Field-effect Solar Cells Research Articles

First-principles investigations of the controllable electronic properties and contact types of type II MoTe2/MoS2 van der Waals heterostructures.

Two-dimensional (2D) van der Waals (vdW) heterostructures are considered as promising candidates for realizing multifunctional applications, including photodetectors, field effect transistors and solar cells. In this work, we performed first-principles calculations to design a 2D vdW MoTe2/MoS2 heterostructure and investigate its electronic properties, contact types and the impact of an electric field and in-plane biaxial strain. We find that the MoTe2/MoS2 heterostructure is predicted to be structurally, thermally and mechanically stable. It is obvious that the weak vdW interactions are mainly dominated at the interface of the MoTe2/MoS2 heterostructure and thus it can be synthesized in recent experiments by the transfer method or chemical vapor deposition. The construction of the vdW MoTe2/MoS2 heterostructure forms a staggered type II band alignment, effectively separating the electrons and holes at the interface and thereby extending the carrier lifetime. Interestingly, the electronic properties and contact types of the type II vdW MoTe2/MoS2 heterostructure can be tailored under the application of external conditions, including an electric field and in-plane biaxial strain. The semiconductor-semimetal-metal transition and type II-type I conversion can be achieved in the vdW MoTe2/MoS2 heterostructure. Our findings underscore the potential of the vdW MoTe2/MoS2 heterostructure for the design and fabrication of multifunctional applications, including electronics and optoelectronics.

Impact of Connectivity on the Electronic Structure of N-Heterotriangulenes

N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer, both N-HTAs adopt a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) are assigned. The optical gap (S0 → S1 transition) is found to be 3.4 eV for N-HTA 550 and 2.5 eV for N-HTA 557. Thus, the introduction of the −C═C– double bond in N-HTA 557 resulted in a pronounced decrease of the optical gap size by 0.9 eV due to the larger π-conjugated electron system compared to N-HTA 550. Structural variations or substitution patterns in N-HTAs foster the opportunity for tailoring their electronic properties.

Low temperature CVD growth of WSe2 enabled by moisture-assisted defects in the precursor powder

Two-dimensional transition metal dichalcogenides (TMDs) have been proposed for a wide variety of applications, such as neuromorphic computing, flexible field effect transistors, photonics, and solar cells, among others. However, for most of these applications to be feasible, it is necessary to integrate these materials with the current existing silicon technology. Although chemical vapor deposition is a promising method for the growth of high-quality and large-area TMD crystals, the high temperatures necessary for the growth make this technique incompatible with the processes used in the semiconductor industry. Herein, we demonstrate the possibility of low-temperature growth of TMDs, using tungsten selenide (WSe2) as a model, by simply using moisture-assisted defective tungsten oxide (WO3) precursor powders during the growth of these materials. Density functional theory calculations reveal the mechanism by which moisture promotes the defect formation on the precursor crystal structure and how it dictates the reduction of the temperature of the growth. The results were compared with the standard growth at high temperatures and with a precursor mixture with alkali salts to show the high quality of the WSe2 grown at temperatures as low as 550 °C. To conclude, the work improves the understanding of nucleation and growth mechanisms of WSe2 at low temperatures and provides a useful strategy for the growth of TMDs at temperatures required for the back-end-of-line compatibility with current silicon technology.

Cation Effect on Anion Exchange in CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals

Colloidal quantum dot and nanocrystals have been shown to be promising materials for device applications because of theirunique size-dependent properties and solution processability. We present general methods to engineer theelectronic, optoelectronic, electromechanical, and electrothermal properties of nanocrystals through theirsurface and interface engineering. Unconventional materials design and strategy are introduced tofabricate semiconducting devices such as field effect transistors, photodetectors, solar cells, and wearablesensors including temperature sensors, strain sensors, and pressure sensors. This technique provides apathway for designing low cost, high performance, and multifunctional electronic skins or wearabledevices.

Electronic Properties of 6,13-Diazapentacene Adsorbed on Au(111): A Quantitative Determination of Transport, Singlet and Triplet States, and Electronic Spectra

The electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based field effect transistors and solar cells. Here, we investigated the ad...

