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In this paper we report a simple and effective approach towards improving the performance of molecular bulk heterojunction (BHJ) photovoltaics through incorporation of a new, nonvolatile processing additive; (3-chloropropyl)trimethoxysilane (CP3MS). A small amount of the additive CP3MS combined with post-annealing treatment significantly enhanced the power conversion efficiency (PCE) of dialkylated diketopyrrolopyrrole chromophore (SMDPPEH)-based molecular BHJ solar cells. The PCE increased from 2.75% for a device prepared without the additive or annealing to 4.55% for a device containing 0.1% CP3MS that had been subjected to post-annealing treatment at 100 °C for 10 min. CP3MS performed an interesting dual function when incorporated as an additive in molecular BHJ devices. The first was that it controlled the morphology: the addition of 0.1% CP3MS to the SMDPPEH:PC61BM blend was sufficient to improve the film's crystallinity and morphology. The second function was the spontaneous migration of the CP3MS molecules from the bulk to the interface between the active layer and the Al cathode, forming an ultrathin interlayer that acted as a buffer layer suppressing charge recombination and enhancing charge transport at the interface. Analyses using atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, together with examinations of device performance, confirmed the dual-functional nature of CP3MS as an additive. CP3MS also showed its promising ability to enhance the performance of different blend systems. The performance of benzodithiophene (BDT)-based molecular solar cells enhanced from 3.05% without additive to 3.8% with the incorporation of 0.1% CP3MS.

We describe a new and highly efficient plasma-assisted electrochemical exfoliation method, involving a plasma-generated graphite cathode and a graphite anode, for the production of graphene sheets from electrodes in a basic electrolyte solution in a short reaction time.

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The fullerene derivative, indene-C60 bisadduct (ICBA), has been introduced into poly(3-hexylthiophene) (P3HT) to improve the efficiency of P3HT-based devices. However, we found that ICBA is not suitable for most low bandgap polymers. In this study, we have correlated the cell performance with surface energy between the donor and acceptor materials in a bulk-heterojunction cell. These results show that higher photocurrent can be attributed to the morphology improvement induced by larger surface energy difference (Δγ) between the low bandgap polymer and fullerene. These results also suggest that synthetic strategies which adjust the Δγ between donor and acceptor should be considered.

In this study we used convergent syntheses to prepare two novel acceptor–donor–acceptor (A–D–A) small molecules (BT4OT, BT6OT), each containing an electron-rich benzotrithiophene (BT) unit as the core, flanked by octylthiophene units, and end-capped with electron-deficient cyanoacetate units. The number of octylthiophene units affected the optical, electrochemical, morphological, and photovoltaic properties of BT4OT and BT6OT. Moreover, BT4OT and BT6OT possess low-energy highest occupied molecular orbitals (HOMOs), providing them with good air stability and their bulk heterojunction (BHJ) photovoltaic devices with high open-circuit voltages (Voc). A solar cell device containing BT6OT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in a 1:0.75 ratio (w/w) exhibited a power conversion efficiency (PCE) of 3.61% with a short-circuit current density (Jsc) of 7.39 mA cm−2, a value of Voc of 0.88 V, and a fill factor (FF) of 56.9%. After adding 0.25 vol% of 1-chloronaphthalene (CN) as a processing additive during the formation of the blend film of BT6OT:PC71BM (1:0.75, w/w), the PCE increased significantly to 5.05% with values of Jsc of 9.94 mA cm−2, Voc of 0.86 V, and FF of 59.1% as a result of suppressed nanophase molecular aggregation.

