Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements.

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

Self-Assembled TaOX/2H-TaS2 as a van der Waals Platform of a Multilevel Memristor Circuit Integrated with a ß-Ga2O3 Transistor.

Two-dimensional (2D)-layered material tantalum disulfide (2H-TaS2) is known to be a van der Waals conductor at room temperature. Here, 2D-layered TaS2 has been partially oxidized by utraviolet-ozone (UV-O3) annealing to form a 12-nm-thin TaOX on conducting TaS2, so that the TaOX/2H-TaS2 structure might be self-assembled. Utilizing the TaOX/2H-TaS2 structure as a platform, each device of a ß-Ga2O3 channel MOSFET and a TaOX memristor has been successfully fabricated. An insulator structure of Pt/TaOX/2H-TaS2 shows good a dielectric constant (k ∼ 21) and strength (∼3 MV/cm) of achieved TaOX, which is enough to support a ß-Ga2O3 transistor channel. Based on the quality of TaOX and low trap density of the TaOX/ß-Ga2O3 interface, which is achieved via another UV-O3 annealing, excellent device properties such as little hysteresis (<∼0.04 V), band-like transport, and a steep subthreshold swing of ∼85 mV/dec are achieved. With a Cu electrode on top of the TaOX/2H-TaS2 structure, the TaOX acts as a memristor operating around ∼2 V for nonvolatile bipolar and unipolar mode memories. The functionalities of the TaOX/2H-TaS2 platform become more distinguished finally when the Cu/TaOX/2H-TaS2 memristor and ß-Ga2O3 MOSFET are integrated to form a resistive memory switching circuit. The circuit nicely demonstrates the multilevel memory functions.

Prospective Cohort Study with a 2-Year Follow-up of Clinical Results, Fusion Rate, and Muscle Bulk for Uniportal Full Endoscopic Posterolateral Transforaminal Lumbar Interbody Fusion.

STUDY DESIGN: Retrospective cohort study. PURPOSE: Postoperative evaluation of the cross-sectional area of paraspinal muscle and clinical findings in patients who had interlaminar route uniportal full endoscopic posterolateral transforaminal lumbar interbody fusion (EPTLIF) after 2 years. OVERVIEW OF LITERATURE: There are limited short-term follow-up studies on efficacy, safety, and physiological changes with a 2-year follow-up. There is no study on paraspinal muscle cross-sectional area change in patients who had undergone uniportal EPTLIF. METHODS: We evaluated patients who underwent EPTLIF with a minimum 24-month follow-up. Clinical parameters of the Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) were measured at the preoperative, 1-week postoperative mark, postoperative 3-month mark, and final follow-up. Preoperative and 1-year postoperative magnetic resonance imaging measurement of preoperative and postoperative Kjaer grade, right and left psoas muscle mass area, and right and left paraspinal muscle mass area was performed. RESULTS: EPTLIF with a minimum 24-month follow-up of 35 levels was included. The complication rate was 6%, and the mean Bridwell's fusion grade was 1.37 (1-2). There was statistically significant improvement at 1 week, 3 months, and 2 years in VAS (4.11±1.23, 4.94±1.30, and 5.46±1.29) and in ODI (40.34±10.06, 46.69±9.14, and 49.63±8.68), respectively (p <0.05). Successful operation rate with excellent and good MacNab's criteria at 2 years was 97%. There was an increment of statistically significant bilateral psoas muscle cross-sectional area, right side (70.03±149.1 mm²) and left side (67.59±113.2 mm²) (p <0.05). CONCLUSIONS: Uniportal EPTLIF achieved good fusion and improved clinical outcomes with favorable paraspinal musculature bulk at the 2-year follow-up.

Remodeling of Epidural Fluid Hematoma after Uniportal Lumbar Endoscopic Unilateral Laminotomy with Bilateral Decompression: Comparative Clinical and Radiological Outcomes with a Minimum Follow-up of 2 Years.