Screen Engineered Field Effect Cu₂O Based Solar Cells

We demonstrate cuprous oxide (Cu2O) based screen engineered field effect solar cells with a record breaking efficiency, exceeding 3.486%, for Cu2O based p-n homojunction. In this architecture, CuO nanowire interphase is successfully employed in the Cu2O fabrication by effectively serving as a simultaneous ohmic current collector. These screen engineered field effect photovoltaic principles are essential in developing promising photovoltaics architectures for hard-to-dope materials that, in principle, enable extremely low-cost, high efficiency solar cells.

Gate-Leakage Current Mechanisms in Silicon Field-Effect Solar Cells

Field-effect solar cells (FESCs) are increasingly attractive for 2-D heterojunction solar cells and especially for gate-tunable Schottky-junction solar cells. A prerequisite for applying FESCs is that the gate-leakage (GL) power must be far less than the output power. However, the conduction mechanism of the GL current in a FESC has yet to be described clearly, thereby leading to excessive gate power consumption. In this work, the GL current mechanisms for a silicon FESC are extracted and analyzed using temperature-dependent current-voltage measurements under dark and illuminated conditions. It shows that the GL current for a FESC in the dark is dominated by both temperature-dependent Poole-Frenkel (P-F) emission and hopping conduction, whereas, under illumination changes to the temperature-independent space-charge-limited (SCL) current. Furthermore, whether the incident light power of 62.5, 100, or 125 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is used, or the gate dielectric of SiO2 is replaced by HfO2, the dominant GL current mechanism is consistent. Finally, the trap energy level is approximately calculated as 0.38-0.72 eV based on a P-F emission analysis; the mean trap spacing of 0.01-0.18 nm and the activation energy of approximately 0.02 and 0.05 eV are extracted by a hopping conduction analysis. Meanwhile, a rigorous SCL conduction analysis provides a carrier mobility of 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in SiO2 and 5.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in HfO2.

(Invited) Engineering the Structure and Properties of Nanocrystals to Design Electronic Devices and Sensors

Colloidal nanocrystals have been shown to be promising materials for device applications because of their unique size-dependent properties and solution processability. We present general methods to engineer the electronic, optoelectronic, electromechanical, and electrothermal properties of nanocrystals through their surface and interface engineering. Unconventional materials design and strategy are introduced to fabricate semiconducting devices such as field effect transistors, photodetectors, solar cells, and wearable sensors including temperature sensors, strain sensors, and pressure sensors. This technique provides a pathway for designing low cost, high performance, and multifunctional electronic skins or wearable devices.

Tunable self-organization in n-type liquid crystalline dibenzocoronene tetracarboxdiimides for high photoconductivity

ABSTRACT A comprehensive study on the self-organised mesomorphism and charge-transport properties of three n-type liquid crystalline (LC) dibenzocoronene tetracarboxdiimide derivatives (Dibenzo-CDI 1, 2, 3) bearing branched alkyl chains with varied side-chain length and branching point (α or β site) has been carried out. All three Dibenzo-CDI compounds possessed high solubility in common organic solvents and low-lying lowest unoccupied molecular orbital energy levels below −4.0 eV indicating their air-stable electron-transport capacity under ambient conditions. It is discovered that the side-chain length has a moderate influence on the molecular arrangement and thus leads to only limited variation of electron-transport properties. In contrast, the branching position plays a critical role on tuning molecular packing and orientation in the bulk mesophases, thereby resulting in dramatic differences in electron mobilities. Remarkably, Dibenzo-CDI 3 demonstrates good photoconductivity by time-of-flight method with high electron mobility up to 0.075 cm2/Vs, close to the highest value reported in certain n-type LC derivatives. All these results reveal that LC Dibenzo-CDI derivatives are potential candidates for high-performance solution-processable electronics, such as air-stable n-type organic field-effect transistors (OFETs) and solar cells.

Photoconductive Properties of Dibenzotetrathiafulvalene-Tetracyanoquinodimethane (DBTTF-TCNQ) Nanorods Prepared by the Reprecipitation Method.

