Temperature accelerated life test on commercial concentrator III–V triple‐junction solar cells and reliability analysis as a function of the operating temperature

A temperature accelerated life test on commercial concentrator lattice‐matched GaInP/GaInAs/Ge triple‐junction solar cells has been carried out. The acceleration of the aging has been accomplished by subjecting the solar cells at temperatures markedly higher than the nominal working temperature inside a concentrator, and the nominal photo‐current condition (820 X) has been emulated by injecting current in darkness. Three tests at different temperatures have been carried out. The failure distributions across the three test temperatures have been fitted to an Arrhenius–Weibull model. An Arrhenius activation energy of 1.59 eV was determined from the fit. The reliability functions and parameters of these solar cells at two nominal working conditions (80 and 100 °C) have been obtained. In both cases, the instantaneous failure rate function monotonically increases, that is, the failures are of the wear‐out kind. We have also observed that the reliability data are very sensitive to the nominal temperature condition. In fact, at a nominal working condition of 820 X and 80 °C, assuming that the concentration module works 5 h per day, the warranty time obtained for a failure population of 5% has been 113 years. However, for a nominal working condition of 820 X and 100 °C, the warranty time obtained for a failure population of 5% has been 7 years. Therefore, in order to offer a long‐term warranty, the working temperature could be a key factor in the design of the concentration photovoltaic systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

High‐irradiance degradation tests on concentrator GaAs solar cells

The present work summarises the results of an experiment of light‐soaking high‐concentrator MOVPE‐grown GaAs solar cells under monochromatic light (808 nm). The irradiance level was set so that the short‐circuit current obtained was 1100 times that produced with the AM1ċ5D spectrum at 1 kW/m². This test caused no morphological changes in the devices. The main phenomenon discovered has been a slight increase with time of the reverse current I₀₂. This increase is analogous to that observed in similar degradation experiments based on high forward currents. In general, the results of these tests show that the drop in performance is very limited, supporting the idea that concentrator GaAs solar cells are rugged devices, capable of achieving long lifetimes in field operation. Copyright © 2003 John Wiley & Sons, Ltd.     

The potential of III‐V semiconductors as terrestrial photovoltaic devices

III‐V semiconductors, GaAs and in particular InGaP, are used in many different electronic applications, such as high power and high frequency devices, laser diodes and high brightness LED. Their direct bandgap and high reliability make them ideal candidates for the realisation of high efficiency solar cells: in the past years they have been successfully used as power sources for satellites in space, where they are able to produce electricity from sunlight with an overall efficiency of around 30%. Nowadays, the use of arsenides and phosphides as photovoltaic (PV) devices is confined only to space applications since their price is much higher than conventional Si flat panel modules, the leading PV market technology. But with the introduction of multijunction solar cells capable of operating in high concentration solar light, the area and, therefore, the cost of these cells can be reduced and will eventually find an application and market also on Earth. This article will review the situation of semiconductor solar cell materials, focusing on Si, GaAs, InGaP and multijunction solar cells and will discuss future trends and possibilities of bringing III‐V technology from space to Earth. Copyright © 2006 John Wiley & Sons, Ltd.     

Explanation for the dark I–V curve of III–V concentrator solar cells

The measurement of the dark I–V curve is one of the most straightforward methods for characterizing solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. In this paper, an explanation of the dark I–V curve for concentrator III–V solar cells is presented using a 3D (three‐dimensional) model in order to provide a proper data fit that provides meaningful physical parameters that are also compatible and coherent with a data fit from illumination curves. The influence on the dark I–V curve of the most significant series resistance components of concentrator solar cells is also analysed concluding that only the vertical component as well as the front contact‐specific resistance can be assessable by means of this characterization method while both emitter and metal sheet resistances cannot be detected. For comparison purposes, the same experimental data have been fitted by means of a traditional two‐diode model showing that, although an accurate dark I–V curve fitting can be achieved, the extracted parameters are unable to reproduce illumination data since lumped models assume the same ohmic losses distribution for both dark and illumination conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Semi‐quantitative temperature accelerated life test (ALT) for the reliability qualification of concentrator solar cells and cell on carriers