Although organic solar cells having a bilayer configuration provide continuous conducting pathways for carrier transport to the requisite electrodes, their efficiencies have remained low because of the short exciton diffusion lengths of organic semiconductors. In this paper, we describe unique spatial organic solar cells featuring nanobowl array structures that capture more light and generate more power than planar organic solar cells. To construct bilayer solar cells, we used electrochemical deposition (with polystyrene beads as the template) to fabricate poly(3,4-ethylenedioxythiophene) nanobowl arrays, functioning as the hole extraction layer, on indium tin oxide substrates and then deposited copper phthalocyanine and a fullerene (C60 or C70) to function as the active layer, onto the nanobowl arrays. By implementing this spatial structure, we could control the active layer's thickness, such that it would be suitable for exciton dissociation, while maintaining a high absorption of incident light (by increasing the absorber volume without decreasing the area of the donor–acceptor interface, such that the light path in the active layer was increased) and ensuring high exciton dissociation efficiency (by enlarging the donor–acceptor interface). Relative to an equally thick planar control active layer the photocurrent generated by such bilayer solar cells increased by approximately 90%.

A series of nucleobase-grafted polycaprolactones have been prepared which exhibits a high complementary ability in solution and solid states owing to the formation of uracil–adenine pairs by induced hierarchical self-assembly. In addition, they can self-organize into spherical structure and result in excellent biomaterial properties.

We systematically investigated the effect of the molecular weight of additives on the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by using different concentrations and molecular weights of polyethylene glycol (PEG) and ethylene glycol (EG). The conductivity enhancement depends on both the molecular weight and concentration of PEG used. The conductivity of PEDOT:PSS was enhanced from 0.3 S cm−1 to 805 S cm−1 with 2% PEG but to only 640 S cm−1 with 6% EG. PEGs with molecular weight higher than 400 have too low mobility to impart the required screening effect, and hence, the conductivity enhancement is less. Through FTIR, XPS and AFM investigations, the mechanism for the conductivity enhancement is found to be charge screening between PEDOT and PSS followed by phase separation and reorientation of PEDOT chains leading to bigger and better connected particles. The molecular weight and concentration of PEG also affect solar cell performances even though the conductivities are the same. Due to their high conductivity and high transmittance, ITO-free organic solar cell devices fabricated using PEDOT:PSS treated with 2% PEG anodes exhibited performance almost equal to that of the ITO counterparts.

In this study, oxidation polymerization was used to synthesize poly(3-hexylthiophene) (P3HT)-modified TiO2 nanoparticles (NPs) and then the effect of the interface between the TiO2 NPs and the P3HT coating on the system's photocatalytic ability under visible light was investigated. The photocatalytic properties of the TiO2 NPs were enhanced from the UV to the visible after incorporation of P3HT. Moreover, charge transfer between the TiO2 NPs and the P3HT coating was promoted when a self-assembled monolayer (SAM) of 2-thiophenecarboxylic acid was present at their interface; this SAM decreased the barrier for charge transfer between the TiO2 NPs and the P3HT coating, leading to improved photocatalytic activity under visible light.

The production of graphite oxide from graphite usually requires strong oxidants, concentrated acids, and a reaction time of the order of 100 h. In this study, we adopted a highly efficient cathodic plasma (CP) process in which the vapor plasma envelope calorific effect provides instant oxidation and expansion of graphite for producing plasma-expanded graphite oxides (PEGOs) from recycled graphite electrodes (GEs) or high purity graphite (HG), within a reaction time of 10 min without the need for strong oxidants or concentrated acids. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the dramatic structural change from GEs or HG to graphite oxides after the CP process. Furthermore, scanning electron microscopy and transmission electron microscopy revealed that the graphite oxide possessed a spheroidal morphology, with dimensions of 1–3 μm, as a result of melting and subsequent quenching during the plasma electrolysis process. We obtained a stable, homogeneous dispersion of PEGOs in N-methyl-2-pyrrolidone after sonication and filtering of the centrifuged PEGOs. We used these spheroidal graphite oxide particles as effective adsorbents for the removal of pollutants (e.g., Methylene Blue) from aqueous solutions. These PEGOs also served as good precursors for the preparation of graphite nanoplatelets. CP processing appears to be an effective and environmentally friendly means for mass-producing graphite oxide.

