Nanyang Technological University
Singapore, Singapore | Singapore
Main Specialties: Other
Additional Specialties: Spectroscopy, PV
7PubMed Central Citations
Small 2016 Jan 10;12(4):534-46. Epub 2015 Dec 10.
School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore.
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Phys Chem Chem Phys 2015 Oct 17;17(39):26111-20. Epub 2015 Sep 17.
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore.
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ACS Appl Mater Interfaces 2015 Sep 18;7(38):21245-53. Epub 2015 Sep 18.
State Key Laboratory and Institute of Elemento-Organic Chemistry and Centre for Nanoscale Science and Technology, Institute of Polymer Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University , Tianjin, 300071, China.
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Nanotechnology 2015 Aug 3;26(34):342001. Epub 2015 Aug 3.
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.
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Small 2015 Aug 30;11(29):3606-13. Epub 2015 Mar 30.
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
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Adv. Energy Mater. 2015, 5, 1500829
Advanced Energy Materials
Organic–inorganic hybrid perovskite solar cells based on CH3NH3PbI3 have achieved great success with efficiencies exceeding 20%. However, there are increasing concerns over some reported efficiencies as the cells are susceptible to current–voltage (I–V) hysteresis effects. It is therefore essential that the origins and mechanisms of the I–V hysteresis can clearly be understood to minimize or eradicate these hysteresis effects completely for reliable quantification. Here, a detailed electro-optical study is presented that indicates the hysteresis originates from lingering processes persisting from sub-second to tens of seconds. Photocurrent transients, photoluminescence, electroluminescence, quasi-steady state photoinduced absorption processes, and X-ray diffraction in the perovskite solar cell configuration have been monitored. The slow processes originate from the structural response of the CH3NH3PbI3 upon E-field application and/or charge accumulation, possibly involving methylammonium ions rotation/displacement and lattice distortion. The charge accumulation can arise from inefficient charge transfer at the perovskite interfaces, where it plays a pivotal role in the hysteresis. These findings underpin the significance of efficient charge transfer in reducing the hysteresis effects. Further improvements of CH3NH3PbI3-based perovskite solar cells are possible through careful surface engineering of existing TiO2 or through a judicious choice of alternative interfacial layers.
ACS Nano, 2014, 8 (10), pp 10101–10110
The origins of performance enhancement in hybrid plasmonic organic photovoltaic devices are often embroiled in a complex interaction of light scattering, localized surface plasmon resonances, exciton–plasmon energy transfer and even nonplasmonic effects. To clearly deconvolve the plasmonic contributions from a single nanostructure, we herein investigate the influence of a single silver nanowire (NW) on the charge carriers in bulk heterojunction polymer solar cells using spatially resolved optical spectroscopy, and correlate to electrical device characterization. Polarization-dependent photocurrent enhancements with a maximum of ∼36% over the reference are observed when the transverse mode of the plasmonic excitations in the Ag NW is activated. The ensuing higher absorbance and light scattering induced by the electronic motion perpendicular to the NW long axis lead to increased exciton and polaron densities instead of direct surface plasmon-exciton energy transfer. Finite-difference time-domain simulations also validate these findings. Importantly, our study at the single nanostructure level explores the fundamental limits of plasmonic enhancement achievable in organic solar cells with a single plasmonic nanostructure.
J. Phys. Chem. C, 2014, 118 (21), pp 11285–11291
The Journal of Physical Chemistry C
Organic solar cell (OSC) devices based on predominantly poly(3-hexylthiophene-2,5-diyl) (P3HT) nanofibers (NFs) exhibit inferior device performance compared to that of their conventional nanodomain P3HT:PCBM systems, which is credited to the low interfibrillar mobility between the NFs [Kurniawan, M.; et al. J. Phys. Chem. C2012, 116, 18015]. To improve the charge transport of these devices, external electric field (E-field) treatment of the active layer is performed in a bid to align the random polymer chains between the NFs perpendicular to the electrode. Extensive device testing revealed a 22.7% improvement in power conversion efficiency and higher mobilities (37.5% improvement) for the E-field-treated devices compared to those for the control. Transient absorption spectroscopy shows an improved initial generation of carriers and formation of polarons in the E-field-treated samples over those in the control samples in the femtosecond–nanosecond time scale. However, in the absence of any sweep-out voltage in the E-field-treated films, a higher recombination rate in the nanosecond–microsecond time scale is observed. Concomitant with the improved device efficiencies and higher mobilities measured in the E-field-treated devices and the higher recombination rate over the nanosecond–microsecond time scale in the E-field-treated films, we assert that the E-field treatment improved charge mobility and transport of P3HT-NF:PCBM through improved orientation of the polymer chains in the amorphous P3HT phase coexisting with the NFs.
Nat.Comm.,4, 2004 (2013)
There has been much controversy over the incorporation of organic-ligand-encapsulated plasmonic nanoparticles in the active layer of bulk heterojunction organic solar cells, where both enhancement and detraction in performance have been reported. Here through comprehensive transient optical spectroscopy and electrical characterization, we demonstrate evidence of traps responsible for performance degradation in plasmonic organic solar cells fabricated with oleylamine-capped silver nanoparticles blended in the poly (3-hexylthiophene):[6,6]-phenyl-C 61-butyric acid methyl ester active layer. Despite an initial increase in exciton generation promoted by the presence of silver nanoparticles, transient absorption spectroscopy reveals no increase in the later free polaron population—attributed to fast trapping of polarons by nearby nanoparticles. The increased trap-assisted recombination is also reconfirmed by light intensity-dependent electrical measurements. These new insights into the photophysics and charge dynamics of plasmonic organic solar cells would resolve the existing controversy and provide clear guidelines for device design and fabrication.