STUDY DESIGN: Retrospective cohort study. PURPOSE: To evaluate the clinical and radiological effects of epidural fluid hematoma in the medium term after lumbar endoscopic decompression. OVERVIEW OF LITERATURE: There is limited literature comparing the effect of postoperative epidural fluid hematoma after uniportal endoscopic decompression. METHODS: Magnetic resonance imaging (MRI) and clinical evaluation were performed for patients with single-level uniportal endoscopic lumbar decompression with a minimum follow-up of 2 years. RESULTS: A total of 126 patients were recruited with a minimum follow-up of 26 months. The incidence of epidural fluid hematoma was 27%. Postoperative MRI revealed a significant improvement in the postoperative dura sac area at postoperative day 1 and at the upper endplate at 6 months in the hematoma cohort (39.69±15.72 and 26.89±16.58 mm2) as compared with the nonhematoma cohort (48.92±21.36 and 35.1±20.44 mm2), respectively (p <0.05); and at the lower endplate on postoperative 1 day in the hematoma cohort (51.18±24.69 mm2) compared to the nonhematoma cohort (63.91±27.92 mm2) (p <0.05). No significant difference was observed in the dura sac area at postoperative 1 year in both cohorts. The hematoma cohort had statistically significant higher postoperative 1-week Visual Analog Scale (VAS; 3.32±0.68) pain and Oswestry Disability Index (ODI; 32.65±5.56) scores than the nonhematoma cohort (2.99±0.50 and 30.02±4.84, respectively; p <0.05). No significant difference was found at the final follow-up VAS, ODI, and MRI dura sac area. CONCLUSIONS: Epidural fluid hematoma is a common early postoperative MRI finding in lumbar endoscopic unilateral laminotomy with bilateral decompression. Conservative management is the preferred treatment option for patients who do not have a neurological deficit. Symptoms last only a few days and are self-limiting. A common endpoint is a remodeled fluid hematoma and the subsequent expansion of the dura sac area.

Catalyst-free synthesis of sub-5 nm silicon nanowire arrays with massive lattice contraction and wide bandgap.

The need for miniaturized and high-performance devices has attracted enormous attention to the development of quantum silicon nanowires. However, the preparation of abundant quantities of silicon nanowires with the effective quantum-confined dimension remains challenging. Here, we prepare highly dense and vertically aligned sub-5 nm silicon nanowires with length/diameter aspect ratios greater than 10,000 by developing a catalyst-free chemical vapor etching process. We observe an unusual lattice reduction of up to 20% within ultra-narrow silicon nanowires and good oxidation stability in air compared to conventional silicon. Moreover, the material exhibits a direct optical bandgap of 4.16 eV and quasi-particle bandgap of 4.75 eV with the large exciton binding energy of 0.59 eV, indicating the significant phonon and electronic confinement. The results may provide an opportunity to investigate the chemistry and physics of highly confined silicon quantum nanostructures and may explore their potential uses in nanoelectronics, optoelectronics, and energy systems.

Remodeling Pattern of Spinal Canal after Full Endoscopic Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression: One Year Repetitive MRI and Clinical Follow-Up Evaluation.

Objective: There is limited literature on repetitive postoperative MRI and clinical evaluation after Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression. Methods: Clinical visual analog scale, Oswestry Disability Index, McNab's criteria evaluation and MRI evaluation of the axial cut spinal canal area of the upper end plate, mid disc and lower end plate were performed for patients who underwent single-level Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression. From the evaluation of the axial cut MRI, four types of patterns of remodeling were identified: type A: continuous expanded spinal canal, type B: restenosis with delayed expansion, type C: progressive expansion and type D: restenosis. Result: A total of 126 patients with single-level Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression were recruited with a minimum follow-up of 26 months. Thirty-six type A, fifty type B, thirty type C and ten type D patterns of spinal canal remodeling were observed. All four types of patterns of remodeling had statistically significant improvement in VAS at final follow-up compared to the preoperative state with type A (5.59 ± 1.58), B (5.58 ± 1.71), C (5.58 ± 1.71) and D (5.27 ± 1.68), p < 0.05. ODI was significantly improved at final follow-up with type A (49.19 ± 10.51), B (50.00 ± 11.29), C (45.60 ± 10.58) and D (45.60 ± 10.58), p < 0.05. A significant MRI axial cut increment of the spinal canal area was found at the upper endplate at postoperative day one and one year with type A (39.16 ± 22.73; 28.00 ± 42.57) mm2, B (47.42 ± 18.77; 42.38 ± 19.29) mm2, C (51.45 ± 18.16; 49.49 ± 18.41) mm2 and D (49.10 ± 23.05; 38.18 ± 18.94) mm2, respectively, p < 0.05. Similar significant increment was found at the mid-disc at postoperative day one, 6 months and one year with type A (55.16 ± 27.51; 37.23 ± 25.88; 44.86 ± 25.73) mm2, B (72.83 ± 23.87; 49.79 ± 21.93; 62.94 ± 24.43) mm2, C (66.85 ± 34.48; 54.92 ± 30.70; 64.33 ± 31.82) mm2 and D (71.65 ± 16.87; 41.55 ± 12.92; 49.83 ± 13.31) mm2 and the lower endplate at postoperative day one and one year with type A (49.89 ± 34.50; 41.04 ± 28.56) mm2, B (63.63 ± 23.70; 54.72 ± 24.29) mm2, C (58.50 ± 24.27; 55.32 ± 22.49) mm2 and D (81.43 ± 16.81; 58.40 ± 18.05) mm2 at postoperative day one and one year, respectively, p < 0.05. Conclusions: After full endoscopic lumbar decompression, despite achieving sufficient decompression immediately postoperatively, varying severity of asymptomatic restenosis was found in postoperative six months MRI without clinical significance. Further remodeling with a varying degree of increment of the spinal canal area occurs at postoperative one year with overall good clinical outcomes.