Charge-transfer complex crystals have been extensively studied because of their metallic conductivity, photoconductivity, ambipolar charge transport, and high career mobility. Numerous studies of their applications for organic electric devices such as organic field effect transistors and solar cells have reported. However, bulky single crystals of charge-transfer complexes are difficult to handle, specifically to be made into a form of a thin film. Recently, nano/micro crystallization of charge-transfer crystal is attracted to realize thin film applications. In this paper, charge transfer complex nanorods composed of dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) were prepared by the reprecipitation method. The as-formed nanorods possess a kinetically metastable crystal structure different from the thermodynamically stable bulk crystal prepared by slow evaporation of the solvent. From photoconductive measurement, nanorod stacks show a significant photosensitivity (354.57 μA/W) on par with bulk crystal (417.14 μA/W). These results suggest dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) nanorods have a favorable crystal structure for carrier transport due to the difference of molecular stacking assembly.

Synthesis of low band-gap 2D conjugated polymers and their application for organic field effect transistors and solar cells

Two donor-acceptor (D-A) 2-dimensional (2D) conjugated polymers P1 and P2 based on isoindigo (ID) and thienoisoindigo (TID) as acceptor unit and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b´]dithiophene (BDT-T) as donor unit were prepared via Stille coupling and characterized by solution-possessed organic field effect transistors (OFETs) and organic solar cells (OSCs), respectively. By changing the electron-withdrawing component of the conjugated polymer backbone from ID to TID, there have also been diversities in the optical absorption, thermal stability, molecular structure, electrochemical energy level, charge mobility and photovoltaic properties of these two polymers. P1 and P2 exhibited intrinsic p-type semiconductor characteristic with hole mobilities of 3.0 × 10−2 and 1.2 × 10−2 cm2 V−1 s−1, respectively. When blended with the [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), the highest power conversion efficiency (PCE) of P1 and P2 were 2.40% and 1.28%, respectively. Our results suggest that ID and TID units are useful building blocks for the further development of efficient organic optical-electrical materials.

Physico-chemical investigation of ZnS thin-film deposited from ligand-free nanocrystals synthesized by non-hydrolytic thio-sol–gel

Ultra-small and monodispersed zinc sulfide nanocrystals (NCs) (d ≤ 3 nm) have been prepared without the use of any surfactants by a synthetic route using benzyl mercaptan as a source of sulfur. The prepared NCs are dispersible in highly polar solvents and display the capability to closely pack-up in a bulky film. The NCs were characterized by TEM, XRD and UV–vis optical absorption as well as by steady-state and time-resolved photoluminescence (PL) spectroscopies. Uniform films of ZnS were spin-coated on glass and ITO-glass substrates using a NCs dispersion in N,N-dimethylformamide. The NCs and the resulting films were characterized by morphological and optoelectronic probing techniques such as AFM, SEM, diffuse reflectance, PL and photoelectron spectroscopy in air. These physical investigations confirmed that the chalcogenide NCs grown by this method have the potential to be utilized directly as photocatalysts and are potentially useful building-blocks/starting materials for the fabrication of semiconductor thin films for optoelectronic applications such as LED, luminescent screens, field effect transistor and solar cells. Insights on the chemistry involved in the NCs growth have been provided revealing that their formation proceeds through a mechanism involving a thioether elimination reaction.

Charge injection and hopping transport in bridged-dithiophene-triazole-bridged-dithiophene (DT-Tr-DT) conducting oligomers: A DFT approach

Here, we have reported a series of exciting conducting organic heterocyclic oligomers possessing the cyclopentadithiophene-4-thione-triazole-cyclopentadithiophene-4-thione (DT-Tr-DT) as basic unit based on the donor-acceptor (D-A) approach. The basic unit is substituted with different electron-withdrawing and donating groups to tune its structural and electronic properties. Density functional theory (DFT) and time-dependent DFT methodologies were utilized to investigate the ground and excited state calculation. Here, our main focus is on the HOMO-LUMO gaps, electronic transition, charge transfer rate and the reorganization energies (λ). Our study reveals that the designed oligomers under investigation exhibit excellent charge transfer rates. Furthermore, it has also been reported that oligomers substituted with the electron-donating groups exhibit lower ΔH-L values compared to the electron-withdrawing groups. Therefore, oligomers substituted with electron-donating moieties will become potential candidates for fabrication of organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), photovoltaic cells and solar cells.

