Molecular electrical doping of organic semiconductors has emerged as key strategy to improve the performance of organic (opto-)electronic devices and to extend their range of functionality. However, despite impressive success in practical applications, the fundamental processes underlying molecular electrical doping are still not fully understood to date. Notably, this is reflected in the observation that the two important classes of organic semiconductors, small conjugated molecules and conjugated polymers, exhibit a vastly different phenomenology upon doping with no satisfactory explanation put forward to date. The present contribution provides an overview on pertinent literature and recent own work focusing on the role of ground-state charge transfer complex formation upon doping prototypical organic semiconductors. In particular, in the prospect of providing a unifying picture of molecular electrical doping, profound differences between the fundamental mechanisms at work in small molecular and polymeric compounds of identical chemical composition and similar microstructure are discussed.

We present multi-photon OLEDs where enhanced light emission was achieved by stacking two OLEDs utilizing a regular device architecture (top cathode) and an intermediate charge carrier generation layer (CGL) for monolithic device interconnection. With respect to future printing processes for organic optoelectronic devices, all functional layers were deposited from solution. The CGL comprises a low-work function zinc oxide layer that was applied from solution under ambient conditions and at moderate processing temperatures and a high-work function interlayer that was realized from various solution processable precursor-based metal oxides, like molybdenum-, vanadium- and tungsten-oxide. Since every injected electron-hole pair generates two photons, the luminance and the current efficiency of the tandem OLED at a given device current are doubled while the power efficiency remains constant. At a given luminance, the lower operating current in the tandem device reduces electrical stress and improves the device life-time. ToF-SIMS, TEM/FIB and EDX analyses provided evidence of a distinct layer sequence without intermixing upon solution deposition.

In this work we focus on the formation of different kinds of polarons upon doping of thin films of poly(3-hexylthiophene) (P3HT). We elucidate profoundly the cyclic voltammogram to fit the number of oxidation peaks and, hence, to calculate the number of charge carriers per thiophene unit. These values are correlated with in-situ spectroelectrochemical measurements ranging from UV-VIS to mid-IR. In-situ UV-VIS measurements show a gradual decrease of the HOMO-LUMO transition. At the same time a new broad absorption band arises, that continuously increases up to a certain potential, above which it disappears. With in-situ spectroelectrochemical mid-IR experiments conducted in the attenuated total reflection (ATR) mode, we observe a broad absorption band, which shifts its maximum as oxidation proceeds. Interestingly, this shifting occurs at the same applied potential as the disappearing of this arised absorption band in the UV-VIS. Moreover, new doping induced infrared active vibration (IRAV) modes appear which hardly change in the different oxidation levels. Electron paramagnetic resonance (EPR) measurements are performed, indicating the persistent formation of radical cations up to a certain potential, above which the EPR signal changes its shape. These results emphasize the formation of polarons which are converted into another species as oxidation proceeds. We present a possible model which supports our spectroscopic data. Additionally, all these measurements have been confirmed by in-situ chemical doping experiments using iodine as an oxidation agent.

It has been discovered that polythiophene molecules (P3AT) aggregate into 1D nanostructures by solution-induced crystallization. In the case of p-doped P3AT, the doped polymer cations could have steric arrangements that are different from their neutral forms. Herein the doping-induced molecular packing processes are studied by absorption spectroscopy and atomic force microscopy (AFM). The doping-induced conformation change and Coulomb interactions influence the molecular packing of the P3AT nanostructures. On the other hand, the P3AT π-π stacked structures are more efficient at delocalizing charges, which could also enhance the charge transfer from dopants. The absorption spectra show distinctive bands for molecular packing and doping products, respectively. By systematically changing the dopant concentration, quantitative kinetic studies are carried out to correlate the correlated growth dynamics of the two absorption bands. p-doping are shown to substantially facilitate the π-π stacking of the conjugated polymers into 1D aggregates even at marginally low p-dopant concentrations. The doped 1D polymer nanostructures are also compared with the non-doped ones by AFM to reveal their morphological differences. This investigation will greatly strengthen our understanding on the chemical doping process for conjugated polymers.

