Energy level alignment and morphology of interfaces between molecular and polymeric organic semiconductors

Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic–organic conductor–semiconductor and semiconductor–semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C₆₀ and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C₆₀ on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation.     

Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors

The lowest unoccupied molecular orbital (LUMO) energies of a variety of molecular organic semiconductors have been evaluated using inverse photoelectron spectroscopy (IPES) data and are compared with data determined from the optical energy gaps, electrochemical reduction potentials, and density functional theory (DFT) calculations. A linear fit to the electrochemical reduction potential (relative to an internal ferrocene reference) vs. the LUMO energy determined by IPES gives a slope and intercept of ?1.19 ± 0.08 eV/V and ?4.78 ± 0.17 eV, respectively, and 0.92 ± 0.04 and ?0.44 ± 0.11 eV, respectively, based on the DFT calculated LUMO energies. From these fits, we estimate the LUMO and exciton binding energies of a wide range of organic semiconductors.     

Simultaneous control of molecular orientation and patterning of small-molecule organic semiconductors for organic transistors

Triethylsilylethynyl anthradithiophene (TES-ADT) has been shown to be a promising soluble semiconductor for the active layer of organic field-effect transistors (OFETs) due to its solution processability, chemical stability and excellent electrical properties. However, there are still some problems that need to be resolved for the utilization of TES-ADT in OFETs. One of these problems is a patterning issue to minimize crosstalk between neighboring TES-ADT FETs. To this end, TES-ADT crystals of various shapes need to be patterned at the desired positions. Here, we demonstrated a simple method to fabricate patterned TES-ADT crystals by using a PDMS mold containing 1,2–dichloroethane (DCE) solvent. This method serves the dual purpose of preparing a variety of pattern shapes while simultaneously changing as-spun TES-ADT thin films into crystal patterns. The top-contact OFETs with the TES-ADT crystal patterns exhibited high performance, reaching a field-effect mobility of ∼0.3 cm² V⁻¹s⁻¹.     

Effect of crystal density on sublimation properties of molecular organic semiconductors

Thermal evaporation is an essential process employed in the fabrication of optoelectronic devices based on small molecular organic materials. Knowing the evaporation properties (e.g. sublimation enthalpy, vapor pressure) of archetypal compounds and being able to predict these properties of new compounds is therefore important for the design of processes and deposition apparatus. To address this lack of reliable, easily reproducible information we used thermogravimetry to characterize the sublimation properties of pentacene; tris(8-hydroxyquinolino) aluminum (Alq₃); 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA); metal-free phthalocyanine (H₂Pc); boron subphthalocyanine chloride (SubPc); iron phthalocyanine (FePc); copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc). A linear relationship was found between enthalpy and vapor pressure, and crystal density, allowing for the estimation of thermophysical properties of untested compounds of a similar class using a simple model.     

Photoluminescence and recombination luminescence in amorphous molecular semiconductors doped with organic dyes

Photoconductivity, photoluminescence (PL), and thermally stimulated luminescence of photoconductive poly-N-epoxypropylcarbazole and poly-N-vinylcarbazole films and non-photoconductive polyvinylbutyral, polyvinyl alcohol, polystyrene, and polyethylene films doped with cationic, anionic, and neutral dyes are studied. It is found that the PL of cationic dyes in photoconductive polymer films is enhanced in comparison to nonphotoconductive ones. The PL enhancement correlates with an increase in photoconductivity, with the quenching effect of an external electric field on the PL intensity, and with an increase in the intensity of the recombination luminescence. It is assumed that this enhancement is related to the presence of predimer traps for holes in the vicinity of dye ions in the films of carbazolyl-containing polymers. A model describing the trap formation upon the photoexcitation of holes into predimer states is suggested.     

某些有机晶体的倍频效应与分子偶极矩之间的关系

初步论证了有机共轭分子的偶极矩与其倍频效应的联系,并采用EHMO法对4-硝基吡啶-1-氧的3位和2位甲基衍生物及4-硝基苯胺的2位和8位卤素衍生物的分子偶极矩进行了理论计算,合成并测定了它们的倍频性能和偶极矩.实验结果与理论预测规律相吻合.     

