CN104393086B - 复合光伏电池 - Google Patents

复合光伏电池 Download PDF

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CN104393086B
CN104393086B CN201410541385.2A CN201410541385A CN104393086B CN 104393086 B CN104393086 B CN 104393086B CN 201410541385 A CN201410541385 A CN 201410541385A CN 104393086 B CN104393086 B CN 104393086B
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CN104393086A (zh
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佐藤俊
佐藤俊一
西山伸彦
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Ricoh Co Ltd
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Abstract

一种复合光伏电池，包括：第一衬底；由第一复合半导体材料制成并形成在第一衬底上的一个或多个第一光电转换单元；形成在一个或多个第一光电转换单元上的隧道结层；以及一个或多个第二光电转换单元，由与第一衬底的材料晶格失配的第二复合半导体材料制成，经隧道结层连接到一个或多个第一光电转换单元，并相对于一个或多个第一光电转换单元而位于光入射方向中的入射侧。一个或多个第一光电转换单元和一个或多个第二光电转换单元的带隙在光入射方向上从入射侧到背侧变小。隧道结层包括位于入射侧的p型层和位于背侧的n型层，该p型层是p+型(Al)GaInAs层，该n型层是n+型InP层，相对于InP层具有拉伸应变的n+型GaInP层，或相对于InP具有拉伸应变的n+型Ga(In)PSb层。

Description

复合光伏电池
技术领域
本发明涉及一种复合光伏电池。
背景技术
已知的一种聚光光伏电池，其采用廉价的聚光透镜或反射镜来聚拢阳光并将所聚拢的阳光转化成电能。通过相对容易地控制复合半导体材料的构成比能够改变复合光伏电池的带隙。因此，正在进行的研究是通过吸收更宽波长范围的阳光来提高复合光伏电池的能量转换效率。
此外，为了改善转换效率(例如参见非专利文献1)，一种光伏电池将与GaAs晶格匹配的GaInP单元和GaAs单元沉积到GaAs衬底上，并且经晶格弛豫缓冲层将与GaAs具有约2％的晶格失配的GaInAs单元沉积到GaInP单元和GaAs单元上。
此外，一种光伏电池将GaInP顶部单元和高掺杂p+型层形成在GaAs衬底上，将GaInAs底部单元和高掺杂n+型层形成在InP衬底上，并且将p+型层和n+型层连接(粘合)在一起以形成一个隧道结层。该GaInP顶部单元和GaInAs底部单元经该隧道结层彼此串联粘合在一起(例如参见专利文献1)。
如果该隧道结层的连接界面的电阻是高的，隧道电流会更难流过隧道结层。结果，转换效率降低。
由于隧道结层的连接界面的电阻不会完全降低，专利文献1所公开的光伏电池不会得到足够的转换效率。
专利文献1：US 2012/0138116
非专利文献1：Proceedings of the 29th IEEE photovoltaic specialistsconference(第29届IEEE光伏专家会议论文集)(2010)第412-417页。
发明内容
本发明是在鉴于上述问题而做出的，本发明的至少一个实施例之目的是提供一种具有改善的转换效率的复合光伏电池。
本发明的一个方面提供一种复合光伏电池，其包括：第一衬底；构造成由第一复合半导体材料制成并形成在第一衬底上的一个或多个第一光电转换单元；形成在一个或多个第一光电转换单元上的隧道结层；以及一个或多个第二光电转换单元，构造成由与第一衬底的材料晶格失配的第二复合半导体材料制成，经隧道结层连接到该一个或多个第一光电转换单元，并相对于该一个或多个第一光电转换单元而言位于光入射方向中的入射侧。其中，该一个或多个第一光电转换单元和该一个或多个第二光电转换单元的带隙在光入射方向上从入射侧到背侧变小；并且，该隧道结层包括位于入射侧的p型层以及位于背侧的n型层，该p型层是p+型(Al)GaInAs层，该n型层是n+型InP层，相对于InP层具有拉伸应变的n+型GaInP层，或者相对于InP具有拉伸应变的n+型Ga(In)PSb层。
附图说明
图1A和1B示出根据第一实施例的复合光伏电池。
图2示出根据第一实施例的复合光伏电池。
图3A示出根据对比例的隧道结层的能带图。
图3B示出根据第一实施例的隧道结层的能带图。
图4示出该复合光伏电池的I-V曲线。
图5A和5B示出根据第一实施例的复合光伏电池的制造过程。
