A high-efficiency tandem solar cell comprises of top and bottom solar cells. The bottom cell has a bandgap of ≈1.1 eV and is designed to absorb longer wavelengths of photons. A Cu(InGa)Se₂ (CIGS) is one of the candidates for a bottom solar cell, but its bandgap is not constant and is graded with depth. This study discusses suitable bandgap grading of CIGS cells for bottom cell applications. Three CIGS devices with different bandgap gradings are prepared for bottom cell evaluation. One device has a single-back-graded bandgap type whereas the others have double-graded bandgap types, which have been used for high-efficiency CIGS devices. The three CIGS devices exhibit similar conversion efficiencies of ≈20% but different solar cell parameters, due to their different shapes of bandgap grading. For the bottom cell evaluations, current–voltage measurements are performed using 720 nm (≈1.72 eV) and 800 nm (≈1.55 eV) long-pass filters. It is found that a CIGS device with a double-graded bandgap is unsuitable and/or shows reduced performance with a decreased top cell bandgap. By contrast, CIGS devices with a single-back-graded bandgap can be adopted as a bottom cell under top cell bandgaps of both ≈1.55 and ≈1.72 eV.

中文翻译：

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适用于串联电池应用的Cu（InGa）Se2底部吸收层的带隙分级研究

高效串联太阳能电池包括顶部和底部太阳能电池。底部电池的带隙约为1.1 eV，旨在吸收更长波长的光子。Cu（InGa）Se ₂（CIGS）是底部太阳能电池的候选材料之一，但其带隙不是恒定的，而是随深度分级的。这项研究讨论了适合底部细胞应用的CIGS细胞的带隙分级。准备了三种带隙等级不同的CIGS器件，用于底部电池评估。一种器件具有单反梯度带隙类型，而其他器件则具有双梯度带隙类型，这些器件已用于高效CIGS器件。三种CIGS器件的带隙渐变形状不同，因此它们具有约20％的相似转换效率，但具有不同的太阳能电池参数。对于底部电池评估，使用720 nm（≈1.72eV）和800 nm（≈1.55eV）长通滤波器执行电流-电压测量。发现具有双级带隙的CIGS器件是不合适的和/或显示的性能降低而顶部单元带隙减小。相比之下，具有单反分级带隙的CIGS器件可以用作≈1.55eV和≈1.72eV的顶部电池带隙下的底部电池。