Monolayer MoS2 is an emerging two-dimensional semiconductor with wide-ranging potential applications in the next generation electronic and optoelectronic devices. Understanding the influences of the supporting substrates on the physical properties of grown MoS2 is an important step toward its applications. Here we synthesized two typical rhomboid shaped MoS2 on MoO2 and triangle shaped MoS2 on SiO2/Si substrates and characterized them by multiple means of X-Ray Photoemission Spectroscopy, Atomic Force Microscopy, Electrostatic Force Microscopy, Raman and Photoluminescence techniques. We found that triangle shaped MoS2 exhibits different core level spectra compared with rhomboid shaped MoS2, attributed to dissimilar charge transfer with the underlying SiO2 substrate. Interestingly, the triangle shaped MoS2 single crystals exhibit distinct electrostatic and photoluminescence properties at center and edges. The underlying mechanism is proposed that partial decoupling of MoS2 at edges from SiO2 substrate induces different doping level and strain effects, resulting in anomalous physical properties at edges. The results reported here demonstrate that doping and strain effects induced by substrates have a significant influence on physical properties of monolayer triangle shaped MoS2，which can be generally applicable to other transition metal dichalcogenides materials.

•Polyvinyl alcohol (PVA) polymer is demonstrated as an efficient capping layer to improve the 2D device performance.•Field-effect mobilities can be increased from 0.28 cm2/Vs to 269.2 cm2/Vs after applying the PVA capping layer.•An energy band diagram based on Schottky barrier modulation is proposed to understand the device mechanism.Two-dimensional (2D) MoS2 field-effect transistors (FETs) have attracted many attentions due to their intriguing electronic, optical, and mechanical properties. In this work, the electrical properties of multilayer MoS2 FETs are significantly enhanced by using water-soluble polyvinyl alcohol (PVA) polymer as the capping layer. The key parameter, field-effect mobilities (μ), can be increased from 0.28 cm2/Vs to 269.2 cm2/Vs after applying the PVA capping layer, which means it has almost three orders of magnitudes increase. An energy band diagram based on Schottky barrier modulation is proposed to understand the device mechanism. The results represent a significant step towards applications of 2D MoS2 FETs for future integrated circuit, sensors, and flexible electronics.Download high-res image (197KB)Download full-size image

•A novel method is reported to improve the mobilities in 2D MoS2 transistors.•The synergy process with UV and ozone plasma treatment is developed for 2D nanoelectronics.•An energy band model based on Schottky barrier modulation is proposed to understand the underlying mechanism.Mobility engineering through physical or chemical process is a fruitful approach for the atomically-layered two-dimensional electronic applications. Unfortunately, the usual process with either illumination or oxygen treatment would greatly deteriorate the mobility in two-dimensional MoS2 field-effect transistor (FET). Here, in this work, we report that the mobility can be abnormally enhanced to an order of magnitude by the synergy of ultraviolet illumination (UV) and ozone plasma treatment in multilayer MoS2 FET. This abnormal mobility enhancement is attributed to the trap passivation due to the photo-generated excess carriers during UV/ozone plasma treatment. An energy band model based on Schottky barrier modulation is proposed to understand the underlying mechanism. Raman spectra results indicate that the oxygen ions are incorporated into the surface of MoS2 (some of them are in the form of ultra-thin Mo-oxide) and can further confirm this proposed mechanism. Our results can thus provide a simple approach for mobility engineering in MoS2-based FET and can be easily expanded to other 2D electronic devices, which represents a significant step toward applications of 2D layered materials in advanced cost-effective electronics.

•High-purity black α-phase formamidinium lead iodide (FAPbI3) perovskite film was prepared via doctor blading in air.•The α-phase FAPbI3 perovskite has a large domain size over 200 μm with (00l) preferential orientation.•The FAPbI3 photodetectors were fabricated via doctor blading and exhibited a responsivity as high as 11.46 AW−1.•The FAPbI3 photodetectors showed a ratio of photocurrent/dark current up to 105 and a response speed as fast as 5.4 ms.High-purity black α-phase formamidinium lead iodide (FAPbI3, FA is NH2CHNH+) perovskite polycrystalline film was prepared using low-cost, high-output doctor-blading deposition technique in ambient condition without further annealing process and any additives. The resulting α-phase FAPbI3 perovskite has a large domain size over 200 μm with (00l) preferential crystallographic orientation. The photodetectors with an extremely simple structure were fabricated via doctor-blading, resulting in a responsivity as high as 11.46 AW−1, a ratio of photocurrent/dark current (Ilight/Idark) as large as 105 and a response speed as fast as 5.4 ms. The results suggest that low-cost doctor-blading technique in ambient condition potentially pave a way to eliminate the yellow δ-phase FAPbI3 and get a high-quality black α-FAPbI3 perovskite film, as well as fabricate efficient FAPbI3 perovskite optoelectronic devices.Download high-res image (250KB)Download full-size image

