Simultaneous measurement of carrier density and mobility of organic semiconductors using capacitance techniques

We present a method to measure both the majority carrier density and mobility in organic semiconductors from the voltage and frequency dependence of capacitance (C–V–f). Poly(3-hexylthiophene) (P3HT) is used as the prototypical material. The carrier density, and its spatial distribution in a planar device structure, is obtained from a subset of the C–V–f data by conventional capacitance–voltage analysis. We show that the validity of the carrier density extraction depends critically on the measurement frequency. Namely, one should make sure that the measurement frequency is lower than the modified dielectric relaxation frequency, which is characteristically low in organic semiconductors due to their low carrier mobility. Our method further exploits the voltage dependence of the modified dielectric relaxation frequency to measure the conductivity and carrier mobility. This mobility extraction method requires no complex fitting or simulation. Nor does it assume any particular dispersive model of mobility a priori. The carrier density, mobility, and conductivity of P3HT all increase with temperature from 250 to 300 K. The activation energies of mobility and conductivity are 0.15 ± 0.01 and 0.24 ± 0.03 eV, respectively.     

Effects of iodine doping on small molecule organic semiconductors for high charge carrier mobility and photoconductivity

Here we report the effects of iodine doping on small molecule organic semiconductors. Thin films of semiconducting p-DTS(FBTTh₂)₂ doped with 1–5 wt% iodine were fabricated and their photo-physical, crystallographic, morphological, and electrical properties were systematically analyzed. The doping significantly increased the energetic distance between the highest occupied molecular orbital (HOMO) and Fermi level of p-DTS(FBTTh₂)₂, typical for p-type doping. In addition, depletion mode transistor measurements showed an increase in the hole concentration with increasing dopant concentration. From grazing incidence X-ray diffraction (GIXD) analyses of iodine-doped p-DTS(FBTTh₂)₂ films, we observed significant changes in the crystal orientation at the optimal doping ratio of 1 wt%. Atomic force microscopy (AFM) analyses showed morphological changes with respect to dopant concentrations, which were in good agreement with the GIXD results. As a result, accumulation mode transistor measurements demonstrated an increase in the hole mobility by 54% at the optimized doping concentration compared to an undoped device. Furthermore, photoconductive device operation revealed that iodine-doping can induce dramatically enhanced photo-responsivity as high as 2.08 A/W. We demonstrate that iodine doping can be a simple and effective method for enhancing the performance of small molecule-based electronic devices, by optimizing the energy level configuration as well as enhancing intermolecular interactions.     

BODIPY derivatives as n-type organic semiconductors: Isomer effect on carrier mobility

In the present work, two dipyrro-boradiazaindacenes (BODIPY) derivatives functioning as novel high-performance organic semiconductors are investigated by theoretical method. These two isomeric complexes are demonstrated to have large electron-transfer mobility, which means they are favor to be n-type organic semiconductors. The highest electron-transfer mobility appears at the same packing style in two crystals. The intermolecular distances of the packing style are nearly same, 4.994 Å in crystal 1 and 5.283 Å in crystal 2. However, their electron-transfer mobility changes significantly. The mobility of crystal 2 with better planar molecular structure is 0.291 cm² V⁻¹ s⁻¹, which is 13 times larger than that of crystal 1 as 0.022 cm² V⁻¹ s⁻¹. The significant difference of carrier mobility is ascribed to the little structural difference of these two isomers. It has been demonstrated that both crystal 1 and 2 show remarkable anisotropic behavior. This study will undoubtedly provide a new understanding of isomerization on designing novel organic semiconductors.     

On the mobility of polycrystalline semiconductors

A simple phenomenological model based on tunnel and thermionic emission across grain boundary barrier has been developed.The present model has been applied to polycrystalline silicon assuming that the dangling bonds at the grain boundaries behave as electron traps. The calculations have been carried out in two different ways; one, assuming that the interface states density, N_is (cm^?2 eV^?1), is constant across the energy gap; and second with the boundary states, N_T (cm^?2), localized around a very narrów energy range at E_T.In the first case no differences in the mobility reduction have been found between n and p type polysilicon, but for the assumption of states localized at an energy E_Tin the upper half of the gap, the barrier height is larger in n-type than in p-type material and consequently the calculated mobility of n-type polysilicon becomes lower than the p-type mobility.In general, the mobility increases with the dopant concentration approaching the monocrystalline behaviour for very large dopings, in qualitative agreement with other approaches and with available experiments.     

The role of the density of states on the hole mobility of disordered organic semiconductors

It was recently demonstrated that the hopping mobility in disordered organic semiconductors depends both on the charge carrier concentration and on the density of states but their relative influence is still an open question. The mobility is almost constant below a certain concentration and increases at large concentration where the density of states is conventionally assumed exponential. Hence the experimental hole mobility in polymer FETs is at least one order of magnitude larger than the hole mobility in LEDs based on the same materials. In order to investigate this issue, the mobility of disordered organic semiconductors is calculated by numerically solving the variable range hopping (VRH) equations in a wide range of carrier concentrations ranging from low concentrations, typical of hole-only diodes, to high concentrations, typical of field effect transistors. The exact mobility is numerically calculated for various density of states (DOS) and it is compared with the experimental data in order to investigate the dependence of the hole mobility on the DOS. Our calculations show that the strong dependence of the hole mobility on the charge carrier density is strictly correlated to the shape of the DOS and that a single gaussian, in general does not explain the mobility behavior in the whole range of concentrations; depending on the material, it could be accurately approximated by a single gaussian, an exponential, or by a combination of both functions.     

