CN110491957A - 应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层 - Google Patents
应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层 Download PDFInfo
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Abstract
本发明公开了一种应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，位于晶格失配太阳能电池的晶格失配外延材料和衬底(或与衬底晶格常数相同的外延层)之间，由多套具有不同反射波段的复合DBR叠置而成，每套复合DBR由多对叠置在一起的DBR对构成，且相邻DBR对之间的晶格常数呈梯度变化，每对DBR对包含两层半导体材料层，该两层半导体材料层的折射率不同但晶格常数相同或存在能够达到应变补偿的失配。本发明将具有宽谱反射的复合DBR有机融合入晶格渐变缓冲层中，既可以大幅降低晶格失配外延材料生长引入的大量位错等缺陷密度，又可以充分发挥反射镜的作用，同时缩减工艺步骤、生长时长和原材料损耗，有利于降低成本。
Description
技术领域
本发明涉及太阳能光伏发电的技术领域，尤其是指一种应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层。
背景技术
随着现代工业技术的发展，能源在人类社会生存和发展中已经成为决定社会进步的最为重要的物质基础，随着社会的发展，煤炭、石油、天然气等对环境污染严重并且不可再生能源逐渐减少，因此发展绿色新能源迫在眉睫。太阳能是取之不尽、用之不竭的清洁能源，因此研究并发展太阳能电池技术将在一定程度上缓解未来能源的短缺。从光伏发电技术的发展历程来看，太阳能电池大体可以分为三大类：第一代晶硅太阳能电池、第二代薄膜太阳能电池和第三代砷化镓多结太阳能电池。其中，GaInP、GaInAs和Ge子电池组成的GaInP/GaInAs/Ge三结太阳能电池作为传统砷化镓多结电池的主流结构，500倍聚光下转化效率超过41％，远高于晶硅电池，并且具有进一步提升的空间。
传统三结电池结构上整体保持晶格匹配，带隙组合为1.85/1.40/0.67eV。然而，对于太阳光光谱，这种电池的带隙并不是最佳组合，由于GaInAs子电池和Ge子电池之间较大的带隙差距，这种结构下底电池电流远大于中电池和顶电池，由于底中顶三结子电池是串联在一起的，根据串联结构的电流机制，电流由三个子电池中电流最小的决定，这种结构造成了很大一部分太阳光能量损失，限制了电池性能的提高。
理论分析表明，为了提高三结电池光电转换效率，需要降低中电池和顶电池吸收区域的带隙，让中电池和顶电池吸收更多的光，从而提高中、顶子电池电流降低底电池电流，最终可以实现电流匹配的三结太阳能电池。据此分析，提出了晶格失配结构的MM(Metamorphic,变质材料)结构太阳能电池，MM结构三结电池最先应用于CPV市场，其转换效率可达42％以上；近些年来，MM结构三结电池产品应用于空间电池，转换效率可达32％以上，并且其辐照后衰减可达常规三结电池的水平，即效率衰减低于18％，优势远高于其它类光伏电池产品。但外延晶格失配材料时，如果失配外延层厚度小于临界厚度，在形变能的作用下晶格常数会与衬底保持一致，一旦超过临界厚度，其晶格常数将恢复到固有值，从而产生大量失配位错，降低材料质量。一般地，临界厚度的大小与失配度有关，晶格失配度越大，临界厚度越小。对于与晶格具有较大失配度的异质外延材料，提高材料的外延质量成为其应用于更为广泛领域和进一步提升器件性能的一个瓶颈。
为解决此问题，目前常用的方法是采用能释放应力的渐变缓冲层(一般是InGaAs或者GaInP材料)连接晶格失配的Ge衬底和InGaAs材料。例如，要生长x＝0.1的Ga0.9In0.1As材料，可以在Ge衬底和Ga0.9In0.1As材料之间生长一系列组分渐变的Ga1-xInxAs缓冲层，组分x由与Ge衬底晶格匹配的0.01连续变化至0.1，Ga1-xInxAs缓冲层可释放晶格失配产生的应力，降低Ga0.9In0.1As材料中产生的位错等缺陷。
