be flexible. In order to embody fully flexible optoelectronic devices, conventional transparent anode materials represented by indium-tin oxide (ITO) should be replaced with any other flexible anode materials due to its brittleness. Furthermore, the increasing price due to scarcity of indium makes it difficult for its use in low-cost, large-area optoelectronics. Here, we embody ITO-free organic light-emitting diodes (OLEDs) using self-organized conducting polymers as transparent anodes without any hole injection layer (HIL). The self-organized conducting polymers are prepared based on poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) in which 5wt% DMSO is added to enhance their conductivity. As a key component, perfluorinated ionomer (PFI) is included to achieve a high work function and hydrophobicity by inducing surface-enriched PFI layer. They show excellent tuning of work-function. The maximum work-function of the anode is 5.8 eV which is the highest value among the flexible anodes which have been reported until now. Moreover, transparency is higher than 90 % in all the visible range. Our OLED devices based on our polymeric anodes showed highly improved current efficiency and stability even with a simplified structure compared to conventional ITO/HIL devices. Even the stability of OPVs based on our polymeric anodes was improved. Furthermore, we analyzed hole-injection characteristics between the modified polymeric anode layer and a hole-transport layer using dark injection space-charge limited current (DI-SCLC) measurement. We concluded that the improved properties of our polymeric anode based devices come from highly efficient hole injection. We also demonstrated flexible solid-state lighting devices using our self-organized polymeric anode with a high work function.

Excimer emission shows a broad spectrum and large full width at half maximum (FWHM), and these features can be advantageous for white OLED applications to achieve a high color rendering index. The excimer emission is also bathochromically shifted compared with usual fluorescence so that there is almost no overlap between the emission and absorption. Less self-quenching of the excimer emission also leads to a high fluorescence quantum yield. In this paper, we describe the synthesis and characterization of an efficient emitting compound, in which two identical π-conjugated luminophores, such as pyrenes, anthracenes, or carbazoles, are closely stacked each other through bonding to the 1- and 8-positions of naphthalene ring, resulting in significant single-molecule excimer emission even in diluted solution. We fully investigated the application of the single molecule excimer-emitting compounds as an OLED material.

Poor charge carrier mobility in organic material is major impediment to the development of organic thin film transistor (OTFT) based active matrix organic light emitting display (AMOLED) which has potential to revolutionize future display systems and electronic papers. Maximum charge carrier mobility of 15cm2/Vs and 1cm2/Vs have been obtained in organic single crystal and organic thin films, respectively. The different molecular environment in organic molecular semiconductors thin films give rise to locally varying polarization energies which leads to a Gaussian density of states(GDOS). The charge carrier mobility in these disordered systems is low due to hopping transport and the degree of carrier localization depends on the location of the transport states in density of states. The central region of GDOS has high density of closely spaced states which provide a high degree of overlap between orbitals and hence possibilities of extended state like conduction is possible in the central region of the GDOS. We have fabricated high performance pentacene and CuPc organic thin film transistors for studying mobility as a function of carrier concentration of charge carriers located at different region of GDOS. Hopping mobilities in structurally disordered organic materials are often many order of magnitude smaller than the mobilities due to transport via extended states. In this work we have demonstrated that the central region of GDOS can be accessed by appropriately biasing the OTFTs. The OTFTs are operated in both negative and positive source-drain bias(VDS) region. The device physics in OTFTs for both regions are totally different causing variation of field effect mobility by three to four orders of magnitude. The limitation in case of OTFT is due to the depleted region, which reduces the background carrier concentration in the device. The highest charge carrier mobility can be obtained by biasing the OTFT in the positive VDS region and carrier mobility as high as 100cm2/Vs can be achieved by accessing different region of GDOS.

Abstract --- We report highly efficient deep blue-emissive polymer light-emitting diodes (PLEDs) achieved by introducing two nanoscale interfacial layer, made of poly(fluorene-co-triphenylamine) (PFO-TPA) and cesium carbonate (Cs2CO3), between (1) the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) PEDOT:PSS and blue poly[9,9-diarylfluorene-co-2,5-bisphenyl-1,3,4-oxadiazole] (P1) and (2) the aluminum (Al) cathode and the P1 emitter, individually. PFO-TPA with highest occupied molecular orbital level (-5.3 eV) lies between those of PEDOT:PSS (-5.1 eV) and P1 emitter (-5.7 eV), forming a stepwise energy ladder to facilitate the hole injection. Also, the thermally crosslinking of styryl groups in PFO-TPA inhibits the solvation of an interlayer in constructing the multilayer device architecture of PLEDs. While applying a Cs2CO3, firstly, it enhances the injection of electrons by providing a lower electron injection barrier. Secondly, applied Cs2CO3 buffer decreases the PL intensity slowly down to ~96 % of the pristine P1 film, located at 422 nm, is less efficiency quenched than the Calcium (Ca). Therefore the overall electron injection attributed by Cs2CO3 buffer is higher. Thirdly, the device with Cs2CO3 buffer did not show the low-energy emission band originated from the fluorenone defects which are often introduced by Ca, thus stabilized blue emission from devices with high brightness can be demonstrated. Based on the hole-transporting PFO-TPA and the Cs2CO3/Al cathode, the device of the optimal configuration has a decent deep blue emission centered at 430-450 nm of the Commission Internationale de l&’Eclairage chromaticity coordinates, (0.15, 0.14), with a maximum brightness of 35054.2 cd/m2 and luminous efficiency of 14.0 cd/A (at 2975.0 cd/m2).

This work investigates the photoresponse mechanism in N-type pentacene-based organic field-effect transistors (OFETs). The N-type OFETs were illuminated with various wavelengths and intensities, which changes the transfer curve from n-type to ambipolar or p-type. It was observed that the electron current was decreased by 95.2%, and then accompanied with the shift in turn-on voltage (Von) from 18.5 V to 76.8 V when illuminated for ten minutes, indicating that the photogenerated carriers affect the accumulated or injected electron in channel. The variation of accumulated carriers can be clarified by the capacitance-voltage measurements and the transfer curve biased at the small drain-source voltage. The results suggest that the photogenerated carriers will diminish the accumulated electron in channel and then the performance of transistors like a resistor which is verified by the saturation characteristics. This study highlights the influence on the transformation of transfer curves using optical modulation while manipulated the N-type pentacene-based OFETs.

Self-assembled monolayers (SAM) can be chemically attached to semiconductor surfaces to form hybrid devices, which open wide application opportunities, ranging from transistors, solar cells to chemical sensors. The proper functioning of these devices largely depends on the interface quality. However, this surface is difficult to probe using conventional electrical methods because of the large leakage current through the thin SAM layer and the damages caused by device fabrication. We circumvented this difficulty by adding an insulator layer over the initial SAM layer using controlled atomic layer deposition, which well preserves the SAM/Si interface and diminishes leakage to permit electrical characterization of the SAM/Si interface. Conductance-voltage measurements are successfully performed on SAM/Si surfaces using a mercury probe setup to obtain the D_it distribution. It is found that superb interface quality (D_it ~ 10¹¹ cm^-2eV^-1 for Si(111)), comparable to Si high temperature oxidation, could be achieved with wet chemistry [1]. But the interface quality depends on the details of the preparation methods, including the Si surface orientation, the surface coverage of functional groups and so on. The incorporation of this electrical interface characterization techniques in addition to more traditional methods (e.g, XPS and FTIR) is shown to bring much higher sensitivity to the Si/SAM interfaces.
[1] W. Peng, O. Seitz, R. A. Chapman, E. M. Vogel, and Y. J. Chabal, Appl. Phys. Lett. 101, (2012).

Due to its large mobility pentacene is a very popular semiconductor. But pentacene is easily oxidized in air, causing the mobility to decrease rapidly. This has been studied extensively and data from electrical measurements are consistent with mass spectrometry [1] and optical spectroscopy results [2]. The pursuit of high-mobility air-stable organic semiconductors has recently led to several promising thioacenes, such as DNTT (dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene) [3] and its derivatives. Besides larger mobilities these compounds are expected to provide better oxidation resistance than pentacene due to their large ionization potential.
To confirm this we have studied the evolution of the mobility of thin-film transistors based on DNTT and C10-DNTT [4] in comparison to pentacene over months of continuous air exposure and correlated the results with the occurrence of oxidation products in LDI-TOF mass spectra.
The TFTs were made in a bottom-gate top-contact structure with Al gates, a ~5-nm-thick AlOx/SAM gate dielectric, a 25-nm-thick vacuum-deposited semiconductor and Au source/drain contacts without encapsulation [5]. Mobilities measured after TFT fabrication (DNTT: 2.5 cm2/Vs, C10-DNTT 3.8 cm2/Vs) indeed exceed that of pentacene (0.8 cm2/Vs) and degrade much slower (after 13 months in air: DNTT: 1.5 cm2/Vs, C10-DNTT 0.9 cm2/Vs, pentacene: 0.004 cm2/Vs). The slower mobility decay correlates with much smaller amounts of oxidation products in the mass spectra.
The good air stability of the thioacenes reveals an interesting phenomenon, i.e. an increase of the effective mobility during the first few hours after fabrication. A similar observation was made by Kalb et al. for pentacene TFTs in vacuum and ascribed to a time-dependent reduction of the contact resistance due to defect healing [6]. In pentacene TFTs exposed to air this effect is usually not seen due to the rapid air-induced mobility degradation. But the effect is very pronounced in thienoacene TFTs; in our DNTT TFTs the contact resistance measured by TLM on TFTs with channel lengths from 1 to 100 µm [7] decreases from 450 to 200 Omega;cm within 10 days, causing the linear mobility to increase from 0.25 to 0.41 cm2/Vs when L = 1 µm.
To demonstrate the impact of the time-dependent contact-resistance reduction on the dynamic performance of the air-stable thienoacene TFTs we fabricated 11-stage ring oscillators on flexible polyethylene naphthalate (PEN). For DNTT TFTs with L = 1 µm the stage delay decreases from 920 to 410 ns at a supply voltage of 3 V during the first 10 days of air exposure after fabrication. These findings may be useful to determine the optimum point in time after fabrication for encapsulation.
[1] de Angelis, Chem. Phys. Lett. 2009,468,4004 [2] Vollmer, Surf. Sci. 2006,600,193 [3] Yamamoto, JACS 2007,129,2224 [4] Kang, Adv. Mat. 2011,23,1222 [5] Kraft, J. Mater. Chem. 2010,20,6416 [6] Kalb, Phys. Rev. B 2007,76,184112 [7] Ante, Small 2012,8,73

Fluorinated analogues of PMMA, such as poly(1H,1H,2H,2H-perfluorodecylmethacrylate) (PFDMA), serve as e-beam resists that are compatible with organic electronic materials. Organic semiconductors cannot be patterned with conventional (organic) resists because the organic solvents used to deposit, develop and strip the resist damage the organic semiconductor at best or, more likely, removes it completely. PFDMA can be processed from fluorous solvents such as hydrofluoroethers (HFEs). HFEs are immiscible with organic and aqueous solutions so can, for example, be spin-coated on top of organic semiconductors without damaging them. Our group has previously demonstrated the use of fluorinated materials as photoresists for organic electronic materials [1] and fabricated relatively complex circuits with these materials [2]. Now we extend this concept to e-beam patterning and are able to attain sub-micron resolutions. Although e-beam patterning of organics is unlikely to be of commercial interest, it is a valuable tool for the study of organic semiconductor materials. For example, it can be used in the fabrication of thin film transistors with channel lengths on the same scale as the microstructure (i.e. grain size). The use of PFDMA and HFEs is considerably more simple and versatile than other techniques proposed for e-beam patterning of organics such as a “resist-bridge” system[3].
[1] P. G. Taylor, J.-K. Lee, A. a. Zakhidov, M. Chatzichristidi, H. H. Fong, J. a. DeFranco, G. G. Malliaras, and C. K. Ober, “Orthogonal Patterning of PEDOT:PSS for Organic Electronics using Hydrofluoroether Solvents,” Advanced Materials, vol. 21, no. 22, pp. 2314-2317, Jun. 2009.
[2] A. a. Zakhidov, J.-K. Lee, J. a. DeFranco, H. H. Fong, P. G. Taylor, M. Chatzichristidi, C. K. Ober, and G. G. Malliaras, “Orthogonal processing: A new strategy for organic electronics,” Chemical Science, vol. 2, no. 6, p. 1178, 2011.
[3] F. Ante, D. Kälblein, U. Zschieschang, T. W. Canzler, A. Werner, K. Takimiya, M. Ikeda, T. Sekitani, T. Someya, and H. Klauk, “Contact doping and ultrathin gate dielectrics for nanoscale organic thin-film transistors.,” Small (Weinheim an der Bergstrasse, Germany), vol. 7, no. 9, pp. 1186-91, May 2011.

We report the solution-processed nickel oxide (NiOx) film prepared by a sol-gel method as a novel hole transport layer (HTL) for efficient and stable phosphorescent organic light-emitting diodes (OLEDs). After spin-coating and annealing, a highly transparent and uniform NiOx film is formed. The nickel and oxygen compositions are characterized by x-ray photoelectron spectroscopy (XPS) and the film roughness is checked by atomic force microscopy (AFM). The NiOx HTL yields an efficient device with similar or better carrier injection and stability compared to the conventional organic counterpart. The non-radiative quenching effect caused by NiOx/organic interface is also studied for improving the efficiency roll-off. Furthermore, NiOx is compatible with solution-processed small molecule phosphorescent emitting layer, which entitles us the capability of fabricating all solution-processed OLEDs. We have demonstrated such devices with decent efficiency and improved stability. This is the first time that the solution-processed NiOx film is reported as a potential HTL applied in phosphorescent OLED and our study explores a new way for synthesizing novel HTLs for OLEDs.

All solution-processed phosphorescent small molecular organic light-emitting diodes (SMOLEDs) were achieved by combining low temperature sol-gel processed vanadium pentoxide (V2O5), solution-processed small molecular emitting layer with charge transport assisting dopants and an electron transport layer/hole blocking layer (ETL/HBL). V2O5 was able to yield efficient phosphorescent SMOLEDs with improved stability compared to conventional poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) OLEDs with similar device fashion. Detailed studies were conducted on the morphology and transport property of V2O5 with various annealing conditions and treatment. Furthermore, by controlling the doping ratio of the hole/electron transport assisting dopants, we were able to independently tune the hole/electron transport capability of the emitting layer. Therefore a controlled recombination zone position and optimized efficiency would be easy to achieve. The low temperature solution process of such devices is compatible with flexible substrates and printing applications. It also shed light on how to fabricate efficient and stable solution-processed multiplayer OLED devices.

