Carrier Injection Properties Research Articles

Enhancing Carrier Behavior via Controlled Molecular Film Formation Engineering Leads to Significant Improvement in Electroluminescence

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge‐trap states called water/oxygen‐induced traps that significantly hinder carrier mobility. While the water/oxygen‐induced traps in non‐doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of "nano‐cluster" structures on both the surface and interior of doped films in target molecule 10‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐1'‐(4‐fluorophenyl)‐10H‐spiro[acridine‐9,9'‐xanthene] (DspiroO‐F‐TRZ), which is modified with a fluorophenyl group. These "nano‐cluster" structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF‐OLED based on DspiroO‐F‐TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from 'nano‐cluster' structure enhancements toward improved electroluminescence performance.

Enhancing Carrier Behavior via Controlled Molecular Film Formation Engineering Leads to Significant Improvement in Electroluminescence.

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge-trap states called water/oxygen-induced traps that significantly hinder carrier mobility. While the water/oxygen-induced traps in non-doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of "nano-cluster" structures on both the surface and interior of doped films in target molecule 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1'-(4-fluorophenyl)-10H-spiro[acridine-9,9'-xanthene] (DspiroO-F-TRZ), which is modified with a fluorophenyl group. These "nano-cluster" structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF-OLED based on DspiroO-F-TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from 'nano-cluster' structure enhancements toward improved electroluminescence performance.

Accurate Extraction of Schottky Barrier Height and Universality of Fermi Level De‐Pinning of van der Waals Contacts

AbstractDue to Fermi level pinning (FLP), metal‐semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal—an essential requirement for several applications, including light‐emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky–Mott limit of de‐pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this study a simple technique is proposed to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H‐TaSe2, semi‐metallic graphene, and degenerately doped semiconducting SnSe2, it is demonstrated that vdW contacts exhibit a universal de‐pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, a) pulsed electroluminescence from monolayer WS2 using few‐layer graphene (FLG) contact, and b) efficient carrier injection to WS2 and WSe2 channels in both n‐type and p‐type field effect transistor modes using 2H‐TaSe2 contact.

Tuning Interfacial Properties by Spontaneously Generated Organic Interlayers in Top‐Contact‐Structured Organic Transistors

Adv. Funct. Mater. 2020, 30, 2002979 In the originally published version of the article, the authors did not fully acknowledge the existing study reported by Frey group. Since the studies in Frey group are highly similar to our results, the reviews or references to those studies need to be updated in our article. The authors apologize for any inconvenience this error may have caused. Added Sentences in Introduction Section However, this approach is also not easy to use due to its inconvenience and the disadvantage that it can only be applied to bottom-contact-structured OFET devices. For this reason, there are few relevant studies on the interfacial optimization of the top-contact OFET devices until now. Recently, the Frey group reported an approach for the spontaneous interfacial modification of electrodes in organic electronic devices by blending additives in the polymer active layer that migrate to the electrodes and reduce contact resistance.[38,39] More specifically, in OFETs they applied hexa(ethyleneglycol)-dithiol and 4-fluorobenzyl mercaptan as thiol additives, and demonstrated that it is possible to efficiently control the interface between the top-contacted source/drain electrode and reduce the level of RC by simply adding the additives to the polymer semiconducting layer. This study is an interesting result in the field of existing interface control, but there is a side that was not sufficiently considered: the aspect doping by the blended thiol molecules. [38] T. Sarkar, B. Shamieh, R. Verbeek, A. J. Kronemeijer, G. H. Gelinck, G. L. Frey, Adv. Funct. Mater. 2020, 30, 1805617. [39] J. Vinokur, I. Deckman, T. Sarkar, L. Nouzman, B. Shamieh, G. L. Frey, Adv. Mater, 2018, 30, 1706803. Modified Sentences in Introduction Section (page 2) Here, we explored an approach for spontaneously controlling the interfaces between the top-contact S/D electrode and the active layer in OFET devices without the doping effect due to the presence of additives, resulting in improved charge carrier injection properties to the polymer semiconducting films. Deleted Sentence in Results and Discussion Section (page 8) Although the interfacial optimization is known to be important for improving the device performances, there are few relevant studies because no suitable experimental method for this has been proposed for top-contact OFET devices until now.

