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1.
Nat Commun ; 11(1): 617, 2020 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-32001688

RESUMO

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial ~200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.

2.
Nanoscale ; 11(40): 18550-18558, 2019 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-31363719

RESUMO

Introduction of in situ HCl etching to an epitaxial growth process has been shown to suppress radial growth and improve the morphology and optical properties of nanowires. In this paper, we investigate the dynamics of photo-generated charge carriers in a series of indium phosphide nanowires grown with varied HCl fluxes. Time resolved photo-induced luminescence, transient absorption and time resolved terahertz spectroscopy were employed to investigate charge trapping and recombination processes in the nanowires. Since the excitation photons generate charges predominantly in less than a half length of the nanowires, we can selectively assess the charge carrier dynamics at their top and bottom. We found that the photoluminescence decay is dominated by the decay of the mobile hole population due to trapping, which is affected by the HCl etching. The hole trapping rate is in general faster at the top of the nanowires than at the bottom. In contrast, electrons remain highly mobile until they recombine non-radiatively with the trapped holes. The slowest hole trapping as well as the least efficient non-radiative recombination was recorded for etching using the HCl molar fraction of χHCl = 5.4 × 10-5.

3.
Faraday Discuss ; 216(0): 191-210, 2019 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-31016293

RESUMO

Ultrafast dynamics of photoinduced charge transfer processes in light-harvesting systems based on Earth-abundant transition metal complexes are of current interest for the development of molecular devices for solar energy conversion applications. A combination of ultrafast spectroscopy and first principles quantum chemical calculations of a recently synthesized iron carbene complex is used to elucidate the ultrafast excited state evolution processes in these systems with particular emphasis on investigating the underlying reasons why these complexes show promise in terms of significantly extended lifetimes of charge transfer excited states. Together, our results challenge the traditional excited state landscape for iron-based light harvesting transition metal complexes through radically different ground and excited state properties in alternative oxidation states. This includes intriguing indications of rich band-selective excited state dynamics on ultrafast timescales that are interpreted in terms of excitation energy dependence for excitations into a manifold of charge-transfer states. Some implications of the observed excited state properties and photoinduced dynamics for the utilization of iron carbene complexes for solar energy conversion applications are finally discussed.

4.
J Am Chem Soc ; 141(19): 7743-7750, 2019 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-31017418

RESUMO

Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices still show power conversion efficiencies far behind those of commercialized solar cells. One of the main reasons is the large driving force required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer donor PTO2 and a nonfullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large, and the induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement by judicious modulation of ESP.

5.
Polymers (Basel) ; 11(3)2019 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-30960392

RESUMO

In this work, we studied influence of post thermal annealing on the performance and charge photogeneration processes of PffBT4T-2OD/PC71BM solar cells. As-prepared device exhibits a high-power conversion efficiency of 9.5%, much higher than that after thermal annealing. To understand this phenomenon, we studied charge photogeneration processes in these solar cells by means of time resolved spectroscopy. We associate the degradation of solar cell performance with the reduction of exciton dissociation efficiency and with increased bimolecular recombination of photogenerated charges as a result of annealing. We correlate the generation of localized PffBT4T-2OD polarons observed via spectro-electrochemical measurements with enhancement of the bimolecular charge recombination of annealed solar cells.

6.
Nano Lett ; 19(3): 1632-1642, 2019 03 13.
Artigo em Inglês | MEDLINE | ID: mdl-30779588

RESUMO

Low-dimensional narrow-band-gap III-V semiconductors are key building blocks for the next generation of high-performance nanoelectronics, nanophotonics, and quantum devices. Realizing these various applications requires an efficient methodology that enables the material dimensional control during the synthesis process and the mass production of these materials with perfect crystallinity, reproducibility, low cost, and outstanding electronic and optoelectronic properties. Although advances in one- and two-dimensional narrow-band-gap III-V semiconductors synthesis, the progress toward reliable methods that can satisfy all of these requirements has been limited. Here, we demonstrate an approach that provides a precise control of the dimension of InAs from one-dimensional nanowires to wafer-scale free-standing two-dimensional nanosheets, which have a high degree of crystallinity and outstanding electrical and optical properties, using molecular-beam epitaxy by controlling catalyst alloy segregation. In our approach, two-dimensional InAs nanosheets can be obtained directly from one-dimensional InAs nanowires by silver-indium alloy segregation, which is much easier than the previously reported methods, such as the traditional buffering technique and select-area epitaxial growth. Detailed transmission electron microscopy investigations provide solid evidence that the catalyst alloy segregation is the origination of the InAs dimensional transformation from one-dimensional nanowires to two-dimensional nanosheets and even to three-dimensional complex crosses. Using this method, we find that the wafer-scale free-standing InAs nanosheets can be grown on various substrates including Si, MgO, sapphire, GaAs, etc. The InAs nanosheets grown at high temperature are pure-phase single crystals and have a high electron mobility and a long time-resolved terahertz kinetics lifetime. Our work will open up a conceptually new and general technology route toward the effective controlling of the dimension of the low-dimensional III-V semiconductors. It may also enable the low-cost fabrication of free-standing nanosheet-based devices on an industrial scale.

