Organic Semiconductor Devices Research Articles

Multiphoton Lithography of Organic Semiconductor Devices for 3D Printing of Flexible Electronic Circuits, Biosensors, and Bioelectronics.

In recent years, 3D printing of electronics have received growing attention due to their potential applications in emerging fields such as nanoelectronics and nanophotonics. Multiphoton lithography (MPL) is considered the state-of-the-art amongst the microfabrication techniques with true 3D fabrication capability owing to its excellent level of spatial and temporal control. Here, a homogenous and transparent photosensitive resin doped with an organic semiconductor material (OS), which is compatible with MPL process, is introduced to fabricate a variety of 3D OS composite microstructures (OSCMs) and microelectronic devices. Inclusion of 0.5 wt% OS in the resin enhances the electrical conductivity of the composite polymer about 10 orders of magnitude and compared to other MPL-based methods, the resultant OSCMs offer high specific electrical conductivity. As a model protein, laminin is incorporated into these OSCMs without a significant loss of activity. The OSCMs are biocompatible and support cell adhesion and growth. Glucose-oxidase-encapsulated OSCMs offer a highly sensitive glucose sensing platform with nearly tenfold higher sensitivity compared to previous glucose biosensors. In addition, this biosensor exhibits excellent specificity and high reproducibility. Overall, these results demonstrate the great potential of these novel MPL-fabricated OSCM devices for a wide range of applications from flexible bioelectronics/biosensors, to nanoelectronics and organ-on-a-chip devices.

Selective doping of a single ambipolar organic semiconductor to obtain P- and N-type semiconductors

•A well-balanced ambipolar organic semiconductor is synthesized as host semiconductor •Proper doping of the single host semiconductor yields both p- and n-type semiconductors •Organic PN homojunctions and corresponding photodiodes are constructed and characterized The idea of acquiring p- and n-type organic semiconductors (OSCs) through controllable doping of a single OSC is highly attractive. However, realization of this idea remains challenging due to the difficulties in achieving effective p- and n-doping on a single OSC simultaneously. Herein, by synthesizing a solution-processable conjugated polymer with well-balanced ambipolar transport as the host semiconductor and using appropriate dopants, we demonstrate the feasibility of obtaining p- and n-type OSCs from a single OSC. The successful doping control of the polarity and electrical property of the OSC is confirmed by electrical characterizations on doped films and devices. Then, solution-processed organic PN homojunctions based on the p- and n-doped OSC films are constructed, which provide good platforms for studying organic semiconductor physics and for fabricating functional devices like photodiodes. The results provide a paradigm for utilizing doping technique in organic semiconductor devices similar to that in silicon-based devices. The idea of acquiring p- and n-type organic semiconductors (OSCs) through controllable doping of a single OSC is highly attractive. However, realization of this idea remains challenging due to the difficulties in achieving effective p- and n-doping on a single OSC simultaneously. Herein, by synthesizing a solution-processable conjugated polymer with well-balanced ambipolar transport as the host semiconductor and using appropriate dopants, we demonstrate the feasibility of obtaining p- and n-type OSCs from a single OSC. The successful doping control of the polarity and electrical property of the OSC is confirmed by electrical characterizations on doped films and devices. Then, solution-processed organic PN homojunctions based on the p- and n-doped OSC films are constructed, which provide good platforms for studying organic semiconductor physics and for fabricating functional devices like photodiodes. The results provide a paradigm for utilizing doping technique in organic semiconductor devices similar to that in silicon-based devices.

45‐2: Invited Paper: Towards High‐Performance Organic Transistors for Display and other Applications

Organic semiconductor devices have potential for displays as light emitter and organic backplane transistors. We present vertical transistor structures with very short channel length without micropatterning. These devices are well suited to drive organic light‐emitting diodes (OLED), allowing all‐organic flexible displays. We have achieved kA/cm2 current densities useful for bright OLED displays and record operating frequencies >40MHz /2/. Vertical organic light‐emitting transistors show efficient emission in different colors, combining OLED operation and switching and fully compatible with vacuum mass production. Vertical transistors have GHz potential using highly ordered layers grown quasi‐epitaxially.

