7 research outputs found
Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites
The
rapid relaxation of above-band-gap āhotā carriers
(HCs) imposes the key efficiency limit in lead-halide perovskite (LHP)
solar cells. Recent studies have indicated that HC cooling in these
systems may be sensitive to materials composition, as well as the
energy and density of excited states. However, the key parameters
underpinning the cooling mechanism are currently under debate. Here
we use a sequence of ultrafast optical pulses (visible pumpāinfrared
pushāinfrared probe) to directly compare the intraband cooling
dynamics in five common LHPs: FAPbI<sub>3</sub>, FAPbBr<sub>3</sub>, MAPbI<sub>3</sub>, MAPbBr<sub>3</sub>, and CsPbBr<sub>3</sub>.
We observe ā¼100ā900 fs cooling times, with slower cooling
at higher HC densities. This effect is strongest in the all-inorganic
Cs-based system, compared to the hybrid analogues with organic cations.
These observations, together with band structure calculations, allow
us to quantify the origin of the āhot-phonon bottleneckā
in LHPs and assert the thermodynamic contribution of a symmetry-breaking
organic cation toward rapid HC cooling
PerovskiteāPerovskite Homojunctions via Compositional Doping
One
of the most important properties of semiconductors is the possibility
of controlling their electronic behavior via intentional doping. Despite
the unprecedented progress in the understanding of hybrid metal halide
perovskites, extrinsic doping of perovskite remains nearly unexplored
and perovskiteāperovskite homojunctions have not been reported.
Here we present a perovskiteāperovskite homojunction obtained
by vacuum deposition of stoichiometrically tuned methylammonium lead
iodide (MAPI) films. Doping is realized by adjusting the relative
deposition rates of MAI and PbI<sub>2</sub>, obtaining p-type (MAI
excess) and n-type (MAI defect) MAPI. The successful stoichiometry
change in the thin films is confirmed by infrared spectroscopy, which
allows us to determine the MA content in the films. We analyzed the
resulting thin-film junction by cross-sectional scanning Kelvin probe
microscopy (SKPM) and found a contact potential difference (CPD) of
250 mV between the two differently doped perovskite layers. Planar
diodes built with the perovskiteāperovskite homojunction show
the feasibility of our approach for implementation in devices
Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide
Solution-processed
nickel oxide (sNiO) is known to be an excellent charge-selective interlayer
in optoelectronic devices. Its beneficial properties can be further
enhanced by an oxygen plasma (OP) treatment. In order to elucidate
the mechanism behind this improvement, we use infrared transmission
and X-ray photoelectron spectroscopy to probe the bulk and surface
properties of the sNiO. We find that increasing the annealing temperature
of the sNiO not only increases the structural order of the material
but also reduces the concentration of nickel hydroxide species present
in the bulk and on the surface of the film. This results in a decrease
of the work function, while an additional OP treatment raises the
work function to between 5.5 and 5.6 eV. For all annealing temperatures
investigated, the consequences of the OP treatment are identified
as reactions of both NiO and Ī²-NiĀ(OH)<sub>2</sub> to form thin
Ī²-NiOOH phases in the first atomic layers. Our results emphasize
the importance of understanding the correlation between the preparation
and resulting properties of sNiO layers and provides further insight
into the interpretation of interface properties of NiO
Dopant Diffusion in Sequentially Doped Poly(3-hexylthiophene) Studied by Infrared and Photoelectron Spectroscopy
The
diffusivity of dopants in semiconducting polymers is of high
interest as it enables methods of sequential doping but also affects
device stability. In this study, we investigate the diffusion of a
bulky sequentially deposited p-dopant in polyĀ(3-hexylthiophene) (P3HT)
thin films using nondestructive <i>in situ</i> infrared
(IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant
diffusion into the polymer film at varying coverage by differentially
evaluating electron transfer in the bulk and at the surface. Thereby
it is possible to determine dopant coverages at which both electron
transfer and incorporation of dopants are saturated. By use of PES,
neutral and charged dopants can be distinguished, revealing that charged
dopants are less mobile in the diffusion process than neutral molecules.
