| 1. |
- Chen, Junsheng, et al.
(författare)
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Cation-Dependent Hot Carrier Cooling in Halide Perovskite Nanocrystals
- 2019
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 141:8, s. 3532-3540
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Tidskriftsartikel (refereegranskat)abstract
- Lead halide perovskites (LHPs) nanocrystals (NCs), owing to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here, we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH 3 NH 3 + ), and formamidinium (FA, CH(NH 2 ) 2 + ) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy-dependent HC cooling. We investigate the details of HC cooling in CsPbBr 3 , MAPbBr 3 , and FAPbBr 3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV), all the three LHP NCs show fast HC relaxation (<0.4 ps) with the cooling time and rate in the following order: CsPbBr 3 (0.39 ps, 2.9 meV/fs) > MAPbBr 3 (0.27 ps, 4.6 meV/fs) > FAPbBr 3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb-Br frameworks compared to the Cs. The revealed cation-dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.
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| 2. |
- Chen, Junsheng, et al.
(författare)
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Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals
- 2017
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:20, s. 5119-5124
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Tidskriftsartikel (refereegranskat)abstract
- Cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable two-photon-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared with one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Because of inherent nonlinearity, the size dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited, leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr3 NCs is likely a general feature of the perovskite NCs and can be tuned via NC size distribution to influence their performance within NC-based nonlinear optical materials and devices.
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| 3. |
- Chen, Junsheng, et al.
(författare)
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Size-And Wavelength-Dependent Two-Photon Absorption Cross-Section of CsPbBr3 Perovskite Quantum Dots
- 2017
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:10, s. 2316-2321
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Tidskriftsartikel (refereegranskat)abstract
- All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr3 QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr3 QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr3 nanocrystals.
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| 4. |
- Karki, Khadga Jung, et al.
(författare)
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Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy.
- 2013
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usually done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods.
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| 5. |
- Qenawy, Mohamed, et al.
(författare)
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Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core-Shell Quantum Dots Attached to ZnO
- 2013
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:11, s. 1760-1765
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Tidskriftsartikel (refereegranskat)abstract
- Core-shell (CS) quantum dots (QDs) are promising light absorbers for solar cell applications mainly because of their enhanced photostability compared with bare QDs. Moreover, the superb photostability can be combined with a low number of defects by using CSQDs with a gradient composition change from the core to the shell. Here, we study electron injection from the gradient CSQDs to ZnO nanoparticles. We observe the typical exponential injection rate dependence on the shell thickness (beta = 0.51 angstrom(-1)) and discuss it in light of previously published results on step-like CSQDs. Despite the rapid drop in injection rates with shell thickness, we find that there exists an optimum thickness of the shell layer at similar to 1 nm, which combines high injection efficiency (>90%) with a superior passivation of QDs.
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| 6. |
- Qenawy, Mohamed, et al.
(författare)
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Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:44, s. 25802-25808
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Tidskriftsartikel (refereegranskat)abstract
- The performance of the current quantum dot (QD) solar cells is limited by several deficiencies. One of them is the existence of surface traps, especially hole traps, which are blocking the hole injection into the electrolyte. The trapping can be efficiently suppressed by growing a shell of wider band gap material around the core dot. Optimum parameters of such a shell layer for photovoltaic applications are, however, not established. We study effects of the shell formation on the ultrafast carrier dynamics and the performance of QD-sensitized solar cells. We can disentangle electron and hole dynamics and demonstrate that the QD shell diminishes surface hole trapping. By combining the knowledge about the hole trapping and electron injection into metal oxide we can clearly correlate the electron and hole dynamics with the solar cell efficiency as a function of the shell thickness. We conclude that the optimal shell thickness is 1.3 nm for this system.
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| 7. |
- Zheng, Kaibo, et al.
(författare)
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Direct Experimental Evidence for Photoinduced Strong-Coupling Polarons in Organolead Halide Perovskite Nanoparticles
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:22, s. 4535-4539
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Tidskriftsartikel (refereegranskat)abstract
- Echoing the roaring success of their bulk counterparts, nano-objects built from organolead halide perovskites (OLHP) present bright prospects for surpassing the performances of their conventional organic and inorganic analogues in photodriven technologies. Unraveling the photoinduced charge dynamics is essential for optimizing the optoelectronic functionalities. However, mapping the carrier-lattice interactions remains challenging, owing to their manifestations on multiple length scales and time scales. By correlating ultrafast time-resolved optical and X-ray absorption measurements, this work reveals the photoinduced formation of strong-coupling polarons in CH3NH3PbBr3 nanoparticles. Such polarons originate from the self-trapping of electrons in the Coulombic field caused by the displaced inorganic nuclei and the oriented organic cations. The transient structural change detected at the Pb L3 X-ray absorption edge is well-captured by a distortion with average bond elongation in the [PbBr6]2- motif. General implications for designing novel OLHP nanomaterials targeting the active utilization of these quasi-particles are outlined.