Quantifying the Excitonic Static Disorder in Organic Semiconductors

AbstractOrganic semiconductors are disordered molecular solids, and as a result, their internal charge generation dynamics, charge transport dynamics, and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. This is particularly pertinent for emerging photovoltaic technology where the extractable power is directly dependent on these dynamics. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, it is shown that the static disorder of an organic semiconductor can be obtained from its photovoltaic external quantum efficiency spectrum at wavelengths near the absorption onset. A detailed methodology is presented, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the authors show that minimizing the limiting effects of optical interference is crucial for achieving high‐accuracy quantifications. Finally, transparent devices are employed to estimate the excitonic static disorder in several technologically relevant organic semiconductor donor–acceptor blends, including the high‐efficiency organic photovoltaic system PM6:Y6.

Highly efficient hole injection from Au electrode to fullerene-doped triphenylamine derivative layer

Triphenylamine derivatives are superior hole-transport materials. For their application to high-functional organic semiconductor devices, efficient hole injection at the electrode/triphenylamine derivative interface is required. Herein, we report the design and evaluation of a Au/fullerene-doped α-phenyl-4′-[(4-methoxyphenyl)phenylamino]stilbene (TPA) buffer layer/TPA/Au layered device. It exhibits rectification conductivity, indicating that hole injection occurs more easily at the Au/fullerene-doped TPA interface than at the Au/TPA interface. The Richardson-Schottky analysis of the device reveals that the hole injection barrier (ΦB) at the Au/fullerene-doped TPA interface decreases to 0.021 eV upon using C70 as a dopant, and ΦB of Au/TPA is as large as 0.37 eV. The reduced ΦB of 0.021 eV satisfies the condition for ohmic contact at room temperature (ΦBle 0.025 eV). Notably, C70 doping has a higher barrier-reduction effect than C60 doping. Furthermore, a noteworthy hole-injection mechanism, in which the ion–dipole interaction between TPA and fullerenes plays an important role in reducing the barrier height, is considered based on cyclic voltammetry. These results should facilitate the design of an electrode/organic semiconductor interface for realizing low-voltage driven organic devices.

Optical and electrical analysis in various organic-inorganic stacked structures

Understanding heterojunctions in layered organic semiconductor devices is crucial in achieving efficient optoelectronic and advanced organic electronic devices. In this paper, we investigated the formation of charge transfer complex (CTC) and the generation of free carriers at the interface of the transition metal oxide (TMO)/organic semiconductor (OS) bi-layer structures. First, the relationship between the CTC and conductivity is analyzed for bi-layers composed of molybdenum oxide (MoO 3 ) and OSs with different highest occupied molecular orbital (HOMO) levels, respectively. Same analysis is done for bi-layers involving N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and various TMOs with different work-functions, respectively. It is investigated that CTC is formed at the interface of the TMO/organic bi-layer structure and the conductivity in the TMO is increased. The absorbance intensities of the CTCs at the heterojunctions are proposed to depend on the density of the OS radicals formed from the charge transfer process. It is demonstrated that conductivity is low at bi-layer interfaces with high density of CTC. This is because the CTC aligned at the interface acts as a hindrance to electron mobility. Consequently, it is demonstrated that MoO 3 /2TNATA and MoO 3 /NPB exhibited low conductivity despite the formation of more CTCs than MoO 3 /CBP. We propose that the CTCs at the OS/TMO heterojunction with low charge separation efficiency, causes negative charges to align at the interface. This negatively charged electric field at interface act as resistance to the mobility of electrons inside the TMO layer. The investigation is key in achieving efficient organic electronic devices and stacked organic semiconductor functional structures. • Heterojunctions with organic hosts and transition metal oxides are investigated. • The formation of CTC and free carriers at the heterojunctions are analyzed. • Conductivity is low at the heterojunctions with high CTCs. • CTCs with low charge separation efficiency cause resistance to electron flow.

