Light-trapping enhancement in thin film solar cells with photonic crystals

Photovoltaics (or solar cell) has been an active area for research and development, driven by the world\u27s constantly increasing needs for power. Among the current solar technologies, thin film solar cell promises lower cost, but at the expense of lower power conversion efficiency. The efficiency of thin film solar cell can be improved using light confinement schemes commonly referred to as light-trapping. In this thesis, we develop novel light-trapping schemes utilizing photonic crystals (PCs). The optical modeling is performed with a rigorous scattering matrix approach, where Maxwell\u27s equations are solved in Fourier space, and simulations are carried out on parallel computation environment. Although the concepts apply to any thin film solar cell structures, hydrogenated amorphous silicon (a-Si:H) single junction thin film solar cell is used for simulation due to widely available optical property data.;In the solar cell structure we design, a one dimensional (1D) photonic crystal or distributed Bragg reflector (DBR) is used as back reflector. The DBR consists of alternating layers of SiO2 and Si or Indium Tin Oxide (ITO) and Si to provide high reflectivity with little loss. A layer of two dimensional (2D) photonic crystal slab between the a-Si:H absorber layer and the DBR can diffract light at oblique angles, so that total internal reflection can occur inside the absorber layer. The light path length inside the absorber layer will be greatly increased, so will the absorption. The parameters for photonic crystals are optimized through computer simulations to obtain the maximum absorption and path length enhancement. The simulations show significantly enhanced photon harvesting between 600--775 nm below the band edge. The path length enhancement can reach several hundred at resonant peaks, far exceeding the classical limit predicted for randomly roughened scattering surfaces

Person Search with Natural Language Description

Searching persons in large-scale image databases with the query of natural language description has important applications in video surveillance. Existing methods mainly focused on searching persons with image-based or attribute-based queries, which have major limitations for a practical usage. In this paper, we study the problem of person search with natural language description. Given the textual description of a person, the algorithm of the person search is required to rank all the samples in the person database then retrieve the most relevant sample corresponding to the queried description. Since there is no person dataset or benchmark with textual description available, we collect a large-scale person description dataset with detailed natural language annotations and person samples from various sources, termed as CUHK Person Description Dataset (CUHK-PEDES). A wide range of possible models and baselines have been evaluated and compared on the person search benchmark. An Recurrent Neural Network with Gated Neural Attention mechanism (GNA-RNN) is proposed to establish the state-of-the art performance on person search

Experimental investigation of non-linear constitutive behavior of PZT piezoceramics

