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目录 contents

    摘要

    首先从热力学角度讨论减少光伏器件效率损失的原理和途径,然后介绍半导体纳米线阵列有效实现陷光和降低发射角的结构设计,其中渐变尺寸的非均匀纳米线阵列具有低发射角、陷光的原理性优势,成为可见到近红外太阳能高效太阳能转换的重要研究方向。

    Abstract

    In this paper, a thermodynamic analysis on current photovoltaics wsa given within the Shockley-Queisser model. Then the latest progresses of designing semiconductor nanowire arrays are introduced to achieve effective light trapping and reduced emission angle. Among them, non-uniform nanowire arrays with gradient shapes hold both advantages of ultralow emission angle and light trapping, therefore has attracted much research interest toward ultrahigh efficiency of solar energy conversion from visible to near-infrared wavelength.

  • 引言

    21世纪人类面临的挑战之一是从基于化石燃料的能源系统向基于可持续和可再生资源的能源系统过[1],在能源系统转换中,直接将太阳光转化为电能的太阳能电池将发挥核心作用。自本世纪初以来,光伏装机容量呈指数增长,在迅速下降的成本和效率提高的推动下,2018年光伏发电量高达136 TWh [2]。为了让光伏发电对能源结构优化做出更大贡献,不断提高光电转换效率,同时减少材料使用和降低成本十分重要。如图1所示,通过使用热力学方法评估太阳能转换效能,光伏电池的效率损失可以归结为两个方面:约58%的能量损失和约23%的熵损[3]。其中能量损失包括不可避免的卡诺循环损失、光的不完全吸收以及光生载流子的热能化损失;熵损失包括吸收和发射时产生的固有光子损失、自发辐射引起的非互易性光损失、缺乏角度限制引起的自发辐射光子损失、不完全陷光的光损失和非辐射激子复合导致的量子效率损失。40多年来光伏效率的提升一直得益于对半导体光伏结构电子层面的优化。例如:通过减少材料电子缺陷密度以抑制光生载流子的热能化损失,以及通过采用多级的能带结构减少带隙与电子-空穴准费米面劈裂的差距,光伏实现了单结28.9%和多结38.8%的高效[4]。现在,针对熵损失机制,一个关键并且有着巨大潜力的挑战是更好地管理太阳能电池内部的光。如果能系统地解决当前光伏发电的热力学效率损失,再结合新兴的材料制造方法,将允许开发效率在50%~70%范围内的超高效太阳能电池。

    图 1
                            太阳能转换中的热力学损失。对于传统的单结太阳能电池,实现的最大效率为28.3%(以绿色表示)。浅蓝色表示与能量相关的损失,深蓝色表示与熵相关的损失,减少能量和熵损失问题的解决方案列在右栏中;引自参考文献3

    图 1 太阳能转换中的热力学损失。对于传统的单结太阳能电池,实现的最大效率为28.3%(以绿色表示)。浅蓝色表示与能量相关的损失,深蓝色表示与熵相关的损失,减少能量和熵损失问题的解决方案列在右栏中;引自参考文献3

    Fig. 1 Thermodynamic losses in solar-energy conversion. The maximum effciency realized for a conventional single-junction solar cell is 28.3% (indicated in green). Light blue bars indicate energy-related losses and dark blue bars indicate entropy-related losses. The solutions to reducing the energy- and entropy-loss problems are listed in the right-hand column. ref 3, © 2012 NM

    最近几年,通过结构设计,对亚波长光学元件,如微光子抛物面光导向器、平面超材料光导结构和纳米线阵列结[5,6,7,8]等进行光管理,可以将任何自发辐射发射的光子重新引导回对应的太阳光入射立体角范围内。其中,纳米线阵列还能通过激发多种共振模式来提高电池内部的光学浓[9,10]。此外,高纵横比的纳米线阵列也能将光吸收和载流子收集分解成正交的空间方向,这提高了在载流子扩散长度短的材料中的载流子收集能[11,12,13,14,15,16]。且它们还通过径向应变松弛放宽了晶格匹配要求,从而在更便宜的晶格失配的衬底上也能实现高质量单晶纳米线的生[17,18]。这些优势使得基于纳米线阵列结构的太阳能电池成为近年来的研究热点。本文的第一部分将分析太阳能光伏结构的熵损失机制,以及纳米结构通过光管理减少相应熵损失的方案。第二部分首先介绍通过半导体纳米线阵列的结构设计形成导-共振模式的陷光效应,实现对光吸收和发射角的控制。然后介绍能够涵盖可见和近红外波段光能转换的纳米线阵列结构:通过锥形结构或多级纳米线结构,以及非均匀、非周期排列纳米线阵列来实现增加共振模式数量和拓宽共振吸收的波长范围。

  • 1 太阳能光伏结构的热力学效率分析

    1961年,Shockley和Queisser基于开路条件下器件的光子吸收和发射的细致平衡原理,提出一个用于确定半导体p-n结太阳能电池极限效率的理论框[19]。该方法表示,假设每个吸收光子产生一个电子-空穴对,在完全光吸收和不考虑非辐射复合的理想情况下,对于传统几何光学下具有光谱优化带隙的太阳能电池(Eg=1.4 eV,接近GaAs的带隙),可实现的最大效率为33%,这就是我们熟悉的Shockley-Queisser限制。S-Q模型为这些年中光伏器件取得的许多进步奠定了理论基础,然而,为了达到更高的功率转换效率,必须寻求新的途径突破这个限制。近年来,人们详细分析了该模型下光伏发电的热力学损失,并从中寻求进一步提升效率的机遇。

    众所周知,最大化开路电压是实现光伏电池高转换效率的关键。在S-Q模型的热力学分析中,光伏电池的开路电压Voc可由式(1)计算得[3]

    Voc=Eg(1-TTsun)-kT-ln(QE)+ln(4n2I)+ln(ΩemitΩsun) ,
    (1)

    其中,Eg为带隙能量,T为电池温度,Tsun为太阳温度,QE为量子效率,n为折射率,I为光浓度因子,Ωemit为自发辐射光子的发射立体角,Ωsun为太阳光入射立体角。公式右边第一项表示光子能量的转换,它包括基于卡诺定理的5%的热力学损失,以及考虑通过光子自发辐射而不是黑体辐射引起的7%的能量损失。还需要说明的是,为了保证电能向外电路的高效传输,太阳能电池的实际工作电压比估算的开路电压低约100 meV。

    式(1)右侧方括号中的第一项表示非辐射激子复合引起量子效率下降的损失,这是由晶体缺陷,杂质和电池内部及表面处的其他载流子陷阱引起的。通过材料质量的不断改进,光电量子效率的提升可以直接转化为更高的开路电压。

    式(1)方括号中的第二项和第三项提供了减少开路电压熵损失的机会。其中,第二项描述了太阳能电池内部不完全陷光的影响,这对于能量接近半导体带隙Eg的光的充分吸收尤为重要。第三项反映了半导体中自发辐射对光子吸收和再辐射的熵增加。从此公式可以看出,如果能使电池内部光浓度因子I达到或超过4n2;同时通过光子结构的设计限制光伏电池自发辐射的发射角Ωemit,使其接近太阳光的入射立体角Ωsun,太阳能电池的效率有望突破S-Q极限。近年来,基于半导体纳米线阵列的太阳能电池结构为这个方案的实现提供了可行性途径。