Properties and Applications of Copper(I) Thiocyanate Hole‐Transport Interlayers Processed from Different Solvents

AbstractCopper(I) thiocyanate (CuSCN) is an effective interlayer material for hole injection and transport in organic electronic devices but its solution processing has conventionally utilized undesirable di‐n‐alkyl sulfide solvents such as diethyl‐ (DES) and dipropyl‐sulfide (DPS). Herein, this paper reports on the use of N,N‐dimethylformamide (DMF) and 1‐methyl‐2‐pyrrolidinone (NMP) as alternative solvents for CuSCN interlayers and performs a detailed comparison of the resulting properties relative to films processed from DES and DPS and two other recent alternatives, dimethyl sulfoxide (DMSO) and ammonium hydroxide. The surface roughness, polymorphism, and surface chemistry of the resulting CuSCN layers are reported. The interlayer surface energy and ionization potential that are key to the overlying semiconductor microstructure and interfacial energy barrier, and hence to charge transport and injection, are also discussed. Finally, systematic device tests using well‐known organic semiconductors in light‐emitting diode, solar cell and field‐effect transistor structures demonstrate the overall suitability of DMSO and DMF as solvents for CuSCN interlayer deposition to achieve better device performance. This study broadens the applicability of CuSCN as a highly efficient hole injection/transport material for organic semiconductor devices by expanding the documented range of suitable CuSCN solvents.

Virtual source emission model leads to improved organic thin film transistor

Better fit to real-world data of organic semiconductor devices finds a managanese interlayer boosts transistor performance.

Image-Force-Stabilized Interfacial Dipole Layer Impedes Charge Injection Into Disordered Organic Semiconductors

We show using three-dimensional kinetic Monte Carlo simulations that the injection of charge carriers from a metallic electrode into a disordered organic semiconductor is under nominally Ohmic injection conditions strongly impeded by the short-range Coulomb interactions between the charge carriers in the image-force-stabilized interfacial dipole layer. In contrast, master equation and conventional one-dimensional drift-diffusion simulations underestimate these Coulomb interactions due to their mean-field approximation, and are found not to reveal the effect. The simulations predict a reduction of the current density in organic semiconductor devices when the nominal injection barrier is taken very small or even negative, consistent with recent experimental results [Kotadiya et al., Nat. Mater. 17, 329 (2018)]. However, whereas in that work a modification of the energetic disorder near the interface is assumed, we find that the effect is already obtained after including charge-charge interactions beyond a one-dimensional and mean-field approximation.

A coarse‐grained electrothermal model for organic semiconductor devices

We derive a coarse‐grained model for the electrothermal interaction of organic semiconductors. The model combines stationary drift‐diffusion‐ based electrothermal models with thermistor‐type models on subregions of the device and suitable transmission conditions. Moreover, we prove existence of a solution using a regularization argument and Schauder's fixed point theorem. In doing so, we extend recent work by taking into account the statistical relation given by the Gauss–Fermi integral and mobility functions depending on the temperature, charge‐carrier density, and field strength, which is required for a proper description of organic devices.

The future of solution processing toward organic semiconductor devices: a substrate and integration perspective

This review summarizes the solution-processing techniques toward organic semiconductor devices from the perspective of the physical state of the film-forming substrates.

Surface adhesion engineering for robust organic semiconductor devices

Strategies for enhancing interfacial adhesion of organic semiconductors by adhesive layer introduction, adhesive group functionalization, adhesive integrated agent, and physical mixing, have shown promising applications in various fields.

De Novo Calculation of the Charge Carrier Mobility in Amorphous Small Molecule Organic Semiconductors.