An emerging advantage of organic semiconductors is their ability to conduct ions in applications such as light-emitting electrochemical cells (LECs), photovoltaic devices, actuators, and electrochromic devices, among others. This ability of organic materials to conduct both ionic and electronic currents in the solid state sets these materials apart from their inorganic counterparts, potentially enabling disruptive technologies not modeled after existing semiconductor devices, yet these benefits are under-utilized, in part because the fundamental electrochemical processes in these materials are not well characterized. In particular, evidence suggests that the profiles of ions and electrochemical doping in the polymer film during operation significantly impacts the performance and stability of the device. However, ion transport and electrochemical doping in conjugated polymer films are not well understood. Here, we present our findings from direct profiling of ion distributions in dynamic and fixed-junction LECs following application of voltage, via Time-of-Flight Secondary Ion Mass Spectrometry. LECs were composed of a polymer film sandwiched between an ITO layer and a gold layer. Dynamic-junction LECs were fabricated using a lithium triflate/trimethylolpropane ethoxylate electrolyte, while fixed-junction LECs contained the polymerizable ionic liquid alkyltrioctylammonium alkylsulfonate. Ion distributions of the devices were characterized with regard to film thickness, salt concentration, applied voltage, and relaxation over time. Results provide insight into correlation between ion profiles and device performance and potential approaches to tuning electrochemical doping processes in LECs.

Organic semiconductors have been promoted as the potentially superior candidate as they are compatible with very low cost patterning technologies based on printing. However, they suffer from low mobility and poor air stability compared with inorganic alternatives. Hence, a number of groups are investigating the use of nanostructured inorganic-organic materials, with the aim of retaining the patterning advantages of the organic host, while enhancing performance by a path through the inorganic material.
Here we report a stretched contact printing technique to assemble 1-D nanostructures with controlled orientation and density from either as-grown substrates or solution-based dispersions. The process is based on a simple one-step stretching of an elastic substrate covered with high-aspect ratio nanostructures. During the stretching process, the randomly orientated nanostructures gradually transform to highly aligned films. Subsequently, these stretching-aligned 1-D nanostructures can be transferred to other rigid or flexible substrates, forming a single layer of well-oriented network. Our results show that up to 90% of the printed NWs are aligned within ± 15 degree of the primary stretching direction. Moreover, the feasibility of incorporating these stretching-aligned networks with polymer semiconductors for generating hybrid thin film transistors devices is demonstrated. Through addition of such ordered networks to polymer semiconductors, the flow of electric current can be accelerated, resulting in an enhancement of transistor mobility and of device stability in ambient air conditions.

The structure and morphology of organic semiconductor thin films strongly influence their electronic and optoelectronic properties and device performance. Using a combination of x-ray, electron and optical methods, molecular-level physical and electronic structure has determined for polyarylamine films grown from solution on a range of metal and oxide substrates. These wide-gap semiconducting polymers are hole conductors and efficient emitters of visible light when irradiated with UV and x-rays. This visible luminescence emission has been used in parallel with photoelectron emission to probe the properties of these films. Electron spectroscopy using laboratory and synchrotron excitation sources has confirmed the chemical integrity and morphology of the films and the electronic structure of the substrate, interface and overlayer. Using a tuneable x-ray source with energies resonant with the K-edges of the light elements within the polymer, the unoccupied molecular states and the orientation of molecules within the films are probed. This x-ray absorption information can be obtained both from photoelectrons and luminescence and the latter has been detected spectroscopically and using a hyperspectral microscope. A laterally-resolved variation in chemical composition and molecular orientation is revealed for these solution-processed polymer films. There is a strong molecular orientation relative to the substrate surface that depends on the film thickness and on the substrate quality.

Interfaces between polymers and inorganic materials can be found at all levels of organic electronics and photovoltaics, including in the active electronic or photovoltaic layers, at electrical contacts or interconnection lines, and within the protective encapsulation and transparent barrier layers. In the case of protective barriers, the integrity of such polymer-inorganic interfaces are of critical importance to the lifetime of devices and their modules, since interfacial damage in the form of loss of adhesion or delamination can degrade device performance and create fast diffusion pathways for potentially harmful environmental species such as moisture and oxygen. The need to understand the kinetics of environmentally-assisted degradation of polymer-inorganic interfaces is underscored by adverse operating conditions frequently encountered, including temperature cycling, mechanical stress, active chemical species, surface weathering, and exposure to solar ultraviolet light. We present research methodologies for quantifying the effect of interfacial chemistry together with the role of environment, temperature and mechanical stress on the adhesion of model polymer-inorganic interfaces. Model interfaces include inorganic oxide and nitride layers adjacent to principally cross-linked acrylate organic layers. Measurements of adhesive strength are reported as a function of both in situ environmental conditions and pre-exposure to accelerated aging conditions. We demonstrate that adhesive strength is degraded after exposure to ultraviolet light, and use XPS measurements to correlate this degradation to photo-induced chemical changes at the interface. We also present measurements of the rate of interface debonding as a function of the applied mechanical loads, which give insight into the kinetics of defect evolution and provide the basis for kinetic models that can allow for quantitative predictions of device lifetime.

To achieve a high exciton production efficiency at a singlet excited level (S₁) in OLEDs, we employed a mechanism of thermally activated delayed fluorescence (TADF). ¹⁾ In order to obtain a high efficiency of TADF, rather small energy gap (ΔE _ST) between S₁ and triplet excited (T₁) levels should be necessary for light-emitting materials, which can be attained by small orbital overlapping between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). We have designed and synthesized a high-efficiency purely organic luminescent material, 2,4-bis{3-(9H-carbazol-9-yl)-9H-carbazol-9-yl}-6-phenyl-1,3,5-triazine (CC2TA)²⁾ comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting TADF. The molecular design of CC2TA allows clear spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to an exceptionally small ΔE_ST of 0.06 eV together with a high triplet energy (T₁ = 2.85 eV). Furthermore, a high external electroluminescence quantum efficiency as high as 11 ± 1% has been achieved in the sky-blue organic light-emitting diodes employing CC2TA as an emitter.
1. A. Endo, M. Ogasawara, and C. Adachi, Adv. Mater., 2009, 21, 4802.
2. S. Y. Lee, T. Yasuda, H. Nomura, and C. Adachi, Appl. Phys. Lett., 2012, 101 093306.

One of the main prerequisites to promote industrial commercialization of organic electronics is to improve the speed of the organic transistors which is rather low compared to its inorganic counterpart. One promising approach to improve the performance is downscaling. However, downscaling of OTFTs is challenging: the key issue is the miniaturization of all critical layers and feature sizes, such as gate dielectric thickness, channel length as well as electrode overlap.
A self-alignment process developed by Palfinger et al. [1] is a promising technique to fulfill these requirements. In a first step an aluminum gate electrode is structured by photolithography or nanoimprint lithography for submicron features. Ultrathin dielectric layers with low gate leakage were built up with two different approaches: the first is the usage of organophosphonic acid SAMs with different functional groups grown on the plasma oxidized aluminum gate. As demonstrated by Vuillaume et al. [2] SAM dielectrics might achieve a dielectric strength comparable to that of SiO2. In a second approach a 30 nm thin electrically dense and photopatternable polymeric dielectric layer was used. The photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) is crosslinked by a Photo-Fries rearrangement under UV-exposure (254 nm) without adding any photoinitiator.
The source/drain-electrodes are defined by backside exposure of a photoresist through the nanoscaled gate electrode acting as the photomask. This self-aligned architecture provides channel lengths in the sub-µm range with minimized parasitic geometric overlap down to several 10 nm. The Bottom Contact Source/Drain electrodes were modified by organothiol or -selenol SAMs in order to reduce the contact resistance of the electrodes.
The properties of all functional layers like film morphology or thickness and orientation of SAMs were studied by TEM, AFM, ellipsometry, contact angle measurements and angular resolved XPS. Finally, the influence of these film properties on the semiconductor growth, (within this work pentacene was used), charge carrier mobility and onset voltage will be discussed.
[1] U. Palfinger, C. Auner, H. Gold, A. Haase, J. Kraxner, T. Haber, M. Sezen, W. Grogger, G. Domann, G. Jakopic, J. R. Krenn, B. Stadlober, Adv. Mater. 22, 5115 (2010)
[2] D. Vuillaume, C. Boulas, J. Collet, J. Davidovits, F. Rondelez, Appl. Phys. Lett. 69, 1646 (1996)

Organic light-emitting diodes (OLEDs) have drawn intense attention during the past decades due to their potential applications in solid-state lighting and flat-panel displays. The use of roll-to-roll printing techniques is the best way to improve its processability and reduce the production costs. For instance, gravure printing has been used to fabricate large-area polymer OLEDs and it is promising for large area optoelectronic applications. Recently, solution-based small molecule OLEDs are drawing more and more research attention, as such a technology combines advantages of facile synthesis and purification, besides the low-cost solution processing of polymers.
The viscosity of small molecules in solution is far below the optimal printing range of the gravure printing technique giving rise to problems in film homogeneity. Therefore, we made use of an ultra-high molecular weight (UHMW) polymer as a matrix for the green small molecule system (Merck KGaA) in order to tune the rheological properties of the functional ink. The UHMW Polymer/small molecule blend was prepared in different solvents at various concentrations to characterize its share rate dependent viscosity and wetting of the substrate. In first instance, spin coated devices were prepared to investigate the effect of the solvent and polymer concentration on the performance of the device. In a second step, viscosity measurements were correlated to the gravure printing outcome in terms of film homogeneity and thickness. Finally, we demonstrate the use of the gravure printing technique in the fabrication of flexible small molecule based OLEDs with low turn-on voltage (~ 5V) and high luminance (~10^3 cd/m^2) on Polyethylene terephtalate (PET) foil.

Ultra-low voltage Organic Field-Effect Transistors (OTFTs) fabricated onto flexible and transparent plastic substrates and designed for high frequency operations are presented. Operating voltages in the range of 1-2 V were achieved thanks to an ultra-thin, hybrid organic/inorganic dielectric which can be fabricated with a highly-reliable process at the nano-scale. Such insulating layer is obtained by coating an ultra-thin alumina layer (5 nm), previously thermally-grown on the aluminum gate, with an organic dielectric, namely Parylene C, deposited at room temperature by Chemical Vapor Deposition in very thin films (25 nm). This process can be easily up-scaled to industrial size for large-area production of organic devices on plastic substrates.
Thanks to the employment of a transparent substrate, a self-alignment process was employed for patterning source and drain electrodes. At first the aluminum gate electrode is patterned in order to reproduce the layout of the transistor channel. After the fabrication of the double layer gate dielectric, source and drain electrodes are patterned by a standard lift-off technique using the gate electrode as a mask during the photolithographic process. This procedure allowed a dramatic reduction of the overlap between source/drain ad gate electrodes. A total overlap length of 1 µm (over a channel width of 40 µm) was obtained and verified by means of Scanning-Electronic Microscopy.
All the fabricated devices are characterized by very low leakage currents (~10 pA), Ion/Ioff up to 10^5, hole mobility up to 0.1 cm^2 V^-1 sec^-1 and almost negligible hysteresis. Moreover, as a consequence of the reduced overlap, parasitic capacitances were minimized, and a cutoff frequency up to 100 kHz was obtained. It is noteworthy that these results, which are particularly remarkable for OTFTs fabricated onto plastic substrates and operating in ambient conditions, were obtained with commercially available, unpurified organic semiconductors and without any significant optimization of the channel length. In addition, the employment of an ultra-thin dielectric layer allows for further reductions of the channel length without reaching the short-channel conditions; as a consequence, further increases of the cutoff frequencies without sacrificing the electrical performances of the devices can be predicted.
The employment of such devices for relatively high frequency applications was investigated by fabricating basic electronic circuitries, including pseudo-CMOS and CMOS organic inverters, and ring oscillators. For organic inverters remarkable gains (up to 50 V/V) and noise margins were obtained; thanks to these results, ring oscillators with low distortion and stage-delay have been fabricated.

Organic light-emitting diodes (OLEDs) have raised interest due to possible applications in large area solid state lighting and TFT-displays. Potentially low manufacturing costs and the feasibility of flexible optoelectronic components, by the use of flexible substrates, are usually cited as main advantages of OLEDs compared to inorganic light-emitting diodes. One obstacle for inexpensive printing processed flexible OLEDs is the current use of low work-function alkaline earth metals like calcium or barium as cathode materials. These metals are highly reactive, which is why they cannot be easily solution processed but have to be prepared in UHV. Furthermore, residual humidity oxidizes these cathode materials and the lifetime of OLEDs is limited.
In this work, we investigate two organic polymers, Polyethylenimine (PEI) and Polyethylenimine-ethoxylated (PEIE), for their applicability as electron injection layers in OLEDs. PEI and PEIE are insulators and it is known that both polymers reduce the work-function of various metals when spin coated on top [1]. In order to study the electron injecting behavior of PEI and PEIE, both polymers are dissolved in 2-Methoxyethanol with varying concentrations between 0.05 and 0.6 weight %. These solutions are then spin coated on top of Silver and Aluminum substrates, and the change in work-function is determined by kelvin-probe measurements and ultraviolet photoemission spectroscopy (UPS). X-ray photoelectron spectroscopy (XPS) is used to determine the thickness of the polymer films as a function of polymer concentration. Additionally, AFM measurements are carried out in order to characterize the homogeneity and morphology of the polymer layers.
Taking these results into account, we prepare OLEDs on flexible substrates using PEI and PEIE as electron injection layers. For this purpose, layers of PEDOT and of a blue emitting polymer are printed on flexible pre-structured ITO PET substrates. Afterwards, PEI or PEIE is spin coated on top. Finally, electrodes of silver or aluminum are evaporated on top of the stack. In the case of aluminum electrodes, turn on voltages of about 4V (PEI and PEIE) are observed, whereas for silver electrodes turn on voltages of about 5V (PEI) and 7V (PEIE) are found. Compared to this, reference devices using calcium and aluminum as cathode material show a slightly lower turn on voltage of about 3.5V.

[1] Zhou et al.: A universal method to produce low-work function electrodes for organic electronics. Science, 336:327-332, April 2012

n-Type doping of organic semiconductors has emerged as a promising means of achieving higher device efficiency and stability. Despite many initial successes in this field, n-type dopants lag behind their p-type counterparts in both number and efficacy due to their inherent sensitivity to oxygen and, in some cases, water. This presentation discloses a series of novel, air- and water-stable, n-type dopants in which the active dopant species is unmasked by thermal or photochemical reaction. The chemical synthesis of these dopants, evaluation of their performance and doping ability in thin films, their effect on the solid-state morphology of doped films, as well as both computational and experimental insights to their mechanism of action will be presented. Relationships between the molecular structure of the dopants, the thermodynamic and kinetic parameters of their doping activity, and their performance in doped films will be discussed.

