High Hole Mobility Research Articles

Removal of secondary phases and its effect on the transport behavior of Cu2ZnSn1-xGexS4 kesterite nanoparticles

Hole-transporting materials are very important for achieving high efficiency in perovskite solar cells. Thin films made of Cu2ZnSn1-xGexS4 have attracted the attention of researchers due to their potential as hole-transporting layers. However, defects and impurities define their effectiveness in solar cells. In this work, we present a facile wet chemical approach to remove the ZnS and Cu8GeS6 impurities from small (∼11 nm) Cu2ZnSn1-xGexS4 (x = 0.0 to 0.7) nanoparticles synthesized by a hydrothermal process. The wet chemical process converted the nanostructures Cu-rich Zn-poor from their initial Zn-rich Cu-poor stoichiometry. Moreover, the bandgap energies of the nanostructures were reduced by about 0.1 eV after chemical treatment due to change the oxidation state of Cu from Cu1+ to Cu2+. The kesterite films prepared by spin coating of chemically treated nanoparticle inks revealed enhanced electrical conductivity and hole concentration in comparison to the films prepared using untreated nanoparticle inks. While the highest hole concentration of about 6.52x1018 cm−3 was obtained for the films made of Cu2ZnSn1-xGexS4 nanoparticles with the highest x value (x = 0.7), the highest hole mobility (18.9 cm2V−1s−1) was achieved for x = 0.3. The effects of secondary phase elimination on carrier concentration and mobility variation have been discussed.

Orientation and mobility control of 4HCB organic film for flexible X-ray detectors with high performance

• The space-confined melt process is used to produce highly orientated organic semiconducting crystalline films with a large area. • Confined molecular packing significantly improves the hole mobility. • The 4HCB based X-ray detectors can achieve a high sensitivity with low dark current, result in outstanding direct imaging capabilities. • The device shows steady flexibility, with no degradation in detecting function after 100 cycles of bending. In comparison to inorganic counterparts, organic semiconducting (OSC) crystalline films are promising for building large-area and flexible ionizing radiation detectors for X-ray imaging or dosimetry due to their tissue equivalence, simple processing and large-scale production accessibility. Fabrication processes, however, hinder the ability to generate aligned and large-area films with high carrier mobility. In this work, the space-confined melt process is used to produce highly orientated 4HCB (4-hydroxycyanobenzene) OSC films with a large area of 15 × 18 mm 2 . The out-of-plane direction of the 4HCB film is <001>, and the benzene rings are found to be extensively overlapped inside the in-plane direction, according to the XRD patterns. The film exhibits a high resistivity up to 10 12 Ω cm, and high hole mobility of 10.62 cm 2 V −1 s −1 . Furthermore, the 4HCB (80-μm-thick film) based X-ray detectors can achieve a sensitivity of 93 μC Gyair −1 cm −2 and on/off ratio of 157. The device also shows steady flexibility, with no degradation in detecting function after 100 cycles of bending. Finally, the proposed 4HCB film detectors demonstrated a high-resolution X-ray imaging capability. The imaging of several materials with sharp edges (copper and polytetrafluoroethylene) has been obtained. This work has developed a fast but efficient approach for producing large-area, highly oriented OSC films for high-performance X-ray detectors.

High Fill Factor and Reduced Hysteresis Perovskite Solar Cells Using Small-Molecule-Engineered Nickel Oxide as the Hole Transport Layer

Nickel oxide (NiOx) is an emerging hole transport layer (HTL) material in halide perovskite solar cells (PSCs), combining high hole mobility, transparency, and stability. Current limitations of the device performance are mainly related to the inefficient hole extraction caused by contact problems between NiOx and the perovskite layer. Based on its expected strong interaction with both the NiOx surface and the perovskite layer, we selected 4-dimethylaminopyridine (DMAP) as a molecular passivation agent for the HTL. Photoelectron spectroscopy and photophysical studies demonstrate that DMAP passivation creates a more favorable band alignment at the NiOx/perovskite interface. This leads to decreased carrier recombination near the interface and enhanced hole transfer. In addition, X-ray diffraction reveals reduced strain, improved crystalline quality, and a redistribution of excess PbI2 in perovskite layers grown on DMAP-passivated NiOx. As a consequence, PSCs with the DMAP-modified HTL exhibit a strongly increased fill factor and power conversion efficiency with values close to 80 and 18%, respectively. Moreover, they show negligible hysteresis and enhanced environmental stability compared to devices with untreated HTLs.

