Organic-inorganic halide perovskite has emerged as promising candidate materials for next-generation energy harvesting and light-emitting applications with the advantages of low processing cost, high defects tolerance, and excellent power conversion efficiency. The instability of these perovskite-based materials under illumination, however, remains a major technical barrier for commercialization. Various techniques have been applied to improve the photo-stability of perovskites. Since the dynamic of photo-generated charged carriers and photo-activated mobile ions affect the stable performance of these applications, a comprehensive understanding of how illumination affect perovskites are of vital importance to improve the performance of perovskite-based optoelectronic applications. In this report, the recent progress of the light soak study on three kinds of perovskites is presented, using depth-resolved, temporal-resolved, and detection-wavelength selective spectroscopic imaging techniques. These works clarify different dominate roles in different perovskite structures and demonstrate the advantages of the imaging spectroscopy in studying the carrier dynamics of perovskite-based materials under light soaking, which is of crucial importance for their applications.
Lead halide perovskites are widely applied in not only photovoltaics, but also on-chip light source, nanolaser, and photon detection. In order to promote the incorporation of perovskite into integrated devices, microscale color patterning flexibility is a very important step. Femtosecond (fs) laser fabrication has shown significant advantages of high spatial resolution, low surround damage, and high processing efficiency over the other laser fabrication. Compared to the state-of-art techniques, the straightforward fs-direct laser writing (fs-DLW) also has advantages of mask-free, simple one step, and contactless. Here, a specially designed formamidinium lead mixed-halide nanoplatelet (FAPb(BrxI1-x)3 NP) with gradient bandgap is fabricated by chemical vapor deposition method. Then, spatially resolved modulation of the fluorescence by fs-DLW is demonstrated on the as-grown NP. The fluorescence color is modulated from red to green under a controlled laser pulse, due to the replacement of iodide ions by bromide ions. Specifically, the as-grown NP (thickness≈800 nm) is with a gradual bromide-iodide composition along the depth, mainly exhibits an emission of 710-nm from the bottom iodine rich phase. After halide substitution induced by fs-DLW, new fluorescence peaks appear in the wavelength range of 540 to 700 nm, which is controlled by the fs-DLW conditions. The fluorescent color is spatially modulated from red to green, enabling microscale resolved multicolor emission, implying the potential applications in micro-encryption, sensors, multicolor displays, lasers, and light-emitting devices.
GaN/InGaN multiple quantum wells (MQW) is a promising material for high-efficiency solid-state lighting. Ultrafast optical pump-probe spectroscopy is an important characterization technique for examining fundamental phenomena in semiconductor nanostructure with sub-picosecond resolution. In this study, ultrafast exciton and charge carrier dynamics in GaN/InGaN MQW planar layer and nanorod are investigated using femtosecond transient absorption (TA) techniques at room temperature. Here nanorods are fabricated by etching the GaN/InGaN MQW planar layers using nanosphere lithography and reactive ion etching. Photoluminescence efficiency of the nanorods have been proved to be much higher than that of the planar layers, but the mechanism of the nanorod structure improvement of PL efficiency is not adequately studied. By comparing the TA profile of the GaN/InGaN MQW planar layers and nanorods, the impact of surface states and nanorods lateral confinement in the ultrafast carrier dynamics of GaN/InGaN MQW is revealed. The nanorod sidewall surface states have a strong influence on the InGaN quantum well carrier dynamics. The ultrafast relaxation processes studied in this GaN/InGaN MQW nanostructure is essential for further optimization of device application.
KEYWORDS: Near field optics, Photovoltaics, Solar energy, Solar cells, Near field, Finite-difference time-domain method, Photonic crystals, Active optics, Luminescence, Photovoltaic materials, Absorption, Silicon, Absorbance
Solar spectrum management using up/down conversion is an important method to improve the photovoltaic energy conversion efficiency. It asks for a monochromatic luminescence absorption at the band edge of the photovoltaic device to reduce both the sub-band-gap and over-band-gap energy losses. Here, we demonstrate an energy selective optical contacting concept to improve the luminescence transfer efficiency for spectrum management. By increasing both the luminescence emission and re-absorption ability through photonic resonance, an efficient photon transfer channel could be established between the luminescence emitter and the photovoltaic component in a near-field region. This concept is not only able to compensate the insufficient band edge absorption ability of the photovoltaic device, but also to break the far-field limitation of luminescence radiation. The energy selection on the optical spectrum naturally imposed by the mode resonance is also helpful to improve the monochromaticity of the luminescence yield. In this paper, a photonic crystal cavity is used to realize the optical contacting concept between a thin silicon film and spectrum converter. The optical power and photon flux transferred between different components are calculated analytically using the electromagnetic Green’s function. The corresponding radiative dipole moment is estimated by the fluctuation-dissipation theorem. The example shows an over 80 times enhancement in the luminescence absorbance by the silicon layer, illustrating the great potential of this concept to be applied on nano-structured photovoltaic devices.
