•Anatase TiO2 film was synthesized by a low-temperature and in situ growth method.•Champion Eff. 10.33%, excellent repeatability and negligible hysteresis are achieved.•Fast electron extraction and transfer are mainly responsible for high performance.Hybrid organic/inorganic perovskite solar cells (PVSCs) are promising alternatives to traditional photovoltaic devices because of their low cost, high power conversion efficiency, and simple production process. Therefore, achieving a low-temperature and high-efficiency procedure is necessary to realize the large-scale production of PVSCs. This study developed a facile low-temperature process (70 °C) for the synthesis of anatase TiO2 thin films on the FTO substrate through an in situ growth method. The as-synthesized anatase TiO2 thin films were applied on efficient compact layers of all low-temperature (sub-100 °C)-processed PVSCs. Under optimization, the achieved low-temperature PVSC device exhibited a champion conversion efficiency of up to 10.33%, with excellent repeatability and negligible hysteresis. Results revealed that fast electron extraction and transfer between perovskite and TiO2 films and effective suppression of charge recombination are the main attributes for the excellent performance of the PVSC device. This work provides a new alternative to achieve low-temperature and high-efficiency photovoltaic devices.Download high-res image (168KB)Download full-size image

Recently, van der Waals heterostructures (vdWHs) have trigged intensive interest due to their novel electronic and optoelectronic properties. The vdWHs could be achieved by stacking two dimensional layered materials (2DLMs) on top of another and vertically kept by van der Waals forces. Furthermore, organic semiconductors are also known to interact via van der Waals forces, which offer an alternative for the fabrication of organic–inorganic p–n vdWHs. However, the performances of organic–inorganic p–n vdWHs produced so far are rather poor, owing to the unmatched electrical property between the 2DLMs and organic polycrystalline films. To make improvements in such novel heterostructure architectures, here we adopt high quality organic single crystals instead of polycrystalline films to construct a pentacene/MoS2 p–n vdWH. The vdWHs show a much higher current density and better anti-ambipolar characteristics with a highest transconductance of 211 nS. Moreover, device configuration-dependent transfer characteristics are demonstrated and a mechanism of a gate bias modulated vertical space charge zone existing at the vertical p–n vdWHs interface is proposed. These findings provide a new route to optimize the organic–inorganic p–n vdWHs and a guideline for studying the intrinsic properties of vdWHs.

In this study, we demonstrate a simple strategy for obtaining pinhole-free, homogeneous, well-crystallized perovskite films under ambient conditions. The preparation of perovskite film with high light-harvesting efficiency and long carrier lifetime is verified. By applying this film in TiO2-based perovskite solar cells (PVSCs), we achieved a high power-conversion efficiency (PCE) of 13.07%, which is doubled with respect to that of the PVSC not subjected to the same improvement procedure (6.54%). High open-circuit photovoltage, photocurrent density, and fill factor are the main contributions to the high PCE that results from low trap density and high recombination resistance of the resultant perovskite films. This work paves a new means for fabricating high-performance perovskite films and PVSC devices in an ambient atmosphere.Keywords: ambient conditions; homogeneous; methylamine iodide; perovskite solar cell; pinhole free

High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achieve high performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrier injection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the ideal crystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physical vapor transport (PVT) method grown directly on SiO2. The size, thickness, and density of Pn crystals are systematically studied. Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gas flow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical size and thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal with bottom gate and top contact configuration showed mobility as high as 5.6 cm2 V–1 s–1, indicating that the proposed novel architecture of organic molecular crystals may pave the way toward the application of large-sized single crystals of Pn in organic electronics.

