| 1. |
- Qian, Deping, et al.
(author)
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Design rules for minimizing voltage losses in high-efficiency organic solar cells
- 2018
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In: Nature Materials. - : NATURE PUBLISHING GROUP. - 1476-1122 .- 1476-4660. ; 17:8, s. 703-
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Journal article (peer-reviewed)abstract
- The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.
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| 2. |
- Zhang, Bin, et al.
(author)
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Effect of exciton transfer on recombination dynamics in vertically nonuniform GaAsSb epilayers
- 2019
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In: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 114:25
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Journal article (peer-reviewed)abstract
- Low-temperature photoluminescence (PL), photoreflectance (PR), and temperature dependent time-resolved PL spectroscopies are employed to investigate optical emission processes and exciton dynamics in graded GaAsSb epilayers. The nonuniformity in the Sb composition along the growth direction is disclosed by low-temperature PL and PR measurements. Furthermore, significant differences in PL dynamics are found at low temperatures for the PL emissions originating from spatial regions with the low and high Sb compositions, with a fast decay and a slow rise at the early stage of the PL transient, respectively. This finding is attributed to exciton transfer from the low Sb region to the high Sb region. The obtained results are important for a general understanding of optical transitions and exciton/carrier dynamics in material systems with a graded alloy composition.
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| 3. |
- Chen, Shula, et al.
(author)
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Core-shell carrier and exciton transfer in GaAs/GaNAs coaxial nanowires
- 2016
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In: Journal of Vacuum Science & Technology B. - : American Institute of Physics (AIP). - 1071-1023 .- 1520-8567. ; 34:4, s. 04J104-
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Journal article (peer-reviewed)abstract
- Comprehensive studies of GaAs/GaNAs coaxial nanowires grown on Si substrates are carried out by temperature-dependent photoluminescence (PL) and PL excitation, to evaluate effects of the shell formation on carrier recombination. The PL emission from the GaAs core is found to transform into a series of sharp PL lines upon radial growth of the GaNAs shell, pointing toward the formation of localization potentials in the core. This hampers carrier transfer at low temperatures from the core in spite of its wider bandgap. Carrier injection from the core to the optically active shell is found to become thermally activated at Tamp;gt;60 K, which implies that the localization potentials are rather shallow. (C) 2016 American Vacuum Society.
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| 4. |
- Chen, Shula, et al.
(author)
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Dilute Nitride Nanowire Lasers Based on a GaAs/GaNAs Core/Shell Structure
- 2017
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In: Nano letters (Print). - : AMER CHEMICAL SOC. - 1530-6984 .- 1530-6992. ; 17:3, s. 1775-1781
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Journal article (peer-reviewed)abstract
- Nanowire (NW) lasers operating in the near infrared spectral range are of significant technological importance for applications in telecommunications, sensing, and medical diagnostics. So far, lasing within this spectral range has been achieved using GaAs/AlGaAs, GaAs/GaAsP, and InGaAs/GaAs core/shell NWs. Another promising III-V material, not yet explored in its lasing capacity, is the dilute nitride GaNAs. In this work, we demonstrate, for the first time, optically pumped lasing from the GaNAs shell of a single GaAs/GaNAs core/shell NW. The characteristic "S"-shaped pump power dependence of the lasing intensity, with the concomitant line width narrowing, is observed, which yields a threshold gain, g(th), of 3300 cm(-1) and a spontaneous emission coupling factor beta, of 0.045. The dominant lasing peak is identified to arise from the HE21b, cavity mode, as determined from its pronounced emission polarization along the NW axis combined with theoretical calculations of lasing threshold for guided modes inside the nanowire. Even without intentional pas sivation of the NW surface, the lasing emission can be sustained up to 150 K. This is facilitated by the improved surface quality due to nitrogen incorporation, which partly suppresses the surface-related nonradiative recombination centers via nitridation. Our work therefore represents the first step toward development of room-temperature infrared NW lasers based on dilute nitrides with extended tunability in the lasing wavelength.
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| 5. |
- Chen, Shula, et al.
(author)
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Effects of Strong Band-Tail States on Exciton Recombination Dynamics in Dilute Nitride GaP/GaNP Core/Shell Nanowires
- 2018
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In: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 122:33, s. 19212-19218
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Journal article (peer-reviewed)abstract
- Exciton dynamics in dilute nitride GaP/GaNP core/shell nanowires (NWs) with pronounced band-tail states formed by nitrogen clusters is investigated using time-resolved photoluminescence (PL) spectroscopy. The emission of excitons localized at the N-related states in the GaNP shell is found to exhibit a stretched exponential decay, with the 1/e lifetime dramatically shortened with decreasing excitation wavelength and reduced shell thickness. The observed PL transient behavior is explained by markedly different exciton lifetimes between the surface and bulk regions of the GaNP shell, that is, similar to 20 ps versus similar to 10 ns, respectively. Despite being trapped at the deep localized N states, the photoexcited excitons are concluded to suffer from pronounced surface recombination via tunneling to the surface states within a distance of 10 nm from the surface, which results in the depth-dependent PL dynamics. The surface recombination rate is, however, lower than that previously reported for GaP, indicative of partial passivation of the surface states by nitrogen. From temperature-dependent PL measurements, characteristic thermal activation energies for the surface and bulk-related nonradiative recombination channels are deduced. The obtained results provide insight into the exciton/carrier dynamics in NW systems with strong localization or alloy disorder, which is important for future nanophotonic and photovoltaic applications of such structures.
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| 6. |
- Chen, Shula L., et al.