  • 1.1 减少不完全陷光的熵损失

    太阳能电池将光能转换为电能的第一步是将尽可能多的光限制在电池内部,因此,陷光能力对太阳能电池的效率有着很大的影响。例如,平面太阳能电池内部没有光陷阱,光浓度因子恒为1,这导致了100 mV的开路电压损耗。通过在平面太阳能电池表面形成金字塔结构表面纹理,能使光在电池内部得到多重内反射,从而增强光浓度。对于经典几何光学极限中具有各向同性发射的太阳能电池,可实现的最大光浓度是4n2的Yablonovitch极[20],此时不完全陷光可能形成的熵损失将被消除掉;而纳米结构的光伏材料可以使得光浓度因子在一定的光谱范围内超出4n2[9,10],此时,传统的几何光学限制被打破,需要通过波动光学来建立新的限[21]

    以包含半导体纳米线的光伏结构为例,如图2所示,它提供了多种共振模式来实现对入射光的局域效[22]图2(a)在纳米线中激发六极对称的光学共振模式(Mie resonance modes),在薄膜层中激发导—共振模式(guided resonance modes)。图2(b)在薄膜层中出现了法布里—珀罗驻波共振模式(Fabry-Perot standing-wave resonance modes),它是由薄膜的上表面和金属背反射器之间的光约束产生。图2(c)硅层中出现了导-共[23],因为周期性纳米线阵列可以充当确保正常入射平面波与硅层中波导模式相位匹配耦合的光栅,所以光场分布显示出周期性强度变化,这是导-共振模式的特征。当入射光被重新导向到作为陷光层的纳米线阵列时,在图2(d)中还出现了一些不同的共[24],从而在陷光层中激发出横向传播的波,这些模式延伸到下面的半导体层中,形成增强吸收。上述共振模式为高效率的陷光设计提供多种可行的途径。

    图 2
                            薄膜光伏电池中多种共振模式激发引起的吸收增强,其中电池由金属背反射器、1 μm厚结晶硅薄膜和周期性结晶硅纳米线阵列组成,图中白色虚线表示结晶硅的结构。(a)λ=880 nm,垂直入射,显示光学共振模式与导-共振模式的混合激发;(b)λ=1031 nm,入射角为28°,显示法布里—珀罗共振模式的激发;(c)λ=946 nm,垂直入射,显示硅层中导—共振模式的激发;(d)λ=1011 nm,垂直入射,显示出一种不同的共振,使得在陷光层中激发横向传播的波。(a)-(d)引自参考文献22

    图 2 薄膜光伏电池中多种共振模式激发引起的吸收增强,其中电池由金属背反射器、1 μm厚结晶硅薄膜和周期性结晶硅纳米线阵列组成,图中白色虚线表示结晶硅的结构。(a)λ=880 nm,垂直入射,显示光学共振模式与导-共振模式的混合激发;(b)λ=1031 nm,入射角为28°,显示法布里—珀罗共振模式的激发;(c)λ=946 nm,垂直入射,显示硅层中导—共振模式的激发;(d)λ=1011 nm,垂直入射,显示出一种不同的共振,使得在陷光层中激发横向传播的波。(a)-(d)引自参考文献22

    Fig. 2 Absorption enhancement caused by the excitation of multiple optical resonance modes in a thin PV cell. It shows a 1-μm-thick c-Si film with a perfect back mirror and a light-trapping layer consisting of a periodic array of c-Si nanowires (NWs) on the top surface. The dashed white lines outline the c-Si structure. (a) λ=880 nm, normal incidence, showing the excitation of a mixture of a Mie resonance with a guided resonance; (b) λ=1031 nm, 28° angle of incidence, showing the excitation of a Fabry-Perot resonance; (c) λ=946 nm, normal incidence, showing the excitation of a guided resonance in the Si layer; (d) λ=1011 nm, normal incidence, showing a diffracted resonance by which a laterally propagating wave is excited in the light-trapping layer. a-d, ref. 22, © 2014 NM

  • 1.2 减少自发辐射热力学熵损失

    太阳能电池在吸收光子的同时,其自身会通过发射光子的形式向外辐射热量。通常太阳能电池结构自发辐射光子的立体角Ωemit远大于太阳光的入射立体角Ωsun, 立体角之间的极大差距最高可导致室温下315 mV的开路电压损失。通过结构设计减小Ωemit,可以提高辐射光子的再利用率,改变发射和吸收的方向性以改善太阳能电池的开路电压已在实验中得到证[25,26,27,28,29]

    传统平面太阳能电池减少开路电压熵损失的最常用方法是通过增加背反射器及使用聚光结构。我们知道,对于传统平面太阳能电池,吸收立体角Ωabs对应于光的入射立体角,即Ωabs=Ωsun=2π(1-cos(θs))=6.82×10-5 sr(θs为太阳光入射角),而Ωemit=4π,增加背反射器可将Ωemit减小到2π[30]。为了更进一步提高开路电压,可以增加聚光结构,把分散的太阳光利用光学原理聚集到平面太阳能电池表面,使吸收立体角超过入射光立体角并接近电池的发射立体角(图3(a)),然而聚光结构将增加电池系统的复杂性和成本。

    图 3
                            半导体纳米线阵列结构可以减小吸收角和发射角之间的差距,使器件实现>40%的功率转换效率。(a)具有聚光结构的传统平面太阳能电池能增加吸收角Ωabs以接近发射角Ωemit,从而减少由它们的失配引起的熵损失;(b)同样,纳米结构太阳能电池也可以降低Ωabs和Ωemit之间的差异;(c)-(e)分别为传统平面太阳能电池、具有背反射器的平面太阳能电池和理想的纳米结构太阳能电池的吸收和发射角;(f)对应于三种结构(c-e)的I-V曲线,显示开路电压的增加是源自发射角逐渐趋近于吸收角。(a)-(f)引自参考文献8

    图 3 半导体纳米线阵列结构可以减小吸收角和发射角之间的差距,使器件实现>40%的功率转换效率。(a)具有聚光结构的传统平面太阳能电池能增加吸收角Ωabs以接近发射角Ωemit,从而减少由它们的失配引起的熵损失;(b)同样,纳米结构太阳能电池也可以降低ΩabsΩemit之间的差异;(c)-(e)分别为传统平面太阳能电池、具有背反射器的平面太阳能电池和理想的纳米结构太阳能电池的吸收和发射角;(f)对应于三种结构(c-e)的I-V曲线,显示开路电压的增加是源自发射角逐渐趋近于吸收角。(a)-(f)引自参考文献8

    Fig. 3 Semiconductor nanowire arrays(NWAs) can reduce the mismatch of angles between absorption and emission, which results in an ideal PV nanostructure achieving >40% power conversion effciency. (a) A traditional planar solar cell with concentrator increases Ωabs to approach Ωemit, thus reducing the entropy generation caused by their mismatch; (b) Similarly, a nanostructured solar cell can reduce the difference between Ωabs and Ωemit; Emission and absorption for (c) slab without back reflector, (d) slab with back reflector, and (e) ideal nanostructured cell; (f) I-V curves corresponding to the three structures, shows the increased Voc as θemitθs. (a)-(f), ref. 8, © 2015 SR