A novel uracil-functionalized poly(3-thiophene) as a hole-injecting/transporting layer in an organic light-emitting device is able to form physical crosslinkages resulting in high thermal stability, non-corrosion, excellent hole injection/transport and electron-blocking capabilities in the solid state, and it achieves up to 10 times higher performance than that of conventional poly(3-thiophene)s under similar experimental conditions.

In this study, we used facile synthetic routes to construct two well-defined starburst donor/acceptor conjugated small molecules with broad absorption features; in TPAKP-2 and TPAKP-3, triphenylamine (TPA) moieties served as electron donor core units and dialkylated diketopyrrolopyrrole (DKP) moieties with symmetrical thiophene units served as electron acceptors, in 1:2 and 1:3 ratios, respectively. Our investigation of the photophysical properties indicated that the absorption bands of TPAKP-2, and TPAKP-3 extended up to 793 nm, with low optical band gaps of 1.56 and 1.65 eV respectively. Under illumination with AM 1.5 white light (100 mW cm−2), we investigated the performance of bulk heterojunction (BHJ) photovoltaic devices incorporating an active layer of an electron-donor small molecule (TPAKP-2 or TPAKP-3) blended with an electron acceptor: [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) at various weight ratios. The photovoltaic device containing the donor TPAKP-3 and the acceptor PC71BM at a 1:3 weight ratio exhibited the best power conversion efficiency (1.81%), with an open circuit voltage of 0.66 V, a short circuit current density of 7.93 mA cm−2, and a fill factor of 34.7%.

Hollow spherical polyaniline (hsPANI) particles are synthesized and deposited on an ITO/glass substrate to prepare a counter electrode (designated as hsPANI-CE) for a dye-sensitized solar cell (DSSC). The structure and crystallization of the hsPANI particles are characterized by using high resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectra. A power-conversion efficiency (η) of 6.84% is obtained for the DSSC with the hsPANI-CE, while it is 6.02% in the case of the DSSC with a CE based on pristine PANI (designated as PANI-CE). Such enhancement is attributed to the hsPANI film having a larger active surface area (A) of 0.191 cm2, compared to that of the PANI film (A = 0.126 cm2), both values being estimated by a rotating disk electrode (RDE). Cyclic voltammetric (CV) curves have evidenced that the electro-catalytic ability of the hsPANI-CE for the reduction of tri-iodide (I3−) ions is higher than that of the PANI-CE. As a reference, the DSSC with a Pt-sputtered CE gives an η of 7.17%. Electrochemical impedance spectroscopic (EIS) spectra are used to substantiate the photovoltaic behaviors. The results suggest that the film consisting of hsPANI particles can be a potential catalytic layer for the replacement of Pt in the CE of a DSSC.

Atomically thin molybdenum disulfide (MoS(2)) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS(2) atomic thin layers is still lacking. Here we report that wafer-scale MoS(2) thin layers can be obtained using MoO(3) thin films as a starting material followed by a two-step thermal process, reduction of MoO(3) at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS(2) films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.

In this study, we developed a simple and efficient approach—using graphene coatings on ITO as transparent electrodes—for inducing the growth of 29H,31H-phthalocyanine (H2Pc) nanofiber arrays through a thermal evaporation process, with potential use in organic field emitters (FEs). By controlling the surface energy and temperature of the electrodes during evaporation, H2Pc molecules readily self-assemble, forming an out-of-plane morphology on reduced graphene oxide surfaces (rGO; surface energy: ca. 50 mJ m−2). The devices fabricated on rGO/indium tin oxide (FE-rGO) exhibited not only excellent FE performance but also outstanding anti-degradation capability during stability tests. This facile approach toward rGO coatings opens a new avenue for the transformation of small organic molecule films into vertically standing nanostructures on transparent electrodes, with various organic electronics applications.

In this perspective, we describe a new type of photovoltaic all-carbon composites, in which single walled carbon nanotubes (SWCNTs) and fullerenes are directly co-assembled and processed in water using graphene oxide (GO) as a surfactant. Solar cells made with C60/SWCNTs/reduced GO as the active layer and an additional evaporated C60 blocking layer yield a power conversion efficiency of 0.21%, which can be significantly increased to 0.85% by replacing C60 with the better absorber C70. These results suggest the possibility of making a new generation of solution processed all-carbon solar cells, in which all the functional layers are made of graphitic nanomaterials.