Molecular doping is an important strategy to optimize organic electronic devices. Transport layers in OLEDs and solar cells are typically doped at less than few percent weight with molecular dopants to finely tune the carrier density and thus modulate carrier transport. The microscopic mechanism is believed to be the formation of charge transfer complexes between the dopant and the semiconductor. The formation of this hybrid states, as well as the complete charge transfer from dopant to semiconductor, strongly relies on the wavefunction overlap between the molecules. For example, it is known that in efficient doping of polymers it is important to have a cofacial arrangement between the dopant and the π orbital system of the polymer backbone. This scenario may be more unpredictable when the conjugated polymer is made of donor acceptor moieties in the repeat unit, which recently is one of the most common chemical architectures pursued for high performance polymer devices. The central question is how does doping work in these alternating copolymers?
In this communication, following our recent studies on doping of conjugated copolymers (1,2), we present an explanation for the low doping efficiency in donor-acceptor copolymers. By performing a combined experimental and theoretical study, focussed on infrared active molecular vibrations, we demonstrate how the geometrical position and orientation of the dopant F4-TCNQ (Tetrafluoro-tetracyano-quinodimethane) influences the doping efficiency in the copolymer PCPDTBT. We first analyze the remarkable changes in the conjugated copolymer vibrational modes upon doping with low molar ratio concentrations of F4-TCNQ, ranging from 1 to 7% , where 1% correspond to one dopant molecule every 100 copolymer repeat units. Appreciable changes in the vibrational spectrum due to transfer of charge from the polymer to the dopant acceptor only occur above 4%. We contrast these results with the same experiments performed on the homopolymer PCPDT, i.e. the polymer based on the donor moiety of PCPDTBT only. Surprisingly, although the homopolymer has the same ionization potential as PCPDTBT, signatures of infrared active vibrational modes from charges are recorded for a doping concentration as low as 1%. We have based our interpretation on density-functional-theory calculations and modelled the vibrational modes of complexes between the polymers and F4-TCNQ. Theory shows how the experimental spectra are a sum of different geometrical configurations in the dopant/polymer complexes. Remarkably, complete charge transfer between the copolymer PCPDTBT and the dopant occurs only when the dopant is docking to the donor moiety of the chain, unravelling one of the reasons for low doping efficiency (3). We further show experiments using the dopant F6-TCNNQ and discuss the role of molecular size. (1) Deschler, et al. PRL 107, 127402 (2011). (2) Tunc et al. Org. Elect. 13, 290 (2012). (3) Di Nuzzo et al. Nature Comm. 6, 6440 (2015).

The ability to precisely control the equilibrium carrier concentration in organic semiconducting devices is of great interest. As early as 1977, it was shown that the conductivity of polyacetylene could be systematically controlled over 11 orders of magnitude by doping using a range of halogens. Today, thermally evaporated organic light-emitting diodes (OLEDs) benefit from the use of doped transport layers; lowering operation voltages, reducing the device&’s sensitivity to electrode work functions, and enhancing device lifetime. However, in solution-processed organic optoelectronic devices the choice and accessibility of doped injection or transport layers is more limited. The ability to solution process doped layers is of extreme importance for high throughput production of organic electronic devices via roll-to-roll or ink-jet printing. In this talk, I will discuss the approach of using Lewis acids to modify the absorption and charge transport properties of π-conjugated systems with an available lone pair of electrons. By modulating the stoichiometry and strength of the added Lewis acid, a wide-range of optical properties were accessible without the need for rigorous synthesis. By formation of the Lewis acid-base adduct, electron density could be withdrawn from the π-system, narrowing the band gap. Using the Lewis acid tris(pentafluorophenyl)borane (BCF), we demonstrate that the absorption, the photoluminescence and electroluminescence, and charge transport of conjugated polymers can be modulated. The adduct formation leads to lower energy absorption and emission transitions, extended PL lifetimes, and increased solid state quantum yields. These properties allowed the strategy to be successfully demonstrated in PLED devices to modify the electroluminescence (EL) characteristics while keeping the luminance efficiency constant. Furthermore, addition of the Lewis acid effectively p-dopes the hole transport in the parent polymer, leading to increases in the free hole density and thus the charge-carrier mobility. This methodology is advantageous since the polymer, BCF, and the adduct have excellent solubility in organic solvents, negating the need for polar co-solvents that result in substandard and thin polymer layers.

