Carrier Mobility Transport Research Articles

Precise Structural Regulation and Band-Gap Engineering of Curved Graphene Nanoribbons.

Graphene nanoribbons (GNRs)─quasi-one-dimensional graphene cutouts─have drawn growing attention as promising candidates for next-generation electronic and spintronic materials. Theoretical and experimental studies have demonstrated that the electronic and magnetic properties of GNRs critically depend on their widths and edge topologies. Thus, the preparation of structurally defined GNRs is highly desirable not only for their fundamental physicochemical studies but also for their future technological development in carbon-based nanoelectronics. In the past decade, significant efforts have been made to construct a wide variety of GNRs with well-defined widths and edge structures via bottom-up synthesis. In addition to extensively studied planar GNRs consisting of armchair, zigzag, or gulf edges, curved GNRs (cGNRs) bearing cove ([4]helicene unit) or fjord ([5]helicene unit) regions along the ribbon edges have received increasing interest after we presented the first attempt to synthesize the fully cove-edged GNRs in 2015. Profiting from their novel edge topologies, cGNRs usually exhibit an unprecedented narrow band gap and high carrier transport mobility in comparison to the planar GNRs with similar widths. Moreover, cGNRs with particular out-of-plane-distorted structures are expected to provide further opportunities in nonlinear optics and asymmetric catalysis. However, the synthesis of cGNRs bearing cove or fjord edges remains underdeveloped due to the absence of efficient synthetic strategies/methods and suitable molecular precursor design.In this Account, we present the recent advances in the bottom-up synthesis and characterization of structurally defined cGNRs containing cove or fjord edges, mainly from our research group. First, the synthetic strategies toward cGNRs bearing cove edges are described, including the design of molecular monomers and polymer precursors as well as the corresponding polymerization methods, such as Ullmann coupling, Yamamoto coupling, A2B2-type Diels-Alder polymerization, followed by Scholl-type cyclodehydrogenation. The synthesis of typical model compounds is also described to support the understanding of the related cGNRs. In addition, the synthesis of cGNRs containing fjord edges from other research groups via the regioselective Scholl reaction, Hopf cyclization or regioselective photochemical cyclodehydrochlorination approach is presented. Second, we discuss the optoelectronic properties of the as-synthesized cGNRs and reveal the design principle to obtain cGNRs with high charge carrier mobilities. Finally, the challenges and prospects in the design and synthesis of cGNRs are offered. We anticipate that this Account will further stimulate the development of cGNRs through a collaborative effort between different disciplines.

Influence of Sputtering Temperature of TiO2 Deposited onto Reduced Graphene Oxide Nanosheet as Efficient Photoanodes in Dye-Sensitized Solar Cells.

Renewable solar energy is the key target to reduce fossil fuel consumption, minimize global warming issues, and indirectly minimizes erratic weather patterns. Herein, the authors synthesized an ultrathin reduced graphene oxide (rGO) nanosheet with ~47 nm via an improved Hummer’s method. The TiO2 was deposited by RF sputtering onto an rGO nanosheet with a variation of temperature to enhance the photogenerated electron or charge carrier mobility transport for the photoanode component. The morphology, topologies, element composition, crystallinity as well as dye-sensitized solar cells’ (DSSCs) performance were determined accordingly. Based on the results, FTIR spectra revealed presence of Ti-O-C bonds in every rGO-TiO2 nanocomposite samples at 800 cm–1. Besides, XRD revealed that a broad peak of anatase TiO2 was detected at ~25.4° after incorporation with the rGO. Furthermore, it was discovered that sputtering temperature of 120 °C created a desired power conversion energy (PCE) of 7.27% based on the J-V plot. Further increase of the sputtering temperature to 160 °C and 200 °C led to excessive TiO2 growth on the rGO nanosheet, thus resulting in undesirable charge recombination formed at the photoanode in the DSSC device.

GeN3 monolayer: A promising 2D high-efficiency photo- hydrolytic catalyst with High carrier mobility transport anisotropy

The investigations of two-dimensional photocatalysts are promising subjects in clean and renewable energy. Herein, we proposed a novel two-dimensional GeN3 monolayer. The stability of GeN3 monolayer was examined via formation energy, phonon calculations and ab initio molecular dynamics simulations. Excellent photocatalytic properties are revealed: (a) GeN3 monolayer is a semiconductor with appropriate band gap of 1.962 eV that the valence and conduction bands straddle the redox potential of water. (b) GeN3 monolayer has extremely high carrier mobility with anisotropic character (its electron mobility is 1.55 × 104 cm2 V−1 s−1 along the armchair direction, whereas the hole mobility reaches 1.6 × 103 cm2 V−1 s−1 along the zigzag direction), which boost the separation of electron-hole pairs. (c) The GeN3 monolayer shows strong light absorption coefficients (up to 105 cm−1) in the visible regions, leading to high solar-to-hydrogen efficiency (12.63 %). (d) Furthermore, the band gap of the GeN3 monolayer could be engineered from indirect to direct under external strain and electric field. This work discloses a novel 2D GeN3 monolayer whose fascinating properties make it a promising photocatalyst for hydrolyzing to produce hydrogen.