Light-emitting electrochemical cells (LECs) are one of the simplest type of molecular electroluminescent devices. Due to their peculiar mechanism LECs can be prepared from solution and operate with air stable electrodes making them suitable for low cost and large area lighting applications.1-3 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
Using fast JL-V scans and in-situ photoluminescence spectroscopy evidences of dynamical doping on these devices are found.5, 6 By means of improved device architecture and an optimized operation scheme LEC devices can reach very high performances with long lifetimes and high efficiencies at very bright luminances.7-9 However their efficiencies are still below those obtained in organic light-emitting diodes (OLEDs). Here we show the effect of the quenching of the excitons on the device efficiency. A strong dependence of the efficiency on the current density is found and the different mechanisms of the quenching of the excitons are analyzed and studied. The efficiency of these devices can be tuned by means of controlling the current density and by limiting the quenching pathways.
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2. E. S. Handy, A. J. Pal and M. F. Rubner, J. Am. Chem. Soc. 121, 3525-3528 (1999).
3. J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. Abruña and G. G. Malliaras, J. Mat. Chem. 17, 2976-2989 (2007).
4. R. D. Costa, E. Orti, H. J. Bolink, F. Monti, G. Accorsi and N. Armaroli, Angew. Chem. Int. Ed. 51, 8178-8211 (2012).
5. M. Lenes, G. Garcia-Belmonte, D. Tordera, A. Pertegas, J. Bisquert and H. J. Bolink, Adv. Funct. Mater. 21, 1581-1586 (2011).
6. S. B. Meier, D. Hartmann, D. Tordera, H. J. Bolink, A. Winnacker and W. Sarfert, Phys. Chem. Chem. Phys. 14, 10886-10890 (2012).
7. D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Orti, W. Sarfert and H. J. Bolink, Adv. Mater. 24, 897-900 (2012).
8. D. Tordera, M. Delgado, E. Ortí, H. J. Bolink, J. Frey, M. K. Nazeeruddin and E. Baranoff, Chem. Mater. 24, 1896-1903 (2012).
9. D. Tordera, A. Pertegas, N. M. Shavaleev, R. Scopelliti, E. Orti, H. J. Bolink, E. Baranoff, M. Gratzel and M. K. Nazeeruddin, J. Mater. Chem. 22 (36), 19264-19268 (2012).

In multilayer white organic light-emitting diodes (OLEDs) the electronic processes in the various layers -injection and motion of charges as well as generation, transfer and radiative decay of excitons- should be concerted such that efficient, stable and colour-balanced electroluminescence occurs. We show that it is feasible to carry out Monte Carlo simulations including all these molecular-scale processes, as demonstrated for a hybrid multilayer OLED stack combining red and green phosphorescent layers with a blue fluorescent layer. The simulations elucidate the crucial role of exciton transfer from green to red. The simulated current density and emission profile agree surprisingly well with experiment. The experimental emission profile was obtained with nanometre resolution from the measured angle- and polarization-dependent emission spectra. By varying the material parameters used, obtained from electrical and optical studies, we identify the factors most strongly influencing the OLED performance. The perpendicular and lateral confinement of the exciton generation to regions of molecular-scale dimensions, revealed by this study, demonstrate the necessity of molecular-scale instead of conventional continuum simulation.

In this work we present the fabrication, characterization and ink formulation of gravure printed polymer light-emitting electrochemical cells (LECs). LECs are a promising alternative to organic light-emitting diodes (OLEDs) towards large-area devices for low end applications. One of the main advantages resides in that a low work function metal is not needed as a cathode, which usually limits the lifetime of OLEDs. These light emitting devices are fabricated by sandwiching a blend of a semiconducting polymer with a solid polymer electrolyte (SPE) between two electrodes, regardless of their work function. When applying a voltage to the device, the ionic species in the active film will help to form p or n doped layers at the corresponding electrode. Following the injection of carriers, the light emission will come from the semiconductor through the formation and successive recombination of excitons in the intrinsic layer between the p and n doped regions.
The gravure printing technique is set to become a common technique for the mass production of large area organic electronic applications. The ability of an ink formulation to be successfully printed by this technique depends on its rheological properties and its interaction with the substrate surface. Therefore, given a certain semiconducting polymer, the choice of the SPE will be determining not only the LEC performance but also the quality of the gravure printed films. Therefore, we correlate the LEC ink formulation to the film quality and device performance by changing the molecular weight of the SPE. The properties of the formulation are characterized by viscosity and contact angle measurements while the properties of the film are studied by impedance spectroscopy and atomic force microscopy. Finally, gravure printed flexible LECs with luminance values ~ 1000 cd/m^2 were fabricated with the optimal formulation.

Reduction of operating voltages of organic field-effect transistors (OFETs) is mandatory to enable their adoption in real applications. Many methods are under investigation, with the goal of preserving the overall fabrication process compatibility with plastic substrates (e.g. low temperature). In this work we have developed a new strategy for a “gentle” deposition of a high-k oxide on the top of an organic layer using a physical vapor deposition method. A fine tuning of the kinetic energy of the species is allowed by Pulsed Laser Deposition (PLD), hence representing a suitable solution for the realization of oxide dielectric layer without damaging the organic layer underneath. Using this technique, high-k alumina was employed as the dielectric of high-performance low-voltage n-type and p-type Top-Gated organic Thin Film Transistors (oTFTs) exhibiting exceptionally low leakage currents.
A wide-range screening of the main PLD parameters on the devices performances is reported, demonstrating well operating oTFTs with a dielectric layer thickness scaled down to asymp;30 nm and specific capacitances well above 100 nF/cm2. Moreover, the importance of the insertion of an ultrathin buffer layer in order to preserve a perfect device functionality is also discussed.
The scaling of the top-gated TFTs&’ operating voltages along with such a low level of parasite currents through the dielectric layer open the way to the fabrication of low power organic electronics with drastically reduced static power consumption. As a demonstration, a low-voltage complementary inverter was realized and characterized in this work. Finally, the ability of alumina coating to provide an intrinsic encapsulation of organic films, preventing from the air operating device degradation was also tested.

Organic field effect transistors (OFETs) are the essential elements in the new application areas of organic devices such as low operating power flexible sensors, displays, radio frequency identification (RFID). In these OFETs, low operating voltage have been achieved by using high permittivity (k) polymer dielectric or some inorganic dielectric materials, however, the high heat treatment temperature and inert annealing environment limit their application on polymer substrate as well as large area fabrications. Besides, to prevent the gate leakage current, these dielectric layer in the OFETs tends to be relatively thick (a few hundreds of nanometers) which will limit their integrations with other devices on a chip. In this work, we developed low operating power Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) OFETs on both n-type heavily doped silicon and paper substrates with ultra thin BST dielectric, less than 20 nm. The BST thin film is prepared by sol gel method at room temperature in ambient air and treated by UV-Ozone. The BST thin films with barium composition ranges from 0.3-0.7 are examined on heavily doped n-type silicon and the Ba_0.7Sr_0.3TiO₃ thin film gives the largest mobility of 2.27 cm²V^-1s^-1. We also fabricated the OFETs on paper substrates and the carrier mobility can be as high as 0.8 cm²V^-1s^-1. Two kinds of inverters based (saturated and depleted) will also be investigated and discussed. This low fabrication temperature and air fabricating atmosphere for sol gel derived gate dielectric as well as low driving voltage of the OFETs on paper substrate have provided promising solutions for applications based on low cost, large area and plastic/flexible/dedicate substrates.

Crystalline poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer with the highest ferroelectricity among organic material has attracted lots of interest for applications in flexible devices as well as the fundamental aspect. The ferroelectric phase of P(VDF-TrFE) thin films can be achieved through annealing above the Curie temperature, which result in the highest crystalline phase immediately below the melting point. The annealing process induces the rough morphology consisting of nano-rods and voids in highly crystalline P(VDF-TrFE), causing degradation of ferroelectric properties; however, there have been few studies to improve the surface roughness and consequential ferroelectrical properties of P(VDF-TrFE) films yet. In this study, we developed a fabrication method to improve morphology of P(VDF-TrFE) thin films using double coating process, which fills voids between highly crystalline nano-rods with additional amorphous P(VDF-TrFE) layer. We report the both improved ferroelectricity and electrical strengths of highly crystalline P(VDF-TrFE) such as fatigue behavior and retention.
P(VDF-TrFE) thin films were firstly coated on indium tin oxide/ glass substrates using spin coating technique. After fabricating P(VDF-TrFE) nano-rods by annealing at 120 celsius, the sample soaked in P(VDF-TrFE) solution at various condition was spin-coated again. Double coated samples showed the increase of diameter of nano-rods and consequently decrease of the size of void. Surface roughness of a double-coated sample was improved by up to 3 times than that of single coated sample.(20 nm to 7 nm in root mean square value) The remnant polarization value of highly crystalline P(VDF-TrFE) increased by 20% using double coating method. The double coated P(VDF-TrFE) exhibits the better fatigue behavior up to 10 times and retention property than low crystalline sample. In this presentation, we will discuss the ferroelectric properties of P(VDF-TrFE) thin films related to the morphology.

We report an White AC field-induced polymer electroluminescent (WFIPEL) device consisting of a white fluorophor poly(fluorene-benzothiadiazole-quinoline) [PF-BT-QL], combined with a red phosphor, Bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) [Ir(MDQ)2(acac)]. To our surprise, all devices exhibit high color quality no matter what the Ir(MDQ)2(acac) concentration is, even at high Ir(MDQ)2(acac) concentration up to 30% by weight, which has previously been suggested to be impossible for white organic light-emitting devices (WOLEDs). A deep understanding of the emission mechanism is gained by the unique charge carriers injection and thus the energy transfer in AC-driven condition. The observed dye-concentration independent emission characteristics can be applicable to most hybrid white system in staggered WFIPEL device configurations or similar organic electronic devices where dye concentration is an issue. Moreover, this finding proves high-quality white emission could be easily obtained by AC field-induced device with a simple-single EML device structure as compared to typical OLED structures.

For the construction of highly conductive printed electrodes on a polymeric substrate having a low glass transition temperature, the development of a low temperature sinterable metallic ink has been a crucial issue in association with various sintering techniques, such as dry sintering techniques using laser, plasma and microwave, or wet sintering techniques using chemical solutions such as electrolytes. For the achievement of low temperature sintering, these sintering techniques are composed of two sintering steps where the first step turns printed metallic parts to be low conductive ones and then the second step eventually turn them into highly conductive ones. Although these two step sintering techniques have shown promising results, the question how to prepare initially low conductive metallic parts at a temperature as low as 150 °C before applying the developed dry or wet sintering techniques as the second sintering step remains not resolved yet. In this work, we propose a novel concept of silver ink, whose sintering is triggered at a low temperature and completed with the aid of its own exothermic chemical reaction. Although individual components of this self-sinterable silver ink, Ag2O and a specific silver metallo-organic compound, exhibit endothermic behaviours, their mixture form shows a strong exothermic chemical reaction when heated at 150 °C. It is found that the dissociated form of the used silver metallo-organic compound not only reduces Ag2O to Ag through its recursive chemical reaction but also is attributed to the source of an exothermic chemical reaction. The conductivity of the developed self-sinterable silver ink is around 2.07 S/m, which might be sufficiently high for the second sintering step.

Sublimation of π-conjugated molecules in high vacuum is technologically important for growing active thin films in organic field effect transistors (OFETs). In such devices the charge transport is confined within the first few monolayers grown on the gate dielectric, and it is two-dimensional (2D) in stacked molecular layers. Molecular thin film growth is far more complex than atomistic growth described by classical growth modes. Organic semiconductor molecules have roto-vibrational degrees of freedom, anisotropic shape, and intermolecular interactions governed by dispersive forces. As high-vacuum sublimation is a non-equilibrium process, energy barriers at the surface and in the film determine self-affine morphology with time- and space correlations.
The correlation between charge transport, molecular order and morphology has been widely explored. A still open question concerns the nature of disorder generated during the early stages of growth, and how it affects charge transport in OFETs.
We demonstrate that thin films of the relevant n-type organic semiconductor N,N&’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis dicarboximide (PDI8-CN2), grown in a quasi layer-by-layer mode at two different substrate temperatures (25°C and 120°C), exhibits a marked difference in molecular organization, crystal order and density, which also reflects in the distribution of charge carriers in a transistor. We elucidate the nature of orientational defects at 25°C, consisting of coexisting domains with either standing molecules or planar/tilted/bent molecules, in the first monolayers of PDI8-CN2 thin films at the gate dielectric interface in working devices. Although these defects tend to heal dynamically, their formation at the early stages has an impact on the charge transport properties of the device: they are the cause of depletion of charge carriers in the first monolayer, leading to the second monolayer being dominant in charge transport. This suggests that the defects effectively behave as shallow traps for electrons.
Conversely, at 120°C the trend of charge mobility vs thickness is consistent with ideal 2D charge transport, where electron percolation occurs within each monolayer and no substantial charge transfer between adjacent monolayers occurs. In this case most of the charge carrier density is in the first monolayer. This highlights the complexity of the growth phenomena and their subtle effects on the device response, and how their control is technologically challenging.
These results, which are not intuitively explained by electrostatics arguments, have been obtained by applying in situ real time structural and electrical characterisation together with ex situ AFM measurements.

Although several methods of fabricating organic semiconducting wires (OSWs) are available, they should be aligned and patterned at desired positions and orientations for practical device application of OSWs. To print the highly aligned organic nanowires (ONWs) at desired position, we used our own home-built nanowire printer, “Electrohydrodynamic nanowire printer (E-nanowire printer)”. We successfully fabricated the aligned patterns of poly(9-vinylcarbazole) (PVK) nanowires using E-nanowire printer with the pitch of 50 mu;m and the diameter of 290 nm. We applied the E-nanowire printing to fabricate the highly aligned nanowire field-effect transistors (FETs) in large-area (7 cm × 7 cm) based on poly(3-hexylthiopehene): poly(ethylene oxide) (P3HT:PEO) blend (80:20, w/w) which showed a moderate mobility of 0.0148 cm²/Vs. After annealing the device at 150 °C for 30 minutes, the hole mobility was decreased to 0.0029 cm²/Vs. To indentify the origin of mobility degradation and structure of E-nanowire printed P3HT:PEO (70:30, w/w) nanowires, TEM analysis, elemental analysis using electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS) were conducted based on sulfur atoms in P3HT chain and oxygen atoms in PEO chain. According to TEM and elemental analysis results, P3HT:PEO blend nanowires have a core-shell structure along the wire axis. The origin of core-shell structure of E-nanowire printed nanowire can be explained by phase separation of polymer blends. There were some small PEO-rich phases which were randomly distributed inside of the P3HT core. These small phases will act as the defect sites of the charge transport in the channel and cause the variation of mobility in P3HT:PEO blend nanowire FETs. Furthermore, we compared the phase morphology with various processing parameters such as polymer composition, solvent evaporation time, applied voltage. Controlling phase morphology, we successfully achieved better electrical properties in nanowire FETs without any heating process. Our strategy to print the organic nanowires and control the phase morphology of them in precisely controlled manner can be one of most promising approaches for realizing flexible or textile electronics in large area.

The self-organization of π-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 a side-chain liquid crystal (SCLC) semiconducting polymer where (i) the backbone is a π-conjugated polymer and (ii) the side groups are π-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 and Atomic Force Microscopy (AFM). Moreover, their photophysical properties and the preliminary charge transport results will also be depicted in view of applications for organic optoelectronics.