The strong electron(hole)-phonon coupling in semiconducting polymers lends itself to the application of vibrational spectroscopies to investigate both the precise effect of electrons or holes on the polymer chain and to determine the degree of chemical, electrochemical or electrostatic doping. Raman microscopy is particularly useful to monitor local doping and spatially resolve doping profiles, e.g., within a transistor channel, as has been shown for operating electrolyte-gated single-walled carbon nanotube network transistors (Adv. Mater. 26 7986-92, 2014).
Here we demonstrate how Raman microscopy can be used to monitor the doping level in electrochemically and chemically doped semiconducting polymers ranging from regio-regular poly(3-hexylthiophene), which shows a significant broadening of the symmetric C=C stretching mode around 1460 cm^-1 with increasing p-doping, to high-mobility donor-acceptor polymers such as the diketopyrrolo-pyrrole thiophene copolymer DPPT-TT. Using the specific spectral features of the charged semiconducting polymers and the mapping capabilities of the Raman microscope it is possible to investigate the efficiency of chemical dopants and to image carrier density and doping profiles in-situ, for example, in electrochemical transistors. The effect of doping on the Raman modes of low and high mobility polymers may also provide further insight into the origin of the different charge transport regimes in these semiconductors.

The low ionization potentials of highly reducing organic n-dopants and host radical anions makes n-doping a much greater challenge than p-doping. However, many modern electronics such as transistors, complementary circuits, light-emitting diodes, photovoltaics, and thermoelectrics either require or benefit from both n- and p-type conduction. It is a long-standing and partially realized goal of the organic electronics community to mass produce devices by exploiting modern printing processes. To accomplish this will require new materials, device architectures, and processing methods. The goal of this work was to advance the state of solution-processable conductive n-type organic materials for use in printed organic solar cells, transistors, and thermoelectric devices.
A bottom-up mechanistic approach was used to design new organic n-dopants and n-dopable host semiconductors. In previous work, the n-doping mechanism of 1,3-dimethylbenzimidazole (DMBI-H) derived dopants was studied in solution, and it was discovered that DMBI-H dopants react with fullerenes by hydride transfer. Following this study, a new class of dimeric dopants ((DMBI)2) were developed to eliminate the dependence of the doping reaction on the hydrogenation thermodynamics of the host. The (DMBI)2 dopants were employed in a thorough synthetic, spectroscopic, and electrical study of new n-dopable conjugated polymers. Ultimately, several polymers with higher conductivities when n-doped than current state-of-the-art materials were identified. The results of this study indicate that the polaron delocalization length is the most relevant parameter to optimize to achieve high conductivity n-doped polymer films. Building upon this work, a class of self-n-doped polymers with remarkable stability in air were developed. Finally, as a demonstration of the utility of the new materials reported in this work, solution-processed organic solar cells with n-doped layers were fabricated.

Although open shell organic molecules (free radicals) are of fundamental interest, with few exceptions, they are generally reactive and unstable. We report solution-processed stable diradicaloids with temperature tunable electrical conductivity via a mechanism of self-doping, a result that is promising for advanced device applications. Electrical measurements show a remarkable electrical self-doping for the diradicaloids at room temperature (RT) with the doping strength highly tunable and reversible with temperature (T), attributed to the formation of radical ion species within the solid state. The self-doping in diradicaloids is confirmed by intentional doping with an external dopant and T-dependent X-ray photoelectron spectroscopy that shows an increase of the nitrogen cations accompanied with a stoichiometric change of the nitrogen in the triazinyl rings at higher temperatures.

We report the status of our research program to develop solution-processable air-stable p-doped polymers with ultrahigh workfunctions larger than 5.4 eV and up to 6.0 eV in collaboration with our industry partner. Previously it was thought that ultrahigh workfunctions beyond 5.3 eV are not possible in ambient because of oxidation of the water couple. We report that these problems can be alleviated to produce novel hole-injection layers (HILs) are able to provide ohmic injection into solution-processed organic semiconductors with deep ionization potentials up to 6.0 eV, and have sufficient air and thermal stability to afford a useful processing window. We also demonstrate that suitable members of this family of HILs also provide higher open-circuit voltages, and surprisingly also higher fill factors, than conventional PEDT:PSSH for organic solar cells fabricated with photoactive layers that have larger ionization potentials than 5.0 eV. In one example with PCDTTBT: PCBM as photoactive layer, power conversion efficiencies are improved by 40%from 5% to 7% simply by using these HILs in place of PEDT:PSSH. We will also report the first organic CMOS circuit elements built by electrode differentiation leveraging on our hole- and electron-injection interlayer technologies.