An adequate qualification of concentrator photovoltaic solar cells and cell‐on‐carriers is essential to increase their industrial development. The lack of qualification tests for measuring their reliability together with the fact that conventional accelerated life tests are laborious and time consuming are open issues. Accordingly, in this paper, we propose a semi‐quantitative temperature‐accelerated life test to qualify solar cells and cell‐on‐carriers that can assure a minimum life when failure mechanisms are accelerated by temperature under emulated nominal working conditions with an activation energy >0.9 eV. A properly designed semi‐quantitative accelerated life test should be able to determine if the device under test will satisfy its reliability requirements with an acceptable uncertainty level. The applicability, procedure, and design of the proposed test are detailed in the paper. Copyright © 2015 John Wiley & Sons, Ltd.     

Techno‐economic analysis of three different substrate removal and reuse strategies for III‐V solar cells

The high cost of wafers suitable for epitaxial deposition of III‐V solar cells has been a primary barrier to widespread use of these cells in low‐concentration and one‐sun terrestrial solar applications. A possible solution is to reuse the substrate many times, thus spreading its cost across many cells. We performed a bottom‐up techno‐economic analysis of three different strategies for substrate reuse in high‐volume manufacturing: epitaxial lift‐off, spalling, and the use of a porous germanium release layer. The analysis shows that the potential cost reduction resulting from substrate reuse is limited in all three strategies––not by the number of reuse cycles achievable, but by the costs that are incurred in each cycle to prepare the substrate for another epitaxial deposition. The dominant substrate‐preparation cost component is different for each of the three strategies, and the cost‐ranking of these strategies is subject to change if future developments substantially reduce the cost of epitaxial deposition. Copyright © 2016 John Wiley & Sons, Ltd.     

Economic competitiveness of III–V on silicon tandem one‐sun photovoltaic solar modules in favorable future scenarios

Tandem modules combining a III–V top cell with a Si bottom cell offer the potential to increase the solar energy conversion efficiency of one‐sun photovoltaic modules beyond 25%, while fully utilizing the global investment that has been made in Si photovoltaics manufacturing. At present, the cost of III–V cells is far too high for this approach to be competitive for one‐sun terrestrial power applications. We investigated the system‐level economic benefits of both GaAs/Si and InGaP/Si tandem modules in favorable future scenarios where the cost of III–V cells is substantially reduced, perhaps to less than the cost of Si cells. We found, somewhat unexpectedly, that these tandems can reduce installed system cost only when the area‐related balance‐of‐system cost is high, such as for area‐constrained residential rooftop systems in the USA. When area‐related balance‐of‐system cost is lower, such as for utility‐scale systems, the tandem module offers no benefit. This is because a system using tandem modules is more expensive than one using single‐junction Si modules when III–V cells are expensive, and a system using tandem modules is more expensive than one using single‐junction III–V modules when III–V cells are inexpensive. Copyright © 2016 John Wiley & Sons, Ltd.     

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

We report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10⁻⁴ to mid 10⁻⁵ Ohm cm² range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulating single‐junction GaAs solar cells including photon recycling

To improve and accelerate further developments of III‐V solar cells an accurate and reliable modelling is necessary. In this work we present a comprehensive two‐ dimensional numerical model of single‐junction GaAs solar cells. Using this numerical model we achieved good correlation between measurements and simulations of single‐junction GaAs solar cells with different internal structures. We introduced the concept of optical coupling matrix in the numerical model to account for photon recycling effects. In addition, it is shown that thermionic currents have to be considered at the III‐V hetero‐interfaces. With this numerical model a powerful and flexible tool for solar cell simulation of III‐V compound semiconductor materials is now available. Copyright © 2006 John Wiley & Sons, Ltd.     

Two‐terminal metal‐inter‐connected multijunction III–V solar cells

A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar‐array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice‐matched two‐terminal, four‐junction device that consists of an inverted top two‐junction (2J) cell with 1.85 eV GaInP/1.42 eV GaAs, and an upright lower 2J cell with ~1 eV GaInAsP/0.74 eV GaInAs aimed for concentrator applications. The fabrication process and simulation of the metal topology are discussed along with the results of GaAs/GaInAs 2J and (GaInP + GaAs)/GaInAs three‐junction bonded cells. Bonding‐related issues are also addressed along with optical coupling across the bonding interface. Copyright © 2014 John Wiley & Sons, Ltd.     

