晶格失配对GaInP/In_xGa_(1-x)As/In_yGa_(1-y)As倒装三结太阳电池性能影响的分析

Analyses of the effect of mismatch on the performance of inverted GaInP/In_xGa_(1-x)As/In_yGa_(1-y)As triple-junction solar cells

传统GaInP/(In)GaAs/Ge三结太阳电池因受其带隙组合的限制,转换效率再提升空间不大.倒装结构三结太阳电池因其更优的带隙组合期望可以得到更高的效率.基于细致平衡原理,结合P-N结形成机理,应用MATLAB语言对双晶格失配GaInP(1.90 eV)/In_xGa_(1-x)As/In_yGa_(1-y)As倒装结构三结太阳电池底、中电池的不同带隙组合进行模拟优化.模拟结果表明在AM1.5D,500倍聚光(500 suns)下,禁带宽度组合为1.90/1.38/0.94 eV的带隙最优,综合材料成本与试验条件,当顶、中电池最优厚度组合为4μm和3.2μm时理论转化效率高达51.22%,此时两个异质结的晶格失配度分别为0.17%和2.36%.忽略渐变缓冲层生长后底电池位错的影响,通过计算0.17%的晶格失配引入1.70×105cm^(-2)的插入位错密度,对比单晶格失配GaInP/GaAs/In_(0.32)Ga_(0.68)As(0.99 eV)倒装结构三结太阳电池光电转化效率仍提高了0.3%.

The traditional lattice matched GaInP/（In） GaAs/Ge triple-junction （3J） solar cell has no much room to enhance its practical achievable conversion efficiency because of its inappropriate ensemble of bandgap energies. According to the P-N junction formation mechanism and the close equilibrium condition, we explore a series of computational codes in the framework of MATLAB to simulate and optimize the inverted structure of series-connected 3J solar cells with a fixed top bandgap of 1.90 eV on GaAs substrate. In this paper, structural optimization is conducted in the real device design, because the realistic （QE） is closely related to a set of material parameters in the subcell, i.e., the absorbtion coefficient of material, subcell thickness, minority carrier diffusion length, surface recombination velocity, etc. The results indicate improved inverted 3J solar cells with nearly optimized bandgaps of 1.90, 1.38, and 0.94 eV, by utilizing two independently lattice-mismatches （0.17% and 2.36% misfit respectively） to the GaAs substrate. A theoretical efficiency of 51.25% at 500 suns is demonstrated with this inverted design with the optimal thickness （4 μm GaInP top and 3.1 μm InGaAs middle）. By contrast, the efficiency with the infinite thickness of subcells is reduced by 1%, which is mainly attributed to the effect of minority carrier recombination on Jsc. Exactly speaking, if photo-generated carriers make a contribution to Jsc, they must be collected effectively by the P-N junction before recombining. A new model is proposed based on the effect of dislocation on the metamorphic structure properties by regarding dislocation as minority-carrier recombination center. Our calculation indicates that threading dislocations density in the middle junction is approximate to 1.70×105 cm-2 when dislocations in the gradient buffer layer are neglected. The theoretical efficiency is increased by 0.3% compared with the inverted design containing a single metamorphic junction. As a result, based on the two metamorphic combinations, a solar cell with an area of 30.25 mm2 is prepared. The efficiency of the designed cell with two lattice-mismatched junctions reaches 40.01% at 500 suns （AM1.5D, 38.4 W/cm2, 25℃）, which is 0.4% higher than that of the single metamorphic junction 3J solar cell.