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Fermi Level Pinning by Gap States in Organic Semiconductors
Abstract
We measure the gap density of states and the Fermi level position in thin-film transistors based on pentacene and dinaphtho[2,3-b:2^{'},3^{'}-f]thieno[3,2-b]thiophene (DNTT) films grown on various surfaces using Kelvin probe force microscopy. It is found that the density of states in the gap of pentacene is extremely sensitive to the underlying interface and governs the Fermi level energy in the gap. The density of gap states in pentacene films grown on bare silicon dioxide (SiO_{2}) was found to be larger by 1 order of magnitude compared to that in pentacene grown on SiO_{2} treated with hexamethyldisilazane and larger by 2 orders of magnitude compared to that of pentacene grown on aluminum oxide (AlO_{x}) treated with a self-assembled monolayer (SAM) of n-tetradecylphosphonic acid (HC_{14}-PA). When DNTT was grown on HC_{14}-PA-SAM-treated AlO_{x}, the gap density of states was even smaller, so that the Fermi level pinning was significantly reduced. The correlation between the measured gap density of states and the transistor performance is demonstrated and discussed.
... The key approximations and findings associated with this large slope enhancement of the S vs. log(p) (Pisarenko) characteristics are discussed below in the context of published angle-resolved photoemission spectroscopy (ARPES) data (which can be compared with the homogeneous and inhomogeneous FWHM broadening of the LDOS found in this model). 22,38,46 Furthermore, as there is currently a lot of interest in the thermoelectric properties of soft materials, [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] and such materials are particularly susceptible to the effects of dynamic disorder, [18][19][20][21][22][23][24][25][26] we extend our discussion on this enhanced behavior of the Seebeck coefficient to related topics, such as: (i) the role played by dynamic thermal disorder versus static thermal disorder on TE transport in soft materials, 28,52,55 (ii) the role played by dopant-induced energetic disorder on TE transport in the presence of thermal disorder (both static and dynamic), 30,[55][56][57][58][59]63,64 (iii) the net contributions of dynamical-induced trapping versus carrierinduced vibrational softening to this large slope enhancement of the Seebeck coefficient, 53,88,89 (iv) the overall contribution of this carrier-induced vibrational softening in S on the vibrational entropy and its potential for producing sizeable reduction in the thermal conductivity at high carrier concentrations, 13,[89][90][91][92][93][94] and (v) the extent to which our findings are supported by recent experimentally deduced behaviors of the Fermi level shifting 61,[95][96][97][98][99] and closely associated changes in the energy distribution of conducting carriers 100 (both of which are critically important in assessing TE properties). 81,84 ...
... The overall better performance of the dynamic disorder model in predicting thermoelectric properties is further supported by its accuracy in predicting the apparent saturation of the Seebeck coefficient (S) [55][56][57][58][59]63,64 and the electrical conductivity (σ) 61 in materials with bulk parts-per-million (ppm) doping (shown in Fig. 4(a) and Fig. 4(b)), where both thermal disorder and a larger degree of static energetic disorder (∆) due to the presence of dopant species 48,49 (incorporated here by considering a random distribution of site (molecular) energies i with variance ∆ in Eq. (1)) are known to be detrimental to charge 11,14,17,[35][36][37][38]44,45,48,57,61,95,96 and TE transport. [27][28][29][30][31][32][53][54][55][56][57][58][59] See the Appendix A (below) for more details on how the effects of static dopant-induced energetic disorder was incorporated in our model. ...
A model is developed that accounts for the effects of thermal disorder (both static and dynamic) in predicting the thermoelectric (TE) performance of weakly bonded semiconductors. With dynamic disorder included, the model is found to fit well with experimental results found in the literature for the density-of-states and the energy-dependent carrier mobility, which are key for assessing TE properties. The model is then used to analyze the concentration-dependent TE properties of the prototypical small molecular semiconductor rubrene. At low (e.g., intrinsic) carrier concentrations, where Fermi level pinning occurs, dynamic disorder is found to reduce electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF) to values that are much lower than those traditionally predicted by static disorder models. As carrier concentration (p) increases, S exhibits nonlinear behavior, increasing well above the conventional S vs log(p) relationship before reaching a peak value (Speak∼1550μV/K). A critical carrier concentration (pcrit.≈4.299×10−4 molar ratio) is observed near Speak at which thermoelectric transport transitions from trap-limited behavior at low concentrations to conventional band behavior at high concentrations. Above this value, σ and PF are reduced compared to the perfect crystal and static-only conditions, causing a drop in the maximum PF by factors of 3 and 2.3, respectively. This PF reduction, while not as large as the PF reduction that occurs for low carrier concentration, is found to occur in a high concentration regime (p>pcrit.) that contains the PF maximum and has remained inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.
... First, in Fig. 1E and movie S1, the source/drain electrodes can be easily peeled off by removing the PDMS template, and there is almost no damage or residue on the PTCDI-C8 film. This indicates that there is no strong chemical interaction between the oxide electrode and the semiconductor (26). A series of material characterization techniques [including AFM, XPS, and x-ray diffraction (XRD) measurements] further confirmed that the peeling process is nondestructive (see fig. ...
Intrinsic gain is a vital figure of merit in transistors, closely related to signal amplification, operation voltage, power consumption, and circuit simplification. However, organic thin-film transistors (OTFTs) targeted at high gain have suffered from challenges such as narrow subthreshold operating voltage, low-quality interface, and uncontrollable barrier. Here, we report a van der Waals metal-barrier interlayer-semiconductor junction–based OTFT, which shows ultrahigh performance including ultrahigh gain of ~10 ⁴ , low saturation voltage, negligible hysteresis, and good stability. The high-quality van der Waals–contacted junctions are mainly attributed to patterning EGaIn liquid metal electrodes by low-energy microfluidic processes. The wide-bandgap semiconductor Ga 2 O 3 as barrier interlayer is achieved by in situ surface oxidation of EGaIn electrodes, allowing for an adjustable barrier height and expected thermionic emission properties. The organic inverters with a high gain of 5130 and a simplified current stabilizer are further demonstrated, paving a way for high-gain and low-power organic electronics.
... The DOS is sensitive to the variations in of the O/C ratioas a gradual opening of the gap around the Fermi level has been observed as oxygen is incorporated into the material [25]. A slight opening of the gap is observed during the first oxidation degrees; however, it is up to GO (-O, 0.17) , that a behavior opposite to the initial one is presented since the gap begins to close and extends until it is minimal at GO (-O, 0.22) , the density of states evaluated at the Fermi level is greater than zero, meaning that, with a minimum amount of energy, the conduction band states can be occupied by the electrons of the valence band [68,69]. This is in full agreement with the bandgap of 0.021 eV obtained in section 2 of Fig. 4b. ...
... where dV G dV CH is extracted from the backward sweep of the experimentally measured E F -hysteresis curve 35,36,51,52 . Figure 4b shows the backward E F -hysteresis curves measured on the flat (red) and the nanobubble (blue series) regions with different heights of 1.8, 4, and 6.7 nm, as presented in Fig. 3b. ...
The electrical stability and reliability of two-dimensional (2D) crystal-based devices are mainly determined by charge traps in the device defects. Although nanobubble structures as defect sources in 2D materials strongly affect the device performance, the local charge-trapping behaviors in nanobubbles are poorly understood. Here, we report a Fermi-level hysteresis imaging strategy using Kelvin probe force microscopy to study the origins of charge trapping in nanobubbles of MoS2 on SiO2. We observe that the Fermi-level hysteresis is larger in nanobubbles than in flat regions and increases with the height in a nanobubble, in agreement with our oxide trap band model. We also perform the local transfer curve measurements on the nanobubble structures of MoS2 on SiO2, which exhibit enhanced current-hysteresis windows and reliable programming/erasing operations. Our results provide fundamental knowledge on the local charge-trapping mechanism in nanobubbles, and the capability to directly image hysteresis can be powerful tool for the development of 2D material-based memory devices.
... However, in organic electronics, the metallization within the first few molecular layers of organic semiconducting thin films during electrode deposition was confirmed to induce gap states, interfacial chemical and physical defects 10 . As a consequence, an interface dipole ( ) can be spontaneously formed between the organic semiconductor and the metal surface; thus, the Fermi level ( ) was always found to be pinned, so that the theoretically calculated ∅ values are not consistent with the experimental observation 11 . ...
Fermi-level pinning (FLP) effect was widely observed in thin-film transistors (TFTs) with van der Waals (vdW) layered semiconductors (organic or two-dimensional) when contact electrodes were thermally evaporated1-3. Intensive investigation was implemented for formation of FLP-free interfacial states by eliminating chemical disorder and crystal defects arising from metal deposition4-9. However, technical and principal challenges are still existing towards high-yield, wafer-scalable and low-cost integration of TFT devices. Herein, we developed a general, scaling-up strategy to fabricate large-scale, high-performance FLP-free organic TFT (OTFT) arrays by using printed vdW contacts consisting of MXene composite electrodes and 2, 7-dioctyl [1] benzothieno [3, 2-b] [1] benzothiophene (C8BTBT). Room-temperature processes allow for a physically stacked junction without any structural or chemical damages. The OTFT arrays can be printed on a large-area silicon wafer or plastic film with 100% yield, exhibit ultrahigh field-effect mobility ({\mu}_FE) over 17.0 square centimetres per volt per second (cm2 V-1s-1), high on/off ratio exceeding 108, relatively low contact resistance of 3k ohm micrometres. The underlying mechanism for the high device performance was unveiled by Kelvin Probe Force Microscopy (KPFM) combined with theoretical simulation. The results indicate that work function (W_F) of the printed electrodes can be tuned at a wide range of 4.8-5.6 eV, thus significantly lowering the charge-injection barrier at the contact interfaces with ideal FLP-free character (the interfacial factor reaches 0.99 \pm 0.02). This study paves a general strategy for achieving large-scale, high-performance thin-film electronics.