图6示出根据第一实施例的复合光伏电池的制造过程。
图7示出根据第一实施例的复合光伏电池的制造过程。
图8示出根据第一实施例的复合光伏电池的制造过程。
图9示出根据第二实施例的复合光伏电池。
图10示出根据第三实施例的复合光伏电池。
图11示出根据第四实施例的复合光伏电池。
图12示出根据第五实施例的复合光伏电池。
图13示出根据第六实施例的复合光伏电池。
具体实施方式
下面将参照附图来说明本发明的实施例。
第一实施例
图1A、1B和2示出根据第一实施例的复合光伏电池100的截面图。
该复合光伏电池100包括电极101、电极102、InP衬底103、GaInPAs104、隧道结层105、GaAs单元106、隧道结层107、GaInP单元108和接触层109。
隧道结层105形成在GaInPAs单元104和GaAs单元106之间。隧道结层107形成在GaAs单元106和GaInP单元108之间。GaInPAs单元104、GaAs单元106和GaInP单元108彼此电学和光学串联连接，通过隧道结层105和107而被连接在一起。在此，GaInPAs单元104、GaAs单元106和GaInP单元108中的每一个都是一个光电转换单元。因此，能够产生流过具有彼此不同带隙的多个光电转换单元的电流。
光电转换单元的带隙从顶侧到底侧沿着光入射方向变窄(变小)，这由图1的箭头表示出。GaInP单元108、GaAs单元106、GaInPAs单元104的带隙分别为1.9eV、1.42eV和1.0eV。
每一个隧道结层105和107为薄p+n+结层，其中，p+型半导体层和n+型半导体层连接。在该隧道结层中，因为高浓度掺杂，n+型半导体层的导带和p+型半导体层的价带退化。因为导带和价带彼此搭接并在之间夹着一个费米能级，由此载流子的隧穿概率增加。因此，能够使隧道电流从p+型半导体层流向n+型半导体层。
隧道结层105优选包括p+型GaInAs层105A和n+型InP层105B连接的隧道结层(参见图1A)。否则，该隧道结层105优选包括p+型AlGaInAs层105A1具有比GaAs和n+型InP层105B窄的带隙的隧道结层(参见图1B)。下面，p+型GaInAs层105A和p+型AlGaInAs层105A1可称为p+型(Al)GaInAs。
通过形成p+型(Al)GaInAs层与n+型InP层105B连接的隧道结层105，能够充分减小隧道结层105的连接界面的电阻并使得隧道电流更容易地从n+型半导体层流向p+型半导体层。该隧道结层105的连接界面是这样的表面：p+型(Al)GaInAs层与n+型InP层105B在此连接。
隧道结层105具有这样的结构：如图2所示，p+型GaInAs层105A、n+型InP层105B和p+型GaAs层105C堆叠在一起。该p+型GaAs层105C为高掺杂p型GaAs层。该GaAs层105C设置在p+型GaInAs层105A和GaAs单元106之间。在一种情况下，图2中的复合光伏电池120所包括的隧道结层105具有三层结构隧道结层，包括p+型GaAs层105C、p+型GaInAs层105A和n+型InP层105B，从而，相对于图1中的复合光伏电池100的二层结构隧道结层所包括的p+型GaInAs层105A，这能够减小p+型GaInAs层105A的厚度。
n+型InP层105B的杂质例如可以是Si(硅)或Te(碲)。p+型GaInAs层105A、p+型AlGaInAs层105A1和p+型GaAs层105C的杂质例如可以为C(碳)。
接着，参见图3来说明隧穿概率和隧道电流，图3示出隧道结层的能带图。图3A示出根据对比例的隧道结层(p+GaAs/n+InP)的能带图。图3B为根据第一实施例的隧道结层105(p+GaAs/p+GaInAs/n+InP)能带图。p+型GaInAs层105A的厚度为5nm，并且In(铟)构成比为20％。在图3A和3B中，水平轴表示隧道结层的厚度[μm]，并且垂直轴表示能量[eV]。
在图3A中，空穴从p+型GaAs层的价带隧穿到n+型InP层的导带。电子从n+型InP层的导带隧穿到p+型GaAs层的价带。在图3B中，空穴从p+型GaInAs层105A的价带隧穿到n+型InP层105B的导带。电子从n+型InP层105B的导带隧穿到p+型GaInAs层的价带105A。
如图3B所示，p+型GaInAs层105A的价带具有向上地突出的凸部。这是因为p+型GaInAs层105A具有比p+型GaAs层105C窄的带隙。因此，该p+型GaInAs层105A的导带的能级低于p+型GaAs层105C的导带的能级，并且该p+型GaInAs层105A的价带的能级高于p+型GaAs层105C的价带的能级。
因为如图3B所示的p+型GaInAs层105A的价带和n+型InP层105B的导带之间的能级差窄于图3A所示的p+型GaAs层的价带和n+型InP层的导带之间的能级差，所以图3B所示的隧道结层105的耗尽层变得比图3A所示的隧道结层的耗尽层窄。