为了保障失配材料晶格完全弛豫，避免晶格位错增多，组分渐变缓冲层的组分变化速率不能太快，所以缓冲层的厚度都比较厚。但该缓冲层在光电方面没有特殊作用，因此人们在保证缓冲层作用的基础上对减薄缓冲层方面做了相关研究，与此不同，本发明切换思路，将晶格渐变缓冲层与分布式布拉格反射镜(DBR)相结合，旨在开发晶格渐变缓冲层潜在优势使其同时具有光学作用。
DBR结构在半导体器件(包括发光二极管LED以及太阳能电池)中的应用已经比较成熟，其特有的光子反射能力对半导体器件的性能有极大提升。例如在三结GaAs太阳能电池中，一方面，DBR可显著降低吸收系数。反射掉一部分到达底电池的过多光子，防止其转换成热量释放到电池系统中，对于提高电池稳定性和延长电池寿命非常重要。为了进一步降低吸收系数，本发明采用多套DBR拓宽光子反射范围，增加反射光子数量。另一方面，DBR对于抗辐照性能的提升有显著效果。由于大量辐照实验结果表明GaInAs子电池的抗辐照性能较GaInP子电池差很多，有分析认为其原因在于As原子半径较大，高能粒子辐照后其位置不易复原导致。在中子电池下面设置DBR，通过调节DBR结构反射相应波段的太阳光，使第一次没有被GaInAs材料所吸收的光子反射回去被二次吸收，相当于变相地增加了GaInAs的有效吸收厚度，可以有效降低GaInAs子电池设计厚度，有利于提高抗辐照性能。在LED领域，通过DBR增强出光效率，对提高亮度非常重要。
另外，若将另外一些优化运用在在本发明上，可以进一步提升本发明在改善器件性能方面的优势。例如，在材料选择上，采用具有很好的可塑性和使薄膜硬化的效果的稀氮材料，使得穿透位错等缺陷改纵向为横向传播同时应力得以释放，对于过滤位错有极好的作用。此外，将DBR设计为应变补偿的周期结构，提高少数载流子的收集同时也可以很好地发挥位错阻挡层的作用。
综上，这种具有宽谱反射功能的晶格渐变缓冲层引入到晶格失配结构多结太阳能电池既可以达到降低吸收系数的要求，又能提升电池的抗辐照性能，还可以改善穿透位错导致的外延晶体质量变差的问题，同时缩减工艺步骤、生长时长和原材料损耗，有利于降低成本。总之，可最大程度地发挥MM结构多结电池的优势，提高电池效率。
发明内容
本发明的目的在于克服现有技术的缺点与不足，提出了一种应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层(亦可称为晶格渐变宽谱反射镜)，具有宽谱反射作用，通过将渐变缓冲层和复合DBR的功能合而为一，既可以降低失配引入的外延层中的缺陷密度，提高材料质量，提升GaInAs子电池抗辐照性能，又可以拓宽反射谱波长范围，降低吸收系数，进而提高电池稳定性，尤其有利于在航空电源上的应用，同时，缩减工艺步骤、生长时长和原材料损耗，有利于降低成本。
为实现上述目的，本发明所提供的技术方案为：应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，所述晶格渐变缓冲层位于晶格失配太阳能电池的晶格失配外延材料和衬底之间，或晶格失配外延材料与衬底晶格常数相同的外延层之间，由多套具有不同反射波段的复合DBR叠置而成，每套复合DBR由多对叠置在一起的DBR对构成，且相邻DBR对之间的晶格常数呈梯度变化，每对DBR对包含两层半导体材料层，该两层半导体材料层的折射率不同但晶格常数相同或存在能够达到应变补偿的失配。
进一步，邻近于衬底的第一对DBR对的晶格常数与衬底匹配，最后一对DBR对与晶格失配外延材料晶格匹配，而它们之间的DBR对的晶格常数从第一对DBR对往最后一对DBR对方向梯度增加。
进一步，所述复合DBR至少有两套，且各套DBR反射波段(λ1－Δλ)～(λ1+Δλ)至(λn－Δλ)～(λn+Δλ)，其中综合范围(λ1－Δλ)～(λn+Δλ)根据相邻子电池带隙决定，λ1为第一套复合DBR的中心反射波长，λn为第n套复合DBR的中心反射波长，n为整数，Δλ为某套复合DBR反射范围的1/2，中心反射波长λ间隔在10～80nm范围内，每套周期为3～20对。