Analysis of charge transfer complex at the interface between organic and inorganic semiconductors

Improving the electrical performance of organic semiconductors is critical to use them for optoelectronic applications. In this study, we analyze the mechanism of charge transfer complex (CTC) formation at the interface between organic and inorganic semiconductors through extensive optical and electrical measurements. N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and molybdenum oxide (MoO₃) were sequentially deposited to form a donor/accepter heterojunction structure. The CTC formation and conductivity of the films were determined using UV–visible spectroscopy and transmission line method, respectively. Compared with the single layer devices, the donor/accepter heterojunction exhibits significantly enhanced conductivity. In addition, the conductivity and CTC generation efficiency of the heterojunction display strong dependence on NPB layer thickness, which originates from the variation of dipole interactions at the heterojunction interface. These results provide useful insights on interfacial doping properties, which is potentially beneficial for enhancing the understanding of organic/inorganic interfaces.     

Synthesis of cyanated tetracenes as the organic semiconductors

Two novel and air-stable cyanated tetracene derivatives, 5-cyanotetracene (1CT) and 5,11-dicyanotetracene (2CT), were synthesized as high-performance organic semiconductors. The stability of 2CT was evaluated by NMR and the electrochemical property was investigated by cyclic voltammetry (CV) and UV–vis spectrum. The reorganization energy of 2CT predicted by UB3LYP/6-311g(d,p) is 0.0881 eV, which is the lowest among existing compounds. The X-ray crystallographic analysis revealed that the 2CT single crystal has a promising face-to-face packing with a relative short intermolecular distance of 3.403 Å. Based on the theoretical model we previously developed, the calculated hole mobilities of these air-stable cyanated tetracene derivatives in a–b plane are 2.9 cm² V⁻¹ s⁻¹ for 1CT and 2.2 cm² V⁻¹ s⁻¹ for 2CT, respectively. These oxygen-resisted organics may offer potential to fabricate the flexible electronics under air conditions.     

Pinning energies of organic semiconductors in high-efficiency organic solar cells

With the emergence of new materials for high-efficiency organic solar cells(OSCs), understanding and finetuning the interface energetics become increasingly important. Precise determination of the so-called pinning energies, one of the critical characteristics of the material to predict the energy level alignment(ELA) at either electrode/organic or organic/organic interfaces, are urgently needed for the new materials. Here, pinning energies of a wide variety of newly developed donors and nonfull...     

Drift velocity and ionization coefficient for holes in single-valley semiconductors

An analytical theory has been developed for drift velocity (V_d) and ionization coefficient (_h) of holes in silicon. Based on Boltzmann transport equation, expressions for drift velocity (V_d) and ionization coefficient (_h) are derived. The theoretical approach is based on calculation of the collision operator for ionization probability, approximated by a delta function. It is observed that the values of drift velocity (V_d) and ionization coefficient (_h) are in good agreement with experimental results for ionization length (l_io = 70 Å) and ionization energy (ε_i = 2.5 eV). This confirms the validity of the developed theoretical model for drift velocity and ionization coefficient of holes.     

O2 and organic semiconductors: Electronic effects

Contact potential difference (CPD) measurements of the relative work functions of a range of organic semiconductor thin films show that oxygen causes effective p-type doping (with work functions increasing 0.1–0.3 eV). This doping effect is found to be reversible by exposure to high vacuum or heating in inert atmosphere. The mechanism of doping is explained by a model, based on a reversible formation of an O-substrate charge transfer state. Conductivity measurements of p-phthalocyanine films at variable temperatures support this doping model. The oxygen doping effect is consistent with filling of tail states in the gap, as shown by the increase of activation energy of hole transport with decreased O-doping, and by the good fit between experimental data and simulations of the in-gap density of states. A model hybrid solar cell configuration also shows the effect of doping by O₂ and corroborates the fact that O-doping fills the tail states in the system.     

Temperature and field-dependence of hopping conduction in organic semiconductors

Electrical characteristics of the hopping transport in organic semiconductors are studied theoretically. Based on percolation theory of hopping between localized states, an analytical mobility model is obtained. This model is applied to the analysis of both the electric field dependence and the temperature dependence of the mobility. The results agree quantitatively with recent experimental data.     

Molecular orientation and thermal stability of thin-film organic semiconductors

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.     

Effect of electron-electron interactions on the ionization rate of charge carriers in semiconductors

The effect of the presence of a large carrier density on the ionization rate of carriers in a semiconductor is investigated. Thus e-e interactions have been considered in addition to normal scattering events, i.e. generation of optical phonons and electron-hole pair production. For a charge density n = 10²¹/m³ and above the ionization rate decreases appreciably due to e-e interactions.     