A new supramolecular material, N-(6-(3-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)propanamido)pyridin-2-yl)undec-10-enamide (U-DPy) possesses an extremely high association constant (Ka >107 M−1), which is able to dimerize through a self-complementary sextuple hydrogen-bonding array. As the end groups of an oligomer, U-DPy forms a self-assembling polymer system. These new mono- and difunctional telechelic supramolecular polymers have highly prominent properties, leading to a substantial increase in viscosity, thermal stability and unique morphology changes in the solid state. More surprisingly, they can self-organize into lamellar crystallization and a spherical structure. In addition, upon increasing the concentration in the solution state, the size of the resulting spherical structure is gradually increased. These spherical structures are constructed from supramolecular polymers with a high degree of polymerization.

In this study, we used electrochemical deposition with a polystyrene (PS) bead template to prepare thin layers of poly(3,4-ethylenedioxythiophene) (PEDOT) that has a nanometer-scale honeycomb periodicity. To achieve dual-color electrochromic films, we then used pulsed electrochemical deposition to form polyaniline (PANI) layers on the rims of the PEDOT honeycomb film. Modifying the depositing charge density of PANI allowed tuning of the color of the composite films. We estimated the coloration efficiencies with respect to the values expected for corresponding amounts of pristine single layers of PEDOT and PANI. As a result of improved contact between the polymer film and the electrolyte, the coloration efficiencies of the structured composite films were much closer to their theoretical values than those of the corresponding bilayer composite films. Moreover, the switching times of the structured composite films were much faster than those of the bilayer composite films prepared without the nanostructures.

We proposed a simple yet robust film treatment method with methanol having only one hydroxyl group to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by four orders of magnitude. Different methods of film treatment: immersing PEDOT:PSS film in the methanol solution; dropping methanol on the film; and a combination of these are employed and the results are compared. The conductivity of PEDOT:PSS films was enhanced from 0.3 S cm−1 to 1362 S cm−1 after film treatment with methanol. Other alcohols like ethanol and propanol were also used to treat the PEDOT:PSS film and showed inferior conductivity enhancement compared to methanol. The conductivity enhancement was greatly affected by the hydrophilicity and dielectric constant of the alcohols used. The mechanism of conductivity enhancement was investigated through various characterization techniques including FTIR, XPS and AFM. Removal of the insulator PSS from the film, and morphology and conformational changes are the mechanisms for the conductivity enhancement. The treated films also showed high transmittance and low sheet resistance desirable for a standalone electrode. ITO-free polymer solar cells were fabricated using PEDOT:PSS electrodes treated with methanol and showed almost equal performance to ITO electrodes.

In this study we developed N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)-based organic thin film transistors (OTFTs) for sensing applications in aqueous media. We converted an n-channel PTCDI-C8 into a stable device when a fluoropolymer (CYTOP) was positioned between the dielectric and the active layer. The resulting device was stable for up to 2000 h; its electron mobility (μ) improved to 0.29 ± 0.04 cm2 V−1 s−1 from 0.005 cm2 V−1 s−1 for the device lacking CYTOP, with the on/off ratio (Ion/Ioff) enhanced by one order of magnitude (from 103 to 104). By monitoring the shifts in drain current (IDS) and threshold voltage (Vth), we used these OTFTs as sensors for different concentrations of potassium phosphate buffer and mercuric(II) acetate solutions at neutral pH. We also used them to investigated the influence of charge on the detection of insulin at pH 4.5 (where insulin possesses a net positive charge) and pH 7.5 (net negative charge).

