24 research outputs found
Experimental Evidence for Defect Tolerance in Pb-Halide Perovskites
The term defect tolerance (DT) is used often to rationalize the exceptional
optoelectronic properties of Halide Perovskites, HaPs, and their devices. Even
though DT lacked direct experimental evidence, it became fact in the field. DT
in semiconductors implies tolerance to structural defects without the
electrical and optical effects (e.g., traps), associated with such defects. We
present first direct experimental evidence for DT in Pb HaPs by comparing the
structural quality of 2D, 2D_3D, and 3D Pb HaP crystals with their
optoelectronic characteristics using high sensitivity methods. Importantly, we
get information from the material bulk, because we sample at least a few 100
nm, up to several micrometer, from the sample surface, which allows assessing
intrinsic bulk (and not only surface) properties of HaPs. The results point to
DT in 3D, to a lesser extent in 2D_3D, but not in 2D Pb HaPs. We ascribe such
dimension dependent DT to the higher number of (near)neighboring species,
available to compensate for structural defect effects in the 3D than in the 2D
HaP crystals. Overall, our data provide an experimental basis to rationalize DT
in Pb HaPs. These experiments and findings can guide the search for, and design
of other materials with DT
Internal electric fields control triplet formation in halide perovskite-sensitized photon upconverters
Halide perovskite-based photon upconverters utilize perovskite thin films to sensitize triplet exciton formation in a small-molecule layer, driving triplet-triplet annihilation upconversion. Despite having excellent carrier mobility, these systems suffer from inefficient triplet formation at the perovskite/annihilator interface. We studied triplet formation in formamidinium-methylammonium lead iodide/rubrene bilayers using photoluminescence and surface photovoltage methods. By studying systems constructed on glass as well as hole-selective substrates, comprising self-assembled layers of the carbazole derivative 2PACz ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) on indium-doped tin oxide, we saw how changes in the carrier dynamics induced by the hole-selective substrate perturbed triplet formation at the perovskite/rubrene interface. We propose that an internal electric field, caused by hole transfer at the perovskite/rubrene interface, strongly affects triplet exciton formation, accelerating exciton-forming electron-hole encounters at the interface but also limiting the hole density in rubrene at high excitation densities. Controlling this field is a promising path to improving triplet formation in perovskite/annihilator upconverters
Impact of intentional photo-oxidation of a donor polymer and PC70BM on solar cell performance
A short lifetime is the main factor hindering the wider implementation of low-cost organic photovoltaics in large-area and outdoor applications. Ingress of oxygen and water vapour through non-ideal encapsulation layers is a known cause of degradation for polymer/fullerene based solar cells. To better understand the origin of this performance degradation, we study the effect of intentional exposure of the photo-active layer to simulated sunlight (AM1.5) in air both on the solar cell performance and on the molecular semiconductor materials. Cathode-free thin films of a blend of the electron donor polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and the electron acceptor fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) were exposed to simulated sunlight in air. Fourier-transform infrared spectra demonstrate the formation of carbonyl photo-oxidation products in the blend films, as well as in the pristine polymer and fullerene films. Solar cells prepared with photo-oxidized active layers show increasingly degraded electrical performance (lower short circuit current, open circuit voltage and fill factor) with increasing exposure time. The increased diode ideality factor indicates that trap-assisted recombination hinders device operation after exposure. The external quantum efficiency decreases drastically with increasing exposure time over the whole photon energy range, while the UV-vis absorption spectra of the blend films only show a mild photo-induced bleaching. This demonstrates that not only the photo-induced degradation of the solar cell performance is not predominantly caused by the loss in light absorption, but charge transport and collection are also hampered. This is explained by the fact that photo-oxidation of PC70BM causes bonds in its conjugated cage to break, as evidenced by the decreased â* intensity in C1s-NEXAFS spectra of PC70BM films. This degradation of unoccupied states of PC70BMÂ will hinder the transport of photo-generated electrons to the electrode. Surface photovoltage spectroscopy gives direct evidence for gap states at the surface of a PC70BM film, formed after 2 hours of exposure and resulting in upward band bending at the PC70BM/air surface. These observations indicate that the photo-oxidation of PC70BM is likely to be the main cause of the performance degradation observed when the photoactive layer of a TQ1:PC70BM solar cell is intentionally exposed to light in air
Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells
Direct comparison between perovskite-structured
hybrid organicâinorganic
methylammonium lead bromide (MAPbBr<sub>3</sub>) and all-inorganic
cesium lead bromide (CsPbBr<sub>3</sub>), allows identifying possible
fundamental differences in their structural, thermal and electronic
characteristics. Both materials possess a similar direct optical band
gap, but CsPbBr<sub>3</sub> demonstrates a higher thermal stability
than MAPbBr<sub>3</sub>. In order to compare device properties, we
fabricated solar cells, with similarly synthesized MAPbBr<sub>3</sub> or CsPbBr<sub>3</sub>, over mesoporous titania scaffolds. Both cell
types demonstrated comparable photovoltaic performances under AM1.5
illumination, reaching power conversion efficiencies of âŒ6%
with a poly aryl amine-based derivative as hole transport material.
