Direct observation of ICT cations at the HTL/transparent semiconductor interface

The cation density at the interface of a transparent anode and an organic layer has been measured for several hole transport (HT) materials. The number of cations at the interface of ITO:MoO_x with rubrene, NPB, m-MTDATA and TCTA was found to range from 8 × 10¹³ to 1.5 × 10¹⁴ per cm² in freshly prepared devices. These values decreased by about 25% after one month. These cations are part of the dipole layer that results from the transfer of electrons from an organic layer, whose adiabatic ionization potential is less than the work function of the anode.     

Interfacial modifying layer-driven high-performance organic thin-film transistors and their nitrogen dioxide gas sensors

Organic thin-film transistors (OTFTs) based on bottom-gate bottom-contact configuration were fabricated by inserting two kinds of modifying layers at the interface of source/drain electrode and organic semiconductor, while nitrogen dioxide (NO₂) sensing capability was also evaluated based on the obtained OTFTs. Compared to OTFT without interfacial layer, the field-effect mobility (μ) was enhanced from 0.018 cm²/Vs to 0.15 cm²/Vs by incorporating with MoO_x interfacial layer. Moreover, when exposed to 30 ppm NO₂, the saturation current and μ of OTFT with MoO_x interfacial layer increase 22.7% and 26.7%, respectively, while in original OTFT, the values are only 3.0% and 3.7%, respectively. The mechanism of performance improvement of OTFT sensor was systematically studied by focusing on the interface of source/drain electrode and organic semiconductor. The reduced contact resistance leads to higher μ, meanwhile, pentacene morphology modulation on MoO_x contributes to better diffusion of NO₂ molecules. As a result, higher μ and more diffused gas molecules enhance the gas sensing property of the transistor.     

Performance improvement mechanisms of organic thin-film transistors using MoOx-doped pentacene as channel layer

Organic thin-film transistors (OTFTs) with various MoO_x-doped pentacene channel layers were fabricated and investigated. Compared the OTFTs with the 0.50 mol% MoO_x-doped pentacene to the conventional OTFTs without MoO_x dopant, the maximum output current was increased from −11.6 to −37.9 μA, the effective field-effect mobility was enhanced from 0.71 to 1.60 cm²/V-s, the threshold voltage was reduced from −21.2 to −14.8 V, and the on/off current ratio slightly decreased from 3.6 × 10⁶ to 1.2 × 10⁶. The performance improvement was attributed to the highest occupied molecular orbital (HOMO) of the MoO_x-doped pentacene gradually approached to the Au work function with increasing the doping percentage of MoO_x, which led to reduce the contact resistance and to enhance the p-type characteristics of the MoO_x-doped OTFTs by increasing the hole density and enhancing the hole-injection efficiency. However, the output current and the field-effect mobility decreased with an increase of the MoO_x doping percentage, if the doping mole percentage of MoO_x was higher than 0.50%. This behavior was attributed to the Fermi level pinning effect, gradual increase of hole concentration and significant degradation of crystallinity.     

Low dark current inverted organic photodetectors employing MoOx:Al cathode interlayer

We report low dark current small molecule organic photodetectors (OPDs) with an inverted geometry for image sensor applications. Adopting a very thin MoO_x:Al cathode interlayer (CIL) in the inverted OPD with a reflective top electrode results in a remarkably low dark current density (J_d) of 5.6 nA/cm² at reverse bias of 3 V, while maintaining high external quantum efficiency (EQE) of 56.1% at visible wavelengths. The effectiveness of the CIL on the diode performance has been further identified by application to inverted OPDs with a semi-transparent top electrode, leading to a significantly low J_d of 0.25 nA/cm², moderately high EQE_{540 nm} of 25.8%, and subsequently high detectivity of 8.95 × 10¹² Jones at reverse bias of 3 V. Possible origins of reduced dark currents in the OPD by using the MoO_x:Al CIL are further described in terms of the change of interfacial energy barrier and surface morphology.     