Highly Efficient Blue Electroluminescent Materials Based on Phenanthro[9,10-d] imidazole

Phenanthro[9,10-d]-imidazole( PI) unit has attracted extensive attentions in organic optoelectronic field due to its intrinsic wide band gap,high luminescence efficiency,good thermal stability,balanced charge carrier injection and transport property as well as low fabrication cost. In this manuscript,the recently reported blue electroluminescent PI derivatives were reviewed. The structure characteristics of PI were demonstrated,and the structure-property relationships of some PI derivatives were summarized systemically. At last,all the reported highly efficient nondoped deep blue emitters were summed up in order to comprehensively evaluate the potentials of PI derivatives in organic light emitting diodes( OLEDs) in the future.

Effects of physical orientation of dye molecules and molecular orbitals on performance of solid-state dye sensitized solar cells.

Performance of Dye-sensitized devices depends on the photon absorption and carrier injection properties of the sensitizer (dye). The orientation of the dye molecule affects the photon absorption cross-section, injection efficiency and carrier transport. These effects are studied, using three variants of cyanine dyes in n-TiO2/Dye/p-CuSCN heterojunction. The results show correlation of dye-molecule's orientation on the short-circuit-photocurrent (Isc). The open-circuit-voltage (Voc) is also subjective. The orientation of the dye molecule influence the photon-harvesting efficiency and obstruct the hole-conductor penetrating onto the working-electrode. Additionally, Cumulative effects of e-e, e-h, spin-coupling and HOMO/LUMO distribution are identified.

Carrier behaviors of 6,13-Bis (triisopropylsilylethynyl) pentacene device with self-assembled monolayer

Abstract Since the discovery of organic semiconductors, material and device processing has been enhanced by improvements in carrier injection, carrier mobility, and device stability. However, these improvements are insufficient in the case of organic devices employing organic semiconductor materials. To improve the performance of organic devices, we need better understanding of the carrier injection property at the electrode-organic layer interface. In this paper, we studied carrier injection and accumulation in organic semiconductor by using electrical and optical measurements, i.e., I-V, C-V, AFM, and FTIR measurements. We used 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) as the organic semiconductor layer. In order to enhance the carrier injection into TIPS-pentacene, we focused on the binding effect of the self-assembled monolayer, i.e., (3-Mercaptopropyl) trimethoxysilane (MPTMS) and pentafluorobenzenethiol (PFBT), deposited on the surface of the substrate.

Direct writing of silver nanowire electrodes via dragging mode electrohydrodynamic jet printing for organic thin film transistors

Abstract AgNWs-based electrodes were directly patterned using an electrohydrodynamic (EHD) jet printing technique. We investigated EHD jet printing for AgNWs ink in detail, and established the optimum printing conditions in dragging mode for controlling the dimensions and conductivity of the AgNWs network, although the cone-jet printing mode has been the most conventionally used mode for EHD jet printing. The printed AgNWs were used as source/drain (S/D) electrodes of an organic thin film transistor (OTFT) with bottom-contact geometry, and yielded an average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.48 cm2/V, −11.2 V, and ∼106, respectively. In addition, we investigate the interface morphologies between pentacene and AgNWs S/D electrode to figure out charge carrier injection property of AgNWs S/D electrode, by comparing vacuum-deposited Au ones.

Zirconium Tetrakis(8-hydroxyquinolinolate) and Lithium Schiff-Base Cluster Complex for Efficient Charge Injection and Transfer in Green PHOLED processed by OVPD

ABSTRACTTypical electron transport (2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)) and injection (Cs2CO3) materials are successfully replaced by zirconium tetrakis(8-hydroxyquinolinolate) (Zrq4) and lithium 2-((o-tolylimino)methyl)-phenolate (EI-111) in simplified OLED (organic light-emitting diodes) processed by organic vapor phase deposition (OVPD). The performance of combining Zrq4 and EI-111 is analyzed in unipolar devices and compared to devices with configurations of Zrq4/Cs2CO3, TPBi/EI-111 and TPBi/Cs2CO3. Current density-voltage (J-V) measurements are performed and correlated to different carrier injection and transport properties. The investigated material combinations are implemented in the simplified OLED structures and compared to each other. To account for the high HOMO level of Zrq4, 5 nm of TPBi are added to confine holes and excitons in the emissive layer (EML) and to improve device performance. After tailoring the organic stack for OLED with Zrq4, a remarkable boost in device efficiency is observed. The luminous efficacy increased from 3.0 to 21.9 lm/W and the EQE from 2.1 to 11.0 % for a device with Zrq4/EI-111. Furthermore, OLED having Zrq4/Cs2CO3 show an even greater enhancement to 26.3 lm/W and 11.7 %.