因此，相比于图3A所示的隧道结层，图3B所示的第一实施例的隧道结层105具有高的载流子隧穿概率以及在连接界面处低的电阻。在此，存在于价带凸部的载流子也有助于增加隧穿概率。在此，p+型AlGaInAs层包括Al(铝)并具有比GaAs窄的带隙，可以替代p+型GaInAs层105A。p+型AlGaAs层包括Al(铝)，可以代替图2所示的p+型GaAs层105C用于隧道结层105。这种情况下，p+型AlGaInAs层可以代替p+型GaInAs层105A。AlGaAs和GaAs可以表示为(Al)GaAs。因此，(Al)GaAs包括GaAs和AlGaAs。有必要设计一种多结光伏电池，例如复合光伏电池100，以便电池104、106和109所产生的电流彼此具有相同值。例如，复合光伏电池100设计为，GaAs单元106不吸收在能被该GaAs单元106所吸收的波长范围内所有的光，而是将一部分光传输到位于入射光方向背侧的GaInPAs单元104中。在这种情况下，为了在位于GaAs单元106和GaInPAs单元104之间的隧道结层105处抑制所传输的光的吸收，优选通过采用具有比GaAs更宽带隙的材料例如AlGaAs来形成形成隧道结层105。这同样应用于其它实施例。
隧道结层107包括p+型Al(x)GaAs层107A和n+型Ga(x)InP层107B。
该GaInP单元108包括n型Al(x)InP层108A、n型Ga(x)InP层108B、p型Ga(x)InP层108C和p型Al(x)InP层108D，它们沿着光入射方向依次形成。n型Al(x)InP层108A为窗口层。p型Al(x)InP层108D是背面场(BSF)层。GaInP单元108可以包括位于光入射侧的抗反射涂层等。
调节n型Ga(x)InP层108B和p型Ga(x)InP层108C的Ga(镓)的构成比，使得GaInP单元108的带隙变为1.9eV。可以通过调节混合晶体半导体的构成比来容易地控制物理性质，例如二元、三元或四元III-V族复合半导体的带隙、晶格常数等。因此，能够设定这样的波长范围：光电转换单元利用III-V族复合半导体任意地吸收光。
n型Ga(x)InP层108B的杂质例如可以是Si(硅)。p型Ga(x)InP层108C的杂质例如可以为Zn(锌)。
GaAs单元106包括n型[Al(x)Ga](y)InP层106A、n型GaAs层106B、p型GaAs层106C和p型Ga(x)InP层106D，它们沿着光入射方向依次形成。该n型[Al(x)Ga](y)InP层106A为窗口层。p型Ga(x)InP层106D为背面场(BSF)层。
n型GaAs层106B的杂质例如可以为Si(硅)。p型GaAs层106C的杂质例如可以为Zn(锌)。
GaInPAs单元104包括n型InP层104A、n型Ga(x)InP(y)As层104B、p型Ga(x)InP(y)As层104C和p型InP层104D，它们沿着光入射方向依次形成。n型InP层104A是窗口层。p型InP层104D为背面场(BSF)层。
调整n型Ga(x)InP(y)As层104B和p型Ga(x)InP(y)As层104C的Ga(镓)的构成比x和构成比y，使得GaInPAs单元104的带隙变为1.0eV。
n型Ga(x)InP(y)As层104B的杂质例如可以为Si(硅)。p型Ga(x)InP(y)As层104C的杂质例如可以为Zn(锌)。
p型Ga(x)InP层(p型BSF层)106D接触隧道结层105的p+型GaInAs层105A(参见图1A)，p+型AlGaInAs层105A1(参见图1B)或p+型GaAs层105C(参见图2)。p型Al(x)InP层(p型BSF层)108D接触隧道结层107的p+型Al(x)GaAs层107A。n型[Al(x)Ga](y)InP层(n型窗口层)106A接触隧道结层107的n+型Ga(x)InP层107B。n型InP层(n型窗口层)104A接触隧道结层105的n+型InP层105B。通过形成接触BSF层或窗口层并具有与上述BSF层或窗口层相同导电类型的隧道结层105和106，能够使电流顺利地流过光电转换单元，即流经GaInPAs单元104、GaAs单元106和GaInP单元108。
该光电转换单元的材料不限于上述材料，例如InP、GaAs和GaInP。然而，优选通过采用InP晶格匹配材料形成位于InP衬底103和n+型InP层105B之间的GaInPAs单元104。此外，优选通过采用GaAs晶格匹配材料形成位于p+型GaInAs层105A和接触层109之间的GaAs单元106和GaInP单元108。
该接触层109例如可以是n+型GaAs层。优选形成该接触层109，使得接触层109具有比GaInP单元108小的平面尺寸。这是为了减少接触层109的光吸收并增加GaInP单元108的入射光。
电极101和102可以由Ti、Pt、Au等制造。电极101和102的每一个可以包括由上述材料之一制成的单层或包括由上述材料制成的多层。
