16 research outputs found
Improving the Stability of Bulk Heterojunction Solar Cells by Incorporating pH-Neutral PEDOT:PSS as the Hole Transport Layer
In the application of traditional
bulk heterojunction polymer solar cells, to prevent the etching of
ITO by the acidic polyÂ(3,4-ethylenedioxythiophene):polyÂ(styrenesulfonate)
(PEDOT:PSS) and thereby improve the device stability, pH-neutral PEDOT:PSS
is introduced as the hole transport layer (HTL). After treating the
neutral PEDOT:PSS with UV-ozone and with an oxygen plasma, the average
power conversion efficiency (PCE) of the device increases from 3.44%
to 6.60%. Such surface treatments reduce the energy level offset between
the HTL and the active layer, which increases the open circuit voltage
and enhances hole transportation, leading to the PCE improvement.
Moreover, the devices with the neutral PEDOT:PSS HTL are more stable
in air than those with the acidic PEDOT:PSS HTL. The PCE of the devices
with the acidic PEDOT:PSS HTL decreases by 20% after 7 days and 45%
after 50 days under ambient conditions, whereas the PCE of the devices
with the pH-neutral PEDOT:PSS HTL decreases by only 9 and 20% after
7 and 50 days, respectively. X-ray photoelectron spectroscopy shows
that the acidic PEDOT:PSS etches the indium from the indium–tin−oxide
(ITO) electrode, which is responsible for the degradation of the device.
In comparison, the diffusion of the indium is much slower in the devices
with the pH-neutral PEDOT:PSS HTL
All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good Stability and Uniformity Using Surface-Energy Patterns
An array of inkjet-printed metal-oxide
thin-film transistors (TFTs)
is demonstrated for the first time with the assistance of surface-energy
patterns prepared by printing pure solvent to etch the ultrathin hydrophobic
layer. The surface-energy patterns not only restrained the spreading
of inks but also provided a facile way to regulate the morphology
of metal oxide films without optimizing ink formulation. The fully
printed InGaO TFT devices in the array exhibited excellent electron
transport characteristics with a maximum mobility of 11.7 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, negligible hysteresis,
good uniformity, and good stability under bias stress. The new route
lights a general way toward fully inkjet-printed metal-oxide TFT arrays
High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing
Solvent additive processing is important
in optimizing an active
layer’s morphology and thus improving the performance of organic
solar cells (OSCs). In this study, we find that how 1,8-diiodooctane
(DIO) additive is removed plays a critical role in determining the
film morphology of the bulk heterojunction OSCs in inverted structure
based on a porphyrin small molecule. Different from the cases reported
for polymer-based OSCs in conventional structures, the inverted OSCs
upon the quick removal of the additive either by quick vacuuming or
methanol washing exhibit poorer performance. In contrast, the devices
after keeping the active layers in ambient pressure with additive
dwelling for about 1 h (namely, additive annealing) show an enhanced
power conversion efficiency up to 7.78% with a large short circuit
current of 19.25 mA/cm<sup>2</sup>, which are among the best in small
molecule-based solar cells. The detailed morphology analyses using
UV-vis
absorption spectroscopy, grazing incidence X-ray diffraction, resonant
soft X-ray scattering, and atomic force microscopy demonstrate that
the active layer shows smaller-sized phase separation but improved
structure order upon additive annealing. On the contrary, the quick
removal of the additive either by quick vacuuming or methanol washing
keeps the active layers in an earlier stage of large scaled phase
separation
Coffee-Ring Defined Short Channels for Inkjet-Printed Metal Oxide Thin-Film Transistors
Short-channel electronic
devices several micrometers in length are difficult to implement by
direct inkjet printing due to the limitation of position accuracy
of the common inkjet printer system and the spread of functional ink
on substrates. In this report, metal oxide thin-film transistors (TFTs)
with channel lengths of 3.5 ± 0.7 μm were successfully
fabricated with a common inkjet printer without any photolithography
steps. Hydrophobic CYTOP coffee stripes, made by inkjet-printing and
plasma-treating processes, were utilized to define the channel area
of TFTs with channel lengths as short as ∼3.5 μm by dewetting
the inks of the source/drain (S/D) precursors. Furthermore, by introduction
of an ultrathin layer of PVA to modify the S/D surfaces, the spreading
of precursor ink of the InO<sub><i>x</i></sub> semiconductor
layer was well-controlled. The inkjet-printed short-channel TFTs exhibited
a maximum mobility of 4.9 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and an on/off ratio of larger than 10<sup>9</sup>. This approach of fabricating short-channel TFTs by inkjet printing
will promote the large-area fabrication of short-channel TFTs in a
cost-effective manner
High Efficiency and High <i>V</i><sub>oc</sub> Inverted Polymer Solar Cells Based on a Low-Lying HOMO Polycarbazole Donor and a Hydrophilic Polycarbazole Interlayer on ITO Cathode
In this work, polyÂ[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4,7-di-2-thienyl-5,6-bisÂ(dodecyloxy)-2,1,3-benzothiadiazole)]
(PCDTBT12) was synthesized as the polymer donor for photovoltaic application.