Contact-Engineered Electrical Properties of MoS2 Field-Effect Transistors via Selectively Deposited Thiol-Molecules.

Although 2D molybdenum disulfide (MoS2 ) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high-performance 2D MoS2 -based devices. In this regard, many studies have been conducted to improve the carrier-injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time-consuming and low-yield transfer processes on sub-micrometer MoS2 flakes. Here, a simple contact-engineering method is suggested, introducing chemically adsorbed thiol-molecules as thin tunneling barriers between the metal electrodes and MoS2 channels. The selectively deposited thiol-molecules via the vapor-deposition process provide additional tunneling paths at the contact regions, improving the carrier-injection properties with lower activation energies in MoS2 field-effect transistors. Additionally, by inserting thiol-molecules at the only one contact region, asymmetric carrier-injection is feasible depending on the temperature and gate bias.

The effect of injection properties of contacts on the dynamics of unipolar space-charge limited currents

The influence of the carrier injection properties of electrode contacts on unipolar space-charge limited currents (SCLC) in perfect insulators is theoretically studied. The usual electric transport model consisting of the charge continuity equation and the Poisson equation is supplemented by boundary conditions describing carrier injection. The dependence of the electric behavior on the saturation current density, which is the most important parameter of the boundary condition, is investigated, mainly with FEM simulations. The scenarios in the focus are transient DC current when a constant voltage is switched on, and dielectric (or impedance) spectroscopy (AC). For the latter, higher harmonics up to the 5th order are relevant due to the intrinsic nonlinearity of SCLC. A simplified ordinary differential equation model for transient contact charging is derived, in an analogous way as it is usually done for the early SCLC transient with ideal ohmic contacts. Finally, a specific case with traps that may quench the transit-time peak is briefly discussed to illustrate trapping effects. It turns out that the SCLC dynamics can be separated in three time (or frequency) regimes. Two of them are separated as usual by the time of flight, τ toƒ , of carriers through the sample. A third region at shorter times t c ≪τ toƒ (or higher frequencies) exists, where τ c characterizes the time for space charge formation at the injecting contact. The contact injection properties thus mainly influence the short-time and high-frequency behavior, where the characteristic time decreases with increasing saturation current. This behavior is reflected by the appearance of specific structures in the absorption current and the impedance spectrum. For instance, an additional current maximum associated with the fast diffusive expansion of the locally injected space charge may appear at early times.

Coarsening of one-step deposited organolead triiodide perovskite films via Ostwald ripening for high efficiency planar-heterojunction solar cells.

Large organolead triiodide perovskite (OTP) grains with little intragranular defects are beneficial to minimize carrier recombination, hence boosting cell performance. However, OTP films deposited by the widely used one-step spin-coating route are usually composed of small grains, because the poor thermal stability of OTP inherently restricts the processing window (temperature, time) during the film preparation, thus limiting grain coarsening in the film. Herein, the remarkable grain coarsening via Ostwald ripening in one-step deposited OTP films has been successfully realized by a facile and effective post-synthesis high-temperature heating treatment assisted with spin-coated CH3NH3I. By systematically investigating the heating treatment parameters, a high-quality OTP film with an enlarged average grain size from ∼280 nm to 1.2 μm, greatly enhanced crystallinity, and excellent stoichiometry is achieved. Benefiting from such improved features, this modified film shows significantly reduced defect states corresponding to the decrease of recombination centers, as well as enhanced carrier transport and injection properties, which lead to the dramatically boosted efficiency from 14.54% to 16.88% for planar-heterojunction solar cells. More importantly, the improved OTP film quality provides the possibility of thickening the absorber layer of cells to realize more sufficient absorption without serious aggravation of charge recombination. By further optimizing the thickness of the coarsened OTP films, highly efficient cells with relatively excellent reproducibility and an optimal efficiency of 19.24% are achieved.