    事实上,经过合理设计的半导体纳米线阵列结构也能获得与聚光结构相同的效果。如图3(b)所示,通过增大Ωabs或减小Ωemit以试图实现Ωemit=Ωabs,可以减少熵产生。从器件的角度来看,Ωabs与光产生的电流密度JL正相关,且JL=IL/A,而Ωemit与反向饱和电流密度JR正相关,JR=IR/A,其中,IL为光电流,IR为暗电流,A为电池的面积。因为光伏器件的开路电压取决于光电流IL与暗电流IR之间的比率,因此增加Ωabs与降低Ωemit对开路电压具有相同的影响。

    根据Kirchhoff定律,太阳能电池的发射率和吸收率在热平衡方面是相等[30,31]。传统的平面太阳能电池可以吸收来自所有方向的入射光子,因此向所有方向发射光子(图3(c))。增加背反射器可以减少电池背面的发射(图3(d)),且对入射光的吸收没有影响,即光电流不变,但暗电流减少(注:光电流可能会由于低吸收或较薄材料中的路径长度增加而略有增加,导致开路电压小幅度增加)。而理想的纳米结构仅允许电池在对应于太阳光入射角度范围内吸收和发射(图3(e))。这些器件的电流—电压特性表明(图3(f)),与传统平面器件相比,增加背反射器的效率提高了约2%,理想纳米结构的效率提高了约11%,从而单p-n结光伏器件实现约42%的超高效[8]

    综上,从减少熵损失的角度,提升太阳能电池效率主要有以下两个方面:首先,尽可能多地将光局域在电池内部,提高光浓度因子,减少不完全陷光产生的熵损失;其次,限制光伏电池自发辐射的发射角,使其接近太阳光的入射立体角,提高自发辐射光子的再利用率。传统的平面电池采用金字塔表面纹理来陷光,使用聚光结构来限制发射角,然而,宏观表面纹理结构需要的硅厚度接近300 μm,聚光结构也将增加电池系统的复杂性和成本。半导体纳米线阵列结构为最小化热力学损失的光管理方法提供了一个新的方案,它通过激发多种共振模式来局域光,同时限制吸收角和发射角,实现Ωemit=Ωabs=Ωsun. 计算表明,基于纳米结构的单p-n结太阳能电池的效率理论上可以达到41.7%。

  • 2 半导体纳米线阵列的光管理

    半导体纳米线阵列的光管理是通过调整纳米线几何参数来实现的。首先,纳米线阵列中的光学共振模式强烈依赖于纳米线直径,而较少依赖于纳米线的周期和长[32,33,34,35],表明这些模式是单个纳米线所固有的,并且报道指出,它们源于单个纳米线中的HE1q共振模[35,36,37],改变纳米线的直径将导致共振波长的变[38,39,40,41]。其次,纳米线阵列的周期在相邻纳米线之间的耦合也起着重要作[35,42,43],它控制光学耦合的主导模式从单纳米线的泄漏模式(leaky modes)到阵列结构的光学布洛克模式(photonic bloch modes)的转[35,44]。利用后者的共振耦合效应,不仅能显著增强吸收,还能有效地抑制垂直于纳米线轴向的偶极子发[44,45]。本节首先介绍尺寸均一的周期纳米线阵列光学共振模式,并提出适用于单结太阳能电池的最优结构参数。为了实现对纳米线光伏结构发射角的控制,同时拓展太阳光共振吸收波长至近红外波段,本节的第二部分介绍非均匀尺寸和随机半导体纳米线阵列的研究进展。

  • 2.1 均匀尺寸纳米线阵列

    半导体纳米线可以看成半径在几十到几百纳米、长度在几到几十微米的电介质圆柱体。当光从纳米线顶部照射时,由于直径比波长小的多,纳米线和周围环境较大的折射率之比,会将光限制在光传播方向(z方向)和至少一个横向方向(x和/或y方向)上。当纳米线的尺寸和形状合适时,会在纳米线内部激发光学共振模式,这些共振模式的强度与金属纳米结构中的共振模式相[46]。它们也出现在深亚波长结构(~10 nm)中,并且已经使纳米级光电器件的性能得到改善,这些器件可以密集地集成到类似尺寸的半导体电子元件[47]。在太阳能电池结构中,如果使用纳米线阵列作为激活区,可以通过激发局部光学共振模式、导-共振模式和反射共振模式来增强吸收,光被吸收或散射的方式取决于局部光学共振模式的性质。在陷光层的设计中,对不同类型的局部光学共振模式进行分类并区分它们对光吸收的贡献程度是十分重要的。

    纳米线的局部光学共振模式可以按照Gustav Mie[46]采用的麦克斯韦方程方法来分析。使用适当的边界条件求解麦克斯韦方[48],共振模式的激发发生在半径为ɑ的无限长介电圆柱体[47]

    (1κ2-1γ2)2(βma)2= k02(n2J'm(κa)κJm(κa)-n02H'm(γa)γHm(γa))×(J'm(κa)κJm(κa)-H'm(γa)γHm(γa))
    (2)

    其中,γ和к分别为纳米线外部和内部横向波矢量,n0n分别为纳米线外部和内部的折射率,βk0是沿纳米线轴向和自由空间的波矢,JmHmm阶第一类贝塞尔函数和第二类修正贝塞尔函数,上标符号表示相关参数的微分。

    当入射光垂直于纳米线轴线照射时,β=0,n0=1,此时,根据式(2),纳米线中激发的模式仅为横磁模式(transverse-magnetic modes, 电场平行于纳米线轴向方向)和横电模式(transverse-electric modes, 电场垂直于纳米线轴向方向)。它们分别用TMpq和TEpq来表示,其中p为方位模式数,表示纳米线周围的有效波长数;q是特征值方程的第q个根,它也对应于径向模式数,即纳米线内径向场最大值的数量,pq均为整数。图4(a)示出了由纳米线激发的几个最低阶横磁模式(横电模式基本与横磁模式相同)。在分析纳米线的散射特性时,将纳米线视为微米级圆柱谐振器,这些谐振器通过来自周边的多个全内反射来捕获循环轨道中的光(回音壁模式),因此,只要有pp为整数)个波长与圆柱形纳米线的周长进行匹配,就会发生共振。并且,局部光学共振模式不限于在完美的圆柱形纳米线中激发,它是高折射率纳米结构的一般特[13]图4(b)给出了不同横截面形状的纳米线在共振波长处的场强分布,可以看出,这些形状的纳米线中也出现了两个场强最大值,类似于圆柱形纳米线中的TM11模式。此外,这些共振模式可以在任何半导体材料(如Si、GaAs、CdTe和CIGS[13])以及透明氧化[24]中激发,即使是高折射率结构中的纳米级空隙也可以提供用于陷光目的的共[49],这些共振的普遍发生展现了在材料选择和合成方法上的巨大灵活性。

    图 4
                            纳米线内部激发的不同光学模式及其对光吸收的贡献。(a)纳米线激发的最低阶横磁模式(p=0、1、2和3,从左到右示出);(b)不同横截面形状的纳米线内部的电场分布,入射光选择由纳米线上方的横磁照射,激发波长为p=1的共振波长;(c)纳米线激发的HE1q模式的共振波长和纳米线直径函数的吸收等高线图;(d)GaAs NW阵列的吸收效率随着入射波长的变化而变化,FR为0.05,插图显示了GaAs的复折射率;(a)-(d)分别引自参考文献47、13、39和40