Advanced Optical Materials, 1: 319–326
Advanced Optical Materials
The carrier recombination dynamics in ZnSe nanowires (NWs) remain poorly understood despite more than a decade of research since their inception in 2001. Herein, through a comprehensive pump fluence- and temperature-dependent two-photon excitation (TPE) study, a clear picture of the carrier relaxation pathways, intrinsic lifetimes, exciton oscillator strengths, and exciton-phonon interactions is presented for this NW system. Contrary to a common perception that the higher pump intensities needed to achieve two-photon-excited photoluminescence correspond to a higher exciton density threshold (nth) for two-photon pumped lasing, it is found that a much lower nth is needed to achieve lasing with TPE compared to single-photon excitation (SPE) of the same ZnSe NWs. This measurement is further supported by the greatly enhanced lasing action photostability characteristics of the ZnSe NWs under TPE. These findings have significant implications on the design and the tailoring of the optoelectronic properties of nanowire lasers.
Phys. Rev. B 87, 115309
Physical Review B
The origins of the commonly observed green emission (GE) from ZnO nanostructures remain highly controversial despite extensive studies over the past few decades. Herein, through a comprehensive ultrafast optical spectroscopy study, new insights into its origin and the charge trapping dynamics at the GE centers in ZnO nanowires prepared by the vapor transport method are gained. Transient absorption spectroscopy (TAS) revealed a sub-band-gap absorption bleaching band arising from the state filling of the electrons in the conduction band and holes trapped in the GE centers. The GE originates from the recombination between the electrons in the conduction band and/or shallow donor levels and the holes trapped at the GE centers (which are located at ∼0.88 eV above the valence band). Importantly, an ultrafast excitonic Auger-type hole trapping process to the GE centers occurring in a subpicosecond time scale was also uncovered by TAS—shedding new light on the mechanism behind the fast and efficient charge trapping of photoexcited carriers. The knowledge gained is crucial for the development of ZnO-based optoelectronic devices.
Appl. Phys. Lett. 101, 091104 (2012)
Applied Physics Letters
Ultrafast optical-pump terahertz probe spectroscopy was performed over a graduated size distribution of CdSnanobelts to investigate the size and surface effects on the transient photoconductivity. It was found that the nanobelt size has a profound influence on the carrier localization and photoconductivitydynamics, brought about by the carrier trapping at surface defects. The strong carrier localization in the nanobelt is ascribed to the internal surface boundaries arising from the surface depletion layer. The increased thickness of surface depletion layer due to a continuous trapping of photocarriers at surface defects results in more pronounced carrier localization after photoexcitation.
J. Phys. Chem. C, 2012, 116 (28), pp 14820–14825
The Journal of Physical Chemistry C
Efficiency enhancement in plasmonic bulk heterojunction (PCDTBT:PCBM) organic solar cells (OSCs) is demonstrated with the integration of large-area periodic Ag nanotriangle (NT) arrays (that were fabricated using the cost-effective, high-throughput nanosphere lithography technique) in the OSC device. The improvements to the power conversion efficiency (from 4.24 to 4.52%) and to the short circuit current density (by ∼12%) are attributed to an increase in exciton generation induced by the strong local E-field and the scattering generated by the localized surface plasmon resonance of the hexagonal NT arrays. These findings are validated by a range of steady-state and transient optical spectroscopy and correlated with device performance data. Importantly, our work demonstrates the feasibility of integrating a simple cost-effective, tailorable, and scalable nanofabrication technique with existing OSC fabrication processes.
A cost-effective approach to enhancing broadband light trapping in ultrathin bulk heterojunction organic photovoltaic (OPV) devices is proposed. This is achieved by simply inserting an array of Al nanodisks at the interface of the ITO anode and the organic active layer; forming circular plasmonic nanopatch cavities (between the nanodisks and the Al cathode) that sandwich the active layer. Through interactions between the surface plasmon polaritons localized at the nanodisk and the cathode, a tunable broadband resonance peak spanning 450–700 nm in the scattering cross-section spectrum is formed, thereby enhancing the electromagnetic field in the active layer. Compared to an OPV device with a 60-nm-thick PCPDTBT/PC60BM layer, our numerical simulations reveal that integrated absorption enhancements of up to 40 % can be achieved in an equivalent device integrated with an array of nanodisks with a diameter of 100 nm and a periodicity of 250 nm. From the analysis of the structure–performance relationships, implications for the design of these nanopatch cavities for light harvesting in ultrathin OPV devices are discussed.
J. Phys. Chem. C, 2012, 116 (10), pp 6453–6458
Solution-processed ultrafine gold nanowires (Au-NWs) have been exploited as plasmonic antennae in organic P3HT:PCBM photovoltaic cells. The careful reduction of the spacer layer thickness which allows the evanescent field to be extended into the photoactive layer and the geometry of the Au-NWs bands which favors the enhanced scattering collectively result in an increased short-circuit current density by 23.2%. The exact nature of the plasmonic effect in Au-NWs incorporated P3HT system and the critical role played by the spacer layer were studied through optical and time-resolved photoluminescence spectroscopy. The improved photocurrent in the Au-NWs integrated devices is due to an enhanced absorption in the photoactive layer which is contributed from an increased plasmon excitation field and far-field scattering of Au-NWs.