Role of oxygen vacancies in TiOx films in electronic structure at interface with an α-NPD layer

Metal oxide buffer layers are a promising method for improving the efficiency and stability of organic photovoltaic cells and organic light-emitting diodes. To design organic electronic devices intelligently, it is necessary to understand the electronic structure at organic/metal-oxide interfaces. In this study, the electronic structure of films of a well-known hole-transporting material, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on TiOx underlayers was studied via ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. New electronic states were found near the Fermi level (EF) upon adsorption of α-NPD when the TiOx underlayer was highly oxygen defective. The observed spectra were not fully explained by density functional theory (DFT) calculations for simple ionic α-NPD molecules. In addition, the vacuum level shift at the interfaces was also different from the shift predicted by several theoretical models that do not consider chemical reactions at the interface. These results suggest that the adsorbate–substrate interaction is complicated when a highly defective metal oxide is used. The stoichiometry of the metal oxide buffer layer may have a large effect on carrier injection or extraction properties at the interface.

Charge injection in metal/organic/metal structures with ZnO:Al/organic interface modified by Zn1−xMgxO:Al layer

Aluminum-doped zinc oxide (ZnO:Al, AZO) electrodes were covered with very thin (∼6nm) Zn1−xMgxO:Al (AMZO) layers grown by atomic layer deposition. They were tested as hole blocking/electron injecting contacts to organic semiconductors. Depending on the ALD growth conditions, the magnesium content at the film surface varied from x=0 to x=0.6. Magnesium was present only at the ZnO:Al surface and subsurface regions and did not diffuse into deeper parts of the layer. The work function of the AZO/AMZO (x=0.3) film was 3.4eV (based on the ultraviolet photoelectron spectroscopy). To investigate carrier injection properties of such contacts, single layer organic structures with either pentacene or 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine layers were prepared. Deposition of the AMZO layers with x=0.3 resulted in a decrease of the reverse currents by 1–2 orders of magnitude and an improvement of the diode rectification. The AMZO layer improved hole blocking/electron injecting properties of the AZO electrodes. The analysis of the current-voltage characteristics by a differential approach revealed a richer injection and recombination mechanisms in the structures containing the additional AMZO layer. Among those mechanisms, monomolecular, bimolecular and superhigh injection were identified.

MoO3/Ag/MoO3 anode for organic light-emitting diodes and its carrier injection property

We report on the application of the dielectric/metal/dielectric (DMD) structure consisting of a molybdenum trioxide (MoO3)/silver (Ag)/MoO3 stack as the transparent electrode in organic light-emitting diodes (OLEDs). Bright emission similar to that of the indium–tin-oxide anode (ITO) device was obtained from the OLEDs with the DMD anode. Also, the barrier height at the interface of DMD/bis[N-(1-naphthyl)-N-phenyl] benzidine (α-NPD) is similar to that at the ITO/α-NPD interface. The DMD electrode is a promising anode for OLEDs.

Blue and blue-green light-emitting cationic iridium complexes: synthesis, characterization, and optoelectronic properties.

Two new cationic iridium complexes, [Ir(ppy)2(phpzpy)]PF6 (complex 1) and [Ir(dfppy)2(phpzpy)]PF6 (complex 2), bearing a 2-(3-phenyl-1H-pyrazol-1-yl)pyridine (phpzpy) ancillary ligand and either 2-phenylpyridine (Hppy) or 2-(2,4-difluorophenyl)pyridine (Hdfppy) cyclometalating ligands, were synthesized and fully characterized. The photophysical and electrochemical properties of these complexes were investigated by means of UV-visible spectroscopy, emission spectroscopy, and cyclic voltammetry. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations were performed to simulate and study the photophysical and electrochemical properties of both complexes. Light-emitting electrochemical cells (LECs) were fabricated by incorporating complexes 1 and 2, which respectively exhibit blue-green (488 and 516 nm) and blue (463 and 491 nm) emission colors, achieved through the meticulous design of the ancillary ligand. The luminance and current efficiency measurements recorded for the LEC based on complex 1 were 1246 cd m(-2) and 0.46 cd A(-1), respectively, and were higher than those measured for complex 2 because of the superior balanced carrier injection and recombination properties of the former.