... Reasons for the very weak field effect could be attributed to: low capacitance density of the 90 nm thermal SiO 2 . Using thinner dielectric with higher dielectric constant, like AlO or ZrO can vastly improve the field effect [46][47][48] or high defect density at the dielectric/semiconductor interface and semiconductor bulk leading to fermi-level pinning as in organic-TFTs [49]. The biggest hurdle might be the microstructure, inducing a high amount of defects. ...
... These trap sites act as energetic obstacles for charge carriers, leading to low charge carrier mobility. 60,61) Considering the transport nature of organic materials, charge transport in the undoped OSC can be improved by doping. Additional charge carriers released by the dopants lead to an increase in mobility by the filling of traps, reducing the number of trap sites to hop for charge carriers. ...
Molecular doping of organic semiconductors (OSCs) has been widely utilized to modulate the charge transport characteristics and charge carrier concentration of active materials for organic electronics such as organic photovoltaics, organic light-emitting diodes, and organic field-effect transistors. For the application of molecular doping to organic electronics, the fundamentals of molecular doping should be thoroughly understood in terms of doping mechanism, host and dopant materials, doping methodologies, and post-doping properties such as doping-induced structural/energetic disorder and doping stability. In this report, the fundamental understanding of molecular doping, types of dopants, doping methods, and their practical applications as organic field-effect transistors, organic photovoltaics, and organic thermoelectric are reviewed. Finally, key strategies for efficient molecular doping may exceed the trade-off relation between device performance and structural disorder.
... Gap states may be charge traps, thus directly influencing the transport properties of the material. The density of gap states of crystalline and polycrystalline organics can be obtained using current voltage [164] or Kelvin probe measurements [165]. However, the density of gap states can also be studied by angular resolved UPS, as in [166] for C 60 crystals. ...
Recently, the scientific community experienced two revolutionary events. The first was the synthesis of single-layer graphene, which boosted research in many different areas. The second was the advent of quantum technologies with the promise to become pervasive in several aspects of everyday life. In this respect, diamonds and nanodiamonds are among the most promising materials to develop quantum devices. Graphene and nanodiamonds can be coupled with other carbon nanostructures to enhance specific properties or be properly functionalized to tune their quantum response. This contribution briefly explores photoelectron spectroscopies and, in particular, X-ray photoelectron spectroscopy (XPS) and then turns to the present applications of this technique for characterizing carbon nanomaterials. XPS is a qualitative and quantitative chemical analysis technique. It is surface-sensitive due to its limited sampling depth, which confines the analysis only to the outer few top-layers of the material surface. This enables researchers to understand the surface composition of the sample and how the chemistry influences its interaction with the environment. Although the chemical analysis remains the main information provided by XPS, modern instruments couple this information with spatial resolution and mapping or with the possibility to analyze the material in operando conditions at nearly atmospheric pressures. Examples of the application of photoelectron spectroscopies to the characterization of carbon nanostructures will be reviewed to present the potentialities of these techniques.
... [33,35] Surveys such as that conducted by Northrup [44] showed that ambient factors, for example hydrogen and oxygen, can be attributed to the formation of the positively charged defects at the pentacene/ dielectrics interface, confirmed by electrical measurements and scanning probe techniques. [45][46][47] These charged defects can be considered as an insulator layer and cause an additional potential barrier for hole-transport between pentacene and electrets, which consequently increases the operating voltages of pentacene-based ONVMs using electrets. [33,35] To reduce the height of the hole-barrier, Wang et al. introduced a small-molecular semiconductor as an interlayer between pentacene and polymer layer, which enables the ONVMs with low programming/erasing voltage and large memory windows. ...
Organic field‐effect transistor (OFET) memory based on pentacene has attracted a lot of attentions due to its promising prospect of application in flexible electronics, while the high programming/erasing (P/E) gate voltages due to the existence of hole barrier at pentacene/polymer interface leaves great challenges for its commercial applications. A high‐performance pentacene‐based OFET nonvolatile memory (ONVM) with polymer blends is reported here as the charge‐trapping layer containing poly(2‐vinyl naphthalene) (PVN) and poly{[N,N'‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5'‐(2,2'‐bithiophene)} (N2200). The presence of N2200, an n‐type semiconductor, in blends significantly improves the memory performance of pentacene‐based memory devices based on the static‐electric effect. The electrons in N2200 are aggregated near the pentacene/polymer interface due to the electric attraction from the positively charged defects in pentacene. Furthermore, those electrons reduce the height of hole barrier and produce local easy‐transportation paths for holes between pentacene and PVN, which enables the electret‐based ONVM device with low P/E voltages, fast P/E speeds, large mobility and stable multilevel data‐storage ability in ambient air. By introducing poly{[N,N‐bis(2‐octyldodecyl) naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5‐(2,2‐bithiophene)} (N2200) into electret, the hole barrier at the pentacene/electret interface in organic memory can be modulated, and local easy‐transportation paths for holes between pentacene and electret are produced. The memory can obtain endurance over 1000 cycles, multilevel data‐storage ability, and stable retention over 104 s by adjusting the composition of N2200 in N2200‐electret blends.
... The deviation of S from the Schottky-Mott limit is known generally as FLP and has many potential physical origins. [55,57,107,109,[175][176][177] There is growing evidence that the benefits of dipolar SAMs and similar approaches are inherently limited by FLP. [77,178] FLP in metal-organic semiconductor interfaces has been most prominently observed and studied through investigations using UPS. ...
To take full advantage of recent and anticipated improvements in the performance of organic semiconductors employed in organic transistors, the high contact resistance arising at the interfaces between the organic semiconductor and the source and drain contacts must be reduced significantly. To date, only a small portion of the accumulated research on organic thin‐film transistors (TFTs) has reported channel‐width‐normalized contact resistances below 100 Ωcm, well above what is regularly demonstrated in transistors based on inorganic semiconductors. A closer look at these cases and the relevant literature strongly suggests that the most significant factor leading to the lowest contact resistances in organic TFTs so far has been the control of the thin‐film morphology of the organic semiconductor. By contrast, approaches aimed at increasing the charge‐carrier density and/or reducing the intrinsic Schottky barrier height have so far played a relatively minor role in achieving the lowest contact resistances. Herein, the possible explanations for these observations are explored, including the prevalence of Fermi‐level pinning and the difficulties in forming optimized interfaces with organic semiconductors. An overview of the research on these topics is provided, and potential device‐engineering solutions are discussed based on recent advancements in the theoretical and experimental work on both organic and inorganic semiconductors. Though organic transistors have long been considered as a potential cornerstone technology for flexible electronics applications, their widespread commercialization in high‐frequency circuits has been substantially impeded by high contact resistance. A review of the state‐of‐the‐art reveals the pivotal role of the organic‐semiconductor morphology and the key challenges now to be overcome, such as Fermi‐level pinning and low charge‐carrier densities.
... These states are found at energies in which the DOS tails into the gap and are thought to originate from defects or structural imperfections. [60,[68][69][70][71] Right at the interfaces with metal electrodes, a hybridization of metal and semiconductor states tends to widen the DOS of organic semiconductors tends to widen even more. ...
Device modeling is an established tool to design and optimize organic electronic devices, be them organic light emitting diodes, organic photovoltaic devices, or organic transistors and thin‐film transistors. Further, reliable device simulations form the basis for elaborate experimental characterizations of crucial mechanisms encountered in such devices. The present contribution collects and compares contemporary model approaches to describe charge transport in devices. These approaches comprise kinetic Monte Carlo, the master equation, drift‐diffusion, and equivalent circuit analysis. This overview particularly aims at highlighting the following three aspects for each method: i) The foundation of a method including inherent assumptions and capabilities, ii) how the nature of organic semiconductors enters the model, and iii) how major tuning handles required to control the device operation are accounted for, namely temperature, external field, and provision of mobile carriers. As these approaches form a hierarchy of models suitable for multiscale modeling, this contribution also points out less established or even missing links between the approaches.
... Compared with evaporated metals, high-energy metal atoms are likely to damage the semiconductor interface and increase the interface's defects, leading to Fermi pinning phenomenon, which may not be conducive to the charge injection. 46,47 The physical contact polymer electrodes used here can effectively optimize the interface quality to achieve low-contact resistance and ideal characteristic curves. The stability of the transistor is one of the critical parameters in practical application. ...
Conductive polymers are considered promising electrode materials for organic transistors, but the reported devices with conductive polymer electrodes generally suffer from considerable contact resistance. Currently, it is still highly challenging to pattern conductive polymer electrodes on organic semiconductor surfaces with good structure and interface quality. Herein, we develop an in situ polymerization strategy to directly pattern the top-contacted polypyrrole (PPy) electrodes on hydrophobic surfaces of organic semiconductors by microchannel templates, which is also applicable on diverse hydrophobic and hydrophilic surfaces. Remarkably, a width-normalized contact resistance as low as 1.01 kΩ·cm is achieved in the PPy-contacted transistors. Both p-type and n-type organic field-effect transistors (OFETs) exhibit ideal electrical characteristics, including almost hysteresis-free, low threshold voltage, and good stability under long-term test. The facile patterning method and high device performance indicate that the in situ polymerization strategy in confined microchannels has application prospects in all-organic, transparent, and flexible electronics.
