On an AlGaInP Light-Emitting Diode With a Modulation-Doped Multiquantum-Well (MD-MQW) Structure

An interesting AlGaInP multiquantum-well (MQW) light-emitting diode (LED) with an n-type modulation-doped (MD) structure, grown by low-pressure metal–organic vapor-phase epitaxy (LP-MOVPE), is fabricated and studied. This n-type MD-MQW LED exhibits lower dynamic resistance, higher luminescence, and higher luminous efficiency than those of a conventional undoped-MQW LED. Experimental results show a higher luminous efficiency of 16.05 lm/W and higher luminescence of 2.53 lm are obtained for the MD-MQW LED which are superior to the corresponding values of 14.49 lm/W and 2.04 lm for the undoped-MQW LED under dc operation. In addition, the n-type MD-MQW LED exhibits a higher quantum efficiency of 7.2% under dc operation as compared with the 6.7% of the undoped-MQW LED. The reduced junction temperature of 12 $~{^circ}$C at 200 mA is also acquired for the MD-MQW LED. Moreover, the brightness reliability of this new device is found to be comparable to the undoped-MQW LED. These positive results could be attributed to the presence of a higher electron concentration in the active region of the MD-MQW structure which causes the suppression of electron thermal velocity especially at a high level injection condition, and a reduced junction heating effect.      

GaAs converters for high power densities of laser illumination

Photovoltaic power converters can be used to generate electricity directly from laser light. In this paper we report the development of GaAs PV power converters with improved conversion efficiency at high power densities. The incorporation of a lateral conduction layer (LCL) on top of the window layer resulted in a considerable gain in efficiency at high illumination levels. Additional performance improvements were obtained by using a metal electrode grid design and antireflection coating optimised for monochromatic and inhomogeneous laser light. Maximum monochromatic (810 nm) optical‐to‐electrical conversion efficiency of 54·9% at 36·5 W/cm² has been achieved. The characteristics of laser power converters with p/n and n/p polarity are discussed in this paper. Moreover, different materials and doping levels were applied in the LCL. The performance of these different device structures at high laser intensity is presented and discussed. It is shown that the lateral series resistance of the cell has a major impact on the overall device performance. Copyright © 2008 John Wiley & Sons, Ltd.     

A Novel Topology for Photovoltaic DC/DC Full-Bridge Converter With Flat Efficiency Under Wide PV Module Voltage and Load Range

In this paper, a novel topology for a photovoltaic (PV) dc/dc converter that can dramatically reduce the power rating and increase the efficiency of a PV system by analyzing PV module characteristics is proposed. Based on the analysis, in the proposed topology, only 30.7% power of the total PV system is needed for a dc/dc converter. Furthermore, the dc/dc converter efficiency curve is flat under wide PV module voltage and all load ranges. In particular, the converter efficiency at the lower duty range is dramatically improved. The total PV system is implemented for a 250-kW PV power conditioning system (PCS). This system has only three dc/dc converters with a 25-kW power rating. It is only one-third of the total PV PCS power. The 25-kW prototype PV dc/dc converter is introduced to experimentally verify the proposed topology. In addition, an experimental result shows that the proposed topology exhibits a good performance.     

On the necessity of joint adoption of both Distributed Maximum Power Point Tracking and Central Maximum Power Point Tracking in PV systems

In this paper, the main causes that are able to limit the efficiency of Distributed Maximum Power Point Tracking (DMPPT) are analyzed in detail. It will be shown that, to get full profit from DMPPT, it is necessary that the bulk inverter voltage belongs to an optimal range whose position and amplitude are functions of the following factors: the number of PV modules and dedicated DC/DC converters in a string, the atmospheric operating conditions characterizing each PV module (irradiance and temperature values), the voltage and current ratings of the physical devices the DC/DC converters are made of, and the adopted DC/DC converter topology. Moreover, it will be given proof of the necessity to couple the DMPPT function with a suitable centralized MPPT function carried out by the inverter through the proper control of its own DC input voltage. Copyright © 2012 John Wiley & Sons, Ltd.     

Annual performance enhancement of building integrated photovoltaic modules by applying phase change materials

The performance of photovoltaic (PV) module outdoors suffers from attained high module temperatures due to irradiation as a result of the negative temperature coefficient of their efficiency. Phase change materials (PCMs) are investigated as an option to regulate photovoltaic module temperature and thereby reduce its electrical efficiency decrease. In this study, a simplified heat balance model is used to calculate the extra energy gain; such a PV/PCM system can bring on an annual basis. With present day commercially available PCM materials, a moderate increase of up to 3% of the total energy output can be expected. When taking into account the additional PCM material cost, a PV/PCM module presently is not economically viable. For an acceptable payback period of 10–20 years to be reached, the heat storage capacity of a PCM would require an increase of about one order of magnitude, which is presently not realistic. Nevertheless, a combination with building climate control in which the PCM plays a double role controlling both the PV temperature as well as the inside climate temperature may be feasible. Copyright © 2011 John Wiley & Sons, Ltd.     