Solution-processed aqueous composite hole injection layer of PEDOT:PSS+MoOx for efficient ultraviolet organic light-emitting diode

Hole injection layer (HIL) plays a crucial role in governing external quantum efficiency (EQE) of ultraviolet organic light-emitting diodes (UV OLEDs). We develop a solution-processed aqueous composite HIL of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) incorporated MoO_x (PEDOT:PSS+MoO_x) and cast successful application to UV OLEDs. PEDOT:PSS+MoO_x is characterized in detail with scanning electron microscopy, atomic force microscopy, UV–visible absorption spectra, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show that PEDOT:PSS+MoO_x features superior film morphology and exceptional electronic properties such as enhanced surface work function and promoted hole injection capacity. With PEDOT:PSS+MoO_x as HIL, the UV OLED gives maximum EQE of 4.4% and radiance of 12.2 mW/cm² as well as improved durability. The electroluminescence peaks at 376 nm with full width at half maximum of 34 nm and stable voltage-dependent spectra. Our results pave a way for exploring efficient UV OLEDs with solution-processable techniques.     

Air stable C60 based n-type organic field effect transistor using a perfluoropolymer insulator

Air stable n-type organic field effect transistors (OFETs) based on C₆₀ are realized using a perfluoropolymer as the gate dielectric layer. The devices showed the field-effect mobility of 0.049 cm²/V s in ambient air. Replacing the gate dielectric material by SiO₂ resulted in no transistor action in ambient air. Perfluorinated gate dielectric layer reduces interface traps significantly for the n-type semiconductor even in air.     

Small-molecular organic solar cells with C60/Al composite anode

By adopting C₆₀/Al composite anode and an inverted device structure of ITO/Alq₃/C₆₀/CuPc/C₆₀/Al, we have achieved a power conversion efficiency of 0.78% under 75 mW/cm² AM1.5G simulated illumination and a shelf lifetime of 950 h from unencapsulated organic solar cells. The improved stability is attributed to efficient protection of the C₆₀ layer in the inverted structure. Replacing the C₆₀/Al anode with C₆₀/Au anode in the inverted structure, produces a power conversion efficiency of 0.64%, comparable to that of the device with C₆₀/Al anode. This indicates that the property of the composite electrode is mainly determined by the thin C₆₀ layer. Use of C₆₀/Al composite anode to fabricate inverted organic light-emitting devices gives rise to an efficiency of the device comparable to that of conventional devices.     

Efficient charge generation layer for tandem OLEDs: Bi-layered MoO3/ZnO-based oxide semiconductor

A novel oxide charge generation layer (CGL) with optical and electrical advantages for tandem organic light-emitting diodes (OLEDs) is proposed. The CGL comprises amorphous Zn-Si-O (a-ZSO) and MoO_3-x. Although a-ZSO has a very small work function of 3.4 eV, it forms Ohmic contact with MoO_3-x with a high work function of 6.6 eV. It is discovered that the interface state appears between a-ZSO and MoO_3-x, which contributes to the formation of quasi-Ohmic contact between the two oxides with dramatically different work functions. High performance of electron and hole injection/transport is achieved in tandem OLEDs with a very small voltage drop of 0.4 V at 100 mA/cm² with a twofold increase in current efficiency. This new CGL provides distinct advantages over conventional organic CGL materials with respect to processing simplicity, cost, and chemical stability.     

Photo-patternable high-k ZrOx dielectrics prepared using zirconium acrylate for low-voltage-operating organic complementary inverters

Solution-processed dielectric materials with a high dielectric constant (k) have attracted considerable attention due to their potential applications in low-voltage-operating organic field-effect transistors (OFETs) for realizing large-area and low-power electronic devices. In terms of device commercialization, the patterning of each film component via a facile route is an important issue. In this study, we introduce a photo-patternable precursor, zirconium acrylate (ZrA), to fabricate photo-patterned high-k zirconium oxide (ZrO_x) dielectric layers with UV light. Solution-processed ZrA films were effectively micro-patterned with UV exposure and developing, and transitioned to ZrO_x through a sol-gel reaction during deep-UV annealing. The UV-assisted and ∼10 nm-thick ZrO_x dielectric films exhibited a high capacitance (917.13 nF/cm² at 1 KHz) and low leakage current density (10⁻⁷ A/cm² at 1.94 MV/cm). Those films could be utilized as gate dielectric layers of OFETs after surface modification with ultrathin cyclic olefin copolymer layers. Finally, we successfully fabricated organic complementary inverters exhibiting hysteresis-free operation and high voltage gains of over 42 at low voltages of ≤3 V.     