... The first component of these dielectrics is a thin metal oxide that can be produced by atomic layer deposition 8,24,25 , anodic oxidation [26][27][28] , UV/ozone-assisted oxidation [29][30][31] , or plasma-assisted oxidation of the surface of the gate electrode 13 . Among the advantages of the plasma-oxidation process are the fact that it does not require electrical contact to the gate metal during the oxidation process 32 (which greatly simplifies the fabrication process), that the oxide is formed only where needed for the TFTs (which eliminates the need for subtractive patterning to open vias for interconnects 22 ) and that the high quality of the native interface between the gate metal and the gate oxide minimizes the hysteresis in the current-voltage characteristics and the subthreshold swing of the TFTs 33,34 . The most popular material combinations for the gate metal and the gate oxide are aluminum/aluminum oxide 15,35 and titanium/titanium oxide 14,27 . ...
A critical requirement for the application of organic thin-film transistors (TFTs) in mobile or wearable applications is low-voltage operation, which can be achieved by employing ultrathin, high-capacitance gate dielectrics. One option is a hybrid dielectric composed of a thin film of aluminum oxide and a molecular self-assembled monolayer in which the aluminum oxide is formed by exposure of the surface of the aluminum gate electrode to a radio-frequency-generated oxygen plasma. This work investigates how the properties of such dielectrics are affected by the plasma power and the duration of the plasma exposure. For various combinations of plasma power and duration, the thickness and the capacitance of the dielectrics, the leakage-current density through the dielectrics, and the current–voltage characteristics of organic TFTs in which these dielectrics serve as the gate insulator have been evaluated. The influence of the plasma parameters on the surface properties of the dielectrics, the thin-film morphology of the vacuum-deposited organic-semiconductor films, and the resulting TFT characteristics has also been investigated.
... 18 For similar systems, alternative mechanisms have been proposed to explain the energy-level alignment, such as integer charge transfer (ICT) induced states 12,17 or induced density of interface states (IDIS). 19,20 However, a general description of such substrate-independent pinning (SIP) is still missing, although SIP is beneficial for many electronic applications, because a layer of a material showing SIP will exhibit the same work function on any substrate and, thus, allows facile charge-injection barrier tuning of subsequently deposited layers. 21−23 PTCDA has not found wide application for energy-level engineering purposes as the substrate independent work function is around 4.60 eV ( Figure 1a), 18,24 which is neither a particularly large nor low work function, and thus, PTCDA thin films are neither superior for hole nor electron injection. ...
Tailor-made electrode work functions are indispensable to control energy-level offsets at the interfaces of (opto-)electronic devices. We show by means of photoelectron spectroscopy that several nanometer thick layers of the organic semiconductor 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-hex-acarbonitrile (HAT-CN) on virtually all substrates provide hole-injecting electrodes with work functions of around 5.60 eV. This substrate-independent energy-level alignment is due to a relatively large density of gap states in HAT-CN thin films, which is clearly visible in the photoemission data. Furthermore, this additional density of occupied states makes the wide-gap semiconductor thin films sufficiently conductive for electrode applications. Moreover, our study highlights a quite intriguing energy-level alignment scenario as the Fermi-level in HAT-CN thin films is located far from the midgap position, this is rather uncommon for undoped organic semiconductor thin films.
... To further improve the interface between the organic semiconductor material and the metal oxide, we employ different interface modifying molecules, which help to passivate the anodized AlO x surface and hence reduce the number of possible defect-states. [23] In Figure 3, transfer curves of OFETs with 8.5 nm thick anodized AlO x as gate dielectric material treated with various self-assembled monolayers (SAM) such as octadecyltrichlorosilane, hexamethyldisilazane, or 12-cyclohexyldodecyl-phosphonic acid (CDPA) are shown. Most remarkably, the electron mobility reaches a value of 1.2 cm 2 V −1 s −1 in the CDPA-treated C 60 OFET structures (channel length L = 100 µm), and the threshold voltage V TH is as low as 1.4 V (Table S1, Supporting Information). ...
Vertical organic transistors are an attractive alternative to realize short channel transistors, which are required for powerful electronic devices and flexible electronic circuits operating at high frequencies. Unfortunately, the vertical device architecture comes along with an increased device fabrication complexity, limiting the potential of this technology for application. A new design of vertical organic field‐effect transistors (VOFETs) with superior electrical performance and simplified processing is reported. By using electrochemical oxidized aluminum oxide (AlOx) as a pseudo self‐aligned charge‐blocking structure in vertical organic transistors, direct leakage current between the source and drain can be effectively suppressed, enabling VOFETs with very low off‐current levels despite the short channel length. The anodization technique is easy to apply and can be surprisingly used on both n‐type and p‐type organic semiconductor thin films with significant signs of degradation. Hence, the anodization technique enables a simplified process of high‐performance p‐type and n‐type VOFETs, paving the road toward complementary circuits made of vertical transistors.
... The characteristic decay of the trap DOS into the band gap is in agreement with other reports on vacuum-deposited films of the organic semiconductor DNTT. [40,41] The results in Figure 3b show a clear correlation between the surface roughness of the gate dielectric and the density of trap states in the organic semiconductor layer. ...
In organic thin‐film transistors (TFTs) fabricated in the inverted (bottom‐gate) device structure, the surface roughness of the gate dielectric onto which the organic‐semiconductor layer is deposited is expected to have a significant effect on the TFT characteristics. To quantitatively evaluate this effect, a method to tune the surface roughness of a gate dielectric consisting of a thin layer of aluminum oxide and an alkylphosphonic acid self‐assembled monolayer over a wide range by controlling a single process parameter, namely the substrate temperature during the deposition of the aluminum gate electrodes, is developed. All other process parameters remain constant in the experiments, so that any differences observed in the TFT performance can be confidently ascribed to effects related to the difference in the gate‐dielectric surface roughness. It is found that an increase in surface roughness leads to a significant decrease in the effective charge‐carrier mobility and an increase in the subthreshold swing. It is shown that a larger gate‐dielectric surface roughness leads to a larger density of grain boundaries in the semiconductor layer, which in turn produces a larger density of localized trap states in the semiconductor. The surface roughness of an ultrathin hybrid gate dielectric of bottom‐gate organic thin‐film transistors is tuned systematically over a wide range. The implications of the degree of surface roughness of the gate dielectric on the charge transport in the organic semiconductor and the semiconductor morphology are investigated.
... Fermi-level pinning can occur when there are interfacial states in the HOMO-LUMO gap of C 60 layers near graphene. [33] To quantitatively analyze the Fermi-level pinning, we used the diode equation in the reverse bias saturation regime, exp [1] The value of Φ B at each V G was then estimated from the plot of ln(I DS /T 2 ) versus 1/(k B T) (Figure 6f). Φ B increased with increasing E F at different rates in the two device types (Figure 6g). ...
Controlling the growth behavior of organic semiconductors (OSCs) is essential because it determines their optoelectronic properties. In order to accomplish this, graphene templates with electronic‐state tunability are used to affect the growth of OSCs by controlling the van der Waals interaction between OSC ad‐molecules and graphene. However, in many graphene‐molecule systems, the charge transfer between an ad‐molecule and a graphene template causes another important interaction. This charge‐transfer‐induced interaction is never considered in the growth scheme of OSCs. Here, the effects of charge transfer on the formation of graphene–OSC heterostructures are investigated, using fullerene (C60) as a model compound. By in situ electrical doping of a graphene template to suppress the charge transfer between C60 ad‐molecules and graphene, the layer‐by‐layer growth of a C60 film on graphene can be achieved. Under this condition, the graphene–C60 interface is free of Fermi‐level pinning; thus, barristors fabricated on the graphene–C60 interface show a nearly ideal Schottky–Mott limit with efficient modulation of the charge‐injection barrier. Moreover, the optimized C60 film exhibits a high field‐effect electron mobility of 2.5 cm2 V−1 s−1. These results provide an efficient route to engineering highly efficient optoelectronic graphene–OSC hybrid material applications. The growth behavior of fullerene (C60) thin films on graphene templates where charge transfer occurs is presented. The number of electrons transferred from graphene to C60 is controlled by in situ electrical gating of graphene during C60 deposition. When this electron transfer is suppressed, high‐crystalline C60 thin films are achieved for highly efficient optoelectronic applications.
... [161,162,166,167] Energetically, these states are found "tailing" into the nominal gap and they are typically attributed to structural imperfections and defects. [161,[168][169][170][171] Notably, this band bending due to the accumulation of additional carriers in the OSC beyond the first monolayer is fundamentally different from the "conventional" band bending described above for (doped) inorganic semiconductors. There, the necessary spacecharge region typically does not arise from accumulating additional carriers but is rather the result of a depletion of mobile carriers leaving behind uncompensated charged dopants. ...
