晶格异变GaInP/GaAs/InGaAsP/InGaAs 四结太阳电池研究

Multiple junction solar cells are indispensible for space power sources and concentration solar plants. Obtaining a wide absorbing spectrum, a high photon energy conversion efficiency, and a high quantum efficiency are important approaches for realizing high conversion efficiency of a solar cell. In this proposal, the dynamic processes of strain relaxation and dislocation generation as well as glide in the bufferlayers will be investigated by x-ray reciprocal space mapping, transmission electron microscopy, and the effects of dislocations on the minority carrier lifetimes by time resolved photoluminescence. Through proper design of the buffer layer structures and the control of the growth processes, the strain in the buffer layers is expected to be fully relaxed, and the lattice constant is well graded from GaAs to InGaAsP while the threading dislocations will be prevented from extending into the InGaAsP layer, reducing the threading dislocation density and improving the monority carrier lifetime of the metamorphic materials.Meanwhile,the GaAs substrate lift-off techniques will also be deveolped. Meanwhile，effetc of the shorter monority carrier diffusion length in the metamorphis materials will be compensated by reducing the thickness of absorbing layer and introducing a DBR reflector beneath it.The key issues in developing high efficiency metamorphic multijunction solar cells will be thorouhly investigated with a target conversion efficiecy of 43%, and the potential new ways will be explored for manufacturing high efficiency photovoltaic devices.

多结太阳电池在空间和聚光太阳能发电方面具有非常重要的应用，增加吸收光谱宽度、充分利用光子能量、提高电池量子效率是实现多结太阳电池高效转换的重要途径。本申请研究GaAs 衬底上生长晶格异变GaInP/GaAs/InGaAsP/InGaAs 四结太阳电池结构，利用XRD 倒易空间mapping 和透射电镜研究晶格失配缓冲层中应力弛豫、位错产生和滑移的动力学过程，利用时间分辨光谱技术揭示位错对少子寿命影响的机制。通过缓冲层设计和生长控制充分弛豫晶格失配应力，实现从GaAs 到InP 晶格常数过渡的同时阻挡穿透位错向InGaAsP外延层的延伸，降低光吸收层中的穿透位错密度从而提高晶格异变材料的少子寿命。设计布拉格反射（DBR）结构补偿由于晶格异变材料中少子扩散长度较短而需要减小吸收层厚度带来的光吸收不足的问题。解决晶格异变多结电池的关键问题，实现聚光下43%的转换效率，探索高效多结电池的新途径。

电流匹配的GaInP/GaAs/InGaAsP/InGaAs四结电池既可吸收较宽的太阳光谱，同时也可以比较高效地转换各段光谱中光子的能量，即可获得较高的转换效率，而GaInP/GaAs电池与InGaAsP/InGaAs电池间存在3.8%的晶格失配, 高质量材料的晶格异变生长技术显得尤为重要。本项目研究晶格异变材料的MOCVD生长和材料特性，制备高质量晶格异变材料，旨在实现晶格异变四结太阳电池。利用AFM、XRD，TEM和PL表征技术，研究了MOCVD生长条件、GaAs衬底偏角对组分梯度渐变的多层AlGaInAs晶格异变缓冲层的表面形貌、位错的产生、滑移和相分离的影响，发现较高的生长温度和大偏角GaAs衬底有利于晶格失配应力的释放、位错的滑移和抑制相分离，掺Si和掺Zn有助于降低穿透位错密度，并对影响机理进行了分析。GaAs上生长的In0.3Ga0.7As的表面粗糙度为5.2 nm, 穿透位错密度小于1106 cm-2，In0.27Ga0.73As单结电池和GaInP/GaAs/InGaAs 倒装三结电池，转换效率分别达到18.2%和20.8%。采用AlGaInAs晶格异变缓冲层在GaAs衬底上生长的InP的表面粗糙度为10.1nm， 穿透位错密度为7106 cm-2，InP外延层上生长的In0.53Ga0.47As材料制作的单结太阳电池漏电严重。