Green and blue electrophosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and plastic substrates. Inverted OLEDs have a top anode and a bottom cathode, making them convenient to integrate with n-type transistor driving electronics. The semitransparent top-anode consists of 20 nm of gold deposited on a MoO3 hole injection layer. The bottom cathode consists of aluminum with a lithium fluoride electron injection layer. Current-voltage characteristics of electron-dominated devices show that this cathode efficiently injects electrons into electron transport materials of either 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) for green OLEDs or 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) for blue OLEDs. The green devices contain an emissive layer comprising tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) doped in a 4,4&’-bis(N-carbazolyl)-1,1&’-biphenyl (CBP) host and show a high current efficacy of 96.3 cd/A at a luminance of 1,387 cd/m2 when an outcoupling layer of N,N&’-Di-[(1-naphthyl)-N,N&’-diphenyl]-(1,1&’-biphenyl)-4,4&’-diamine (α-NPD) is deposited on the anode. The blue devices contain an emissive layer composed of iridium(III)bi[(4,6-di-fluorophenyl)-pyridinato-N,C2&’]picolinate (FIrpic) doped in an N,N&’-dicarbazolyl-3,5-benzene (mCP) host and achieve a current efficacy of 33.6 cd/A at 1,126 cd/m2. TmPyPB is better suited for the blue OLEDs because it has a higher triplet energy than TpPyPB and FIrpic, thus decreasing exciton transfer from the emissive layer to the electron transport layer. These results represent a significant improvement over previously reported inverted OLEDs.

Polyaniline (PANI) is one of the mostly used conducting polymers because it can not only be configured to reveal a wide range of conductivity but also possess multiple oxidation states useful in electronic and electrochemical applications. It is well known that PANI can form nanofibers under sequential polymerization of aniline (ANI) through a reaction with ammonium persulfate (APS) and acid. Here we report a new scheme capable of producing a diversity of micro/nanostrucutures of PANI. It involves the use of a compound drop, which comprises a smaller water-like APS drop immersed in a larger oil-like drop of pure ANI precursors, in controlling the polymerization and subsequent assembly processes. As the pre-requisite for the formation of PANI entails diffusion of APS into ANI, distinct nanostructures such as rods, needles, and islands can be observed as a result of the spatial gradient of APS over the drop. In addition, for the first time we discover long PANI microneedles of several hundred micrometers in length. The observed phenomena could be attributed to protonation-induced coil-stretch transition that can lead PANI to exhibit distinct conformations such as stretched, elongated, and coiled states due to different degrees of protonation. X-ray Photoelectron Spectroscopy (XPS) is also employed to identify how this transition occurs in terms of the amine-to-imine ratio.

The different electrical characteristics of devices with tetraoctylammonium bromide (TOAB) on the different conjugated polymers are investigated. The increase in electroluminescence efficiency of green emissive poly(9,9-dialkylfluorene) derivative (G-PF), super yellow polyphenylene vinylene (SY-PPV), and Poly(3-hexylthiophene-2,5-diyl) (P3HT) based devices is attributed to the interfacial dipole formed by layer structure of TOAB thin films which are evidenced by the diffraction peaks of X-ray diffraction (XRD). The electron injection ability is increased by spin-coating TOAB on the G-PF and SY-PPV except P3HT. The large grain size and high surface roughness of TOAB thin film on P3HT surface which is confirm by the XRD and atomic force microscopy (AFM) is attributed to cause the block the carrier at the interface between P3HT and Al to enhance the exciton formation.
The enhancement of hole injection ability for indium tin oxide (ITO) by spin-coating ammonium salts on ITO is investigated. The work function of ITO is increased via spin-coating the NH4Br aqueous solution on ITO, being evidenced by ultra-violet photoelectron spectroscopy. The formation of H-bonding at the interface between ITO and NH4Br is corrobrated by the surface sensitivity of synchrotron X-ray photoelectronic spectroscopy at low photoelectron take-off angles. We illustrate the increase in work function of ITO with a possible scheme via H-bonding between NH4+ and ITO. Polymer light-emitting diodes (PLED) with ITO/NH4Br compared to that with ITO showed the siginificantly enhanced performance with higher current density (924 mA cm-2 v.s. 781 mA cm-2) and brightness (44224 cd m-2 v.s. 4018 cd m-2) at a bias voltage of 7 V. To confirm the validity of general ammonium salts for enhancing device performance, PLEDs with ITO coated by NH4Cl, NH4H2PO4, and(NH4)2SO4 were fabricated and verified.

Organic thin film transistors (OTFTs) are highly promising devices for mobile consumer electronics. Therefore, low-voltage operation and the successful integration of n-channel and p-channel transistors into logic circuits are essential. Both demands require precise control over the threshold voltage (V_th). Self-assembled monolayers (SAMs) have been proven to enable both low V_th and low leakage currents in OTFTs [1]. Applying co-deposited mixed SAMs V_th can be tuned to desired values using molecules with different dipole moments [2].
Further expanding this approach we developed a procedure to continuously adjust V_th of α,αprime;-dihexyl-sexithiophene based OTFTs by controlled SAM exchange reactions [3].
In our work, we compared the kinetics of monolayer self-assembly of long chained carboxylic acids and phosphonic acids on thin aluminum oxide surfaces by means of static contact angle measurement. Moreover, we tested the possibility of SAM exchange reactions by immersing samples which were fully covered with one molecular species in the solution of the other one followed again by static contact angle measurements. Due to higher binding strength and faster self-assembly kinetics, phosphonic acid anchor groups substitute carboxylic acid molecules on aluminum oxide surfaces, whereas the converse exchange does not occur. This facilitates the formation of mixed or fully exchanged monolayers, where the mixing ratio is a function of exchange time.
We introduced this exchange approach into the fabrication of OTFTs with α,αprime;-dihexyl-sexithiophene as an example for an organic semiconductor. The SAM on top of a thin aluminum oxide layer form a hybrid dielectric. Using SAMs based on n-alkyl carboxylic acids and substituting them by fluorinated alkylphosphonic acids V_th can be shifted from negative to positive values or vice versa if the chemistry of the substituents is interchanged. Due to the different dipole moments of the molecules, V_th is intimately related to the mixing ratio and consequently a function of exchange time, too.
In conclusion, with controlled SAM exchange reactions we can have precise control over the threshold voltage of organic thin film transistors. According to the molecular fashion of the chains and the time of the exchange reaction V_th reflects the degree of substitution and the composition of the mixed SAM. Therefore, our concept enables the fabrication of integrated circuits which are optimized with respect to electric noise and energy consumption.
[1] M. Halik et al., Nature, 2004 431 (7011), 963-966
[2] U. Zschieschang et al., Advanced Materials, 2010 22 (40): p. 4489-4493
[3] T. Lenz et al., Langmuir 2012 28 (39), 13900-13904

Organic light-emitting diodes (OLEDs) have been investigated for more than 20 years and are standing now at the frontier to mass production. Current research focuses on the enhancement of light outcoupling efficiency which is reduced especially due to the energy dissipation to surface plasmons (SPs) by its excitation at the interface to a metallic cathode. It is known that the orientation of emitter molecules in films used in OLEDs has a huge effect on the coupling to SPs. Numerical simulations show that a horizontal emitter orientation with respect to the substrate can enhance the efficiency by up to 50%. An expeditious method to determine the orientation of the transition dipole moment of molecules has been developed using angular dependent photoluminescence spectroscopy. By comparing measurement with simulations, the orientation can be quantitatively determined. Although other methods to measure molecular orientation exist, the presented method not only makes it possible to study molecular orientation in neat thin films but also of small amounts of emitters doped into a matrix material without knowing any detailed information of the optical properties of the dopant. Hence, this method is particularly useful for the investigation of newly developed materials. The method is applied to Iridium-based phosphorescent emitters in order to gain a consistent efficiency analysis of OLEDs and possibilities to enhance light outcoupling.

The extreme temperature stability, pseudo-aromaticity and an unusual electron withdrawing cage structure are well known properties of 1,2-closo-dicarbadodecaboranes (carboranes). Such interesting properties have prompted much interest in the incorporation of carborane into a variety of polymers, principally as a strategy to improve temperature stability and oxidative stability. However much less is known about the effects of carborane incorporation on fully conjugated optoelectronic polymers. Here carboranes have the potential to improve stability, tune band gaps and manipulate film crystallinity.
In this paper we present the synthesis of a novel benzocarborane containing monomer, incorporating organotin at the 1,4 positions. Polymerisation with a variety of conjugated co-monomers enabled the preparation of the first fully conjugated benzocarborane containing polymers in good molecular weight. Analogous polymers containing benzene were also preparared, and the thermal, morphological and optoelectronic properties of both polymers systems are compared and contrasted

Recent research in organic thin film transistors (OTFTs) has yielded improved synthetic and fabrication techniques for the application in consumable electronics. Independent control of injection and transport of charge carriers (both electrons and holes) is important for the fabrication of field effect transistors and complementary logic circuits. In organic heterojunction transistors, the charge accumulation and transport of charge carriers (holes and electrons) usually occur in different layers. Exploring an active and simple mechanism to balance the charge transport is necessary for the application in wide variety of device architectures. In this study, an active mechanism for controlling ambipolar charge transport is developed based on self-assembled gold nanoparticles (Au NPs). We used pentacene as p-channel and C60 as n-channel materials for the organic heterojunction transistors. A self-assembled monolayer of Au NPs (5 nm) was inserted between the gate dielectric and the active layer (pentacene/C60). By controlling the gate bias, electron and hole currents are manipulated to overcome the intrinsic material limitations. The endurance and retention measurements confirm that this method exhibits good electrical reliability and stability. This solution process approach has potential applications for large area printed electronic devices.

Recently, organic electronics is recognized to be highly compatible for implementation of electronic logic circuit due to their advantages over inorganic semiconductor devices such as light-weight, low-cost, flexibility and environmental friendly. The possibility of tuning the threshold voltage (Vth) of organic field-effect transistor (OFET) over a broad range can lead to a precise control of depletion or enhancement operation. Predictably tuning Vth is a key point for achieving functional unipolar inverters which only adopt one type of semiconductor in logic circuits. The variation of Vth can be obtained by the introduction of a second gate, floating gate, suitable functionalization of dielectric layer, or appropriate combination of two organic semiconductors. However, the introduction of a second gate or floating gate requires additional power consumption in the circuit, making this technique useful mainly for dynamical changes of Vth. On the other hand, the functionalization of the dielectric layer or appropriate mixing of semiconductors always requires a systematical selection of materials to obtain a reproducible result. Therefore, exploring a simple processing method to achieve controllable threshold voltage is essential. Organic-inorganic nanocomposites are promising candidates for electronic devices because of the expected co-activation between organic and inorganic components, which has great potential to be applied in OFETs, solar cells, memories and so on. However, demonstration of tunable Vth for OFETs and switching voltage for inverters by using organic-inorganic composites as active layers is scarce.
Herein, we report solution processed composites of conjugated poly(3-hexylthiophene) (P3HT) and gold nanoparticles (Au NPs) modified with 1-dodecanethiol (DDT) for obtaining controllable Vth shifts. Through varying the doping rate of Au NPs, tunable Vth shifts have been achieved from 12 V to 27 V without electrical performance degradation. We integrate such transistors to form two kinds of unipolar inverters and their switching voltages have been successfully controlled. Our findings not only provide better understanding on the relationship between doping concentrations of Au NPs and threshold voltages shifts of P3HT based field-effect transistors and inverters, but also demonstrate their potential applications in solution-processed integrated circuits.

Because of pursuing the sustainable use of resources, the efficient lighting source is desired recently. Organic Light Emitting Diodes (OLEDs) is one of the candidates of its purpose, but production of it is still energy consuming. Light-Emitting Electrochemical Cells (LECs), which normally consists of just solution processable one active layer and two air stable electrodes, is another option. This simple architecture device has possibility as a new lighting source. In addition, it is also known that LECs have unique light emitting mechanisms; both electronic and ionic conduction at the same time. This fact makes it difficult to understand the initial mechanism of the LECs. In this poster, we reveal that the shape of current density, light intensity, and efficacy during the transients of the LECs are universal for a wide range of ionic transition metal complexes used as the active materials. We also show that the universality is not affected by the operated temperature. We suppose it derives from the ionic mobility in the device. From this hypothesis, we calculate the activation energy of some iridium complexes from the data collected with various temperatures and explain them from the physical point of view.

For high external quantum efficiency of organic light-emitting diodes (OLEDs), defined as the ratio of the number of photons emitted into the air to the number of injected electrons, extracting more light from the devices plays a key role because, in conventionally structured OLED, less than 20% of the light generated from the emissive layer can escape from the device as useful radiation. Although many research groups have reported promising results in light extraction from OLEDs with various out-coupling structures, the fabrication of these structures requires complex and expensive equipment with a vacuum system or a complicated lithographic process. Considering practical applications, it is essential that fabrication techniques be applicable to a large area and cost effective. Hence, it is desirable to develop a simple fabrication technique for out-coupling structures. In this study, we developed solution-processible out-coupling structures by adopting a spin-coating process and a sol-gel method. A scattering layer can be easily obtained via a spin-coating process with a SiO₂ sol solution containing TiO₂ particles. Furthermore, a corrugated structure can be quickly fabricated by spin-coating a small amount of a mixture of SiO₂ and TiO_x sol solutions. Introducing the scattering layer and the corrugated structure inside and outside the glass substrate as internal and external extraction layers, OLEDs showed enhanced light extraction efficiency by 30% without a change of electroluminescence spectrum compared to the devices without the structures. The light extraction enhancement is attributed to suppression of the light-trapping caused by total internal reflection at the interface between the glass substrate and the air. Using these solution-processible out-coupling structures, nearly all-solution-processed OLEDs with enhanced light extraction could be demonstrated. It is believed that the solution-processible out-coupling structures demonstrated in this study can be applied to various types of LEDs and other optical devices to enhance light extraction.

The comprehensive goal of this research is to understand the link between molecular structure and charge transport to aid in the design of nanoscale devices, and especially, in the design of molecular/organic based electronics. In molecular electronics, assemblies and aggregates of molecules can control the transport of electrons because of interactions of the molecules in the assemblies themselves, molecular connectivity, and the geometry of the molecules in the assemblies, relative to the surface. The specific goal of this research is to understand how tuning the molecular structure of porphyrinoids affects charge transport of these molecules on Au surfaces . Here we use porphyrins as a platform to study such effects because of their stability, vast potential for tuning their electronic structure, small HOMO-LUMO gap, and overall pi-conjugated structure. Here we will illustrate how systematic changes in the porphyrinoid structure to deliberately vary molecular connectivity, such as variations in pi- conjugation lengths and nearest neighbor interactions, influences charge transport properties of porphyrinoid assemblies. We examine these assemblies by using a combination of Fourier Transform Infrared Spectroscopy (FTIR), Atomic Force Microscopy (AFM), and Scanning Tunneling Microscopy (STM) to characterize the electronic and structural properties of the surfaces.

Polymer dielectrics layer is easily prepared through spin coating of polymer solution and applicable for flexible organic thin film transistors (OTFTs). A lot of research have been done to develop candidate polymers such as poly(4-vinyl phenol) (PVP), polyimide, PMMA, poly(vinyl alcohol), BCB (divinyltetramethylsiloxane-bis(benzocyclobutene) derivative) polymer, and these derivatives. Among the various polymer dielectrics, PVP based cross-linking systems especially received much attention due to their high dielectric constant. However, PVP has lots of free hydroxyl groups which are sensitive to moisture so it prevents to use it in high-humidity environment. Moreover, devices with PVP dielectrics commonly show large hysteresis and large leakage current since the hydroxyl groups act like electron traps. On the other hand, commercially available crosslinked BCB polymer does not have free hydroxyl groups but requires high annealed temperature (more than 200oC), so that it can be a limitation when applying to flexible OTFT with using plastic substrates. Thus, to overcome these challenges, we design a new hydroxyl-free PVP based dielectric that is curable at low temperature at 80oC and highly stable even in water, and has small hysteresis as a device. We report the polymer properties as well as OTFT performance.