进一步，每对DBR对的两层半导体材料层需具有不同折射率，选择As系材料或P系材料，有GaInNAs、AlGaInAs、GaInP或AlGaInP，其厚度设计遵循公式：式中，d为厚度，λ为预计反射波段的中心反射波长，n为相应半导体材料的折射率；其中As系材料折射率大于P系，同系材料中铝组分越高折射率越低。
进一步，每对DBR对的两层半导体材料层的晶格常数能够相同，也允许存在能够达到应变补偿的失配，失配度在0.01％～5％范围内。
本发明与现有技术相比，具有如下优点与有益效果：
1、本发明可连接晶格失配较大的两个结构单元，实现晶格过渡的同时降低位错等缺陷密度。
2、引入多套不同反射波段的复合DBR，其综合反射半宽可从(100±30)nm提高至(150±30)nm，可以有效拓宽DBR反射范围，降低吸收系数，有利于提高电池的稳定性。
3、采用本发明的晶格渐变缓冲层(宽谱反射镜)，改善了单套DBR反射谱光子反射范围窄的问题，反射光子数量增加可以进一步减小子电池有效厚度，提升抗辐照性能。
4、将具有宽谱反射的复合DBR有机融合入晶格渐变缓冲层中，达到提高电池性能和稳定性目的的同时缩减工艺步骤、生长时长和原材料损耗，有利于降低太阳能电池成本。
5、现有成熟外延工艺的半导体材料，如GaInNAs、AlGaInAs、GaInP、AlGaInP可供选择。由于受具体应用的位置和相邻电池带隙的限制，若可以采用具有刚性特质的GaInNAs或晶格强化效应的AlGaInP等材料，组合成DBR应用到MM结构多结太阳能电池中，提高光子吸收的同时可过滤缺陷，更好地发挥位错阻挡层作用。
6、将组成DBR的高折射率(nH)和低折射率(nL)层设计为应变补偿结构，参见图1所示，可释放晶格失配引入的应力，受张应变和压应变作用的界面有助于穿透位错横向滑移，进一步降低位错密度。
经实验，采用Vecco公司的MOCVD制备本发明设计的三结MM结构太阳能电池，生长时长较传统工艺缩短约46min，大幅提升了生产效率且大量节省原材料。通过X射线衍射仪(XRD)测试分析表明，采用该方法制作的三结MM结构太阳能电池外延片中顶电池的半高宽与传统结构无明显差异，说明晶格失配导致的缺陷密度降低到较低水平。白光反射谱测试表明，该晶格渐变缓冲层的反射波范围由(840～940)nm拓宽至(840～1020)nm，整体反射强度没有明显降低。通过对反射率测试分析，相比具有单套DBR结构的三结MM结构电池，本发明制作的MM三结太阳能电池吸收系数由0.927降低到0.91，热性能提高，电池的稳定性显著增强。
经过分析对比，采用本发明制作的三结MM太阳能电池，较传统DBR，抗辐照性能也有明显改善，在AM0光谱下的转换效率提升幅度可达到2.1％(如下表1)，可大幅提高空间电源的输出功率。
表1采用传统DBR、晶格渐变缓冲层(宽谱反射镜)的MM结构三结电池在AM0空间光谱下的抗辐照性能分析
附图说明
图1为本发明所述晶格渐变缓冲层(亦可称为晶格渐变宽谱反射镜)的结构示意图。
图2为采用本发明所述晶格渐变缓冲层(亦可称为晶格渐变宽谱反射镜)的晶格失配三结太阳能电池结构示意图。
具体实施方式
为进一步说明本发明的内容，以下结合具体实施例及附图对本发明进行详细描述。
如图1和图2所示，本实施例所提供的晶格失配三结太阳能电池，包括有Ge衬底1，所述Ge衬底1为单面抛光004取向的p型Ge单晶片；在所述Ge衬底1抛光面上按照层状叠加结构由下至上依次生长GaInP成核层2、GaInAs缓冲层3、第一隧穿结4、晶格渐变缓冲层5(亦可称为晶格渐变宽谱反射镜)、GaInAs子电池6、第二隧穿结7和GaInP子电池8；其中，所述晶格渐变缓冲层5是由两套具有不同反射波段的复合DBR 51、52叠置而成，该两套复合DBR 51、52由多对叠置在一起的DBR对构成，且相邻DBR对之间的晶格常数呈梯度变化，每对DBR对包含两层半导体材料层，该两层半导体材料层的折射率不同但晶格常数相同或允许存在可达到应变补偿的失配，失配度在0.01％～5％范围内，这样的设计既可以大幅降低晶格失配外延材料生长引入的大量位错等缺陷密度，又可以充分发挥反射镜的作用，同时缩减工艺步骤、生长时长和原材料损耗，有利于降低成本。