•Efficient perovskite solar cells (PSCs) were fabricated via two-step solution processes with intramolecular exchange.•The pre-deposited PbI2(DMSO) layer accelerated the formation of high-quality CH3NH3PbI3 thin film.•The efficiency over 14% was achieved for PSCs with a simple structure of ITO/PEDOT:PSSS/CH3NH3PbI3/PCBM/Al.•The fabrication process could match with large-scale, roll-to-roll printing or coating techniques.The high-quality CH3NH3PbI3 perovskite thin film with excellent coverage and uniformity was prepared using an intramolecular exchange technology via a low-temperature, two-step sequential deposition process. The PbI2(DMSO) complex was synthesized at room temperature without any additives and was deposited, then the CH3NH3I solution was deposited subsequently. The further controllable thermal annealing process resulted in the complete formation of flat and uniform CH3NH3PbI3 thin film with large-size grains and (110) preferred crystallographic orientation. The perovskite solar cells (PSCs) with a very simple inverted planar heterojunction structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al and without other buffer layers, e.g., C60, LiF, BCP, etc., were fabricated, resulting in a power conversion efficiency (PCE) as high as 14.26%. The results suggest that the low-temperature, two-step sequential deposition process with intramolecular exchange technology provides a good route to fabricate high-quality perovskite thin film and efficient PSCs, which would match with large-scale, high-output roll-to-roll (R2R) printing/coating techniques.

Monolayer MoS2 is an emerging two-dimensional semiconductor with wide-ranging potential applications in novel electronic and optoelectronic devices. Here, we reported controlled vapor phase growth of hybrid spiral-like MoS2 crystals investigated by multiple means of X-Ray photoemission spectroscopy, scanning electron microscopy, atomic force microscopy, kelvin probe force microscopy, Raman and Photoluminescence techniques. Morphological characterizations reveal an intriguing hybrid spiral-like MoS2 feature whose lower planes are AB Bernal stacking and upper structure is spiral. We ascribe the hybrid spiral-like structure to a screw dislocation drive growth mechanism owing to lower supersaturation and layer-by-layer growth mode. In addition, the electrostatic properties of MoS2 microflakes with hybrid spiral structures are obvious inhomogeneous and dependent on morphology manifested by kelvin probe force microscopy. Our work deepens the understanding of growth mechanisms of CVD-grown MoS2, which is also adoptable to other TMDC materials.

Highly efficient planar heterojunction (PHJ) perovskite solar cells (PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al were fabricated by a low-temperature solution process. As employed silica-coated gold (Au@SiO2) nanorods at the interface between the hole transport layer PEDOT:PSS and the active layer CH3NH3PbI3, the average power conversion efficiency (PCE) showed over 40% enhancement, of which the average PCE was improved from 10.9% for PHJ-PSCs without Au@SiO2 to 15.6% for PHJ-PSCs with Au@SiO2, and the champion one up to 17.6% was achieved. Both experiment and simulation results proved that prominent efficiency enhancement comes from the localized surface plasmon resonance of Au@SiO2 nanorods which could improve the incident light trapping as well as improve the transport and collection of charge carrier, resulting in the enhancement in device parameters. The results suggest that metal nanorods, e.g., Au@SiO2, could be employed to fabricate high-efficiency and low-cost PHJ-PSCs.

•Solution-processed, annealing-free TiO2 NPs layer was fabricated and used in organic photovoltaics (OPVs).•The OPV performance was dramatically enhanced with inserting an annealing-free TiO2 NPs layer.•Solution-processed, annealing-free TiO2 NPs shows greatly potential applications in printable OPVs.A solution-processed, annealing-free TiO2 nanocrystalline particles (TiO2 NPs) as an interface modification layer was inserted in organic photovoltaics (OPVs), in which the widely used polymer poly (3-hexyl thiophene) (P3HT), a low band gap alkoxylphenyl substituted [1,2-b:4,5-b′] dithiophene-based polymer (PBDTPO-DTBO), and a soluble small molecule benzodithiophene derivative (TIBDT) were used as the donor material, respectively. The annealing-free TiO2 NPs could be easily spin-coated upon the surface of organic active layers, and showed comparable properties to thermal-annealed ones. The power conversion efficiencies (PCEs) of OPV devices could be enhanced dramatically with inserting an annealing-free TiO2 NPs layer. The PCEs of OPV devices based on P3HT:PC61BM, PBDTPO-DTBO:PC71BM and TIBDT:PC61BM bulk heterojunctions were improved by 28%, 15% and 27%, respectively, with an annealing-free TiO2 NPs layer, in which the highest PCE of 5.76% was achieved in PBDTPO-DTBO:PC71BM OPVs. The solution-processed, annealing-free TiO2 NPs thin films show great potential applications in the fabrication of large-area OPVs by printing or coating techniques on flexible polymer substrates. In particularly, it would promote to fabricate solution-processed, annealing-free OPV devices with suitable hole transport layer and organic/polymer active materials.