可以基于图4所示的I-V曲线确定复合光伏电池100的转换效率。如果电流和电压变大则转换效率变高，并且如图4所示的I-V曲线具有方形形状。在I-V曲线为三角形的情况下，根据电压的增加不能获得足够的电流。结果，转换效率变低。
因为隧道结层105由p+GaInAs层105A和n+InP层105B形成，所以能够减小隧道结层105的隧道连接界面的电阻。因为复合光伏电池100包括建立了彼此具有不同带隙的多结结构的多个光电转换单元，所以能够吸收宽的波长范围内的阳光。因此，复合光伏电池100的转换效率提高了。
下面，结合图5A、5B、6、7和8来说明根据第一实施例的复合光伏电池120的制造方法。图5A、5B、6、7和8示出根据第一实施例的复合光伏电池120。
如图5所示，叠层体100A形成在GaAs衬底110上，并且叠层体100B形成在InP衬底103上。叠层体100A和100B的制造方法可以是金属有机化学气相沉积(MOCVD)法、分子束外延(MBE)法等。还可以采用InP衬底103和GaAs衬底110之外的衬底。例如，可以采用GaSb衬底和GaAs衬底的组合、Si衬底和GaAs衬底的组合等代替InP衬底103和GaAs衬底110的组合。
该叠层提100A的形成是通过将蚀刻停止层111、接触层112、GaInP单元108、隧道结层107、GaAs单元106、p+型GaAs层105C和p+型GaInAs层105A依次沉积在GaAs衬底110上(参见图5A)。
GaAs单元106和GaInP单元108与GaAs衬底110晶格匹配。GaAs的晶格常数约为优选地，调整包括在III-V族复合半导体材料中的每层的构成比，使得形成在GaAs衬底110上的GaAs单元106和GaInP单元108的晶格常数接近
通过将GaInPAs单元104和n+型InP层105B依次沉积在InP衬底103上来形成叠层体100B(参见图5B)。
GaInPAs单元104与InP衬底103晶格匹配。InP的晶格常数约为优选地，调整包括在III-V族复合半导体材料中的每层的构成比，使得形成在InP衬底103上的GaInPAs单元104的晶格常数接近
因为GaAs单元106和GaInP单元108的晶格常数以及GaInPAs单元104的晶格常数彼此不同，故难以将GaAs单元106、GaInP单元108和GaInPAs单元104形成在单层衬底上。因此，分别形成叠层体100A和叠层体100B。
优选地，调整p+型GaInAs层105A或p+型AlGaInAs层105A1的晶格常数，使得晶格常数更接近GaAs衬底110的晶格常数，而不是InP衬底103的晶格常数。
接着，如图6所示，包括外延生长在GaAs衬底110上的光电转换单元的叠层体100A，与包括外延生长在InP衬底103上的光电转换单元的叠层体100B，直接粘接在一起。
对p+型GaInAs层105A的表面和n+型InP层105B的表面实施清洗工序和表面活化处理。GaInAs层105A是叠层体100A的顶层并且n+型InP层105B是叠层体100B的顶层。然后，p+型GaInAs层105A的表面和n+型InP层105B的表面在真空中粘接在一起。实施氮等离子体(N2等离子体)处理或类似处理作为表面活化处理。优选在约150℃粘接这些表面。
然后，如图7所示，选择性地蚀刻和移除所述GaAs衬底110和所述蚀刻停止层111。
通过湿法蚀刻或类似方法蚀刻GaAs衬底110。在这种情况下，一种蚀刻溶液例如为硫酸(H2SO4)、过氧化氢(H2O2)和水(H2O)的混合溶液。因为混合溶液不溶解用作蚀刻停止层111的GaInP，所以能够在蚀刻停止层111前停止蚀刻工序。因此，只精确蚀刻GaAs衬底110。
通过湿法蚀刻或类似方法蚀刻该蚀刻停止层111。在这种情况下，一种蚀刻溶液例如为盐酸(HCl)和水(H2O)。
通过实施这些蚀刻工序，GaAs衬底110和蚀刻停止层111被从叠层体110A选择性地移除。
然后，在接触层112上施加光刻胶，随后通过实施曝光和显影来将光刻胶图案化，这可以利用光刻机通过公知的光刻工艺来实现。因此，图案化的光刻胶形成在接触层112上。然后，通过实施真空沉积方法或类似方法将电极材料气相沉积在接触层112和图案化的光刻胶上，以沉积金属化膜。通过揭开形成在该图案化的光刻胶上的金属化膜，使得电极101形成在接触层112上。
然后，通过移除不与电极101重叠的部分接触层112来形成接触层109。利用电极101作为掩膜通过湿法蚀刻实施该移除工序。
在这种情况下，蚀刻溶液例如是硫酸(H2SO4)、过氧化氢(H2O2)和水(H2O)的混合溶液。因为混合溶液不溶解GaInP单元108中的AlInP，所以能够在n型Al(x)InP层108A前停止蚀刻工序。因此，只精确蚀刻接触层112。
然后，在InP衬底103下形成电极102。通过实施与电极101相似的工艺来形成电极102。在将InP衬底103研磨到设计厚度之后形成电极102。
通过实施上述工艺，实现了图8所示的复合光伏电池120。
