Improving the efficiency of kesterite (Cu 2 ZnSn(S,Se) 4 ; CZTSSe) solar cells requires understanding the effects of Na doping. This paper investigates these effects by applying a NaF layer at various positions within precursors. The NaF position is important because Na produces Na-related defects in the absorber and suppresses the formation of intrinsic defects. By investigating precursors with various NaF positions, the sulfo-selenization mechanism and the characteristics of defect formation are confirmed. Applying a NaF layer onto a Zn layer in a CZTSSe precursor limits Zn diffusion and suppresses Cu-Zn alloy formation, thus changing the sulfo-selenization mechanism. In addition, the surface NaF layer provides reactive Se and S to the absorber layer by generating Na 2 Se x and Na 2 S x liquid phases during sulfo-selenization, thus limiting the incorporation of Na into the absorber and reducing the Na effects. Efficiency values of 11.16% and 11.19% are obtained for a flexible CZTSSe solar cell by applying NaF between the Zn layer and back contact and between the Cu and Sn layers, respectively. This study presents methods for doping with alkali metals and improving the efficiency of photovoltaics.
In this study, we investigated the effect of the stacking order of metal precursors on the formation of volume defects, such as blisters and nanopores, in CZTSSe thin-film solar cells. We fabricated CZTSSe thin films using three types of metalprecursor combinations, namely, Zn/Cu/Sn/Mo, Cu/Zn/Sn/Mo, and Sn/Cu/Zn/Mo, and studied the blister formation. The blisterformation mechanism was based on the delamination model, taking into consideration the compressive stress and adhesion properties. A compressive stress could be induced during the preferential formation of a ZnSSe shell. Under this stress, the adhesion between the ZnSSe film and the Mo substrate could be maintained by the surface tension of a metallic liquid phase with good wettability, or by the functioning of ZnSSe pillars as anchors, depending on the type of metal precursor used. Additionally, the nanopore formation near the back-contact side was found to be induced by the columnar microstructure of the metal precursor with the Cu/Zn/Mo stacking order and its dezincification. Based on the two volume-defect-formation mechanisms proposed herein, further development of volume-defect-formation suppression technology is expected to be made.
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.
Understanding the defect characteristics that occur near the space‐charge regions (SCRs) of kesterite (CZTSSe) solar cells is important because the recombination loss at the CZTSSe/CdS interface is considered the main cause of their low efficiency. CZTSSe surfaces with different elemental compositions were formed without polishing (C00) and with polishing for 20 s (C20) and 60 s (C60). For C60, a specific region near the SCR was excessively Cu‐rich and Zn‐poor compared to C00 and C20. Various charged defects formed where the elemental variation was large. As the main deep acceptor defect energy level (Ea2) near the SCR increased, the efficiency, open‐circuit voltage deficit, and current density degraded, and this phenomenon was especially rapid for large Ea2 values. As the Ea2 near the SCR became deep, the carrier diffusion length decreased more for the CZTSSe solar cells with a low carrier mobility than for the CuInGaSe2 (CIGSe) solar cells. The large amplitude of the electrostatic potential fluctuation in the CZTSSe solar cells induced a high carrier recombination and a short carrier lifetime. Consequently, the properties of the CZTSSe solar cells were more strongly degraded by defects with deep energy levels near the SCR than those of the CIGSe solar cells.
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