14 research outputs found
Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors
We report electroluminescence from ion-gel gated, field-effect
transistors based on the conjugated polymer, polyÂ(9,9′-dioctylfluorene-co-benzothiadiazole)
gated by an 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl)
imide/poly (styrene-block-ethylene oxide-block-styrene) ion gel, and
investigate the mechanism for light emission. The devices emit light
from near the electron-injecting drain electrode when the drain source
voltage exceeds approximately the energy gap of the polymer (<i>V</i><sub>ds</sub> <i>> E</i><sub>g</sub><i>/e</i>). Charge accumulation spectroscopy is used to demonstrate
the significant
penetration of the negative TFSI<sup>–</sup> ions into the
F8BT assisted by the application of negative gate voltages, where
they lead to significant p-type doping of the bulk of the F8BT film.
In contrast, no evidence for diffusion of positive ions with positive
gate voltages is observed, and this is consistent with the location
of the recombination zone in the proximity of electron injecting electrode
and the absence of a comparable electron current at positive gate
voltages. We conclude that in the light-emission regime the devices
operate more akin to a hole-current dominated light-emitting electrochemical
cell than a transistor
Enhanced Ambipolar Charge Injection with Semiconducting Polymer/Carbon Nanotube Thin Films for Light-Emitting Transistors
We investigate the influence of small amounts of semiconducting single-walled carbon nanotubes (SWNTs) dispersed in polyfluorenes such as poly(9,9-di-<i>n</i>-octylfluorene-<i>alt</i>-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene) (F8) on device characteristics of bottom contact/top gate ambipolar light-emitting field-effect transistors (LEFETs) based on these conjugated polymers. We find that the presence of SWNTs within the semiconducting layer at concentrations below the percolation limit significantly increases both hole and electron injection, even for a large band gap semiconductor like F8, without leading to significant luminescence quenching of the conjugated polymer. As a result of the reduced contact resistance and lower threshold voltages, larger ambipolar currents and thus brighter light emission are observed. We examine possible mechanisms of this effect such as energy level alignment, reduced bulk resistance above the contacts, and field-enhanced injection at the nanotube tips. The observed ambipolar injection improvement is applicable to most conjugated polymers in staggered transistor configurations or similar organic electronic devices where injection barriers are an issue
Electrooptical Spectroscopy of Uniaxially Aligned Polythiophene Films in Field-Effect Transistors
Charge carriers induced in field-effect
transistors based on a
uniaxially aligned polythiophene polymer, polyÂ(2,5-bisÂ(3-alkylthiophen-2-yl)ÂthienoÂ[3,2-b]Âthiophene)
(PBTTT), are investigated by electrooptical charge modulation spectroscopy
(CMS). We used a zone-casting deposition method for uniaxial alignment
of the nanoribbon crystalline domains of the polymer and examined
the optical anisotropy of neutral and charge induced absorptions in
FETs. We find that the charge-induced optical absorptions of the polaronic
carriers at the interface exhibit a similar degree of uniaxial anisotropy
as the absorption spectrum of the neutral polymer bulk suggesting
similar degree of polymer alignment at the interface compared with
the bulk. We observe subtle differences in the spectral shape of the
polaron absorption polarized along and perpendicular to the polymer
chain direction. We also detect an additional charge-induced absorption
peak appearing at high charge carrier concentrations, which is similar
to the charge-induced absorption that is typical for chemically doped
films. These observations provide important insight into the interplay
between polaron transport and polymer microstructure
Fast Holes, Slow Electrons, and Medium Control of Polaron Size and Mobility in the DA Polymer F8BT
The
nature of electron and hole polarons on polyÂ(9,9-di-<i>n</i>-hexylfluorenyl-2,7-diyl) (pF) and a copolymer polyÂ[(9,9-di-<i>n</i>-octylfluorenyl-2,7-diyl)-alt-(benzoÂ[2,1,3]Âthiadiazol-4,8-diyl)]
(F8BT) has been studied by chemical doping, pulse radiolysis, charge
modulation spectroscopy, quantum chemical calculations, and microwave
conductivity. While pF exhibits very similar behavior in all respects
for the electron and the hole, this paper explores the hypothesis
that the donor acceptor (push–pull) nature of F8BT will tend
to localize charges. Optical spectra and quantum chemical calculations
point to an electron localized on the thiadiazole unit in polar liquids