In this paper, we present evidence of balanced electron and hole transport in polymer-fullerene based solar cells by means of embedded indium-tin-oxide nanoelectrodes. Enabled by a controllable electrochemical deposition, the individual nanoelectrodes are uniformly enclosed by a poly(3,4-ethylenedioxythiophene) hole-conducting layer, allowing a relatively short route for holes to reach the anode and hence increasing the effective hole mobility. Consequently, the power conversion efficiency and photogenerated current are maximized with a deposition condition of 50 μC, where the ratio of the electron to hole mobility is nearly unity.

In this study, we developed a multiscale molecular simulation framework including coarse-grained (CG) molecular simulation, reverse-mapping, and morphology evaluation schemes to investigate the nanoscale morphologies of bulk heterojunction (BHJ) blend films comprising poly(3-hexylthiophene) (P3HT) and the methanofullerene derivative PCBM. A stable and phase-separated blend film with the fibrillar P3HT structure was observed after CG simulation of the thermal annealing process, and by the reverse-mapping technique the atomistic details—showing strong π–π interaction between thiophene rings—were retrieved. To evaluate the morphologies of P3HT:PCBM blends, a spatial-discretization scheme was developed. With such a scheme, we estimated the average domain sizes, interface-to-volume ratios, and percolation ratios of the blends at different P3HT:PCBM weight ratios. The average domain sizes determined through these simulations were in excellent agreement with those reported experimentally. Moreover, our simulations indicated that blend films having weight ratios close to 1 : 1 would have the highest interface-to-volume ratio and the most balanced charge carrier transport in both the P3HT and PCBM phases, consistent with the experimental observation that a 1 : 1 weight ratio is optimal for P3HT:PCBM blends. The multiscale molecular simulation framework proposed herein can be extended to investigating the morphologies of other photoactive layers of organic photovoltaic cells.

Nanographite/polyaniline (NG/PANI) composite films were developed and characterized, and the performances of the dye-sensitized solar cells (DSSCs) employing these composite films as the counter electrode (CE) were evaluated in this study. The nanographite/aniline (NG/ANI) particle was firstly synthesized by a reflux method and served as the monomer for the electro-polymerization of the NG/PANI composite films. The surface modification of NG by ANI was confirmed by EDX mapping, TEM image, zeta-potential, and UV-Vis absorption measurements. The electro-polymerized NG/PANI composite films were characterized by Raman spectroscopy, XPS, and conducting-AFM, which verified the successful incorporation of NGs in the PANI films. The electro-catalytic activity of the NG/PANI composite film was evaluated using the positive-feedback mode of scanning electrochemical microscopy (SECM), by which a comparable heterogeneous rate constant (ks0) for the ferrocene/ferrocenium (Fc/Fc+) redox pair was obtained and compared with that of a sputtered Pt. The DSSC employing the NG/PANI (20 mC cm−2) CE exhibited a higher short-circuit current density (JSC) but lower fill factor (FF), and gave a comparable power-conversion efficiency (η) of 7.07%, as compared to that of a DSSC containing a sputtered Pt CE (η = 7.19%).

A transfer printing technique allows the development of flexible photovoltaic devices. Blending poly(ethylene glycol) (PEG) in the photoactive layer causes the PEG molecules to migrate spontaneously to and weaken the bonding at the interface between the polymer film and the Si wafer, thereby facilitating the transfer process from a hard stamp to the receiving flexible substrate. The device fabricated using this transfer printing process had characteristics similar to those of the corresponding device prepared using a spin-coating process. Using hard stamps should help to realize the low-cost, high-throughput roll-to-roll fabrication of flexible solar cells.

In this study, a novel solution-processed small molecule (TQTFA) for use as an electron donor has been incorporated into organic solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C71-butyric acid methyl (PC[70]BM). The combination of TQTFA with P3HT and PC[70]BM allows not only a broad absorption but also tuning of the inter energy level leading to a higher short-circuit current (JSC) and open-circuit voltage (VOC). The best performing devices exhibited a power conversion efficiency of 4.50%. The efficiency is increased by almost 15% when compared to the one without TQTFA.

