16 research outputs found
Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell
The interfacial band alignment among
boron subnaphthalocyanine
chloride (SubNc), boron subphthalocyanine chloride (SubPc), and Îą-sexithiophene
(Îą-6T) is explored using ultraviolet, inverse, and X-ray photoemission
spectroscopies (UPS, IPES, and XPS, respectively). With these tools,
the ionization energy (IE) and electron affinity (EA) for each material
are determined. Layer-by-layer deposition of SubPc and SubNc on Îą-6T
as well as SubPc on SubNc, combined with UPS and IPES, allows for
the direct determination of the energy level alignment at the interfaces
of interest. A small dipole is found at the Îą-6T/SubNc/SubPc
interface, expanding the donor-LUMO to acceptor-HOMO gap and explaining
the large open circuit voltage obtained with these devices. However,
there is a small electron barrier between SubNc and SubPc, which may
limit the efficiency of electron extraction in the current device
configuration. Excess chlorine may be responsible for the high IE
and EA found for SubNc and could potentially be remedied with improved
synthetic methods or further purification
Improved Absorber Phase Stability, Performance, and Lifetime in Inorganic Perovskite Solar Cells with Alkyltrimethoxysilane Strain-Release Layers at the Perovskite/TiO<sub>2</sub> Interface
All-inorganic β-CsPbI3 has superior
chemical and
thermal stability compared to its hybrid counterparts, but the stability
of state-of-the-art β-CsPbI3 perovskite solar cells
(PSCs) under normal operating conditions (i.e., under illumination
in an inert atmosphere) remains inferior to their hybrid counterparts.
Here, we found that the lattice distortion in CsPbI3 near
the perovskite/electron transport layer (ETL) interface can induce
polymorphic transformation in encapsulated CsPbI3 films
aged under illumination. To suppress this lattice distortion, we introduced
alkyltrimethoxysilane strain-release layers (SRLs) at the perovskite/ETL
interface. We found the SRL with the longest alkyl chain is the most
effective at reducing interfacial lattice distortion, leading to enhanced
charge transfer at the perovskite/ETL interface and improved phase/device
stability. Its incorporation in β-CsPbI3 solar cells
resulted in a power-conversion efficiency of 20.1% and an operational
lifetime with an extrapolated T80 of >3000
h for encapsulated devices tested under continuous illumination under
maximum power point tracking conditions
Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT)
We demonstrate new approaches to
the characterization of oxidized
regioregular polyÂ(3-hexylthiophene-2,5-diyl) (P3HT) that results from
electronic equilibration with device-relevant high work function electrical
contacts using high-resolution X-ray (XPS) and ultraviolet (UPS) photoelectron
spectroscopy (PES). Careful interpretation of photoemission signals
from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films
provides the ability to uniquely elucidate the products of charge
transfer between the polymer and the electrical contact, which is
a result of Fermi-level equilibration between the two materials. By
comparing high-resolution S 2p core-level spectra to electrochemically
oxidized P3HT standards, the extent of the contact doping reaction
is quantified, where one in every six thiophene units (ca. 20%) in
the first monolayer is oxidized. Finally, angle-resolved XPS of both
pure P3HT and its blends with phenyl-C<sub>61</sub>-butyric acid methyl
ester (PCBM) confirms that oxidized P3HT species exist near contacts
with work functions greater than ca. 4 eV, providing a means to characterize
the interface and âbulkâ region of the organic semiconductor
in a single film
Air-Exposure-Induced Gas-Molecule Incorporation into Spiro-MeOTAD Films
Combined photoemission and charge-transport
property studies of
the organic hole transport material 2,2â˛,7,7â˛-tetrakisÂ(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9â˛-spirobifluorene
(spiro-MeOTAD) under air exposure and controlled environments of O<sub>2</sub>, H<sub>2</sub>O + N<sub>2</sub>, and N<sub>2</sub> (1 atm
and under dark conditions) reveal the incorporation of gas molecules
causing a decrease in charge mobility. Ultraviolet photoelectron spectroscopy
shows the Fermi level shifts toward the highest occupied molecular
orbital of spiro-MeOTAD when exposed to air, O<sub>2</sub>, and H<sub>2</sub>O resembling p-type doping. However, no traces of oxidized
spiro-MeOTAD<sup>+</sup> are observed by X-ray photoelectron spectroscopy
(XPS) and UVâvisible spectroscopy. The charge-transport properties
were investigated by fabricating organic field-effect transistors
with the 10 nm active layer at the semiconductorâinsulator
interface exposed to different gases. The hole mobility decreases
substantially upon exposure to air, O<sub>2</sub>, and H<sub>2</sub>O. In the case of N<sub>2</sub>, XPS reveals the incorporation of
N<sub>2</sub> molecules into the film, but the decrease in the hole
mobility is much smaller
Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the MoleculeâSilicon Interface
The
interface level alignment of alkyl and alkenyl monolayers,
covalently bound to oxide-free Si substrates of various doping levels,
is studied using X-ray photoelectron spectroscopy. Using shifts in
the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find
that charge distribution around the covalent SiâC bond dipole
changes according to the initial position of the Fermi level within
the Si substrate. This shows that the interface dipole is not fixed
but rather changes with the doping level. These results set limits
to the applicability of simple models to describe level alignment
at interfaces and show that the interface bond and dipole may change
according to the electrostatic potential at the interface
High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid OrganicâInorganic Perovskite Solar Cells
We
investigate the effect of high work function contacts in halide
perovskite absorber-based photovoltaic devices. Photoemission spectroscopy
measurements reveal that band bending is induced in the absorber by