Towards realization of a large‐area light‐emitting diode‐based solar simulator

Solar simulators based on light‐emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of photovoltaic cells with certain characteristics that make them superior to the traditional solar simulators. However, large‐area uniform illumination more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large‐area LED‐based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized AM 1.5 spectrum over an area of 25 cm × 50 cm with better than 10% spatial nonuniformity. We present the spectral output, the spatial uniformity, and the temporal stability of the simulator in both the constant current mode and the pulsed‐mode LED operation, and compare our data with the International Electrotechnical Commission standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5 intensity, this design and further improvements open up the possibility of achieving large‐area, high‐power indoor solar simulation with various desired spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

基于FPGA的LED屏控制器设计

通过对大型户外全彩LED显示屏的研究,基于FPGA设计了一种LED显示屏的控制系统。该系统主要工作基于Altera公司提供的DE1开发板上进行设计,在Quartus II的软件开发环境下,采用层次化设计,用Verilog HDL语言建立分频时钟模块、数据采集和重组模块、扫描驱动模块,最后连接成一个整体的系统模块,进行仿真和调试,完成FPGA控制系统的设计。通过SPI通信协议发送数据,完成了64×64的LED屏的图形显示,从而验证了LED大屏幕的设计方法。本方案实现的显示控制系统方法,满足目前LED大屏幕区域显示和高速处理图像数据的要求,具有稳定性高、设计灵活等特点。     

Tetraphenylimidazole‐Based Excited‐State Intramolecular Proton‐Transfer Molecules for Highly Efficient Blue Electroluminescence

Aiming for highly efficient blue electroluminescence, we have designed and synthesized a novel class of tetraphenylimidazole‐ based excited‐state intramolecular proton‐transfer (ESIPT) molecules with covalently linked charge‐transporting functional groups (carbazole‐ and oxadiazole‐functionalized hydroxyl‐substituted tetraphenylimidazole (HPI), i.e., HPI‐Cbz and HPI‐Oxd, respectively). High T_g (ca. 130 °C) amorphous films of HPI‐Cbz and HPI‐Oxd showed intense and ideal blue‐light emission (λ_max = 462 and 468 nm, Φ_PL = 0.44 and 0.38) with a large Stokes shift of over 160 nm and a narrow full width at half‐maximum of less than 65 nm. Organic light‐emitting devices using HPI‐Cbz and HPI‐Oxd as the emitting layer generated an efficient blue electroluminescence (EL) emission peaking at around 460 nm with excellent CIE coordinates of (x, y) = (0.15, 0.11). A maximum external quantum efficiency of 2.94%, and a maximum brightness of 1 229 cd m⁻² at 100 mA cm⁻², as well as a low turn‐on voltage of 4.8 V were achieved in this work.     

Wafer bonded four‐junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells with 44.7% efficiency

Triple‐junction solar cells from III–V compound semiconductors have thus far delivered the highest solar‐electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four‐junction solar cell architectures with optimum bandgap combination are found for lattice‐mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs‐based top tandem solar cell structure was bonded to an InP‐based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four‐junction solar cell with a new record efficiency of 44.7% at 297‐times concentration of the AM1.5d (ASTM G173‐03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi‐junction solar cells having four and in the future even more junctions. Copyright © 2014 John Wiley & Sons, Ltd.     

>35% 5-junction space solar cells based on the direct bonding technique

Multijunction solar cells are the highest efficiency photovoltaic devices yet demonstrated for both space and terrestrial applications. In recent years five-junction cells based on the direct semiconductor bonding technique (SBT), demonstrates space efficiencies >35% and presents application potentials. In this paper, the major challenges for fabricating SBT 5J cells and their appropriate strategies involving structure tunning, band engineering and material tailoring are stated, and 4-cm² 35.4% (AM0, one sun) 5J SBT cells are presented. Further efforts on detailed optical managements are required to improve the current generating and matching in subcells, to achieve efficiencies 36%–37%, or above.     