Analytical study and evaluation results of power optimizers for distributed power conditioning in photovoltaic arrays

The use of modular or ‘micro’ maximum power point tracking (MPPT) converters at module level in series association, commercially known as “power optimizers”, allows the individual adaptation of each panel to the load, solving part of the problems related to partial shadows and different tilt and/or orientation angles of the photovoltaic (PV) modules. This is particularly relevant in building integrated PV systems. This paper presents useful behavioural analytical studies of cascade MPPT converters and evaluation test results of a prototype developed under a Spanish national research project. On the one hand, this work focuses on the development of new useful expressions which can be used to identify the behaviour of individual MPPT converters applied to each module and connected in series, in a typical grid‐connected PV system. On the other hand, a novel characterization method of MPPT converters is developed, and experimental results of the prototype are obtained: when individual partial shading is applied, and they are connected in a typical grid connected PV array. Copyright © 2011 John Wiley & Sons, Ltd.     

大尺寸LED背光源散热结构的仿真、设计与测试

过高的工作温度会直接降低LED使用寿命,并且会影响其发光强度以及发光效率,导致大尺寸LED背光源光学性能大幅下降。从侧边式LED背光源模组的角度出发,设计并采用IcePak软件仿真散热铝挤的宽度和厚度对背光模组温度的影响,最终给出了适用于大尺寸LED背光模组的散热结构。结果表明,随着铝挤的长度或厚度的增加,LED Bar的温度都会减少,取铝挤长度为200 mm,厚度为2 mm,此时LED Bar的温度为69.4℃,小于70℃,符合设计要求且成本最低。进一步测量对应样机的LED Bar温度,其最高温度为69.7℃,与仿真实验结果非常接近。该结构制造工艺较为简单、成本低廉,并且符合背光模组轻薄化的要求,具有一定的市场价值。     

Schottky diode design for low-voltage rectification

Barrier height and impurity concentration of a power Schottky diode are optimized for maximum rectifying efficiency in DC-DC converter operation. An optimum barrier-height-impurity-concentration combination is calculated for a given output voltage and diode temperature. For a 1.5 – 2 V output converter, the optimum combination is found to be 17 kT/q and 1.5 × 10¹⁶ cm^?3. Based on the theoretical prediction, titanium- and hafnium-barrier diodes were fabricated as suitable diodes for low-voltage converters and compared with conventionally used molybdenum-barrier diodes. In the experiment on a 2-V output DC-DC converter, the new diodes show higher efficiency than molybdenum diodes at up to 85°C. They are fit for use in encapsulated converters because of their smaller heat generation.     

Temperature stability of contact systems for GaSb-based photovoltaic converters

The thermally induced degradation (temperature T ≈ 200°C) of Cr-Au, Cr-Au-Ag-Au, Ti-Pt-Au, and Ti-Pt-Ag contact systems deposited onto the surface of p-GaSb by magnetron sputtering and resistive evaporation are studied. It is found that photovoltaic (PV) converters with a contact grid based on Ti-Pt-Ag are characterized by the maximum thermal stability. PV cells with gold-containing contacts based on Cr-Au and Ti-Pt-Au exhibit a high rate of degradation with increasing temperature, which may require a more effective heat removal system for their operation.     

固态照明用蓝光LED和LD辐射量子效率与电流密度的比较研究

比较了蓝光LED和LD的辐射量子效率随电流密度变化的趋势.LED在低电流密度时具有很高的辐射量子效率,并且在电流密度为24.4 A/cm2时达到了峰值效率,随着电流密度的增大,其量子效率会急剧下降;LD在电流密度低于出光阈值时辐射量子效率为零,待达到出光阈值后迅速上升并在电流密度为4.85×103 A/cm2时超越LED.分析表明,LED效率骤减是由于自身的发光机理限制了辐射速率的提升;LD受激辐射发光机理恰好弥补了LED的不足,其辐射量子效率受非辐射复合速率的影响较小.结果表明,LD具有高电流密度下高辐射量子效率的特性.     