Semiconducting polymers have inherent trap, or gap, states due to structural/morphological imperfections and irregularities, or to impurities. These states can lead to impeded charge transport and, in the case of organic photovoltaic systems, result in an increased probability of charge recombination. It has been shown that low molecular doping levels (<1%) engender the passivation of gap states in organic semiconductor films. When co-deposited during vacuum deposition of C₆₀, an increasing ratio of n-dopants causes an exponential increase in the conductivity until the dopant density approaches the trap density of C₆₀, after which contribution of carriers to the conduction band begins. Contrary to the relatively even spatial distributions of dopant molecules that one would expect to be afforded by evaporation, processing of films from solution which are doped at low levels could result in less homogenous films, which might in turn impact their electronic properties. The details of the solution doping and deposition process can then significantly affect the distribution of the dopants in the film, depending on miscibility of the ion pairs with solvents and host materials.
This presentation reports doping of the widely-studied semi-crystalline donor polymers P3HT, PTB7, & PCDTBT with soluble molecular p-dopants derived from molybdenum tris[dithiolene] exhibiting high electron affinities ~5.5 eV, which resulted in passivation of trap states, but not without marked effects on material order, even at the lowest levels examined (10^-4 wt%). In highly ordered P3HT, the incorporation of the dopant is found to affect phase formation and consistently lower photovoltaic PCE. However with PTB7, and PCDTBT, where charge transport is less dependent on short-range order, doping results in an improvement in current density, and PCE, at dopant ratios of ca. 0.1-0.2 wt%.

The lifetime of the OLED device has been one of the critical hurdles for commercialization, and it is especially true for blue OLED. Notably, the intrinsic chemical stability of host materials for emission layer of blue OLED becomes more relevant due to the high emission energy of the blue guest materials, which may cause the degradation of surrounding host materials and eventually the failure of the entire device. Therefore, understanding the nature of the bonds and its dissociation phenomena of the blue OLED host materials are utmost importance for the better lifetime as well as the performance. Here, we present a systematic computational study on C-N bond dissociation of a series of carbazole(Cz)-dibenzothiophene(DBT) derivatives (Cz(x)DBTs, x=1, 2, 3, and 4), which is essentially a simpler variations of one of the commercialized high triplet host materials for blue OLED: DCzDBT (2,8-di(9H-carbazol-9-yl)dibenzo[b,d]thiophene). Our calculations show that the CzDBTs also have high triplet energies and compatible HOMO/LUMO levels. We found that the extra electron of CzDBT anion was redistributed towards the C-N bond upon bond stretching, forming a partially cleaved bond, and that is why the anions are more susceptible to C-N bond dissociation than cations or neutral species. Based on this, the substitution effect on the C-N bond stability was examined, and the results confirmed that an electron-withdrawing substituent with a strong inductive effect can improve the C-N bond stability of the CzDBTs without compromising their excited states energies.

Energy level alignment at organic heterojunctions plays a crucial role not only in determining the performance of organic electronic devices, but also in correlating the electronic interaction of organic semiconductors. Here photoemission spectroscopy is used to investigate the energy level alignment of organic bulk heterojunction (BHJ) which formed via solution mixing of conjugated polymer (electron donor) and fullerene derivatives (electron acceptor). Owing to the preferential vertical phase segregation with polymer dominated on the surface, we find that an abnormally high [6,6]-phenyl-C₇₁-butyric acid methyl ester (PCBM) to polymer ratio is required to be able to acquire the signals from the both components. By using this approach, we have successfully differentiated the interface dipole and energy level bending at the BHJs by photoemission spectroscopy. In addition, the effective band gap extracted from the energy difference between the highest-occupied molecular-orbital (HOMO_D) of the polymer and the lowest-unoccupied molecular-orbital (LUMO_A) of PCBM have excellent agreement with the values obtained from temperature dependent open-circuit voltage (V_OC) measurement in photovoltaic cells. The results demonstrate a facile approach to determine the energy gap in BHJ thin films, and sight light into the fundamental correlation between the energetic alignment and photovoltage in organic solar cells.