Controllable orientations for Sb2S3 solar cells by vertical VTD method

AbstractAntimony sulfide (Sb2S3) is a promising photoelectric material because of its wide bandgap approximately 1.7 eV for next‐generation solar cells, high optical absorption coefficient, and its green and earth‐abundant constituents. Different to traditional cubic structure photovoltaic materials, Sb2S3 holds one‐dimensional crystal structure and its thin film with [hk1] preferred orientation shows one‐order‐higher carrier transport mobility. However, all the reported Sb2S3 films exhibited [hk0] preferred orientation on CdS‐based superstrate device structure up to now. Thus, it is indispensable to study the controllable‐orientations Sb2S3 film deposition and the relationship between the orientation and performances. In this paper, we develop a vertical vapor transport deposition (V‐VTD) method, which can tune the preferred orientation of Sb2S3 thin film from [hk0] to [hk1] by reaction recipe monitoring. Combining the experiment results, a reasonable deposition/reevaporation competing model is suggested to explain above orientation conversion mechanism. The device efficiency increases from less than 2% to about 4% with the orientation of Sb2S3 film changing from [hk0] to [hk1]. By fine regulating the technique of deposition, the device with [hk1] orientation has better crystallinity, lower interface recombination, and higher built‐in voltage comparing with the [hk0] one. Finally, a champion power conversion efficiency (PCE) of 4.5% has been achieved, and the VOC of 730 mV is the top value among the Sb2S3 solar cells. The present versatile orientation tuning strategy could overcome the bottleneck of strong anisotropic materials and show high potential for noncubic material deposition and related optoelectronic device performance enhancement.

Donor-acceptor covalent organic frameworks for visible light induced free radical polymerization.

Covalent organic frameworks (COFs) are promising materials for applications in photocatalysis, due to their conjugated, porous and chemically stable architectures. Alternating electron donor-acceptor-type structures are known to enhance charge carrier transport mobility and stability in polymers and are therefore also interesting building units for COFs used as photocatalysts but also as photoinitiator. In this work, two donor-acceptor COFs using electron deficient 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline and electron rich thiophene-based thieno[3,2-b]thiophene-2,5-dicarbaldehyde or [2,2'-bithiophene]-5,5'-dicarbaldehyde linkers are presented. The resulting crystalline and porous COFs have been applied as photoinitiator for visible light induced free radical polymerization of methyl methacrylate (MMA) to poly-methyl methacrylate (PMMA). These results pave the way to the development of robust and heterogeneous systems for photochemistry that offers the transfer of radicals induced by visible light.

Few-layer graphene/ZnO nanowires based high performance UV photodetector

A graphene and zinc oxide nanowires (G/ZnO NWs) based ultraviolet (UV) photodetector presents excellent responsivity and photocurrent gain with detectivity. Graphene due to higher charge carrier transport mobility induces faster response to UV illumination at the interface between ZnO and graphene with improved response and decay times as compared to a ZnO NWs device alone. A linear increase is revealed for both the responsivity and photocurrent gain of the G/ZnO NWs device with the applied bias. These results suggest that the G/ZnO NWs device exhibits great promise for highly efficient UV photodetectors.

A new approach to calculate charge carrier transport mobility in organic molecular crystals from imaginary time path integral simulations.

We present a new non-perturbative method to calculate the charge carrier mobility using the imaginary time path integral approach, which is based on the Kubo formula for the conductivity, and a saddle point approximation to perform the analytic continuation. The new method is first tested using a benchmark calculation from the numerical exact hierarchical equations of motion method. Imaginary time path integral Monte Carlo simulations are then performed to explore the temperature dependence of charge carrier delocalization and mobility in organic molecular crystals (OMCs) within the Holstein and Holstein-Peierls models. The effects of nonlocal electron-phonon interaction on mobility in different charge transport regimes are also investigated.

Comparison of the carrier mobility, unipolar conduction, and light emitting characteristics of phosphorescent host–dopant system

We have investigated the charge carrier transport mobility and hole-only current behavior of phosphorescent host–dopant mixture, and evaluated the efficiency and lifetime behavior of organic light emitting device (OLED) containing the corresponding light emitting host–dopant system. The carrier drift mobilities of the phosphorescent host, 4,4′-N,N′-dicarbazole-biphenyl (CBP), doped with green-emitting fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) or bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetyl-acetonate) (Btp2Ir(acac)) red-emitting dopants, were directly investigated with time of flight (TOF) photoconductivity method. The resolved electron mobility of phosphorescent host–dopant layer by TOF-PC method showed the significant reduction at CBP:(Btp2Ir(acac)) layer, which can be explained by the electron trapping. Measured hole-only current data also shows the reduction, which is more significant at CBP:(Btp2Ir(acac)) as expected from larger energy level offset. The efficiency, spectral emitting properties, and device stability of phosphorescent OLEDs with identical host–dopant layer were evaluated. Compared with CBP:(Btp2Ir(acac)), device with CBP:Ir(ppy)3 emitter shows the spectral response and half-lifetime less dependent upon the hole/exciton blocking layer and its thickness. Such device data was well-correlated with probed TOF and hole current behavior.