The presence of dipolar layers determines the functionality of most technologically relevant interfaces. The present contribution reviews how periodic dipole assemblies modify the properties of such interfaces through so‐called collective electrostatic effects. They impact the ionization energies and electron affinities of thin films, change the work function of metallic and semiconducting substrates, and determine the alignment of electronic states at interfaces. Dipolar layers originate either from the assembly of polar molecules or they arise from interfacial charge rearrangements triggered by the deposition of an adsorbate layer. Such charge rearrangements result from the omnipresent Pauli pushback caused by exchange interaction, from covalent bonds, or from charge transfer following the deposition of particularly electron rich (donors) or electron poor molecules (acceptors). A peculiarity of charge‐transfer interfaces is that they enter the realm of Fermi‐level pinning, where the sample work function becomes independent of the substrate and is solely determined by the electronic properties of the adsorbate. Beyond changing work functions and injection barriers, the presence of polar layers also modifies various other physical observables, like core‐level binding energies or tunneling currents in monolayer junctions. All these aspects suggest that polar layers can also be exploited for electrostatically designing the electronic properties of materials. Polar layers crucially impact the electronic properties of interfaces. In this article, their fundamental properties are reviewed, their physical origin is discussed and their impact on interfacial level alignment at metal/organic and semiconductor/organic interfaces is described. Emphasis is put on how they impact physical observables like work functions, core‐level energies, and transport properties and how they could be used for electrostatically designing materials.
... While the trap DOS cannot be measured directly, it can be accessed indirectly by means of a number of experimental techniques, including photoemission spectroscopy [18,19], Kelvin-probe force microscopy [20,21], electrical transport measurements [22], photoconductivity measurements [23,24], electron spin resonance spectroscopy [25], and capacitance-voltage measurements [26]. In the 1980s, Grünewald et al. developed a method, initially intended for thin-film transistors (TFTs) based on hydrogenated amorphous silicon (a-Si:H), to convert a single transfer curve of a field-effect transistor (i.e., the drain current measured as a function of the applied gate-source voltage) to the underlying density-of-states function [27][28][29]. ...
A method for extracting the density and energetic distribution of the trap states in the semiconductor of a field-effect transistor from its measured transfer characteristics is investigated. The method is based on an established extraction scheme [M. Grünewald et al., Phys. Stat. Sol. B 100, K139 (1980)] and extends it to low-voltage thin-film transistors (TFTs). In order to demonstrate the significance of this extension, two types of TFTs are fabricated and analyzed: one with a thick gate dielectric and high operating voltage and one with a thin gate dielectric and low operating voltage. From the measured transfer characteristics of both TFTs, the density of states (DOS) is calculated using both the original and the extended Grünewald method. The results not only confirm the validity of the original Grünewald method for high-voltage transistors, but also indicate the need for the extended Grünewald method for the reliable extraction of the trap DOS in transistors with a thin gate dielectric and low operating voltage.
... As listed in [36]. The Fermi level is pinned near the mid-gap at the interface by surface states arising from the interface, structural and chemical defects [37,38]. The unpinning of the Fermi level can be attained by reducing the density of interface states [39]. ...
The present paper reports on the synthesis and characterization of methylammonium lead iodide perovskite thin film and its applications in heterojunction devices. Perovskite thin films were deposited by a simple spin-coating method using a precursor solution including methyl ammonium iodide and lead iodide onto a glass substrate. The surface morphology study via field emission scanning electron microscopy of the perovskite thin film shows complete surface coverage on glass substrate with negligible pin-holes. UV-visible spectroscopy study revealed a broad absorption range and the exhibition of a band-gap of 1.6 eV. The dark current-voltage (I-V) characteristics of all the devices under study show rectifying behaviour similar to the Schottky diode. Various device parameters such as ideality factor and barrier height are extracted from the I-V curve. At low voltages the devices exhibit Ohmic behaviour, trap free space charge limited conduction governs the charge transport at an intermediate voltage range, while at much higher voltages the devices show trap controlled space charge limited conduction. Furthermore, impedance spectroscopy measurements enable us to extract the various internal parameters of the devices. Correlations between these parameters and I-V characteristics are discussed. The different capacitive process arising in the devices was discussed using the capacitance versus frequency curve.
... S URFACE potential plays a crucial role in understanding the fundamental charge dynamics at interfaces. For instance, it is related to transistor performances [1], surface corrosion [2] and recognition of biomolecular interactions [3]. Kelvin probe force microscopy (KPFM) [4] is a significant technique for measuring the contact potential difference (CPD) between the tip and sample in the non-contact atomic force microscopy (nc-AFM) mode, which models the probe-sample system as a parallel plate capacitor. ...
In this paper, a new open-loop amplitude modulation Kelvin probe force microscopy is presented for nanoscale mapping of the surface potential. In this method, measurement of the contact potential difference (CPD) is performed with a normal probe under two electrostatic excitations. One excitation oscillates the probe in its second resonance mode with an amplitude of less than 5 nm and the other non-resonant excitation modulates this oscillation at a frequency much lower than the probe's first resonant frequency. With this approach, the probe-sample CPD can be measured directly from the periodic variations of the modulated amplitude and the phase without feedback control. Nanoscale mapping of the surface potential of the graphene-onsilicon sample shows that the proposed method has comparable performance to the conventional amplitude modulation Kelvin probe force microscopy. Further experimental results demonstrate that the proposed method can accurately map the surface potentials of various conductor, semiconductor and insulator materials.
... Similar results were found using two SFM probes also for sexithiophene [15]. Charge trapping can be significantly reduced when additional organic layers are introduced [143]. ...
Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF$_2$ are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, C$_{60}$ and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. C$_{60}$ nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of C$_{60}$ islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules "climb" to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.
Organic field‐effect transistors (OFETs) hold great promise for applications in non‐volatile memories, detectors, and artificial synapses due to the good flexibility and biocompatibility. However, certain drawbacks such as high operating voltages and significant degradation in endurance characteristics have hindered their practical implementations. Herein, a novel approach is proposed to enhance the performance of OFETs by incorporating a bi‐functional n‐type polymer semiconductor interlayer, Poly‐{[N,N'‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (N2200), into a pentacene OFET structure. The device exhibits remarkable improvements, with reliable P/E operation cycles of over than 10 ⁴ and a retention time of more than 10 years. On one hand, the inclusion of N2200 as an n‐type semiconductor effectively reduces the height of hole‐injection barrier for trapping and thus reducing the working voltage based on the electrostatic induction theory. On the other hand, n‐type semiconductor N2200 serves as a native hole‐consumption (or hole‐trapping) dielectric, and its narrower bandgap restrains the formation of deep hole‐traps, thus favoring the endurance characteristics of the OFET.
Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.
Amorphous and organic semiconductors have strong topological irregularities with respect to specific ideal structures, which depend on the particular class of such semiconductors. Most of these defects are rather gradual displacements from an ideal surrounding. The disorder leads to defects levels with a broad energy distribution which extends as band tails into the bandgap. Instead of a sharp band edge known from crystalline solids a mobility edge exists separating between extended states in the bands and localized states in the band tails. Amorphous semiconductors, also referred to as semiconducting glasses, comprise the classes of amorphous chalcogenides and tetrahedrally bonded amorphous semiconductors. Amorphous chalcogenides are structurally floppy solids with low average coordination numbers and pronounced pinning of the Fermi level near midgap energy. The more rigid tetrahedrally bonded amorphous semiconductors have larger coordination numbers. They may be well doped p-type and n-type much like crystalline semiconductors. Organic semiconductors comprise small-molecule crystals and polymers. Both have weak intermolecular bonds favoring deviations from ideal alignment. In small-molecule semiconductors the structure of thin films grown on substrates usually deviates from the structure of bulk crystals, with a substantially different molecule ordering at the interface and a strong dependence on the dielectric properties of the substrate. Polymers consist of long chain-like molecules packed largely uniformly in crystalline domains separated by amorphous regions with tangled polymer chains. Besides chemical structure of the chains crystallinity depends on the molecular length.
A method is developed that can emulate the dramatic enhancement of the thermoelectric power factor (PF) traditionally predicted to occur near the band edge of high-performance thermoelectric materials. The method uses photo-excitation of an infrared (IR)-active intramolecular vibration mode in a weakly-bonded (soft) organic material to couple high-mobility band states to tail states, creating sharply-peaked Dirac-delta-like resonant states within the tail density-of-states (DOS) that also enhance the carrier mobility (μ), the number of conducting carriers (N), and the asymmetry in the energy distribution of conducting carriers (σ(E)). The use of these resonant DOS distortions to optimize PF is explored as a function of the number of IR photons (Nph). As Nph is increased to augment the coupling between valence carriers and C–C stretching vibrations, a resonant four-step intramolecular charge transfer process is shown to shift the average energy of conducting carriers from band states to a position in the DOS tail near the intrinsic Fermi level. A critical photon number (Nph = 35) is observed where DOS peaks merge to create a high mobility band of states on one side of the Fermi level and diverge to create a low mobility band of states on the other side. As a consequence, large asymmetries develop in σ(E), causing PF to attain a maximal value when the merged high-mobility DOS peak is located ∼2.4kBT from the Fermi level. Importantly, these DOS distortions provide improvements in PF in the DOS tail and is therefore accessible to carrier concentrations achievable by traditional doping techniques.
With advances in Si-based technology infrastructures and the rapid integration of Si-based optoelectronics, Si-based optoelectronic synaptic devices have the potential to greatly facilitate the large-scale deployment of neuromorphic computing. The incorporation of solution-processable polymer semiconductors into Si-based optoelectronics may enable the cost-effective fabrication of optoelectronic synaptic devices. Poly(3-hexylthiophene) (P3HT) is a semiconducting polymer used to manufacture optoelectronic synaptic transistors with P3HT channels and Si gates. The gate dielectric between them consists of a SiO2 layer. Hybrid inorganic-organic Si/P3HT optoelectronic synaptic transistors can mimic synapses when exposed to optical and electrical stimuli. The Si/P3HT synaptic devices can spatiotemporally integrate optical and electrical stimuli to mimic cross-modal learning.
