| 1. |
- Giovanni, David, et al.
(författare)
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Ultrafast long-range spin-funneling in solution-processed Ruddlesden-Popper halide perovskites
- 2019
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Room-temperature spin-based electronics is the vision of spintronics. Presently, there are few suitable material systems. Herein, we reveal that solution-processed mixed-phase Ruddlesden-Popper perovskite thin-films transcend the challenges of phonon momentum-scattering that limits spin-transfer in conventional semiconductors. This highly disordered system exhibits a remarkable efficient ultrafast funneling of photoexcited spin-polarized excitons from two-dimensional (2D) to three-dimensional (3D) phases at room temperature. We attribute this efficient exciton relaxation pathway towards the lower energy states to originate from the energy transfer mediated by intermediate states. This process bypasses the omnipresent phonon momentum-scattering in typical semiconductors with stringent band dispersion, which causes the loss of spin information during thermalization. Film engineering using graded 2D/3D perovskites allows unidirectional out-of-plane spin-funneling over a thickness of similar to 600 nm. Our findings reveal an intriguing family of solution-processed perovskites with extraordinary spin-preserving energy transport properties that could reinvigorate the concepts of spin-information transfer.
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| 2. |
- Karlsson, Max, 1990-
(författare)
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Dynamics in Blue Emitting Metal Halide Perovskites for Light Emitting Diodes
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lighting comprises a large part of the global electricity consumption as of today, and the use of lighting in illumination and displays is only projected to grow. It is therefore imperative to meet this energy demand, not only by means of greener energy production, but also with materials that are both more efficient to fabricate as well as to use. Low cost and energy efficient light sources therefore play an important role in minimizing further greenhouse emissions from the way we choose to live.Metal halide perovskites are a group of semiconductors that have received a great amount of attention during the past years due to impressive - and continuously increasing - performance as active materials implemented in solar cells and light emitting diodes. This is due to highly desirable optoelectronic properties combined with low-cost, solution-processable fabrication methods. Simple bandgap-tunability is easily achieved by compositional and dimensional engineering, allowing perovskite emission to span a broad wavelength region from ultraviolet to near infrared. As with previous technologies, attaining stable, bright, and pure blue light has proven difficult also in metal halide perovskites. This thesis investigates some of the challenges in achieving blue emission in mixed-halide and mixed-dimensional perovskites for light-emitting-diode applications.Mixed-halide alloying provides the most straightforward way of tuning the bandgap of perovskites. Unfortunately, mixed bromide/chloride-perovskites (used to achieve blue light) suffer from both spectral and temporal instabilities, as well as severe luminescence quenching at the large chloride contents necessary for blue emission. The spectral instability arises from a segregation of halides into regions of differing halide content, and hence different bandgap, resulting in a shift in emission color during operation. Although the origins of the poor temporal stability of perovskite light emitting diodes are manifold, one of the main problems are the low barriers for halide migration under the applied electric field during operation, rapidly degrading the device properties.We first find that compositional heterogeneities, stemming from rapid uncontrolled film growth, both lowers the threshold for further halide segregation as well as serves as centers for non-radiative recombination, resulting in reduced luminescence yield. We show that by carefully moderating the crystallization dynamics it is possible to achieve films with a homogeneous composition, thereby mitigating the negative effects arising from material inhomogeneities. We identify means of how growth environment, stoichiometric tuning and chelating additives can be used to favorably control film formation and provide guidelines that can be more widely applied in the fabrication of perovskite films and devices. We continue by investigating the role of Br/Cl-alloying on device efficiency and stability in green to blue emitting perovskite LEDs. We find that chloride incorporation, while having only a minor impact on efficiency at moderate levels, detrimentally affects device stability even in small amounts. We ascribe this phenomenon to an increased mobility of halogen ions in the mixed-halide lattice resulting from an increased chemically and structurally disordered landscape with reduced migration barriers. We assign this as the major obstacle towards stable blue-emitting mixed-halide perovskite light emitting diodes.In the last work we investigate blue emitting mixed-dimensional Ruddlesden-Popper perovskites (RPPs) comprising of multiple-quantum-well-structures of varying bandgap. Successful implementation in LEDs has been attributed to efficient carrier funneling from large bandgap (donor) regions to low bandgap regions (acceptors) resulting in improved luminescence yields due to trap state filling from the locally increased carrier density. However, due to the enhanced carrier concentrations in acceptor domains, Auger recombination quickly outcompetes radiative recombination mechanisms already at moderate pump fluences or carrier injection densities in RPPs. We show that by moderating the inter-well carrier transfer, while at the same time providing adequate defect passivation, high quantum yields can be maintained even at large carrier densities. We thereby show that RPPs can support a large density of carriers without compromising luminescence efficiency, paving the way for their use in high brightness applications by engineering the funneling and recombination processes in these materials.The work in this thesis provides new insights on various dynamical processes in metal halide perovskites aimed at light emitting applications. The hope is that it will contribute toward the understanding of these systems and help in bringing these materials closer to practical use.