Is Cu3-x P a Semiconductor, a Metal, or a Semimetal?

Despite the recent surge in interest in Cu3-x P for catalysis, batteries, and plasmonics, the electronic nature of Cu3-x P remains unclear. Some studies have shown evidence of semiconducting behavior, whereas others have argued that Cu3-x P is a metallic compound. Here, we attempt to resolve this dilemma on the basis of combinatorial thin-film experiments, electronic structure calculations, and semiclassical Boltzmann transport theory. We find strong evidence that stoichiometric, defect-free Cu3P is an intrinsic semimetal, i.e., a material with a small overlap between the valence and the conduction band. On the other hand, experimentally realizable Cu3-x P films are always p-type semimetals natively doped by copper vacancies regardless of x. It is not implausible that Cu3-x P samples with very small characteristic sizes (such as small nanoparticles) are semiconductors due to quantum confinement effects that result in the opening of a band gap. We observe high hole mobilities (276 cm2/(V s)) in Cu3-x P films at low temperatures, pointing to low ionized impurity scattering rates in spite of a high doping density. We report an optical effect equivalent to the Burstein-Moss shift, and we assign an infrared absorption peak to bulk interband transitions rather than to a surface plasmon resonance. From a materials processing perspective, this study demonstrates the suitability of reactive sputter deposition for detailed high-throughput studies of emerging metal phosphides.

Side‐Chain Methylthio‐Based Position Isomerism of Hole‐Transport Materials for Perovskite Solar Cells: From Theoretical Simulation to Experimental Characterization

AbstractHole transporting materials (HTMs) are imperative for promoting the development of perovskite solar cells (PSCs). Herein, three isomers of RQ4, RQ5, and RQ6 are constructed by methylthio (‐SMe) group in the para, meta, and adjacent sites of terminal benzene on the side‐chain of carbazole‐arylamine derivatives based HTMs, and investigated by means of the theoretical simulation and experimental characterization. As a result of the theoretical simulation, the isomeric HTMs of RQ4‐RQ6 exhibit appropriate highest occupied molecular orbital /lowest unoccupied molecular orbital energy levels and good optical properties. However, by comparison with RQ4 and RQ5, a better planar configuration and closer molecular stacking for RQ6 may be beneficial to promote the hole coupling, interface interaction, and charge transfer at perovskite/HTMs interface. In order to verify the accuracy of the theoretical model, the designed RQ4‐RQ6 are synthesized to be used to assemble PSCs devices. In comparison to isomers RQ4 (20.07%) and RQ5 (18.18%) based devices, the RQ6 based devices has higher power conversion efficiency of 21.03% because of its high hole mobility, the film formation ability, and large charge transfer at perovskite/HTMs interface. The experimental results confirm the reliability of the theoretical simulation and provide an effective strategy to obtain potential HTMs through isomerization of side‐chain functional groups.

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors.

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba1.01 AgSb shows a high room-temperature hole mobility of 344 cm2 V-1 S-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm2 TiO2-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO2/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10-3 cm2 V-1 S-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Cation exchange strategy to construct nanopatterned Zn:NiOx electrode with highly conductive interface for efficient inverted perovskite solar cells

Inorganic hole-transporting materials (HTMs) with excellent hole extraction and transfer properties are highly desirable for inverted perovskite solar cells (PSCs). However, the widely employed oxide HTMs (e.g., NiOx nanocrystal) often suffer from the low p-type conductivity and severe trap-assisted recombination at HTMs/perovskite interface, which significantly limits the final performance of PSCs. Herein, the facile template-assisted cation exchange strategy is developed to fabricate nanopatterned Zn:NiOx as efficient HTMs for PSCs, for the first time. Promisingly, by virtue of the advantageous 1D nanoscale architecture and synergistic substitutional Zn doping, high conductivity, high hole mobility and low interfacial charge trap states density have been achieved by such nanopatterned Zn:NiOx HTMs. These features endow the highly conductive interface for PSCs to accelerate hole extraction and minimize recombination loss. As a consequence, PSCs with nanopatterned Zn:NiOx deliver a champion efficiency of ≈20 % with open-circuit voltage of as high as 1.14 V. This work demonstrates and paves a new insight and avenue for the rational design of efficient oxide HTMs.