GaN/InGaN multiple quantum wells (MQW) and GaN nanorods have been widely studied as a candidate material for high-performance light emitting diodes. In this study, GaN/InGaN MQW on top of GaN nanorods are characterized in nanoscale using confocal microscopy associated with photoluminescence spectroscopy, including steady-state PL, timeresolved PL and fluorescence lifetime imaging (FLIM). Nanorods are fabricated by etching planar GaN/InGaN MQWs on top of a GaN layer on a c-plane sapphire substrate. Photoluminescence efficiency from the GaN/InGaN nanorods is evidently higher than that of the planar structure, indicating the emission improvement. Time-resolved photoluminescence (TRPL) prove that surface defects on GaN nanorod sidewalls have a strong influence on the luminescence property of the GaN/InGaN MWQs. Such surface defects can be eliminated by proper surface passivation. Moreover, densely packed nanorod array and sparsely standing nanorods have been studied for better understanding the individual property and collective effects from adjacent nanorods. The combination of the optical characterization techniques guides optoelectronic materials and device fabrication.
The hot Carrier Solar Cell (HCSC) allows the photon-induced hot carriers (the carriers with energy larger than the band gap) to be collected before they completely thermalise. The absorber of the HCSC should have a large phononic band gap to supress Klemens Decay, which results in a slow carrier cooling speed. In fact, a large phononic band gap likely exists in a binary compound whose constituent elements have a large mass ratio between each other. Binary hydrides with their overwhelming mass ratio of the constituent elements are important absorber candidates. Study on different types of binary hydrides as potential absorber candidates is presented in this paper. Many binary transition metal hydrides have reported theoretical or experimental phonon dispersion charts which show large phononic band gaps. Among these hydrides, the titanium hydride (TiHX) is outstanding because of its low cost, easy fabrication process and is relatively inert to air and water. A TiHX thin film is fabricated by directly hydrogenating an evaporated titanium thin film. Characterisation shows good crystal quality and the hydrogenation process is believed to be successful. Ultrafast transient absorption (TA) spectroscopy is used to study the electron cooling time of TiHX. The result is very noisy due to the low absorption and transmission of the sample. The evolution of the TA curves has been explained by band to band transition using the calculated band structure of TiH2. Though not reliable due to the high noise, decay time fitting at 700nm and 600nm shows a considerably slow carrier cooling speed of the sample.
We report the fabrication of densely packed InGaN/GaN nanorods with high hexagonal periodicity. Nanosphere lithography and reactive ion etching were adopted to fabricate the nanorods from planar multiple quantum wells (MQWs). Compared to the planar MQWs, the nanorods exhibit significant luminescence enhancement. This is mostly attributed to the increased radiative recombination and light extraction efficiency. Both photoluminescence and Raman measurements confirmed in-plane strain relaxation in the MQWs after nanofabrication. A reduction in strain-induced quantum confined Stark effect in the nanorods increased radiative recombination. This work is most crucial to the understanding of optical properties with respect to the carrier transport and recombination in InGaN/GaN nanorods.
Methylammonium Lead Bromide (CH3NH3PbBr3) is a promising material for tandem solar cell due to its high band gap. Ultrafast optical techniques on a time scale of femto- and picosecond are used to investigate the carrier dynamics in CH3NH3PbBr3. An ultrafast cooling of hot carriers occurs in sub-picoseconds in CH3NH3PbBr3 by phonon scattering. Two ultrafast relaxation processes are attributed to optical phonon scattering and acoustic phonon scattering. The relaxation processes are evidently slower when CH3NH3PbBr3 is in contact with compact TiO2 (c-TiO2) layer, suggesting better quality CH3NH3PbBr3. when deposited on c-TiO2. The nanosecond decay in CH3NH3PbBr3 film is ascribed to electron-hole recombination. With the presence of c-TiO2 layer, this process is accelerated due to electron transport across the CH3NH3PbBr3/ c-TiO2 interface.