    图 4 纳米线内部激发的不同光学模式及其对光吸收的贡献。(a)纳米线激发的最低阶横磁模式(p=0、1、2和3,从左到右示出);(b)不同横截面形状的纳米线内部的电场分布,入射光选择由纳米线上方的横磁照射,激发波长为p=1的共振波长;(c)纳米线激发的HE1q模式的共振波长和纳米线直径函数的吸收等高线图;(d)GaAs NW阵列的吸收效率随着入射波长的变化而变化,FR为0.05,插图显示了GaAs的复折射率;(a)-(d)分别引自参考文献47、13、39和40

    Fig. 4 Different optical resonance modes excited inside the nanowires and their contribution to light absorption. (a) Lowest-order transverse magnetic optical modes (p=0, 1, 2, 3, shown from left to right) supported by subwavelength high-refractive-index nanocylinders; (b) Electric field distributions inside the nanowires of different cross-section shapes for transverse magnetic illumination from above. The excitation wavelength was chosen to drive an p=1 resonance; (c) Contour plot of absorption as a function of photon wavelength and nanowire diameter for vertical NWAs; (d) Absorption effciency of GaAs NW arrays change with incident wavelength with FR of 0.05 and the insets illustrate the complex refractive index for GaAs. (a) ref. 47, © 2009 NM; (b) ref. 13, © 2010 NL; (c) ref. 39, © 2012 OL; (d) ref. 40, © 2017 SR

    在光伏应用中,入射光从纳米线顶部照射是我们主要关注的,此时,入射光经过衍射变为泄漏模[39]。由于纳米线的亚波长尺寸,它仅能激发少量泄漏模式,这些模式通常是电磁场分量HzEz的混合,可以用HEpq和EHpq分别表示横磁和横电主导的模式(pq的定义与前面一致)。对于在x方向上电场偏振的入射光,电场关于yz平面反对称,因此只有关于yz平面反对称或者没有方位角对称性的模式才能与该入射光耦合。HE模式具有方位角数l=p-1,EH模式具有l=p+1[48],所以,在垂直纳米线所提供的泄漏模式中,只有无方位角变化的HE1q模式能增强光吸收(如图4(c)所示)。其中,HE11通常被采纳为纳米线阵列共振吸收峰的模[38,39,40,41]图4(d)显示随着纳米线直径的增加,共振吸收峰值在长波处(λ>700 nm)逐渐降低,且共振波长发生了明显的红移演变。对于直径大于120 nm,开始出现第二吸收峰,即HE12模式。这是因为在自由空间平面波的波前受到高折射率纳米线阵列的干扰,在波矢中引入横向分量,使入射光耦合到泄漏模式中。以上对于单根纳米线和具有极低填充比的(filling ratio: FR=0.05)纳米线阵列的模式分析表明,由直径主导的低阶共振模式可以增强纳米线对特定波长入射光的吸收。

    由于纳米线中的泄漏模式仅与直径有关,因此由纳米线组成的周期阵列可以满足光伏器件利用超薄吸收区实现高效光电转换的要求。图5(a)示出了由直径D,周期P和长度L的纳米线组成的纳米线阵列示意图。图5(b)显示在固定填充比(直径D与周期P的比值:D/P)时,不同直径纳米线阵列在300~900 nm的吸收谱。可以看出,在短波长处纳米线阵列的吸收效率随着直径的增加而略有下降,这归因于大直径纳米线的顶端反射增加。在长波长处,具有较大直径的纳米线能激发更多泄漏模式,从而耦合更多的光,所以当纳米线的直径从60 nm增加到180 nm时,阵列的光吸收显著增强。当直径进一步增加到240 nm时,由于顶端反射和长波长处场强的下降,纳米线阵列的吸收率反而降低。另外,纳米线阵列的周期对相邻纳米线之间的耦合也有很大影响,图5(c)显示在固定直径下具有不同填充比的纳米线阵列的吸收谱。在波长λ<700 nm,较大的填充比使得大部分光不能进入相邻纳米线的间隙,只能在纳米线顶端反射掉,所以吸收率随着填充比的增加而减小。在λ>700 nm,小填充比纳米线阵列的吸收不足导致显著的衬底反射和透射,因此吸收率随着填充比的增加而增加。

    图 5
                            固定填充比和固定直径下纳米线阵列的吸收谱。(a)周期性纳米线阵列结构示意图;(b)固定填充比D/P=0.5时,不同直径纳米线阵列的吸收谱;(c)固定直径D=120 nm时,不同填充比下纳米线阵列的吸收谱;(a)-(c)引自参考文献50

    图 5 固定填充比和固定直径下纳米线阵列的吸收谱。(a)周期性纳米线阵列结构示意图;(b)固定填充比D/P=0.5时,不同直径纳米线阵列的吸收谱;(c)固定直径D=120 nm时,不同填充比下纳米线阵列的吸收谱;(a)-(c)引自参考文献50

    Fig. 5 Absorption spectra of NWAs at fixed filling ratios and fixed diameters. (a) Schematic drawing of the periodic NWAs structure; (b) Absorptance of NWAs with different diameters varied from 60 to 240 nm under the fixed filling ratio of 0.5; (c) Absorptance of NWAs with different filling ratios varying from 0.4 to 0.8 under the fixed diameter of 120 nm. (a)-(c), ref. 50, © 2011 NRL

    纳米线阵列的自发辐射发射角也与纳米线的直径相[37,44,45]。根据前面的分析,可以通过反向饱和电流密度来估算发射立体角,图6(a)中给出了当高度为4 μm,周期为400 nm时,不同直径纳米线阵列顶部与底部的反向饱和电流,纳米线阵列的反向饱和电流密度等于顶部与底部反向饱和电流之和。由图可知,相比于传统的平面电池,所有直径的纳米线阵列均有着更低的反向饱和电流密度,表明纳米线阵列具有更好的发射角抑制作用。对于D<170 nm, 纳米线阵列底部区域的发射角被明显限制,然而,顶部区域的发射角随直径的增加而增大,并在170 nm达到最大。对于D>170 nm, 随着直径的增加,向衬底的发射变得占主导地位。图6(b)-(e)示出了在波长为900 nm下(带隙波长附近),直径分别为170 nm和400 nm的纳米线阵列在不同极角θ和方位角φ处的发射率eem. 当D=170 nm时,纳米线阵列向顶部区域的所有方向发射光子(图6(b));而底部区域的发射率却在20°<θ<30°的范围内迅速下降,表明底部发射被抑制在较小的角度范围内(图6(c))。当直径增大到D=400 nm时,纳米线阵列顶部的发射角度范围与D=170 nm相同(图6(d));对于底部区域(图6(e)),在60°的大角度范围内发现了强烈的发射(发射率eem→1);这说明大直径的纳米线阵列并没有很好地限制自发辐射光子的发射角度。

    图 6
                            当纳米线的长度L=4 μm,周期P=400 nm时,不同直径纳米线阵列的发射。(a)不同直径纳米线阵列的顶部和底部反向饱和电流,反向饱和电流密度jR为两者之和;(b)D=170 nm的纳米线的顶部发射率;(c)D=170 nm的纳米线的底部发射率;(d)D=400 nm的纳米线的顶部发射率;(e)D=400 nm的纳米线的底部发射率;(a)-(e)引自参考文献51

    图 6 当纳米线的长度L=4 μm,周期P=400 nm时,不同直径纳米线阵列的发射。(a)不同直径纳米线阵列的顶部和底部反向饱和电流,反向饱和电流密度jR为两者之和;(b)D=170 nm的纳米线的顶部发射率;(c)D=170 nm的纳米线的底部发射率;(d)D=400 nm的纳米线的顶部发射率;(e)D=400 nm的纳米线的底部发射率;(a)-(e)引自参考文献51