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| 8. |
- Zheng, Kaibo, et al.
(författare)
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Exciton Binding Energy and the Nature of Emissive States in Organometal Halide Perovskites.
- 2015
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 6:15, s. 2969-2975
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Tidskriftsartikel (refereegranskat)abstract
- Characteristics of nanoscale materials are often different from the corresponding bulk properties providing new, sometimes unexpected, opportunities for applications. Here we investigate the properties of 8 nm colloidal nanoparticles of MAPbBr3 perovskites and contrast them to the ones of large microcrystallites representing a bulk. X-ray spectroscopies provide an exciton binding energy of 0.32 ± 0.10 eV in the nanoparticles. This is 5 times higher than the value of bulk crystals (0.084 ± 0.010 eV), and readily explains the high fluorescence quantum yield in nanoparticles. In the bulk, at high excitation concentrations, the fluorescence intensity has quadratic behavior following the Saha-Langmuir model due to the nongeminate recombination of charges forming the emissive exciton states. In the nanoparticles, a linear dependence is observed since the excitation concentration per particle is significantly less than one. Even the bulk shows linear emission intensity dependence at lower excitation concentrations. In this case, the average excitation spacing becomes larger than the carrier diffusion length suppressing the nongeminate recombination. From these considerations we obtain the charge carrier diffusion length in MAPbBr3 of 100 nm.
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| 9. |
- Zheng, Kaibo, et al.
(författare)
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Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:5, s. 3077-3084
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Tidskriftsartikel (refereegranskat)abstract
- Organometal halide perovskites have attracted tremendous attention for optoelectronic applications. Charge carrier trapping is one of the dominant processes often deteriorating the performance of devices. Here, we investigate the details of trap behavior in colloidal nanoparticles (NPs) of CH3NH3PbBr3 perovskites with mean size of 8 nm and the corresponding bulk crystals (BCs). We use excitation intensity dependence of photoluminescence (PL) dynamics together with comprehensive simulation of charge carrier trapping and the trap-state dynamics. In the bulk at very low excitation intensities the PL is quenched by trapping. A considerable fraction of the traps become filled if excitation fluence is increased. We identified two different traps, one exhibiting ultralong lifetime (∼70 μs) which leads to efficient accumulation of trap filling even at relatively low excitation intensities. In colloidal NPs, the average number of surface traps is estimated to be 0.7 per NP. It means about 30% excitation would undergo trap-free radiative recombination. The trapping time constant of 7 ns is orders of magnitude longer than the usual trapping times in typical colloidal quantum dots indicating semipassivation of the trap states by a large barrier which slows down the process in the perovskite NPs. We also note that due to the localized character of photogenerated electron-hole pairs in NPs the trapping efficiency is reduced compared to the freely moving charges in BCs. Our results offer insight into the details of photophysics of colloidal perovskite nanoparticles which show promise for light-emitting diode and laser applications.
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| 10. |
- Zhu, Qiushi, et al.
(författare)
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Correlating structure and electronic band-edge properties in organolead halide perovskites nanoparticles
- 2016
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:22, s. 14933-14940
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Tidskriftsartikel (refereegranskat)abstract
- After having emerged as primary contenders in the race for highly efficient optoelectronics materials, organolead halide perovskites (OHLP) are now being investigated in the nanoscale regime as promising building blocks with unique properties. For example, unlike their bulk counterpart, quantum dots of OHLP are brightly luminescent, owing to large exciton binding energies that cannot be rationalized solely on the basis of quantum confinement. Here, we establish the direct correlation between the structure and the electronic band-edge properties of CH3NH3PbBr3 nanoparticles. Complementary structural and spectroscopic measurements probing long-range and local order reveal that lattice strain influences the nature of the valence band and modifies the subtle stereochemical activity of the Pb2+ lone-pair. More generally, this work demonstrates that the stereochemical activity of the lone-pair at the metal site is a specific physicochemical parameter coupled to composition, size and strain, which can be employed to engineer novel functionalities in OHLP nanomaterials.
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