PCDTBT12 possesses a band gap of 1.99 eV, a low-lying HOMO of −5.6
eV, and good hole mobility up to 4.1 × 10<sup>–3</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. With
ZnO as the interlayer on an ITO cathode, a PCDTBT12-based inverted
solar cell showed a high open-circuit voltage of 0.98 V and a good
power conversion efficiency (PCE) of 5.53%, suggesting that PCDTBT12
would be a promising donor material in the fabrication of a subcell
for shorter wavelength absorption in a tandem solar cell. Using PC-P,
a homopolymer of 2,7-carbazole with hydrophilic phosphonate side chains,
as an interlayer polymer on the ITO cathode could further elevate
the efficiency to 6.04% because of increased current (higher efficiency
of 6.2% was achieved for a smaller cell area of 0.045 cm<sup>2</sup>). The efficiencies are the highest ones so far reported for an inverted
solar cell with an organic cathode interlayer. It was proposed that
the hydrophilic side chains of PC-P supplied a subgap state for electron
transport. The two devices showed comparable air stability, and retained
over 96% of their initial PCEs after storage in air for more than
1 month. Therefore, a hydrophilic conjugated polymer as the cathode
interlayer, already shown in outstanding cathode modifications in
conventional polymer solar cells, will play an important role in the
future development of high efficiency and air-stable inverted solar
cells
Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[<i>c</i>][1,2,5]thiadiazole Unit for Polymer Solar Cells
We
designed and synthesized two isomeric nonfullerene acceptors, IFBR-<i>p</i> and IFBR-<i>d</i>. These molecular semiconductors
contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric
5-fluorobenzoÂ[<i>c</i>]Â[1,2,5]Âthiadiazole units, and are
flanked with rhodanine as the peripheral units. The orientation of
the two fluorine atoms (proximal, <i>p</i>, or distal, <i>d</i>), relative to IDT impacts most severely the film morphologies
when blended with the electron-donating polymer PTzBI. Polymer solar
cells based on PTzBI:IFBR-<i>p</i> give rise to a power
conversion efficiency (7.3 ± 0.2%) that is higher than what is
achieved with PTzBI:IFBR-<i>d</i> (5.2 ± 0.1%). This
difference is attributed to the lower tendency for (over)Âcrystallization
by IFBR-<i>p</i> and the resulting more favorable morphology
of the photoactive layer. These results highlight the subtle impact
of substitution regiochemistry on the properties of nonfullerene acceptors
through modulation of their self-assembly tendencies
Homogeneous Surface Profiles of Inkjet-Printed Silver Nanoparticle Films by Regulating Their Drying Microenvironment
Obtaining homogeneous
surface profiles of inkjet-printed silver
nanoparticle films (SNFs) is severely hampered by the classical <i>coffee-ring</i> phenomenon. Traditional approaches to suppress
the coffee-ring effect are achieved by improving the uniformity of
liquid evaporation rate or introducing inward Marangoni force during
ink drying. However, these existing methods involve extra chemicals,
treatments, or equipment that will definitely increase the production
cost or reduce conductivity. In this study, we demonstrate an inexpensive
and efficient method to obtain uniform surface profiles of inkjet-printed
SNFs by rationally regulating the drying microenvironment. To this
end, a number of surface profiles of printed dots and lines, such
as concave, convex, and flat, were first obtained via this method.
