Showing papers by "Yeng Ming Lam published in 2018"

Efficient Room-Temperature Phosphorescence from Organic-Inorganic Hybrid Perovskites by Molecular Engineering.

Abstract: Solution-processed organic-inorganic hybrid perovskites are promising emitters for next-generation optoelectronic devices. Multiple-colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room-temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room-temperature phosphorescence is activated by designing a mixed-cation perovskite system to suppress nonradiative recombination. Multiple-colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room-temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.

Ultrafast spin‐to‐charge conversion at the surface of topological insulator thin films

Abstract: Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all-optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi2 Se3 /ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi2 Se3 or Co, a giant terahertz emission is observed in the heterostructure that originates from spin-to-charge conversion, in which the topological surface states play a crucial role. A 0.12 ps timescale is identified that sets a technological speed limit of spin-to-charge conversion processes in topological insulators. In addition, it is shown that the spin-to-charge conversion efficiency is temperature independent in Bi2 Se3 as expected from the nature of the surface states, paving the way for designing next-generation high-speed optospintronic devices based on topological insulators at room temperature.

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 multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (${\mathrm{PEA})}_{2}{\mathrm{PbI}}_{4}$ (PEA = phenylethylammonium). We determine the binding energy of biexcitons---correlated two-electron, two-hole quasiparticles---to be $44\ifmmode\pm\else\textpm\fi{}5$ meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by $\ensuremath{\sim}25%$ upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.

Acene-based organic semiconductors for organic light-emitting diodes and perovskite solar cells

Abstract: In this work, three novel acene-based organic semiconductors, including 2,7-bis(trimethylstannyl)naphtho[2,1-b:6,5-b′]dithiophene (TPA-NADT-TPA), 4,4′-(anthracene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-ANR-TPA) and N2,N2,N6,N6-tetrakis(4-methoxyphenyl)anthracene-2,6-diamine (DPA-ANR-DPA), are designed and synthesized for use in organic light-emitting diodes (OLEDs) and perovskite solar cells (PSCs). In OLEDs, devices based on TPA-NADT-TPA, TPA-ANR-TPA and DPA-ANR-DPA showed pure blue, blue green, and green emission, respectively. Also, the maximum brightness of the devices with a turn-on voltage of 3.8 V reached 8682 cd m−2 for TPA-NADT-TPA, 11 180 cd m−2 for TPA-ANR-TPA, and 18 600 cd m−2 for DPA-ANR-DPA. These new materials are also employed as hole transporting materials (HTMs) in inverted PSCs, where they were used without additives. The inverted devices based on these HTMs achieved an overall efficiency of 10.27% for TPA-NADT-TPA, 7.54% for TPA-ANR-TPA, and 6.05% for DPA-ANR-DPA under identical conditions (AM 1.5G and 100 mW cm−2). While the PSCs with TPA-NADT-TPA as the HTM achieved the highest efficiency, the DPA-ANR-DPA-based OLED devices showed the brightest emission and efficiency. Based on the obtained promising performance, it is clear that this molecular design presents a new research strategy to develop materials that can be used in multiple types of devices.

Molecular engineering of two-dimensional hybrid perovskites with broadband emission for white light-emitting diodes

Abstract: Two-dimensional (2D) hybrid perovskites exhibiting broadband light emission are attractive as down-converting phosphors in white light-emitting diodes (WLEDs). Despite active exploration of new members of this family of materials, fine-tuning of their emission through structural variation for realizing high color-rendering white light remains largely untapped. Here we report a series of (100)-oriented 2D perovskites whose structures are templated by the organic cations. By controlling the tilting of the inorganic octahedra, we were able to shift the broadband emission from blue to white. A photophysical study further suggests that the coexistence of self-trapped excitons and free excitons contributes to a double-peak broad emission, covering the entire visible spectrum. Using the broad-emitting perovskites as down-converting phosphors, we fabricated WLEDs with white-light emission having a correlated color temperature (CCT) of 6600 K and a high color rendering index (CRI, Ra) of 86.

Structure-controlled optical thermoresponse in Ruddlesden-Popper layered perovskites

Abstract: Ruddlesden-Popper perovskites are highly attractive for light-emitting and photonic applications. In these exceptionally deformable frameworks, structural properties strongly impact on the energetic landscape of the material; thus, it is crucial to establish a correlation between the structure and optoelectronic characteristics. Here, we study the structural transformations induced by phase transitions in the butylammonium-based series (BA)2(MA)n−1[PbnI3n+1] (n = 1 and n = 2). We show how thermally driven lattice contraction and changes in crystal packing affect their characteristic absorption and photoluminescence. These findings provide new insights for functional perovskites’ rational design, highlighting the possibility to tune the structural properties through external stimuli to control their functionalities on-demand.Ruddlesden-Popper perovskites are highly attractive for light-emitting and photonic applications. In these exceptionally deformable frameworks, structural properties strongly impact on the energetic landscape of the material; thus, it is crucial to establish a correlation between the structure and optoelectronic characteristics. Here, we study the structural transformations induced by phase transitions in the butylammonium-based series (BA)2(MA)n−1[PbnI3n+1] (n = 1 and n = 2). We show how thermally driven lattice contraction and changes in crystal packing affect their characteristic absorption and photoluminescence. These findings provide new insights for functional perovskites’ rational design, highlighting the possibility to tune the structural properties through external stimuli to control their functionalities on-demand.

Importance of Electronic Correlations and Unusual Excitonic Effects in Formamidinium Lead Halide Perovskites

Abstract: Perovskite compounds show promise for charge generation in solar cells because of their high density of excitons when exposed to light. New experiments reveal the origin of this behavior, which could point the way to more efficient optoelectronic devices.

Elucidating the effect of additives on the alkyl chain packing of a double tail cationic surfactant

Abstract: Hypothesis Some low molecular weight additives can strongly influence the phase behavior of aqueous surfactant systems, and this offers an important handle to control the properties of surfactant solutions and thus to optimize the stability and performance of various formulations. Experiments The surfactant dioctadecyldimethylammonium chloride (DODAC) self-assembles into two lamellar phases in water, the gel phase (Lβ) and the liquid crystalline phase (Lα). Here, we present approaches to tune the gel-to-liquid crystalline transition temperature (Tm) with the use of additives. The effects of urea, sodium butyrate and butyric acid on the packing behavior of DODAC were determined. The surfactant phases were characterized using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and small/wide angle X-ray scattering (SWAXS). Findings All three additives - urea, sodium butyrate and butyric acid yield a single and stable lamellar phase. Urea and sodium butyrate have only minor effects on Tm, butyric acid gives a large decrease as it stabilizes the Lα phase with respect to the Lβ phase. From the bilayer thickness of the gel phase an interdigitated or tilted packing of the surfactant molecules is suggested. The addition of sodium butyrate gives a highly interdigitated gel structure and resulted in the transition from lamellar liquid crystal to an isotropic L3 phase.