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| 3. |
- Lin, Xihong, et al.
(författare)
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Effect of alloying on the dynamics of coherent acoustic phonons in bismuth double perovskite single crystals
- 2021
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 29:5, s. 7948-7955
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Tidskriftsartikel (refereegranskat)abstract
- The bismuth double perovskite Cs2AgBiBr6 has been regarded as a potential candidate for lead-free perovskite photovoltaics. A detailed study on the coherent acoustic phonon dynamics in the pure, Sb- and T1-alloyed Cs2AgBiBr6 single crystals is performed to understand the effects of alloying on the phonon dynamics and band edge characteristics. The coherent acoustic phonon frequencies are found to be independent of the alloying, while the damping rates are highly dependent on the alloying. Based on the mechanism of coherent acoustic phonon damping, a technique has been successfully developed that can accurately extract the absorption spectra near the indirect band gap for these single crystals with coefficients on the order of 10(2) cm(-1). (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 4. |
- Muduli, Subas Kumar, et al.
(författare)
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Evolution of hydrogen by few-layered black phosphorus under visible illumination
- 2017
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 5:47, s. 24874-24879
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Tidskriftsartikel (refereegranskat)abstract
- Recently, a new class of two-dimensional black phosphorus (BP) with a visible direct band gap is predicted as a potential candidate for photo-catalysis applications. Here, we present the first experimental evidence of hydrogen (H-2) evolution from aqueous solution by using BP (nanosheets and nanoparticles) under visible light illumination. Our experimental results describe that liquid phase exfoliated BP nanosheets and BP nanoparticles exhibit suitable energy level alignments for electron transfer and further proton reduction reactions in the solution under visible light illumination. Density functional theory (DFT) calculations predict that the H-2 evolution activity of bilayer BP is independent of edge or center positions, which is unique in BP as compared to those of other 2D materials.
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| 5. |
- Ning, Weihua, et al.
(författare)
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Long Electron-Hole Diffusion Length in High-Quality Lead-Free Double Perovskite Films
- 2018
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Ingår i: Advanced Materials. - : WILEY-V C H VERLAG GMBH. - 0935-9648 .- 1521-4095. ; 30:20
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Tidskriftsartikel (refereegranskat)abstract
- Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs2AgBiBr6) solar cells using the planar structure are demonstrated. The prepared Cs2AgBiBr6 films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO2 exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.
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| 6. |
- Qing, Jian, et al.
(författare)
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Aligned and Graded Type-II Ruddlesden-Popper Perovskite Films for Efficient Solar Cells
- 2018
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Ingår i: Advanced Energy Materials. - : WILEY-V C H VERLAG GMBH. - 1614-6832 .- 1614-6840. ; 8:21
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Tidskriftsartikel (refereegranskat)abstract
- Recently, Ruddlesden-Popper perovskites (RPPs) have attracted increasing interests due to their promising stability. However, the efficiency of solar cells based on RPPs is much lower than that based on 3D perovskites, mainly attributed to their poor charge transport. Herein, a simple yet universal method for controlling the quality of RPP films by a synergistic effect of two additives in the precursor solution is presented. RPP films achieved by this method show (a) high quality with uniform morphology, enhanced crystallinity, and reduced density of sub-bandgap states, (b) vertically oriented perovskite frameworks that facilitate efficient charge transport, and (c) type-II band alignment that favors self-driven charge separation. Consequently, a hysteresis-free RPP solar cell with a power conversion efficiency exceeding 12%, which is much higher than that of the control device (1.5%), is achieved. The findings will spur new developments in the fabrication of high-quality, aligned, and graded RPP films essential for realizing efficient and stable perovskite solar cells.
