Showing papers by "Yeng Ming Lam published in 2017"

Stable biexcitons in two-dimensional metal-halide perovskites with strong dynamic lattice disorder

Abstract: With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors --- chemical, electronic and structural --- that govern strong multi-exciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA)$_2$PbI$_4$ (PEA = phenylethylammonium). We determine the binding energy of biexcitons --- correlated two-electron, two-hole quasiparticles --- to be $44 \pm 5$\,meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by $\sim25$\% upon cooling to 5\,K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.

Thienylvinylenethienyl and naphthalene core substituted with triphenylamines-highly efficient hole transporting materials and their comparative study for inverted perovskite solar cells

Abstract: In this report, two simple cost efficient solution processable small molecular hole transporting materials (HTMs) are synthesized and used successfully in inverted perovskite devices. These HTMs, namely (E)-4,4'-(ethene-1,2-diylbis(thiophene-5,2-diyl))bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-TVT-TPA) and 4,4'-(naphthalene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-NAP-TPA), are designed by using triphenylamine methoxy as common end capping groups with thienylvinylenethienyl and naphthalene cores respectively. They possess good solubility in common organic solvents. Additionally, they have not only appropriate highest occupied molecular orbital energy levels for good hole injection ability but also sufficient lowest unoccupied molecular orbital for electron blocking capability. The power conversion efficiency (PCE) of these HTMs based devices is found to be of 16.32% for TPA-TVT-TPA and 14.63% for TPA-NAP-TPA. Particularly, TPA-TVT-TPA exhibits an impressive Voc of 1.07 V. The obtained performance is one of the highest performances in inverted perovskite layouts. The cut-price and straightforward synthesis with elegant scale up makes these classes of materials important for the industry to produce high-throughput printed perovskite solar cells for large area applications.

Low-frequency optical phonon modes and carrier mobility in the halide perovskite CH3NH3PbBr3 using terahertz time-domain spectroscopy

Abstract: As a light absorber in photovoltaic applications, hybrid organic-inorganic halide perovskites should have long and balanced diffusion lengths for both the separated electrons and holes before recombination, which necessitates high carrier mobility. In polar semiconductors, the room-temperature carrier mobility is often limited by the scattering between carriers and the lowest-frequency optical phonon modes. Using terahertz time-domain spectroscopy, we examine the temperature evolution of these phonon modes in CH_3NH_3PbBr_3 and obtained high carrier mobility values using Feynman's polaron theory. This method allows us to estimate the upper limit of carrier mobilities without the need to create photogenerated free carriers, and can be applied to other heteropolar semiconductor systems with large polarons.

Facile in situ synthesis of stable luminescent organic–inorganic lead halide perovskite nanoparticles in a polymer matrix

Abstract: Organic–inorganic halide perovskites have attracted a lot of interest for applications such as light-emitting diodes (LEDs), photodetectors and lasers. As this group of materials is highly sensitive to moisture and oxygen, a simple in situ controlled synthesis of these nanoparticles (NPs) in a polymer matrix will open up the possibility for a wide range of light-emitting and color-conversion applications. This paper presents a simple in situ approach for controlled synthesis of organic–inorganic lead halide perovskite NPs as a function of halide composition in polymer matrix (polymethyl methacrylate, PMMA). Chemical mapping using scanning transmission electron microscopy (STEM) shows a varying of size distribution of the lead halide perovskite NPs. Mixed halide NP in PMMA were composed of grains of different compositions. Time-resolved photoluminescence (TRPL) correlated with compositional information from energy dispersive X-ray spectroscopy (EDX) provides evidence for the role of surfaces and grain boundaries on charge recombination in the nanoparticles. The annealing temperature and the molar ratio of the precursors were found to greatly affect the morphology and composition of the perovskite NP in polymer matrix.

Reflective perovskite solar cells for efficient tandem applications

Abstract: Tandem solar cells combining a wide bandgap, efficient perovskite absorber with a low bandgap photovoltaic module, such as a c-Si cell, can potentially achieve a high theoretical efficiency of over 30%. Instead of using the conventional parallel stacking tandem, we report here a reflective tandem configuration, with the perovskite solar cell acting as the spectral filter that absorbs high energy photons, while sub-bandgap photons are reflected to a Si sub-cell using a highly reflective back metal electrode. The perovskite solar cell exhibits a high reflectance of over 60% in the near infrared spectral region, which allows the subsequent silicon cell to absorb photons in this region, resulting in a high current density of 13.03 mA cm−2. In such a tandem configuration, we achieved a combined efficiency of 23.1% using a four-terminal measurement. This result demonstrates the promise of employing perovskite solar cells in a reflective tandem for a high efficiency solar energy conversion system, with an efficiency of up to 30%.

Phonon features in terahertz photoconductivity spectra due to data analysis artifact: A case study on organometallic halide perovskites

Abstract: We propose a simple scenario where the superimposed phonon modes on the photoconductive spectra are experimental artifacts due to the invalid formula used in data analysis. By use of experimental and simulated data of CH_3NH_3PbI_3 perovskites as a case study, we demonstrate that a correction term must be included in the approximated thin-film formula used in the literature; otherwise, parts of the spectra with high background permittivity near the phonon-mode resonances might interfere with the transient photoconductivity. The implication of this work is not limited to perovskites but other materials with strong vibrational modes within the THz spectral range.