Dihedral angle control to improve the charge transport properties of conjugated polymers in organic field effect transistors

Diketopyrrolopyrrole (DPP) and i-Indigo (i-Ind) are two monomers that are widely explored as active materials in organic field effect transistor and solar cells. These two molecules showed impressive charge carrier mobility due to better packing that are facilitated by quadrupoles. We hypothesized that the copolymers of these monomers would also exhibit high charge carrier mobility. However, we envisioned that the dihedral angle at the connecting point between the monomers will play a crucial role in packing as well as charge transport. To understand the impact of dihedral angle on charge transport, we synthesized three copolymers, wherein the DPP was sandwiched between benzenes, thiophenes and furans. The copolymer of i-Indigo and furan comprising DPP showed a band gap of 1.4 eV with a very high dihedral angle of 179°. The polymer was found to pack better and the coherence length was found to be 112 Å. The hole carrier mobility of these polymer was found to be highest among the synthesized polymer i.e. 0.01 cm2/vs. The copolymer comprising benzene did not transport hole and electrons. The dihedral angle at the connecting point between i and Indigo and benzene DPP was 143 Å, which the packing and consequently charge transport properties.

Self-Assembled α-Cyanostilbenes for Advanced Functional Materials.

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [ArCHC(CN)Ar] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

The role of chain conformation in energy transfer properties of single conjugated polymer molecule

Study of the relationship between conformation and photophysics of individual -conjugated polymer chain is one of the most important problems in polymer nanoscience and nanotechnology, which will facilitate the application of conjugated polymer in a range of electronic devices such as organic field-effect transistors, light-emitting diodes, and solar cells. Single-molecule spectroscopy has emerged as a powerful tool to unravel structure and dynamic heterogeneities that are hidden in ensemble average. Identification of the emitting segments through fluorescence of single conjugated polymer molecules and their dependence on the conformation can help reveal the mechanism and the extent of energy transfer process in a single polymer chain. In this paper, the photophysical properties of individual poly[2, 7-(9, 9-dioctylfluorene)-alt-4, 7-bis(thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) conjugated polymer molecules are measured based on the defocused wide-field microscopy of single molecules. The single PFO-DBT molecules are prepared on cleaned glass coverslips by spin-coating solution of poly[methyl methacrylate] (PMMA) containing 110-9 mol/L PFO-DBT molecules in chloroform and toluene, respectively. Defocused imaging of single conjugated polymer molecule is performed based on a wide-field fluorescence microscope system. The change of defocused patterns of individual polymer chain maps the angular distribution of emitted chromophore and thus the emitting dipole orientation. Fluorescence trajectory and corresponding emission dipole moments of single conjugated polymer molecules are analyzed to identify the emitting conjugated segments. It is found that single PFO-DBT conjugated polymer molecules prepared by chloroform solvent show extended conformation. The intrachain energy transfer is dominant in the single conjugated polymer molecules that take extended conformation, which leads to photophysical properties of multiple chromophores. In contrast, single PFO-DBT conjugated polymer molecules prepared by toluene solvent hold folded conformation, which exhibit emission from single chromophore due to efficient interchain energy transfer. The emitting chromophore is not constant in a single PFO-DBT conjugated polymer molecule with folded conformation. About 35% of the single conjugated molecules prepared with toluene show only one constant emitting chromophore before photobleaching. However, about 65% of single conjugated polymer molecules prepared with toluene show two or more sequencely emitting chromophores. It can be concluded that the energy transfer properties of single PFO-DBT conjugated polymer molecule is greatly dependent on the conformation, which can be reflected in its photophysical properties. The study on the influence of single conjugated polymer conformation on energy transfer efficiency can provide the reference for the preparation and performance of optoelectronic devices and molecular devices based on conjugated polymer.

Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures

Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n‑ or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Ω sq−1 to 1260 Ω sq−1 for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm2 V−1 s−1 indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si:H is a viable route for large scale graphene based solar cells or display applications.

ChemInform Abstract: Polythiophene and Oligothiophene Systems Modified by TTF Electroactive Units for Organic Electronics

AbstractReview: [87 refs.

ChemInform Abstract: Synthesis and Reactions of Halo‐Substituted Alkylthiophenes. A Review

AbstractReview: 91 refs.

Synthesis and Reactions of Halo-substituted Alkylthiophenes. A Review

Thiophene chemistry is a flourishing area of organic semiconductors as they are useful for field effect transistors, solar cells and light-emitting diodes.1 This interest stems from the unique opti...