the deposition of the high work function molybdenum trioxide (MoO<sub>3</sub>). We find that direct contact between MoO<sub>3</sub> and
the perovskite leads to a chemical reaction, which diminishes device
functionality. Introducing an ultrathin spiro-MeOTAD buffer layer
prevents the reaction, yet the altered evolution of the energy levels
in the methylammonium lead iodide (MAPbI<sub>3</sub>) layer at the
interface still negatively impacts device performance
Recommended from our members
Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined ExperimentalâTheoretical Study
We
report valence and conduction band densities of states measured
via ultraviolet and inverse photoemission spectroscopies on three
metal halide perovskites, specifically methylammonium lead iodide
and bromide and cesium lead bromide (MAPbI<sub>3</sub>, MAPbBr<sub>3</sub>, CsPbBr<sub>3</sub>), grown at two different institutions
on different substrates. These are compared with theoretical densities
of states (DOS) calculated via density functional theory. The qualitative
agreement achieved between experiment and theory leads to the identification
of valence and conduction band spectral features, and allows a precise
determination of the position of the band edges, ionization energy
and electron affinity of the materials. The comparison reveals an
unusually low DOS at the valence band maximum (VBM) of these compounds,
which confirms and generalizes previous predictions of strong band
dispersion and low DOS at the MAPbI<sub>3</sub> VBM. This low DOS
calls for special attention when using electron spectroscopy to determine
the frontier electronic states of lead halide perovskites
Evidence for near-Surface NiOOH Species in Solution-Processed NiO<sub><i>x</i></sub> Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics
The characterization and implementation of solution-processed,
wide bandgap nickel oxide (NiO<sub><i>x</i></sub>) hole-selective
interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics
(OPVs) are discussed. The surface electrical properties and charge
selectivity of these thin films are strongly dependent upon the surface
chemistry, band edge energies, and midgap state concentrations, as
dictated by the ambient conditions and film pretreatments. Surface
states were correlated with standards for nickel oxide, hydroxide,
and oxyhydroxide components, as determined using monochromatic X-ray
photoelectron spectroscopy. Ultraviolet and inverse photoemission
spectroscopy measurements show changes in the surface chemistries
directly impact the valence band energies. O<sub>2</sub>-plasma treatment
of the as-deposited NiO<sub><i>x</i></sub> films was found
to introduce the dipolar surface species nickel oxyhydroxide (NiOOH),
rather than the p-dopant Ni<sub>2</sub>O<sub>3</sub>, resulting in
an increase of the electrical band gap energy for the near-surface
region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking
properties of the as-deposited and O<sub>2</sub>-plasma treated NiO<sub><i>x</i></sub> films are compared using both electron-only
and BHJ devices. O<sub>2</sub>-plasma-treated NiO<sub><i>x</i></sub> interlayers produce electron-only devices with lower leakage
current and increased turn on voltages. The differences in behavior
of the different pretreated interlayers appears to arise from differences
in local density of states that comprise the valence band of the NiO<sub><i>x</i></sub> interlayers and changes to the band gap
energy, which influence their hole-selectivity. The presence of NiOOH
states in these NiO<sub><i>x</i></sub> films and the resultant
chemical reactions at the oxide/organic interfaces in OPVs is predicted
to play a significant role in controlling OPV device efficiency and
lifetime
Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells
The large band gaps of existing contorted
hexabenzocoronene derivatives
severely limit visible-light absorption, restricting the photocurrents
generated by solar cells utilizing contorted hexabenzocoronene (cHBC).
To decrease the band gap and improve the light-harvesting properties,
we synthesized cHBC derivatives having extended heterocyclic moieties
as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC)
and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader
absorption of the solar spectrum compared to cHBC, with peak absorbances
on the order of 10<sup>5</sup> cm<sup>â1</sup> in the near-ultraviolet
and in the visible. Planar-heterojunction organic solar cells comprising
cTBFDBC or cTBTDBC as the donor and C<sub>70</sub> as the acceptor
surpass those having cHBC in photocurrent generation and power-conversion
efficiency. Interestingly, devices containing cTBFDBC/C<sub>70</sub> exhibit the highest photocurrents despite cTBTDBC having the smallest
band gap of the three cHBC derivatives. X-ray reflectivity of the
active layers indicates a rougher donorâacceptor interface
when cTBFDBC is employed instead of the other two donors. Consistent
with this observation, internal quantum efficiency spectra suggest
improved charge transfer at the donorâacceptor interface when
cTBFDBCî¸as opposed to cTBTDBC or cHBCî¸is used as the
donor
Mixed-Halide Perovskites with Stabilized Bandgaps
One merit of organicâinorganic
hybrid perovskites is their
tunable bandgap by adjusting the halide stoichiometry, an aspect critical
to their application in tandem solar cells, wavelength-tunable light
emitting diodes (LEDs), and lasers. However, the phase separation
of mixed-halide perovskites caused by light or applied bias results
in undesirable recombination at iodide-rich domains, meaning open-circuit
voltage (<i>V</i><sub>OC</sub>) pinning in solar cells and
infrared emission in LEDs. Here, we report an approach to suppress
halide redistribution by self-assembled long-chain organic ammonium
capping layers at nanometer-sized grain surfaces. Using the stable
mixed-halide perovskite films, we are able to fabricate efficient
and wavelength-tunable perovskite LEDs from infrared to green with
high external quantum efficiencies of up to 5%, as well as linearly
tuned <i>V</i><sub>OC</sub> from 1.05 to 1.45 V in solar
cells