| 1. |
- Alvarez, Sol Gutierrez, et al.
(författare)
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Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:37, s. 44742-44750
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Tidskriftsartikel (refereegranskat)abstract
- CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid ( OA) and oleylamine ( OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be similar to 239 +/- 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (+/- 0.1) and 0.69 (+/- 0.03) cm(2)/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.
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| 2. |
- Finkelstein-Shapiro, Daniel, et al.
(författare)
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Understanding radiative transitions and relaxation pathways in plexcitons
- 2021
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Ingår i: Chem. - : Elsevier BV. - 2451-9308 .- 2451-9294. ; 7:4, s. 1092-1107
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Tidskriftsartikel (refereegranskat)abstract
- Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.
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| 3. |
- Honarfar, Alireza, et al.
(författare)
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Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals
- 2021
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:26, s. 14332-14337
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Tidskriftsartikel (refereegranskat)abstract
- Extra charges in semiconductor nanocrystals are of paramount importance for their electrically driven optoelectronic and photovoltaic applications. Optical excitations of such charged nanocrystals lead to rapid recombinationviaan Auger process, which can deteriorate the performance of the corresponding devices. While numerous articles report trion Auger processes in negatively charged nanocrystals, optical studies of well-controlled positive charging of nanocrystals and detailed studies of positive trions remain rare. In this work, we used electrochemistry to achieve positive charging of CdSe nanocrystals, so-called quantum dots (QDs), in a controlled way. Femtosecond transient absorption spectroscopy was applied forin situinvestigation of the charge carrier dynamics after optical excitation of the electrochemically charged QD assembly on TiO2. We observe that without bias (i.e., neutral QDs), sub-picosecond hot carrier cooling is followed by multiple phases of the dynamics corresponding to electron injection and transfer to the TiO2. Positive charging first leads to activation of the hole traps close to the valence band maximum, which opens a rapid recombination channel of the optical excitation. A further increase in the positive bias interrupts the electron injection to TiO2, and if nanocrystals are positively charged, it leads to Auger relaxation in a few hundred picosecond timescale. This study represents a step toward the understanding of the effect of positive charging on the performance of semiconductor nanocrystals under conditions which closely mimic their potential applications.
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| 4. |
- Yan, Yajie, et al.
(författare)
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Implementing an intermittent spin-coating strategy to enable bottom-up crystallization in layered halide perovskites
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional halide perovskites (2D PVSKs) have drawn tremendous attentions owing to their outstanding ambient stability. However, the random orientation of layered crystals severely impedes the out-of-plane carrier transport and limits the solar cell performance. An in-depth understanding coupled with an effective control of the crystallization in 2D PVSKs is the crux for highly efficient and durable devices. In this contribution, we accidentally discovered that the crystallization of 2D PVSKs can be effectively regulated by so-called ′intermittent spin-coating (ISC)′ process. Combined analyses of in(ex)-situ grazing-incidence wide-angle X-ray scattering with time-of-flight secondary ion mass spectrometry distinguish the interface initialized bottom-up crystallization upon ISC treatment from the bi-directional one in the conventional spin-coating process, which results in significantly enhanced crystal orientation and thus facilitated carrier transport as confirmed by both electrical measurements and ultrafast spectroscopies. As a result, the p-i-n architecture planar solar cells based on ISC fabricated paradigm PEA2MA3Pb4I13 deliver a respectable efficiency of 11.2% without any treatment, which is three-fold improvement over their spin-coated counterparts and can be further boosted up to 14.0% by NH4Cl addition, demonstrating the compatibility of ISC method with other film optimization strategies.
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| 5. |
- Yu, Shuang, et al.
(författare)
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Asymmetric Spacer in Dion–Jacobson Halide Perovskites Induces Staggered Alignment to Direct Out-of-Plane Carrier Transport and Enhance Ambient Stability Simultaneously
- 2021
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 31:46
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Tidskriftsartikel (refereegranskat)abstract
- Dion–Jacobson (DJ)-type 2D halide perovskites present superior environmental stability and a narrower bandgap, yet a contradiction between charge transport and stability remains to be resolved. Herein, it is shown that both symmetry and substitution of the organic spacer in DJ perovskites synergistically direct the narrow interlayer spacing, staggered spacer alignment, and regular phase arrangement, thereby promoting out-of-plane carrier transport and ambient stability. Compared to its symmetric para-xylylenediamine (PDMA) counterpart, the asymmetric 2-(4-aminophenyl)ethylamine (PMEA) spacer largely aids in compressing the inorganic octahedra layer to form a non-confinement structure with decreased exciton binding energy, while stacked benzene rings enable a staggered alignment of spacers. Such non-confined structures are less remarkable in meta-substituted diamine-based DJ perovskites than those para-ones, which retard carrier transport from 2D to quasi-2D phases. The preferential PMEA spacer however requires a long relaxation time to form a dense and ordered staggered alignment, which is realized by a slight addition of strong-coordinating DMSO into the DMF solvent, thus decelerating crystallization and further optimizing lamellar orientation. As a result, a best efficiency of ≈12% is achieved in (PMEA)MA3Pb4I13 based p-i-n type planar solar cells. Importantly, such unencapsulated devices can maintain 81% initial efficiencies after storage in ambient conditions (≈60% relative humidity, ≈20 °C) for 700 h.
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| 6. |
- Zhang, Fengying, et al.
(författare)
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Microscopic morphology independence in linear absorption cross-section of CsPbBr3 nanocrystalsMicroscopic morphology independence in linear absorption cross-section of CsPbBr3 nanocrystals
- 2021
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Ingår i: SCIENCE CHINA Materials. - : Springer Science and Business Media LLC. - 2095-8226 .- 2199-4501. ; 64:6, s. 1418-1426
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Tidskriftsartikel (refereegranskat)abstract
- Multiple morphologies of colloidal perovskite nanocrystals (NCs) diversify their optical and electronic properties. Among them, the linear absorption cross-section (σ) is a primary parameter to determine their intrinsic photophysical features, and consequently, application potential. Herein, three morphologies of all-inorganic hybrid colloidal perovskite CsPbBr3 NCs, nanocubes (NBs), nanoplatelets (NLs), and nanowires (NWs), were targeted, and their linear σ values were obtained through femtosecond transient absorption (TA) spectroscopy analysis. At high excitation energy well above the bandgap, the σ per particle of all CsPbBr3 NCs linearly increased with the particle volume (VNC) regardless of the morphology with the value of σ400 = 9.45 × 104 cm−1 × VNC (cm2). Density functional theory (DFT) calculation confirmed the negligible influence of shapes on the optical selection rules. The Einstein spontaneous emission coefficients calculated from the σ values define the intrinsic radiative recombination rate. However, reduced size dependence is observed when the excitation energy is close to the bandgap (i.e., at 460 nm) with the value of σ460 = 2.82 × 108 cm0.65 × (VNC)0.45 (cm2). This should be ascribed to the discrete energy levels as well as lower density of states close to the band edge for perovskite NCs. These results provide in-depth insight into the optical characteristics for perovskite NCs.
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