Multi-cation perovskites prevent carrier reflection from grain surfaces

© 2020, The Author(s), under exclusive licence to Springer Nature Limited. The composition of perovskite has been optimized combinatorially such that it often contains six components (AxByC1−x−yPbXzY3−z) in state-of-art perovskite solar cells. Questions remain regarding the precise role of each component, and the lack of a mechanistic explanation limits the practical exploration of the large and growing chemical space. Here, aided by transient photoluminescence microscopy, we find that, in perovskite single crystals, carrier diffusivity is in fact independent of composition. In polycrystalline thin films, the different compositions play a crucial role in carrier diffusion. We report that methylammonium (MA)-based films show a high carrier diffusivity of 0.047 cm2 s−1, while MA-free mixed caesium-formamidinium (CsFA) films exhibit an order of magnitude lower diffusivity. Elemental composition studies show that CsFA grains display a graded composition. This curtails electron diffusion in these films, as seen in both vertical carrier transport and surface potential studies. Incorporation of MA leads to a uniform grain core-to-edge composition, giving rise to a diffusivity of 0.034 cm2 s−1 in CsMAFA films. A model that invokes competing crystallization processes allows us to account for this finding, and suggests further strategies to achieve homogeneous crystallization for the benefit of perovskite optoelectronics

Naphthalenediimide Cations Inhibit 2D Perovskite Formation and Facilitate Subpicosecond Electron Transfer

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http:// 10.1021/acs.jpcc.0c05521.Layered metal halide perovskites, also called perovskite quantum wells (PQWs), are versatile optoelectronic materials possessing large oscillator strengths, band gaps tuned via the quantum size effect, and promising stability. The majority of examples of PQWs make use of small aryl- and alkylammonium A′-site cations to tune dimensionality and stability, with fewer examples of larger molecules that exhibit frontier orbital energies near those of the inorganic component of the perovskite. Here, we report two new lead-iodide-based systems that incorporate a dye molecule A′-site dication, 2,2′-[naphthalene-1,8:4,5-bis(dicarboximide)-N,N′-diyl]-bis(diethylammonium) (NDIC2), along with either methylammonium or a mixture of methylammonium and formamidinium as A-site cations. From transient absorption spectra, we find that films synthesized with NDIC2, PbI2, and methylammonium inhibit the growth of PQWs and instead result in a mixture of weakly confined perovskite and 1D perovskitoid structures. When both formamidinium and methylammonium are used as A-site cations, we observe spectroscopic signatures of quantum-confined 2D structures similar to PQWs with a polydisperse well width distribution. We observe a rapid (∼700 fs) decay of the photoexcited perovskite carrier population in the presence of NDIC2 and fully quenched photoluminescence: this is consistent with ultrafast perovskite-to-NDIC2 electron transfer. This work explores the interplay between large and small cation molecules in influencing the perovskite structure and how such molecules may offer a route to structures with charges separately localized on inorganic and organic components, raising prospects of using perovskites with electron-accepting ligands for hybrid organic–inorganic optoelectronic devices.This publication is partly based on work supported by the United States Department of the Navy, Office of Naval Research (grant award no.: N00014-17-1-2524) and the United States Air Force Office of Scientific Research (FA9550-18-1-0499). The authors thank the Ontario Graduate Scholarship program (A.H.P.) and the Natural Sciences and Engineering Research Council of Canada (A.H.P. and M.-H.T.). This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). This work was performed in part at the Georgia Tech NMR Center and the CSICOMP NMR Facility at the University of Toronto

Synthetic Control over Quantum Well Width Distribution and Carrier Migration in Low-Dimensional Perovskite Photovoltaics

Metal halide perovskites have achieved photovoltaic efficiencies exceeding 22%, but their widespread use is hindered by their instability in the presence of water and oxygen. To bolster stability, researchers have developed low-dimensional perovskites wherein bulky organic ligands terminate the perovskite lattice, forming quantum wells (QWs) that are protected by the organic layers. In thin films, the width of these QWs exhibits a distribution that results in a spread of bandgaps in the material arising due to varying degrees of quantum confinement across the population. Means to achieve refined control over this QW width distribution, and to examine and understand its influence on photovoltaic performance, are therefore of intense interest. Here we show that moving to the ligand allylammonium enables a narrower distribution of QW widths, creating a flattened energy landscape that leads to ×1.4 and ×1.9 longer diffusion lengths for electrons and holes, respectively. We attribute this to reduced ultrafast shallow hole trapping that originates from the most strongly confined QWs. We observe an increased PCE of 14.4% for allylammonium-based perovskite QW photovoltaics, compared to 11–12% PCEs obtained for analogous devices using phenethylammonium and butylammonium ligands. We then optimize the devices using mixed-cation strategies, achieving 16.5% PCE for allylammonium devices. The devices retain 90% of their initial PCEs after >650 h when stored under ambient atmospheric conditions