Utilization of triplet excited states in organic semiconductors

Organic optoelectronics is an emerging research field, which has attracted extensive interests in the last few decades owing to its practical applications, like organic light-emitting diodes (OLEDs), organic memory devices, organic photovoltaic (OPV), sensors, and organic field-effect transistors[1, 2]. Organic semiconductors play a crucial role in this field. Compared to the traditional inorganic semiconductors, organic semiconductors open a fascinating research direction because of some unique advantages, such as flexible design, low cost, and rich optical and electronic properties. In organic optoelectronics, the excited states greatly determine the photoelectronic properties and application areas as shown in Fig. 1. Based on the electron spin in the molecule, the excited states of organic semiconductors include singlet and triplet states. As we know, the radiative transitions of singlet and triplet excited states are always accompanied by fluorescence and phosphorescence emission, respectively.     

Langevin-recombination-controlled explosive kinetics of electroluminescence in organic semiconductors

The kinetics of electroluminescence in organic semiconductors is investigated theoretically taking into account the strong dependence, characteristic of these materials, of the charge-carrier mobility on the electric field strength. Recombination of electron-hole pairs under the influence of an external electric field and the electric field due to the Coulomb interaction is investigated on the basis of the Langevin theory. It is shown that as a result of the nonuniformity of the field and the field-dependence of the mobility, the recombination kinetics after the external field is switched off is explosive. Fiz. Tekh. Poluprovodn. 33, 945–947 (August 1998)     

Current—Voltage characteristics of bulk semiconductors on impact ionization

A simple theoretical model based on the concept of impact ionization of carriers is worked out to represent the current-voltage characteristics of bulk semiconductors. The underlying idea is that the carriers start attaining velocity saturation and multiplying themselves soon after acquiring a minimum energy required for impact ionization. Consequently, in a certain region of the device the rate of decrease of resistance with the increase of carrier concentration becomes very high giving rise to negative resistance. The above model agrees fairly well with the experimental results.     

Effect of trap states on the electrical doping of organic semiconductors

We demonstrate that direct charge transfer (CT) from trap states of host molecules to the p-dopant molecules raises the doping effect of organic semiconductors (OS). Electrons of the trap states in 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) (E_HOMO = 6.1 eV) are directly transferred to the p-dopant, 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) (ELUMO = 5.4 eV). This doping process enhances the conductivity of doped OS by different ways from the ordinary doping mechanism of generating free hole carriers and filling trap states of doped OS. Trap density and trap energy are analysed by impedance spectroscopy and it is shown that the direct charge transfer from deep trap states of host to dopants enhances the hole mobility of doped OS and the I–V characteristics of hole only devices.     

Contacts between amorphous metals and semiconductors

The letter describes the relative enhancement of the thermal stability of Schottky barriers on both silicon and gallium arsenide that can be achieved using thin films of amorphous Ni-Nb and Ta-Ir alloy, the latter alloy being among the most stable fabricated to date, with less than 5% degradation of the barrier height after 2.5 h at 500°C.     

Assessing the width of Gaussian density of states in organic semiconductors

The Density of States (DOS) is an ingredient of critical importance for the accurate physical understanding of the optoelectronic properties of organic semiconductors. The disordered nature of this class of materials, though, renders the task of determining the DOS far from trivial. Its extraction from experimental measurements is often performed by driving the semiconductor out of thermal equilibrium and therefore requires making assumptions on the charge transport properties of the material under examination. This entanglement of DOS and charge transport models is unfavorable since transport mechanisms in organic semiconductors are themselves still subject of debate. To avoid this, we propose an alternative approach which is based on populating and probing the DOS by means of capacitive coupling in Metal Insulator Semiconductors (MIS) structures while keeping the semiconductor in thermal equilibrium. Assuming a Gaussian shape, we extract the DOS width by numerical fitting of experimental Capacitance–Voltage curves, exploiting the fact that the DOS width affects the spatial distribution of accumulated charge carriers which in turn concurs to define the MIS capacitance. The proposed approach is successfully tested on two benchmark semiconducting polymers, one of n-type and one of p-type and it is validated by verifying the robustness of the extraction procedure with respect to varying the insulator electrical permittivity. Finally, as an example of the usefulness and effectiveness of our approach, we study the static characteristics of thin film transistors based on the aforementioned polymers in the framework of the Extended Gaussian Disorder transport model. Thanks to the extracted DOS widths, the functional dependence of current on the gate voltage is nicely predicted and physical insight on transistor operation is achieved.