The design of circuits utilizing organic complementary metal oxide semiconductor (CMOS) architecture requires the development of both p-channel and n-channel semiconductors with high performance and air-stability. Compared to highly developed p-channel organic semiconductors, the availability of high-performance, air-stable n-channel materials, in particular with solution processability, remains considerably limited. 1 This is attributed to the vulnerability of electrons to trapping by ambient oxidants, such as O 2 and H 2 O. 2 These ambient traps result in significant decreases of the density of mobile electrons in n-channel organic thin-film transistors (OTFTs) and, thus, poor airstability. The difficulties encountered in the development of new, airstable n-channel semiconductors prompted us to seek a new approach to increase the density of mobile electrons to compensate for the trapped electrons under ambient conditions. We hypothesized that controlled n-type doping might fulfill this requirement. Previous attempts at intentional doping have increased film conductivities as a result of increased charge carrier density, 3 with the approach resulting in highly efficient organic light-emitting diodes (OLEDs) 4 and organic photovoltaics (OPVs). 5 However, the design of n-type (vs p-type) dopants is considerably challenging owing to the requirement of highlying dopant highest occupied molecular orbital (HOMO) levels, making n-type dopants unstable against O 2 . 3e A promising strategy involves the formation of the active dopant species through thermal activation or photoactivation, allowing the active n-type dopants to transfer electrons to the host matrix and form stable cations. 3b,d However, most previously reported dopants were processed by vacuum deposition and required high conversion temperatures. These dopants were thus incompatible with solution-processed materials and demonstrated weak doping effects (i.e., insufficient to render the device air-stable). Therefore, it is essential to design new n-type dopants for solution-processed n-channel OTFTs.1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) derivatives have been reported as effective reagents for reductive transformations of organic compounds. 6 These materials are also known to promote hydrogen-and/or electron-transfer reactions via radical formation. 7 Thus, these solution-processable moieties present an ideal class of n-type dopants, as they readily form neutral radicals and H radical. 6 In this Communication, we report the use of (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI, Figure 1a) as an n-type dopant and demonstrate improved airstability of N-DMBI-doped n-channel OTFTs by solution processing.To characterize the doping effect of N-DMBI, film conductivities of a well-known solution-processable n-channel semiconductor, [6,6]-phenyl C 61 butyric acid methyl ester (PCBM), 8 were explored. N-DMBI and PCBM were mixed at varying ratios and spin-coated from chlorobenzene solutions to form thin films. Th...

The discovery of air-stable n-dopants for organic semiconductor materials has been hindered by the necessity of high-energy HOMOs and the air sensitivity of compounds that satisfy this requirement. One strategy for circumventing this problem is to utilize stable precursor molecules that form the active doping complex in situ during the doping process or in a postdeposition thermal- or photo-activation step. Some of us have reported on the use of 1H-benzimidazole (DMBI) and benzimidazolium (DMBI-I) salts as solution- and vacuum-processable n-type dopant precursors, respectively. It was initially suggested that DMBI dopants function as single-electron radical donors wherein the active doping species, the imidazoline radical, is generated in a postdeposition thermal annealing step. Herein we report the results of extensive mechanistic studies on DMBI-doped fullerenes, the results of which suggest a more complicated doping mechanism is operative. Specifically, a reaction between the dopant and host that begins with either hydride or hydrogen atom transfer and which ultimately leads to the formation of host radical anions is responsible for the doping effect. The results of this research will be useful for identifying applications of current organic n-doping technology and will drive the design of next-generation n-type dopants that are air stable and capable of doping low-electron-affinity host materials in organic devices.

