3 research outputs found
Singly and Doubly Occupied Higher Quantum States in Nanocrystals
Filling
the lowest quantum state of the conduction band of colloidal
nanocrystals with a single electron, which is analogous to the filling
the lowest unoccupied molecular orbital in a molecule with a single
electron, has attracted much attention due to the possibility of harnessing
the electron spin for potential spin-based applications. The quantized
energy levels of the artificial atom, in principle, make it possible
for a nanocrystal to be filled with an electron if the Fermi-energy
level is optimally tuned during the nanocrystal growth. Here, we report
the singly occupied quantum state (SOQS) and doubly occupied quantum
state (DOQS) of a colloidal nanocrystal in steady state under ambient
conditions. The number of electrons occupying the lowest quantum state
can be controlled to be zero, one (unpaired), and two (paired) depending
on the nanocrystal growth time via changing the stoichiometry of the
nanocrystal. Electron paramagnetic resonance spectroscopy proved the
nanocrystals with single electron to show superparamagnetic behavior,
which is a direct evidence of the SOQS, whereas the DOQS of the two-
or zero-electron occupied nanocrystals in the 1S<sub>e</sub> exhibit
diamagnetic behavior. In combination with the superconducting quantum
interference device measurement, it turns out that the SOQS of the
HgSe colloidal quantum dots has superparamagnetic property. The appearance
and change of the steady-state mid-IR intraband absorption spectrum
reflect the sequential occupation of the 1S<sub>e</sub> state with
electrons. The magnetic property of the colloidal quantum dot, initially
determined by the chemical synthesis, can be tuned from diamagnetic
to superparamagnetic and vice versa by varying the number of electrons
through postchemical treatment. The switchable magnetic property will
be very useful for further applications such as colloidal nanocrystal
based spintronics, nonvolatile memory, infrared optoelectronics, catalyst,
imaging, and quantum computing
Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells by Synergistic Effect of Nonhygroscopic Doping in Conjugated Polymer-Based Hole-Transporting Layer
A face-on
oriented and p-doped semicrystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-<i>alt</i>-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[<i>c</i>][1,2,5]-thiadiazole)] (PPDT2FBT), was studied as a hole-transport
layer (HTL) in methylammonium lead triiodide-based perovskite solar
cells (PVSCs). PPDT2FBT exhibits a mid-band gap (1.7 eV), high vertical
hole mobility (7.3 × 10<sup>–3</sup> cm<sup>2</sup>/V·s),
and well-aligned frontier energy levels with a perovskite layer for
efficient charge transfer/transport, showing a maximum power conversion
efficiency (PCE) of 16.8%. Upon doping the PPDT2FBT HTL with a nonhygroscopic
Lewis acid, tris(pentafluorophenyl)borane (BCF, 2–6 wt %),
the vertical conductivity was improved by a factor of approximately
2, and the resulting PCE was further improved up to 17.7%, which is
higher than that of standard PVSCs with 2,2′,7,7′-tetrakis(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene
(spiro-OMeTAD) as an HTL. After BCF doping, the clearly enhanced carrier
diffusion coefficient, diffusion length, and lifetime were measured
using intensity-modulated photocurrent and photovoltage spectroscopy.
Furthermore, compared to the standard PVSCs with spiro-OMeTAD, the
temporal device stability was remarkably improved, preserving the
∼60% of the original PCE for 500 h without encapsulation under
light-soaking condition (1 sun AM 1.5G) at 85 °C and 85% humidity,
which is mainly due to the highly crystalline conjugated backbone
of PPDT2FBT and nonhygroscopic nature of BCF. In addition, formamidinium
lead iodide/bromide (FAPbI<sub>3–<i>x</i></sub>Br<i><sub>x</sub></i>)-based PVSCs with the BCF-doped PPDT2FBT as
an HTL was also prepared to show 18.8% PCE, suggesting a wide applicability
of PPDT2FBT HTL for different types of PVSCs
A Planar Cyclopentadithiophene–Benzothiadiazole-Based Copolymer with sp<sup>2</sup>‑Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices
A semicrystalline
p-type thermoelectric conjugated polymer based
on a polymer backbone of cyclopentadithiophene and benzothiadiazole,
poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-<i>b</i>:3,4-<i>b</i>′]dithiophene)-<i>alt</i>-(benzo[<i>c</i>][1,2,5]thiadiazole)] (PCPDTSBT), is designed
and synthesized by replacing normal alkyl side-chains with bis(alkylsulfanyl)methylene
substituents. The sp<sup>2</sup>-hybridized olefinic bis(alkylsulfanyl)methylene
side-chains and the sulfur–sulfur (S–S) chalcogen interactions
extend a chain planarity with strong interchain packing, which is
confirmed by density functional calculations and morphological studies,
i.e., grazing incidence X-ray scattering measurement. The doping,
electrical, morphological, and thermoelectric characteristics of PCPDTSBT
are investigated by comparison with those of poly[(4,4′-bis(2-ethylhexyl)cyclopenta[2,1-<i>b</i>:3,4-<i>b</i>′]dithiophene)-<i>alt</i>-(benzo[<i>c</i>][1,2,5]thiadiazole)] (PCPDTBT) with ethylhexyl
side-chains. Upon doping with a Lewis acid, B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, the maximum electrical conductivity (7.47 S cm<sup>–1</sup>) of PCPDTSBT is ∼1 order higher than that
(0.65 S cm<sup>–1</sup>) of PCPDTBT, and the best power factor
is measured to be 7.73 μW m<sup>–1</sup> K<sup>–2</sup> for PCPDTSBT with doping 9 mol % of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. The Seebeck coefficient–electrical conductivity
relation is analyzed by using a charge transport model for polymers,
suggesting that the doped PCPDTSBT film has superb charge transport
property based on a high crystallinity with olefinic side-chains.
This study emphasizes the importance of side-chain engineering by
using the sp<sup>2</sup>-hybridized olefinic substituents to modulate
interchain packing, crystalline morphology, and the resulting electrical
properties