    Fig. 6 The emission of NWAs with different diameters when the length L=4 μm and the period P=400 nm. (a) The top and bottom reverse saturation currents of the different diameter NWAs, the reverse saturation current density jR is the sum of the two; (b) the top emissivity of the nanowires with D=170 nm; (c) The bottom emissivity of the nanowires with D=170nm; (d) the top emissivity of nanowires with D=400 nm; (e) the bottom emissivity of nanowires with D=400 nm; (a)-(e), ref. 51, © 2015 ACSP

    综上,在陷光方面,均匀尺寸纳米线阵列通过提供有效的共振和入射光散射来提高吸收率,它们取决于纳米线的直径和周期。纳米线阵列在直径180 nm左右,填充比0.5左右时可以实现900 nm以内优化的光吸[23,26,27,28],这与单结太阳能电池的最优能隙对应。通过将最佳光吸收与最佳p-n结相结合,可以使太阳能电池效率高于Shockley-Queisser极限,这一点已经得到了Peter Krogstrup等人在2013年报道的证[52]。他们介绍了一种使用自催化方法在硅衬底上生长的单根GaAs纳米线太阳能电池,实现一个太阳光照射下180 mA/cm2的短路电流密度(40%的太阳能转换效率),比Lambert-Beer定律预测的效率高一个数量级。在同一年,Jesper Wallentin等人使用外延方法制备的InP纳米线阵列太阳能电池,也证明了纳米线阵列可以突破几何光学限[21],这些报道体现了纳米光学结构的原理性优势。不过,由外延方法制备的纳米线阵列太阳能电池目前最高的效率也只有15.3%[53],这一方面是由于外延生长的纳米线阵列的整体晶体质量相对于薄膜还有差距;另一方面,单一尺寸的半导体纳米柱在发射角的限制上,还不是最优设计,这会产生显著的热力学效率损失;同时,均匀尺寸的周期阵列很难兼顾不同波长的减反和陷光效果,尤其很难满足多级太阳能电池对可见至近红外光高效率转换的要求。

  • 2.2 非均匀尺寸纳米线阵列

    在纳米线阵列的设计中,小直径、低填充比的纳米线阵列可以减少材料的使用,但纳米线之间较大的间隙也会增加底端反射。例如,直径40 nm,周期400 nm的均匀纳米线阵列在400~900 nm的反射高达30%[36]。另一方面,大直径的纳米线阵列有利于对长波光的共振吸收,但过大的直径会增加纳米线顶端的反射。在周期为500 nm的情况下,直径400 nm的纳米线阵列在900 nm以内达到了~26%的反射[34]

    近年来,具有锥形尖端的纳米锥结构被用来减少顶端反射,同时兼顾长波长太阳光的吸收和减少材料使用,这已经被一些理论和实验工作证[54,55,56]。Jia Zhu等人用二氧化硅纳米粒子作为模板,在基于氯的反应离子下刻蚀具有渐变有效折射率的氢化多晶硅纳米线和纳米锥结[54],它们的有效折射率分布如图7(a)所示。可以看出,纳米锥阵列结构由于直径从根部到顶部逐渐收缩,形成有效折射率的分级转变。通过这种逐渐降低的有效折射率来提供结构与空气之间接近完美的阻抗匹配,纳米锥阵列在非晶硅带隙边缘的吸收率仍有88%,远高于纳米线阵列(70%)和薄膜(53%)(图7(b))。此外,纳米锥结构对光入射角非常不敏感,图7(c)显示纳米锥阵列在高达60°的入射角下仍能吸收超过90%的光。由此可见,半导体纳米锥阵列既可用作吸收层又可用作抗反射层,这为简化太阳能电池结构,同时提高能量转换效率提供了一种新的方法。

    图 7
                            在ITO玻璃衬底上分别涂覆1 μm厚的非晶硅薄膜、纳米线阵列和纳米锥阵列。其中,(a)三种结构的示意图及空气与三种结构之间界面处的有效折射率分布;(b)三个样品在正常入射光照射下不同波长处的吸收测量结果;(c)三个样品在不同入射角下的吸收测量结果(波长λ=488 nm)。(a)-(c)引自参考文献54

    图 7 在ITO玻璃衬底上分别涂覆1 μm厚的非晶硅薄膜、纳米线阵列和纳米锥阵列。其中,(a)三种结构的示意图及空气与三种结构之间界面处的有效折射率分布;(b)三个样品在正常入射光照射下不同波长处的吸收测量结果;(c)三个样品在不同入射角下的吸收测量结果(波长λ=488 nm)。(a)-(c)引自参考文献54

    Fig. 7 Schematic illustration of 1 µm thick a-Si:H on ITO coated glass substrate, a monolayer of silica nanoparticles on top of a-Si:H thin film, NW arrays, and NC arrays. (a) The effective refractive index profiles of the interfaces between air and three structures; (b) Measured value of absorption on three samples over a large range of wavelengths at normal incidence; (c) Measured results of absorption on three samples over different angles of incidence (at wavelength λ=488 nm). (a)-(c), ref. 54, © 2009 NL

    Ken Xingze Wang等人将锥形阵列与薄膜结合,设计了如图8(a)的三维减反和陷光结构,这一结构不仅将高效吸收的波长延伸到近1100 nm的红外波段,还能同时减小顶端与底端的自发辐射发射角。作者预测,在仅2 µm厚的单晶硅薄膜上下表面引入锥形光栅阵列,光电流密度可以达到34.6 mA/cm2,接近35.5 mA/cm2的Yablonovitch[57]

    图 8
                            在单晶硅薄膜上下表面引入锥形光栅阵列,形成三维减反和陷光结构,可以实现光浓度因子接近Yablonovitch限,引自参考文献57

    图 8 在单晶硅薄膜上下表面引入锥形光栅阵列,形成三维减反和陷光结构,可以实现光浓度因子接近Yablonovitch限,引自参考文献57

    Fig. 8 Introducing a tapered shape grating array on the top and bottom surfaces of the single crystal silicon film to form a three-dimensional anti-reflection and light trapping structure, which can achieve a light concentration factor close to the Yablonovitch limit; ref. 57, © 2012 NL

    与锥形结构类似,Silke L. Diedenhofen等人用金属有机气相外延法(MOVPE)制备带有锥形基座的多级InP纳米线阵[56];Zhiyong Fan等人用自组织阳极氧化铝膜作为模板,在铝箔上使用汽-液-固(vapor-liquid-solid)方法生长有序的双直径Ge纳米线阵[58],均实现了在900 nm内至少98%的超高吸收率(如图9(a),9(b)所示)。均匀和非均匀尺寸纳米线吸收光的能力可以通过计算它们的消光效率来评[59],其公式如下:

    Qext-array = Qext-mo× [ρ(n)×Across],where  if  Qext-array  >  1 , Qext-array=1 ,
    (3)

    其中,Qext-mo是单根纳米线的消光效率,ρ(n)是表面密度数,Across是纳米线的横截面积。图9(c)显示了由五个不同直径组成的非均匀尺寸纳米线和均匀尺寸纳米线的消光效率谱。从图中可以看出,在非均匀尺寸纳米线的消光曲线中出现了五个峰值,其分别对应于具有不同直径的五个子纳米线,而在均匀尺寸纳米线的消光曲线中则只出现了一个峰值。图9(d)显示了均匀和非均匀尺寸纳米线阵列的太阳光谱加权吸收。相比于均匀尺寸纳米线,非均匀尺寸纳米线阵列表现出显著的聚光优势,特别是在表面覆盖率为约8.5%的情况下,非均匀尺寸纳米线阵列可以比均匀尺寸纳米线阵列多收集20.8%的太阳光。与纳米锥结构相似,这种优越性可以通过其连续直径而激发的多个共振模式来解释。