根据第一实施例，说明了复合光伏电池120的制造方法，该复合光伏电池包括隧道结层105，该隧道结层105具有含有p+型GaAs层105C、p+型GaInAs层105A和n+型InP层105B的三层结构。如果隧道结层105中所包括的层数改变，可以通过与上述制造方法相同的制造方法形成包括该改变数量的隧道结层105的复合光伏电池。
例如，在一种情况下，隧道结层105具有两层结构，包括p+型GaInAs层105A和n+型InP层105B，通过与上述制造方法相同的制造方法形成该复合光伏电池100。此外，例如，在另一种情况下，隧道结层105具有两层结构，包括p+型AlGaInAs层105A1和n+型InP层105B，通过与上述制造方法相同的制造方法形成该复合光伏电池100。
复合光伏电池100和120具有隧道结层105，其中p+型GaInAs层105A(或p+型AlGaInAs层105A1)和n+型InP层105B在叠层体100A和叠层体100B的连接界面处连接。因此，连接界面处的电阻减小并且隧道电流更容易流过隧道结层105。因此，能够提高复合光伏电池100和120的转换效率。
第二实施例
在第二实施例中，将说明一种复合光伏电池，其具有不同于第一实施例的复合光伏电池100和120的结构。
图9示出根据第二实施例的复合光伏电池130。
如图9所示，复合光伏电池130包括四个光电转换单元和三个隧道结层。另外，复合光伏电池130包括与图2所示的复合光伏电池120同样的结构。
GaInAs单元116形成在InP衬底103上，并且隧道结层115形成在该GaInAs单元116和GaInPAs单元104之间。
在复合光伏电池130中，光电转换单元的带隙沿着光入射方向从入射侧到背侧变小。GaInP单元108、GaAs单元106、GaInPAs单元104和GaInAs单元116的带隙分别为1.9eV、1.42eV、1.0eV和0.75eV。
隧道结层115包括p+型Al(x)InAs层115A和n+型InP层115B。该p+型Al(x)InAs层115A和n+型InP层115B是高掺杂层。
GaInAs单元116包括n型InP层116A、n型Ga(x)InAs层116B、p型Ga(x)InAs层116C和p型InP层116D，它们沿着光入射方向依次形成。n型InP层116A为窗口层。p型InP层116D为背面场(BSF)层。
调整n型Ga(x)InAs层116B和p型Ga(x)InAs层116C的Ga(镓)的构成比，使得GaInAs单元116的带隙变为0.75eV。
n型Ga(x)InAs层116B的杂质例如可以是Si(硅)。p型Ga(x)InAs层116C的杂质例如可以是Zn(锌)。
复合光伏电池130是四结光伏电池，包括带隙分别为1.9eV、1.42eV、1.0eV和0.75eV的GaInP单元108、GaAs单元106、GaInPAs单元104和GaInAs单元116。如上所述，该四结光伏电池具有良好的平衡带隙。因此，相比于三结光伏电池，该复合光伏电池130可以进一步提高阳光的能量转换效率。
根据第二实施例，能够获得一种具有高转换效率的复合光伏电池130。
第三实施例
在第三实施例中，将说明一种复合光伏电池，其具有不同于第一实施例的复合光伏电池100和120的结构。
图10示出根据第三实施例的复合光伏电池140。
如图10所示，该复合光伏电池140包括隧道结层145和GaInPAs单元144来代替图2中的隧道结层105和GaInPAs单元104。该隧道结层145包括p+型GaInAs层105A、p+型GaAs层105C和n+型Ga(x)InP层105D。该n+型Ga(x)InP层105D相对于InP具有拉伸应变。该n+型Ga(x)InP层105D取代了该n+型InP层105B并作为隧道结层145的n+型隧道结层。GaInPAs单元144包括[Al(x)Ga](y)InAs层104E、n型Ga(x)InP(y)As层104B、p型Ga(x)InP(y)As层104C和p型InP层104D。该[Al(x)Ga](y)InAs层104E取代n型InP层104A并作为GaInPAs单元144的窗口层。而且，该复合光伏电池140包括与图2所示的复合光伏电池120相同的结构。
该n+型Ga(x)InP层105D是高掺杂的。对于该n+型Ga(x)InP层105D，Ga的构成比为10％，拉伸应变为0.7％，并且带隙为1.42eV。必须调整该n+型Ga(x)InP层105D的厚度，使得该n+型Ga(x)InP层105D不具有晶格弛豫。
优选n+型隧道结层(该n+型Ga(x)InP层105D)由带隙大于或等于光电转换单元(例如，GaAs单元106)的带隙的材料制成，位于光入射方向中背侧上的n+型隧道结层105D旁边。该材料除GaInP之外例如可以是GaPSb、GaInPSb、AlInAs、AlGaInAs、AlAsSb、AlGaAsSb、AlPSb、AlGaPSb、AlInPSb等。