The underlying principle of this method was then investigated by analytical
calculations based on the simplified vapor diffusion model. Consequently,
homogeneous surface profiles of inkjet-printed SNFs were achieved
on the basis of aforementioned analytical calculations. Our work provides
a new possibility to regulate the surface profiles of inkjet-printed
nanoparticle-based patterns in the form of low cost and efficiency
High-Performance, Solution-Processed Quantum Dot Light-Emitting Field-Effect Transistors with a Scandium-Incorporated Indium Oxide Semiconductor
Light-emitting
field-effect transistors (LEFETs) have attained
great attention due to their special characteristics of both the switching
capacity and the electroluminescence capacity. However, high-performance
LEFETs with high mobility, high brightness, and high efficiency have
not been realized due to the difficulty in developing high electron
and hole mobility materials with suitable band structures. In this
paper, quantum dot hybrid LEFETs (QD-HLEFETs) combining high-luminous-efficiency
quantum dots (QDs) and a solution-processed scandium-incorporated
indium oxide (Sc:In<sub>2</sub>O<sub>3</sub>) semiconductor were demonstrated.
The red QD-HLEFET showed high electrical and optical performance with
an electron mobility of 0.8 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, a maximum brightness of 13 400 cd/m<sup>2</sup>, and a maximum external quantum efficiency of 8.7%. The high performance
of the QD-HLEFET is attributed to the good energy band matching between
Sc:In<sub>2</sub>O<sub>3</sub> and QDs and the balanced hole and electron
injection (less exciton nonradiative recombination). In addition,
incorporation of Sc into In<sub>2</sub>O<sub>3</sub> can suppress
the oxygen vacancy and free carrier generation and brings about excellent
current and optical modulation (the on/off current ratio is 10<sup>5</sup> and the on/off brightness ratio is 10<sup>6</sup>)
Damage-Free Back Channel Wet-Etch Process in Amorphous Indium–Zinc-Oxide Thin-Film Transistors Using a Carbon-Nanofilm Barrier Layer
Amorphous indium–zinc-oxide
thin film transistors (IZO-TFTs) with damage-free back channel wet-etch
(BCE) process were investigated. A carbon (C) nanofilm was inserted
into the interface between IZO layer and source/drain (S/D) electrodes
as a barrier layer. Transmittance electron microscope images revealed
that the 3 nm-thick C nanofilm exhibited a good corrosion resistance
to a commonly used H<sub>3</sub>PO<sub>4</sub>-based etchant and could
be easily eliminated. The TFT device with a 3 nm-thick C barrier layer
showed a saturated field effect mobility of 14.4 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, a subthreshold swing of 0.21
V/decade, an on-to-off current ratio of 8.3 × 10<sup>10</sup>, and a threshold voltage of 2.0 V. The favorable electrical performance
of this kind of IZO-TFTs was due to the protection of the inserted
C to IZO layer in the back-channel-etch process. Moreover, the low
contact resistance of the devices was proved to be due to the graphitization
of the C nanofilms after annealing. In addition, the hysteresis and
thermal stress testing confirmed that the usage of C barrier nanofilms
is an effective method to fabricate the damage-free BCE-type devices
with high reliability
Critical Impact of Solvent Evaporation on the Resolution of Inkjet Printed Nanoparticles Film
We
first verify the critical role of solvent evaporation on the resolution
of inkjet printing. To confirm our hypothesis, we adjusted the evaporation
rate gradient along the surface of adjacent droplets by controlling
the drying microenvironment. Uneven solvent evaporation flux caused
thermocapillary surface flow inward the space of micrometer-sized
droplets and increase the air pressure, which prevented the neighboring
droplets from coalescence. When reducing the droplet distance by the
solvent evaporation-based method, a uniform profile could be obtained
at the same time. This work brings us a step closer to resolving one
of the critical bottlenecks to commercializing printed electronic
goods