    图 9
                            多级纳米线阵列可以形成全向吸收介质。(a)测量的带有锥形基座纳米线阵列的吸收率,纳米线的长度为3 μm,顶部直径为90 nm,底部直径为270 nm;(b)具有D1=60 nm和D2=130 nm的双直径纳米线阵列及直径为60和130 nm的单直径纳米线阵列的实验吸收光谱;(c)由等量材料组成的多级分段纳米线和圆柱形纳米线的消光效率谱;(d)在不同的表面覆盖率下,由Si纳米锥和Si纳米线阵列收集的太阳光的一小部分。(a)引自参考文献56,(b)引自参考文献58,(c)和(d)引自参考文献59

    图 9 多级纳米线阵列可以形成全向吸收介质。(a)测量的带有锥形基座纳米线阵列的吸收率,纳米线的长度为3 μm,顶部直径为90 nm,底部直径为270 nm;(b)具有D1=60 nm和D2=130 nm的双直径纳米线阵列及直径为60和130 nm的单直径纳米线阵列的实验吸收光谱;(c)由等量材料组成的多级分段纳米线和圆柱形纳米线的消光效率谱;(d)在不同的表面覆盖率下,由Si纳米锥和Si纳米线阵列收集的太阳光的一小部分。(a)引自参考文献56,(b)引自参考文献58,(c)和(d)引自参考文献59

    Fig. 9 Multistage NWAs could form a omnidirectional absorbing medium. (a) Measured absorbance in the arrays of base-tapered InP nanowires. The length of NW is 3 μm, the top diameter is 90 nm, the bottom is 270 nm; (b) Experimental absorption spectra of double-diameter NWAs with D1=60 nm and D2=130 nm and single diameters NWAs of 60 and 130 nm; (c) extinction efficiency spectra of multi-stage nanowires and cylindrical nanowires composed of equal materials; (d) The fraction of sunlight being harvested by the nanocone and nanowire array, under different surface coverage rates. (a) ref. 56, © 2011 ACS NANO; (b) ref. 58, © 2010 NL; (c-d) ref. 59, © 2015 SR

    图10(a)显示了直径连续变化的纳米锥结构在不同波长下的内部场强分[38]。可以看出,在λ≥500 nm时,纳米锥内部均存在径向共振模式,这种径向TM11共振随着波长的增加向下移动到更大直径处,并且由于其与纵向共振的卷积而在垂直方向上具有多个波瓣。对于400 nm波长,纳米锥中没有可见的强径向模式,这是因为半导体材料在该区域中强烈吸收并且光不能深入到纳米锥中以建立径向模式。另外,在500 nm波长下,还在纳米锥的底部看到了TM12模式的特征。在一个阵列周期内排布多个不同直径的纳米线以及随机排列纳米线阵列中也能增加共振数[60,61,62](如图7(b),7(c)所示)。

    图10
                            非均匀尺寸和周期纳米线阵列可以增加共振数量。(a)顶部直径为40 nm,底部直径为100 nm,高度为3 μm的纳米锥阵列分别在400, 500, 600, 700和800 nm入射波长下的电场强度分布;(b)均匀直径和随机直径纳米线阵列的场强分布;(c)均匀排列和随机排列纳米线阵列的场强分布。(a)引自参考文献38,(b)引自参考文献60,(c)引自参考文献61。

    图10 非均匀尺寸和周期纳米线阵列可以增加共振数量。(a)顶部直径为40 nm,底部直径为100 nm,高度为3 μm的纳米锥阵列分别在400, 500, 600, 700和800 nm入射波长下的电场强度分布;(b)均匀直径和随机直径纳米线阵列的场强分布;(c)均匀排列和随机排列纳米线阵列的场强分布。(a)引自参考文献38,(b)引自参考文献60,(c)引自参考文献61。

    Fig. 10 Non-uniform sized and randomly arranged NWAs can increase the number of resonance. (a) The electric field intensity distribution at the incident wavelengths of 400, 500, 600, 700 and 800 nm for the nanocone arrays with a top diameter of 40 nm and a bottom diameter of 100 nm and a height of 3 μm. (b) Field distribution of the uniform diameter and random diameter NWAs; (c) Uniformly arranging and randomly arranging the field distribution of the NWAs. (a) ref. 38, © 2014 OSA; (b) ref. 60, © 2010 OSA; (c) ref. 61, © 2011 OSA

    表1总结了近年来非均匀尺寸纳米线阵列吸收增强的实验和理论研究。从这些非均匀尺寸纳米线阵列吸收的详细分析中,我们得出结论,通过在结构中引入变化的直径或周期,可以在阵列中激发出更多的HE11共振和导-共振,从而实现对太阳光的全向高效吸收。

    Table 1 A summary of methods for achieving high-efficiency light absorption in NWAs.

    GeometrySynthesis Methods

    Diameter

    /nm

    Length

    (μm)

    FR/(%)Rmaxa/(%)ηb/(%)ApproachesRef.
    Experimental/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F011.pngMask+etching

    20~300

    300

    LNW=0.6, LFilm=0.4

    46

    57

    /

    87

    76

    Nanocone54
    /html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image012.png

    GaAs

    Mask+etching

    40~112

    115

    Ltip=0.4, Lbot=1.6

    2

    4.4

    5.1

    22

    28

    68

    65

    Tapered tip55
    /html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image013.png

    InP

    MOVPE

    90~270

    90

    LNW=2, Lbase=1

    3

    5.1

    2.4

    12

    65

    85.4

    53.7

    Tapered base56
    /html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F014.png

    Ge

    Template- assisted VLS

    60~130

    60

    130

    L1=1, L2=1

    2

    2

    35.8

    12.6

    59

    1

    3

    20

    97.7

    86.7

    83.1

    Dual-diameter58
    /html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image015.jpegMetal assisted electroless etching~35

    12

    16

    12

    5

    22

    36

    34

    Ag:92.6

    78.9

    70

    Increased length;

    Ag-back reflector.

    63
    /html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F016.pngTemplate- assisted VLS~2500674.2/

    84.4

    80.8

    AR coating;

    Al2O3 particles;

    Ag-back reflector.