Damage-Free Charge Transfer Doping of 2D Transition Metal Dichalcogenide Channels by van der Waals Stamping of MoO3 and LiF.

To dope 2D semiconductor channels, charge-transfer doping has generally been done by thermal deposition of inorganic or organic thin-film layers on top of the 2D channel in bottom-gate field-effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge-transfer doping process is suggested. A damage-free charge-transfer doping is conducted for 2D MoTe2 (or MoS2 ) channels using a polydimethylsiloxane stamp. MoO3 or LiF is initially deposited on the stamp as a doping medium. Hysteresis-minimized transfer characteristics are achieved from stamp-doped FETs, while other devices with direct thermal deposition-doped channels show large hysteresis. The stamping method seems to induce a van der Waals-like damage-free interface between the channel and doping media. The stamp-induced doping is also well applied for a MoTe2 -based complementary inverter because MoO3 - and LiF-doping by separate stamps effectively modifies two ambipolar MoTe2 channels to p- and n-type, respectively.

Observation of 3D Biexcitons in Pristine-Quality CH3 NH3 PbBr3 Single Crystals.

Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors.

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C60 molecular crystals. Transmission electron microscopy and theoretical analysis of the C60 /BP vdW heterostructure clearly confirm that the BP template results in oriented C60 assembly with higher-order commensurism. Lateral and vertical devices with C60 /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C60 /BP junctions. Effective tuning of the C60 /BP junction barrier from 0.2 to 0.5 eV and maximum on-current density higher than 104  mA cm-2 are achieved with graphite/C60 /BP vertical vdW transistors. Due to the formation of high-quality C60 film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W-1 as well as a fast response time under visible light illumination.

Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells.

Despite the exceptional efficiency of perovskite solar cells (PSCs), further improvements can be made to bring their power conversion efficiencies (PCE) closer to the Shockley-Queisser limit, while the development of cost-effective strategies to produce high-performance devices are needed for them to reach their potential as a widespread energy source. In this context, there is a need to improve existing charge transport layers (CTLs) or introduce new CTLs. In this contribution, we introduced a new polyelectrolyte (lithium poly(styrene sulfonate (PSS))) (Li:PSS) polyelectrolyte as an HTL in inverted PSCs, where Li+ can act as a counter ion for the PSS backbone. The negative charge on the PSS backbone can stabilize the presence of p-type carriers and p-doping at the anode. Simple Li:PSS performed poorly due to poor surface coverage and voids existence in perovskite film as well as low conductivity. PEDOT:PSS was added to increase the conductivity to the simple Li:PSS solution before its use which also resulted in lower performance. Furthermore, a bilayer of PEDOT:PSS and Li:PSS was employed, which outperformed simple PEDOT:PSS due to high quality of perovskite film with large grain size also the large electron injection barrier (ϕe ) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.

Mechanism and Timescales of Reversible p-Doping of Methylammonium Lead Triiodide by Oxygen.

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. ​In this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI3 ) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI3 bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.

Dramatic Reduction of Contact Resistance via Ultrathin LiF in Two-Dimensional MoS2 Field Effect Transistors.

Molybdenum disulfide (MoS2) has been regarded as one of the most important n-type two-dimensional (2D) transition metal dichalcogenide semiconductors for nanoscale electron devices. Relatively high contact resistance (RC) remains as an issue in the 2D-devices yet to be resolved. Reliable technique is very compelling to practically produce low RC values in device electronics, although scientific approaches have been made to obtain a record-low RC. To resolve this practical issue, we here use thermal-evaporated ultrathin LiF between channel and source/drain metal to fabricate 2D-like MoS2 field effect transistors (FETs) with minimum RC. Under 4-bar FET method, RC less than ∼600 Ω·µm is achieved from the LiF/Au contact MoS2 FET. Our normal 2-bar FET with LiF thus shows the same mobility as that of 4-bar FET that should have no RC in principle. On the basis of these results, ultrathin LiF is also applied for transparent conducting oxide contact, successfully enabling transparent MoS2 FETs.