We investigate the relationship between the charge carrier type in organic thin film transistors (OTFTs) and molecular energy levels. We examine a series of functionalized acenes that collectively have their HOMOs range from -4.9 eV to -5.6 eV and LUMOs range from -2.8 eV to -3.7 eV, as measured by cyclic voltammetry. Placed together, these 20 molecules allow us to chart the transition from OTFTs that display only hole transport, to ambipolar, to solely electron transport. Specifically, we note that for octadecyltrimethoxysilane (OTS) treated substrates, with top contact gold electrodes, electron injection and transport occurs when the LUMO < -3.15 eV, while hole injection and transport ceases when the HOMO < -5.6 eV. Ambipolar transport prevails when molecules have HOMO/ LUMO levels within the aforementioned range. This is seen across channel lengths ranging from 50-150 microm and using only gold as electrodes. This empirical plot is the first time such a detailed study has been made on the onset of charge injection and transport for a class of organic semiconductors. It provides guidelines for future molecular design.

Tuning the threshold voltage of a transistor is crucial for realizing robust digital circuits. For silicon transistors, the threshold voltage can be accurately controlled by doping. However, it remains challenging to tune the threshold voltage of single-wall nanotube (SWNT) thin-film transistors. Here, we report a facile method to controllably n-dope SWNTs using 1H-benzoimidazole derivatives processed via either solution coating or vacuum deposition. The threshold voltages of our polythiophene-sorted SWNT thin-film transistors can be tuned accurately and continuously over a wide range. Photoelectron spectroscopy measurements confirmed that the SWNT Fermi level shifted to the conduction band edge with increasing doping concentration. Using this doping approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants. We observed an unprecedented noise margin of 28 V at V DD = 80 V (70% of 1/2V DD ) and a gain of 85. Additionally, robust SWNT complementary metal−oxide−semiconductor inverter (noise margin 72% of 1/2V DD ) and logic gates with rail-torail output voltage swing and subnanowatt power consumption were fabricated onto a highly flexible substrate. nanomaterials | n-doping | inkjet-printed | CMOS circuit F lexible electronics have attracted increasing attention recently due to the plethora of possible and realized applications in radio-frequency identification cards (1, 2), flexible displays (3, 4), and digital processors (5). Solution-processed single-walled carbon nanotubes (SWNTs) are a promising candidate for flexible circuits due to their high charge carrier mobility (6), excellent flexibility/stretchability (7-9), and their compatibility with lowcost, large-area manufacturing processes, such as printing (1, 10) of SWNTs. Their applications in thin-film transistors (TFTs) and integrated logic circuits (11-14) have been demonstrated. However, to achieve robust digital circuits with high immunity against the influence of electronic noise in the system, it is important to be able to control the specific value of the threshold voltage of a transistor during the fabrication process (15,16). This is because transistor threshold voltage determines the input voltage at which a circuit switches between two logic states (trip voltage of an inverter). When the trip voltage is half of the supply voltage, the circuit has the largest noise margin, which is a quantitative measure of the immunity of a logic circuit against noise and a figure of merit to characterize the robustness of the circuit (17, 18). If threshold voltage cannot be controlled during the fabrication process, the resulting circuit might not work reliably due to the electrical noise that is always present in the system. Because SWNTs have ambipolar electrical transport properties (19), accurately tuning the threshold voltage permits the construction of complementary metal−oxide−semiconductor (CMOS) circuits that use both the p-type and n-type character of SWNTs. The advantages of CMOS circuits compared with unipolar ...

Carbon allotropes possess unique and interesting physical, chemical, and electronic properties that make them attractive for next-generation electronic devices and solar cells. In this report, we describe our efforts into the fabrication of the first reported all-carbon solar cell in which all components (the anode, active layer, and cathode) are carbon based. First, we evaluate the active layer, on standard electrodes, which is composed of a bilayer of polymer sorted semiconducting single-walled carbon nanotubes and C(60). This carbon-based active layer with a standard indium tin oxide anode and metallic cathode has a maximum power conversion efficiency of 0.46% under AM1.5 Sun illumination. Next, we describe our efforts in replacing the electrodes with carbon-based electrodes, to demonstrate the first all-carbon solar cell, and discuss the remaining challenges associated with this process.