Further analysis shows that Cs-based devices are as efficient as,
and more stable than methylammonium-based ones, after aging (storing
the cells for 2 weeks in a dry (relative humidity 15â20%) air
atmosphere in the dark) for 2 weeks, under constant illumination (at
maximum power), and under electron beam irradiation
Molecular Length, Monolayer Density, and Charge Transport: Lessons from AlâAlOx/AlkylâPhosphonate/Hg Junctions
A combined electronic transportâstructure characterization of self-assembled monolayers (MLs) of alkylâphosphonate (AP) chains on AlâAlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that âlongâ APs (C14 and C16) form much denser MLs than do âshortâ APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed âshortâ AP MLs attenuate the current relatively more efficiently than those with densely packed, âlongâ ones. Furthermore, âlongâ AP ML junctions showed strong bias variation of the length decay coefficient, ÎČ, while for âshortâ AP ML junctions ÎČ is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed
Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> Films
Halide perovskite-based solar cells
still have limited reproducibility,
stability, and incomplete understanding of how they work. We track
electronic processes in [CH<sub>3</sub>NH<sub>3</sub>]ÂPbI<sub>3</sub>(Cl) (âperovskiteâ) films <i>in vacuo</i>, and in N<sub>2</sub>, air, and O<sub>2</sub>, using impedance spectroscopy
(IS), contact potential difference, and surface photovoltage measurements,
providing direct evidence for perovskite sensitivity to the ambient
environment. Two major characteristics of the perovskite IS response
change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N<sub>2</sub> exposure,
indicating for the first time an electrochemical process in the perovskite,
and -2- orders of magnitude decrease in the film resistance upon transferring
the film from O<sub>2</sub>-rich ambient atmosphere to vacuum. The
same change in ambient conditions also results in a 0.5 V decrease
in the material work function. We suggest that facile adsorption of
oxygen onto the film dedopes it from n-type toward intrinsic. These
effects influence any material characterization, i.e., results may
be ambient-dependent due to changes in the materialâs electrical
properties and electrochemical reactivity, which can also affect material
stability
MobilityâLifetime Products in MAPbI<sub>3</sub> Films
Photovoltaic
solar cells operate under steady-state conditions
that are established during the charge carrier excitation and recombination.
However, to date no model of the steady-state recombination scenario
in halide perovskites has been proposed. In this Letter we present
such a model that is based on a single type of recombination center,
which is deduced from our measurements of the illumination intensity
dependence of the photoconductivity and the ambipolar diffusion length
in those materials. The relation between the present results and those
from time-resolved measurements, such as photoluminescence that are
commonly reported in the literature, is discussed
What Is the Mechanism of MAPbI<sub>3</sub> pâDoping by I<sub>2</sub>? Insights from Optoelectronic Properties
Obtaining insight
into, and ultimately control over, electronic
doping of halide perovskites may improve tuning of their remarkable
optoelectronic properties, reflected in what appear to be low defect
densities and as expressed in various charge transport and optical
parameters. Doping is important for charge transport because it determines
the electrical field within the semiconducting photoabsorber, which
strongly affects collection efficiency of photogenerated charges.
Here we report on intrinsic doping of methylammonium lead tri-iodide,
MAPbI<sub>3</sub>, as thin films of the types used for solar cells
and LEDs, by I<sub>2</sub> vapor at a level that does not affect the
optical absorption and leads to a small (<20 meV, âŒ9 nm)
red shift in the photoluminescence peak. This I<sub>2</sub> vapor
treatment makes the films 10Ă more electronically conductive
in the dark. We show that this change is due to p-type doping because
we find their work function to increase by 150 mV with respect to
the ionization energy (valence band maximum), which does not change
upon I<sub>2</sub> exposure. The majority carrier (hole) diffusion
length increases upon doping, making the material less ambipolar.
Our results are well-explained by I<sub>2</sub> exposure decreasing
the density of donor defects, likely iodide vacancies (V<sub>I</sub>) or defect complexes, containing V<sub>I</sub>. Invoking iodide
interstitials, which are acceptor defects, seems less likely based
on calculations of the formation energies of such defects and is in
agreement with a recent report on pressed pellets