| 1. |
- Bian, Qingzhen, 1988-, et al.
(författare)
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Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes
- 2020
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Damtie, Fikeraddis A., et al.
(författare)
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Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:6
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Tidskriftsartikel (refereegranskat)abstract
- Multiple exciton generation (MEG) is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.
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| 3. |
- Damtie, Fikeraddis A., et al.
(författare)
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Two-dimensional action spectroscopy of excitonic systems : Explicit simulation using a phase-modulation technique
- 2017
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Ingår i: Physical Review A. - 2469-9926. ; 96:5
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states.
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| 4. |
- Ghosh, Supriya, et al.
(författare)
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Light-induced defect healing and strong many-body interactions in formamidinium lead bromide perovskite nanocrystals
- 2020
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:4, s. 1239-1246
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Tidskriftsartikel (refereegranskat)abstract
- Organic lead halide perovskite (OLHP) nanocrystals (NCs) have paved the way to advanced optoelectronic devices through their extraordinary electrical and optical properties. However, understanding of the light-induced complex dynamic phenomena in OLHP NCs remains a subject of debate. Here we used wide field microscopy and time-resolved spectroscopy to correlate the local changes in photophysics and the dynamical behavior of photocarriers. We demonstrate that light-induced brightening of the photoluminescence from the formamidinium lead bromide NC films is related to the film preparation condition and reduction of trap density. The density of trap states is reduced via halide ion migration from interstitial position. Our femtosecond transient absorption study identifies transient Stark effect due to the generation of hot carriers. Because of slow carrier trapping, Auger recombination through many-body carrier-carrier interactions dominates over trion recombination. This work presents unprecedented insights into the light-driven processes enabling better device design in the future.
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| 5. |
- Ghosh, Supriya, et al.
(författare)
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Phonon Coupling with Excitons and Free Carriers in Formamidinium Lead Bromide Perovskite Nanocrystals
- 2018
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:15, s. 4245-4250
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Tidskriftsartikel (refereegranskat)abstract
- Organometal halide perovskites in the form of nanocrystals (NCs) have attracted enormous attention due to their unique optoelectronic and photoluminescence (PL) properties. Here, we examine the phase composition and the temperature dependence of emission line width broadening in formamidinium lead bromide (FAPbBr3) perovskite nanocrystals (NCs) for light-emitting applications and identify different charge-carrier scattering mechanisms. Our results show most of the emission is from the orthorhombic phase. The PL line width broadening at high temperature is dominated by the Fröhlich interaction between the free charge carriers and the optical phonons. At low temperatures, the peak of the PL spectrum exhibits a continuous red shift indicating an increase of excitons contribution at lower temperatures, and concurrently the line width also narrows down due to the inhibition of the optical phonons. From the temperature-dependent measurements, the coupling strength of both the charge phonon interaction and the exciton phonon interaction have been determined. The obtained results indicate that the charge phonon coupling strengths are higher compared to the exciton phonon coupling.
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| 6. |
- Karki, Khadga Jung, et al.
(författare)
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Before Förster. Initial excitation in photosynthetic light harvesting.
- 2019
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Ingår i: Chemical Science. - 2041-6539. ; 10, s. 7923-7928
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Tidskriftsartikel (refereegranskat)abstract
- Electronic 2D spectroscopy allows nontrivial quantum effects to be explored in unprecedented detail. Here, we apply recently developed fluorescence detected coherent 2D spectroscopy to study the light harvesting antenna 2 (LH2) of photosynthetic purple bacteria. We report double quantum coherence 2D spectra which show clear cross peaks indicating correlated excitations. Similar results are found for rephasing and nonrephasing signals. Analysis of signal generating quantum pathways leads to the conclusion that, contrary to the currently prevailing physical picture, the two weakly coupled pigment rings of LH2 share the initial electronic excitation leading to quantum mechanical correlation between the two clearly separate absorption bands. These results are general and have consequences for the interpretation of initially created excited states not only in photosynthesis but in all light absorbing systems composed of weakly interacting pigments where the excitation transfer is commonly described by using Förster theory. Being able to spectrally resolve the nonequilibrium dynamics immediately following photoabsorption may provide a glimpse to the systems' transition into the Förster regime.