Multilevel Inverter Topologies for Stand-Alone PV Systems

This paper shows that versatile stand-alone photovoltaic (PV) systems still demand on at least one battery inverter with improved characteristics of robustness and efficiency, which can be achieved using multilevel topologies. A compilation of the most common topologies of multilevel converters is presented, and it shows which ones are best suitable to implement inverters for stand-alone applications in the range of a few kilowatts. As an example, a prototype of 3 kVA was implemented, and peak efficiency of 96.0% was achieved.     

A high‐efficiency LPE GaAs solar cell at concentrations ranging from 2000 to 4000 suns

III–V solar cells for terrestrial concentration applications are currently becoming of greater and greater interest. From our experience, concentrations higher than 1000 suns are required with these cells to reduce PV electricity cost to such an extent that this alternative could become cost competitive. In this paper, a single‐junction p/n GaAs solar cell, with efficiencies of 23ċ8 and 22ċ5% at concentration ratios of 2700 and 3600 suns respectively, is presented. This GaAs solar cell is well suited for use with non‐imaging optical concentrators, which possess a large aperture angle. Low‐temperature liquid phase epitaxy (LTLPE) has been the growing technique for the semiconductor structure as an attempt to use a simplified, cheap and clean technique, within a renewable energy perspective. The GaAs solar cell presented is compared with the highest efficiency tandem solar cells at concentration levels exceeding 1000 suns. The GaAs solar cell performance maintains high efficiencies up to 4000 suns, while tandem cells seem to drop very quickly after reaching their maximum. Therefore, single‐junction GaAs solar cells are a good candidate for operating at very high concentrations, and LPE is able to supply these high‐quality solar cells to work within terrestrial concentration systems, the main objective of which is the reduction of PV electricity costs. Copyright © 2003 John Wiley & Sons, Ltd.     

快速热退火对ITO薄膜及LED芯片性能的影响

采用金属有机化学气相沉积(MOCVD)技术在蓝宝石衬底上制备了GaN基LED外延层,采用磁控溅射法制备了氧化铟锡(ITO)薄膜,ITO薄膜用于制作与p-GaN的欧姆接触.研究了快速热退火温度为550℃,退火时间为200 s时,不同氧气体积流量对ITO薄膜性能及LED芯片光电性能的影响.结果表明:不通氧气时,ITO薄膜的方块电阻和透过率分别为33 Ω/口和93.1％,LED芯片出现电流拥挤效应,其电光转换效率只有33.3％;氧气体积流量为1 cm3/min时,ITO薄膜的方块电阻和透过率分别为70 Ω/口和95.9％,LED芯片的电流扩展不佳,其正向电压较高,电光转换效率为43.8％;氧气体积流量为0.4 cm3/min时,ITO薄膜的方块电阻和透过率分别为58 Ω/口和95.4％,LED芯片的电流扩展最佳,其亮度最高、正向电压最低,电光转换效率较高,为52.9％.     

Closing a Second Feedback Loop in a DC–DC Converter Based on a Piezoelectric Transformer

Nowadays, piezoelectric transformers (PT) are a good alternative to substitute magnetic materials in AC/DC and DC-DC converters. They have high isolation voltage and operate at higher frequencies than magnetics, with lower losses. However, their optimum operating frequency exhibits a strong dependence on different parameters, such as temperature, load, or even voltage level applied. This is usually an inconvenience, because this drift affects PT gain and efficiency, which can vary enormously within a few hundred of hertz. On the other hand, not only is it necessary to ensure the PT is driven at the proper frequency - in terms of gain and efficiency, including zero voltage switching in the power stage - output voltage must also be regulated. In this paper, two simple feedback loops are implemented in a PT-based DC-DC converter. One of them adjusts the switching frequency to obtain the best gain and efficiency. The other one performs the output voltage regulation.     

High-Power and High-Efficiency InGaN-Based Light Emitters

In this paper, we report on the latest advancements in improving AlGaInN-based visible-light-emitting-diode (LED) efficiency in epitaxy, chip, and package designs. We investigate the fundamental origin of the typical high current ?droop? of efficiency observed in such LEDs. We show that this effect is most likely not caused by incomplete carrier injection or carrier escape but that it is rather a fundamental material property of InGaN/GaN-heterostructure-based light emitters. The droop can be reduced in improved epitaxial LED active-layer designs. We show how this can be achieved by lowering InGaN volume carrier density in multiple quantum wells (MQWs) and thick InGaN layers. Improved epitaxial MQW structures are then combined with a new advanced chip concept. It is optimized for high efficiency at high current operation and arbitrary scalability and can be manufactured at low cost. This is accomplished by improving light-extraction efficiency, homogenizing the emission pattern, reducing forward voltage, and lowering thermal resistance. The improved high current efficiency can be fully exploited by mounting the chip in the highly versatile new OSLON SSL package. It features very stable package materials, a small footprint, and an electrically isolated design decoupling electrical and thermal contacts.     