The design and performance of organic photovoltaic cells is dictated in part by the magnitude of the exciton diffusion length (L_D). Despite the importance of this parameter, there have been few investigations connecting L_D and materials purity. Here, we investigate L_D for the organic small molecule N,Nprime;-bis(naphthalen-1-yl)-N,Nprime;-bis(phenyl)-benzidine (α-NPD) as native impurities are systematically removed from the material. Thin films deposited from the as-synthesized material yield an L_D, as measured by photoluminescence quenching, of (3.9 ± 0.5) nm with a corresponding photoluminescence efficiency (eta;_PL) of (25 ± 1)%. After purification by thermal gradient sublimation, the value of L_D is increased to (4.7 ± 0.5) nm with a corresponding eta;_PL of (33 ± 1)%. Using a model of diffusion by Förster energy transfer, the variation of L_D with purity is predicted as a function of eta;_PL and is in good agreement with measurements. The observed increase in eta;_PL and concomitant increase in the exciton lifetime suggest a reduction in the concentration of exciton quenching impurities with purification. Interestingly, a similar behavior is also observed as a function of the thin film deposition rate. Films grown from the purified material at a high deposition rate give L_D = (5.3 ± 0.8) nm with eta;_PL= (37 ± 1)%. The results of this work highlight the role of impurities in determining L_D, while also providing design rules for materials purity and device processing.

Fluorescent and phosphorescent organic light-emitting diodes (OLEDs) were fabricated using a charge-transfer-featured compound, 6-{3,5-bis-[9-(4-t-butylphenyl)-9H-carbazol-3-yl]-phenoxy}-2-(4-t-butylphenyl)-benzo[de]isoquinoline-1,3-dione (CzPhONI), as a host. CzPhONI exhibits a twisted intramolecular CT character, and the density functional theory calculations have shown that the overlap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of CzPhONI is zero. The HOMO of CzPhONI is merely located on the dicarbazylphenyl moiety, while the LUMO is only distributed on the naphthalimide unit. Therefore, the excited state of CzPhONI should possess small exchange energy, resulting in the small DE_ST, which is beneficial for triplet energy up-conversion.
Fluorescent dyes of rubrene and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and phosphorescent dyes of bis[2-(4-tertbutylphenyl)benzothiazolato-N,C^2prime;]iridium (acetylacetonate) [(t-bt)₂Ir(acac)] and tris(1-phenylisoquinoline)iridium(III) [Ir(piq)₃] were doped into CzPhONI to form the emissive layers of OLEDs. The results showed that the external quantum efficiencies of both fluorescent and phosphorescent OLEDs were exceeding their theoretical limits. Based on the analysis of EL characteristics, the high device performance of fluorescent and phosphorescent OLEDs was attributed to both efficient energy transfer and triplet energy up-conversion, and direct exciton formation was also involved in phosphorescent OLEDs. In addition, the host film possessed high thermal and morphological stabilities due to the attachment of steric bulks on host molecule, resulting in the high doping concentration for both fluorescent and phosphorescent dyes. The use of a intramolecular charge-transfer-featured compound as the host for both fluorescent and phosphorescent emitters is very promising, and this work provides an applicable new route for developing high performance OLEDs.

Organic photovoltaic (OPV) cells employing bulk-heterojunctions (BHJ) have been intensely investigated in the past decade. One of the important hurdles for achieving high efficiencies OPV cells is the limitation imposed by the low hole mobility of the light absorbing polymer. In this study, we highlight two strategies of enhancing hole mobilities and power conversion efficiencies (PCEs) in PTB7:PC₇₁BM based OPV cells. First, we developed a family of fluorenone-based acceptors. They were used as dopants in PTB7:PC₇₁BM BHJs. Small concentrations (~0.5 % by weight) of such dopants are found to boost the PCEs of PTB7:PC₇₁BM from 7.6 % to about 8.3%. The hole transport behaviors of the undoped and doped BHJs were investigated in details by dark-injection space-charge limited current technique and admittance spectroscopy. Additional impacts arising from doping was further examined by measuring the subgap optical absorptions of the BHJs with photothermal deflection spectroscopy. The results suggest that improved hole transport occur after doping. Besides doping, we also investigate a polymer rich BHJs using PTB7:PC₇₁BM as a model system. Such a BHJ was obtained by adjusting the concentration of solvent additive in the BHJ solution. We discovered that the hole mobility such a BHJ was also improved by a factor of 2-3 while OPV employing thick layers of PTB7:PC₇₁BM exceeding a PCE of 7% can be demonstrated.