优选，该窗口层(n型InP层104A)由带隙大于或等于光电转换单元(例如GaAs单元106)的带隙的材料制成，位于光入射方向中入射侧的窗口层旁边。因此，[Al(x)Ga](y)InAs层104E的带隙大于或等于1.42eV，优选大于或等于1.5eV。通过在GaAs单元106(1.42eV)和GaInPAs单元144(1.0eV)之间形成具有上述带隙的窗口层，能够使得长波长光通过GaAs单元106传输进GaInPAs单元144。
该窗口层的材料除AlGaInAs之外例如可以是GaInP、GaPSb、GaInPSb、AlInAs、AlGaInAs、AlAsSb、AlGaAsSb、AlPSb、AlGaPSb、AlPSb、AlInPSb等。
在常规复合光伏电池中，InP(1.35eV)层广泛地应用于n+型隧道结层(晶圆结合层)或窗口层。因为InP(1.35eV)层吸收传输过GaAs单元(1.42eV)的光的一部分，传统复合光伏电池的转换效率降低了。通过选择上述的具有合适带隙的n+型隧道结层(n+型Ga(x)InP层105D)和窗口层([Al(x)Ga](y)InAs层104E)的材料，能够使长波长光传输过GaAs单元106(1.42eV)进入GaInPAs单元144。结果，该复合光伏电池140的转换效率得到提高。
根据第三实施例，能够获得具有高转换效率的复合光伏电池140。
第四实施例
在第四实施例中，将说明一种复合光伏电池，其具有不同于第一实施例的复合光伏电池100和120的结构。
图11示出根据第四实施例的复合光伏电池150。
如图11所示，该复合光伏电池150包括四个光电转换单元和三个隧道结层。如图11所示，该复合光伏电池150包括包括隧道结层145和GaInPAs单元144来代替图2中的隧道结层105和GaInPAs单元104。该隧道结层145包括p+型GaInAs层105A、p+型GaAs层105C和n+型Ga(x)InP层105D。该n+型Ga(x)InP层105D相对于InP具有拉伸应变。该n+型Ga(x)InP层105D取代了该n+型InP层105B并作为隧道结层145的n+型隧道结层。GaInPAs单元144包括[Al(x)Ga](y)InAs层104E、n型Ga(x)InP(y)As层104B、p型Ga(x)InP(y)As层104C和p型InP层104D。该[Al(x)Ga](y)InAs层104E取代n型InP层104A并作为GaInPAs单元144的窗口层。而且，该复合光伏电池150包括与图9所示的复合光伏电池130相同的结构。
GaInAs单元116形成在InP衬底103上，并且隧道结层115形成在该GaInAs单元116和GaInPAs单元144之间。
该隧道结层145包括p+型GaAs层105C、p+型GaInAs层105A和n+型Ga(x)InP层105D。该p+型GaAs层105C、p+型GaInAs层105A和n+型Ga(x)InP层105D为高掺杂层。对于n+型Ga(x)InP层105D，Ga的构成比是10％，相对于InP的拉伸应变是0.7％。
该复合光伏电池150是四结光伏电池，包括带隙分别为1.9eV、1.42eV、1.0eV和0.75eV的GaInP单元108、GaAs单元106、GaInPAs单元144和GaInAs单元116。
通过选择具有合适带隙的n+型隧道结层和窗口层的合适材料并采用上述多结结构，能够提高复合光伏电池150的转换效率。
如图11所示，包括Al(铝)的p+型AlGaAs层取代p+GaAs层105C用于隧道结层145中。在这种情况下，p+型AlGaInAs层可以代替p+型GaInAs层105A。必须设计多结光伏电池，使得各单元产生的电流具有相同的电流值。该复合光伏电池150设计成使得GaAs单元106不吸收能被该GaAs单元106所吸收的波长范围内所有的光，而是将一部分光传输到相对于GaAs单元106位于入射光方向背侧的GaInPAs单元144和GaInAs单元116中。在这种情况下，为了在位于GaAs单元106和GaInPAs单元144之间的隧道结层145处抑制所传输的光的吸收，优选通过采用具有比GaAs更宽带隙的材料例如AlGaAs来形成形成隧道结层145。
在一种情况下，像第一和第二实施例所描述的那样，在InP衬底103上形成单独单元，不必使光分别传输通过GaInP单元108和GaAs单元106。然而，在一种情况下，在InP衬底103上形成多个单元，必须根据单元数量使光分别传输通过GaInP单元108和GaAs单元106。因此，必须减小一个单元和另一个单元间各层的最大光吸收范围，一个单元形成在GaAs衬底110(参见5A)上并位于光入射方向的最后侧，以及另一个单元形成在InP衬底103上并位于光入射方向的最前侧。该最前侧与该最后侧相反。一个单元的例子是GaAs单元106(参见图5A)，其形成在GaAs衬底110上(参见图5A)并位于光入射方向的最后侧。一个单元的例子是GaInPAs单元144(参见图5B和11)，其形成在InP衬底103上(参见图5A)并位于光入射方向的最前侧。根据第四实施例，通过采用由宽带隙的GaInP或Ga(In)PSb制成的n+隧道结层(晶圆结合层)以及由宽带隙的AlGaAs制成的p+层，能够减小光在n+隧道结层处的吸收。因此，能够使光有效地传输通过隧道结层145以进入形成在InP衬底103上的单元。结果，提高了转换效率。