The mutual influence of surface energy and substrate temperature on the saturation mobility in organic semiconductors

We report on a mutual correlation between the substrate temperature during semiconductor deposition and the surface energy of the gate dielectric on the charge carrier mobility in bottom gate top contact organic field effect transistors (OFETs) with N,N′-diphenyl-3,4,9,10-perylene tetracarboxylic diimide (DP-PDI) as organic semiconductor.     

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.     

Automatic electrochemical profiling of Hall mobility in semiconductors

High-resolution automatic profiling of Hall mobility in semiconductors is achieved through the use of an electrolytic contact, both as a Schottky gate and as a means of controlled dissolution. Application to multilayer m.b.e. GaAs is shown as an example.     

Towards reliable charge-mobility benchmark measurements for organic semiconductors

Charge carrier mobility is a figure of merit commonly used to rate organic semiconducting materials for their suitability in applications such as solid-state lighting or photovoltaics. Although large variations are found in published mobility values on identical materials, there is little open discussion in the literature of the reproducibility of these results. We address this with an interlaboratory study of mobility measurements performed on a set of organic semiconductors using the space-charge limited current method. We found mobility measured on nominally identical devices could vary by more than one order of magnitude, with the largest sources of variation being poor electrodes and film thickness variation. Moreover, we found that mobility values extracted from identical data by different scientists would typically vary by a factor of 3. We propose a protocol for analysis and reporting that was found to reduce this analysis variation to as little as 20%. We also present general guidelines for improving the reproducibility of benchmark mobility measurements.     

In-situ/operando characterization techniques for organic semiconductors and devices

The increasing demands of multifunctional organic electronics require advanced organic semiconducting materials to be developed and significant improvements to be made to device performance. Thus, it is necessary to gain an in-depth understanding of the film growth process, electronic states, and dynamic structure-property relationship under realistic operation conditions, which can be obtained by in-situ/operando characterization techniques for organic devices. Here, the up-to-date developments in the in-situ/operando optical, scanning probe microscopy, and spectroscopy techniques that are employed for studies of film morphological evolution, crystal structures, semiconductor-electrolyte interface properties, and charge carrier dynamics are described and summarized. These advanced technologies leverage the traditional static characterizations into an in-situ and interactive manipulation of organic semiconducting films and devices without sacrificing the resolution, which facilitates the exploration of the intrinsic structure-property relationship of organic materials and the optimization of organic devices for advanced applications.     

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...     

Isomers of dialkyl diketo-pyrrolo-pyrrole: Electron-deficient units for organic semiconductors

Diketo-pyrrolo-pyrrole (DPP) is an important electron-deficient unit, and the alkylation of DPP can gives rise to good solubility of organic and polymer semiconductors in solvent. In this paper, we report our observations on the isomers of dialkyl DPP. The alkylation of DPP can take place on both the nitrogen and oxygen atoms, leading to the formation of three isomers. The presence of the isomers is evidenced by the nuclear magnetic resonance (NMR) and mass spectra. The yields of the isomers are affected by the experimental conditions, including the reaction temperature, reaction time and solvent. The isomers have different electronic structures and optical properties as revealed by the UV–Vis absorption spectroscopy, fluorescent spectroscopy, cyclic voltammetry and theoretical simulation using Gaussian 03 program with B3LYP/6-31G(d). Small molecules based on the DPP isomers with dialkylation at different sites were synthesized and investigated as donor materials of organic solar cells (OSCs). The photovoltaic performances of OSCs are consistent with the electronic structures of the DPP isomers.     

Temperature-dependent hole and electron mobility models for CMOScircuit simulation

Semi-empirical hole and electron mobility models with temperature dependence have been proposed for circuit simulation as well as for process characterization. These models are based on the universal dependence of low field mobility on the effective transverse field and cover a wide range of oxide thickness as well as of temperature. The accuracy of our models is justified by comparing them with experimental work reported in the literature as well as that obtained in our laboratory. They are accurate and physical enough to be suited for the circuit simulation of modern VLSI CMOS circuits with gate oxide thickness less then 400 Å in the temperature range of 250-400 K     

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.     

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.     

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.     

Energy-dependent mobility in the small-signal analysis of waves in semiconductors

It is shown that the effect of energy-dependent mobility can be included in the small-signal analysis of semiconductot samples by an appropriate change in the square of the plasma frequency of the carriers.     

Exponential-type density of states with clearly cutting tail for organic semiconductors

It was recently demonstrated that the density of states (DOS) is the key factor to determine charge transport, photoelectric and contact properties in disordered organic semiconductors. However, the density of states in organic semiconductors is unclear at present. Although the Gaussian DOS is the most popular, some works support the exponential DOS or combination of both forms. In this paper, we propose three exponential-type DOS, one has complete exponential tail, and other two cuts tails at some places. The variations of mobility with carrier density are obtained through numerically solving variable range hopping (VRH) equations. It is shown that the relationships of mobility with density and Fermi level are very different among results obtained from Gaussian, un-cutting and cutting exponential-type DOS. The results show that the experimental mobility-density data can be well fitted by using single cutting exponential-type DOS in the wide ranges of density, but cannot be fitted by using single Gaussian and un-cutting exponential-type DOS. Instead of the Gaussian and pure exponential DOS, the DOS with exponential core and clearly cutting tail is recommended.