An official website of the United States government
Abstract Faux‐hawk fullerenes are promising candidates for high‐performance organic field‐effect transistors (OFETs). They show dense molecular packing and high thermal stability. Furthermore, in contrast to most other C60derivates, functionalization of the fullerene core by the fluorinated group C6F4CF2does not increase their lowest unoccupied orbital position, which allows the use of air‐stable molecular n‐dopants to optimize their performance. The influence of n‐doping on the performance of OFETs based on the faux‐hawk fullerene 1,9‐C60(cyclo‐CF2(2‐C6F4)) (C60FHF) is studied. An analytic model for n‐doped transistors is presented and used to clarify the origin of the increase in the subthreshold swing usually observed in doped OFETs. It is shown that the increase in subthreshold swing can be minimized by using a bulk dopant layer at the gate dielectric/C60FHF layer instead of a mixed host:dopant layer. Following an optimization of the OFETs, an average electron mobility of 0.34 cm2 V−1 s−1, a subthreshold swing below 400 mV dec−1for doped transistors, and a contact resistance of 10 kΩ cm is obtained, which is among the best performance for fullerene based n‐type semiconductors.
more »
« less
Organic Doping at Ultralow Concentrations
Abstract Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C60metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C60OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 107are realized. The devices have low subthreshold swing in the range of 80 mV dec−1and a large transconductance of up to 8 mS mm−1.
more »
« less
- Award ID(s):
- 1709479
- PAR ID:
- 10446162
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Optical Materials
- Volume:
- 9
- Issue:
- 14
- ISSN:
- 2195-1071
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used not only to optimize these devices but also to enable new design principles as well. Here, a novel type of OFET is reported—the vertical organic tunnel field-effect transistor. Based on heterogeneously doped drain and source contacts, charge carriers are injected from an n-doped source electrode into the channel by Zener tunneling and are transported toward a p-doped drain electrode. The working mechanism of these transistors is discussed with the help of a tunnel model that takes energetic broadening of transport states in organic semiconductors and roughness of organic layers into account. The proposed device principle opens new ways to optimize OFETs. It is shown that the Zener junction included between the source and drain of the vertical organic tunnel field-effect transistors suppresses short channel effects and improves the saturation of vertical OFETs.more » « less
-
While GaN is a crucial semiconductor material for bright light‐emitting devices, fabrication of p‐type GaN remains challenging since the Mg acceptor commonly used for p‐type doping is not shallow enough. Doping of GaN with Be is a promising path, yet no reliable p‐type GaN has been achieved by Be doping so far. One of the reasons is a poor understanding of point defects in Be‐doped GaN that can be studied by photoluminescence (PL). The yellow (YLBe) band at 2.15 eV is the dominant PL band in Be‐doped GaN. In this work, a blue PL band named the BLBeband is discovered. It has a maximum at 2.6 eV and a lifetime of 0.8 μs at temperatures below 100 K. The BLBeband is observed in GaN samples with relatively high concentrations of Be (>1018 cm−3). Both the YLBeand BLBebands likely originate from the isolated BeGadefect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the BeGa. The 0/+ transition level is located at 0.1–0.2 eV above the valence band. Other broad PL bands in Be‐doped GaN were also observed and preliminarily attributed to Be‐containing complexes.more » « less
-
Abstract Current potentiometric sensing methods are limited to detecting nitrate at parts-per-billion (sub-micromolar) concentrations, and there are no existing potentiometric chemical sensors with ultralow detection limits below the parts-per-trillion (picomolar) level. To address these challenges, we integrate interdigital graphene ion-sensitive field-effect transistors (ISFETs) with a nitrate ion-sensitive membrane (ISM). The work aims to maximize nitrate ion transport through the nitrate ISM, while achieving high device transconductance by evaluating graphene layer thickness, optimizing channel width-to-length ratio (RWL), and enlarging total sensing area. The captured nitrate ions by the nitrate ISM induce surface potential changes that are transduced into electrical signals by graphene, manifested as the Dirac point shifts. The device exhibits Nernst response behavior under ultralow concentrations, achieving a sensitivity of 28 mV/decade and establishing a record low limit of detection of 0.041 ppt (4.8 × 10−13M). Additionally, the sensor showed a wide linear detection range from 0.1 ppt (1.2 × 10−12M) to 100 ppm (1.2 × 10−3M). Furthermore, successful detection of nitrate in tap and snow water was demonstrated with high accuracy, indicating promising applications to drinking water safety and environmental water quality control.more » « less
-
Abstract Bis‐porphyrin nanocages (M2BiCage, M = FeCl, Co, Zn) and their host‐guest complexes with C60and C70were used to examine how molecular porosity and interactions with carbon nanomaterials affect the CO2reduction activity of metalloporphyrin electrocatalysts. The cages were found to adsorb on carbon black to provide electrocatalytic inks with excellent accessibilities of the metal sites (≈50%) even at high metal loadings (2500 nmol cm−2), enabling good activity for reducing CO2to CO. A complex of C70bound inside(FeCl)2BiCageachieves high current densities for CO formation at low overpotentials (|jCO| >7 mA cm−2,η= 320 mV; >13.5 mA cm−2,η= 520 mV) with ≥95% Faradaic efficiency (FECO), andCo2BiCageachieves high turnover frequencies (≈1300 h−1,η= 520 mV) with 90% FECO. In general, blocking the pore with C60or C70improves the catalytic performance of(FeCl)2BiCageand has only small effects onCo2BiCage, indicating that the good catalytic properties of the cages cannot be attributed to their internal pores. Neither enhanced electron transfer rates nor metal‐fullerene interactions appear to underlie the ability of C60/C70to improve the performance of(FeCl)2BiCage, in contrast to effects often proposed for other carbon nanosupports.more » « less