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| 7. |
- Qing, Jian, et al.
(författare)
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High-Quality Ruddlesden-Popper Perovskite Films Based on In Situ Formed Organic Spacer Cations
- 2019
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Ingår i: Advanced Materials. - : WILEY-V C H VERLAG GMBH. - 0935-9648 .- 1521-4095. ; 31:41
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Tidskriftsartikel (refereegranskat)abstract
- Ruddlesden-Popper perovskites (RPPs), consisting of alternating organic spacer layers and inorganic layers, have emerged as a promising alternative to 3D perovskites for both photovoltaic and light-emitting applications. The organic spacer layers provide a wide range of new possibilities to tune the properties and even provide new functionalities for RPPs. However, the preparation of state-of-the-art RPPs requires organic ammonium halides as the starting materials, which need to be ex situ synthesized. A novel approach to prepare high-quality RPP films through in situ formation of organic spacer cations from amines is presented. Compared with control devices fabricated from organic ammonium halides, this new approach results in similar (and even better) device performance for both solar cells and light-emitting diodes. High-quality RPP films are fabricated based on different types of amines, demonstrating the universality of the approach. This approach not only represents a new pathway to fabricate efficient devices based on RPPs, but also provides an effective method to screen new organic spacers with further improved performance.
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| 8. |
- Qing, Jian, et al.
(författare)
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Spacer Cation Alloying in Ruddlesden-Popper Perovskites for Efficient Red Light-Emitting Diodes with Precisely Tunable Wavelengths
- 2021
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Ingår i: Advanced Materials. - : Wiley-V C H Verlag GMBH. - 0935-9648 .- 1521-4095. ; 33:49
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite light-emitting diodes (PeLEDs) have recently shown significant progress with external quantum efficiencies (EQEs) exceeding 20%. However, PeLEDs with pure-red (620-660 nm) light emission, an essential part for full-color displays, remain a great challenge. Herein, a general approach of spacer cation alloying is employed in Ruddlesden-Popper perovskites (RPPs) for efficient red PeLEDs with precisely tunable wavelengths. By simply tuning the alloying ratio of dual spacer cations, the thickness distribution of quantum wells in the RPP films can be precisely modulated without deteriorating their charge-transport ability and energy funneling processes. Consequently, efficient PeLEDs with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak EQEs up to 11.5%, representing the highest values among RPP-based pure-red PeLEDs. This work opens a new route for color tuning, which will spur future developments of pure-red or even pure-blue PeLEDs with high performance.
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| 9. |
- Qing, Jian, et al.
(författare)
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Spacer cation engineering in Ruddlesden-Popper perovskites for efficient red light-emitting diodes with recommendation 2020 color coordinates
- 2023
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Ingår i: Applied Surface Science. - : ELSEVIER. - 0169-4332 .- 1873-5584. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- Ruddlesden-Popper perovskites (RPPs) have been demonstrated as a very promising approach for tuning the emission color of perovskite light-emitting diodes (PeLEDs). However, achieving high-performance red PeLEDs with recommendation 2020 color coordinates is still challenging due to the lack of reasonable control over the properties of RPP films. Here, we demonstrate that the judicious selection of spacer cations in RPPs affords a lever for engineering their film properties, including phase distribution, energy funneling process, trap density, and carrier mobility. Four structurally related spacer cations, benzylammonium (BZA), phenylethylammonium (PEA), 3-phenyl-1-propylammonium (PPA), and phenoxyethylammonium (POEA), are studied. Owing to narrow phase distribution, efficient energy funneling, and low trap density, the POEA-based RPP films enable efficient red PeLEDs with a peak external quantum efficiency of 10.3%, a maximum brightness of 1052 cd m- 2, and excellent spectral stability. Significantly, the electroluminescence spectrum represents CIE 1931 color coordinates of (0.71, 0.29), which meets the recommendation 2020 standard (0.708, 0.292). The findings provide useful guidelines for the rational design of new organic spacer cations for RPPs with high performance.
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| 10. |
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