but becoming more delocalized as the solvent polarity decreases. Indeed,
in the nonpolar solvent benzene, the electron mobility is only 2.7
times lower than that of the hole, which conversely is shown to be
delocalized in all environments and has a similar mobility to polarons
on the homopolymer polyfluorene. Advantageous modifications to the
optoelectronic properties of conjugated polymers that come about by
using alternating donor acceptor repeat units have thus been shown
to not significantly hinder charge transport despite the corrugated
energy landscape along the backbone
Microstructure of Polycrystalline PBTTT Films: Domain Mapping and Structure Formation
We utilize near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM) to study the microstructure and domain structure of polycrystalline films of the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-<i>b</i>]thiophene) (PBTTT). Total electron yield NEXAFS spectroscopy is used to examine the surface structure of the first 1–2 molecular layers, while bulk-sensitive STXM is used to produce maps of domain orientation and order sampled through the entire film thickness. We study different phases of PBTTT including as-cast, terraced and nanoribbon morphologies produced <i>via</i> spin-coating as well as aligned films of as-cast and nanoribbon morphologies produced by zone-casting. For the terraced morphology, domains are observed that are larger than the size of the terraced surface features, and the calculated degree of order is reduced compared to the nanoribbon morphology. For zone-cast films, we find that, although little optical anisotropy is observed in the bulk of as-cast films, a high degree of surface structural anisotropy is observed with NEXAFS spectroscopy, similar to what is observed in annealed nanoribbon films. This observation indicates that the aligned surface structure in unannealed zone-cast films templates the bulk ordering of the aligned nanoribbon phase. STXM domain mapping of aligned nanoribbon films reveals elongated, micrometer-wide domains with each domain misoriented with respect to its neighbor by up to 45°, but with broad domain boundaries. Within each nanoribbon domain, a high degree of X-ray dichroism is observed, indicating correlated ordering throughout the bulk of the film
Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification
Photoinduced
charge separation at hybrid organic–inorganic
interfaces is poorly understood and challenging to control. We investigate
charge separation at a model system of ZnO/polyÂ(3-hexylthiophene)
(P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA)
self-assembled modification to study and enhance the charge separation
efficiency. We find that doping alone lowers the efficiency of charge
separation due to the introduction of defect states at the oxide surface.
However, with the combination of doping and molecular modification,
charge separation efficiency is significantly enhanced due to the
passivation of interfacial traps and improved modifier coverage. This
demonstrates a complex noncumulative effect of doping and surface
modification and shows that with the correct choice of metal oxide
dopant and organic modifier, a poorly performing hybrid interface
can be turned into an efficient one
High Performance, Low Temperature Solution-Processed Barium and Strontium Doped Oxide Thin Film Transistors
Amorphous mixed metal oxides are
emerging as high performance semiconductors
for thin film transistor (TFT) applications, with indium gallium zinc
oxide, InGaZnO (IGZO), being one of the most widely studied and best
performing systems. Here, we investigate alkaline earth (barium or
strontium) doped InBaÂ(Sr)ÂZnO as alternative, semiconducting channel
layers and compare their performance of the electrical stress stability
with IGZO. In films fabricated by solution-processing from metal alkoxide
precursors and annealed to 450 °C we achieve high field-effect
electron mobility up to 26 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. We show that it is possible to solution-process
these materials at low process temperature (225–200 °C
yielding mobilities up to 4.4 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) and demonstrate a facile “ink-on-demand”
process for these materials which utilizes the alcoholysis reaction
of alkyl metal precursors to negate the need for complex synthesis
and purification protocols. Electrical bias stress measurements which
can serve as a figure of merit for performance stability for a TFT
device reveal Sr- and Ba-doped semiconductors to exhibit enhanced
electrical stability and reduced threshold voltage shift compared
to IGZO irrespective of the process temperature and preparation method.
