The conversion efficiency of solar energy in semiconductors is fundamentally limited by ultrafast hot-carrier relaxation processes, and slowing down these processes is critical for improved energy ...harvesting. Here we report formamidinium tin iodide (FASnI3) nanocrystals where quantum confinement effects yield an evolution from a continuous band structure to separate energy states with decreasing nanocrystal size, as observed by transient absorption spectroscopy. The appearance of separate energy levels slows down the relaxation of hot carriers by two orders of magnitude at low injected carrier densities (<1 carrier pair per nanoparticle). The observed build up time of the ground-state bleach at the band edge is two orders of magnitude slower in FASnI3 nanocrystals than in lead halide perovskite bulk and nanocrystals, which we attribute to a phonon bottleneck effect. Our results highlight the promise of lead-free perovskite nanocrystals for high-efficiency photovoltaic applications operating above the Shockley–Queisser limit.Tin-based perovskite nanocrystals with slower than usual relaxation dynamics holds promise for superior lead-free perovskite optoelectronic devices.
Abstract
Understanding and control of ultrafast non-equilibrium processes in semiconductors is key to making use of the full photon energy before relaxation, leading to new ways to break efficiency ...limits for solar energy conversion. In this work, we demonstrate the observation and modulation of slow relaxation in uniformly mixed tin-lead perovskites (MASn
x
Pb
1-x
I
3
and CsSn
x
Pb
1-x
I
3
nanocrystals). Transient absorption measurements reveal that slow cooling mediated by a hot phonon bottleneck effect appears at carrier densities above ~10
18
cm
−3
for tin-lead alloy nanocrystals, and tin addition is found to give rise to suppressed cooling. Within the alloy nanoparticles, the combination of a newly introduced high-energy band, screened Fröhlich interaction, suppressed Klemens decay and reduced thermal conductivity (acoustic phonon transport) with increased tin content contributes to the slowed relaxation. For inorganic nanocrystals where defect states couple strongly with carriers, sodium doping has been confirmed to benefit in maintaining hot carriers by decoupling them from deep defects, leading to a decreased energy-loss rate during thermalization and an enhanced hot phonon bottleneck effect. The slow cooling we observe uncovers the intrinsic photophysics of perovskite nanocrystals, with implications for photovoltaic applications where suppressed cooling could lead to hot-carrier solar cells.
Colloidal lead-halide
perovskite nanocrystals (LHP NCs) have emerged
over the past decade as leading candidates for efficient next-generation
optoelectronic devices, but their properties and ...performance critically
depend on how they are purified. While antisolvents are widely used
for purification, a detailed understanding of how the polarity of
the antisolvent influences the surface chemistry and composition of
the NCs is missing in the field. Here, we fill this knowledge gap
by
studying the surface chemistry of purified CsPbBr
x
I
3-
x
NCs as the model system,
which in itself is considered a promising candidate for pure-red light-emitting
diodes and top-cells for tandem photovoltaics. Interestingly, we find
that as the polarity of the antisolvent increases (from methyl acetate
to acetone to butanol), there is a blueshift in the photoluminescence
(PL) peak of the NCs along with a decrease in PL quantum yield (PLQY).
Through transmission electron microscopy and X-ray photoemission spectroscopy
measurements, we find that these changes in PL properties arise from
antisolvent-induced iodide removal, which leads to a change in halide
composition and, thus, the bandgap. Using detailed nuclear magnetic
resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR)
measurements along with density functional theory calculations, we
propose that more polar antisolvents favor the detachment of the oleic
acid and oleylamine ligands, which undergo amide condensation reactions,
leading to the removal of iodide anions from the NC surface bound
to these ligands. This work shows that careful selection of low-polarity
antisolvents is a critical part of designing the synthesis of NCs
to achieve high PLQYs with minimal defect-mediated phase segregation.
Abstract
I-V-VI
2
ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have ...slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS
2
nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >10
5
cm
−1
just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS
2
nanocrystals. These unusual features arise because of the localised, non-bonding S
p
character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.
Perovskite surface passivation is key to eliminating junction charge-carrier losses and is considered a critical step in fabricating solar cells with efficiencies close to the theoretical limit. ...Various halogenated organic ligands are found to produce efficient and stable solar cells when the perovskite surfaces are thermally activated. Photoemission spectroscopy shows that most of the ligands start to evaporate during heating. When annealed at high temperatures, halogens are cleaved off the ligands, leaving the organic cations anchored to the perovskite and causing charge transfer n-doping. The band-alignment measurement reveals that the formation of n-doped perovskites caused by strongly anchored ligands leads to the formation of ideal heterojunctions with energy offsets benefiting electron extraction and hole blocking, contributing to over 22% improvement in device performance without burn-in degradation.