Organic field-effect transistors (OFETs) as nonvolatile memory units are essential for lightweight and flexible electronics, yet the practical application remains a great challenge. The positively charged defects in pentacene film at the interface between pentacene and polymer caused by environmental conditions, as revealed by theoretical and experimental research works, result in unacceptable high programming/erasing (P/E) gate voltages in pentacene OFETs with polymer charge-trapping dielectric. Here, we report a pentacene OFET in which an n-type semiconductor layer was intercalated between a polymer and a blocking insulator. In this structure, the hole barrier caused by the defect layer can be adjusted by the thickness and charge-carrier density of the n-type semiconductor interlayer based on the electrostatic induction theory. This idea was implemented in an OFET structure Cu/pentacene/poly(2-vinyl naphthalene) (PVN)/ZnO/SiO2/Si(p+), which shows low P/E gate voltages, large field-effect mobility (0.73 cm2 V-1 s-1), fast P/E speeds (responding to a pulse width of 5 × 10-4 s), and long retention time in air.
The electronic level alignment at interfaces plays an important role in the development and optimization of organic semiconductor devices. While organic heterointerfaces are well studied, photoelectron spectroscopy on homointerfaces is challenging. As the emissions of the substrate and the adsorbate layer are dominated by the same spectral features, it is nearly impossible to distinguish their layer origin. In this study, we accept this challenge and analyze the interface between doped and undoped layers consisting of the same hole transport molecule (HTM). We used X-ray and ultraviolet photoelectron spectroscopy to monitor the stepwise thin film deposition of the well-known 4,4′,4″-tris[phenyl-(m-tolyl)amino]triphenylamine (m-MTDATA) molecule as well as a state-of-the-art triarylamine-based molecule synthesized at Merck KGaA. The interpretation of the data is enabled by a fitting procedure based on an energetic disorder model. First, as a test case, heterointerfaces of step by step deposited, differently p-doped HTMs on indium tin oxide are analyzed, revealing the power of the model for an accurate description of the data while enabling detailed discussions of the model by comparison to classical PES data analysis. Second, homointerfaces of the intrinsic HTMs on their p-doped sublayers are studied. Here, we observe an unexpected space charge region in the p-doped sublayer. Within the boundaries of our model, we obtain good fits of spectra by introducing an increased number of electronic states right at the interface in the undoped layer.
Ultrathin DNTT films are unstable due to rapid morphological changes. This work investigates the stability of ultrathin DNTT films and the fabrication of ultrathin DNTT organic transistors.
The contact resistance of organic field-effect transistors is revisited to address its fundamental origin, parametric interplays, and technological implications. In a time when flexible electronics powered by an organic circuit comes close to the market, the revelation of wide-spread carrier mobility overestimation has astonished the broad scientific community, as this may contradict some of the most significant developments made to date. Since the contact resistance was pointed out as the major reason behind the issue, the research into reducing or eliminating this resistance has become more intense and justified than ever. However, there have been other revelations that suggest the benefits of contact resistance in certain structures and applications. Therefore, it seems timely to fairly judge the true meaning and consequences of the contact resistance, and to provide a comprehensive view covering both its positive and negative aspects, which constitutes the main motivation of this article. To maximize the depth of discussion, several important backgrounds for contact effects will be recapitulated before tackling selected practical problems of contact resistance, and before clarifying when it should actually be minimized and when it could otherwise serve as a useful element.
Gap states and Fermi level pinning play an important role in all semiconductor devices, but even more in transition metal dichalcogenide-based devices due to their high surface to volume ratio and the absence of intralayer dangling bonds. Here, we measure Fermi level pinning using Kelvin probe force microscopy, extract the corresponding electronic state distribution within the band gap, and present a systematic comparison between the gap state distribution obtained for exfoliated single layer, bilayer and thick MoS2 FET samples. It is found that the gap state distribution in all cases decreases from the conduction band edge and is in the order of 10¹⁹ eV⁻¹ cm⁻³ and slightly decreases with increasing channel thickness. Strong Fermi level pinning is observed near the conduction band edge, and it decreases as it approaches the middle and lower part of the bandgap.
Poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) has demonstrated outstanding performance as a charge transport layer or an electrode in various electronic devices, including organic solar cells, organic light-emitting diodes, and organic field-effect transistors (OFETs). The electrical properties of these devices are affected by the contact properties at the PEDOT:PSS–semiconductor junction. In this research, we performed work function (WF) engineering of electrohydrodynamic (EHD)-jet-printed PEDOT:PSS and successfully used it as an electrode to fabricate high-performance OFETs and complementary logic circuits. Two types of PEDOT:PSS materials‒one with a high WF (HWF, 5.28 eV) and the other with a low WF (LWF, 4.53 eV)‒were synthesized and EHD-jet-printed. The WF of PEDOT:PSS was deterministically modulated by approximately 0.75 eV through simple mixing of the two synthesized PEDOT:PSS materials in various ratios. OFETs fabricated with HWF and LWF PEDOT:PSS electrodes showed excellent electrical properties, including the ON/OFF switching ratio higher than 107 and the highest carrier mobility greater than 1 cm2·V-1·s-1. Furthermore, the HWF and LWF PEDOT:PSS electrodes were integrated to fabricate complementary metal–oxide–semiconductor (CMOS) NOT, NOR, and NAND circuits.
The contact resistance (Rc) and the effective carrier mobility (eff) are considered as the important indicators of the performance of the organic field-effect transistors (OFETs). Conventionally, the contact resistance is regarded as the interface effect between the metal electrodes and the organic semiconductors, while the carrier mobility is correlated to the crystallinity and - stacking of the organic molecules. In the staggered OFETs, Rc is actually closely correlated to eff through the channel sheet resistance. Besides, the accuracy of the carrier mobility directly extracted from the non-ideal transfer curves with significant contact effect is always questionable. Herein, a diffusion-lead surface doping approach is employed to improve the contact resistance and mobility issues simultaneously. By suppressing the trap states in the sublimated 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) organic semiconductor with the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), we observed a 3-fold increase in the carrier mobility from 0.5 to 1.6 cm2V-1s-1, and the Rc also drops remarkably from 25.7 kΩ-cm to 5.2 kΩ-cm. Moreover, the threshold voltage (VTH), subthreshold swing (SS) and the bias stability of the OFETs are also significantly improved. Based on the detailed characterizations of the C8-BTBT film upon surface doping, including X-ray diffraction (XRD) on the film crystallinity, Kelvin probe force microscopy (KPFM) on the surface potential, trap states investigation by density of states (DOS) measurement and electrical circuit modeling on partially doping analysis, we confirmed the spontaneous charge transfer process due to the diffusion of the F4-TCNQ dopants in the C8-BTBT matrix can lead to an effective trap filling. This technique and findings can be potentially developed into a general approach for improvement of different performance parameters of the OFETs.
MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have potential for application as high-performance, low-cost electrodes in organic field-effect transistors (OFETs) because of their water dispersibility, high conductivity, and work-function tunability. In this study, we successfully fabricated a large-scale, uniform Ti3C2T
x
MXene electrode array on a flexible plastic substrate for application in high-performance OFETs. The work function of the Ti3C2T
x
MXene electrodes was also effectively modulated via chemical doping with NH3. The fabricated OFETs with Ti3C2T
x
MXene electrodes exhibited excellent device performance, such as a maximum carrier mobility of ∼1 cm2·V-1·s-1 and an on-off current ratio of ∼107 for both p-type and n-type OFETs, even though all the electrode and dielectric layers were fabricated on the plastic substrate by solution processing. Furthermore, MXene-electrode-based complementary logic circuits, such as NOT, NAND, and NOR, were fabricated via integration of p-type and n-type OFETs. The proposed approach is expected to expand the application range of MXenes to other OFET-based electronic devices, such as organic light-emitting displays and electronic skins.
The behavior of polar metal organic molecules, chloroaluminum phthalocyanine (ClAlPc), upon ultraviolet (UV) irradiation was investigated to evaluate the stability of the adsorption process on the Ag(111) thin film and bulk crystal. Photoelectron spectroscopy (PES) was mainly employed to measure the molecular energy states (MES) and vacuum level (VL) shift for 1-ML ClAlPc in the Cl-down configuration. A consistent trend was observed showing that ClAlPc in the Cl-down configuration is energetically more stable on the Ag thin-film surface than on the corresponding surface of the Ag bulk crystal. The intermediate adsorption state in tilted configuration during the irradiation impinging is identified by large positive VL shifts and broad spectra line shapes to infer a flipping mechanism from Cl-down to Cl-up configuration. Strain on the Ag thin films from the underlying lattice-mismatched Ge(111) substrate is considered to cause enlarged hollow sites on the Ag(111) thin-films, that anchor the Cl-down configuration more tightly on the thin-film surfaces, as confirmed by density functional theory (DFT) calculations.
A novel alternating TBTBT (“T” - thiophene, “B” - benzothiadiazole) compound has been synthesized from readily available precursors using efficient direct arylation and Stille cross-coupling reactions. Molecular structure of TBTBT has been revealed using single-crystal X-ray diffraction. TBTBT was applied as electron donor material for vacuum-processed small-molecule planar and bulk heterojunction solar cells and ambipolar semiconductor for organic field-effect transistors.
Photoelectron spectroscopy in the core level as well as the valence band region has been used to determine the electronic properties of interfaces between manganese phthalocyanine (MnPc) and the strong electron acceptor F6TCNNQ. It is shown that charge transfer occurs at this interface which results in the oxidation (reduction) of MnPc (F6TCNNQ) at the interface. Our data indicate a full electron transfer with no evidence for hybrid states. The valence band data suggest that the interface remains insulating/semiconducting despite the charge transfer, which indicates localized charges. The boundary between two materials can be a region where the physical properties substantially vary compared to the two materials on their own. Charge transfer across the interface is one reason of such changes and according new functionalities. It is demonstrated that the interface between manganese phthalocyanine and the strong acceptor F6TCNNQ is characterized by a strong charge transfer.