Enhanced Efficiency of Dye-Sensitized Solar Cells with Mesoporous-Macroporous TiO 2 Photoanode Obtained Using ZnO Template

Abstract: Improved light harvesting efficiency can be achieved by enhancing the optical properties of the titanium dioxide (TiO2) photoanode in dye-sensitized solar cells (DSSCs), leading to higher power conversion efficiency. By incorporating submicrometer cavities in TiO2 mesoporous film, using zinc oxide (ZnO) particles as a template, a bimodal pore size structure has been created, called a mesoporous–macroporous nanostructure. This photoanode structure consists of 20-nm TiO2 nanoparticles with two kinds of pores with size of 20 nm (mesopores) and 500 nm (macropores). Energy-dispersive x-ray spectroscopy and x-ray diffraction studies showed no trace of ZnO in the TiO2 after removal by TiCl4 treatment. Higher diffuse transmittance of this film compared with the standard transparent photoanode provides evidence of improved light scattering. When employed in a device, the incident-photon-to-current efficiency of ZnO-assisted devices showed enhancement at longer wavelengths, corresponding to the Mie light scattering effect with the macropores as scattering centers. This resulted in overall higher power conversion efficiency of the DSSC. In this work, a nonvolatile gel ionic liquid was used as the electrolyte to also demonstrate the benefit of this structure in combination with a viscous electrolyte and its promising application to prolong the stability of DSSCs.

Influence of size and shape of sub-micrometer light scattering centers in ZnO-assisted TiO2 photoanode for dye-sensitized solar cells

Abstract: Sub-micrometer cavities have been incorporated in the TiO2 photoanode of dye-sensitized solar cell to enhance its optical property with light scattering effect. These are large pores of several hundred nanometers in size and scatter incident light due to the difference refraction index between the scattering center and the surrounding materials, according to Mie theory. The pores are created using polystyrene (PS) or zinc oxide (ZnO) templates reported previously which resulted in ellipsoidal and spherical shapes, respectively. The effect of size and shape of scattering center was modeled using a numerical analysis finite-difference time-domain (FDTD). The scattering cross-section was not affected significantly with different shapes if the total displacement volume of the scattering center is comparable. Experiments were carried out to evaluate the optical property with varying size of ZnO templates. Photovoltaic effect of dye-sensitized solar cells made from these ZnO-assisted films were investigated with incident-photon-to-current efficiency to understand the effect of scattering center size on the enhancement of absorption. With 380 nm macropores incorporated, the power conversion efficiency has increased by 11% mostly thanks to the improved current density, while 170 nm and 500 nm macropores samples did not have increment in sufficiently wide range of absorbing wavelengths.

Unique Reversible Crystal-to-Crystal Phase Transition – Structural and Functional Properties of Fused Ladder Thienoarenes

Abstract: Donor–acceptor type molecules based on fused ladder thienoarenes, indacenodithiophene (IDT), and dithienocyclopenta–thienothiophene (DTCTT), coupled with benzothiadiazole, are prepared, and their solid-state structures are investigated. They display a rich variety of solid phases ranging from amorphous glass states to crystalline states, upon changes in the central aromatic core and side group structures. Most notably, the DTCTT-based derivatives showed reversible crystal-to-crystal phase transitions in heating and cooling cycles. Unlike what has been seen in π-conjugated molecules, variable temperature XRD revealed that structural change occurs continuously during the transition. A columnar self-assembled structure with slip-stacked π–π interaction is proposed to be involved in the solid state. This research provides the evidence of unique structural behavior of the DTCTT-based molecules through the detailed structural analysis. This unique structural transition paves the way for these materials to have ...

Elucidating the relationship between crystallo-chemistry and optical properties of CIGS nanocrystals

Abstract: The performance of solar cells fabricated using Cu(In,Ga)(S,Se)2 nanocrystal (NC) inks synthesized using the hot injection method has yielded efficiencies up to 12% recently. The efficiency of these devices is highly dependent on the chemical composition and crystallographic quality of the NCs. The former has been extensively discussed as it can be easily correlated to the optical properties of the film, but detailed crystallographic structure of these NCs has scarcely been discussed and it can influence both the optical and electrical properties. Hence both chemical composition and crystal structure should be explored for these NCs in order for this material to be further developed for application in thin film solar cells. In this work, a thorough investigation of the composition and crystal structure of CuIn x Ga1−x Se2 NCs synthesized using the hot injection method over the entire composition range (0 ≤ x ≤ 1) has been conducted. Raman spectroscopy of the NCs complements the information derived from x-ray diffraction (XRD) and electron probe microanalysis (EPMA). EPMA, which was carried out for the first time, indicates good controllability of the NC Ga/(In + Ga) ratio using this synthesis method. Raman spectroscopy reveals that CuInSe2 NCs are a mixture of chalcopyrite and sphalerite with disordered cations, whereas CuGaSe2 NCs are purely chalcopyrite. The lattice parameters determined from XRD were found to deviate from those calculated using Vegard's law for all compositions. Hence, it can be deduced that the lattice is distorted in the crystal. The optical and electrochemical band gap of CuIn x Ga1−x Se2 NCs increases as the Ga content increases. The energy band gap deviates from the theoretical values, which could be related to the contribution from cation disordering and strain. These results help to tailor the opto-electrical properties of semiconductors, which inherently depend on the crystalline quality, strain and composition.

Ink composition, method for forming a conductive member, and conductive device

Abstract: According to embodiments of the present invention, an ink composition for forming a conductive film is provided. The ink composition includes a plurality of nanostructures distributed in at least two cross-sectional dimension ranges, wherein each nanostructure of the plurality of nanostructures is free of a cross-sectional dimension of more than 200 nm; preferably each nanostructure of the plurality of nanostructures is encapsulated with a surfactant. In a preferred embodiment, the nanoparticles have bimodal distributions: small particles (5 - 10 nm) and large particles (30 - 100 nm), wherein the small particles are likely to occupy the empty space between the large particles, and therefore helps in densification of the film. According to further embodiments of the present invention, a conductive member is formed by fusing the plurality of nanostructures of the ink composition to each other.