Typical PLED is a multilayered device having a general structure ITO/HTL/EML/ETL/Cathode, where HTL is a hole transporting layer, EML the emission layer and ETL the electron transport layer.
For improving the performance of these PLEDs, one of the important fields is the development of n-type conjugated polymer semiconductors so in this work three new copolymers having electron and hole carriers was synthesized with the objective of getting into a single polymeric emission layer both EML and the ETL.
These copolymers were prepared through the incorporation of a quinoline units into fluorine chains by the Suzuki cross-coupling polymerization.
The first synthetic route performed aimed to obtain an alternate copolymer (PFQ) using as starting materials 9,9-dioctilfluorene 2,7-diboronic acid and of 5,7-dibromo-8-oxyloctyl-quinoline (1:1). In the second route were used the same materials of the first route with the addition of 9,9-dioctyl-2,7-dibromofluorene (PFQ-S2) (1:0.6:0.4) and the third route have the only difference for the second route the substitution of the 9,9-dioctyl-2,7-dibromofluorene for the 1,4 dibromobenzene (PFQ-S1) (1:0.6:0.4).
All the absorption and photoluminescence spectra of these copolymers were obtained in chloroform solution. The absorption spectra for all copolymer showed two maximum absorption peaks. For PFQ these peaks were observed at 330 nm and 250 nm for PFQ-S1 at 350 nm and 250 nm and for the PFQ-S2 at 360 nm and 250 nm.
All the PL spectra were made due to excitation at the maximum absorption peak for each one respective copolymer and for all were observed a broad band emission with the emission peak maximum for PFQ at 425 nm, PFQ-S1 at 405 nm and for PFQ-S2 at 415 nm.
The analyses of infrared, Nuclear Magnetic Resonance, Gel Permeation Chromatography, Thermogravimetric Analyzer and Differential Scanning Calorimetry were made in order to characterize these copolymers.
The PLEDs were prepared having a multilayer architecture, comprised by a semiconductor polymer layer, poly(3,4-ethylenedioxy thiophene)/poly(4-styrenesulfonate) (PEDOT:PSS), deposited on the anode ITO/glass substrate, previously treated with UV-Ozone for 5 minutes, followed by the synthesized photo and electroactive copolymers and finally, aluminum cathode. Another series of devices were prepared having an additional electron transport layer (ETL) of 8-hydroxyquinoline aluminum salt (Alq3) prior to the aluminum cathode deposition was also prepared for comparison.
The PLEDs containing these new copolymers showed the threshold voltage around 4.0 V, while those prepared having Alq3 presented a threshold voltage around 5.5 V, indicating that the PLEDs built with only with the copolymer do not need an extra ETL.
The electroluminescence (EL) spectra of the PLEDs have shown emission in the whole visible spectra with a peak maximum at 520 nm, red shifted in comparison to the correspondent solution photoluminescence, as expected.

The field of organic semiconductors has progressed considerably in recent years, but improving our understanding of these materials has to potential to bring about significant advances to organic device technology. The next step to improve the performance of organic electronics is to develop our understanding of the mechanisms of doping in organic semiconductors so that the doping level can be precisely controlled. As organic semiconductors and dopants interact on a molecular level, the doping process is more complex and less well understood than doping in inorganic semiconductors. This understanding will be necessary in order to implement these materials in a wider range of electronic devices.
In this report, we used the known electron acceptor F4TCNQ (7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane) as a p-type dopant for TIPS- Pentacene (6,13-Bis(triisopropylsilylethynyl) Pentacene), a small molecule that has been widely used for organic LEDs. In order to study semiconductor properties in a form that allows for flexible electronics applications, we investigated solution processed thin films of the semiconductor-dopant system. Using optical absorption measurements in both film and solution, and Grazing Incidence X-Ray Diffraction (GIXD), we analyze the effects of doping on the crystalline structure of TIPS-Pentacene, and how this relates to the electronic properties of the semiconductor-dopant system. We find that the addition of F4TCNQ disrupts the crystal packing structure of the semiconductor, and show that this loss of crystallinity significantly decreases the conductivity. This is attributed to a large drop in mobility, despite a significant increase in the charge carrier density resulting from the addition of F4TCNQ.
Maintaining the mobility of charge carriers is an important factor in successfully doping an organic semiconductor, and is especially important in small molecule systems. Our results clearly show the strong dependence of charge carrier mobility on the crystallinity of the semiconductor thin film. Consequently, in doping small molecule semiconductors, care must be taken to choose dopants that can be incorporated into the crystal structure without dramatically affecting the crystalline order of the host material. Such knowledge is invaluable in the future design of efficient molecular dopants for organic electronics.

In this work, highly efficient solution-processed AC field-induced polymer electroluminescent (FIPEL) devices using high-k relaxor ferroelectric terpolymer poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) as dielectric layer was investigated. P(VDF-TrFE-CFE) was selected since it possesses higher dielectric constant as compared to common polymer dielectrics including polystyrene (PS, kasymp;2.6), poly(methyl methacrylate) (PMMA, kasymp;3.5), poly(vinyl phenol) (PVP, kasymp;4.2), and poly(vinyl alcohol) (PVA, kasymp;7.8). We found that the threshold voltage for the onset of luminance can be significantly lowered using high-k P(VDF-TrFE-CFE) dielectric layers. Accordingly, high brightness of over 2000 cd m-2 and very low threshold voltage for the onset of AC luminance were achieved. This suggests that greater device performance can be achieved in such devices using high-k dielectric layers.

The molecular orientation and microstructure of films of the high-mobility semiconducting polymer P(NDI2OD-T2) (poly(N,N-bis 2-octyldodecylnaphthalene-1,4,5,8-bis dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene)) are probed using a combination of grazing-incidence wide-angle x-ray scattering (GIWAXS) and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. In particular a novel approach is used whereby the bulk molecular orientation and surface molecular orientation are simultaneously measured on the same sample using NEXAFS spectroscopy in an angle-resolved transmission experiment. Furthermore, the acquisition of bulk-sensitive NEXAFS data enables the information provided by GIWAXS and NEXAFS to be reconciled. By comparing the bulk-sensitive and surface-sensitive NEXAFS data a distinctly different molecular orientation is observed at the surface of the film compared to the bulk. While a more ‘face-on&’ orientation of the conjugated backbone is observed in the bulk of the film, consistent with the lamella orientation observed by GIWAXS, a more ‘edge-on&’ orientation is observed at the surface of the film with surface-sensitive NEXAFS spectroscopy. This distinct edge-on surface orientation explains the high in-plane mobility that is achieved in top-gate P(NDI2OD-T2) field-effect transistors (FETs), while the bulk face-on texture explains the high out-of-plane mobilities that are observed in time-of-flight and diode measurements. These results also stress that GIWAXS lacks the surface sensitivity required to probe the microstructure of the accumulation layer that supports charge transport in organic FETs and hence may not be appropriate for correlating film microstructure and FET charge transport.

Solution-processed metal-oxides (ATO) with self-assembled octadecylphosphonic acid (ODPA) are successfully integrated onto Au coated polyimide (PI) substrates and show excellent dielectric characteristics, with a low leakage current density of less than 10-6 A/cm2 and a high areal capacitance of 180 nF/cm2. High-performance low-voltage flexible pentacene thin film transistors (TFTs) are demonstrated by employing the above substrates and in-situ modified low-cost Cu (M-Cu) as source-drain (S/D) electrode. Under an operation voltage of -2 V, the resultant flexible device shows a high mobility of 1.5 cm2/Vs, an on/off ratio of 2 × 104, a small subthreshold slope (SS) of 161 mV/dec. The obtained results are clearly better than those devices with Au electrodes. The improved device performance can be assigned to the low contact resistance and efficient hole-injection between M-Cu and pentacene. The mechanic stress effect on the device performance of the flexible TFTs is examined and the detailed mechanism is proposed. In conclusion, our results suggest a simple and possible approach to achieve high performance low-voltage flexible OTFTs with low-cost S/D electrode, which is desirable for large scale applications of organic electronics in the near future.
This work is supported by Research Grants Council of Hong Kong (Grant Nos. CUHK2/CRF/08, CUHK418209), and the University Grants Committee of the Hong Kong SAR (Grant No. AoE/P-03/08), and the National Science Foundation of China (Grant Nos. 60990314, 60928009 and 61106093).

Conventional OLEDs have non-polarized light emission property. Therefore, in mobile applications, external brightness is decreased at least 50 % by a polarizer film to enhance the contrast ratio under ambient light conditions. To overcome this disadvantage of conventional OLEDs, several methods proposed to achieve a polarized light emission in OLEDs such as emission layer formed on rubbed polyimide (PI) layer, direct shearing onto the emission layer, and emissive layer formation by the Langmuir-Blodgett (LB) method. However, several conventional methods still have problems for removing electrical defects at organic interfaces. In this work, we demonstrate polarized light emission in OLED from the ordered emissive layer by using a transfer printing method, where the ordered emissive conjugated polymer layer is aligned by unidirectionally chain-ordered self-assembled-monolayer (SAM) formed on the ion-beam irradiated anisotropic substrate. In this experiment, for a light-emitting material, we used a liquid crystalline light-emitting polymer of PFO-DMP. First, the SAM layer was treated on the ion-beam irradiated SiO2 surface (functional substrate) to induce the unidirectional property and to provide low surface energy condition (21 mJ/m2) for facile transfer process from the functional substrate to polydimethylsiloxane (PDMS) surface. After coating the light-emissive polymer film on functional substrate and then heating the film above a transition temperature (Tg), the ordered PFO-DMP film was transferred from the SAM-treated functional substrate to the PDMS surface. Finally, the PFO-DMP layer was transferred from the PDMS surface to PEDOT : PSS layer, used as the charge transport layer to make the OLED. To confirm the degree of the polarization of PFO-DMP film, we measure the polarization dependent absorption spectra before and after the transfer printing process, respectively. The order parameters about before and after the transfer printing process were estimated to be about 0.45 and 0.43, respectively. This result means the ordering behavior of the PFO-DMP layer was induced by macroscopic alkyl chain ordering effect of the SAM due to molecular interaction at the interface and the ordering of the PFO-DMP was preserved well during transfer printing process. Finally, we measure the polarized EL curve of the fabricated devices, which showed that the EL intensity along the alignment direction showed 4 times higher than that perpendicular to the alignment direction.

In recent years, organic light emitting diodes (OLEDs) have gained enormous attention in many research groups due to their technical advantages [1] but there are still technical demands to improve performance of blue OLEDs. Various methods have been developed to optimize blue OLED's performance and balancing ratio of charge carriers in EML and energy transfer between organic molecules still remain technical barriers to obtaining better OLED.
In this study, we fabricated blue OLEDs with quantum well structures (QWS) using three different blue emissive materials such as DPVBi, ADN and DPASN, and BAlq as QWS material. Conventional QWS blue OLEDs used to be composed of emissive layer and charge blocking layer with lower HOMO-LUMO energy level but we designed triple emitting layers for more significant hole-electron recombination in EML and a wider region of exciton generation as forming QWS spontaneously. The structure of triple emitting layered blue OLED is ITO / NPB(700Å) / X(100Å) / BAlq(100Å) /X (100Å) / Bphen(300Å) / Liq(20Å) / Al(1200Å) (X= DPVBi, ADN, DPASN). HOMO-LUMO energy levels of DPVBi, ADN, DPASN and BAlq were 2.8-5.9, 2.6-5.6, 2.3-5.2 and 2.9-5.9 eV, respectively. The maximum luminous efficiency was 5.32cd/A at 3.5V and the current density was 24.2mA/cm2 at 1000cd/m2 along with CIE coordinates of (0.14, 0.20) at 8V in a blue OLED with DPASN/BAlq/DPASN QWS.
QWS blue OLED achieved 36% higher luminous efficiency than 3.89 cd/A of conventional blue OLED. Obviously, the quantum well structure affects hole-electron recombination to generate more excitons in the emitting layer [2] of DPVBi, ADN and DPASN with BAlq in the device. There was no significant improvement of the luminous efficiency (only about 3 and 4%) when using DPVBi and ADN for the additional emitting layer to form a quantum well but we achieved 36% better efficiency in DPASN/BAlg/DPASN blue OLED. This result shows that blue OLEDs can only improve efficiency with proper difference of HOMO-LUMO energy level between the central and surrounding emitting layers. Electroluminescence spectra of these QWS blue OLEDs showed similar peak patterns to extend their original blue peak region (460-465 nm) into the BAlq region at 490 nm. This result suggests that the BAlg layer contributes additional blue emission to the major blue emission from DPVBi, ADN and DPASN.
The charge carrier transport mechanism from the quantum well and the blue emitting layer based on space charge limited current will be examined along with the influence of various layer thicknesses and different quantum well materials. White OLEDs using QWS in the triple emissive layer will be fabricated in the near future to obtain better luminous efficiency and color stability of white emission.
[1] A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and A. B. Holmes, Chem. Rev., 2009, 109, 897-1091
[2] Bernard Geffroy, Philippe le Roy and Christophe Prat, Polym Int, 2006, 55, 572-582

Molecular doping in organic electronics has long positioned itself as a primary means to convert π-conjugated polymers into highly conductive materials, most notably for use as hole injection layers. Partial charge transfer, instead of full charge transfer as found in such highly doped polymers, is now being exploited in a more subtle way to tune carrier injection, mobility, and charge recombination rate. Here, by using density functional theory (DFT) calculations, we have investigated doping characteristics over a very wide range of charge transfer, 0.05e-0.9e. To create a large variation of charge transfer, we paired up pyronin B, one of a few successful n-dopants, with a variety of electron transport molecules with varying electron affinities (EA). In doing so, we first verified that the radical form of pyronin B, not the leuco form, is indeed the dopant in action. We found that there exist two well-defined charge transfer regimes, in each of which the amount of charge transfer varies linearly with the offset between the donor ionization potential (IP) and the acceptor EA. The crossover to the high-transfer regime takes place when the donor IP approaches within 3.5 eV of the acceptor EA. Importantly, both the linearity and the distinct crossover between the two regimes emerge only when appropriate intramolecular geometric changes accompanying each level of charge transfer are fully taken into account.

The field of organic electronics has grown exponentially over the last decade, with extensive research in both material development and device optimization. In the fields of both organic photovoltaics and transistors, our group has demonstrated the ability to tune electronic properties by manipulating functionalization patterns to control solid state ordering, and thus, intermolecular electronic coupling. Our pentacene- and anthradithiophene-based materials both exhibit excellent field effect mobilities in transistor configurations, and good performance as non-fullerene acceptors in polymer-based bulk heterojunction photovoltaics.
We are now exploring new acene- and thienoacene-based materials in the context of developing new semiconductors for both transistor and photovoltaic applications. These materials are highly stable, soluble, and crystalline, indicating a high degree of long-range solid-state order, and allowing complete elucidation of the packing motifs. We use our library of previous crystalline compounds to design new materials to achieve the desired type of solid state ordering—π-face to face interactions for transistors, and more spacious herringbone motifs for photovoltaic materials. Departing from our traditional alkyne-based functionalization strategy, we are able to demonstrate control over HOMO and LUMO energy levels as well as solubility and solid-state order, by manipulation of the solubilizing substituents, allowing access to both donor and acceptor materials.