    64
    Calculation/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image017.png

    Si

    /652.33~3370

    60

    29

    Random array60
    /html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image018.png

    Si

    /60~1402~2010

    52

    31.1

    Random diameter61
    /html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F019.png/2001.112.6/

    5 DBR:95

    1 DBR:78

    72

    Integrated bragg reflectors65
    /html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F020.png/

    D=Db=200

    200

    L=1.1, Lb=0.25

    1.1

    15.4

    12.6

    /

    78

    72

    Branched NWs66

    注:备注:a.最大反射率,b.加权吸收

    表1 纳米线阵列实现高效光吸收的方法总结。

  • 3 结论

    太阳能光伏的研究和应用在过去十年取得了很大进展,相比于此前对光伏材料及其电子学特性的改善,近年来光伏研究的一个重要方向是通过在纳米至微米尺度上对材料进行光子学设计,这有望使太阳能电池效率突破Shockley-Queisser限制,其中一个引人关注的前沿是基于半导体纳米线阵列的光伏结构。

    本文用热力学方法分析了光伏结构的开路电压损失,并针对其中的熵损失机制介绍了半导体纳米线阵列的光管理方案以及相关的研究进展:通过引入多种局域共振的光学模式,增加电池材料激活区的光浓度;同时,利用纳米结构对发射角的限制,减少电池自发辐射的效率损失;而采用渐变尺寸的纳米线构型,能够进一步将纳米线阵列高效率吸收的优势拓展到近红外波段。这些光学层面的综合设计,使得由半导体纳米线阵列形成的光伏结构具有50%~70%的理论转换效率。

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童中英

机 构:

1. 上海理工大学 材料科学与工程学院,上海 200093

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:

1. School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

邮 箱:tzy883721@163.com

作者简介:(Biography):(Biography):童中英(1992-), 女,安徽芜湖人, 硕士研究生,主要研究领域为可见至近红外宽波段光吸收优化. E-mail:tzy883721@163.com

谢天

机 构:

1. 上海理工大学 材料科学与工程学院,上海 200093

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:

1. School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

叶新辉

机 构:

1. 上海理工大学 材料科学与工程学院,上海 200093

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:

1. School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

夏辉

机 构:中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

李菊柱

机 构:

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

3. 上海师范大学 数理学院,上海 200234

Affiliation:

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

3. Mathematics and Science College, Shanghai Normal University, Shanghai 200234, China

张帅君

机 构:

1. 上海理工大学 材料科学与工程学院,上海 200093

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:

1. School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

姜新洋

机 构:

2. 中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

4. 上海科技大学 物质科学与技术学院,上海 201210

Affiliation:

2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

4. School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

陈泽中

机 构:上海理工大学 材料科学与工程学院,上海 200093

Affiliation:School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

角 色:通讯作者

Role:Corresponding author

邮 箱:txli@mail.sitp.ac.cnzzhchen@usst.edu.cn

作者简介:E-mail: txli@mail.sitp.ac.cn; zzhchen@usst.edu.cn

李天信

机 构:中国科学院上海技术物理研究所 红外物理国家重点实验室,上海 200083

Affiliation:State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

角 色:通讯作者

Role:Corresponding author

邮 箱:txli@mail.sitp.ac.cnzzhchen@usst.edu.cn

作者简介:E-mail: txli@mail.sitp.ac.cn; zzhchen@usst.edu.cn

/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F001.png
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/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F006.png
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F007.png
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F008.png
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F009.png
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F010.png
GeometrySynthesis Methods

Diameter

/nm

Length

(μm)

FR/(%)Rmaxa/(%)ηb/(%)ApproachesRef.
Experimental/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F011.pngMask+etching

20~300

300

LNW=0.6, LFilm=0.4

46

57

/

87

76

Nanocone54
/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image012.png

GaAs

Mask+etching

40~112

115

Ltip=0.4, Lbot=1.6

2

4.4

5.1

22

28

68

65

Tapered tip55
/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image013.png

InP

MOVPE

90~270

90

LNW=2, Lbase=1

3

5.1

2.4

12

65

85.4

53.7

Tapered base56
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F014.png

Ge

Template- assisted VLS

60~130

60

130

L1=1, L2=1

2

2

35.8

12.6

59

1

3

20

97.7

86.7

83.1

Dual-diameter58
/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image015.jpegMetal assisted electroless etching~35

12

16

12

5

22

36

34

Ag:92.6

78.9

70

Increased length;

Ag-back reflector.

63
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F016.pngTemplate- assisted VLS~2500674.2/

84.4

80.8

AR coating;

Al2O3 particles;

Ag-back reflector.

64
Calculation/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image017.png

Si

/652.33~3370

60

29

Random array60
/html/hwyhmbcn/2019115/media/6fb96284-ef15-4c89-bc7b-5dc791005b17-image018.png

Si

/60~1402~2010

52

31.1

Random diameter61
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F019.png/2001.112.6/

5 DBR:95

1 DBR:78

72

Integrated bragg reflectors65
/html/hwyhmbcn/2019115/alternativeImage/6fb96284-ef15-4c89-bc7b-5dc791005b17-F020.png/

D=Db=200

200

L=1.1, Lb=0.25

1.1

15.4

12.6

/

78

72

Branched NWs66

图 1 太阳能转换中的热力学损失。对于传统的单结太阳能电池,实现的最大效率为28.3%(以绿色表示)。浅蓝色表示与能量相关的损失,深蓝色表示与熵相关的损失,减少能量和熵损失问题的解决方案列在右栏中;引自参考文献3

Fig. 1 Thermodynamic losses in solar-energy conversion. The maximum effciency realized for a conventional single-junction solar cell is 28.3% (indicated in green). Light blue bars indicate energy-related losses and dark blue bars indicate entropy-related losses. The solutions to reducing the energy- and entropy-loss problems are listed in the right-hand column. ref 3, © 2012 NM

图 2 薄膜光伏电池中多种共振模式激发引起的吸收增强,其中电池由金属背反射器、1 μm厚结晶硅薄膜和周期性结晶硅纳米线阵列组成,图中白色虚线表示结晶硅的结构。(a)λ=880 nm,垂直入射,显示光学共振模式与导-共振模式的混合激发;(b)λ=1031 nm,入射角为28°,显示法布里—珀罗共振模式的激发;(c)λ=946 nm,垂直入射,显示硅层中导—共振模式的激发;(d)λ=1011 nm,垂直入射,显示出一种不同的共振,使得在陷光层中激发横向传播的波。(a)-(d)引自参考文献22

Fig. 2 Absorption enhancement caused by the excitation of multiple optical resonance modes in a thin PV cell. It shows a 1-μm-thick c-Si film with a perfect back mirror and a light-trapping layer consisting of a periodic array of c-Si nanowires (NWs) on the top surface. The dashed white lines outline the c-Si structure. (a) λ=880 nm, normal incidence, showing the excitation of a mixture of a Mie resonance with a guided resonance; (b) λ=1031 nm, 28° angle of incidence, showing the excitation of a Fabry-Perot resonance; (c) λ=946 nm, normal incidence, showing the excitation of a guided resonance in the Si layer; (d) λ=1011 nm, normal incidence, showing a diffracted resonance by which a laterally propagating wave is excited in the light-trapping layer. a-d, ref. 22, © 2014 NM

图 3 半导体纳米线阵列结构可以减小吸收角和发射角之间的差距,使器件实现>40%的功率转换效率。(a)具有聚光结构的传统平面太阳能电池能增加吸收角Ωabs以接近发射角Ωemit,从而减少由它们的失配引起的熵损失;(b)同样,纳米结构太阳能电池也可以降低ΩabsΩemit之间的差异;(c)-(e)分别为传统平面太阳能电池、具有背反射器的平面太阳能电池和理想的纳米结构太阳能电池的吸收和发射角;(f)对应于三种结构(c-e)的I-V曲线,显示开路电压的增加是源自发射角逐渐趋近于吸收角。(a)-(f)引自参考文献8