Large area electronics are currently witnessing a boost with the introduction of new materials, device architectures and methods of integration for various circuit functions. Nevertheless, energy efficiency, reliability and uniformity of performance are still important research topics.
The source-gated transistor (SGT) is a three-terminal thin-film device which relies on a potential barrier deliberately introduced at the source as its current control mechanism. This type of device can produce very high gain with low saturation voltage, and is very tolerant to large variations in geometry, making it ideal for large-area, cost-efficient analog and digital circuits.
The most convenient way of realizing the source barrier is by creating a Schottky contact, but depending on the material system and fabrication process, this approach is not always successful or controllable. In polysilicon devices, we have previously shown that the profile of the Schottky barrier can be tuned by ion implantation at the metal-semiconductor interface, leading to changes in transconductance, drain current activation energy, and saturation characteristics.
Here, we use results from solution-processed devices together with numerical simulations to investigate means of fabricating transistors which behave like SGTs without the need of rigorous control of the properties of the Schottky source contact. Potential routes to achieving this type of devices include heterostructures, contact area (as opposed to electrode) modification, doping, and interface engineering. We generically call transistors with barriers realized in this fashion bulk unipolar SGTs (BUSGTs).
BUSGTs may have superior dynamic range of the on current, larger on/off ratio and lower temperature dependence of the drain current than Schottky barrier SGTs (SBSGTs). Moreover, their architecture makes them suited to both staggered electrode configurations (top or bottom contact; bottom or top gate), which could provide some integration advantages.
SGT properties make them suited for a variety of sensing and control applications of printed, solution processed and flexible electronics: from noise-tolerant digital to high-gain low-power analog.

We demonstrate that it is possible to tune charge carrier balance in a bulk-heterojunction (BHJ) solar cell. To achieve this, we investigate the impacts of a solvent additive, 1,8-diiodooctane (DIO) on both hole and electron transports in a state of the art bulk-heterojunction (BHJ) system, namely PTB7:PC₇₁BM. For a polymer:fullerene weight ratio of 1:1.5, besides changes in the BHJ film morphology, the electron mobility in the blend film increases by two orders of magnitude from around 10^-5 to 10^-3 cm² V^-1 s^-1 with the DIO concentration while almost no change is found in the hole mobility (~10^-4 cm² V^-1 s^-1). For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC₇₁BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 - 5 vol%, the electron and hole mobilities are balanced, and under these conditions, an optimized power conversion efficiencies (PCE) between 7-7.5% can be obtained. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of electrons. At DIO concentrations much smaller than 3 vol%, the BHJ film possesses low electron mobilities corresponding to longer hopping distances of 2.4 - 3.1 nm. On the other hand, at DIO concentrations much larger than 3 vol%, the BHJ film possesses high electron mobilities corresponding to smaller hopping distances of 0.7 - 1.1 nm. Our work suggest that hole mobility is the bottleneck on the PCE, and the amount of fullerene is in excess. By increasing the DIO concentration in the processing solutions, we demonstrate that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.

Charge traps in polymer gate dielectrics determine the electrical stability of organic field-effect transistors (OFETs), and polar alkoxy groups are wellknown extrinsic charge traps. However, the actual location of intrinsic charge traps in nonpolar polymer gate dielectrics has been poorly understood yet. Here, we demonstrate that the skeletal structure of polymer chain plays an important role in determining the electrical stability. To verify it, we prepared linear and branched polystyrene (l-PS and b-PS) and blended them, in which branched segments provide much larger free volume than the other segments. The current-insulating performance and field-effect mobility increased with decease of b-PS portion. In particular, the bias-stress stability was remarkably varied according to the change of b-PS portion even though all measurements excluded reactive components such as oxygen and water; the increase of b-PS resulted in time-dependent decay of mobility and threshold voltage under bias stress. This indicates that the branched segments in b-PS provide intrinsic and metastable charge trap sites. Our result suggests that the skeletal structure of polymeric chains in gate dielectric is one of the important factors affecting intrinsic long-term operational stability of OFET devices.