We investigated blue OLEDs with dual-emitting-layers with one layer fluorescent doped and the other phosphorescent doped on two different host materials with respect to their electrical and optical performance as a function of various fluorescent dopant concentrations. The fluorescent emitting layer (F-EML) is adjacent to the hole transport layer (HTL) and is created by 4,4-bis(2,2-diphenylyinyl)-1,10-biphenyl (DPVBi) doped with 4,4&’-bis(9-ethyl-3-carbazovinylene)-1,1&’biphenyl (BCzVBi) while the phosphorescent emitting layer (P-EML) is adjacent to the electron transport layer (ETL) and employs 1,3-Bis(N-carbazolyl)benzene (mCP) doped with iridium (III) bis(4,6-difluorophenylpyridinato) Picolate (FIrpic). The fluorescent and phosphorescent EML thicknesses are 15nm each. The dopant concentrations of BCzVBi are 5wt%, 8wt%, 12wt%, and 15wt%, respectively and that of FIrpic is fixed at 10wt%.
The current density and luminance of the blue OLEDs increase with increasing bias voltage and doping concentration. The hopping distances between dopant molecules are reduced and thus, the current density increases as the doping concentration increases. However, there is little increase between the 12wt% and 15wt% fluorescent doped devices. At 10 V the current density and luminance of the 15wt% doped device were 141.59 mA/cm2 and 6,582cd/m2, respectively and 134.61mA/cm2 and 6,401cd/m2 for the 12wt% doped device. The highest luminous efficiency was achieved from the 12wt% doped device with 5.44cd/A at 58.71mA/cm2. The luminous efficiency of the blue OLEDs slightly increased with increasing doping concentration, whereas it decreases at over 12wt% doping due to exciton quenching. In the electroluminescent spectra, the major and shoulder peaks are found at 448nm, 476nm and 496nm, which are predominantly generated from BCzVBi and FIrpic because the singlet excitons at the F-EML resulted in fluorescent radiation or were transferred to the triplet state of the P-EML where light emission is generated. With increasing dopant concentrations it is more favorable to transfer to the triplet energy state at the P-EML. This can be seen from the increased intensity of the shoulder peak at 496nm from FIrpic and the decrease of the 448nm peak from BCzVBi. All measured CIEy coordinates move toward sky blue as a function of doping concentration. The CIExy coordinates at 6V of the OLEDs in order of decreasing fluorescent dopant concentration are (0.143, 0.285), (0.143, 0.286), (0.142, 0.276), and (0.143, 0.255).
We introduced a blue OLED consisting of fluorescent and phosphorescent dual emitting layers to understand how these emitting layers affect the devices&’ performance when the fluorescent doping concentration is changed. As the fluorescent doping concentration increases, emission is more dominantly generated at the P-EML by transferring the excitons from the F-EML so that enhanced efficiency and red shifted emissions were found.

Organic light-emitting diodes (OLEDs) attract a high level of research enthusiasm due to their thin-film effects, high-contrast, light-weight, fast-response time, wide-viewing-angle and low-voltage attributes. [1] Especially, there has been much effort to improve the internal and external quantum efficiency of blue organic light-emitting devices (BOLEDs) by changing various emitting materials [2].

BOLEDs using BCzVBi for blue dopant has a device structure: NPB(600Å) / NPB:BCzVBi-7%(100Å) / ADN:BCzVBi-7%(300Å) / BAlq:BCzVBi-7%(100Å) / BAlq(200Å) / Liq(20Å) / Al(1200Å) to optimize probability of exciton generation. Maximum luminescence and luminous efficiency of BOLED were 14610 cd/m2 at 10V and 7.22 cd/A at 4.5V, respectively, with CIE coordinates of (0.14, 0.17).
More hole-electron recombination occurred in not only emitting layer (EML) but also in the hole transport (HTL) and electron transport layers (ETL). Doping BCzVBi in the EML, HTL/EML, and HTL/EML/ETL resulted in luminous efficiencies of 6.21 cd/A, 6.79 cd/A, 7.04 cd/A at 5V, respectively, without significant shift of CIE coordinates at (0.14, 0.17). The corresponding external quantum efficiencies (EQE) were 4.71%, 4.93% and 5.21%, respectively. These devices also showed similar current densities of 1.58 mA/cm2, 1.50 mA/cm2, 1.47 mA/cm2 at 5V, respectively. BCzVBi has two peaks in the electroluminescence (EL) spectra, at 456 nm and 476 nm. The intensity of the peak at 456 nm was changed depending on where BCzVBi was doped. The intensity was decreased when BCzVBi was doped in HTL but increased with ETL doping. In all cases, peak intensity at 476 nm stays at the same level. This result indicates that BCzVBi, a p-type organic semiconductor, has a better chance to generate excitons in the ETL than HTL of BOLED device.
Efforts to improve efficiency of blue OLED is a very important component of improving the electrical and optical performance of white OLED&’s. Organic semiconductors with wide energy band gap [3] as a host material should be developed to apply for doping fluorescent and phosphorescent materials [4] into the HTL, EML and ETL in near future. Furthermore, energy transition mechanism between host and dopant will be examined through electroluminescence spectra of blue OLED considering n-and p-type organic materials in near future.
[1] R.F Service, Science 273, 878 (1996).
[2] Tae Gu Kim, Trans. Electr. Electron. Mater. 11, 85 (2010).
[3] Kyung-Ryang Wee, J.Org.Chem 74, 21 (2009)
[4] Nail M. Shavaleev, Inorg. Chem. 51, 2263 (2012)

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.

We have designed and synthesized new host materials (3Czs) that has two carbazole moieties on the various positions of core carbazole, dibezofuran, and others. The higher T g value (>140 °C) of 3Czs compared to mCP expects significantly enhanced morphological and thermal stability of 3Czs. The devices with the optimized structure, ITO/HATCN(50 Å)/NPB (550 Å)/TAPC (100 Å)/3Czs:FIrpic 10 wt% (250 Å)/BmPyPB (300 Å)/LiF (5 Å)/Al (1000 Å), are characterized by high performance with a current efficiency of 30 cd/A, a power efficiency of 22 lm/W, an external quantum efficiency of 18%, and CIE coordinates of (0.13, 0.30) at 1000 cd/m2 . The performance is better than that of the mCP device which exhibits 12 % and 20 cd/A as an external quantum and current efficiencies in the same device structure. The new host materials shows one of the best results in FIrpic-doped blue PhOLEDs as well as high thermal stability and we continuously have optimized those device structures. Although, mainly, FIrpic-doped blue light-emitting devices are studied in here, such these large gap host materials may also be of use for other deeper and newly synthesized phosphorescent blue dopants. Further applications of new host materials, 3Czs in other dopants are underway and the results will be reported in due course.

Interfacial properties between organic film and electrodes play a crucially important role on the performance of organic devices. We studied electrical and morphological properties of pentacene thin films on surface-modified gold (Au) electrodes. After deposition of gold electrodes using thermal evaporation under high vacuum conditions, they were immersed in 10-mM thiophenol (TP) and pentafluorobenzenethiol (PFBT) solutions dissolved in toluene for 2 minutes. Then, pentacene thin (110 nm) films were thermally evaporated on top of the as-deposited Au and TP- or PFBT-modified Au electrodes. From the analysis of pentacene film morphology, we observed that the pentacene molecules deposited on as-deposited Au electrodes lie down but those on TP- or PFBT-modified Au stand up. This morphological change affects the electrical properties of pentacene devices which have the top electrode consisting of molybdenum trioxide (10 nm) and aluminium (100 nm). Assuming the trap inside the pentacene film is exponentially distributed, we can extract the trap energy (Et) by fitting current-voltage characteristics at various temperatures. The Et for a pentacene film on as-deposited Au is calculated to be 28.7 meV but the Et for that on TP- and PFBT-modified gold is calculated to be 19.7 and 19.2 meV, respectively.

Organic light-emitting diodes (OLEDs) are of current scientific and commercial interests for applications in full-color display panels, flexible displays, and solid-state lighting. Compared to conventional fluorescent OLEDs, considerable efforts have been made in developing phosphorescent OLEDs (PHOLEDs). Although green and red-emitting PHOLEDs have been intensely studied in the past, blue PHOLEDs are still challenging because of the lack of appropriate host and dopant materials. Depending on the properties of blue dopants, the host molecules must be designed and synthesized for satisfying efficient singlet and triplet energy transfer in the solid state.
In this presentation we will demonstrate the synthesis of new carbazole-based host materials exhibiting high triplet energy, good thermal stability, and good miscibility with blue iridium-based dopants in the solid state. By steering the kinds and positions of substituents in carbazole ring, the molecular energy levels of the host materials were finely tuned accompanying with triplet energy levels. The optical and photophysical properties of solution and film states were investigated precisely. Then, we employed the newly synthesized host materials for fabricating multilayered PHOLEDs for comparing their properties.

Recently, we demonstrated the effect of exciton lifetime on the nature of charge transport in organic heterostructures based on an exciton-polaron coupling mechanism.[1] Our experiments have shown that the total current through an archetypal NPD/Alq3 bilayer device can be modulated substantially through the addition of a capping layer. Considering this, it is possible to realize an organic light emitting device (OLED) based sensor that uses electrical current as the primary signal to monitor external dielectric conditions. Here we demonstrate a sensor using a top-emitting metal-dielectric-metal OLED that shows substantial variations in current upon touch. This demonstration could have interesting implications for OLED based touchscreen displays as a pixel-level sensor with a very high resolution and also have applications in other sensor modalities.
1] Kanika L. Agrawal, Matthew E. Sykes, K. H. An, Bradley Frieberg, P.F. Green and Max Shtein, Influence of exciton lifetime on charge carrier dynamics in an organic heterostructure, Phy. Rev. Lett. 2012 (Under Review)

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 heaver elements can reside at the relatively stable square planar center of the phthalocyanines. 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 NEXAFS, commonly attributable to π* and σ* 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 π* 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. Fe- and Co- resemble the K-edges; StoBe DFT shows that the lowest-energy allowed resonances are indeed molecular π* states, with a high degree of mixing with the dxz and dyz orbitals of the metals. In contrast, the intense, in-plane resonance of the Cu-PC L-edge LUMO resembles a molecular σ* state. Confirmed by StoBe, the dx2-y2 character at the Cu center is responsible for this intense in-plane resonance. NEXAFS thus directly probes the electronic structure, illuminating the uniqueness of Cu- compared to H2, Fe-, and Co- phthalocyanines.

Organic semiconductors hold a great promise of enabling new technology based on low cost and flexible electronic devices. Charge transport is limited in such devices by the nature of the metal-semiconductor interfaces where charge is injected into the semiconductor film. This work demonstrates the unique potential of harnessing externally excited plasmonic states at the metal-semiconductor for the investigation of physical phenomena at the nanoscale in organic field-effect transistors (OFETs). The plasmonic states manifest themselves through surface enhanced Raman scattering (SERS) observed from pentacene films under thermally evaporated Au films. Comparing experiments with density-functional theoretical calculations of the Raman spectrum of pentacene indicates disorder in the pentacene film induced by evaporation of Au onto the pentacene film.. The Gold atoms are shown to disrupt the aromaticity of the pentacene molecules leaving a disordered sp² Carbon structure. Changes in the Raman spectra are further tracked after biasing the devices. The SERS effect is shown to further detect disorder induced in the film by bias-stress. Raman maps across the pentacene-Au interface provide a powerful visualization tool for correlating the device performance to structural changes of the molecule. The effect of the plasmonic states is further investigated via photocurrent measurements. Attempts are made to model the contribution of plasmonic states to the photocurrent response. In summary, the plasmonic states are used to develop a non-invasive in-situ tool that can create a combined electro-optical characterization system offering insight into the effects of both fabrication and operating conditions on device performance and lifetime of OFETs.

The charge-generating mechanism in bulk heterojunction, organic photovoltaic devices is dominated by exciton dissociation at the interface formed between the donor and acceptor species. The mechanism assumes that the donor is the dominant light absorber in the system, with diffusion of the exciton to the interface limited by its lifetime and the diffusion length. But is this the only route to generating charges in these systems?
This presentation will examine an alternative energy transfer mechanism in a bilayer of poly(p-phenylenevinylene) and C60, using flash photolysis, time-resolved microwave conductivity (fp-TRMC) to probe the efficiency of charge generation and recombination. Using additional interfacial layers at the interface between the polymer and fullerene, the role of long-range energy (exciton) transfer will be examined in detail.

My lecture will focus on stability and thermal processing of the photoactive layer of bulk heterojunction organic solar cells. Comparative studies will be presented of different materials systems, with the aim of materials design guidelines for enhanced photoactive layer stability. My lecture will include consideration of the role of triplet states in causing singlet oxygen mediated photodegradation, including the parameters determining triplet generation, and correlations with singlet oxygen yields and materials photodegradation rates. I will go on to consider the role of polymer polarons in inducing material degradation. I will then go on to consider the morphological evolution of photoactive layers under thermal annealing conditions, and strategies to address this both to enhance film stability and as a processing aid to enhance device efficiencies. Finally, I will present some recent results on different topic - the potential of piezoelectric effects to enhance the efficiency of hybrid metal oxide / polymer solar cells.

Organic photovoltaics (OPV) is a promising candidate technology for the low-cost fabrication of modules to harvest solar energy. Although OPV technology has significantly matured over the past few years, there remain many challenges in OPV manufacturing, from materials selection, to device design, to design and control of the fabrication process. Structure-property-performance relationships for polymer-fullerene blend OPV devices are still underdeveloped, and relationships based on one system are not necessarily transferrable to new, higher-performance systems. This talk will describe our efforts to develop measurements that support OPV manufacturing. An important issue in materials selection is the use of OPV systems that are more manufacturable because they perform well even when the polymer-fullerene blend is coated at thicknesses greater than 100 nm. Some systems that work well in thicker films appear to have slowed bimolecular recombination, but the structural origin of this feature is unclear. I will discuss our efforts to identify it using a suite of comprehensive structural characterization techniques. Solution formulation and the design of the coating process could also benefit from more structural characterization - during the film solidification process itself. Using a blade coating process as a prototype for slot-die coating, we have developed several techniques to observe the structure of OPV films in-situ as they dry. We use these techniques to identify the mechanisms by which different additives to polymer-fullerene blends influence the structure of the final films. Using in-situ techniques provides far more information about the solidification process than can be obtained by measuring already-dried films, providing a valuable tool to guide the selection of formulation and processing parameters.