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| 7. |
- Karki, Khadga Jung, et al.
(författare)
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Digital cavities and their potential applications
- 2013
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Ingår i: Journal of Instrumentation. - 1748-0221. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The concept of a digital cavity is presented. The functionality of a tunable radio-frequency/microwave cavity with unrestricted Q-factor is implemented. The theoretical aspects of the cavity and its potential applications in high resolution spectroscopy and synchronization of clocks together with examples in signal processing and data acquisition are discussed.
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| 8. |
- Kumar, Pushpendra, et al.
(författare)
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Kinetics of near-infrared-to-visible upconversion in rubrene : An initial excitation of triplets
- 2021
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 104:14
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Tidskriftsartikel (refereegranskat)abstract
- Upconversion (UC) in a molecular system is a process in which excitons produced by a multiple absorption of low-energy photons at long wavelengths undergo fusion to produce high energy excitons that consequently recombine to emit anti-Stokes shifted photons. Molecular systems for UC typically require a sensitizer. However, recent experiments show that UC in rubrene occurs even without the presence of the sensitizer. In this system, intermediate states are assumed to absorb photons at near-infrared wavelengths, which either absorb additional photons to populate the emissive singlet state or undergo fusion to generate triplets. The triplets can again undergo fusion to populate the excited singlet state. The final emission is around 600 nm. These models have been tested against the intensity dependence of the UC emission. Here, we have measured the kinetics of UC in rubrene by using intensity modulated photoexcitation at 800 nm to better understand the underlying mechanism. The models of UC that have been proposed so far do not agree with our measurements. Our results show that the yield of UC lags behind excitation significantly, indicating that triplet states are directly excited from the ground state, and their fusion, which depends on the population, becomes prominent after a certain build up time. While the intermediate states could form dynamically after the UC has been initiated and enhance the process, further sensitive absorption measurements are necessary to understand the role of the intermediate states in the process. Our results are important in finding new routes to enhance UC in pristine organic semiconductors for applications in photovoltaics, lasers, bioimaging, optical devices, and lighting.
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| 9. |
- Nariyangadu, Sesha Bamini, et al.
(författare)
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New nonlinear optical crystal of rhodamine 590 acid phthalate
- 2020
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 5:33, s. 20863-20873
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis and crystal structure of rhodamine 590 acid phthalate (RhAP) have been reported. This novel solid-state rhodamine derivative not only has a longer fluorescence lifetime compared to rhodamine solid-state matrixes where emission is quenched but also possesses strong nonlinear optical characteristics. The static and dynamic first- and second-order hyperpolarizabilities were calculated using the time-dependent density functional theory at the B3LYP/6-31+G∗ level. The computed static values of β and γof RhAP by the X-ray diffraction (XRD) structure were 31.9 × 10-30 and 199.0 × 10-36 esu, respectively. These values were about 62 times larger than the corresponding values in urea, an already well-known nonlinear optical material. The second-order hyperpolarizability of the compound was determined experimentally by measuring the two-photon absorption cross section using intensity-modulated light fields. The reported compound, excitable at near-infrared, exhibited frequency upconversion with the two-photon absorption coefficient enhanced by two orders of magnitude compared to that of the dye solution. Hosting the dye in the solid, at high concentrations, exploits the nonlinearity of the dye itself as well as results in significant excitonic effects including formation of broad exciton band and superradiance.
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| 10. |
- Shi, Qi, et al.
(författare)
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Variations in the Composition of the Phases Lead to the Differences in the Optoelectronic Properties of MAPbBr3 Thin Films and Crystals
- 2018
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:38, s. 21817-21823
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskite-based photoactive devices have been debated. We have used two-photon PL to study the temperature-dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature, the tetragonal (TE) phase is dominant in the BCs, while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL spectra from the TFs, while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature-dependent narrowing of the PL linewidths shows a stronger coupling between the longitudinal optical phonons and the free carriers in the OR phase as compared to the TE phase, implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs, provided that the sample is shielded from other types of degradation.
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