•Surface modification of PEDOT: PSS with DMSO in polymer solar cells.•Lateral electrical conduction effect in DMSO-treated solar cells.•Lowered resistance of the solar cell leads to increased efficiency of 6.52%.Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) films were post-treated with polar solvent dimethylsulfoxide (DMSO) by spin-coating method and polymer solar cells (PSCs) based on poly [N-9″]-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were fabricated to investigate the effect of the treatment. By post-modifying the PEDOT: PSS layer, the conductivity of the PEDOT: PSS film was largely improved. Using electrochemical impedance spectroscopy (IS) we observed that the series resistance of the device decreased greatly after the treatment. With DMSO-treated PEDOT: PSS transport layer, the power conversion efficiency (PCE) of the PSC based on PCDTBT: PC71BM raised from 5.95% to 6.52% with both increase in Jsc and FF. We systematically studied charge transport property via space-charge-limited-current (SCLC) and our results suggest that the increment in device efficiency can be attributed to the increased hole-mobility and thus more balanced charge transport benefits the enhancement of polymer solar cell efficiency. We also noted that if measured without a shadow mask much more overestimation will take place in the DMSO-treated device as a result of lateral electrical conduction. We suggest that when we apply the highly conductive PEDOT: PSS layer in the PSCs, careful measurement should be carried out to avoid inaccuracy.

•Heterojunction was formed by co-evaporating CH3NH3I and PbI2 on the top of small molecules layer.•Hybrid photo transistors showed ideal optical-electrical performance with photo responsivity as high as 33 A/W.•Temperature dependent measurements illustrated the carriers’ distribution and charge transport in the photo transistor.To enhance photosensitive performances for an organic thin film transistor, we fabricated a hybrid structured transistor with C8BTBT film as organic active layer, onto which, a CH3NH3PbI3 layer was formed through the vacuum deposition. The phototransistor showed the best photo responsivity as high as 33 A/W, much higher than most other organic based thin film transistors, in addition to keeping fast response time and well gate tunable ability. The working mechanism were further investigated with the temperature dependence measurement. The organic-perovskite hybrid transistor may open up a path way for the optimization of organic photo sensitive transistors.

We report the observation of a screw-dislocation-driven spiral growth of DMDPC organic thin films using physical vapor deposition. The existence of screw dislocations was clearly confirmed by the observations of outcropped steps, single monolayer height helical periodicity and spiral fringes.

•A new system of co-additives of DPE and DMSO was developed.•The effect of the co-additives was systematically investigated.•The highest PCE of PCDTBT:PC71BM based solar cells reached 7.13% by using the co-additives.Two high boiling point solvents namely diphenylether (DPE) and dimethylsulfoxide (DMSO) were employed as co-additives in fabricating poly [N-9″]-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzothiadiazole) (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) based polymer solar cells (PSCs). It was found that the power conversion efficiency (PCE) can be improved to 7.13% for the PSCs processed with co-additives in comparison with ~6.5% for the PSCs processed with either DPE or DMSO additives. The enhanced PCE is benefited from simultaneous increase in Voc, Jsc and FF due to the co-additives fabricating process and the two additives are found to improve the photovoltaic performance with different mechanisms.

•Monolayer pentacene FETs being built for achievement of high sensitivity gas sensors.•Monolayer FETs applying to NO2 is precedently enhanced for three orders of magnitude.•Good results attributed to 2D electrical transport characters and strong NO2 affinity.By optimization of structural and physical–chemical properties at a precision level of molecules, organic sensing devices seek to realize the state of the art in monolayer based device applications. In our work, pentacene ultrathin film transistor has been integrated into the implement of a gas sensor based on an increase in its mobility and a shift of its threshold voltage. A limit of detection of pentacene monolayer field-effect transistor was found to be at sub ppm level when it is applied for detection of NO2. Compared with a thick layer sensor device, the pentacene monolayer NO2 sensor has boosted up sensing response with three orders of magnitude. An enhancement of high selectivity and response mechanism can be understood with a reasonable description emphasizing on the 2D transport characters and a series of gauzy interplay for monolayer pentacene film with NO2 analyte.