This enhancement in stability can be attributed to the higher Gibbs
energy of oxidation of barium and strontium compared to gallium
Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors
We
have investigated the degradation effects of ozone exposure
on organic field-effect transistors based on 2,8-difluoro-5,11-bisÂ(triethylsilylethynyl)Âanthradithiophene
as the organic semiconducting channel layer, as well as on thin films
of this widely used, high-mobility, small molecule semiconductor.
Electrical <i>I</i>–<i>V</i> measurements
showed a loss of transistor characteristic behavior. We present <sup>1</sup>H Nuclear Magnetic Resonance (NMR) spectroscopy results as
well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform
Infrared (FTIR) spectroscopy measurements showing the oxidation of
the parent molecule, from which we suggest various possible reaction
paths
Structure Influence on Charge Transport in Naphthalenediimide–Thiophene Copolymers
Reported here is a characterization
of a series of NDI–thiophene
copolymers with one, two, three, and four thiophene units synthesized
using Stille polycondensation of dibromo-naphthalene diimide and the
trimethylstannylthiophene monomers. The effect of extension of the
thiophene donor group is studied in terms of structure-charge transport
correlation. The influence of side chains located on the thiophene
units of copolymers with two and four thiophene units per monomer
is also investigated. Charge transport of both signs is studied experimentally
in field-effect transistors. Microstructural data obtained by near-edge
X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle
X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet
photoelectron spectroscopy (UPS) and UV–vis spectroscopy data
are employed in the measurement of energy levels, and changes with
annealing temperature are also discussed. Most of the polymers reach
excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting
an especially balanced ambipolar charge transport of 0.03 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. An odd–even
effect in hole mobility is observed with higher values for polymers
with an even number of thiophene units. The reported findings indicate
that the final charge transport properties are a result of the interplay
of many factors, including crystallinity, planarity and linearity
of chain, spacing between acceptor units and packing of solubilizing branched side
chains
Charge Mobility Enhancement for Conjugated DPP-Selenophene Polymer by Simply Replacing One Bulky Branching Alkyl Chain with Linear One at Each DPP Unit
We
demonstrate a simple, but efficient, approach for improving
the semiconducting performances of DPP-based conjugated D-A polymers.
This approach involves the replacement of one bulky branching alkyl
chain with the linear one at each DPP unit in regular polymer PDPPSe-10
and PDPPSe-12. The UV–vis absorption, Raman spectra, PDS data,
and theoretical calculations support that the replacement of bulky
branching chains with linear ones can weaken the steric hindrance,
and accordingly conjugated backbones become more planar and rigid.
GIWAXS data show that the incorporation of linear alkyl chains as
in PDPPSe-10 and PDPPSe-12 is beneficial for side-chain interdigitation
and interchain dense packing, leading to improvement of interchain
packing order and thin film crystallinity by comparing with PDPPSe,
which contains branching alkyl chains. On the basis of field-effect
transistor (FET) studies, charge mobilities of PDPPSe-10 and PDPPSe-12
are remarkably enhanced. Hole mobilities of PDPPSe-10 and PDPPSe-12
in air are boosted to 8.1 and 9.4 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, which are about 6 and 7 times, respectively,
than that of PDPPSe (1.35 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>). Furthermore, both PDPPSe-10 and PDPPSe-12 behave
as ambipolar semiconductors under a nitrogen atmosphere with increased
hole/electron mobilities up to 6.5/0.48 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and 7.9/0.79 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, respectively