Abstract Experimental Investigation of Non-linear Constitutive Behavior of PZT Piezoceramics The uni-axial non-linear large signal behavior of PIC 151 soft PZT ceramics was experimentally investigated under a pure electric field, a pure compressive stress and combined electromechanical loading conditions, respectively. Polarisation and strain vs. E field hysteresis loops were observed under a pure cyclic electric field load. The corresponding strain vs. polarisation (S P) curves also exhibited a significant hysteresis. The material response was found to depend on the loading rate and amplitude of the applied E field. A higher loading rate resulted in a smaller coercive field. Ageing effects caused the remnant polarisation and strain to decrease with time after removing the E field load. As subjected to a pure compressive stress load, the material exhibited non-linear stress-strain behavior. In addition, a non-linear depolarisation curve was observed for the pre-poled specimen. Permanent changes of polarisation and strain induced by the mechanical load could be brought back to their initial values by a subsequent application of an electric field to repolarise the material. Loading rate dependence was also found in the non-linear stress - strain behavior. When being subjected to a constant load, this material exhibited significant time-dependent effects. Polarisation and strain exhibited creep-like behavior with the passage of the external load hold time. Most pronounced time-dependent effects were observed as the load was close to the coercive field or the coercive stress. Polarisation and strain versus electric field hysteresis loops were measured under various levels of a preload compressive stress. It turned out that the superimposed compression load reduced the remnant polarisation, decreased the coercive field and also had a significant impact on the dielectric and piezoelectric properties. High field dielectric permittivity and piezoelectric coefficients were found to be enhanced by the compressive preload within a small range. The improved performance was accompanied by an unfavorable larger hysteresis, which was attributed to larger extrinsic contribution due to more non-180° domain switching induced by the prestress. The effects of a bias electric field on the non-linear stress strain and stress depolarisation response were also studied. The non-linear curves were effectively closed upon application of an electric field parallel to the pre-poling direction. Larger stresses were needed to initiate and forward the ferroelastic domain switching. The inverse of this trend occurred when the specimen was subjected to a bias electric field anti-parallel to the pre-poling direction. Zusammenfassung Experimentelle Untersuchung des nichtlinearen konstitutiven Verhaltens von PZT Piezokeramiken Das uniaxiale nichtlineare Großsignalverhalten der weichen PZT Keramik PIC 151 wurde unter Belastung mit einem elektrischen Feld, unter Druckspannungsbelastung und unter kombinierter elektromechanischer Belastung experimentell untersucht. Unter zyklischer Belastung mit einem elektrischen Feld wurden bei der Polarisation und der Dehnung in Abhängigkeit von der elektrischen Feldstärke Hystereseschleifen beobachtet. Ebenso trat bei der Dehnung in Abhängigkeit von der Polarisation (S P) eine deutliche Hysterese auf. Es hat sich herausgestellt, dass das Materialverhalten sowohl von der Belastungsgeschwindigkeit als auch von der Amplitude des elektrischen Feldes abhängt. Eine höhere Belastungsgeschwindigkeit führte zu einer kleineren Koerzitivfeldstärke. Alterungseffekte führten mit der Zeit zu einer Abnahme der remanenten Polarisation und der Dehnung nach der Belastung mit einem elektrischen Feld. Unter reiner Druckspannungsbelastung zeigte das Material ein nichtlineares Spannungs-Dehnungsverhalten. Außerdem wurde bei vorpolarisierten Proben eine nichtlineare Depolarisationskurve beobachtet. Permanente Änderungen in der Polarisation und Dehnung, die durch die mechanische Belastung verursacht wurden, konnten durch ein nachfolgendes Anlegen eines elektrischen Feldes, mit dem das Material wieder polarisiert wurde, rückgängig gemacht werden. Beim nichtlinearen Spannungs-Dehnungverhalten stellte sich auch eine Abhängigkeit von der Belastungsgeschwindigkeit heraus. War das Material einer konstanten Belastung unterworfen, traten zeitabhängige Effekte deutlich in Erscheinung. Solange die äußere Belastung aufrechterhalten war, trat bei der Polarisation und der Dehnung ein Kriechverhalten auf. Am ausgeprägtesten waren diese zeitabhängigen Effekte bei einer Belastung nahe der Koerzitivfeldstärke und der Koerzitivspannung. Für unterschiedliche Druckvorspannungen wurden Hystereseschleifen für die Polarisation und die Dehnung in Abhängigkeit von der elektrischen Feldstärke gemessen. Es stellte sich heraus, dass die überlagerte Druckbelastung sowohl die remanente Polarisation als auch die Koerzitivfeldstärke verminderte und einen deutlichen Einfluss auf die dielektrischen und piezoelektrischen Eigenschaften des Materials ausübte. Es wurde festgestellt, dass bei hohen Feldstärken die dielektrische Permittivität und die piezoelektrischen Koeffizienten durch die Druckvorspannung in einem kleinen Bereich erhöht werden. Diese vorteilhaften Eigenschaften waren allerdings begleitet von unerwünscht großen Hysteresen, die einem größeren extrinsischen Beitrag von nicht 180° Umklappprozessen der Domänen aufgrund der Vorspannung zugeschrieben wurden. Der Einfluss einer anliegenden konstanten elektrischen Feldstärke auf das nichtlineare Spannungs-Dehnungsverhalten bzw. Spannungs-Depolarisationsveralten wurde ebenso untersucht. Durch Anlegen eines elektrischen Feldes in Richtung der Vorpolarisation konnten die nichtlinearen Kurven praktisch geschlossen werden. Größere Spannungen waren notwendig, um das ferroelastische Umklappen der Domänen auszulösen und voranzutreiben. Das umgekehrte Verhalten trat in Erscheinung, wenn an die Probe eine ansteigende elektrische Feldstärke entgegen zur Vorpolarisation angelegt wurde

TMac: Temporal Multi-Modal Graph Learning for Acoustic Event Classification

Audiovisual data is everywhere in this digital age, which raises higher requirements for the deep learning models developed on them. To well handle the information of the multi-modal data is the key to a better audiovisual modal. We observe that these audiovisual data naturally have temporal attributes, such as the time information for each frame in the video. More concretely, such data is inherently multi-modal according to both audio and visual cues, which proceed in a strict chronological order. It indicates that temporal information is important in multi-modal acoustic event modeling for both intra- and inter-modal. However, existing methods deal with each modal feature independently and simply fuse them together, which neglects the mining of temporal relation and thus leads to sub-optimal performance. With this motivation, we propose a Temporal Multi-modal graph learning method for Acoustic event Classification, called TMac, by modeling such temporal information via graph learning techniques. In particular, we construct a temporal graph for each acoustic event, dividing its audio data and video data into multiple segments. Each segment can be considered as a node, and the temporal relationships between nodes can be considered as timestamps on their edges. In this case, we can smoothly capture the dynamic information in intra-modal and inter-modal. Several experiments are conducted to demonstrate TMac outperforms other SOTA models in performance. Our code is available at https://github.com/MGitHubL/TMac.Comment: This work has been accepted by ACM MM 2023 for publicatio

Electric field and temperature scaling of polarization reversal in silicon doped hafnium oxide ferroelectric thin films

HfO2-based binary lead-free ferroelectrics show promising properties for non-volatile memory applications, providing that their polarization reversal behavior is fully understood. In this work, temperature-dependent polarization hysteresis measured over a wide applied field range has been investigated for Si-doped HfO2 ferroelectric thin films. Our study indicates that in the low and medium electric field regimes (E < twofold coercive field, 2E(c)), the reversal process is dominated by the thermal activation on domain wall motion and domain nucleation; while in the high-field regime (E > 2E(c)), a non-equilibrium nucleation-limited-switching mechanism dominates the reversal process. The optimum field for ferroelectric random access memory (FeRAM) applications was determined to be around 2.0 MV/cm, which translates into a 2.0 V potential applied across the 10 nm thick films

Acoustic control system in medicine

This article describes a device, which allows to control various elements using sound signals of different tones remotely. Its possible application are also described in various control systems