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

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

Colloidal quantum dot photodetectors with 10-ns response time and 80% quantum efficiency at 1,550 nm

Fast and sensitive infrared (IR) photodetection is of interest for depth imaging that is fundamental to machine vision, augmented reality, and autonomous driving. Colloidal quantum dots (CQDs) are appealing candidates for this goal: in contrast with III-V semiconductors, they offer facile tuning of IR absorption and enable ease of integration via solution processing. So far, the best short-wave IR CQD photodetectors have been limited to 70-ns response time and quantum efficiency of 17% at 1,450 nm. To advance the field using CQDs, large-diameter CQDs are needed that combine passivation with efficient charge transport. Here, we report an efficient ligand-exchange route that tailors the halide passivants and introduces an added exchange step crucial to efficient passivation, removal of unwanted organics, and charge transport. In devices, the CQD solids give rise to external quantum efficiency greater than 80% at 1,550 nm, a measured detectivity of 8 x 10(11) Jones, and a 10-ns response time

Deep-Blue Perovskite Single-Mode Lasing through Efficient Vapor-Assisted Chlorination

This is the peer-reviewed version of the following article: Pina, J. M., Parmar, D. H., Bappi, G., Zhou, C., Choubisa, H., Vafaie, M., Najarian, A. M., Bertens, K., Sagar, L. K., Dong, Y., Gao, Y., Hoogland, S., Saidaminov, M. I., Sargent, E. H., Deep‐Blue Perovskite Single‐Mode Lasing through Efficient Vapor‐Assisted Chlorination. Adv. Mater. 2021, 33, 2006697, which has been published in final form at https://doi.org/10.1002/adma.202006697. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Metal halide perovskites have emerged as promising candidates for solution-processed laser gain materials, with impressive performance in the green and red spectral regions. Despite exciting progress, deep-blue-an important wavelength for laser applications-remains underexplored; indeed, cavity integration and single-mode lasing from large-bandgap perovskites have yet to be achieved. Here, a vapor-assisted chlorination strategy that enables synthesis of low-dimensional CsPbCl3  thin films exhibiting deep-blue emission is reported. Using this approach,  high-quality perovskite thin films having a low surface roughness (RMS ≈ 1.3 nm) and efficient charge transfer properties are achieved. These enable us to document low-threshold amplified spontaneous emission. Levering the high quality of the gain medium,  vertical-cavity surface-emitting lasers with a low lasing threshold of 6.5 µJ cm-2  are fabricated. This report of deep-blue perovskite single-mode lasing showcases the prospect of increasing the range of deep-blue laser sources.This work was supported financially by the R&D Center U.S. San Jose Laboratory, A Division of Sony Corporation of America (2018 Sony Research Award Program Ref. No. 2019‐0669)

Orthogonal colloidal quantum dot inks enable efficient multilayer optoelectronic devices

Surface ligands enable control over the dispersibility of colloidal quantum dots (CQDs) via steric and electrostatic stabilization. Today's device-grade CQD inks have consistently relied on highly polar solvents: this enables facile single-step deposition of multi-hundred-nanometer-thick CQD films; but it prevents the realization of CQD film stacks made up of CQDs having different compositions, since polar solvents redisperse underlying films. Here we introduce aromatic ligands to achieve process-orthogonal CQD inks, and enable thereby multifunctional multilayer CQD solids. We explore the effect of the anchoring group of the aromatic ligand on the solubility of CQD inks in weakly-polar solvents, and find that a judicious selection of the anchoring group induces a dipole that provides additional CQD-solvent interactions. This enables colloidal stability without relying on bulky insulating ligands. We showcase the benefit of this ink as the hole transport layer in CQD optoelectronics, achieving an external quantum efficiency of 84% at 1210 nm.This work was supported 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. Computations were performed on the Niagara supercomputer at the SciNet HPC Consortium. SciNet is funded by: the Canada Foundation for Innovation; the Government of Ontario; Ontario Research Fund—Research Excellence; and the University of Toronto. This research was also supported by the National Research Foundation (NRF) of Korea (NRF-2020R1A6A3A03038131). We thank D. Kopilovic, E. Palmiano, L. Levina, and R. Wolowiec for the technical support. The authors acknowledge the financial support from QD Solar Inc

Facet-Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodetectors

Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as-synthesized CQDs results in random orientational relationships among facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal-phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post-synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10x increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D* > 10(12) Jones, each at 1550 nm. The photodetectors feature a 7 ns response time for a 0.01 mm(2) area-the fastest reported for solution-processed short-wavelength infrared PDs.1

Inverted perovskite solar cells with low-loss hole-selective contact on textured substrates

Inverted perovskite solar cells (PSCs) promise enhanced operating stability compared to their normal-structure counterparts. To improve efficiency further, it is crucial to combine effective light management with low interfacial losses. Here we develop a conformal self-assembled monolayer as the hole-selective contact on light-managing textured substrates. Molecular dynamics simulations indicate cluster formation during phosphonic acid adsorption leads to incomplete SAM coverage. We devise a co-adsorbent strategy that disassembles high-order clusters, thus homogenizing the distribution of phosphonic acid molecules, thereby minimizing interfacial recombination and improving electronic structures. We report a lab-measured power-conversion efficiency (PCE) of 25.3% and a certified quasi-steady-state PCE of 24.8% for inverted PSCs, with a photocurrent approaching 95% of the Shockley-Queisser maximum. An encapsulated device having a PCE of 24.6% at room temperature retains 95% of its peak performance when stressed at 65°C and 50% relative humidity following > 1000 hours of maximum power point tracking under 1-sun illumination

Low-loss contacts on textured substrates for inverted perovskite solar cells

<p>Inverted perovskite solar cells (PSCs) promise enhanced operating stability compared to their normal-structure counterparts. To improve efficiency further, it is crucial to combine effective light management with low interfacial losses. Here we develop a conformal self-assembled monolayer as the hole-selective contact on light-managing textured substrates. Molecular dynamics simulations indicate cluster formation during phosphonic acid adsorption leads to incomplete SAM coverage. We devise a co-adsorbent strategy that disassembles high-order clusters, thus homogenizing the distribution of phosphonic acid molecules, thereby minimizing interfacial recombination and improving electronic structures. We report a lab-measured power-conversion efficiency (PCE) of 25.3% and a certified quasi-steady-state PCE of 24.8% for inverted PSCs, with a photocurrent approaching 95% of the Shockley-Queisser maximum. An encapsulated device having a PCE of 24.6% at room temperature retains 95% of its peak performance when stressed at 65°C and 50% relative humidity following > 1000 hours of maximum power point tracking under 1-sun illumination. </p&gt