High-Performance van der Waals Junction Field-Effect Transistors Utilizing Organic Molecule/Transition Metal Dichalcogenide Interface.

Two-dimensional (2D) transition metal dichalcogenide (TMD) hetero PN junctions with a van der Waals (vdW) interface have received much attention, because PN diodes are basically important to control the vertical current across the junction. Interestingly, the same vdW PN junction structure can be utilized for junction field-effect transistors (JFETs) where in-plane current is controlled along the junction. However, 2D vdW JFETs seem rarely reported, despite their own advantages to achieve when good vdW junction is secured. Here, we present high-performance p-MoTe2 JFETs using almost perfect vdW organic Alq3/p-MoTe2 junctions and demonstrate organic NPB/n-MoS2 JFETs. The p- and n-channel JFETs stably show high mobilities of 60-80 and ∼800 cm2/V s, respectively, along with a high ON/OFF current ratio (>1 × 105) and minimal gate leakage at 5 V even after a few months. Such performances are attributed to a quality vdW junction at organic layer/TMD interfaces.

Enhanced Photoinduced Carrier Generation Efficiency through Surface Band Bending in Topological Insulator Bi2Se3 Thin Films by the Oxidized Layer.

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi2Se3 films is enhanced, rather than degraded, after oxidation in air for 24 h, resulting in a maximum responsivity of 140 mA W-1. This responsivity is substantially higher than previously reported values for Bi2Se3. Furthermore, a change in the photoresponse time of Bi2Se3 due to air exposure is systematically observed. Based on variations in the Fermi level and work function, using photoelectron spectroscopy, it is confirmed that the responsivity is improved from the junction effect of the Bi-based surface oxidized layer.

Self-Powered Visible-Invisible Multiband Detection and Imaging Achieved Using High-Performance 2D MoTe2/MoS2 Semivertical Heterojunction Photodiodes.

Two-dimensional (2D) van der Waals (vdW) heterostructures herald new opportunities for conducting fundamental studies of new physical/chemical phenomena and developing diverse nanodevice applications. In particular, vdW heterojunction p-n diodes exhibit great potential as high-performance photodetectors, which play a key role in many optoelectronic applications. Here, we report on 2D MoTe2/MoS2 multilayer semivertical vdW heterojunction p-n diodes and their optoelectronic application in self-powered visible-invisible multiband detection and imaging. Our MoTe2/MoS2 p-n diode exhibits an excellent electrical performance with an ideality factor of less than 1.5 and a high rectification (ON/OFF) ratio of more than 104. In addition, the photodiode exhibits broad spectral photodetection capability over the range from violet (405 nm) to near-infrared (1310 nm) wavelengths and a remarkable linear dynamic range of 130 dB within an optical power density range of 10-5 to 1 W/cm2 in the photovoltaic mode. Together with these favorable static photoresponses and electrical behaviors, very fast photo- and electrical switching behaviors are clearly observed with negligible changes at modulation frequencies greater than 100 kHz. In particular, inspired by the photoswitching results for periodic red (638 nm) and near-infrared (1310 nm) illumination at 100 kHz, we successfully demonstrate a prototype self-powered visible-invisible multiband image sensor based on the MoTe2/MoS2 p-n photodiode as a pixel. Our findings can pave the way for more advanced developments in optoelectronic systems based on 2D vdW heterostructures.

Band Alignment Engineering between Planar SnO2 and Halide Perovskites via Two-Step Annealing.

Managing defects in SnO2 is critical for improving the power conversion efficiency (PCE) of halide perovskite-based solar cells. However, typically reported SnO2-based perovskite solar cells have inherent defects in the SnO2 layer, which lead to a lower PCE and hysteresis. Here, we report that a dual-coating approach for SnO2 with different annealing temperatures can simultaneously form a SnO2 layer with high crystallinity and uniform surface coverage. Along with these enhanced physical properties, the dual-coated SnO2 layer shows favorable band alignment with a mixed halide perovskite. After careful optimization of the dual-coating method, the average PCE of the perovskite solar cell based on the dual-coated SnO2 layer increases from 18.07 to 19.23% with a best-performing cell of 20.03%. Note that a facile two-step coating and annealing method can open new avenues to develop SnO2-based perovskite solar cells with stabilized and improved photovoltaic performances.