Recent years have seen a rapid increase in organic solar cell performance. Much of this progress is due to a better understanding of fundamental processes and organic molecules specifically tailored for organic solar cells. One particularly promising class of donor molecules are dicyanovinyl-capped oligothiophenes (DCV2-nT) which we use in vacuum processed organic solar cells based on the p-i-n concept, i.e. having an intrinsic photovoltaic active layer sandwiched between p- and n-doped wide gap transport layers[1].
These DCV2-nTs are of special interest, because they offer a large freedom in well-controlled structural variations. These variations in turn affect both energy levels and microstructure in pristine films and films mixed with C60 as electron acceptor. Thus, systematic variations in length of the thiophene backbone (3-6) and the position, number, and length of alkyl side chains (0-4) allow for experimentally accessing fundamental processes in organic photovoltaics (OPV).
To investigate structure-property relationships for this material class we use a combination of X-ray methods, transport measurements, quasi-steady state photoinduced absorption (PIA), and standard solar cell characterisation methods as well as variations in heating the substrate during film deposition. What we find is that increasing the DCV2-nT backbone length n from 3 to 6 leads to a transition from energy-transfer to charge-transfer dominated processes at the interface between DCV2-nT and C60. The results from PIA furthermore indicate that for ngE;4 the generation rate of cations (polarons) and generation efficiency of triplet excitons in these materials is strongly increased with increasing temperature whereas the polaron lifetime decreases by more than one order of magnitude from 10K to room temperature. Similarly, drastic changes in the solid-state microstructure and the resulting optoelectronic properties of pristine DCV2-nT and DCV2-nT:C60 films are observed for a given backbone length with even smallest modification in alkyl side chains. This is particularly evident when changing number and position of Me-side chains, i.e. the shortest possible alkyl-length, in DCV2-5T. These subtle modifications result in a completely different microstructure and interactions with C60 that change the efficiency of exciton splitting and charge transport as well as absorption of mixed films.
Combining the results of the intrinsic (i.e. structural changes) and extrinsic (i.e. processing conditions) tuning of the microstructure of the photovoltaic active layer, we are able to obtain certified power conversion efficiencies exceeding 7% for single heterojunction devices based on DCV5T-Me(3,3)-C60.
[1] K. Walzer et al., Chem. Rev. 107, 1233 (2007)

The use of small volumes of high boiling liquids as ‘solvent additives&’ is a pre-electrode deposition processing method that has been implemented in most high performing polymer/fullerene based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in solution processable small molecule (SPSM) BHJ systems with similar drastic effects on several key device metrics and ultimately the power conversion efficiency (PCE). The significant differences in solvent additive processing between polymer and SPSM based BHJs are that the amount of additive used in SPSMs is nearly an order of magnitude less and SPSMS have a much smaller additive processing window. A recent SPSM delineates the sensitive processing window where when 0.4 v/v% of additive is used the PCE increases from 1.8 to 7% while a rapid deterioration occurs above the optimized solvent additive amount to less than 1%. The effects of solvent additives on the microstructure and composition fluctuations were examined via several techniques such as GIWAXS, DSIMS, and EF-TEM to develop robust structure-processing relationships to correlate to device performance.

The performance of bulk heterojunction (BHJ) organic solar cells depends strongly upon their ability to separate electrons and holes from one another. A BHJ OPV must be able not only to split a bound exciton into an electron and hole, it must also be able to transport carriers to the correct electrode with low rates of recombination. The energy levels of the donor (D) and acceptor (A) materials in a BHJ solar cell play an important role in the separation of the electron and hole, with efficient D-A pairs providing an energetic incentive for the exciton to dissociate, resulting in the electron and hole residing on different molecules. Recent work has suggested that the energy levels of electrons and holes are lower in the pure D and A phases than they are in the mixed D-A phase. Such a difference in carrier energy levels between the pure and mixed phases of the BHJ OPV would drive carriers out of the mixed phase and into the pure phases for transport to the electrodes. This three phase morphology, consisting of the mixed phase and two pure phases, would create efficient carrier transport channels that reduce carrier recombination by separating electrons and holes from one another. In order for these phases to effectively transport the carriers, there must be enough of each phase to form a percolating pathway. In this work, we present results which highlight the impact of the three phase morphology on the energy levels of the D and A phases, and on the performance of the BHJ solar cell. We measure the energy levels of polymers and fullerenes in the pure and mixed phases using ultraviolet photoelectron spectroscopy (UPS), photoelectron spectroscopy in air (PESA) and cyclic voltammetry (CV) to describe how the carrier energy levels vary across the three phases. We also measure the volume fraction of pure phases in polymer:fullerene blends, using UV-VIS absorption to measure the fraction of aggregated polymer, and using x-ray diffraction to measure the onset of fullerene aggregation. With these measurements we demonstrate the importance of have all three phases present at a sufficient volume fraction in order for carriers to be separated and collected efficiently in BHJ solar cells.

For bulk heterojunction (BHJ) photovoltaic devices to realize commercial applications, effective strategies to maximize the performance have to be developed and fundamentally understood. In BHJ-type solar cells, the ability to control and optimize the active layer morphology is a critical issue to improve device efficiency, and this is usually achieved by optimizing the processing conditions, eg. using varied annealing procedures and choosing the right solvent additive. A family of structurally similar low band gap polymers were synthesized where their absorption can be well tuned by the choice of monomer units. The morphology of these materials, and their blends with phenyl-C71-butyric acid methyl ester (PCBM) were characterized using x-ray scattering and electron microscopy methods. We observed that by using solvent additive, the morphology could be tuned to produce an interconnected polymer fibril network throughout the film. Details of this morphology, e.g. the inter-fibrillar distance and fibril dimensions are strongly affected by slight chemical structure modifications of the polymer. The best performance comes from the morphology with smaller fibril diameter and inter-fibrillar distance, which can only be obtained by combined processing and structural optimization. Using the conventional binary mixture approach for OPVs, these polymers (deep wavelength absorption) were mixed with short wavelength absorption conjugated polymer and further mixed with PCBM to form ternary blends to enhance the absorption. An enhancement in the device performance was observed and the multi-component thin film morphology was also characterized in detail.

Probably the fastest emerging direction in the field of organic electronics deals with the interface with life sciences. Under this umbrella of “Organic bioelectronics”, applications for organics are envisioned in neural implants, medical diagnostics, tissue engineering and drug delivery. These new applications bring into perspective some unique features of organic materials that help them gain an edge over competing material technologies. The first one is their ability to conduct ionic carriers in addition to electronic ones. This mixed conductivity allows the design of new devices that transduce signals across the biology/electronic materials interface. The second is their biocompatibility. Organics have been used as scaffolds for cell culture and also as means to improve the lifetime of implanted devices. In this talk, the main applications of organics in bioelectronics will be reviewed and the current status of understanding the underlying structure vs. properties relationships will be discussed.

Light-emitting electrochemical cells are promising candidates for solid-state lighting because of their potential cost-efficiency combined with good performance. The presence of mobile ions in the luminescent layer significantly boosts the performance of these single-layer solution-processed devices to competitive values. Two different types of LECs can be distinguished: polymer LECs in which a salt is admixed to a conjugated polymer and a small molecule alternative that consists of an intrinsically ionic semiconductor, typically an ionic Transition Metal Complex (iTMC). The compositional differences and the complexity of these devices has lead to prominent ambiguity regarding both the operational mechanism(s) and the similarity between both types of LECs. Here, we resolve this ambiguity.
We studied the operational mechanism of an Ir(III)-iTMC by fluorescence microscopy and scanning Kelvin probe microscopy (SKPM) on a planar electrode geometry. Electrochemical doping was identified by fluorescence quenching in regions extending from the contacts after application of sufficient bias voltage. In between electroluminescence was observed. SKPM further proved the presence of a p-i-n junction by the appearance of a relatively large potential drop far away from the contacts. Recent experimental and numerical work has shown identical behavior in planar polymer LECs.
In addition to the behavior of planar cells, also the transient current, light output and efficacy of ‘standard&’ stacked devices were studied. Transients of polymer and iTMC-based LECs were studied as a function of temperature. Despite large differences in turn-on time, ranging from seconds to hours, the transients of both types follow a universal shape. By normalizing to the turn-on time also the temperature dependence can be scaled out. Moreover, we conclude that the turn-on time of both LEC types is dominated by the ion conductivity since the turn-on time exhibits the same activation energy as the ion conductivity.
Combining the above, we show that iTMC and polymer LECs follow the same operational model despite marked differences in active layer constituents, turn-on time and performance. This operation is dominated by a relatively slow electrochemical doping process leading to the formation of a dynamic light emitting p-i-n junction.

Large amplification factor and fast response are essential for transistors in applications ranging from switching elements to measurement of fast biological events such as neuronal action potentials. To this end, organic electrochemical transistors (OECTs) based on conducting polymers such as PEDOT:PSS {poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)} have been recently targeted for applications in environmental and biological sensing due to their efficient ionic to electronic signal transduction. OECT operation relies on the modulation of drain current by the de-doping of PEDOT upon application of a gate bias (which causes a drift of cations into the bulk of the channel). Efficient operation therefore relies on fast electronic pathways for holes, and hydrated phases for mobile ion drift. However, relatively little is known of the solid state and hydrated structure of PEDOT:PSS for electronic and ionic mobility. Through active material structure-property investigations as well as micron-scale fabrication and optimization we present a solution processed organic electrochemical transistor with a transconductance exceeding 4 mS. This device outperforms both traditional high performance semiconductor devices based on all-inorganic CMOS devices and low dimensionality graphene and nanowire devices. OECTs are found to have a broad and invariant frequency response from steady state to 5 kHz, which can be described and further optimized using an analytical model that highlights the operational advantages of this class of devices. The effect of channel geometry, materials morphology/microstructure, and device layout are addressed in order to test the efficacy of the presented model, and to improve device characteristics and understanding.

Light-emitting electrochemical cells (LECs) offer a number of important advantages over competing emissive technologies -- notably the utilization of air-stabile electrodes and thick and uneven active materials. This unique opportunity opens the door for a recently demonstrated ambient fabrication of low-cost and flexible light-emitting sheets using R2R coating techniques [1], and I will present our recent results in this field.
A critical drawback with LECs has been a short operational lifetime. We have for a long time worked to resolve this problem and also been able to identify a number of lifetime-limiting chemical [2] and electrochemical [3] side reactions. By following motivated and straightforward design principles to minimize the extent of these side reactions, we are now able to repeatedly realize LEC devices that emit with significant brightness (>100 cd/m2) and good efficiency (>2 lm/W for red emission, >10 lm/W for green emission) for several months of uninterrupted operation.[4,5]
In another development, we have performed a parallel optical probing and scanning Kelvin probe microscopy study on planar LEC devices during operation, and the acquired light emission and potential profiles present irrefutable evidence for that electrochemical doping takes place in-situ in the active material, and that a dynamic p-n junction structure can self-assemble in an LEC during operation.[6]
Finally, we have conceptualized and demonstrated a truly metal-free and “all-plastic” LEC device comprising a graphene cathode and a conducting-polymer anode.[7] Both electrodes in this device architecture are transparent and the light emission is accordingly omni-directional. Moreover, all parts of the device can be processed from solution, which -- in combination with the elimination of expensive and/or reactive metal materials -- promises to pave the way for a low-cost production of functional light-emitting devices.
References
[1] Sandström, A., Nature Communications, 2012, 3, 1002.
[2] Waring;gberg, T., et al., Advanced Materials, 2008, 20, 1744.
[3] Fang, J., et al., Journal of the American Chemical Society, 2008, 130, 4562.
[4] Fang, J., et al. Advanced Functional Materials, 2009, 19, 2671.
[5] Asadpoordarvish, et al. Appl. Phys. Lett. 2012, 100, 193508.
[6] Matyba, P., et al., Nature Materials, 2009, 8, 672.
[7] Matyba, et al., ACS Nano, 2010, 4, 637.

Phosphorescence in organic light-emitting diodes is a practical and general pathway to high external quantum efficiencies. To date, however, efficient phosphorescence from organic molecules can only be obtained if heavy metals (mostly iridium, but also platinum, palladium, or osmium) are incorporated into the molecular core of the emitting species to enhance spin orbit coupling and enable emission from the excited triplet state. For purely organic molecules, phosphorescence is very weak and its observation remains an exception.
The cost of Ir- and Pt-based materials may hamper the adoption of OLED technology in traditionally low margin applications like solid-state lighting. Here, we describe a novel, purely organic small molecule N,N'-bis(4-benzoyl-phenyl)-N,N'-diphenyl-benzidine [(BzP)PB] that exhibits efficient phosphorescence at room temperature. In our study, we use poly(methyl methacrylate) (PMMA) as a rigid, solid glass matrix for (BzP)PB. The singlet-triplet splitting of (BzP)PB with 0.2 eV is comparably small, which we hold responsible for effective intersystem crossing. Both at low (77 K) and room temperature, (BzP)PB exhibits high efficiency emission from both singlet and triplet state - an effect we term biluminescence - with their characteristic time constants in the nanosecond and millisecond time range, respectively. The ratio between fluorescence and phosphorescence is approximately 3:1, with interestingly no decrease in phosphorescence yield going from low to room temperature. At 293 K, the photoluminescence quantum yield (PLQY) of the integrated emission is as high as 74%, with a lower limit for the phosphorescence quantum yield of about 41%. We further demonstrate that the emissive triplet state of (BzP)PB can effectively participate as a donor in Förster resonant energy transfer to a fluorescent acceptor, here 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB). Here we demonstrate that through introduction of the fluorescent acceptor, the donor triplet excited state lifetime can effectively shortened by one order of magnitude. Considering the PLQY of the emitter molecule, OLEDs based on this molecule could reach external quantum efficiencies higher than 10%, clearly exceeding the 5% limit of fluorescent OLEDs. In addition, biluminescent molecules are highly attractive for white OLEDs, having an ultra broadband spectrum. We will discuss OLED performance data, which is currently work in progress.
To our knowledge, this is the first demonstration of highly efficient room temperature phosphorescence in a purely organic, solid state, disordered material. Furthermore, biluminescence opens a new path for organic optoelectronics. Future applications of biluminescent materials are emitters for OLEDs, ultra-broadband emitters, exciton probes, and bio-, temperature and chemical sensors.

The miscibility of donor/acceptor mixtures can strongly affect the performance of organic photovoltaic devices comprised of such mixtures. For instance, the miscibility can strongly affect the morphological evolution thereby dictating the relationship between processing conditions and performance of devices. Furthermore, recent energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. The role of mixing within nanophases on charge transport of organic semiconductor mixtures, however, is not fully understood. Through the combination of Flory-Huggins theory and energy-filtered transmission electron microscopy, we have estimated the miscibility limit of polythiophene/fullerene blends. We have also demonstrated the interplay between miscibility and percolation to describe field-effect mobilities derived from thin-film transistors as a measure of the conductive pathways present in a model organic semiconductor mixture (amorphous polythiophene/fullerene blends). This approach examines electron transport in the fullerene-rich phase of the active layer of polythiophene/fullerene solar cells. Measured electron mobilities follow 3D percolation behavior for small fullerene content and increases significantly above high fullerene volume fractions where the components are immiscible. Thus, our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. However, strongly immiscible systems would readily phase separate into large domains, preventing efficient charge separation in organic photovoltaics. As a consequence, an optimum degree of miscibility between donor/acceptor mixtures exists for the application of such mixtures to organic solar cells.