Organic field‐effect transistors (OFETs) are the central building blocks of organic electronics, but still suffer from low performance and manufacturing difficulties. This is due in part to the absence of doping, which is mostly excluded from OFET applications for the concern about uncontrollable dopant diffusion. Doping enabled the modern semiconductor industry to build essential components like Ohmic contacts and P–N junctions, empowering devices to function as designed. Recent breakthroughs in organic semiconductors and doping techniques demonstrated that doping can also be a key enabler for high‐performance OFETs. However, the knowledge of organic doping remains limited particularly for OFET use. Therefore, this review addresses OFET doping from a device perspective. The paper overviews doping basics and roles in advanced complementary technologies. These fundamentals help to understand why and how doping provides the desired transistor characteristics. Typical OFETs without doping are discussed, with consideration for operating principle and problems caused by the absence of doping. Achievements for channel, contact, and overall doping are also examined to clarify the corresponding doping roles. Finally, doping mechanisms, techniques, and dopants associated with OFET applications are reviewed. This paper promotes fundamental understanding of OFET doping for the development of high‐performance OFETs with doped components. Doping specifically for organic transistor applications from a device perspective is reviewed herein. Doping fundamentals, various doping roles in transistors, different operating principles, relevant issues caused by the absence of doping in typical organic transistors, organic transistor doping achievements to date, doping mechanisms, techniques, and dopants are systematically discussed for the first time.
Organic transistors with different structures are investigated to address the applicability and reliability of parameter extraction. A dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene channel is coupled with pristine or functionalized gold bottom and top contacts to reveal a geometrical impact on the device performance and nonidealities. Scanning Kelvin probe microscopy is employed as a key method to quantify the channel and contact potential in operation. Taking full account of the contact effects and including an explicit threshold voltage in calculation are shown to be critical to access the intrinsic carrier mobility, while simple derivative‐based extraction may over‐ or underestimate it. Further analytical developments correlate individual physical parameters, leading to the discovery that pentafluorobenzenethiol self‐assembled on gold predominantly affects the carrier mobility rather than the injection barrier.
Our scientific understanding of the nanoscale world is continuously growing ever since atomic force microscopy (AFM) has enabled us to “see” materials at this length scale. Beyond morphology, functional imaging is becoming standard practice as new AFM-based techniques are continuously extending its capabilities. Resolving material properties with high spatial accuracy is now extremely critical, as future next-generation energy harvesting and storage systems are comprised of complex and intricate nanoscale features. Here, we review recent research discoveries that implemented AFM methods to measure and determine how the electrical, chemical, and/or optical properties influence the overall device behavior. We dedicate a portion of this Review to perovskite solar cells, which are of primary interest to photovoltaic research, and highlight the remarkable progress made towards understanding and controlling their instabilities. We conclude with a summary and outlook anticipating the most pressing materials related challenges associated with solar cells and batteries, and how that will likely be overcome in the near future by nanoimaging through AFM.
This article reviews experimental studies on ‘bridging electronic structure and charge transport
property of organic semiconductors’ performed using ultraviolet photoelectron spectroscopy (UPS)
and related methods mainly in Chiba University, Japan, in particular on the investigation of the
origin and the role of electronic states existing in the highest occupied molecular orbital band–
lowest unoccupied molecular orbital band (HOMO–LUMO) gap. We summarize experimental
observations including direct measurements of ‘invisible’ gap states with ultrahigh sensitivity
UPS, which demonstrate that there exist intrinsic gap states in organic semiconductors. We firstly
describe the nature of organic molecular solids to understand features of organic semiconductors
because such intrinsic gap states are a result of the interplay of these features, which give the
principal difference between the organic semiconductor and inorganic counterpart. We then
discuss (i) the origin and role of the band gap states in relation to intermolecular interaction/
band dispersion and electron–phonon coupling, (ii) the Fermi level pinning issue in organic
semiconductors, and (iii) the method of computing the Fermi level position within the HOMO–
LUMO gap for experimental groups. The gap states of organic semiconductors appear easily when
a weak perturbation is applied to the organic system, namely by contact with other material, by
injecting a charge, by elevating temperature, and by exposure to 1 atm gas. What we finally found
is that tailing states of HOMO and LUMO always exist, and their energy distributions must not be
symmetric; they thus produce a larger Fermi level shift from the mid gap position than previously
thought. Furthermore, as shown by computational work, Fermi level pinning, which is a wellknown
phenomena in semiconductor devices field, occurs in weakly interacting organic/conductor
systems without any gap states if the system temperature is not zero (T > 0). We also describe the
experimental knowhow of the ultrahigh-sensitivity UPS of organic systems, which are very fragile
upon ultraviolet-light irradiation, and some updates on our previously published results.
Exposure to moisture and elevated temperatures usually results in significant degradation of organic thin film transistor (OTFT) performance. Typical observations include reduced mobility, unstable threshold voltage and the appearance of hysteresis in electrical characteristics. In this contribution we investigate the effects of environmental conditions on OTFTs based on DNTT, a high-mobility, small-molecule, organic semiconductor, with polystyrene (PS) as the gate insulator. Device characteristics were measured after consecutive 30-min exposures to a relative humidity (RH) that was gradually increased from 20% to 80% with temperature fixed at 20 °C and for temperatures increasing from 20 °C to 90 °C with RH held at 10%. Despite significant negative shifts in turn-on and threshold voltages, only slight changes in the hole mobility were observed at the highest RH and temperature. The DNTT density of states (DoS) extracted from transfer characteristics in the linear regime using the Grünewald approach showed little change with environmental conditions. In all cases, the DoS decreased from ∼1 × 10²⁰ down to ∼1 × 10¹⁷ cm⁻³ eV⁻¹ in the 0.45 eV energy range above the hole mobility edge. Some evidence was obtained for a weak trap feature between ∼0.25 and 0.35 eV above the mobility edge. These results confirm the high stability of DNTT as a semiconducting material and that OTFT instability observed here is associated almost entirely with a flatband voltage shift caused by hole trapping in the polystyrene gate dielectric or at the polystyrene/DNTT interface.
A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from N,N’-bis-(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (N1400), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature one-dimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the N1400 film. The morphological and crystalline structures of the N1400 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the N1400-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm2 V–1 s–1, more than a factor of two higher than the mobility of 0.25 cm2 V–1 s–1 submitted to conventional thermal annealing and the mobility of 0.29 cm2 V–1 s–1 from the horizontally applied temperature gradient.
Amorphous and organic semiconductors have strong topological irregularities with respect to specific ideal structures, which depend on the particular class of such semiconductors. Most of these defects are rather gradual displacements from an ideal surrounding. The disorder leads to defects levels with a broad energy distribution which extends as band tails into the bandgap. Instead of a sharp band edge known from crystalline solids a mobility edge exists separating between extended states in the bands and localized states in the band tails.
Amorphous semiconductors, also referred to as semiconducting glasses, comprise the classes of amorphous chalcogenides and tetrahedrally bonded amorphous semiconductors. Amorphous chalcogenides are structurally floppy solids with low average coordination numbers and pronounced pinning of the Fermi level near midgap energy. The more rigid tetrahedrally bonded amorphous semiconductors have larger coordination numbers. They may be well doped p-type and n-type much like crystalline semiconductors.
Organic semiconductors comprise small-molecule crystals and polymers. Both have weak intermolecular bonds favoring deviations from ideal alignment. In small-molecule semiconductors the structure of thin films grown on substrates usually deviates from the structure of bulk crystals, with a substantially different molecule ordering at the interface and a strong dependence on the dielectric properties of the substrate. Polymers consist of long chain-like molecules packed largely uniformly in crystalline domains separated by amorphous regions with tangled polymer chains. Besides chemical structure of the chains crystallinity depends on the molecular length.
The transport properties of high-performance thin-film transistors (TFT) made with a regioregularpoly(thiophene) semiconductor (PQT-12) are reported. The room-temperature field-effect mobility of the devices varied between 0.004 and 0.1 cm2∕V s and was controlled through thermal processing of the material, which modified the structural order. The transport properties of TFTs were studied as a function of temperature. The field-effect mobility is thermally activated in all films at T<200 K and the activation energy depends on the charge density in the channel. The experimental data are compared to theoretical models for transport, and we argue that a model based on the existence of a mobility edge and an exponential distribution of traps provides the best interpretation of the data. The differences in room-temperature mobility are attributed to different widths of the shallow localized state distribution at the edge of the valence band due to structural disorder in the film. The free carrier mobility of the mobile states in the ordered regions of the film is the same in all structural modifications and is estimated to be between 1 and 4 cm2∕V s.
The authors report on the modeling of the water related trap state in pentacene single crystal field-effect transistors that is created by a prolonged application of a gate voltage [
C. Goldmann et al., Appl. Phys. Lett. 88, 063501 (2006)
]. The authors find a trap state narrow in energy to be appropriate to explain the steplike feature measured in the subthreshold region of the transfer characteristic. The trap state forms in an interface layer next to the gate insulator and is centered at 430±50 meV above the valence band edge. The density increases from (2 to 10.5)×1018/cm3 during gate bias stress. The knowledge of the details of this defect state can help to identify the physical and chemical origin of the created trap state.