(author)
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Suppression of non-radiative surface recombination by N incorporation in GaAs/GaNAs core/shell nanowires
- 2015
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5
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Journal article (peer-reviewed)abstract
- III-V semiconductor nanowires (NWs) such as GaAs NWs form an interesting artificial materials system promising for applications in advanced optoelectronic and photonic devices, thanks to the advantages offered by the 1D architecture and the possibility to combine it with the main-stream silicon technology. Alloying of GaAs with nitrogen can further enhance performance and extend device functionality via band-structure and lattice engineering. However, due to a large surface-to-volume ratio, III-V NWs suffer from severe non-radiative carrier recombination at/near NWs surfaces that significantly degrades optical quality. Here we show that increasing nitrogen composition in novel GaAs/GaNAs core/shell NWs can strongly suppress the detrimental surface recombination. This conclusion is based on our experimental finding that lifetimes of photo-generated free excitons and free carriers increase with increasing N composition, as revealed from our time-resolved photoluminescence (PL) studies. This is accompanied by a sizable enhancement in the PL intensity of the GaAs/GaNAs core/shell NWs at room temperature. The observed N-induced suppression of the surface recombination is concluded to be a result of an N-induced modification of the surface states that are responsible for the nonradiative recombination. Our results, therefore, demonstrate the great potential of incorporating GaNAs in III-V NWs to achieve efficient nano-scale light emitters.
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| 7. |
- Chen, Shula, et al.
(author)
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Near-Infrared Lasing at 1 mu m from a Dilute-Nitride-Based Multishell Nanowire
- 2019
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In: Nano letters (Print). - : AMER CHEMICAL SOC. - 1530-6984 .- 1530-6992. ; 19:2, s. 885-890
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Journal article (peer-reviewed)abstract
- A coherent photon source emitting at near-infrared (NIR) wavelengths is at the heart of a wide variety of applications ranging from telecommunications and optical gas sensing to biological imaging and metrology. NIR-emitting semiconductor nanowires (NWs), acting both as a miniaturized optical resonator and as a photonic gain medium, are among the best-suited nanomaterials to achieve such goals. In this study, we demonstrate the NIR lasing at 1 mu m from GaAs/GaNAs/GaAs core/shell/cap dilute nitride nanowires with only 2.5% nitrogen. The achieved lasing is characterized by an S-shape pump-power dependence and narrowing of the emission line width. Through examining the lasing performance from a set of different single NWs, a threshold gain, g(th), of 4100-4800 cm(-1), was derived with a spontaneous emission coupling factor, beta, up to 0.8, which demonstrates the great potential of such nanophotonic material. The lasing mode was found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Based on temperature dependence of the lasing emission, a high characteristic temperature, T-0, of 160 (+/- 10) K is estimated. Our results, therefore, demonstrate a promising alternative route to achieve room-temperature NIR NW lasers thanks to the excellent alloy tunability and superior optical performance of such dilute nitride materials.
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| 8. |
- Chen, Shula, et al.
(author)
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Room-temperature polarized spin-photon interface based on a semiconductor nanodisk-in-nanopillar structure driven by few defects
- 2018
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In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9
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Journal article (peer-reviewed)abstract
- Owing to their superior optical properties, semiconductor nanopillars/nanowires in one-dimensional (1D) geometry are building blocks for nano-photonics. They also hold potential for efficient polarized spin-light conversion in future spin nano-photonics. Unfortunately, spin generation in 1D systems so far remains inefficient at room temperature. Here we propose an approach that can significantly enhance the radiative efficiency of the electrons with the desired spin while suppressing that with the unwanted spin, which simultaneously ensures strong spin and light polarization. We demonstrate high optical polarization of 20%, inferring high electron spin polarization up to 60% at room temperature in a 1D system based on a GaNAs nanodisk-in-GaAs nanopillar structure, facilitated by spin-dependent recombination via merely 2-3 defects in each nanodisk. Our approach points to a promising direction for realization of an interface for efficient spin-photon quantum information transfer at room temperature-a key element for future spin-photonic applications.
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| 9. |
- Chen, Yafeng, et al.
(author)
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Temperature-dependent radiative and non-radiative dynamics of photo-excited carriers in extremely high-density and small InGaN nanodisks fabricated by neutral-beam etching using bio-nano-templates
- 2018
-
In: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 123:20
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Journal article (peer-reviewed)abstract
- Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 x 10(11) cm(-2). The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process. Published by AIP Publishing.
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| 10. |
- Higo, Akio, et al.
(author)
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Optical Study of Sub-10 nm In0.3Ga0.7N Quantum Nanodisks in GaN Nanopillars
- 2017
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In: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 4:7, s. 1851-1857
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Journal article (peer-reviewed)abstract
- We have demonstrated the fabrication of homogeneously distributed In0.3Ga0.7N/GaN quantum nanodisks (QNDs) with a high density and average diameter of 10 nm or less in 30-nm-high nanopillars. The scalable top-down nanofabrication process used biotemplates that were spin-coated on an In0.3Ga0.7N/GaN single quantum well (SQW) followed by low-damage dry etching on ferritins with 7 nm diameter iron cores. The photoluminescence measurements at 70 K showed a blue shift of quantum energy of 420 meV from the In0.3Ga0.7N/GaN SQW to the QND. The internal quantum efficiency of the In0.3Ga0.7N/GaN QND was 100 times that of the SQW. A significant reduction in the quantum-confined Stark effect in the QND structure was observed, which concurred with the numerical simulation using a 3D Schrödinger equation. These results pave the way for the fabrication of large-scale III–N quantum devices using nanoprocessing, which is vital for optoelectronic communication devices.
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