另外，邻近于衬底的第一对DBR对的晶格常数与衬底匹配，最后一对DBR对与晶格失配外延材料晶格匹配，而它们之间的DBR对的晶格常数从第一对DBR对往最后一对DBR对方向梯度增加；各套DBR反射波段(λ1－Δλ)～(λ1+Δλ)至(λn－Δλ)～(λn+Δλ)，其中综合范围(λ1－Δλ)～(λn+Δλ)根据相邻子电池带隙决定，λ1为第一套复合DBR的中心反射波长，λn为第n套复合DBR的中心反射波长，n为整数，Δλ为某套复合DBR反射范围的1/2，中心反射波长λ间隔在10～80nm范围内，每套周期为3～20对；每对DBR对的两层半导体材料层需具有不同折射率，可选择As系材料或P系材料，有GaInNAs、AlGaInAs、GaInP、AlGaInP等合金材料，其厚度设计遵循公式：式中，d为厚度，λ为预计反射波段的中心反射波长，n为相应半导体材料的折射率；其中As系材料折射率大于P系，同系材料中铝组分越高折射率越低。
在本实施例，所述晶格渐变缓冲层5由In组分逐对梯度增加的GaInAs/AlGaInAsDBR和GaInNAs/AlGaInAs DBR构成，第一对GaInAs/AlGaInAs DBR晶格常数与衬底相同，最后一对GaInNAs/AlGaInAsDBR与GaInAs子电池相同，均为p型掺杂层，空穴浓度为1×1018/cm3。所述GaInAs/AlGaInAs DBR的反射波长范围为840～940nm，其中GaInAs/AlGaInAs组合层的对数为12对；设计GaInAs材料与AlGaInAs材料为应变补偿结构，晶格失配度0.3％。所述GaInNAs/AlGaInAs DBR的反射波长范围为930～1020nm，其中GaInNAs/AlGaInAs组合层的对数为4对；设计GaInNAs材料与AlGaInAs材料为应变补偿结构，晶格失配度0.3％。
所述GaInP成核层2、GaInAs缓冲层3、第一隧穿结4均与Ge衬底1保持晶格匹配。
所述GaInP成核层2为n型掺杂层，电子浓度为2×1018/cm3，厚度为5nm。
所述GaInAs缓冲层3为n型掺杂层，电子浓度为4×1018/cm3，厚度为500nm。
所述第一隧穿结4为p-AlGaInAs/n-GaInP结构，其中p-AlGaInAs/n-GaInP的厚度均为12nm。
所述GaInAs子电池6的电池总厚度为1800nm，GaInAs材料的光学带隙为1.3eV。
所述第二隧穿结7为p-AlGaInAs/n-GaInP结构，其中p-AlGaInAs/n-GaInP的厚度均为12nm。
所述GaInP子电池8的电池总厚度为600nm，GaInP子电池材料的光学带隙为1.85eV。
综上所述，本发明结合晶格渐变缓冲层和DBR(分布式布拉格反射镜，DistributedBrag Reflector)的特点，将渐变缓冲层和DBR的功能合而为一引入到MM结构多结太阳能电池，既可以起到晶格渐变缓冲层的作用，降低失配引入的外延层中的缺陷密度，提高材料质量，又可以拓宽反射谱波长范围，提升GaInAs子电池抗辐照性能，降低吸收系数，解决进入底电池过多的光子转化成热能影响电池稳定性的问题，同时，缩减工艺步骤、生长时长和原材料损耗，有利于降低成本。总之，本发明可以更加充分地发挥晶格失配太阳能电池的光电转换效率，值得推广。
以上所述实施例只为本发明之较佳实施例，并非以此限制本发明的实施范围，故凡依本发明之形状、原理所作的变化，均应涵盖在本发明的保护范围内。
Claims (5)