Potential mapping of organic thin-film transistors (TFTs) has been carried out using originally developed atomic-force-microscope potentiometry (AFMP). The technique is suitable for the accurate measurement at metal–semiconductor boundaries of working TFTs. Potential drops near metal–organic boundaries are observed for both source and drain Au top contacts of a pentacene TFT. The approximate width of the steeper potential slope is 400 nm, which is larger than the spatial resolution of AFMP. The potential drop is considered to be due to a damaged area with low carrier mobility caused by the Au evaporation, which is also reproduced by device simulation.
Crystalline domain size and temperature dependences of the carrier mobility of commonly used pentacene polycrystalline films on SiO2 have been studied by four-point-probe field-effect transistor measurements. The mobility is found to be proportional to the crystalline domain size and thermally activated. This behavior is well explained by a polycrystalline model with the diffusion theory, and thereby the barrier height at boundary and the mobility in domain are calculated to be 150 meV and 1.0 cm2/V s, respectively. The in-domain mobility is lower than those expected in single crystals, which suggests that there exist some other limiting factors of carrier transport than the domain boundaries.
We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap trap density of states trap DOS of small-molecule organic semiconduc-tors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited current measurements. This was done by comparing data from a large number of samples including thin-film transistors TFT's, single crystal field-effect transistors SC-FET's and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small-molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase in the trap DOS very close 0.15 eV to the mobility edge with a characteristic slope of 10–20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase in the trap DOS. Moreover, we show that the trap DOS in TFT's with small-molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small-molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.
We have developed an organic thin-film transistor (TFT) technology that aims at providing a good balance of static and dynamic performance parameters. An inverted staggered (bottom-gate, top-contact) device structure with patterned metal gates, a room-temperature-deposited gate dielectric providing a capacitance of 0.7 μ F / cm <sup>2</sup> , and vacuum-deposited pentacene as the semiconductor were employed. The TFTs have a channel length of 10 μ m , a carrier mobility of 0.4 cm <sup>2</sup>/ V s , an on/off current ratio of 10<sup>7</sup> , a subthreshold swing of 100 mV /decade, and a transconductance per channel width of 40 μ S / mm . Ring oscillators operate with supply voltages as low as 2 V and with signal propagation delays as low as 200 μ s per stage.
The electronic structure of pentacene/graphite, pentacene/Cu-phthalocyanine (CuPc)/graphite, and pentacene/SiO(2)/Si(100) was studied by ultraviolet photoelectron spectroscopy as a function of the thickness of the pentacene film. We observed that the binding energy position of the highest occupied molecular orbital (HOMO) of pentacene becomes significantly lower on CuPc and SiO(2) surfaces than that on graphite. Furthermore, a splitting of the UPS band was observed only for the HOMO of thicker crystalline films on CuPc and SiO(2) surfaces, where the molecules are oriented with their long axis nearly perpendicular to the substrate surface. The splitting is ascribed to the intermolecular band dispersion, and the decrease in the threshold ionization energy on CuPc and SiO(2) surfaces originates from the HOMO-band dispersion as well as the increase in the relaxation/polarization energies, which may be caused by the better molecular packing structure with a nearly standing molecular orientation.
The bias stress effect in pentacene organic thin-film transistors has been investigated. The transistors utilize a thin gate dielectric based on an organic self-assembled monolayer and thus can be operated at low voltages. The bias stress-induced threshold voltage shift has been analyzed for different drain-source voltages. By fitting the time-dependent threshold voltage shift to a stretched exponential function, both the maximum (equilibrium) threshold voltage shift and the time constant of the threshold voltage shift were determined for each drain-source voltage. It was found that both the equilibrium threshold voltage shift and the time constant decrease significantly with increasing drain-source voltage. This suggests that when a drain-source voltage is applied to the transistor during gate bias stress, the tilting of the HOMO and LUMO bands along the channel creates a pathway for the fast release of trapped carriers.
We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene single crystals and thin films. It is found that O(2) and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O(2) at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface properties.
The density of trap states in the bandgap of semiconducting organic single crystals has been measured quantitatively and with high energy resolution by means of the experimental method of temperature-dependent space-charge-limited-current spectroscopy (TD-SCLC). This spectroscopy has been applied to study bulk rubrene single crystals, which are shown by this technique to be of high chemical and structural quality. A density of deep trap states as low as ~ 10^{15} cm^{-3} is measured in the purest crystals, and the exponentially varying shallow trap density near the band edge could be identified (1 decade in the density of states per ~25 meV). Furthermore, we have induced and spectroscopically identified an oxygen related sharp hole bulk trap state at 0.27 eV above the valence band.
The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak-chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.
A facile three-step synthetic procedure for highly pi-extended heteroarenes, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) and dinaphtho[2,3-b:2',3'-f]selenopheno[3,2-b]selenophene (DNSS), was established. Solution UV-vis spectra and electrochemistry indicated that they have relatively low-lying HOMO levels and large HOMO-LUMO energy gaps. OFET devices fabricated with evaporated thin-films of DNTT and DNSS showed excellent FET characteristics in air, and the highest field-effect mobility of DNTT- and DNSS-based OFETs is 2.9 and 1.9 cm(2) V-1 s(-1), respectively.
The slow formation of hole bipolarons is proposed to explain the
bias-stress effect that is observed in some polymer thin-film
transistors (TFT). The primary evidence is the observation that holes
are removed from the channel at a rate proportional to the square of
their concentration. As bound hole pairs form, they deplete the TFT
channel of mobile holes, which reduces the TFT current and causes a
threshold voltage shift. The slow rate of bipolaron formation is
attributed to the mutual Coulomb repulsion of the holes. Contrasting
stress effects in different polymers are related to the magnitude of the
bipolaron binding energy. Present experiments cannot distinguish whether
the bipolarons are intrinsic, or associated with disorder, interfaces,
or impurities.
Organic thin-film transistors based on the vacuum-deposited small-molecule conjugated semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) have been fabricated and characterized. The transistors have field-effect mobilities as large as 2cm2/Vs and an on/off ratio of 108. Owing to the large ionization potential of DNTT, the TFTs show excellent stability for periods of several months of storage in ambient air. Unipolar ring oscillators based on DNTT TFTs with a channel length of 10μm oscillate with a signal propagation delay as short as 7μsec per stage at a supply voltage of 5V. We also show that DNTT TFTs with usefully small channel width/length ratio are able to drive blue organic LEDs to a brightness well above that required for active-matrix displays.
Grain morphology, crystal structure and transistor characteristics of pentacene films have been studied using molecular beam deposition under various conditions: growth temperature ranging from −5 to 80 °C and growth rate from 0.15 to 2 nm/min. Grain morphologies observed with atomic force microscopy are categorized into five groups, which are summarized in a phase diagram. Above the growth temperature of 40 °C, many ‘recessed regions’ which extend independently of molecular steps are observed on the film surfaces. Carrier mobilities of the films estimated from transistor measurements are distributed from 1×10−5 to 0.20 cm2/Vs. A comparative study between the structural and electrical results indicates that the mobility does not explicitly depend either on grain size or on specific grain morphology. Lamellar grains associated with flat-lying molecules among differently oriented grains, gaps between grains and the recessed regions are considered to restrict the overall carrier mobility.
First-principles pseudopotential density functional calculations are reported for hydrogen- and oxygen-related defects in crystalline pentacene (C22H14). It is shown that defects that perturb a carbon atom so as to remove its pz orbital from participation in the π system give rise to a state in the gap. One such defect is formed by adding a H atom to create a C22H15 molecule with one fourfold C atom. In the neutral-charge state a single electron occupies a π orbital having reduced amplitude on the perturbed carbon atom. Charged defects correspond to addition or removal of an electron from this orbital. The possibility that these defects give rise to the bias-stress effect in pentacene-based thin film transistors is discussed.
We report on direct high lateral resolution measurements of density of states in pentacene thin film transistors using Kelvin probe force microscopy. The measurements were conducted on passivated (hexamethyldisilazane) and unpassivated field effect transistors with 10- and 30-nm-thick pentacene polycrystalline layers. The analysis takes into account both the band bending in the organic film and the trapped charge at the SiO2–pentacene interface. We found that the density of states for the highest occupied molecular orbital band of pentacene film on the treated substrate is Gaussian with a width (variance) of σ=0.07±0.01 eV and an exponential tail. The concentration of the density of states in the gap for pentacene on bare SiO2 substrate was larger by one order of magnitude, had a different energy distribution, and induced Fermi level pinning. The results are discussed in view of their effect on pentacene thin film transistors’ performance.
Temperature-dependent measurements of thin-film transistors were performed to gain insight in the electronic transport of polycrystalline pentacene. Devices were fabricated with plasma-enhanced chemical vapor deposited silicon nitride gate dielectrics. The influence of the dielectric roughness and the deposition temperature of the thermally evaporated pentacene films were studied. Although films on rougher gate dielectrics and films prepared at low deposition temperatures exhibit similar grain size, the electronic properties are different. Increasing the dielectric roughness reduces the free carrier mobility, while low substrate temperature leads to more and deeper hole traps. © 2003 American Institute of Physics.