Buffer-modified C60/pentacene as organic charge generation layer based on Al and MoO3

We demonstrate tandem organic light-emitting diodes (TOLEDs) with excellent performance using Al and MoO₃ buffer-modified C₆₀/pentacene as charge generation layer (CGL). Al and MoO₃ were used as the electron and hole injection layers of C60/pentacene CGL, respectively. Green phosphorescence TOLEDs with the structure of ITO/NPB/mCP:Ir(ppy)₃/TPBi/Al/C₆₀/pentacene/MoO₃/NPB/mCP:Ir(ppy)₃/TPBi/Cs₂CO₃/Al were fabricated. The results show that the inserted Al and MoO₃ can effectively increase the charge injection capacity of organic CGL, resulting the improvement of luminance and current efficiency of TOLEDs. The turn-on voltage of TOLEDs is much lower than that of single-unit device, and the current efficiency is more than 2 times larger than that of the single-unit device. TOLEDs can exhibit excellent photoelectric performance when the thicknesses of Al, C₆₀, pentacene and MoO₃ are 3 nm, 15 nm, 25 nm and 1 nm, respectively. The maximum luminance and current efficiency are 7 920.0 cd/m² and 16.4 cd/A, respectively. This work is significant to build new CGL structures for realizing high-performance TOLEDs.     

Design of a MoOx/Au/MoOx transparent electrode for high-performance OLEDs

A MoO_x(top)/Au/MoO_x(bottom) multilayer was systematically designed for transparent electrodes in green OLEDs in terms of optical transmission and series resistance of the device. The enhancement in optical transmission of MoO_x/Au/MoO_x (MAM) structures is a result of a series of events, including the optical interference within the multilayers and the interaction of light with surface plasmon polaritons in the metal layer. For the maximum transmission, the optical interference occurring within the multilayers was simulated using a transfer matrix model to determine the optimum thickness of MoO_x layers, and then the thickness of the Au interlayer was experimentally optimized for extraordinary optical transmission. In addition, the series resistance added by the top MoO_x was characterized to confirm its negligible impact on the performance of the device. The optimum MoO_x (40 nm)/Au (10 nm)/MoO_x (40 nm) structure showed much higher transmission in the green-red region and lower sheet resistance than indium tin oxide (ITO). We have fabricated MAM-based OLEDs the driving voltage of which was significantly reduced to ∼5.5 V at a current density of 20 mA/cm², and the current efficiency (11.46 Cd/A) was higher than that (10.91 Cd/A) of ITO-based OLEDs, demonstrating that the MAM electrode is a potential replacement for ITO in optical devices.     

High-performance n-type organic field-effect transistors fabricated by ink-jet printing using a C60 derivative

We report on the performance of ink-jet-printed n-type organic thin-film transistors (OTFTs) based on a C₆₀ derivative, namely, C₆₀-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C₆₀TH-Hx). The new devices exhibit excellent n-channel performance, with a highest mobility of 2.8 × 10^?2 cm² V^?1 s^?1, an I_On/I_Off ratio of about 1 × 10⁶, and a threshold voltage of 7 V. The C₆₀TH-Hx films show large crystalline domains that result from the influence of an evaporation-induced flow, thus leading to high electron mobility in the ink-jet-printed devices.     

Low-voltage p-channel, n-channel and ambipolar organic thin-film transistors based on an ultrathin inorganic/polymer hybrid gate dielectric layer

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated low-voltage operating (below 3V) p-channel, n-channel and ambipolar OTFTs based on pentacene or/and C₆₀ as the active layers, respectively, with an ultrathin AlO_X/poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) hybrid layer as the gate dielectric. Benefited from the enhanced crystallinity of C₆₀ layer and greatly reduced density of electron trapping states at the interface of channel/dielectric due to the insertion of ultrathin pentacene layer between C₆₀ and P(MMA–GMA), high electron mobility can be achieved in present pentacene/C₆₀ heterostructure based ambipolar OTFTs. The effect of the thickness of pentacene layer and the deposition sequence of pentacene and C₆₀ on the device performance of OTFTs was studied. The highest electron mobility of 3.50 cm²/V s and hole mobility of 0.25 cm²/V s were achieved in the ambipolar OTFT with a pentacene (3.0 nm)/C₆₀ (30 nm) heterostructure.     