1.应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，其特征在于：所述晶格渐变缓冲层位于晶格失配太阳能电池的晶格失配外延材料和衬底之间，或晶格失配外延材料与衬底晶格常数相同的外延层之间，由多套具有不同反射波段的复合DBR叠置而成，每套复合DBR由多对叠置在一起的DBR对构成，且相邻DBR对之间的晶格常数呈梯度变化，每对DBR对包含两层半导体材料层，该两层半导体材料层的折射率不同但晶格常数相同或存在能够达到应变补偿的失配。
2.根据权利要求1所述的应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，其特征在于：邻近于衬底的第一对DBR对的晶格常数与衬底匹配，最后一对DBR对与晶格失配外延材料晶格匹配，而它们之间的DBR对的晶格常数从第一对DBR对往最后一对DBR对方向梯度增加。
3.根据权利要求1所述的应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，其特征在于：所述复合DBR至少有两套，且各套DBR反射波段(λ1－Δλ)～(λ1+Δλ)至(λn－Δλ)～(λn+Δλ)，其中综合范围(λ1－Δλ)～(λn+Δλ)根据相邻子电池带隙决定，λ1为第一套复合DBR的中心反射波长，λn为第n套复合DBR的中心反射波长，n为整数，Δλ为某套复合DBR反射范围的1/2，中心反射波长λ间隔在10～80nm范围内，每套周期为3～20对。
4.根据权利要求1所述的应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，其特征在于：每对DBR对的两层半导体材料层需具有不同折射率，选择As系材料或P系材料，有GaInNAs、AlGaInAs、GaInP或AlGaInP，其厚度设计遵循公式：式中，d为厚度，λ为预计反射波段的中心反射波长，n为相应半导体材料的折射率；其中As系材料折射率大于P系，同系材料中铝组分越高折射率越低。
5.根据权利要求1所述的应用于晶格失配太阳能电池外延生长的晶格渐变缓冲层，其特征在于：每对DBR对的两层半导体材料层的晶格常数能够相同，也允许存在能够达到应变补偿的失配，失配度在0.01％～5％范围内。
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| US11107940B2 (en) | 2016-05-11 | 2021-08-31 | Solaero Technologies Corp. | Multijunction solar cells having a graded-index structure |
| EP3876291A1 (en) * | 2020-03-06 | 2021-09-08 | SolAero Technologies Corp., a corporation of the state of Delaware | Distributed bragg reflector structures in multijunction solar cells |
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| EP3836231A1 (en) * | 2019-12-11 | 2021-06-16 | SolAero Technologies Corp., a corporation of the state of Delaware | Multijunction solar cells having a graded-index structure |
| EP3876291A1 (en) * | 2020-03-06 | 2021-09-08 | SolAero Technologies Corp., a corporation of the state of Delaware | Distributed bragg reflector structures in multijunction solar cells |
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