荧光粉浓度和电流强度对白光LED特性的影响

讨论了荧光粉浓度及驱动电流强度对白光LED特性的影响。采用软件模拟实验和实际封装测试相结合的研究方法进行分析研究。对荧光粉浓度变化对白光LED光通量和相关色温(CCT)的影响进行了三维光线追迹模拟,并且进行了实际的封装验证。另外对白光LED的节温和显色性也做了深入细致的研究。研究结果表明:CCT随着荧光粉浓度的增大而减小,光通量则先上升后下降。同时由荧光粉浓度和驱动电流强度变化所引起的节温升高会降低荧光粉的转换效率。对显色性而言,采用高浓度荧光粉封装的白光LED有相对低的显色指数;并且显色指数随着驱动电流强度的增加而升高,最终趋于稳定。     

荧光粉配比对大功率白光LED发光特性的影响

利用黄色、红色和黄绿色3种荧光粉混合的方法制备了一系列大功率平面发光LED光源,深入研究了黄色、红色和黄绿色3种荧光粉分别对大功率白光LED光源的发光效率、显色指数以及色温的影响规律。研究结果表明,随着黄色荧光粉含量的增加,其发光效率明显提高,最高可达140 lm/W,而显色指数和色温略有下降。随着红色荧光粉含量的增加,其显色指数明显提高,最高可达85,而发光效率和色温明显降低。随着黄绿色荧光粉含量的增加,其发光效率、显色指数以及色温均不同程度地略有下降,但是其对大功率白光LED的色容差起到很好的调节作用。     

n型硅双面发电光伏组件及其应用

n型硅双面发电光伏(PV)电池组件具有光致衰减小、弱光响应好、温度系数低等优势,正面和反面均具有把光能转换成电能的能力.通过研究双面发电PV组件的结构和制备技术,成功研发了高效率低成本的n型硅双面发电PV组件.与单面发电光伏组件相比,双面发电PV组件输出功率更大,从而降低其在光伏系统应用中的综合成本.对双面发电光伏组件在白石子、绿草地和沙地等三种典型环境进行测试并获得实验数据,结果表明,在白石子环境下双面发电光伏组件较单面组件的发电量增加20％,发电量增量最高;其次是沙地环境,发电量增量为8.06％;绿草地环境的发电量增量最低,发电量增量仅为4.69％.通过实验应用研究得出不同反射环境下的发电量数据,可供后续双面发电PV组件电站系统设计做参考.     

Overall efficiency enhancement and cost optimization of semitransparent photovoltaic thermal air collector

A semitransparent photovoltaic‐thermal (PV/T) air collector can produce electricity and heat simultaneously. To maximize the thermal and overall efficiency of the semitransparent PV/T air collector, its availability should be maximum; this can be determined through a Markov analysis. In this paper, a Markov model is developed to select an optimized number of semitransparent PV modules in service with five states and two states by considering two parameters, namely failure rate ( λ ) and repair rate (µ). Three artificial neural network (ANN) models are developed to obtain the minimum cost, minimum temperature, and maximum thermal efficiency of the semitransparent PV/T air collector by setting its type appropriately and optimizing the number of photovoltaic modules and cost. An attempt is also made to achieve maximum thermal and overall efficiency for the semitransparent PV/T air collector by using ANN after obtaining its minimum temperature and available solar radiation.     

A design and qualification of LED flip Chip-on-Board module with tunable color temperatures

with the increasing requirements on guaranteeing the color uniformity and improving the luminous efficacy and manufacturing efficiency, a wafer level chip scale packaging (WLCSP) technology has been developed by thermally impressing a thin multiple phosphor film on a LED wafer, then being segmented into individual LED chips. In this paper, a high power white LED Chip-on-Board (COB) module with high color rendering index (CRI, Ra > 93) and tunable correlated color temperatures (CCTs) is prepared by the flip chip technology. In this COB module, the tunable color temperatures are achieved by using two types of white LED CSPs with different target CCTs of 3000 K and 5000 K. The thermal and photochromatic properties and the photochromatic stability of the COB module are studied through both experiments and simulations. The results show that: 1) The measured spectral power distribution (SPD) intensity of the prepared COB module is smaller than the arithmetic sum of SPD intensities of its each series connected CSPs, which may be attribute to the light absorption happened among CSPs; 2) The junction temperature and driven current have the different effects on the photochromatic properties of COB module (i.e. luminous flux, CCT and CRI); 3) The nonlinearity of luminous flux as a function of driven current and junction temperature should be considered in its modeling.