Surface doping of rubrene single crystals by molecular electron donors and acceptors.

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

Interface engineering strategy for multisource spintronic devices via TMPS4 modulation of black-phosphorus.

Interface engineering is an effective strategy for significantly providing performance improvements in logic operations and information storage techniques, given the quantum effects characteristic of pristine two-dimensional ferromagnetic (FM) materials. Furthermore, the van der Waals (vdW) heterostructures enable a more efficient design of logic components. Herein, two novel designs of van der Waals heterostructures are proposed, black-phosphorus (BP)/TMPS4(TM = Cr, Fe-Mn), where the CrPS4 relies on odd and even layers for FM state - antiferromagnetic (AFM) state alternation, which is used to modulate BP with high hole mobility but no magnetism. Based on the first principles, the simulation results show that the two heterojunctions exhibit high stability, carrier mobility, and thermoelectric effects, of which the BP/CrPS4 heterojunction is specifically modulated to a type II electronic bandstructure. These two structures are applied in multi-source logic devices. The device performances show that the devices have spin Sebeck effect (SSE), perfect spin filtering effect (SFE), high extinction ratio (1347), and high thermal magnetoresistivity (1011). The above results suggest BP/TMPS4 bilayers as promising candidates for spin-based vdW devices and facilitate the future development of atomically thin magnetic information storage.

Ferromagnetic and half-metallic phase transition by doping in a one-dimensional narrow-bandgap W6PCl17 semiconductor.

Based on first-principles calculations, we predict a one-dimensional (1D) semiconductor with cluster-type structure, namely phosphorus-centered tungsten chloride W6PCl17. The corresponding single-chain system can be prepared from its bulk counterpart by an exfoliation method and it exhibits good thermal and dynamical stability. 1D single-chain W6PCl17 is a narrow direct semiconductor with a bandgap of 0.58 eV. The unique electronic structure endows single-chain W6PCl17 with the p-type transport characteristic, manifested as a large hole mobility of 801.53 cm2 V-1 s-1. Remarkably, our calculations show that electron doping can easily induce itinerant ferromagnetism in single-chain W6PCl17 due to the extremely flat band feature near the Fermi level. Such ferromagnetic phase transition expectedly occurs at an experimentally achievable doping concentration. Importantly, a saturated magnetic moment of 1μB per electron is obtained over a large range of doping concentrations (from 0.02 to 5 electrons per formula unit), accompanied by the stable existence of half-metallic characteristics. A detailed analysis of the doping electronic structures indicates that the doping magnetism is mainly contributed by the d orbitals of partial W atoms. Our findings demonstrate that single-chain W6PCl17 is a typical 1D electronic and spintronic material expected to be synthesized experimentally in the future.

Large magnetoresistance in phosphorus-sulfur compounds (TMPS4) based temperature regulated spin-caloritronic devices

Two-dimensional (2D) transition metal phosphorus-sulfur compounds (TMPS 4 ) have attracted great theoretical and experimental attention because of their broadband modulation and high hole mobility that can be used in nanodevices. Therefore, their Curie temperature and thermal properties deserve further study for applications in electronic devices. The electromagnetic properties and spin transport properties of 2D transition metal phosphorus-sulfur compounds are systematically investigated by density functional theory (DFT) and DFT combined with the nonequilibrium Green's function method. We find that TMPS 4 materials cover almost all spin electronic behaviors, including half-metals, magnetic and nonmagnetic semiconductors, and metals. The simulation results show that the magnetic tunnel junction constructed based on TMPS 4 has a perfect thermal spin filtering effect and a negative differential resistance effect. Especially, we obtain up to 10 10 % thermal colossal magnetoresistance and almost 100% spin polarization, as well as a high current on/off ratio of ∼10 7 . Based on these results, it is proved that TMPS 4 can be a substrate material for the development of high performance for spin-caloritronic and spintronic devices.