Organic semiconductors (OSC) are key components in applications such as organic photovoltaics, organic sensors, transistors and organic light emitting diodes (OLED). OSC devices, especially OLEDs, often consist of multiple layers comprising one or more species of organic molecules. The unique properties of each molecular species and their interaction determine charge transport in OSCs—a key factor for device performance. The small charge carrier mobility of OSCs compared to inorganic semiconductors remains a major limitation of OSC device performance. Virtual design can support experimental R&D towards accelerated R&D of OSC compounds with improved charge transport. Here we benchmark a de novo multiscale workflow to compute the charge carrier mobility solely on the basis of the molecular structure: We generate virtual models of OSC thin films with atomistic resolution, compute the electronic structure of molecules in the thin films using a quantum embedding procedure and simulate charge transport with kinetic Monte-Carlo protocol. We show that for 15 common amorphous OSC the computed zero-field and field-dependent mobility are in good agreement with experimental data, proving this approach to be an effective virtual design tool for OSC materials and devices.

Formation of MoO3/Organic Interfaces

AbstractMolybdenum trioxide (MoO3) has been extensively used in numerous organic semiconductor devices for hole injection and extraction. In this paper, photoelectron spectroscopy combined with cleavage and sputter depth profile has been used to probe the structures of buried MoO3/organic semiconductor interfaces. Organics used in this work include: tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA), N,N′‐bis(naphthalene‐l‐yl)‐N,N′‐bis(phenyl)benzidine (NPB), 4,4′‐bis(carbazol‐9‐yl)‐2,2′‐biphenyl (CBP), and 1,3‐bis(N‐carbazolyl) benzene (mCP). It is found that there are two distinct types of interfaces: sharp interfaces (type‐I) where the oxide layer has limited or no diffusion when deposited on organics that have a high glass transition temperature such as TCTA and NPB; mixed interfaces (type‐II) where the formation of interfaces is followed by significant diffusion and reaction on organics having low glass transition temperatures such as CBP and mCP. The causes for these two types of interfaces are discussed.

Density Functional Theory Investigation of the Doping Effects of Bromine and Fluorine on the Electronic and Optical Properties of Neutral and Ionic Perylene

Perylene and its derivatives are some of the promising organic semiconductors. They have found vast applications in many areas such as photovoltaic systems, organic light-emitting diodes, and so on. The instability of organic molecules under ambient conditions is one factor deterring the commercialization of organic semiconductor devices. Currently, most of the investigation of Perylene and its derivatives concentrated on its diimide and bisimide derivatives. In this work, an investigation of the effects of doping Bromine and Fluorine on the electronic and non-linear optical properties was carried out based on Density Functional Theory (DFT) as implemented in the Gaussian 09 software package. We computed the Molecular geometries of the molecules, HOMO-LUMO energy gap, global chemical indices and non-linear optical properties using the same method. The bond lengths and angles of the mono-halogenated molecules at different charge states were found to be less than that of the isolated Perylene. 1-fluoroperylene was found to be the most stable amongst the studied molecule for having the least bond angles and bond lengths. In the calculation of the energy bandgap neutral 1-fluoroperylene was observed to have the highest energy gap 3.0414 eV and 3.0507 eV for 6-31++G(d,p) and 6-311++G(d,p) basis sets respectively. These results were found to agree with the existing literature. This reconfirmed 1-fluoroperylene as the most stable molecule. The computations of the ionic molecules reported small values of the energy gap. The molecule with the most chemical hardness was obtained to be the neutral 1-fluoroperylene with a chemical hardness of 1.5253eV. All the ionic molecules results were found to be more reactive than their neutral form for having lower values of chemical hardness. For NLO calculations, the results showed an increment in their values with the ionic hybrid molecules having the largest values. In the case of first-order hyper-polarizability, 1-bromoperylene (neutral), 1-fluoroperylene (neutral), 1-bromoperylene (anionic), 1-fluoroperylene (anionic), 1-bromoperylene (cationic) and 1-fluoroperylene (cationic) were found to be 73.93%, 1.71%, 83.9%, 39.2%,38.7% and 41.7% larger than that of Urea respectively. These calculated results make these hybrid molecules suitable for a wide range of optoelectronic applications.

Accurate Analysis of Anisotropic Carrier Mobility and Structure-property Relationships in Organic BOXD Crystalline Materials.