We have investigated the influence of thermal annealing on the performance of polymer bilayer solar cell devices incorporating poly{2,6-(4,4-bis[2-ethylhexyl]-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)} (PCPDTTBT) as the donor and two kinds of fullerenes (C60, C70) as acceptors. The higher absorption of C70 increased the external quantum efficiency in the spectral range 400–600 nm. We observed morphological changes of the polymer films when the pre-annealing temperature was near the crystalline temperature (Tc, 207 °C). These nanostructural transformations resulted in a modified interfacial morphology of the donor phases and, therefore, greatly influenced the device performance. Post-annealing treatment reorganized the interface between the donor and acceptor phases, leading to better contact. The highest power conversion efficiency (2.85%) was obtained when we performed device pre- and post-annealing both at 200 °C for 30 min; the open-circuit voltage was 0.69 V and the short-circuit current was 8.42 mA cm−2.

In this study, we fabricated inverted polymer solar cells featuring titanium dioxide (TiO2) as the electron collection layer and vanadium (V) oxide (V2O5) as the hole collection layer. TiO2 films (anatase phase) were prepared by combining electrochemical deposition with high-pressure crystallization. The low temperature process used to obtain the TiO2 films minimized interdiffusion of Ti and In species between the TiO2 and ITO films and maintained the conductivity of the indium tin oxide substrate. The inverted device reached a power conversion efficiency of 3.22% and exhibited much better stability under ambient conditions relative to that of the corresponding conventional device.

The influence of solvent on the electrochemical property and conductivity of PEDOT has been investigated by electrochemical and physical characterizations. The PEDOT treated with different solvents reveals a different reversibility of ionic transport and cycling stability, which is associated with the conformational rearrangement from aggregated to linear polymer chains, as evidenced from atomic force microscopy (AFM). The conductivity of the PEDOT thin film can also be enhanced while the degree of linear polymer chains is higher. The application of highly conductive PEDOT thin film as a buffer layer in the polymer photovoltaic devices was realized. A PCE of 4.27% for photovoltaic devices based on P3HT–PCBM under simulated sun light is achieved by using a PEDOT thin film treated with DMSO.

In this article, we demonstrate a simple and reliable stamping technique for fabricating multi-layer solar cells. A poly(di-methylsilane) (PDMS) stamp is used for transferring the active layers onto the substrate. An intermediate solvent treatment is introduced to temporarily modify the PDMS surface; therefore, the polymer film can be uniformly formed on top of the PDMS surface. This method is involves non-contaminative and non-invasive processes, therefore it can avoid possible degradation or contamination of the polymer film and the PDMS stamp can be reused. Devices realized through this stamping technique both by direct and inverted structures exhibited power conversion efficiencies of 3.2 and 2.83% respectively.

In this Letter, ambipolar transport properties of a bilayer of In2O3 and a pentacene heterostructure have been realized. While In2O3 thin film transistors exhibited a n-channel behavior, pentacene presumed p-channel characteristics on bare SiO2∕p-Si substrates. However, when a bilayer of In2O3/pentacene was realized on the gate dielectrics, the hybrid structure exhibited both n- and p-channel conductions, depicting an ambipolar transistor behavior. When two identical ambipolar transistors were integrated to establish an inverter structure, a voltage gain of 10 was obtained. The results indicate that these heterostructures can be utilized for the complementary circuits.

We report an effective way to equilibrate hole and electron transport in ambipolar pentacene thin film transistors (TFTs). It was achieved by inserting an ultra thin lithium fluoride (LiF) layer at the electrode/organic interface to form a bifunctional source/drain electrode. It can be observed that the presence of ultrathin LiF layer will facilitate the injection of electrons from source/drain electrodes to organic semiconductor, resulting in a significant enhancement of the n-channel conduction, without sacrificing p-channel conduction for ambipolar pentacene TFTs. These factors make them potential candidates for the organic complementary circuits and inverter circuits of these ambipolar TFTs are also demonstrated.