Organic light emitting diodes using phosphorescent dyes (PHOLEDs) have excellent performance and an internal quantum efficiency approaching 100%. To maximize performance, PHOLED devices use a conductive organic host material with a phosphorescent guest that is sufficiently dispersed to avoid concentration quenching. One of the most widely used organic compounds is green phosphorescent fac-tris (2-phenylpyridine) iridium, [Ir(ppy)₃]. In this work, we used the effect of the resonance vibration of the substrate during thermal deposition in high vacuum environment of the co-deposition of [Ir(ppy)₃] into host organic material, for reducing the clusters growth formed in the co-deposited film. It is found that the distribution of the [Ir(ppy)₃] concentration in the host material is more homogeneous in the case of the films co-deposited on vibrating substrate, as confirmed by means of scanning transmission electron microscopy (STEM) equipped with HAADF (High-Angle Annular Dark-Field) and EDS (Energy Dispersive X-Ray Spectroscopy) detectors. This analysis technique, employed for the first time in co-deposited organic thin films, permits to obtain simultaneously an image and its respective chemical information, allowing to undoubtedly characterizing their distribution and morphology.

The fabrication of OLEDs by high throughput printing techniques requires the development of solution processable electron injection layers. In this context, two classes of organic materials, aliphatic amines such as polyethylenimine (PEI) and amino-functionalized polyfluorenes such as PFN, have raised interest. However, processing of PEI poses a big challenge as films need to be very thin (<5 nm) in order to reach a high device performance [1,2]. In contrast, processing of PFN is easier but OLEDs that use it as electron injection layer exhibit limited power efficiencies due to high operational voltages [3,4].
In this work, we introduce a novel amino-functionalized polyfluorene, that consists of multiple PEI-like tertiary amine side-chains connected to the polyfluorene backbone via an amide, as an electron injection material. As a result of its molecular structure, layer thicknesses of up to 20 nm can be used in OLEDs while high power efficiencies and low operational voltages are maintained.
We solution process OLEDs that use a PPV derivative as emitting layer and either PEI, PFN or our new material in combination with silver as cathode layer. OLEDs that use our polyfluorene exhibit a current efficiency of ~ 7.5 cd/A compared to ~ 6 cd/A for PFN and ~ 7 cd/A for PEI. At the same time, due to the chemical structure of our material, operational voltages are lowered by more than 1 V compared to PFN. These results can be correlated to kelvin probe measurements that show that the new polyfluorene reduces the work-function of silver substrates by ~ 0.9 eV, exceeding the reduction observed for PFN and PEI by ~ 0.5 and 0.2 eV, respectively. AFM measurements furthermore confirm that film formation of our new material is similar to PFN and thus larger thicknesses can be used in devices.
These results show that our new polyfluorene combines the advantages of PEI and PFN, namely simple processing and a good OLED performance.
REFERENCES
[1] Zhou et al., Science 2012, 336:327-332, 2012.
[2] Stolz et al., ACS Applied Materials and Interfaces 2014, 6:6616-6622, 2014.
[3] Zeng et al., Advanced Materials 2007, 19: 810-814
[4] Zheng et al., Nature Communications2013,4:1971

Ammonia (NH₃) gas sensors based on organic field-effect transistor (OFET) using poly(methyl methacry) (PMMA) blending with zinc oxide (ZnO) nanoparticles as a gate dielectric layer were fabricated. Compared to those with the pure PMMA dielectric layer, the sensing properties of these devices using ZnO/PMMA blend as the gate dielectric layer were significantly improved when the sensors exposed to various concentrations of NH₃, and the percentage response under 75 ppm NH₃ was nearly 10 folds higher than that using pure PMMA. The results showed that there was a remarkable shift in the threshold-voltage as well as a change in the field-effect mobility after exposed to NH₃ gas. By analyzing the morphologies of the dielectrics and pentacene (which acted as the organic semiconductor) films and the electrical characteristics of OFETs, it was found that ZnO/PMMA blend gate dielectric layer was responsible for the enhanced sensing properties. As the interaction force between polar ZnO surface and NH₃ (which is polar molecule) is relatively strong, the introduction of ZnO nanoparticles to the surface of dielectric can increase the number of NH₃ molecules absorbed on the dielectric/semiconductor interface. As a consequence, more hole-trapping sites are obtained on blend dielectric surface than on pure PMMA dielectric. Besides, the decreased grain size of pentacene was formed on the ZnO/PMMA blend dielectric, facilitating NH₃ to diffuse into the conducting channel and then interact with the ZnO nanoparticles. Moreover, the environmental stability of the OFET sensors with ZnO/PMMA blend dielectric was measured, and the sensing property maintained after stored in atmosphere for 40 days. In order to confirm the functionality of the ZnO/PMMA blend dielectric in OFET based sensors, copper phthalocyanine (CuPc) was employed as the organic semiconducting layer in OFET for NO₂ detection. Compared to those with the pure PMMA dielectric, significant enhancement of NO₂ sensing property was also observed. Encouraged by the above results, a simple blending method to improve the sensing properties of OFET based gas sensors was proposed. These performance improved OFET-sensors based on ZnO/PMMA blend dielectric pave a novel way to the formation and modulation of OFETs based gas sensor as well as potential for low-cost, fast, and portable electronic nose.