Organic solar cells have shown rapid performance increase in recent years. For the development of new high-performance materials, reliable quantification of the loss mechanisms is indispensable. One of the main limiting factors in the power-conversion process in organic photovoltaics is the recombination of photogenerated charges. In an organic bulk-heterojunction solar cell, absorbed photons are converted into free holes and electrons that are transported through the donor and acceptor phases, respectively. However, free charge carriers may recombine bimolecularly instead of being extracted at the electrodes.
We present a steady-state method to probe bimolecular recombination in organic solar cells. The technique is applicable to thin-film solar cells at any temperature and does not require a separate measurement setup other than conventional solar-cell testing equipment. The key element in our method is the derivation of a simple analytical expression that directly gives access to the recombination strength by employing the concept of charge neutralization, a process that is competing with charge recombination. The analytical equation requires only standard transport parameters as input. We demonstrate the suitability of our technique by applying it to solar cells of poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(2&’-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as well as other organic photovoltaic material systems. From these measurements, insight in the origin of reduced bimolecular recombination can be obtained. We expect that the simplicity of our technique renders it eminently suitable for screening charge recombination in new materials for organic solar cells.

The nature of the interface structure between donor and acceptor are known to be critical for fullerene-based bulk heterojunction (BHJ) solar cells, yet have not been widely studied due to limitations of common characterization techniques. We show that processing solvents are an effective way to control the interface structure (sharp, fractal, or diffuse) of the active layer and thus impact device performance. Six different solvents or solvent mixtures, CF, DCB, CF/DIO, DCB/DIO, CF/DCB and DCB/CF/DIO, are used as processing solvents in diketopyrrolopyrrole (DPP) based polymer PDPP3T with PC71BM blends to investigate the impact of solvents on interface properties. Interface roughness is revealed by analysing the scaling of high-q data of resonant soft x-ray scattering profiles. We find that with the presence of DIO, rough interfaces are always observed. While rough interfaces provide shorter average distances for excitons to reach donor/acceptor interfaces, they also enhance recombination and are thus not ideal.[1,2] When CF is used as one component, a sharp or slightly diffuse interface is induced. However, over-pure domains (especially mixed CF with DIO) are also created that seem to negatively impact performance. Overall, the mixture of DCB/CF/DIO yields the highest PCE of 6.7%. Our work provides a comprehensive picture of how to control donor/acceptor interface structure with solvents where this structure is patterned by the aggregation in the solution. It also reveals that use of DIO leads to the highest performing devices by only positively impacting domain size and purity. If a solvent mixture could be found that would simultaneously also lead to sharp interfaces, further improvements could be realized.
[1] H. Yan, S. Swaraj, C. Wang, I. Hwang, N. C. Greenham, C. Groves, H. Ade, C. R. McNeill, Adv. Funct. Mater. 2010, 20, 4329-4337.
[2] B. P. Lyons, N. Clarke, C. Groves, Energy Environ. Sci. 2012, 5, 7657.

Blends of poly(3-hexyl thiophene), P3HT, and [6,6]-phenyl-C₆₁-butyric acid methyl ester, PCBM, often comprise the active layers of bulk-heterojunction polymer solar cells. Due to the chemical incompatibility between P3HT and PCBM, they tend to phase separate. Given that the active layers are typically ca. 100 nm thick, the energy difference between the surface and the organic semiconductor-substrate interface can also induce composition variations in the vertical direction of the active-layer thin films. We have quantified the interfacial segregation characteristics of P3HT:PCBM active layers via near-edge X-ray absorption fine structure spectroscopy.^[1] We found the active layer-cathode interface to comprise 97 wt% P3HT whereas the buried active layer-anode interface to comprise 65 wt% P3HT relative to the bulk composition of 55 wt% P3HT. By delaminating, flipping, and transferring the flipped active layers, we have been able to build devices having reversed interfacial segregation characteristics without the need to modify the electrodes or alter the device architectures. We found that the device characteristics of P3HT:PCBM solar cells are not sensitive to the preferential segregation of P3HT at the active layer-cathode interface, despite the fact that P3HT is the polymer donor that primarily transports holes.
In order to understand why preferential segregation of P3HT at the active layer-cathode interface does not affect device characteristics, we systematically inserted thin, continuous films of P3HT having varying thicknesses at the active layer-cathode interface.^[2] These insertions resulted in slight reductions in short-circuit current densities (J_sc) with increasing P3HT thicknesses. The systematic insertion of thin, continuous films of PCBM at the active layer-anode interface, however, resulted in substantial disruption in J_sc. We attribute this asymmetric reduction in J_sc to the presence of a broader distribution of tail states in P3HT compared with in PCBM. The broader distribution of tail states in P3HT can assist in the recombination of holes in these tail states with electrons at the active layer-cathode interface. This process is in turn compensated by hole injection from the cathode into these tail states, effectively promoting current flow through the polymer solar cell.
[1] H. Wang, etc, Chemistry of Materials, 23, 2020, 2011.
[2] H. Wang, etc, Advanced Energy Materials, doi: 10.1002/aenm.201200361, 2012.

Metal oxide contact layers have emerged as attractive alternatives to PEDOT:PSS in order to increase long-term stability in high-performance, bulk heterojunction (BHJ) organic solar cells. In particular, low-temperature, solution-processed molybdenum oxide (MoOx) has recently been shown to form efficient BHJ solar cells with significantly-improved electrical stability. However, active layer film morphology and device performance are sensitive to the thermal treatment of the metal oxide film. Using a combination of novel transient photocurrent measurements and morphological characteristics, we have determined the impact of MoOx interlayers on the power conversion efficiency of P3HT:PCBM solar cells as a function of processing conditions. We probed the photocurrent dynamics as a function of excitation light intensity and applied bias to investigate the effects of trapping and de-trapping of charges within the devices. Importantly, we can correlate the transient behavior with the vertical phase composition of the polymer:fullerene active layer, which we resolved using dynamic secondary-ion mass spectroscopy (DSIMS). Our experimental results have been reinforced by drift-diffusion simulations incorporating energy band tails and recombination. When constrained by the experimentally-determined vertical composition profiles, the simulations reproduced the measured current-voltage responses of the cells. This allows us to identify the extent to which charge distribution, carrier mobility, and contact barriers contribute to the overall shape of the photocurrent.

Hybrid composites of semiconducting polymers with metal oxides and solution processed inorganic semiconductors show huge potential for solar cell device optimization owing to the vast library of organic and inorganic materials available for each device component. One example of this versatility is our newly developed architecture consisting of mesoporous TiO2 co-functionalized with an organic surface adsorbed fullerene (C60-SAM) and dye absorber, along with a polymer hole-conductor (Grancini et al., Adv. Funct. Mater. 2012, 22, 2160). We have also recently demonstrated high performance meso-superstructured solar cells utilizing a novel organic-inorganic mixed halide perovskite structure as the light harvesting antenna and charge generation centre on a TiO2 and insulating Al2O3 scaffold (Lee et al., Science 2012, 338, 643).
In the work presented here, we have combined these novel strategies to fabricate polymer-based perovskite devices which show power conversion efficiencies approaching 7% when incorporating a fullerene SAM (~5% without the C60-SAM), with the open-circuit voltage consistently ~0.2V higher than devices without C60-SAM functionalization. Remarkably, this voltage increase is retained whether semiconducting TiO2 or insulating Al2O3 is used as a mesoporous scaffold for perovskite formation. Quasi-steady state photoinduced absorption (PIA) measurements show that, using either oxide, a long-lived characteristic C60 anion peak is seen at ~1050nm corresponding to electron injection from the absorbing perovskite material to the C60-SAM. Transient photocurrent decay measurements also show that electron transport is an order of magnitude faster with C60-SAM functionalization. These results suggest that the molecular electron acceptor acts as a charge reservoir, preventing electrons from being trapped in the TiO2 states.
We extend the work to present a comparative study with two different C60-SAM structures (mono and bis), where we can tune the energy alignment at the oxide-perovskite interface by increasing the LUMO of the C60 acceptor. This allows the open-circuit voltages to be further increased and demonstrates the potential of the C60-SAM system to improve device performance. The use of this novel perovskite absorber, combined with our recently acquired knowledge about C60-SAM fullerenes, establishes an exciting and highly tuneable route forward for "dual absorbing" polymer-perovskite hybrid photovoltaics.

Photochromic systems are capable of undergoing efficient and reversible photochemical reactions, i.e. to switch between two or more (meta)stable isomers featuring markedly different properties. Such bi- or multi-stable building blocks can therefore be employed to translate an incoming stimulus such as light into a macroscopic property change of the material.
In particular, azobenzenes (AZO) are known to undergo reversible photoinduced isomerization between trans and cis form which can exhibit different optical and electrical properties.
We demonstrate that (i) a photochromic bi-stable AZObenzene-Self-Assembled Monolayer (AZO-SAM) chemisorbed on flat Au source and drain electrodes can mediate the injection through the variation of the tunneling barrier across the SAM making it possible to modulate reversibly the charge injection at the Au electrodes/semiconductor interface in an organic transistor.
(ii) by blending poly(3-hexylthiophene) with gold nanoparticles (AuNP) coated with an AZO-SAM, charge traps are generated within the semiconductor layer. The light-induced isomerization between the trans and cis state of the azobenzene molecules coating the AuNP induces a variation of the tunneling barrier, which controls the efficiency of the charge trapping/detrapping process within the active layer.
Diarylethenes (DAEs) are another class of photochromic systems which are very popular scaffolds in molecular electronics because their open and closed isomers feature different HOMO and LUMO levels depending on the specific irradiation wavelength (UV or white light). Hence other properties such as absorption, emission as well as redox characteristics can be phototuned as well. In our study, the blend between DAE derivatives featuring different energy levels 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).Taking into account the hole transport levels of P3HT, we designed and synthesized a DAE molecule, DAE_1, featuring different ionization energy (IE) in its open and closed form with respect to the IE of P3HT. To explore the role of the energy levels in the two different DAE isomers with respect to the IE of P3HT we have extended our study to DAE_2 with higher IE of both isomers.
In summary, in our work [1-3] the source-drain current through the channel can be therefore gated both electrically (through gate control), like in a conventional OFET, and optically through photochemical control exerted either at the charge injection or transport level. Such a proof of concept is instrumental to the field of organic electronics which searches for solutions to integrate novel and additional functionalities in a single device.

[1] E. Orgiu et al., Nature Chem., 2012, 4, 675-679.
[2] Raimondo C. et al., PNAS, 2012, 109, 12375-12380.
[3] Crivillers N. et al., Adv. Mater. 2011, 23, 1447-1452.

Small molecule organic semiconductors have many advantages over their polymer counterparts for the fabrication of organoelectronic devices, including high purity and well-defined electronic properties. In order to implement low-cost solution processing of these materials, dewetting must be eliminated to ensure film uniformity. Additionally, it is necessary to have control over their spatial organization on the nanoscale to tailor the electronic percolation pathway. We have developed a supramolecular method to address these issues (1,2). A quaterthiophene semiconductor (4T) containing alkyl and phenolic moieties was hydrogen-bonded to the 4-vinylpyridine groups of a block copolymer, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) or a homopolymer, P4VP. These supramolecules can be readily cast into uniform films where the 4Ts form oriented nanostructures without hindering the charge mobility of the semiconductor. The assembly and properties of these supramolecules depend on several parameters, including the crystallization of the semiconductor to ensure high charge mobility and the ordering of the polymer to direct the nanoscopic assembly. To investigate this effect, a family of organic semiconductors with different melting temperatures was investigated to understand how the interplay between these two competing processes affects the overall morphology and electronic properties of the system. In general, asymmetrically-functionalized small molecules are necessary for a supramolecular approach, which can be synthetically demanding. With this in mind, we extended this approach to blends of symmetrically and asymmetrically-functionalized organic semiconductors at various mixing ratios. Our results suggested that favorable interactions between the symmetric and asymmetric molecules were sufficient to coassemble them into the nanoscopic domains of the supramolecules. Fundamentally, present studies shine light on how to synergize two self-assembly processes, i.e. molecular ordering of organic semiconductors and microphase separation of the block copolymer, and provide useful guidance toward directed hierarchical assemblies in multi-component systems. Since the polymers and small molecules can be readily exchanged for others, these studies may open a new route for the fabrication of functional, nanostructured thin films of organic semiconductors using solution processing.
References:
(1) Rancatore et al., ACS Nano 2010, 4, 2721.
(2) Rancatore et al., Macromolecules 2012, 45, 8292.

Well-defined organic nanowires may provide important insights into the fundamental processes of charge generation and transport in organic semiconductors under nanoscopic confinement. We demonstrate that self-assembled nanowires comprising exactly a single stack of either electron-rich or electron poor π-conjugated segments at their core are obtained by using oligopeptide side groups as ‘supramolecular synthons&’ to promote helical, one-dimensional aggregation. Solution-spinning of the nanofibrils resulted in the formation of microfibers. These exhibited hierarchical structure formation with a remarkably high degree of internal order, ranging from the supramolecular level (hydrogen-bonding, π-π stacking) over nanostructure formation (arrays of aligned and densely packed nanofibrils) up to the microscopic length scale (controlled and tunable microfiber diameter and uniform morphology). Moreover, the nanofibrils underwent photo-induced charge carrier generation in the absence of oxygen or a dopant, both in solution or in films and microfibers. The processes appears to proceed by exciton generation within the 'ordered phase' (nanofibrils), followed by electron transfer to 'disordered domains' (non-aggregated molecules or defects) that exhibit molecular orbital levels different from the nanofibrils and can thus serve as electron acceptors. The process resulted in the formation of polaron charge carriers delocalized on the nanofibrils that exhibited variable range hopping behavior. Furthermore, probing the nanofibrils in two-point devices proved that they were macroscopic semiconductors, exhibited photo-conductivity, and followed space-charge-limited conductivity characteristics. Our results, thus, provide an example of a universal organic nanowire model system that successfully links molecular design, supramolecular self-assembly into well-defined nanofibrils, photo-induced charge carrier generation, investigations of local charge carrier mobility, and finally macroscopic charge transport. These findings pave the way for incorporating such organic nanowires as defined charge transport paths into novel materials for future ‘nanoelectronic&’ or photovoltaic devices.

Vertically oriented arrays of semiconductor nanowires are of great technological importance. While the synthetic approaches towards such structures have matured for inorganic materials and led to breakthroughs in numerous applications, the growth for the organic counterparts remains challenging. Here, we demonstrate a simple, solution-based approach for growing single-crystalline, vertically oriented arrays of a variety of organic semiconductors using CVD graphene as the substrate. Using tetraaniline as an example, we demonstrate the exquisite control over the densities, sizes and orientations of the crystal arrays. X-ray diffraction (XRD) and selected area electron diffraction (SAED) indicate the crystals grow along the interfacial stacking direction vertically from the graphene substrates. The anisotropic electrical properties of the organic crystals were investigated via conductive atomic force microscope (cAFM) measurements. Furthermore, the crystallization process shows high selectivity towards graphene over other substrates, which allows control over their precise deposition locations by patterning the graphene layer, rendering this approach suitable for large area device fabrication. Integration of the tetraaniline crystals into pre-patterned graphene field-effect transistors (FET) leads to a positive shift of its Dirac point, indicative of p-doping. With the graphene layer conveniently serving as the bottom electrode, such heterostructures have great implications in electronics, photonics, and biological sensors.