High Efficiency CuInS_2 Thin Film Solar Cells

利用CuInS_2薄膜作为光吸收层，采用有利于大面积电池大规模生产的连续工艺，获得了12.25%的效率。在金属前驱体中加入少量的Ga，然后通过快速热处理将其硫化，得到结晶性良好的平坦吸收层。采用由夹在硫化镉缓冲层和吸收层之间的非常薄的锌化合物层组成的复合缓冲层，降低了异质结的二极管因数和暗电流密度。在由化学浴沉积的锌(S，O，OH)和(S，O，OH)薄缓冲层组成的异质结中，观察到导带偏移分别为0.4 eV(能量悬崖)和0.8 eV(能量尖峰)。具有前者(后者)异质结的太阳能电池的效率为7%(10.1%)。在前一种(后一种)情况下，我们需要对异质结进行热处理(用碘溶液对吸收体表面进行表面处理)。薄膜太阳电池的各种特性，如电容-电压曲线、电流-电压曲线和光谱光响应都可以用一个理论器件模型来解释，其中我们假设在异质界面上存在一个薄的n型CunS_2层。实验发现，开路电压随结上电场的减小而增大，这与理论上的预期相符。

A 12.25 % efficiency solar cell has been achieved using the CuInS_2 thin film as an optical absorber layer produced by a sequential process that is advantageous to mass production of large area cells. Here we added a small amount of Ga to a metallic precursor and then sulfurized it by rapid thermal process to obtain a flat absorber layer with good crystallinity.The diode factor and the dark current density of a heterojunction was decreased by using a composite buffer layer which consists of a very thin Zn compound layer sandwiched between a CdS buffer layer and an absorber layer. The efficiency of the cell was thus improved to 12.1 %.In the heterojunction consisting of chemical bath deposited Zn( S, O, OH ) and In( S, O, OH ) thin buffer layer instead of toxic CdS, we observed conduction band offsets of 0.4 eV ( energy cliff ) and 0.8 eV ( energy spike ), respectively. The solar cell with the former ( latter ) heterojunction exhibited a 7 % ( 10.1 % ) efficiency. In the former ( latter ) case, we needed annealing of the heterojunction ( surface treatment of the absorber surface by an iodide solution ).The various characteristics of the thin film solar cell such as capacitance-voltage curve, current-voltage curve and spectral photoresponse could be explained by a theretical device model, where we assume the existence of a thin n-type CulnS_2 layer at the heterointerface. The experimental finding that the open-circuit voltage increases with decreasing the electric field at the junction agreed fairly well with the theoretical expectation.