•Single molecular layer FETs being built with recorded mobility of 0.3 cm2 V−1 s−1.•2D electrical transport mechanism in organic small molecular film.•Untypical mobility-electric field relation was recorded and reasonably explained.We employed a novel electrode-contact architecture to enable operation of single monolayer pentacene-based field-effect transistor with a high electrical performance, whose mobility reaches as high as 0.31 cm2 V−1 s−1, the highest among any pentacene-based monolayer devices ever reported. A temperature dependent charge transport was systematically carried out to elucidate the carrier transport mechanisms within even single layer of molecules. The carrier mobilities are found to exhibit a pure Arrhenius type at the high temperature regime (above 170 K), by contrast, a pronounced turning point has been observed when the measurement temperature is below 170 K, possible mechanisms were ascribed to the distribution attribute of trapped states among the grain boundary. Furthermore, an electric field dependent mobility characterization shows a unique non-Poole–Frenkel type behavior at room temperature, which shows much different from the case occurred in multiple-layer devices where the quantity of permitted percolation routes among different monolayers guarantees an mobility enhancement as an electric field increasing. But for the case of single monolayer, the electric field-induced potential reduction effect is competing with a drop of percolation path arising from the directional movement of carrier under a strong electric field. The depth understanding of carrier transport within one monolayer may be helpful for optimizing the design of OFETs for better device applications.

We report on the fabrication of an organic field-effect transistor (OFET) of a monolayer pentacene thin film with top-contact electrodes for the aim of ammonia (NH3) gas detection by monitoring changes in its drain current. A top-contact configuration, in which source and drain electrodes on a flexible stamp [poly(dimethylsiloxane)] were directly contacted with the monolayer pentacene film, was applied to maintain pentacene arrangement ordering and enhance the monolayer OFET detection performance. After exposure to NH3 gas, the carrier mobility at the monolayer OFET channel decreased down to one-third of its original value, leading to a several orders of magnitude decrease in the drain current, which tremendously enhanced the gas detection sensitivity. This sensitivity enhancement to a limit of the 10 ppm level was attributed to an increase of charge trapping in the carrier channel, and the amount of trapped states was experimentally evaluated by the threshold voltage shift induced by the absorbed NH3 molecular analyte. In contrast, a conventional device with a 50-nm-thick pentacene layer displayed much higher mobility but lower response to NH3 gas, arising from the impediment of analyte penetrating into the conductive channel, owing to the thick pentacene film.Keywords: field-effect transistor; monolayer; NH3; pentacene; sensor; top contact;

•Several methods for fabrication of flower-like TiO2 nanostructures are discussed.•Morphologies, properties and functions of different flower-like TiO2 nanostructures are summarized.•Application progresses of different flower-like TiO2 structures are also analyzed.Flower-like TiO2 materials, with their advantages of high specific surface area, developed pore structure, and high photocatalytic activity, have been widely used in environmental management and air purification, sterilization, and surface self-cleaning, among other areas. This paper summarizes several methods used to fabricate the flower-like TiO2 nanostructures, such as the hydrothermal, solvothermal, microemulsion, sol–gel, hydrolysis, and electrodeposition oxidation methods. In addition, the morphologies, properties, and performance of different flower-like TiO2 structures are discussed. Meanwhile, the application progresses of different flower-like TiO2 structures are also analyzed.

A two-step growth method, namely, initial crystallization from pentacene monolayer grains to nanosized single crystals (nanocrystals) and subsequent physical vapor transport (PVT) growth on the nanocrystals as seeds, is demonstrated to fabricate larger microsized pentacene single crystals. The pentacene monolayer film deposited on a bare SiO2 substrate was found to self-assemble to nanoparticles during a post-annealing process. Selected area electron difffraction demonstrates unambiguously the nanoparticles and the microsized particles grown by the following PVT are both single crystals but with different crystal phases. Such a two-step growth technology maintains the intrinsic interface states between the SiO2 dielectric and the pentacene active layer and provides a more flexible way to in-situ prepare oraganic single crystals for device fabrication. The detailed investigation of morphology evolution and the discussion of formation mechanism of nanocrystal seeds indicate that the pentacene molecular transport during annealing is controlled by molecular surface diffusion. A crystal growth diagram based on thermodynamic analysis is proposed to qualitatively interpret the molecular transport from the monolayer film to the nanocrystals of pentacene.

