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Liu Y.,Institute of High Performance Computing of Singapore | Eddie Chua K.T.,Harvard - Smithsonian Center for Astrophysics | Sum T.C.,Nanyang Technological University | Sum T.C.,Singapore Berkeley Research Initiative for Sustainable Energy SinBeRISE | Gan C.K.,Institute of High Performance Computing of Singapore
Physical Chemistry Chemical Physics

We present a lattice dynamics study of orthorhombic antimony sulphide (Sb2S3) obtained using density-functional calculations in conjunction with the supercell force-constant method. The effect of Born effective charges is taken into account using a mixed-space approach, resulting in the splitting of longitudinal and transverse optical (LO-TO) phonon branches near the zone center. Zone-center frequencies agree well with Raman scattering experiments. Due to the slow decay of the interatomic force constants (IFC), a minimal 2 × 4 × 2 supercell (Pnma setting) with 320 atoms is crucial for an accurate determination of the dispersion relations. Smaller supercells result in artificial acoustic phonon softening and unphysical lifting of degeneracies along high symmetry directions. We propose a scheme to investigate the convergence of the IFC with respect to the supercell sizes. The phonon softening can be attributed to the periodic images that affect the accuracy of the force constants, and the truncation of long-ranged forces. The commensuration of the q-vectors with the supercell size is crucial to preserve degeneracies in Sb2S3 crystals. © 2014 the Owner Societies. Source

Chandra R.D.,Nanyang Technological University | Rao M.,Nanyang Technological University | Zhang K.,Nanyang Technological University | Prabhakar R.R.,Nanyang Technological University | And 6 more authors.
ACS Applied Materials and Interfaces

Solution processed zinc tin oxide (ZTO) thin film transistors (TFTs) were fabricated by varying the Zn/Sn composition. The addition of Sn to the zinc oxide (ZnO) films resulted in improved electrical characteristics, with devices of Zn0.7Sn0.3O composition showing the highest mobility of 7.7 cm2/(V s). An improvement in subthreshold swings was also observed, indicative of a reduction of the interfacial trap densities. Mobility studies at low temperature have been carried out, which indicated that the activation energy was reduced with Sn incorporation. Kelvin probe force microscopy was performed on the films to evaluate work function and correlated to the metal-semiconductor barrier indicating Zn0.7Sn0.3O films had the smallest barrier for charge injection. Organic-inorganic hybrid complementary inverters with a maximum gain of 10 were fabricated by integrating ZTO TFTs with poly-3-hexylthiophene (P3HT) transistors. © 2013 American Chemical Society. Source

Solanki A.,Nanyang Technological University | Wu B.,Nanyang Technological University | Wu B.,Singapore Berkeley Research Initiative for Sustainable Energy SinBeRISE | Salim T.,Nanyang Technological University | And 2 more authors.
Physical Chemistry Chemical Physics

The addition of a small amount of high boiling point solvent in organic donor/acceptor blends to control their morphology is a viable approach to enhance the power conversion efficiency of bulk heterojunction (BHJ) organic solar cells. Herein, through transient absorption spectroscopy (TAS) correlated with physical characterizations and device studies, we investigate the effects of a family of thiol-based additives (i.e., 1,5-pentanedithiol (PDT), 1,6-hexanedithiol (HDT) and 1,8-octanedithiol (ODT)) in P3HT:PCBM blend films in a bid to establish a morphology-function-charge dynamics relationship with their photovoltaic performances. The performance of solar cell devices (ηHDT = 2.8%, ηODT = 2.8%, ηPDT = 1.7%) is related to the additive-induced phase separation and the degree of ordering of P3HT. TAS uncovers a more efficient initial exciton and polaron generation in the additive-treated blend samples compared to the non-additive treated control sample. HDT and ODT-added blends exhibit decay dynamics and performances similar to those of the thermally annealed samples. However, the PDT-added blend exhibits a strong trap-assisted recombination in the subsequent nanosecond-microsecond timescales. We attribute this to the loss of charge carriers in the larger isolated P3HT domains due to the lack of percolation paths to the electrode. Our findings illustrate that understanding the complex interplay of the crystalline order, intermixed phases and percolation pathways is key to optimizing the performance of thermal-annealing free, additive-treated organic solar cells. This journal is © the Owner Societies. Source

Li M.,Nanyang Technological University | Xing G.,Nanyang Technological University | Xing G.,Singapore Berkeley Research Initiative for Sustainable Energy SinBeRISE | Xing G.,University of New South Wales | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics

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. © 2013 American Physical Society. Source

Solanki A.,Nanyang Technological University | Wu B.,Nanyang Technological University | Salim T.,Nanyang Technological University | Yeow E.K.L.,Nanyang Technological University | And 3 more authors.
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. C 2012, 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. © 2014 American Chemical Society. Source

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