We further compare the diffusivity in semicrystalline and fully amorphous
P3HT. We find that at high coverage semicrystalline P3HT seems to
yield a higher capacity for dopants than fully amorphous P3HT. A temperature-dependent
measurement of sequential doping shows directly that the incorporation
of dopants is thermally activated and requires temperatures close
to room temperature
Controlled Molecular Orientation of Inkjet Printed Semiconducting Polymer Fibers by Crystallization Templating
Here we present the
controlled deposition of highly aligned polyĀ(3-hexylthiophene-2,5-diyl)
(P3HT) fibers by inkjet printing. The functional ink consists of the
crystallization agent 1,3,5-trichlorobenzene (TCB), the carrier solvent
chlorobenzene, and the semiconducting polymer P3HT. The inkjet printing
process was designed in such a way that the drying zone migrates in
the printing direction, effectively growing the TCB out of solution
and forcing the P3HT chains to align in the printing direction. The
films are deposited in arbitrary shapes on a variety of substrates,
thus demonstrating the full freedom of design necessary for the digital
fabrication of future integrated circuits. We demonstrate by optical
and structural investigations that P3HT arranges in a nontrivial empty-coreāshell
structure with the long molecular axis in the fiber direction while
the short axis extends in a radial fashion. Such arrangement induces
a fourfold increase in field-effect mobility along the fiber direction
as compared to the isotropic printed reference
Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity
Nickel oxide (NiO)
is a widely used material for efficient hole extraction in optoelectronic
devices. However, its surface characteristics strongly depend on the
processing history and exposure to adsorbates. To achieve controllability
of the electronic and chemical properties of solution-processed nickel
oxide (sNiO), we functionalize its surface with a self-assembled monolayer
(SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared
and photoelectron spectroscopy shows the chemisorption of the molecules
with a nominal layer thickness of around one monolayer and gives an
insight into the chemical composition of the SAM. Density functional
theory calculations reveal the possible binding configurations. By
the application of the SAM, we increase the sNiO work function by
up to 0.8 eV. When incorporated in organic solar cells, the increase
in work function and improved energy level alignment to the donor
does not lead to a higher fill factor of these cells. Instead, we
observe the formation of a transport barrier, which can be reduced
by increasing the conductivity of the sNiO through doping with copper
oxide. We conclude that the widespread assumption of maximizing the
fill factor by only matching the work function of the oxide charge
extraction layer with the energy levels in the active material is
a too narrow approach. Successful implementation of interface modifiers
is only possible with a sufficiently high charge carrier concentration
in the oxide interlayer to support efficient charge transfer across
the interface
Dipolar SAMs Reduce Charge Carrier Injection Barriers in nāChannel Organic Field Effect Transistors
In this work we examine small conjugated
molecules bearing a thiol
headgroup as self assembled monolayers (SAM). Functional groups in
the SAM-active molecule shift the work function of gold to n-channel
semiconductor regimes and improve the wettability of the surface.
We examine the effect of the presence of methylene linkers on the
orientation of the molecule within the SAM. 3,4,5-Trimethoxythiophenol
(TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH<sub>2</sub>-SH) were
first subjected to computational analysis, predicting work function
shifts of ā430 and ā310 meV. Contact angle measurements
show an increase in the wetting envelope compared to that of pristine
gold. Infrared (IR) measurements show tilt angles of 22 and 63Ā°,
with the methylene-linked molecule (TMP-CH<sub>2</sub>-SH) attaining
a flatter orientation. The actual work function shift as measured
with photoemission spectroscopy (XPS/UPS) is even larger, ā600
and ā430 meV, respectively. The contact resistance between
gold electrodes and polyĀ[<i>N</i>,<i>N</i>ā²-bisĀ(2-octyldodecyl)-naphthalene-1,4:5,8-bisĀ(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5ā²-(2,2ā²-bithiophene) (Polyera Aktive
Ink, N2200) in n-type OFETs is demonstrated to decrease by 3 orders
of magnitude due to the use of TMP-SH and TMP-CH<sub>2</sub>-SH. The
effective mobility was enhanced by two orders of magnitude, significantly
decreasing the contact resistance to match the mobilities reported
for N2200 with optimized electrodes