The charge transport process in organic thin-film transistors (OTFTs) has always been under intensive research while the mechanism is still controversial, even in the small-molecule OTFTs. In this study, we report on a transport model for interpreting the charge transport mechanism in polycrystalline pentacene TFTs. By investigating the growth mode of pentacene films and the distribution of charge carriers in the channel of OTFTs, a transport model involving two-dimensional single-layered grains and grain boundaries (GBs), namely the two-dimensional grain boundary (TDGB) model, is proposed for describing the charge transport process in polycrystalline pentacene TFTs. An analytical expression for the field-effect mobility could be obtained from the model. The model is applied to explain the mobility dependence on the grain sizes at the first monolayer of pentacene films, temperature and on gate voltage in polycrystalline pentacene TFTs, with good agreement obtained. In addition, the values of some crucial parameters are given by the simulations based on the model.

A binary-component self-assembled monolayer (SAM) comprising tetrathiafulvalene (TTF) and n-tetradecane (n-C14H30) molecules has been investigated by a scanning tunneling microscope (STM) on a highly oriented pyrolytic graphite (HOPG) surface at room temperature. High-resolution STM images of the SAM reveal that the two different kinds of molecules spontaneously form periodic ordered strip-like phase separation structure on the HOPG substrate. The phenomenon can be qualitatively understood in terms of a phase field model, in which the interplay of three ingredients, including free energy of the binary component solution monolayer, phase boundary energy, and surface stress, is suggested to play an important role in determining the equilibrium sizes of strip-like domains of the TTF and n-C14H30 in the ordered phase separation structure. In addition, scanning tunneling spectrum (STS) measurements have also been performed by operating an STM tip to locate on the individual TTF or n-C14H30 molecule in the SAM on the HOPG substrate. The STS at the TTF molecule shows a distinct rectifying behavior, while at the n-C14H30 molecule it shows an intrinsic small current increase with the change of bias voltage and a slight asymmetry.

We have investigated the effects of hydroxyl-free polystyrene (PS) as buffer layer on pentacene-based low-voltage organic thin-film transistors (OTFTs). The PS buffer layer is formed on the HfO2 layer evaporated on Si substrate by spin-coating method prior to pentacene deposition, existence of which results in a dramatic increase of field effect mobility from 0.09 to 0.59 cm2/Vs and negligible hysteresis. The improved mobility and hysteresis of the OTFTs can be attributed to the formation of smooth and nonpolar hydroxyl-free PS/HfO2 gate dielectric surface. The PS insulator buffer layer can also effectively reduce gate leakage current by more than 70%. The results demonstrate that using appropriate polymer buffer layer is favorable to improve the performance of the OTFTs operating at low voltages with high mobility and good electrical stability.

Two novel conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) and isoindigo units linked with a 3-octylthieno[3,2-b]thiophene unit as a π-bridge were designed and synthesized. The two polymers named PBDT-TT-IID and PBDTT-TT-IID with the same acceptor unit and different donor unit showed similar molecular weight, absorption spectrum range, and molecular energy levels. Introduction of a thienyl group in PBDTT-TT-IID increased the conjugation degree of the backbone, expanded the absorption spectrum and improved the hole mobility. The film absorption spectrum of PBDTT-TT-IID exhibited a strong shoulder peak at 697 nm, which is attributed to the strong aggregation and highly ordered packing of the polymer chains in the solid state. The power conversion efficiencies (PCEs) of the polymer solar cells (PSCs) based on PBDT-TT-IID and PBDTT-TT-IID achieved 6.31% and 8.05%, respectively. This is first time that efficiency over 8% based on isoindigo conjugated polymers in conventional devices, and it also has the possibility of further improvement. Moreover, these two polymers showed extremely high charge carrier transport including hole and electron mobilities on the 10−2 cm2 V−1 s−1 magnitude. This shows their potential as high efficiency materials in polymer solar cells.