Visualization of exciton transport in ordered and disordered molecular solids

Akselrod, Gleb M.; Deotare, Parag B.; Thompson, Nicholas J.; Lee, Jiye; Tisdale, William A.; Baldo, Marc A.; Menon, Vinod M.; Bulović, Vladimir

doi:10.1038/ncomms4646

Cited by 305 publications

(420 citation statements)

References 36 publications

(50 reference statements)

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“…1 and 2 over the full range of accessible delays indicates that carrier transport is within the diffusive regime. Carrier trapping, which would lead to subdiffusive behavior, 44 therefore has a negligible influence on the carrier kinetics within the first nanosecond after excitation. This is in agreement with studies of CH 3 NH 3 PbI 3 thin films using transient absorption microscopy, 23 in which a diffusive model accounted for the spatiotemporal carrier occupation over a 3 ns time interval after excitation.…”

Section: −1 S Is Plotted Versus 8πmentioning

confidence: 99%

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

Webber

Clegg

Mason

et al. 2017

Applied Physics Letters

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We report the application of femtosecond four-wave mixing (FWM) to the study of carrier transport in solution-processed CH3NH3PbI3. The diffusion coefficient was extracted through direct detection of the lateral diffusion of carriers utilizing the transient grating technique, coupled with simultaneous measurement of decay kinetics exploiting the versatility of the boxcar excitation beam geometry. The observation of exponential decay of the transient grating versus interpulse delay indicates diffusive transport with negligible trapping within the first nanosecond following excitation. The in-plane transport geometry in our experiments enabled the diffusion length to be compared directly with the grain size, indicating that carriers move across multiple grain boundaries prior to recombination. Our experiments illustrate the broad utility of FWM spectroscopy for rapid characterization of macroscopic film transport properties.Organo-lead trihalide perovskites possess a unique combination of electronic and optical properties, making them attractive for applications in light-emitting diodes, 1,2 lasers, 3-5 optical sensors, 6-8 and most notably high performance solar cells where the efficiencies have rapidly increased, reaching over 22% in just a few years. 9In addition, these hybrid perovskites can be solutionprocessed and applied as thin films to a variety of substrates, 10,11 pointing to the potential for large-scale, low-cost solar cell production.12-16 The transport properties of electrons and holes within the perovskite absorber material are crucial to the performance of photovoltaics and other optoelectronic technologies using these materials. The first observation of micron-scale carrier diffusion lengths in CH 3 NH 3 PbI 3−x Cl x 17-19 stimulated a comprehensive research effort aimed at understanding the nature of carrier transport.20-33 Carrier mobilities and/or diffusion lengths have been studied using electrical techniques (e.g. AC Photo Hall, 25 spacecharge-limited current, 31 impedance spectroscopy, 21 , and spatially-resolved electron-beam-induced current 27 ) as well as optical techniques that rely on electrical contacts such as photoluminescence quenching 17,18 and scanning photocurrent microscopy.26,33 Some of these techniques offer the ability to probe transport in a working solar cell device, however imperfectly characterized interface energetics and ambiguities tied to the model-dependent extraction of transport characteristics impede the determination of the physical processes limiting device performance.All-optical techniques provide an effective approach to studying carrier transport within a wide range of photovoltaic materials as no carrier extraction layers or ohmic contacts are required. Time-domain terahertz spectroscopy (TDTHz) and time-resolved microwave conductivity (TRMC) have provided valuable insight into carrier scattering processes in the organometal halide perovskites in recent years. 24,28,29 For such techniques, the quantitative determination of mobility requires modeling of the...

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Section: −1 S Is Plotted Versus 8πmentioning

confidence: 99%

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

Webber

Clegg

Mason

et al. 2017

Applied Physics Letters

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“…[80][81][82][83] As shown in Fig. 4, the spatial spread of the signal at each delay time can be fitted with a Gaussian function parameterized by variance s. The exciton diffusion length L at delay time t is then related to the exciton density characterized by variance s,…”

Section: (5) Direct Visualization Of Tetmentioning

confidence: 99%

“…80 They also demonstrated that the mechanism of exciton transport depends strongly on the nanoscale morphology. Note that these measurements were performed at long timescales, up to 7 ms with a temporal resolution of 100 ns.…”

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confidence: 99%

Triplet transport in thin films: fundamentals and applications

Tang

2017

Chem. Commun.

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Triplet excitons are key players in multi-excitonic processes like singlet fission and triplet-triplet annihilation based photon upconversion, which may be useful in next-generation photovoltaic devices, photocatalysis and bioimaging. Here, we present an overview of experimental and theoretical work on triplet energy transfer, with a focus on triplet transport in thin films. We start with the theory describing Dexter-mediated triplet energy transfer and the fundamental parameters controlling this process. Then we summarize current experimental methods used to measure the triplet exciton diffusion length. Finally, the use of hierarchically ordered structures to improve the triplet diffusion length is presented, before concluding with an outlook on the remaining challenges.

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“…IXs were realized in wide single quantum wells (WSQW) [1][2][3][4] and in coupled quantum wells (CQW) [5][6][7][8] [2-4, 6, 7]. Their long lifetimes allow IXs to travel over large distances before recombination, providing the opportunity to study exciton transport by optical imaging [9][10][11][12][13][14][15] and explore excitonic circuit devices based on exciton transport, see [16] and references therein.Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19].…”

mentioning

confidence: 99%

“…Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19].…”

mentioning

confidence: 99%

Transport of indirect excitons in ZnO quantum wells

et al. 2015

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We report on spatially-and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.An indirect exciton (IX) in a semiconductor quantum well (QW) structure is composed of an electron and a hole confined to spatially separated QW layers. IXs were realized in wide single quantum wells (WSQW) [1][2][3][4] and in coupled quantum wells (CQW) [5][6][7][8] [2-4, 6, 7]. Their long lifetimes allow IXs to travel over large distances before recombination, providing the opportunity to study exciton transport by optical imaging [9][10][11][12][13][14][15] and explore excitonic circuit devices based on exciton transport, see [16] and references therein.Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19]. These properties make materials with robust IXs particularly interesting. However, so far, studies of IX transport mainly concerned GaAs-based CQW. In this paper, we probe transport of IXs in ZnO/MgZnO WSQW structures. IXs in these structures are much more robust than in GaAs structures: their binding energy ∼ 30 meV [4] is considerably higher than that in GaAs/AlGaAs and GaAs/AlAs CQW (∼ 4 and ∼ 10 meV, respectively [7,20]). The binding energy of IXs is smaller than that of excitons in bulk ZnO (∼ 60 meV), however it is large enough to make the IXs stable at room temperature. At the same time, the measurements reported in this work show that transport lengths of IXs in WSQW ZnO structures reach ∼ 4 µm. In comparison, for excitons in bulk ZnO and direct excitons in ZnO structures, transport lengths are within ∼ 0.2 µm [21,22].In this work, we study polar and semipolar ZnO/MgZnO QW structures. The samples were grown by molecular beam epitaxy as in Refs. [4,23] Fig. 1a. The charges on the interfaces between ZnO and MgZnO result in a built-in electric field in the structure, which is stronger for polar samples [4,23]. The built-in electric field pulls the electron and the hole toward opposite borders of the QW, resulting in the spatial separation required for an IX.

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Visualization of exciton transport in ordered and disordered molecular solids

Cited by 305 publications

References 36 publications

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

Triplet transport in thin films: fundamentals and applications

Transport of indirect excitons in ZnO quantum wells

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