7.
Science ; 363(6424): 249-253, 2019 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-30498167

RESUMO

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}-, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the 2LMCT state of [Fe(phtmeimb)2]+ in bimolecular quenching studies with methylviologen and diphenylamine.

8.
J Phys Chem Lett ; 9(12): 3161-3166, 2018 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-29806455

RESUMO

Despite the enormous amount of attention CH3NH3PbI3 has received, we are still lacking an in-depth understanding of its basic properties. In particular, the directional mechanical and structural characteristics of this material have remained elusive. Here, we investigate these properties by monitoring the propagation of longitudinal and shear phonons following the absorption of a femtosecond pulse along various crystalline directions of a CH3NH3PbI3 single crystal. We first extract the sound velocities of longitudinal and transverse phonons along these directions of the crystal. Our study then reveals the negative directional thermal expansion of CH3NH3PbI3, which is responsible for strong coherent shear phonon generation. Finally, from these observations, we perform elastic characterization of this material, revealing a large directional Poisson's ratio, which reaches 0.7 and that we associate with the weak mechanical stability of this material. Our results also provide guidelines to fabricate a transducer of high-frequency transverse phonons.

9.
Nano Lett ; 17(7): 4248-4254, 2017 07 12.
Artigo em Inglês | MEDLINE | ID: mdl-28654299

RESUMO

Understanding of recombination and photoconductivity dynamics of photogenerated charge carriers in GaxIn1-xP NWs is essential for their optoelectronic applications. In this letter, we have studied a series of GaxIn1-xP NWs with varied Ga composition. Time-resolved photoinduced luminescence, femtosecond transient absorption, and time-resolved THz transmission measurements were performed to assess radiative and nonradiative recombination and photoconductivity dynamics of photogenerated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping, whereas electrons are highly mobile until they recombine nonradiatively. We also resolve gradual decrease of mobility of photogenerated electrons assigned to electron trapping and detrapping in a distribution of trap states. We identify that the nonradiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as nonradiative recombination become faster with increasing Ga composition in GaxIn1-xP NWs. We have estimated early time electron mobility in GaxIn1-xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley.

10.
Nat Commun ; 8(1): 34, 2017 06 26.
Artigo em Inglês | MEDLINE | ID: mdl-28652597

RESUMO

Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals.Understanding crystal phase transition in materials is of fundamental importance. Using luminescence spectroscopy and super-resolution imaging, Dobrovolsky et al. study the transition from the tetragonal to orthorhombic crystal phase in methylammonium lead triiodide nanowires at low temperature.

11.
Nat Commun ; 8: 14398, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28176755

RESUMO

Despite the great amount of attention CH3NH3PbI3 has received for its solar cell application, intrinsic properties of this material are still largely unknown. Mobility of charges is a quintessential property in this aspect; however, there is still no clear understanding of electron transport, as reported values span over three orders of magnitude. Here we develop a method to measure the electron and hole deformation potentials using coherent acoustic phonons generated by femtosecond laser pulses. We apply this method to characterize a CH3NH3PbI3 single crystal. We measure the acoustic phonon properties and characterize electron-acoustic phonon scattering. Then, using the deformation potential theory, we calculate the carrier intrinsic mobility and compare it to the reported experimental and theoretical values. Our results reveal high electron and hole mobilities of 2,800 and 9,400 cm2 V-1 s-1, respectively. Comparison with literature values of mobility demonstrates the potential role played by polarons in charge transport in CH3NH3PbI3.

12.
Nanotechnology ; 27(45): 455704, 2016 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-27713183

RESUMO

In this paper we have investigated the dynamics of photo-generated charge carriers in a series of aerotaxy-grown GaAs nanowires (NWs) with different levels of Zn doping. Time-resolved photo-induced luminescence and transient absorption have been employed to investigate radiative (band edge transition) and non-radiative charge recombination processes, respectively. We find that the photo-luminescence (PL) lifetime of intrinsic GaAs NWs is significantly increased after growing an AlGaAs shell over them, indicating that an AlGaAs shell can effectively passivate the surface of aerotaxy-grown GaAs NWs. We observe that PL decay time as well as PL intensity decrease with increasing Zn doping, which can be attributed to thermally activated electron trapping with the trap density increased due to the Zn doping level.

13.
Nano Lett ; 16(9): 5701-7, 2016 09 14.
Artigo em Inglês | MEDLINE | ID: mdl-27564139

RESUMO

We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires.