Temperature and gate voltage dependent transport measurements on single grain boundaries in the organic semiconductor sexithiophene (6T) are described. Isolated grain boundaries are formed by vacuum deposition of pairs of 6T grains between Au electrodes 1.5–2.0 μm apart on SiO2/Si substrates; grain boundary formation is monitored using atomic force microscopy. The Si substrate serves as the gate electrode. We show from the activation energy, threshold voltage, and field effect resistance of the grain boundary junction that carrier transport is limited by the grain boundary. The activation energy of room temperature transport is of the order of 100 meV at carrier densities of ∼1018 cm−3 and decreases with increasing carrier concentration (gate voltage). We also observe that longer grain boundaries with smaller misorientation angles result in larger currents through a grain boundary. We relate our data to two models, one that assumes acceptor-like traps localized at a grain boundary and another that assumes localized donor-like traps. Using the activation energy as a measure of the potential well (acceptor model) or barrier (donor model) at a grain boundary, we calculate trap densities of the order of 1012 cm−2 assuming discrete trap energies of 0.015 and 0.34 eV relative to the valence band in the acceptor and donor models, respectively. We note that the calculated values of the trap density are sensitive to the estimated value of the trap level. © 2001 American Institute of Physics.
The influence of environmental conditions on the electronic transport and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ and ex situ measurements of pentacene thin film transistors were carried out to study the influence of dry oxygen and moisture on the device operation. The staggered thin film transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Exposing the pentacene films to oxygen leads to the creation of acceptorlike states in the band gap. The acceptorlike states cause a shift of the onset of the drain current towards positive gate voltages. A simple analytical model will be presented which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film. Exposing the pentacene film to moisture causes a drop of the charge carrier mobility, a reduction of the threshold voltage, and a shift of the onset voltage. Besides the creation of acceptorlike states in the pentacene film the interface between the drain and source electrodes and the pentacene film is affected by moisture. The injection of holes in the highest occupied molecular orbital level of the pentacene film is inhibited, which causes an apparent drop of the charge carrier mobility and a reduction of the threshold voltage.
We show novel and selective means to modify the dielectric surfaces in organic TFTs. Modification schemes include alkylphosphonic acid monolayers that have a strong affinity for alumina surfaces. Monolayers form robust, extremely uniform thin films and are deposited through simple spin-coating with a dilute solution of the monolayer precursor in solvent. Adding monolayers to organic TFTs has resulted in polycrystalline devices with mobilities nearly equal to single-crystal values while maintaining acceptable values of other device parameters (for example, the threshold voltage, on/off ratio, and subthreshold slope) required for fully functional integrated circuits.
Molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3) is investigated as a p-dopant for organic semiconductors. With an electron affinity of 5.6 eV, Mo(tfd)3 is a strong oxidizing agent suitable for the oxidation of several hole transport materials (HTMs). Ultraviolet photoemission spectroscopy confirms p-doping of the standard HTM N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD). Strong enhancement of hole injection at α-NPD/Au interfaces is achieved via doping-induced formation of a narrow depletion region in the organic semiconductor. Variable-temperature current−voltage measurements on α-NPD: Mo(tfd)3 (0−3.8 mol %) yield an activation energy for polaron transport that decreases with increasing doping concentration, which is consistent with the effect of the doping-induced filling of traps on hopping transport. Good stability of Mo(tfd)3 versus diffusion in the α-NPD host matrix is demonstrated by Rutherford backscattering for temperatures up to 110 °C. Density functional theory (DFT) calculations are performed to obtain geometries and electronic structures of isolated neutral and anionic Mo(tfd)3 molecules.
We have prepared organic thin-film transistors (OTFTs) featuring pentacene molecules deposited at various substrate temperatures onto either hexamethyldisilazane (HMDS)- or poly(α-methylsyrene) (PαMS)-treated SiO2 surfaces. As a result, we obtained different grain boundary densities in the conducting channel. Since the surface-modified devices featured similar grain boundary densities in their active layers, but displayed different electrical performances, we suspected that different trap states probably existed at the grain boundaries for the two different kinds of OTFTs. In addition, the surface morphologies of the initial layers featured grain boundaries that were rather blurred for the thin films prepared on the PαMS-treated substrates, whereas shallow boundaries appeared for the pentacene layers on the HMDS-treated surfaces. Therefore, we deduced that the different surface treatment processes resulted in different Schwoebel (step-edge) barriers, and hence, different morphologies. These results suggested that different trap states existed at the grain boundaries of the two types of surface-treated devices, leading to variations in the electrical performance, even though the grain boundary densities were similar.
In this Review, we summarize recent work on modeling of organic/metal and organic/organic interfaces. Some of the models discussed have a semiempirical approach, that is, experimentally derived values are used in combination with theory, and others rely completely of calculations. The models are categorized according to the types of interfaces they apply to, and the strength of the interaction at the interface has been used as the main factor. We explain the basics of the models, their use, and give examples on how the models correlate with experimental results. We stress that given the complexity of organic/metal and organic/organic interface formation, it is crucial to know the exact way in which the interface was formed before choosing the model that is applicable, as none of the models presented covers the whole range of interface interaction strengths (weak physisorption to strong chemisorption).
Using high sensitivity electric force microscopy we are investigating the electronic properties at the semiconductor-dielectric interface in pentacene thin film devices. It is believed that the conduction takes place within the first few monolayers of the organic and is adversely affected by the presence of charge traps. We find that charge traps in polycrystalline pentacene are distributed inhomogeneously but do not appear to be associated with grain boundaries as is generally supposed. We will also report on ongoing studies of thin (1-3 monolayers) devices, where the relationship between the topography and the location of the charge traps is more easily interpreted.
We report here on the rational synthesis, processing, and dielectric properties of novel layer-by-layer organic/inorganic hybrid multilayer dielectric films enabled by polarizable π-electron phosphonic acid building blocks and ultrathin ZrO(2) layers. These new zirconia-based self-assembled nanodielectric (Zr-SAND) films (5-12 nm thick) are readily fabricated via solution processes under ambient atmosphere. Attractive Zr-SAND properties include amenability to accurate control of film thickness, large-area uniformity, well-defined nanostructure, exceptionally large electrical capacitance (up to 750 nF/cm(2)), excellent insulating properties (leakage current densities as low as 10(-7) A/cm(2)), and excellent thermal stability. Thin-film transistors (TFTs) fabricated with pentacene and PDIF-CN(2) as representative organic semiconductors and zinc-tin-oxide (Zn-Sn-O) as a representative inorganic semiconductor function well at low voltages (<±4.0 V). Furthermore, the TFT performance parameters of representative organic semiconductors deposited on Zr-SAND films, functionalized on the surface with various alkylphosphonic acid self-assembled monolayers, are investigated and shown to correlate closely with the alkylphosphonic acid chain dimensions.
Mixed alkyl/fluoroalkyl phosphonic acid self-assembled monolayers have been prepared as ultra-thin dielectrics in low-voltage organic thin-film transistors and complementary circuits. Mixed monolayers enable continuous threshold-voltage tuning simply by adjusting the molecular mixing ratio Continuous threshold-voltage control makes it possible to place the switching voltage of the circuits at precisely half the supply voltage, producing the maximum noise margin.
We developed a novel method for obtaining the distribution of trapped carriers over their degree of localization in organic transistors, based on the fine analysis of electron spin resonance spectra at low enough temperatures where all carriers are localized. To apply the method to pentacene thin-film transistors, we proved through continuous wave saturation experiments that all carriers are localized at below 50 K. We analyzed the spectra at 20 K and found that the major groups of traps comprise localized states having wave functions spanning around 1.5 and 5 molecules and a continuous distribution of states with spatial extent in the range between 6 and 20 molecules.
A new method to deactivate the impurity states at the interface in single crystal devices is presented that incorporates the impurity states into the dielectric gate barrier. Various new developments in organic single crystal field effect transistor (FET) devices have revealed that the surface layer of the semiconductor, which forms the conduction channel between the source and the drain electrode, contains a broad distribution of interface states. Organic dielectrics such as parylelne have been used as gate dielectric to minimize the trap density. The bulk mobility can be increased by reducing the number of traps through controlled processing. Pentacene is used for rollable displays in prototypes of commercial devices and exhibits the highest reported electronic mobility of 35 cm2V1-s-1 at room temperature.
We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.
Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity or low-bandgap organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric--such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)--can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10(-3) to 10(-2) cm(2) V(-1) s(-1) have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor-dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.
Among all organic semiconductors, pentacene has been shown to have the highest thin film mobility reported to date. The crystalline structure of the first few pentacene layers deposited on a dielectric substrate is strongly dependent on the dielectric surface properties, directly affecting the charge mobility of pentacene thin film OTFTs. Herein, we report that there is a direct correlation between the crystalline structure of the initial submonolayer of a pentacene film and the mobility of the corresponding 60-nm-thick films showing terrace-like structure, as confirmed by 2D grazing-incidence X-ray diffraction and atomic force microscopy. Specifically, multilayered pentacene films, grown from single crystal-like faceted islands on HMDS-treated surface, have shown much higher charge mobility (mu = 3.4 +/- 0.5 cm2/Vs) than those with polycrystalline dendritic islands (mu = 0.5 +/- 0.15 cm2/Vs) on OTS-treated ones.
Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO(2) interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.
We report on high-resolution electronic measurements of doped organic thin-film transistors using Kelvin probe force microscopy. Measurements conducted on field effect transistors made of N,NI-diphenyl-N,NI-bis(1-naphthyl)-1,1I-biphenyl-4,4I-diamine p-doped with tetrafluoro-tetracyanoquinodimethane have allowed us to determine the rich structure of the doping-induced density of states. In addition, the doping process changes only slightly the Fermi energy position with respect to the highest occupied molecular orbital level center. The moderate change is explained by two counter-acting effects on the Fermi energy position: the doping-induced additional charge and the broadening of the density of states.