Photoemission spectroscopy reveals halogen cleavage from bulky cation organics and the formation process of 2D-like perovskites on the parent perovskite as well as their impact on band alignment and device performance.
Abstract
Interlayer decoupling plays an essential role in realizing unprecedented properties in atomically thin materials, but it remains relatively unexplored in the bulk. It is unclear how to ...realize a large crystal that behaves as its monolayer counterpart by artificial manipulation. Here, we construct a superlattice consisting of alternating layers of NbSe
2
and highly porous hydroxide, as a proof of principle for realizing interlayer decoupling in bulk materials. In (NaOH)
0.5
NbSe
2
, the electric decoupling is manifested by an ideal 1D insulating state along the interlayer direction. Vibration decoupling is demonstrated through the absence of interlayer models in the Raman spectrum, dominant local modes in heat capacity, low interlayer coupling energy and out-of-plane thermal conductivity (0.28 W/mK at RT) that are reduced to a few percent of NbSe
2
’s. Consequently, a drastic enhancement of CDW transition temperature (>110 K) and Pauling-breaking 2D superconductivity is observed, suggesting that the bulk crystal behaves similarly to an exfoliated NbSe
2
monolayer. Our findings provide a route to achieve intrinsic 2D properties on a large-scale without exfoliation.
People increasingly search for health information through the media and make decisions about their health based on these health stories. The mainstream media, including newspapers, are often the ...first source for the public to obtain health information. This study aims to assess the health stories reported in the health edition of People’s Daily in 2019 with four tools of the Media Doctor Toolkit (MDT), which can be an effective tool to evaluate the quality of public health stories. Based on the results, we attempt to address the gap in media coverage in terms of reporting on public health issues, and promote media to display the image of medical staff objectively, both of which can improve relationship of doctors, nurses and patients.
A prospective quantitative analysis of the quality of health stories reported in the health edition of People’s Daily from 1 February to September 31, 2019 was conducted. Forty-eight articles were collected and divided into four groups according to the MDT standards. Four rating tools were adapted from the MDT to assess the quality of the groups with corresponding criteria.
Forty-eight unique health stories were assessed with four MDT rating tools. The mean total satisfactory score was 80%. Health advice had the highest average satisfactory score, 84%, compared with health policy group and public health problems and their solutions, at 80% and 77%, respectively. Health news group had the lowest score at 73%.
This study provides a description of the quality of health stories on People’s Daily. The overall quality of People’s Daily was fairly good, although there was a wide range of quality between groups. The health edition of People’s Daily covers a wide range of health topics such as new medical methods, doctor-patient relationship, advanced nursing practice, lifestyle change of health promotion etc. which promote excellence in providing the latest health information, representing medical staff image, resolve the disharmonious factors in the relationship between doctors and patients, and creating a good medical environment for the public. The findings of this study also provide insight into problems in health reporting of People’s Daily.
Recent advances in heterojunction and interfacial engineering of perovskite solar cells (PSCs) have enabled great progress in developing highly efficient and stable devices. Nevertheless, the effect ...of halide choice on the formation mechanism, crystallography, and photoelectric properties of the low‐dimensional phase still requires further detailed study. In this work, we present key insights into the significance of halide choice when designing passivation strategies comprising large organic spacer salts, clarifying the effect of anions on the formation of quasi‐2D/3D heterojunctions. To demonstrate the importance of halide influences, we employ novel neo‐pentylammonium halide salts with different halide anions (neoPAX, X=I, Br, or Cl). We find that regardless of halide selection, iodide‐based (neoPA)
2
(FA)
(
n
‐1)
Pb
n
I
(3
n
+1)
phases are formed above the perovskite substrate, while the added halide anions diffuse and passivate the perovskite bulk. In addition, we also find the halide choice has an influence on the degree of dimensionality (
n
). Comparing the three halides, we find that chloride‐based salts exhibit superior crystallographic, enhanced carrier transport, and extraction compared to the iodide and bromide analogs. As a result, we report high power conversion efficiency in quasi‐2D/3D PSCs, which are optimal when using chloride salts, reaching up to 23.35%, and improving long‐term stability.
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