The excellent light harvesting properties and potentially low cost fabrication of organometal halide perovskites have attracted great attention in their application as solar cell device. Apart from the general advantages of organic-inorganic perovskite, CH3NH3PbBr3 has a larger bandgap (~2.3eV) suitable to be the top cell in a tandem solar device. Here we use steady-state and time-resolved photoluminescence (PL) techniques to investigate the photophysical behaviour of CH3NH3PbBr3 perovskite including its carrier dynamics under continuous illumination. Samples were studied under different illumination conditions and the following observations were made: (1) defect assisted recombination is dominant under low excitation under nano-second scale measurement, (2) bimolecular and Auger recombinations dominate under high excitation under the minute timescale measurement, (3) the magnitude PL decay traces decrease over time under continuous excitation. We propose that both the density of photo-generated free carriers and the density of mobile ions have an impact on the carrier dynamic of CH3NH3PbBr3. This finding provides insights into the photophysical properties of perovskite materials.
Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer promising applications in photonics, photovoltaics and photocatalysis. Herein we study the optical properties and electron dynamics in CNDs using steady state and time-resolved spectroscopy. The photoluminescence (PL) is determined to originate from both core and surface. The massive surface fluorophores result in a broad spectral fluorescence. In addition to various synthesis techniques, it is demonstrated that the PL of CNDs can be extended from the blue to the near infrared by thermal assisted growth. Directional electron transfer was observed as fast as femtosecond in CND-graphene oxide nanocomposites from CND into graphene oxide. These results suggest CNDs can be promising in many applications.
A hot carrier solar cell device that consists of heterogeneous nano-particle arrays has been proposed. It has been demonstrated that such array has good properties both as a light absorber and as a carrier conductor. The photo-generated hot carrier populations can be potentially retained due to the strong acoustic impendence between the component nano-particles, which localizes the lattice vibrational energy. The electronic and phononic properties of the nano-particles have demonstrated the potential of generating a hot carrier population. It has been demonstrated that by modulating the structure of the array, it is possible to maintain a fine electrical conductivity while strongly block the lattice heat conductivity. This helps to minimize the entropy generation during the energy conversion process, providing possibilities of realizing the high-efficiency hot carrier solar cell.
Using time-resolved and time-integrated photoluminescence and spectrally resolved two-colour three-pulse photon
echo spectroscopy we study the quantum confinement and dephasing properties of near spherical Si QDs with an
average size of 4.3 nm. Filling of the low energy states and parabolic confinement of the quantum dot structures can be
inferred from the dependence of the photoluminescence intensity on the detection wavelengths. A dephasing time of 1 -
1.8 ps which is slightly dependent on the quantum dot energy states can be measured. We show that the dephasing time
of the electrons in the quantum dots is strongly influenced by the density of excited carriers.
We report the two photon luminescence (TPL) and second harmonic generation (SHG) characteristics of Zinc Oxide (ZnO) in ceramic, thick film and nano-rod. All samples were prepared from commercially available analytical pure ZnO powder. Sintering, physical vapour deposition (PVD), and hydrothermal methods were used in preparing the three types of samples respectively. The comparison among the three showed, while a degree of similarities between the ceramic and the nano-rod, a significant difference in the film. Possible reasons for the wavelength downshift in the film sample are discussed. Images acquired by TPL and SHG microscopy are presented, both ceramic and film samples show granular structure and a reverse bright-dark contract was observed from TPF to SHG image between the grain region and the granular boundaries.
The average fluence distribution of excimer laser is an important parameter in various applications of excimer laser. But it is more difficulty to measure the parameter due to UV wavelength of excimer laser, very short pulse width, energy undulation among various pulses and or so. Basing on the anisotropy Seeback tensor coefficients, a light-thermo radiation detector made from high Tc superconductor YBa2Cu3O7-(delta ) was fabricated. The novel advantages of this detector are that it functions at room temperature, with very low noise, very fast response and higher sensitivity within a very wide wavelength from ultraviolet to infrared. A measuring system of average fluence distribution of excimer laser is formed with the YBCO light-thermo-detector, a lock-in amplifier and a computer. The principle of the measurement system is analyzed, XeCl laser fluence distribution is measured.
The mechanism of the marking of excimer laser to titanium enamelware is studied in this paper. This mechanism is different from the principle that matter in surface is vaporized by laser and engraved, it based on the principle that matter of surface is melted and then resolidify very quickly so as to change color and other characters. A visible and fine pattern can be gained in this way, and fluence threshold is lower. The option condition is gained from the experiment.
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