Bifunctional Surface Engineering on SnO2 Reduces Energy Loss in Perovskite Solar Cells

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Energy letters, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://doi.org/10.1021/acsenergylett.0c01566Tin oxide (SnO2) has recently emerged as a promising electron transport layer for perovskite solar cells (PSCs) in light of the material’s optical and electronic properties and its low-temperature processing. However, SnO2 films are prone to surface defect formation, which results in energy loss in PSCs. We report that surface treatment using ammonium fluoride (NH4F) leads to reduced surface defects and that it also induces chemical doping of the SnO2 substrate simultaneously. The effects of NH4F treatment on SnO2 properties are revealed by surface chemical analysis, computational studies, and energy level investigations, and PSCs with the treatment achieve photovoltaic performance of 23.2% in light of higher voltage than in relevant controls.This research was made possible by Ontario Research Fund-Research Excellence program (ORF7 Ministry of Research and Innovation, Ontario Research Fund-Research Excellence Round 7) and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. This work was in part supported by the U.S. Department of the Navy, Office of Naval Research (N00014-17-1-2524). This research was also supported by the National Research Foundation of Korea (NRF) funded by NRF-2019R1A6A3A03032792

3D-2D Neural Nets for Phase Retrieval in Noisy Interferometric Imaging

In recent years, neural networks have been used to solve phase retrieval problems in imaging with superior accuracy and speed than traditional techniques, especially in the presence of noise. However, in the context of interferometric imaging, phase noise has been largely unaddressed by existing neural network architectures. Such noise arises naturally in an interferometer due to mechanical instabilities or atmospheric turbulence, limiting measurement acquisition times and posing a challenge in scenarios with limited light intensity, such as remote sensing. Here, we introduce a 3D-2D Phase Retrieval U-Net (PRUNe) that takes noisy and randomly phase-shifted interferograms as inputs, and outputs a single 2D phase image. A 3D downsampling convolutional encoder captures correlations within and between frames to produce a 2D latent space, which is upsampled by a 2D decoder into a phase image. We test our model against a state-of-the-art singular value decomposition algorithm and find PRUNe reconstructions consistently show more accurate and smooth reconstructions, with a x2.5 - 4 lower mean squared error at multiple signal-to-noise ratios for interferograms with low (< 1 photon/pixel) and high (~100 photons/pixel) signal intensity. Our model presents a faster and more accurate approach to perform phase retrieval in extremely low light intensity interferometry in presence of phase noise, and will find application in other multi-frame noisy imaging techniques

Multication perovskite 2D/3D interfaces form via progressive dimensional reduction

Many best-performing perovskite photovoltaics use 2D/3D interfaces to improve efficiency and stability, yet the mechanism of interface assembly is unclear. Here, Proppe et al. use in-situ GIWAXS to resolve this transformation, observing progressive dimensional reduction from 3D to 2D perovskites

Ligand-induced symmetry breaking, size and morphology in colloidal lead sulfide QDs: from classic to thiourea precursors

Colloidal lead chalcogenide quantum dots (CQDs) exhibit promising optoelectronic properties for applications in solar cell devices and as thermoelectrics. Herein, we report and discuss a ferroelectric structural distortion, at the picometer scale resolution, in PbS CQDs prepared using both classic and new synthetic pathways. The investigation was performed using synchrotron X-ray total scattering data and advanced methods of analysis that rely on a homo-core-shell model and evaluate the atomic arrangement, stoichiometry, size and morphology of nanocrystals. The CQDs show comparable size-dependent relative elongation, up to 0.7 % of one body diagonal of the cubic rock-salt structure, which corresponds to a rhombohedral lattice deformation. The findings suggest a joint role for the oleate ligands (which induce surface tensile strain) and the Pb(II) lone pair as the driving forces of the deformation. Pb displacements along the [111] direction, which provoke a ferrolectric distortion related to the lattice change, fall in the 0.0 \u2013 0.1 \uc5 range. Overall, the findings suggest the local nature of the metal off-centering, leading to different average displacements which depend on the synthetic conditions

An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics

In lead-halide perovskites, antibonding states at the valence band maximum (VBM)-the result of Pb 6s-I 5p coupling-enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4s-Se 4p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~1012 cm-3. We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m-2; and 60 thermal cycles from -40 to 85 °C.This work is supported by the National Natural Science Foundation of China (21922512, 21875264, 61725401), the Youth Innovation Promotion Association CAS (2017050). The work of Y.M., Y.H., A.P., and E.H.S. is supported by the US Department of the Navy, Office of Naval Research (Grant Award NO. N00014-17-1-2524)