Limiting factors on the semiconductor structure of III–V multijunction solar cells for ultra‐high concentration (1000–5000 suns)

The main limiting factors of multijunction solar cells operating under ultra‐high concentration (>1000 suns) are examined by means of 2D physically based numerical modelling. The validation of the model is carried out by fitting calibrated light concentration measurements. Because the series resistance is the most important constraint in the electrical performance of the solar cell under ultra‐high irradiance, it is analysed and quantified detailing different contributions such as: (i) the electrical properties of the emitter; (ii) window layer of the top cell; and (iii) the band discontinuities formed at heterojunctions. We found the role of window layer to be important at very high concentrations (above 700 suns), while at ultra‐high concentrations, (above 1000 suns) a gain in efficiency (~ 1% absolute) can be obtained by a proper structural design of the window layer. In the case of the heterojunctions included in the multijunction solar cell, the impact of a high‐band offset can be mitigated by increasing the doping level density thus favouring the tunnelling effect. Moreover, the influence of different recombination mechanisms and high‐injection effects at ultra‐high irradiance is discussed. Finally, an optimisation of the complete solar cell taking into account the ohmic contacts to work under ultra‐high irradiances (from 1000 to 5000 suns) is presented as well as the implications on the use of ultra‐high irradiance in different multijunction solar cell architectures. Copyright © 2016 John Wiley & Sons, Ltd.     

Exciton‐Adjustable Interlayers for High Efficiency,Low Efficiency Roll‐Off,and Lifetime Improved Warm White Organic Light‐Emitting Diodes (WOLEDs) Based on a Delayed Fluorescence Assistant Host

Recently, a new route to achieve 100% internal quantum efficiency white organic light‐emitting diodes (WOLEDs) is proposed by utilizing noble‐metal‐free thermally activated delayed fluorescence (TADF) emitters due to the radiative contributions of triplet excitons by effective reverse intersystem crossing. However, a systematic understanding of their reliability and internal degradation mechanisms is still deficient. Here, it demonstrates high performance and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host via a strategic exciton management by multi‐interlayers. By introducing such interlayers, carrier recombination zone could be controlled to suppress the generally unavoidable quenching of long‐range triplet excitons, successfully achieving remarkable external quantum efficiency of 15.1%, maximum power efficiency of 48.9 lm W⁻¹, and extended LT50 lifetime (time to 50% of initial luminance of 1000 cd m⁻²) exceeding 2000 h. To this knowledge, this is the first pioneering work for realizing high efficiency, low efficiency roll‐off, and operational stable WOLEDs based on a TADF assistant host. The current findings also indicate that broadening the carrier recombination region in both interlayers and yellow emitting layer as well as restraining exciplex quenching at carrier blocking interface make significant roles on reduced efficiency roll‐off and enhanced operational lifetime.     

A monolithic three‐terminal GaInAsP/GaInAs tandem solar cell

We describe the design and performance of a three‐terminal tandem solar cell for low‐concentration terrestrial applications. Designed for operation under a GaAs filter, the tandem demonstrates cumulative conversion efficiencies of 10.2 and 11.9% at 1 sun and 45 suns, respectively, under the concentrated direct spectrum. The middle terminal is shared between the two subcells and allows them to be operated independently at their respective maximum power points. Copyright © 2009 John Wiley & Sons, Ltd.     

Scrutinizing Design Principles toward Efficient,Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro‐substituted cyclometallated ligands and electron donating groups (methyl and tert ‐butyl)‐substituted diimine ancillary (N^{^}N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)₂(N^{^}N)][PF₆] (4Fppy = 2‐(4‐fluorophenyl)pyridine) with the N^{^}N ligand being either 2,2′‐bipyridine ( 1 ), 4.4′‐dimethyl‐2,2′‐bipyridine ( 2 ), 5.5′‐dimethyl‐2,2′‐bipyridine ( 3 ), or 4.4′‐di‐tert ‐butyl‐2,2′‐bipyridine ( 4 ) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529–547 nm and photoluminescence quantum yields in the range of 50.6%–59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on ( 1 ) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m^?2 resulting in 16 cd A^?1 and 8.6 lm W^?1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on ( 3 ) and ( 4 ) perform lower, with luminance and lifetime of 1314 cd m^?2, 45.7 h and 1193 cd m^?2, 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir‐iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the N^{^}N ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.