根据第四实施例，能够获得具有高转换效率的复合光伏电池150。在此，Ga(In)PSb包括GaPSb和GaInPSb。
第五实施例
在第五实施例中，将说明一种复合光伏电池，其具有不同于第一实施例的复合光伏电池100和120的结构。
图12示出根据第五实施例的复合光伏电池160。
如图12所示，该复合光伏电池160包括隧道结层145和GaInPAs单元144来代替图2中的隧道结层105和GaInPAs单元104。该隧道结层145包括p+型GaInAs层105A、p+型GaAs层105C和n+型Ga(x)InP层105D。该n+型Ga(x)InP层105D相对于InP具有拉伸应变。该n+型Ga(x)InP层105D取代了该n+型InP层105B并作为隧道结层145的n+型隧道结层。GaInPAs单元144包括[Al(x)Ga](y)InAs层104E、n型Ga(x)InP(y)As层104B、p型Ga(x)InP(y)As层104C和p型InP层104D。该[Al(x)Ga](y)InAs层104E取代n型InP层104A并作为GaInPAs单元144的窗口层。该复合光伏电池160包括GaInAs单元166以取代GaAs单元106。GaInAs单元166包括n型[Al(x)Ga](y)InP层106A、n型Ga(x)InAs层106E、p型Ga(x)InAs层106F和p型Ga(x)InP层106D。该n型Ga(x)InAs层106E和p型Ga(x)InAs层106F具有压缩应力。除包括n型Ga(x)InAs层106E、p型Ga(x)InAs层106F的GaInAs单元166之外，复合光伏电池160具有与图10所示复合光伏电池140相同的结构。
该n+型Ga(x)InP层105D是高掺杂的。对于该n+型Ga(x)InP层105D，Ga的构成比为7.0％，拉伸应变是0.5％，并且带隙是1.40eV。
对于n型Ga(x)InAs层106E和p型Ga(x)InAs层106F，In(铟)的构成比是1.5％，相对于InP拉伸应变是0.1％，带隙为1.40eV。n型Ga(x)InAs层106E和p型Ga(x)InAs层106F具有彼此相同的In构成比、拉伸应变和带隙。在一种情况下，拉伸应变为约0.1％，Ga(x)InAs层106E和106F具有能够使Ga(x)InAs层106E和106F吸收足够光并作为光电转换单元的厚度。
在常规复合光伏电池中，InP(1.35eV)层广泛地应用于n+型隧道结层(晶圆结合层)或窗口层。因为InP(1.35eV)层吸收传输过GaAs单元(1.42eV)的光的一部分，传统复合光伏电池的转换效率降低了。通过选择具有合适带隙的上述n+型隧道结层(n+型Ga(x)InP层105D)和窗口层([Al(x)Ga](y)InAs层104E)的材料，能够使长波长光传输过GaInAs单元166(1.40eV)进入GaInPAs单元144。结果，复合光伏电池160的转换效率提高了。
根据第五实施例，通过采用GaInAs单元166(1.40eV)来代替GaAs单元106(1.42eV)，能够采用具有比根据第三和第四实施例的n+型隧道结层(晶圆结合层)和窗口层窄带隙的n+型隧道结层(晶圆结合层)和窗口层。微小的应力对于实现具有拉伸应变的GaInP层是足够的。因此，容易形成具有拉伸应变的GaInP层。通过采用GaInAs单元166(1.40eV)来代替GaAs单元106(1.42eV)，即使用InP层(1.35eV)作为晶圆结合层和窗口层，也能够减小晶圆结合层和窗口层处的光吸收。
根据第五实施例，能够获得具有高转换效率的复合光伏电池160。
第六实施例
在第六实施例中，将说明一种复合光伏电池，其具有不同于第一实施例的复合光伏电池100和120的结构。
图13示出根据第六实施例的复合光伏电池170。
如图13所示，该复合光伏电池170包括四个光电转换单元和三个隧道结层。如图13所示，该复合光伏电池170包括包括隧道结层145和GaInPAs单元144来代替图2中的隧道结层105和GaInPAs单元104。该隧道结层145包括p+型GaInAs层105A、p+型GaAs层105C和n+型Ga(x)InP层105D。该n+型Ga(x)InP层105D相对于InP具有拉伸应变。该n+型Ga(x)InP层105D取代了该n+型InP层105B并作为隧道结层145的n+型隧道结层。GaInPAs单元144包括[Al(x)Ga](y)InAs层104E、n型Ga(x)InP(y)As层104B、p型Ga(x)InP(y)As层104C和p型InP层104D。该[Al(x)Ga](y)InAs层104E取代n型InP层104A，并作为GaInPAs单元144的窗口层。该复合光伏电池170包括GaInAs单元166以取代GaAs单元106。GaInAs单元166包括n型[Al(x)Ga](y)InP层106A、n型Ga(x)InAs层106E、p型Ga(x)InAs层106F和p型Ga(x)InP层106D。该n型Ga(x)InAs层106E和p型Ga(x)InAs层106F具有压缩应力。除包括n型Ga(x)InAs层106E、p型Ga(x)InAs层106F的GaInAs单元166不同之外，复合光伏电池170具有与图11所示复合光伏电池150相同的结构。