Active Regulation Volume Change of Micrometer-Size Li2S Cathode with High Materials Utilization for All-Solid-State Li/S Batteries through an Interfacial Redox Mediator

Low electronic and ionic transport, limited cathode active material utilization, and significant volume change have pledged the practical application of all-solid-state Li/S batteries (ASSLSBs). Herein, an unprecedented Li₂S-Li_xIn₂S₃ cathode is designed whereby In₂S₃ reacts with Li₂S under high-energy ball milling. In situ electron diffraction and ex situ XPS are implanted to probe the reaction mechanism of Li₂S-Li_xIn₂S₃ in ASSLSBs. The results indicate that Li_xIn₂S₃ serves as a mobility mediator for both charge-carriers (Li⁺ and e⁻) and redox mediator for Li₂S activation, ensuring efficient electronic and ionic transportation at the cathode interface and inhibiting ≈ 70% relative volumetric change in the cathode, as confirmed by in situ TEM. Thus, the Li₂S-Li_xIn₂S₃ cathode delivers an initial areal capacity of 3.47 mAh cm⁻² at 4.0 mg_Li2S cm⁻² with 78% utilization of Li₂S. A solid-state cell with Li₂S-Li_xIn₂S₃ cathode carries 82.35% capacity retention over 200 cycles at 0.192 mA cm⁻² and a remarkable rate capability up to 0.64 mA cm⁻² at RT. Besides, Li₂S-Li_xIn₂S₃ exhibits the highest initial areal capacity of 4.08 mAh cm⁻² with ≈74.01% capacity retention over 50 cycles versus 6.6 mg_Li2S cm⁻² at 0.192 mA cm⁻² at RT. The proposed strategy of the redox mediator minimized volumetric change and realized outstanding electrochemical performance for ASSLSBs.     

Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells

The authors demonstrate an effective anode interfacial layer based on aqueous solution-processed MoO₃ (sMoO₃) in poly (3-hexylthiophene) (P3HT) and indene-C60 bisadduct (ICBA) based bulk-heterojunction organic solar cells (PSCs). Various sMoO₃ concentration (0.03–0.25 wt%) was obtained by dissolving MoO₃ powder into deionized water directly with weak solubility. The characteristics of sMoO₃ films evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM) suggest that the sMoO₃ films continuously cover the entire indium tin oxide (ITO) surface. The sMoO₃ based PSCs exhibit comparable power conversion efficiency with poly (3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) based devices. However, even more importantly, the stability of sMoO₃ based devices have been greatly improved in air under continual light-illumination at 52 mW/cm². Further evaluations on Mo valence states and work function of sMoO₃ films by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) demonstrate that the aqueous solution-processed MoO₃ could act as an better anode interfacial layer than the conventional PEDOT:PSS.     

Amorphization Boost Multi-Ions Storage for High-Performance Aqueous Batteries

Regarding the complex properties of various cations, the design of aqueous batteries that can simultaneously store multi-ions with high capacity and satisfactory rate performance is a great challenge. Here an amorphization strategy to boost cation-ion storage capacities of anode materials is reported. In monovalent (H⁺, Li⁺, K⁺), divalent (Mg²⁺, Ca²⁺, Zn²⁺) and even trivalent (Al³⁺) aqueous electrolytes, the capacity of the resulting amorphous MoO_x is more than quadruple than that of crystalline MoO_x and exceeds those of other reported multiple-ion storage materials. Both experimental and theoretical calculations reveal the generation of ample active sites and isotropic ions in the amorphous phase, which accelerates cation migration within the electrode bulk. Amorphous MoO_x can be coupled with multi-ion storage cathodes to realize electrochemical energy storage devices with different carriers, promising high energy and power densities. The power density exceeded 15000 W kg⁻¹, demonstrating the great potential of amorphous MoO_x in advanced aqueous batteries.     