•Efficient electron-blocking layer (EBL)-free perovskite solar cells (PSCs) were fabricated.•The EBL-free PSCs were simply structured with ITO/CH3NH3PbI3/PCBM/Al.•The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PSCs.•The open-circuit voltage (Voc) up to 1.06 V was obtained in EBL-free PSCs.Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH3NH3PbI3/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (Voc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and Voc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.

•A facile solution-based method was developed to synthesize TiO2 nanoparticles (NPs).•The PSCs with a TiO2 NPs buffer layer have excellent improvement in PCE and stability.•The conventional structure PSC with TiO2 NPs based on P3HT:PCBM shows a PCE of 4.24%.•The TiO2 NPs play as an efficient ETL and HBL as well as optical spacer layer in PSCs.•TiO2 NPs have potential applications in PSCs, especially for large-area printed PSCs.TiO2 sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO2 nanoparticles (TiO2 NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H2O in the process of peptization both the crystallinity and particle size of TiO2 NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO2 NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al showed the short-circuit current (Jsc) of 12.83 mA/cm2 and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO2 buffer layer. The stability measurement showed that PSC devices with a TiO2 NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H2O diffusion into the active layers. The observations indicate that TiO2 NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.Graphical abstract

•Low-temperature pre-synthesized TiO2 nanoparticles (NPs) were applied in inverted PSCs.•The power conversion efficiency (PCE) increases under exposure in air at the first 24 h.•The inverted PSCs with TiO2 NPs as the ETL showed a PCE of 4.56% and good stability.•Mott-Schottky capacitance was successfully to analyze both regular and inverted PSCs.•TiO2 NPs showed great potentials to fabricate efficient, stable, and flexible inverted PSCs.The performance of both inverted and conventional polymer solar cells (PSCs) were examined with a low-temperature, solution-processed synthesized TiO2 nanoparticles (TiO2 NPs) as the electron extraction layer. The performance of inverted PSCs based on P3HT:PCBM bulk-heterojunction with a TiO2 NPs layer was dramatically improved and the highest power conversion efficiency (PCE) of 4.56% was achieved via 24 h exposure in air, which is one of the highest PCEs for P3HT:PCBM bulk-heterojunction PSCs using TiO2 as electron extraction layer. Meanwhile, the performance of inverted PSCs was superior to regular PSCs. Mott-Schottky capacitance analysis was carried out for both inverted and regular PSCs to obtain the built-in potential, the depletion width, as well as the doping level of the active layer, which all support the performance improvement of PSCs devices with inverted structure. In addition, inverted PSCs show excellent stability in air without encapsulation. The PCE can retain 87% of its original values after 400 h exposure in air, which is much better than that of regular PSCs. The results indicate that solution-processed TiO2 NPs shows great potential applications in the fabrication of highly efficient and stable inverted PSCs as well as large-area, flexible printed PSCs.Graphical abstract

•PBDFTDTBT is a benzo[1,2-b:4,5-b′]difuran-based donor–acceptor (D–A) conjugated polymer.•PBDFTDTBT is used to fabricate organic field-effect transistors (OFETs) with good performance.•PBDFTDTBT OFETs show a hole mobility of 0.05 cm2/Vs and an on/off ratio of 4.6 × 105.•OFETs have a photosensitivity (Ilight/Idark) of 1.2 × 105 under white light illumination.•PBDFTDTBT OFETs are very stable and have no obvious degeneration for 3 months in air.Organic field-effect transistors (OFETs) were fabricated through a solution process with a donor–acceptor (D–A) conjugated polymer poly{4,8-bis(2′-ethylhexylthiophene)benzo [1,2-b;3,4-b′]difuran-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)} (PBDFTDTBT) as the active layer, which is a highly efficient D–A conjugated polymer as a donor in polymer solar cells with a power conversion efficiency (PCE) over 6.0%. The OFET devices showed a hole mobility of 0.05 cm2/Vs and an on/off ratio of 4.6 × 105. Those are one of the best performance parameters for OFETs based on D–A conjugated polymers including benzo[1,2-b:4,5-b′]dithiophene (BDT) or benzo[1,2-b:4,5-b′]difuran (BDF) unit. The photoresponse of OFETs was investigated by modulating light with various intensities. The devices produced a photosensitivity (Ilight/Idark) of 1.2 × 105 and a photoresponsivity of 360 mA W−1 under white light illumination. The drain current in saturation region increases gradually with increasing illumination intensity. The threshold voltage exhibited a positive shift from −15.6 V in darkness to 27.8 V under illumination, which can be attributed to the well-known photovoltaic effect resulting from the transport of photogenerated holes and trapping of photogenerated electrons near the source electrode in organic phototransistors. Meanwhile, the devices showed good stability and with no obvious degeneration for 3 months in air. The study suggests that D–A conjugated polymers including BDF unit can be potentially applied in OFETs and organic phototransistors in addition to highly efficient polymer solar cells.Graphical abstract