Thin film transistors (TFTs) of indium oxide (In2O3) and tin oxide (SnO2) were fabricated on SiO2 gate dielectric using reactive evaporation process. Structural investigation of the films revealed that In2O3 films were polycrystalline in nature with preferred (222) orientation and SnO2 films exhibited amorphous nature. The x-ray photoelectric spectroscopy measurements suggest that SnO2 films were oxygen rich and presume mixed oxidation states of Sn, namely Sn2+ and Sn4+. While the In2O3 based TFTs possess n-type channel conduction, SnO2 based TFTs exhibited anomalous p-type conductivity. Integration of these n- and p-type devices resulted in complementary inverter with a gain of 11.

Ambipolar conduction of a pentacene-based field-effect transistor can be attributed to dual interface engineering, which occurs at the dielectric∕semiconductor interface and electrode∕semiconductor interface. While the former was realized by utilizing a hydroxyl-free gate dielectric, the latter was made feasible by the use of appropriate metal source and drain electrodes. The field-effect hole and electron mobilities of 0.026 and 0.0023cm2∕Vs, respectively, were extracted from the transfer characteristics of pentacene organic field-effect transistors utilizing polymethyl methacrylate as the trap-reduction interfacial modified layer and Al as the source and drain (S∕D) electrodes. We demonstrated a complementarylike inverter by using two identical ambipolar transistors and it can be operated both in the first and third quadrants with a high output voltage gain of around 10.

Bottom gate and top contact thin film transistors were fabricated using In2O3 thin films as active channel layers. Thin films of varying thicknesses in the range 5?20?nm were deposited on an SiO2 gate dielectric by the thermal evaporation process in the presence of high purity oxygen. The results of atomic force microscopy show that all the films exhibit dense grain distribution with a root-mean-square roughness in the range 0.6?8.0?nm. Irrespective of the thickness of the channel layer, the on/off ratio of the device is 104. The channel mobility and resistivity were found to be a strong function of the thickness of the active layer. The Levinson model was used to calculate the trap density and the grain boundary mobility. The low processing temperature shows the possibility of utilizing these devices on flexible substrates such as polymer substrates.

In this article, we report the fabrication of SnO2 thin film transistors (TFTs) fabricated by reactive evaporation. Different from the previous reports, the fabricated TFTs exhibit p-type conductivity in its undoped form. The postdeposition annealing temperature was tuned to achieve p-channel SnO2 TFTs. The on/off ratio and the field-effect mobility were ∼103 and 0.011cm2∕Vs, respectively. To demonstrate inverter circuit, two devices with different threshold voltages were combined and an output gain of 2.8 was achieved. The realization of p-channel oxide TFTs would open up new challenges in the area of transparent electronics.

Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.

Pentacene thin-film transistors operated at low voltages have been realized by employing hybrid dielectrics, poly(4-vinylphenol) (PVP)-capped MgO. The MgO film was deposited by reactive evaporation of magnesium in oxygen. The PVP layer plays a role in modifying the surface morphology of the MgO layer, and the images of atomic force microscopy for the PVP-capped MgO films show a root-mean-square roughness of 1 nm. The hybrid dielectric has a dielectric constant of 7.05 with a dielectric strength. The fabricated pentacene organic thin-film transistors exhibit a mobility of 0.66 cm 2 /V s, a threshold voltage of -2.67 V, and an on/off ratio of 10 3 .

In2O3 thin films have been grown by reactive evaporation of indium in ambient oxygen. The films were structurally characterized by x-ray diffraction (XRD) and atomic force microscopy techniques. The results of XRD revealed that the films were polycrystalline in nature with preferred (222) orientation. The as-grown films were subjected to various annealing treatments to modulate the conductivity of the films for thin film transistors (TFTs). TFTs fabricated on SiO2 gate dielectric exhibited an on/off ratio of 104 and a field-effect mobility of 27cm2∕Vs. High on-state current makes them potential candidates for flat-panel display devices.

Organic field-effect transistors have been the subject of much recent inquiry due to their unique properties. Here, the authors report an ambipolar vertical organic field-effect transistor, which consists of a capacitor cell vertically stacked with an organic active cell, separated by a thin source electrode. By inserting a nanoscale transition-metal-oxide layer at the source/organic interface, the authors fabricated the organic ambipolar transistors with low working voltage and high current output. The thin transition-metal oxide and partial oxidization metal grains form a unique nanostructure that balances the injection barrier height of two types of carriers at the source/organic contact.