Charge mobility is an essential factor of organic crystalline materials. Although many investigators have made important progress, the exact relationship between the crystal structure and carrier mobility remains to be clarified. Fortunately, a series of bis-1,3,4-oxadiazole derivatives have been successfully prepared and reported. They have similar main molecular fragments but different crystal packing modes, which provide an ideal research objective for studying the effect of molecular packing on charge mobility in organic photoelectric conversion systems. In this work, the charge mobilities of these molecules are systematically evaluated from the perspective of first-principles calculation, and the effect of a molecular overlap on orbital overlap integral and final charge carrier mobility is fully discussed. It can be seen that the small intermolecular distance (less than 6 Å) is the decisive factor to achieve high electron mobility in π stacking, and better mobility can be obtained by increasing the hole migration distance appropriately. A larger dihedral angle of anisotropy is an important point limiting the charge mobility in the herringbone arrangement. It is hoped that the correlation results between the crystal structure and mobility can assist the experimental study and provide an effective way to improve the photoelectric conversion efficiency of the organic semiconductor devices and multiple basis for multiscale material system characterization and material information.

Improvement of charge transport in organic TIPS semiconductor device using crystal alignment eco-friendly plastic cellulose nano-whisker

Improvement of charge transport in organic TIPS semiconductor device using crystal alignment eco-friendly plastic cellulose nano-whisker

Magnetic Field Effects of Charge Transfer Excitons in Organic Semiconductor Devices

AbstractThe topic of this review is the effect of magnetic field on photo‐current (PC) in organic semiconductor devices. Magnetic field modifies the fraction of the singlet/triplet population in the charge transfer excitons (CTE) whose lifetime is spin configuration dependent yielding magneto‐PC (MPC). We review MPC in the framework of the polaron pair model and emphasize the effect of CTE lifetime on MPC. In addition to a detailed calculation of the MPC response we find a range of physical parameters in which the response follows a simple ′fit function′. For example, a sum of wide and narrow Lorentzian functions, MPC(B)=cwB2/(B2+bw2)+cnB2/(B2+bn2), in the case of hyperfine interaction; or a Voigt profile V=L*G, where the sign * stands for the convolution operation between a Lorentzian (L) and a Gaussian (G) functions, in the case of thermal spin polarization. We provide expressions for the fit parameters in terms of physical parameters.

De Novo Simulation of Charge Transport through Organic Single-Carrier Devices.

In amorphous organic semiconductor devices, electrons and holes are transported through layers of small organic molecules or polymers. The overall performance of the device depends both on the material and the device configuration. Measuring a single device configuration requires a large effort of synthesizing the molecules and fabricating the device, rendering the search for promising materials in the vast molecular space both nontrivial and time-consuming. This effort could be greatly reduced by computing the device characteristics from the first principles. Here, we compute transport characteristics of unipolar single-layer devices of prototypical hole- and electron-transporting materials, N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) and 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) using a first-principles multiscale approach that requires only the molecular constituents and the device geometry. This approach of generating a digital twin of the entire device can be extended to multilayer stacks and enables the computer design of materials and devices to facilitate systematic improvement of organic light-emitting diode (OLED) devices.

On the minimum thickness of doped electron/hole transport layers in organic semiconductor devices

Doped hole (respectively electron) transport layers [HTLs (respectively ETLs)] are commonly used in evaporated organic devices to achieve high work function hole contact (respectively low work function electron contact) in organic LEDs to inject large current, in solar cells to increase the open circuit voltage, and in photodetectors to minimize the dark current. However, optimization of the HTL thickness results from a delicate trade-off. Indeed, on the one hand, to minimize the impact of HTLs on light propagation and series resistance effects, it is commonly admitted that HTLs must be kept as thin as possible. In this work, a model, validated by drift and diffusion simulations, has shown that, depending of the doping level, a minimum thickness between 10 and 20 nm was needed to prevent the transport layer work function from degradation due to field effects. Experiments have been performed on template p-only devices featuring a single HTL of various thicknesses and doping, confirming the validity of the model. Finally, simulations have been performed on a p-i-n device featuring both HTL and ETL. These results constitute precious indications for the design of efficient evaporated organic LEDs, solar cells, or photodetectors.