该n+型Ga(x)InP层105D是高掺杂的。对于该n+型Ga(x)InP层105D，Ga的构成比为7.0％，拉伸应变是0.5％，并且带隙是1.40eV。
对于n型Ga(x)InAs层106E和p型Ga(x)InAs层106F，In(铟)的构成比是1.5％相对于InP拉伸应变是0.1％，带隙为1.40eV。n型Ga(x)InAs层106E和p型Ga(x)InAs层106F具有彼此相同的In构成比、拉伸应变和带隙。
该复合光伏电池170是四结光伏电池，包括带隙分别为1.9eV、1.42eV、1.0eV和0.75eV的GaInP单元108、GaInAs单元166、GaInPAs单元144和GaInAs单元116。
通过选择具有合适带隙的n+型隧道结层和窗口层的合适材料并采用上述多结结构，用GaInAs单元166(1.40eV)代替GaAs单元106(1.402eV)，能够进一步提高复合光伏电池170的转换效率。
根据第六实施例，能够获得具有高转换效率的复合光伏电池140。
该复合光伏电池不限于上述具体实施例，在不偏离本发明范围的情况下可以进行改变和修改。
本发明基于并要求于2013年7月30日提交的日本优先权专利申请No.2013-157477以及于2014年6月9日提交的日本优先权专利申请No.2014118296的优先权，在此通过引用而将其全部内容并入本文。

Claims (10)

1.一种复合光伏电池，包括：
第一衬底；
构造成由第一复合半导体材料制成并形成在第一衬底上的一个或多个第一光电转换单元；
形成在该一个或多个第一光电转换单元上的隧道结层；以及
一个或多个第二光电转换单元，构造成由与第一衬底的材料晶格失配的第二复合半导体材料制成，经隧道结层连接到该一个或多个第一光电转换单元，并相对于该一个或多个第一光电转换单元而位于光入射方向中的入射侧，
其中，该一个或多个第一光电转换单元和该一个或多个第二光电转换单元的带隙在光入射方向上从入射侧到背侧变小；
该隧道结层包括位于入射侧的p型层以及位于背侧的n型层，该p型层是p+型(Al)GaInAs层，该n型层是n+型InP层、相对于InP具有拉伸应变的n+型GaInP层、或者相对于InP具有拉伸应变的n+型Ga(In)PSb层，并且
其中，该隧道结层的入射侧的所述p+型(Al)GaInAs层的晶格常数低于该隧道结层的背侧的为n+型InP层、相对于InP具有拉伸应变的n+型GaInP层、或者相对于InP具有拉伸应变的n+型Ga(In)PSb层的n+型层的晶格常数；
其中，隧道结层还包括位于p+型(Al)GaInAs层的入射侧的p+型(Al)GaAs层；并且
p+型(Al)GaInAs层的带隙小于p+型(Al)GaAs层的带隙。
2.权利要求1所述的复合光伏电池，其中，该n+型GaInP层或者该n+型Ga(In)PSb层的带隙大于或等于该一个或多个第二光电转换单元中位于光入射方向上的最后侧的第二光电转换单元的带隙。
3.权利要求1所述的复合光伏电池，其中，p+型(Al)GaInAs层的晶格常数比InP的晶格常数更接近GaAs的晶格常数。
4.权利要求1所述的复合光伏电池，其中，n+型GaInP层或n+Ga(In)PSb层的晶格常数比GaAs的晶格常数更接近InP的晶格常数。
5.权利要求1所述的复合光伏电池，其中，该一个或多个第一光电转换单元中位于光入射方向上的最前侧的第一光电转换单元包括与隧道结层接触的窗口层；并且
该窗口层的带隙大于或等于该一个或多个第二光电转换单元中位于最后侧的第二光电转换单元的带隙。
6.权利要求5所述的复合光伏电池，其中，窗口层包括GaInP、GaPSb、GaInPSb、AlInAs、AlGaInAs、AlAsSb、AlGaAsSb、AlPSb、AlGaPSb、AlPSb和AlInPSb中的任意一种材料。
7.权利要求1所述的复合光伏电池，其中，该一个或多个第二光电转换单元中位于最后侧的第二光电转换单元包括相对于GaAs具有压缩应力的GaInAs层。
8.权利要求1所述的复合光伏电池，其中，该一个或多个第一光电转换单元由InP晶格匹配材料制成；并且
该一个或多个第二光电转换单元由GaAs晶格匹配材料制成。
9.权利要求1所述的复合光伏电池，其中，该一个或多个第一光电转换单元和该一个或多个第二光电转换单元的数量大于或等于3。
10.权利要求1所述的复合光伏电池，其中，该第一衬底是InP衬底；并且
该一个或多个第一光电转换单元的数量大于或等于2。
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