This work describes the construction and characterization of self-assembled thin films of N,N&’-(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI) on indium-tin oxide (ITO) substrates using the zirconium phosphonate technique (ZP) . Films with up to 20 layers were grown by deposition of alternating layers of zirconium cations and the imide¹ PPDI.
The films were immersed in a deaerated solution of 1,4-diazabicyclo[2.2.2]octane (DABCO) (10mM) in acetonitrile and irradiated with a high pressure mercury lamp . In situ reduction of the dye was observed, generating a light purple film (initially red) containing PPDI radical anions. This radical formation can be attributed by the electron-transfer from DABCOsup3; to PPDI. The formation of the corresponding anion radical was observed, with the absorption maxima at 719 nm, 812 nm and 969 nm, corresponding to the closest values reported by Marcon and Brochsztain² for a chemically generated PDI^#9679;- using sodium dithionite.
The stability observed for the PDI radical anions immobilized in zirconium phosphonate films can be mainly attributed to two factors, namely ring stacking, resulting in spin pairing, and the highly organized zirconium phosphonate framework.
Cyclic voltammetry (CV) data showed the typical two step redox process usually observed with PDI derivatives, giving first an anion radical (PDI^#9679;-, E_1/2 = -491 mV) followed by a dianion species (PDI^2-, E_1/2 = -900 mV). The peak currents were proportional to the number of layers in the films. The surface coverage calculated by integrating the area under the CV was 2.8 x 10¹⁴ molecules/cm² per layer.
These results suggest that regular PPDI/ZP films were formed, with the same amount of dye incorporated at each deposition cycle, making these films potential candidates for the construction of new materials with technological applications such as corrosion protection coatings, biosensors, solar cells or OLEDs.

Since its advent decades ago, the field of molecular electronics has come a long way, evolving into a vast interdisciplinary and applied area of research. Possessing an amazingly diverse range of structures and properties, single molecules are now well expected to be the functional building blocks in future computing technologies, as well as bio and ferroelectronics. The utility of molecular bridging with organic groups in an active or passive role between two solid-state contacts has been extended into the field of electrochemistry with one of the attached contacts being replaced by an electroactive organic redox group. This single contact electrochemical avenue has widened the reach of molecular electronics towards organic light emitting diodes and molecular photovoltaics, organic radical batteries, and biosensor. To simulate electron transport in such systems, we propose an electrochemical charge transfer model utilizing the non-equilibrium Green&’s function (NEGF) formulation of the Landauer quantum transport picture; with the electrochemically active redox group described by a Marcus-Gerischer density of states distribution. The formulation is implemented in the study of ultra/fast linear sweep voltammetry, applied to capture electron transfer between a metal substrate and a redox group bridged by a molecular chain. Based on this formulation, the electronic coupling independent voltammetric spectra is predicted from which the redox reorganization energy may be extracted. Moreover, two methods based on voltammetric peak potentials and the degree of reaction completion are examined as possible techniques to measure the electronic coupling between the redox group and substrate. In general, the results are expected to aid in the bridging of the molecular electronics and electrochemistry communities.

This work reports the mobility enhancement of poly(3-hexylthiophene) (P3HT) based organic thin film transistors (OTFTs) by incorporating gold nanorods (Au NRs). Through varying the doping concentration and surface modifier of the Au NRs in P3HT matrix, the P3HT/Au composite with 0.5 mg mL^-1 pyridine capped Au NRs exhibits a hole mobility of 0.059 cm² V^-1 s^-1, which is 8 times higher than that of pristine P3HT. This remarkable improvement of mobility is originated from the enhanced crystallinity and optimized orientation of P3HT after doping with Au NRs. In addition, the appropriate surface modification can produce more efficient hole conduction of Au NRs.