Fig. 3 Semiconductor nanowire arrays(NWAs) can reduce the mismatch of angles between absorption and emission, which results in an ideal PV nanostructure achieving >40% power conversion effciency. (a) A traditional planar solar cell with concentrator increases Ωabs to approach Ωemit, thus reducing the entropy generation caused by their mismatch; (b) Similarly, a nanostructured solar cell can reduce the difference between Ωabs and Ωemit; Emission and absorption for (c) slab without back reflector, (d) slab with back reflector, and (e) ideal nanostructured cell; (f) I-V curves corresponding to the three structures, shows the increased Voc as θemitθs. (a)-(f), ref. 8, © 2015 SR

图 4 纳米线内部激发的不同光学模式及其对光吸收的贡献。(a)纳米线激发的最低阶横磁模式(p=0、1、2和3,从左到右示出);(b)不同横截面形状的纳米线内部的电场分布,入射光选择由纳米线上方的横磁照射,激发波长为p=1的共振波长;(c)纳米线激发的HE1q模式的共振波长和纳米线直径函数的吸收等高线图;(d)GaAs NW阵列的吸收效率随着入射波长的变化而变化,FR为0.05,插图显示了GaAs的复折射率;(a)-(d)分别引自参考文献47、13、39和40

Fig. 4 Different optical resonance modes excited inside the nanowires and their contribution to light absorption. (a) Lowest-order transverse magnetic optical modes (p=0, 1, 2, 3, shown from left to right) supported by subwavelength high-refractive-index nanocylinders; (b) Electric field distributions inside the nanowires of different cross-section shapes for transverse magnetic illumination from above. The excitation wavelength was chosen to drive an p=1 resonance; (c) Contour plot of absorption as a function of photon wavelength and nanowire diameter for vertical NWAs; (d) Absorption effciency of GaAs NW arrays change with incident wavelength with FR of 0.05 and the insets illustrate the complex refractive index for GaAs. (a) ref. 47, © 2009 NM; (b) ref. 13, © 2010 NL; (c) ref. 39, © 2012 OL; (d) ref. 40, © 2017 SR

图 5 固定填充比和固定直径下纳米线阵列的吸收谱。(a)周期性纳米线阵列结构示意图;(b)固定填充比D/P=0.5时,不同直径纳米线阵列的吸收谱;(c)固定直径D=120 nm时,不同填充比下纳米线阵列的吸收谱;(a)-(c)引自参考文献50

Fig. 5 Absorption spectra of NWAs at fixed filling ratios and fixed diameters. (a) Schematic drawing of the periodic NWAs structure; (b) Absorptance of NWAs with different diameters varied from 60 to 240 nm under the fixed filling ratio of 0.5; (c) Absorptance of NWAs with different filling ratios varying from 0.4 to 0.8 under the fixed diameter of 120 nm. (a)-(c), ref. 50, © 2011 NRL

图 6 当纳米线的长度L=4 μm,周期P=400 nm时,不同直径纳米线阵列的发射。(a)不同直径纳米线阵列的顶部和底部反向饱和电流,反向饱和电流密度jR为两者之和;(b)D=170 nm的纳米线的顶部发射率;(c)D=170 nm的纳米线的底部发射率;(d)D=400 nm的纳米线的顶部发射率;(e)D=400 nm的纳米线的底部发射率;(a)-(e)引自参考文献51

Fig. 6 The emission of NWAs with different diameters when the length L=4 μm and the period P=400 nm. (a) The top and bottom reverse saturation currents of the different diameter NWAs, the reverse saturation current density jR is the sum of the two; (b) the top emissivity of the nanowires with D=170 nm; (c) The bottom emissivity of the nanowires with D=170nm; (d) the top emissivity of nanowires with D=400 nm; (e) the bottom emissivity of nanowires with D=400 nm; (a)-(e), ref. 51, © 2015 ACSP

图 7 在ITO玻璃衬底上分别涂覆1 μm厚的非晶硅薄膜、纳米线阵列和纳米锥阵列。其中,(a)三种结构的示意图及空气与三种结构之间界面处的有效折射率分布;(b)三个样品在正常入射光照射下不同波长处的吸收测量结果;(c)三个样品在不同入射角下的吸收测量结果(波长λ=488 nm)。(a)-(c)引自参考文献54

Fig. 7 Schematic illustration of 1 µm thick a-Si:H on ITO coated glass substrate, a monolayer of silica nanoparticles on top of a-Si:H thin film, NW arrays, and NC arrays. (a) The effective refractive index profiles of the interfaces between air and three structures; (b) Measured value of absorption on three samples over a large range of wavelengths at normal incidence; (c) Measured results of absorption on three samples over different angles of incidence (at wavelength λ=488 nm). (a)-(c), ref. 54, © 2009 NL

图 8 在单晶硅薄膜上下表面引入锥形光栅阵列,形成三维减反和陷光结构,可以实现光浓度因子接近Yablonovitch限,引自参考文献57

Fig. 8 Introducing a tapered shape grating array on the top and bottom surfaces of the single crystal silicon film to form a three-dimensional anti-reflection and light trapping structure, which can achieve a light concentration factor close to the Yablonovitch limit; ref. 57, © 2012 NL

图 9 多级纳米线阵列可以形成全向吸收介质。(a)测量的带有锥形基座纳米线阵列的吸收率,纳米线的长度为3 μm,顶部直径为90 nm,底部直径为270 nm;(b)具有D1=60 nm和D2=130 nm的双直径纳米线阵列及直径为60和130 nm的单直径纳米线阵列的实验吸收光谱;(c)由等量材料组成的多级分段纳米线和圆柱形纳米线的消光效率谱;(d)在不同的表面覆盖率下,由Si纳米锥和Si纳米线阵列收集的太阳光的一小部分。(a)引自参考文献56,(b)引自参考文献58,(c)和(d)引自参考文献59

Fig. 9 Multistage NWAs could form a omnidirectional absorbing medium. (a) Measured absorbance in the arrays of base-tapered InP nanowires. The length of NW is 3 μm, the top diameter is 90 nm, the bottom is 270 nm; (b) Experimental absorption spectra of double-diameter NWAs with D1=60 nm and D2=130 nm and single diameters NWAs of 60 and 130 nm; (c) extinction efficiency spectra of multi-stage nanowires and cylindrical nanowires composed of equal materials; (d) The fraction of sunlight being harvested by the nanocone and nanowire array, under different surface coverage rates. (a) ref. 56, © 2011 ACS NANO; (b) ref. 58, © 2010 NL; (c-d) ref. 59, © 2015 SR

图10 非均匀尺寸和周期纳米线阵列可以增加共振数量。(a)顶部直径为40 nm,底部直径为100 nm,高度为3 μm的纳米锥阵列分别在400, 500, 600, 700和800 nm入射波长下的电场强度分布;(b)均匀直径和随机直径纳米线阵列的场强分布;(c)均匀排列和随机排列纳米线阵列的场强分布。(a)引自参考文献38,(b)引自参考文献60,(c)引自参考文献61。

Fig. 10 Non-uniform sized and randomly arranged NWAs can increase the number of resonance. (a) The electric field intensity distribution at the incident wavelengths of 400, 500, 600, 700 and 800 nm for the nanocone arrays with a top diameter of 40 nm and a bottom diameter of 100 nm and a height of 3 μm. (b) Field distribution of the uniform diameter and random diameter NWAs; (c) Uniformly arranging and randomly arranging the field distribution of the NWAs. (a) ref. 38, © 2014 OSA; (b) ref. 60, © 2010 OSA; (c) ref. 61, © 2011 OSA

Table 1 A summary of methods for achieving high-efficiency light absorption in NWAs.

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备注:a.最大反射率,b.加权吸收

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