14.
J Am Chem Soc ; 138(34): 10935-44, 2016 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-27479751

RESUMO

Growing interests have been devoted to the design of polymer acceptors as potential replacement for fullerene derivatives for high-performance all polymer solar cells (all-PSCs). One key factor that is limiting the efficiency of all-PSCs is the low fill factor (FF) (normally <0.65), which is strongly correlated with the mobility and film morphology of polymer:polymer blends. In this work, we find a facile method to modulate the crystallinity of the well-known naphthalene diimide (NDI) based polymer N2200, by replacing a certain amount of bithiophene (2T) units in the N2200 backbone by single thiophene (T) units and synthesizing a series of random polymers PNDI-Tx, where x is the percentage of the single T. The acceptor PNDI-T10 is properly miscible with the low band gap donor polymer PTB7-Th, and the nanostructured blend promotes efficient exciton dissociation and charge transport. Solvent annealing (SA) enables higher hole and electron mobilities, and further suppresses the bimolecular recombination. As expected, the PTB7-Th:PNDI-T10 solar cells attain a high PCE of 7.6%, which is a 2-fold increase compared to that of PTB7-Th:N2200 solar cells. The FF of 0.71 reaches the highest value among all-PSCs to date. Our work demonstrates a rational design for fine-tuned crystallinity of polymer acceptors, and reveals the high potential of all-PSCs through structure and morphology engineering of semicrystalline polymer:polymer blends.

15.
Nano Lett ; 16(8): 4792-8, 2016 08 10.
Artigo em Inglês | MEDLINE | ID: mdl-27352041

RESUMO

We have developed and demonstrated an experimental method, based on the picosecond acoustics technique, to perform nondestructive complete mechanical characterization of nanowires, that is, the determination of the complete elasticity tensor. By means of femtosecond pump-probe spectroscopy, coherent acoustic phonons were generated in an ensemble of nanowires and their dynamics was resolved. Specific phonon modes were identified and the detection mechanism was addressed via wavelength dependent experiments. We calculated the exact phonon dispersion relation of the nanowires by fitting the experimentally observed frequencies, thus allowing the extraction of the complete elasticity tensor. The elasticity tensor and the nanowire diameter were determined for zinc blende GaAs nanowires and were found to be in a good agreement with literature data and independent measurements. Finally, we have applied this technique to characterize wurtzite GaAs nanowires, a metastable phase in bulk, for which no experimental values of elastic constants are currently available. Our results agree well with previous first principle calculations. The proposed approach to the complete and nondestructive mechanical characterization of nanowires will allow the efficient mechanical study of new crystal phases emerging in nanostructures, as well as size-dependent properties of nanostructured materials.

16.
J Phys Chem Lett ; 7(7): 1096-101, 2016 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-26942559

RESUMO

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.

17.
J Am Chem Soc ; 137(51): 16043-8, 2015 Dec 30.
Artigo em Inglês | MEDLINE | ID: mdl-26636183

RESUMO

Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.

18.
Nano Lett ; 15(11): 7238-44, 2015 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-26421505

RESUMO

Measuring lifetime of photogenerated charges in semiconductor nanowires (NW) is important for understanding light-induced processes in these materials and is relevant for their photovoltaic and photodetector applications. In this paper, we investigate the dynamics of photogenerated charge carriers in a series of as-grown InP NW with different levels of sulfur (S) doping. We observe that photoluminescence (PL) decay time as well as integrated PL intensity decreases with increasing S doping. We attribute these observations to hole trapping with the trap density increased due to S-doping level followed by nonradiative recombination of trapped charges. This assignment is proven by observation of the trap saturation in three independent experiments: via excitation power and repetition rate PL lifetime dependencies and by PL pump-probe experiment.

19.
Phys Chem Chem Phys ; 17(38): 24978-87, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26343504

RESUMO

A light-induced photoluminescence (PL) enhancement in surface-deposited methylammonium lead iodide (CH3NH3PbI3) perovskites was investigated in detail using time-resolved luminescence microscopy. We found the PL intensity to increase up to three orders of magnitude upon light illumination with an excitation power density of 0.01-1 W cm(-2). The PL enhancement is accompanied by an increase of the PL lifetime from several nanoseconds to several hundred nanoseconds and also by an increase of the initial amplitude of the PL decay. The latter suggests excited state quenching at the subpicosecond timescale. We propose a model where the trapping sites responsible for non-radiative charge recombination can be de-activated by a photochemical reaction involving oxygen. The reaction zone is spatially limited by the excitation light-penetration depth and diffusion length of the charge carriers. The latter increases in the course of the light-curing process making the reaction zone spreading from the surface towards the interior of the crystal. The PL enhancement can be reversed by switching on/off the excitation light or switching the atmosphere between oxygen and nitrogen. Slow diffusion of the reactants and products and equilibrium between the active and "cured" trapping sites are proposed to be the reasons for peculiar responses of PL to such varied experimental conditions.

20.
Nano Lett ; 15(3): 1603-8, 2015 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-25706329

RESUMO

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.


Assuntos
Compostos de Cálcio/química , Compostos de Cálcio/efeitos da radiação , Cristalização/métodos , Medições Luminescentes/métodos , Nanopartículas/química , Nanopartículas/efeitos da radiação , Óxidos/química , Óxidos/efeitos da radiação , Titânio/química , Titânio/efeitos da radiação , Absorção de Radiação , Luz , Teste de Materiais , Nanopartículas/ultraestrutura , Tamanho da Partícula
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