Tze Chien Sum

Low-temperature solution-processed wavelength-tunable perovskites for lasing

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing.

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Advancements in perovskite solar cells : photophysics behind the photovoltaics

Solution-processed organic–inorganic perovskite solar cells are hailed as the recent major breakthrough in low-cost photovoltaics. Power conversion efficiencies approaching those of crystalline Si solar cells (exceeding 15%) have been reported. Remarkably, such phenomenal performances were achieved in a matter of 5 years – up from ∼3.8% back in 2009. Since then, the field has expanded exponentially. In this perspective, we review the basic working mechanisms of perovskite solar cells in relation to their intrinsic properties and fundamental photophysics. The current state-of-the-art and the open questions in this maturing field are also highlighted.

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Room-Temperature Near-Infrared High-Q Perovskite Whispering-Gallery Planar Nanolasers

Near-infrared (NIR) solid-state micro/nanolasers are important building blocks for true integration of optoelectronic circuitry. Although significant progress has been made in III-V nanowire lasers with achieving NIR lasing at room temperature, challenges remain including low quantum efficiencies and high Auger losses. Importantly, the obstacles toward integrating one-dimensional nanowires on the planar ubiquitous Si platform need to be effectively tackled. Here we demonstrate a new family of planar room-temperature NIR nanolasers based on organic-inorganic perovskite CH3NH3PbI(3-a)X(a) (X = I, Br, Cl) nanoplatelets. Their large exciton binding energies, long diffusion lengths, and naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold optically pumped in-plane lasing. We show that these remarkable wavelength tunable whispering-gallery nanolasers can be easily integrated onto conductive platforms (Si, Au, indium tin oxide, and so forth). Our findings open up a new class of wavelength tunable planar nanomaterials potentially suitable for on-chip integration.

Correlated d 0 ferromagnetism and photoluminescence in undoped ZnO nanowires

We report the correlated d(0) ferromagnetism and photoluminescence in undoped single-crystalline ZnO nanowires synthesized by using a vapor transport method. We systematically tune the oxygen deficiency in the ZnO nanowires from 4% to 20% by adjusting the growth conditions, i.e., selecting different catalyst (Au or Ag) and varying the growth temperature. Our study suggests that oxygen vacancies induce characteristic photoluminescence and significantly boost the room-temperature ferromagnetism. Such undoped ZnO nanowires with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic nanodevices.

Surface plasmon enhanced band edge luminescence of ZnO nanorods by capping Au nanoparticles

The author observe sixfold enhancement in the near band gap emission of ZnO nanorods by employing surface plasmon of Au nanoparticles, while the defect-related emission is completely suppressed. Time-resolved photoluminescence indicates that the decay process becomes much faster by Au capping. The remarkable enhancement of the ultraviolet emission intensities and transition rates is ascribed to the charge transfer and efficient coupling between ZnO nanorods and Au surface plasmons. The suppression of the green emission might be due to a combined effect of Au surface plasmon and passivation of the ZnO nanorod surface traps.

The Physics of ultrafast saturable absorption in graphene

The ultrafast saturable absorption in graphene is experimentally and theoretically investigated in the femtosecond (fs) time regime. This phenomenon is well-modeled with valence band depletion, conduction band filling and ultrafast intraband carrier thermalization. The latter is dominated by intraband carrier-carrier scattering with a scattering time of 8 ( +/- 3) fs, which is far beyond the time resolution of other ultrafast techniques with hundred fs laser pulses. Our results strongly suggest that graphene is an excellent atomic layer saturable absorber.

Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers

Semiconductor nanowires have received considerable attention in the past decade driven by both unprecedented physics derived from the quantum size effect and strong isotropy and advanced applications as potential building blocks for nanoscale electronics and optoelectronic devices. Recently, organic-inorganic hybrid perovskites have been shown to exhibit high optical absorption coefficient, optimal direct band gap, and long electron/hole diffusion lengths, leading to high-performance photovoltaic devices. Herein, we present the vapor phase synthesis free-standing CH3NH3PbI3, CH3NH3PbBr3, and CH3NH3PbIxCl3(-x) perovskite nanowires with high crystallinity. These rectangular cross-sectional perovskite nanowires have good optical properties and long electron hole diffusion length, which ensure adequate gain and efficient optical feedback. Indeed, we have demonstrated optical-pumped room-temperature CH3NH3PbI3 nanowire lasers with near-infrared wavelength of 777 nm, low threshold of 11 μJ/cm(2), and a quality factor as high as 405. Our research advocates the promise of optoelectronic devices based on organic-inorganic perovskite nanowires.

A room temperature low-threshold ultraviolet plasmonic nanolaser

Constrained by large ohmic and radiation losses, plasmonic nanolasers operated at visible regime are usually achieved either with a high threshold (10(2)-10(4) MW cm(-2)) or at cryogenic temperatures (4-120 K). Particularly, the bending-back effect of surface plasmon (SP) dispersion at high energy makes the SP lasing below 450 nm more challenging. Here we demonstrate the first strong room temperature ultraviolet (~370 nm) SP polariton laser with an extremely low threshold (~3.5 MW cm(-2)). We find that a closed-contact planar semiconductor-insulator-metal interface greatly lessens the scattering loss, and more importantly, efficiently promotes the exciton-SP energy transfer thus furnishes adequate optical gain to compensate the loss. An excitation polarization-dependent lasing action is observed and interpreted with a microscopic energy-transfer process from excitons to SPs. Our work advances the fundamental understanding of hybrid plasmonic waveguide laser and provides a solution of realizing room temperature UV nanolasers for biological applications and information technologies.