Effects of substrate topography on current injection and light emission properties of organic light emitting devices

Substrate topography plays a critical role in the function of nano-scale materials and devices. We study small molecular organic light emitting devices (OLEDs) deposited onto non-planar substrates, where the substrate’s radius of curvature in some regions approaches the thickness of the active device layers. As a result, the electric field profile inside the organic charge transport layers is modified, influencing carrier injection, transport, and light emission properties. Experiments and numerical modeling suggest that charge balance and electroluminescence efficiency potentially can be improved in electron injection-limited OLED architectures via substrate geometry. These findings elucidate the optoelectronic behavior (and degradation) of OLEDs on imperfect substrates, and suggest a strategy based on substrate topography for controlling device behavior.

High efficiency, blue emitting materials based on phenanthro[9,10-d]imidazole derivatives

The blue light emitting materials based on a fluoro phenanthro [9,10-d] imidazole derivatives prepared by a facial synthetic process exhibit good thermal stability, highly efficient fluorescence and balanced carrier injection. The multi-layered device based on fluoro phenanthroimidazole derivatives shows a higher luminance in a lower turn-on voltage. The device performance implies that the phenanthroimidazole unit is an excellent building block for tuning the carrier injection properties as well as blue emission.

Carrier injection properties in spin–orbit coupling structure based on a correlated electronic ferromagnetic system

Carrier injection effects on phase-field domain structures are displayed between BiFeO3 and La0.4Gd0.1Sr0.5CoO3 thin films prepared by pulsed laser deposition, epitaxially grown on LaAlO3 (100) substrates. The leakage current and magnetoresistance (MR) in the fabricated BiFeO3/La0.4Gd0.1Sr0.5CoO3/LaAlO3 heterostructure were measured under 0.2, 0.4 and 0.6 T at 80–300 K. The BiFeO3/LGSCO heterojunction exhibits the carrier transfer of the metal–insulator transition and a positive MR effect at 219–250 K. The electric-conductivity mechanism below the Curie temperature, TC, is dominated by Poole–Frenkel emission. The BiFeO3/LGSCO pn junction shows rectifying behavior between 80–300 K. The energy bands of the heterojunction were modified by the morphology of the interface, the magnetic domain, spin polarization and interface tensile strain due to external magnetic perturbations. The strain and magnetic field-modified domain wall and the carrier density at the BiFeO3/LGCO interface were characterized by atomic force microscopy. The domain and phase separation of LGSCO/BiFeO3 changed with strain, structural defects, grain size and boundary and the modified concentration of carriers. Additionally, the BiFeO3/La1−xGdxSrCoO3 heterostructure shows a positive colossal MR effect at 80–300 K and an metal–insulator phase transition was observed around TC 219.5 K for 0.6 T, 220 K for 0.4 T and 249.7 K for 0.2 T.

Computational investigation of charge injection and transport properties of a series of thiophene–pyrrole based oligo-azomethines

The present study explores the structural, charge carrier injection and transport properties of a series of thiophene-pyrrole based oligo-azomethines using density functional theory (DFT) methods. Our findings show that the presence of a bulky substituent adversely affects these properties. However, the electronic effect of substituents may be utilized to tune these properties by substitutions at suitable positions. Values of frontier orbitals, ionization energies, and electron affinities are calculated for each compound to predict the ease of charge injection from metal electrodes to these azomethines and the stabilities of their ionic forms. In addition to having large injection barriers, lack of stability of the anions may hinder the electron injection. However, most of the compounds have excellent hole injection capability. Computation of reorganization energies and electronic couplings followed by charge transfer rates and mobilities show large carrier mobilities for some of the studied compounds. Considering both the injection capability and carrier mobilities, it is found that a thiophene-pyrrole azomethine without any substituent and substituted azomethines with a methyl, methoxy or amine group at the 3 position of the pyrrole ring may act as efficient materials for the hole transport layer.