Abstract
The efficiency of thin-film chalcogenide solar cells is dependent on their window layer thickness. However, the application of an ultrathin window layer is difficult because of the limited capability of the deposition process. This paper reports the use of atomic layer deposition (ALD) processes for fabrication of thin window layers for Cu(Inx,Ga1−x)Se2 (CIGS) thin-film solar cells, replacing conventional sputtering techniques. We fabricated a viable ultrathin 12 nm window layer on a CdS buffer layer from the uniform conformal coating provided by ALD. CIGS solar cells with an ALD ZnO window layer exhibited superior photovoltaic performances to those of cells with a sputtered intrinsic ZnO (i-ZnO) window layer. The short-circuit current of the former solar cells improved with the reduction in light loss caused by using a thinner ZnO window layer with a wider band gap. Ultrathin uniform A-ZnO window layers also proved more effective than sputtered i-ZnO layers at improving the open-circuit voltage of the CIGS solar cells, because of the additional buffering effect caused by their semiconducting nature. In addition, because of the precise control of the material structure provided by ALD, CIGS solar cells with A-ZnO window layers exhibited a narrow deviation of photovoltaic properties, advantageous for large-scale mass production purposes.
Highlights
Thin-film chalcogenide solar cells such as Cu(Inx,Ga1−x )Se2 (CIGS), Cu2 ZnSnSe4(CZTSe), and SnS typically require an intrinsic ZnO (i-ZnO) window layer, deposited between their CdS buffer layer and transparent conducting oxide (TCO) layers, to improve device performance without increasing light absorption loss [1,2,3,4]
The thickness of the ZnO window layer is closely related to its parasitic absorption loss, which induces a decrease in the short-circuit current (JSC ) of CIGS solar cells
Cross-sectional STEMHADDF and energy dispersive spectroscopy (EDS) images of the interface of the CdS buffer layer and the CIGS absorber layer were acquired to confirm the formation of a window layer (Figure 1)
Summary
(CZTSe), and SnS typically require an intrinsic ZnO (i-ZnO) window layer, deposited between their CdS buffer layer and transparent conducting oxide (TCO) layers, to improve device performance without increasing light absorption loss [1,2,3,4]. This i-ZnO window layer effectively blocks the short-circuit pathways through the voids in the CdS buffer layer, and enhances the shunt resistance (Rsh ), fill factor (FF), and open-circuit voltage (VOC ) of the solar cells.
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