Synthesis and Mesophases Study on 2-(8-bromooctanoic acid ester)-3,6,7,10,11-penta-pentyloxytriphenylene

A novel triphenylene-based discotic liquid crystal compound 2-(8-bromooctanoic acid ester)-3,6,7,10,11-penta-pentyloxytriphenylene was synthesized in this study.The chemical structure of the compound was characterized by1HNMR spectra, FT-IR spectra and we confirmed its structure. The mesophases were investigated by using different scanning calorimetry DSC and polarizing optical microscopy POM, the results showed that the target product had Liquid crystal properties and the temperature range of 90-101 °C, and showed a clear columnar phase. The carrier transport mobility was measured using time-of-flight TOF method, μ=1.09*10-2cm2V-1s-1.

Organic Photovoltaic Devices Based on an Acceptor of Solution-Processable Functionalized Graphene

We prepared the exfoliation of graphite, which was necessary for the production of graphene sheets that are desirable for the fabrication of nano-composites. Then a Solution-Processable Functionalized Graphene (SPFGraphene) with functionalization groups doped with P3HT hybrid thin film-based organic photovoltaic cells (OPVCs) was systematically identified using a general device structure of, ITO/PEDOT:PSS/P3HT:SPFGraphene/LiF/Al. The effect of annealing on the photoelectric properties of the SPFGraphene was analyzed by Fourier transform infrared FT-IR spectroscopy and solar cell performance. After treatment at different annealing temperatures, with an increase in the SPFGraphene content, the short-circuit current density J(SC) and power conversion efficiency PCE of the hybrid devices increased first, reaching the peak efficiency for the 10 wt% SPFGraphene content, and then decreased. After annealing at 160 degrees C, the device containing 10 wt% SPFGraphene showed the open-circuit voltage V(OC) of 0.73 V, the J(SC) value of 3.98 mA cm(-2), fill factor (FF) value of 0.36, the PCE value of 1.046%. After thermal annealing at 210 degrees C, with the removal of the functional groups and recovery of the pi-conjugated areas, the conductivity of the graphene sheet and the charge carrier-transport mobility increased greatly, the J(SC) value of the 10 wt% SPFGraphene content device increased to 4.2 mA cm(-2), the V(OC) value decreased to 0.59 V, which may be attributed to the altered work-function value of the functionalized graphene and low quasi-Fermi levels for electrons and holes, the FF value was 0.27, and the PCE was 0.669%, which is lower than the former one. The results indicated that annealing at the appropriate temperature can improve the device performance greatly, and the functionalized graphene is expected to be a competitive candidate in organic photovoltaic applications because it is soluble, cheap, easily prepared, stable, and inert against the ambient conditions.

Enhanced photocurrent and efficiency of poly(3-hexylthiophene)/fullerene photovoltaic devices by the incorporation of gold nanoparticles

We have investigated the role of gold nanoparticle incorporated in the bulk-heterojunction active layer of poly(3-hexylthiophene) (P3HT)/fullerene. Since the performance of this organic photovoltaic device is strongly related to the charge carrier transport mobility of bulk heterojunction that arises from the molecular ordering of P3HT semiconductor, we have set the range of nanoparticle concentration to the infinitesimal level, which does not affect the thermally-induced ordering and conductivity of P3HT/fullerene composite film. While the increase of power conversion efficiency is mainly attributed to the enhancement of photocurrent, both the fill factor and the open circuit voltage of photovoltaic device were not sensitive to the concentration of gold nanoparticles, which does not coincide with the previous report on the case of poly(3-octylthiophene)/fullerene/nanoparticle mixture. We suggest that an enhancement of the photoinduced short-circuit current and efficiency can be explained by the increased absorption of nanoparticle–polymer composite film with the condition of the consistent molecular ordering of donor-acceptor bulk-heterojunction. At the weight fraction of 6.25 × 10 −8 for gold nanoparticle/P3HT, about 50% of enhancement ( i.e. from 1.43 to 2.17%) was obtained in the power efficiency.

Carrier Transport Behaviour of Molecular DopedPoly(N-Vinylcarbozole) in Polymer Light-Emitting Diodes

Single-layer polymer light-emitting diodes areprepared from blends of poly(N-vinylcarbozole) (PVK) doped withtris(8-hydroxy-quinoline) aluminium (Alq3) of 2 wt% (sample a)and 0.2 wt% (sample b). The onset of PVK transientelectroluminescence (EL) is delayed with respect to that of Alq3in sample a under pulsed excitation, while the EL onsets of Alq3 andPVK in sample b are simultaneous. The total carrier mobility of theAlq3-rich regions in sample a is larger than that of the PVK-richregions. However, the total carrier mobility is homogeneous in sample b.The phase image of atomic force microscopy and photoluminescencespectra of samples a and b indicate that the separated phase ofsamples a and b exists in the PVK-rich and Alq3-richregions. The variance of the doping concentration and separated phasein blends results in the different carrier transport mobility ofAlq3-rich and PVK-rich regions.