Tris(pentafluorophenyl)borane–water complex doped Spiro-TTB for high-efficiency and stable perovskite solar cells

An Brønsted acid of tris(pentafluorophenyl)borane–H2O complexes as a p-type dopant is used for doping Spiro-TTB, with a high hole mobility (1.22 × 10−2 cm2 V−1 s−1), so that the device presents high photovoltaic efficiency and stable operation.

Graphene quantum dots (GQD) and edge-functionalized GQDs as hole transport materials in perovskite solar cells for producing renewable energy: a DFT and TD-DFT study.

This study investigated the potential suitability of graphene quantum dots (GQD) and certain edge-functionalized GQDs (GQD-3Xs) as hole transport materials (HTMs) in perovskite solar cells (PSCs). The criteria for appropriate HTMs were evaluated, including solubility, hole mobility, light harvesting efficiency (LHE), exciton binding energy (Eb), hole reorganization energy (λh), hole mobility, and HTM performance. It was found that several of the compounds had higher hole mobility than Spiro-OMeTAD, a commonly used HTM in PSCs. The open circuit voltage and fill factor of the suitable GQD and GQD-3Xs were found to be within appropriate ranges for HTM performance in MAPbI3 PSCs. GQD-COOH and GQD-COOCH3 were identified as the most suitable HTMs due to their high solubility, small λh, and appropriate performance.

Monolayer Ge2Te2P4 as a promising photocatalyst for solar driven water-splitting: a DFT study.

The buckling hexagonal structure of Ge2Te2P4 was studied by first-principles calculations. The newly proposed structure was proven to be stable by analyzing its cohesive energy, phonon dispersion, elastic constants and AIMD results. Poisson's ratio of the Ge2Te2P4 monolayer is in the range 0.16-0.18, and Young's modulus is in the range 40.16-43.74 N m-1. The substituted Te atoms enhance the sp2 orbitals which strengthen the σ-bonds and therefore the thickness of the Ge2Te2P4 monolayer is smaller than that of monolayer GeP3. The Ge2Te2P4 monolayer has an indirect band gap of 1.85 eV, which can be narrowed by strains. The compressive band gaps from -2% to -4% change the electronic structure from the indirect band gap into the direct band gap. Strains can also increase the light absorption rate α(ω) in the visible region, which is 2-3 × 105 cm-1 at equilibrium. The Ge2Te2P4 monolayer has a suitable band gap and an appropriate VBM and CBM position for hydrogen generation. Under strain rate of 4% and higher, the VBM and CBM remain at suitable positions for hydrogen production. Another advantage of the Ge2Te2P4 monolayer is that its charge carrier mobilities are really high. The highest electron mobility is 1301.47 cm2 V-1 s-1, and the highest hole mobility is 28627.24 cm2 V-1 s-1, which are much higher than the mobility in monolayer GeP3. The Ge2Te2P4 monolayer has advantages for photocatalytic applications and it is necessary to perform further study on the material.

Isoquinoline-1,3-dione-derived conjugated polymers for field-effect transistors: synthesis, properties, and the effect of inner aromatic bridges

A series of novel isoquinoline-1,3-dione (IQD)-derived conjugated polymers were designed and synthesized. They showed promising high hole mobilities up to 1.08 cm2 V−1 s−1, which are among the highest in the IQD-based polymers reported so far.