Lamellar MXene Composite Aerogels with Sandwiched Carbon Nanotubes Enable Stable Lithium–Sulfur Batteries with a High Sulfur Loading

Realizing long cycling stability under a high sulfur loading is an essential requirement for the practical use of lithium–sulfur (Li–S) batteries. Here, a lamellar aerogel composed of Ti₃C₂T_x MXene/carbon nanotube (CNT) sandwiches is prepared by unidirectional freeze-drying to boost the cycling stability of high sulfur loading batteries. The produced materials are denoted parallel-aligned MXene/CNT (PA-MXene/CNT) due to the unique parallel-aligned structure. The lamellae of MXene/CNT/MXene sandwich form multiple physical barriers, coupled with chemical trapping and catalytic activity of MXenes, effectively suppressing lithium polysulfide (LiPS) shuttling under high sulfur loading, and more importantly, substantially improving the LiPS confinement ability of 3D hosts free of micro- and mesopores. The assembled Li–S battery delivers a high capacity of 712 mAh g⁻¹ with a sulfur loading of 7 mg cm⁻², and a superior cycling stability with 0.025% capacity decay per cycle over 800 cycles at 0.5 C. Even with sulfur loading of 10 mg cm⁻², a high areal capacity of above 6 mAh cm⁻² is obtained after 300 cycles. This work presents a typical example for the rational design of a high sulfur loading host, which is critical for the practical use of Li–S batteries     

Using metal/organic junction engineering to prepare an efficient organic base-modulation triode and its inverter

In this study, we investigated the influence of a buffer layer of molybdic oxide (MoO₃) at the metal/organic junction on the behavior of organic base-modulation triodes. The performance of devices featuring MoO₃/Al as the emitter electrode was enhanced relative to that of corresponding devices with Au and Ag, presumably because of the reduced in the contact barrier and the prevention of metal diffusion into the organic layer. The device exhibited an output current of ?16.1 μA at V_B = ?5 V and a current ON/OFF ratio of 10³. Using this architecture, we constructed resistance–load inverters that exhibited a calculated gain of 6.     

All solution-processed organic single-crystal transistors with high mobility and low-voltage operation

High-mobility organic single-crystal field-effect transistors of 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]-dithiophene (C₁₀-DNBDT) operating at low driving voltage are fabricated by an all-solution process. A field-effect mobility as high as 6.9 cm²/V s is achieved at a driving voltage below 5 V, a voltage as low as in battery-operated devices, for example. A low density of trap states is realized at the surface of the solution-processed organic single-crystal films, so that the typical subthreshold swing is less than 0.4 V/decade even on a reasonably thick amorphous polymer gate dielectrics with reliable insulation. The high carrier mobility and low interface trap density at the surface of the C₁₀-DNBDT crystals are both responsible for the development of the high-performance all-solution processed transistors.     

Effect of annealing-induced oxidation of molybdenum oxide on organic photovoltaic device performance

Molybdenum oxide (MoO_x) has been widely used as a hole transport layer in organic photovoltaic cells (OPVs), whose performance can be improved by inserting a MoO_x layer between an organic active layer and a transparent anode because of efficient carrier dissociation. In this study, the influence of thermally annealed MoO_x on the photovoltaic performance of OPVs was first investigated using low-bandgap polymer and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) blend films as the active layer. We used three low-bandgap polymers: poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7), and poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b,3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7-Th). Power conversion efficiencies were drastically increased for all investigated polymers when the as-deposited MoO_x layer was annealed at 160 °C for 5 min. In particular, a high efficiency of 6.57% was achieved when PTB7 was used; for comparison, the efficiency of a reference device with an as-deposited MoO_x layer (not subjected to annealing) was 1.40%. Specifically, the short-circuit current density and fill factor were remarkably improved after annealing, which means that efficient carrier dissociation was achieved in the active layer. We evaluated optical absorption and surface morphology to elucidate reasons behind the improved photovoltaic performance, and these parameters only slightly changed after annealing. In contrast, angle-dependent X-ray photoelectron spectroscopy revealed that the MoO_x layer was oxidized after annealing. In general, the oxygen vacancies of MoO_x act as carrier traps; a reduction in the number of carrier traps causes high hole mobility in the organic layer, which, in turn, results in an improved photovoltaic performance. Therefore, our results indicate that the annealing-induced oxidation of MoO_x is useful for achieving high photovoltaic performance.