ZnO:Ga-graded ITO electrodes to control interface between PCBM and ITO in planar perovskite solar cells.

Ga-doped ZnO (GZO)-graded layer, facilitating electron extraction from electron transport layer, was integrated on the surface of transparent indium tin oxide (ITO) cathode by using graded sputtering technique to improve the performance of planar n-i-p perovskite solar cells (PSCs). The thickness of graded GZO layer was controlled to optimize GZO-indium tin oxide (ITO) combined electrode for planar n-i-p PSCs. At optimized graded thickness of 15 nm, the GZO-ITO combined electrode showed an optical transmittance of 95%, a resistivity of 2.3 × 10-4 Ohm cm, a sheet resistance of 15.6 Ohm/square, and work function of 4.23 eV, which is well matched with the 4.0-eV lowest unoccupied molecular orbital of [6,6]-phenyl-C61-butyric acid methyl ester. Owing to enhanced extraction of electron by the graded GZO, the n-i-p PSC with GZO-ITO combined electrode showed higher power conversion efficiency (PCE) of 9.67% than the PCE (5.25%) of PSC with only ITO electrode without GZO-graded layer. In addition, the GZO integrated-ITO electrode acts as transparent electrode and electron extraction layer simultaneously due to graded mixing of the GZO at the surface region of ITO electrode.

Electronic Structure of Nonionic Surfactant-Modified PEDOT:PSS and Its Application in Perovskite Solar Cells with Reduced Interface Recombination.

The interfacial properties of organolead halide perovskite solar cells (PSCs) affect the exciton and charge-transport dynamics significantly. Thus, proper modification of the interfaces between perovskite and charge-transport layers is an efficient method to increase the power conversion efficiency (PCE) of PSCs. In this work, we explore the effect of a nonionic surfactant, that is, Triton X-100 (TX) additive, in the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. The electronic structure of TX-modified PEDOT:PSS is investigated with ultraviolet/X-ray photoelectron spectroscopy and X-ray absorption spectroscopy with various TX concentrations. The surface of the TX-modified PEDOT:PSS layer showed high TX content, and thus the semimetallic properties of PEDOT:PSS were suppressed conspicuously by its insulating nature. With the TX-modified PEDOT:PSS, the PCE of methylammonium lead iodide (MAPbI3) PSCs increased significantly. To elucidate the origin of the improved device performance, the electrical properties and photoluminescence were investigated comprehensively. Consequently, it was found that the TX additive inhibits interface recombination between PEDOT:PSS and MAPbI3, which is caused by the suppression of semimetallic properties of the PEDOT:PSS surface. Hence, we fabricated flexible PSCs successfully using a graphene electrode and TX-modified PEDOT:PSS.

Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle Atomic Layer Deposition and Their Rollable Applications.

Amorphous InGaZnO semiconductors have been rapidly developed as active charge-transport materials in thin film transistors (TFTs) because of their cost effectiveness, flexibility, and homogeneous characteristics for large-area applications. Recently, InZnSnO (IZTO) with superior mobility (higher than 20 cm2 V-1 s-1) has been suggested as a promising oxide semiconductor material for high-resolution, large-area displays. However, the electrical and physical characteristics of IZTO have not been fully characterized. In this study, thin IZTO films were grown using a novel atomic layer deposition (ALD) supercycle process consisting of alternating subcycles of single-oxide deposition. By varying the number of deposition subcycles, it was determined that the insertion of a Sn-O cycle improved the mobility and reliability of IZTO-based TFTs. Specifically, the IZTO TFT obtained using one In-O cycle, one Zn-O cycle, and one Sn-O exhibited the best performance (saturation mobility of 27.8 cm2 V-1 s-1 and threshold voltage shift of 1.8 V after applying positive-bias temperature stress conditions). Next, the production of rollable and flexible devices was demonstrated by fabricating ALD-processed IZTO TFTs on polymer substrates. The electrical characteristics of these TFTs were retained without drastic degradation for 240,000 bending cycles. These results indicate that the supercycle ALD technique is effective for synthesizing multicomponent oxide TFTs for electronic applications requiring high mobility and mechanical flexibility.

Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n-MoS2 and p-MoTe2 Transistors.

Since transition metal dichalcogenide (TMD) semiconductors are found as two-dimensional van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. However, overall reports indicate that threshold voltage ( Vth) of those FETs are located far away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HAT-CN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposition, compared to its initial performance. When the same TMD FETs of the CMOS structure are integrated into an OLED pixel circuit for ambipolar switching, the circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which was not switched off without HAT-CN.