Electronic level modelling of graphene-borophene lateral heterostructures as anodes in Li-ion batteries

Covalent stitching of dissimilar semi-infinite two-dimensional (2D) nanosheets as the lateral heterostructures is relatively rare due to the various challenges involved in the experimental synthesis. In this study, we have integrated the graphene and borophene monolayers and created a series of lateral heterostructures inspired by their recent experimental synthesis. Further, we have systematically explored the geometrical, thermal, mechanical properties and electronic structure of these lateral heterostructures using the density functional theory calculations. Results authenticate that graphene and borophene lateral heterostructures (GBLHs) are dynamically and thermally stable. Further, they exhibit anisotropic mechanical properties including the negative Poisson's ratio. Electronic structure calculations reveal that the GBLHs can be classified as either semi-metals or metals and the conductivity depending on the width of the graphene and borophene chains comprises the heterostructure. Further, massless Dirac fermions and anisotropic high hole and electron mobilities are the prominent electronic features of the semi-metallic GBLHs. Finally, we have investigated the application potentials of GBLHs as the anode materials for lithium-ion rechargeable batteries (LIBs). Computed results illustrate that the GBLHs can serve as potential anode materials in the LIBs with intriguing features like metallicity, optimal adsorption energies, low diffusion barrier, enhanced specific storage capacity (1394.5 mA h g−1) and an average open circuit voltage (∼0.52 eV).

Influence of the cooling rate in annealing post-treatment on small molecule with siloxane side chains for solution-processed organic field-effect transistors

In this study, a donor-acceptor-type small molecule possessing a narrow bandgap with siloxane-terminated side chains, namely, S-LBG, was synthesized and characterized to be used as a semiconducting layer in organic field-effect transistors (OFETs). The diketopyrrolopyrrole (DPP)-based small molecule contains silaindacenodithiophene as an electron-donor core and 2-(1,1-dicyanomethylene)rhodanine as an electron-accepting end group. S-LBG films show highly crystalline structures after thermal annealing treatment owing to the strong intermolecular interactions between its siloxane side chains; furthermore, a smooth-surface film with a large polycrystalline grain size can be fabricated by controlling the cooling rate after annealing. Solution-processed OFETs using cooled S-LBG films at a rate of 1 °C/min and exhibits a high hole mobility of 0.102 cm2 V−1 s−1 with a π–π stacking distance of 0.659 nm.

Synthesis and Characterization of a Non-Conjugated Backbone Polymer Bearing [1]Benzothieno[3,2-&lt;i&gt;b&lt;/i&gt;][1]benzothiophene with a Herringbone Packing Motif

Conjugated polymers encompassing delocalized π-electrons in their backbones exhibit high photoabsorption and efficient charge carrier transport through the intermolecular π-π stacking suitable for organic photovoltaics (OPV). By contrast, small molecules such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, which demonstrates high field-effect transistor hole mobilities owing to their 2-dimensional herringbone packing, have not yet been applied to OPVs. Here we report a non-conjugated backbone polymer bearing phenyl- and decyl-substituted BTBT (Ph-BTBT-10) as the pendant conjugated unit, which was synthesized by ring opening polymerization of epoxy append. The polymer showed an X-ray diffraction pattern ascribed to the layered lamellar and less ordered herringbone packing. In addition, a model molecule of Ph-BTBT-10 showed charge separation upon photoexcitation when blended with a non-fullerene acceptor (ITIC) or fullerene (PCBM).

Biaxial strain improving carrier mobility for inorganic perovskite: ab initio Boltzmann transport equation

Inorganic halide perovskites have attracted interest due to their high efficiency and low cost. Considering the uncertainty of experimental measurements, it was important to predict the upper limit of carrier mobility. In this study, the ab initio Boltzmann transport equation, including all electron–phonon interactions, was used to accurately predict the mobilities of CsPbI3, CsSnI3, CsPbBr3, and CsSnBr3. Using the iterative Boltzmann transport equation (IBTE), the calculated mobility for CsPbI3 is µ e = 512/µ h = 379 cm2 V−1 s−1, and Sn-based perovskite exhibited high hole mobility. The longitudinal optical phonons associated with the stretching between halogen anions and divalent metal cations were revealed to be the dominant scattering source for the carriers. Furthermore, the effect of biaxial strain on mobility was investigated. We observed that biaxial compressive strain could improve the mobility of CsPbI3 and CsPbBr3. Surprisingly, under a compressive strain of , the mobilities of CsPbI3 using IBTE approach were improved to µ e = 1176/µ h = 936 cm2 V−1 s−1. It was revealed that the compressive strain could decrease the effective mass of CsPbI3 and CsPbBr3.