The authors demonstrate the operation of an organic single-crystal complementary circuit in the form of a simple inverter. The device is constructed from a high mobility p-type organic single-crystal transistor of tetramethylpentacene (TMPC) and a n-type single-crystal transistor of N,N′-di[2,4-difluorophenyl]-3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Field-effect mobilities of up to 1.0cm2∕Vs are reported for TMPC devices, while a mobility of 0.006cm2∕Vs is reported for a n-type PTCDI single-crystal device. Considering that organic single-crystal inverters have not yet been explored, they are representative of potential candidates for use in high-performance complementary circuits.

We compare organic donor/acceptor heterojunction photovoltaic devices based on copper phthalocyanine (CuPc) as the donor and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) or buckminsterfullerene (C60) as the acceptor. The effect of the two acceptors on open-circuit voltage and short-circuit current density is studied. Improved interface properties, optimal energy level matching, and more balanced charge transport are believed to be the reasons for better performance of CuPc/PCBM heterojunction device, in terms of photocurrent and fill factor. The energy conversion efficiency of the devices under AM1.5G solar illumination at 130mW∕cm2 is improved to 1.18%, up from 0.74%.

Recently, electrical bistability was demonstrated in polymer thin films incorporated with metal nanoparticles [J. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma, and Y. Yang, Nat. Mater. 3, 918 (2004)]. In this letter, we show the evidence that electrons are the dominant charge carriers in these bistable devices. Direct integration of bistable polymer layer with a light-emitting polymer layer shows a unique light-emitting property modulated by the electrical bistability. A unique negative differential resistance induced by the charged gold nanoparticles is observed due to the charge trapping effect from the nanoparticles when interfaced with the light-emitting layer.

Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.

An effective connecting structure for tandem organic light-emitting devices is reported. The connecting structure consists of a thin metal layer as the common electrode, a hole-injection layer containing MoO3 on one side of the common electrode, and an electron-injection layer involving Cs2CO3 on the other side. Such a connecting structure permits efficient opposite hole and electron injection into two adjacent emitting units and gives tandem devices superior electrical and optical performances. Furthermore, the present connecting structure involves no sputtering or handling of reactive metals during device fabrication and can be prepared purely by thermal evaporation, thus rendering device processing more feasible.

We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between ...

A high-performance organic active matrix pixel was fabricated by using a metal oxide (V2O5) coupling layer that effectively integrates an organic light-emitting diode (OLED) on top of an organic field-effect transistor (OFET). The field-effect mobility of the OFET approached 0.5cm2V−1s−1 and the ON/OFF current ratio was >103. The brightness of the OLED was on the order of 2000cd∕m2, with an efficiency above 3.3cd∕A. The present work describes in detail a methodology for sizing and stacking an OFET in bottom-emitting active matrix pixel circuits. The confinement of pixel dimension ensures the uniformity of light emission. The material for coupling layer can be tailored to achieve maximum device efficiency. A unique active matrix pixel circuit is proposed that renders both the OFET and OLED their individual performance after integration.

We report organic solar cells fabricated with small-molecule organic semiconductor tetracene∕C60 heterojunction as the photoactive layer. The external power conversion efficiency of the devices under AM 1.5 solar illumination at 100mW∕cm2 (1 sun) is 2.3±0.5% with relatively high open-circuit voltage (Voc=0.58±0.06V) compared to most of the other small-molecular donor-acceptor (D-A) heterojunction solar cells reported so far. Using atomic force microscopy and x-ray diffraction we found that tetracene thin films consist of submicron-sized grains with rough surface and well defined molecular order. Therefore, using high mobility polycrystalline tetracene thin films for D-A heterojunction devices dramatically increases area of tetracene and C60 interface for exciton diffusion to reduce the recombination.