Sn1–xGexOy and Zn1–xGexOy by Atomic Layer Deposition─Growth Dynamics, Film Properties, and Compositional Tuning for Charge Selective Transport in (Ag,Cu)(In,Ga)Se2 Solar Cells

Two inorganic electron-selective layers (ESLs), Sn1–xGexOy (TGO) and Zn1–xGexOy (ZGO), were developed by using atomic layer deposition (ALD) with in situ quartz crystal monitoring. To ensure (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cell compatibility, a 120 °C ALD process was developed for GeOy using Ge(N(CH3)2)4 and H2O as precursors. In the ALD supercycle approach, the GeOy ALD cycle was interchanged with either ZnO or SnOy cycles to deposit TGO and ZGO with varying conduction band positions (Ec), respectively. The material properties were experimentally verified using X-ray photoelectron spectroscopy and optical absorption and by employing these films as ESLs in ACIGS solar cells. There, the open-circuit voltage initially increased as the Ge content of the TGO and ZGO films increased due to the ESL Ec simultaneously shifting up from the low position in ZnO or SnOy to match the ACIGS Ec. As the Ge content increased further, the fill factor (FF) of these devices decreased since the ESL Ec became positioned significantly above the ACIGS Ec, forming an energy barrier as seen from ACIGS. As a result, the efficiency of the ACIGS solar cell peaked for an intermediate Ge content for both TGO and ZGO. Using good TGO and ZGO compositions in ACIGS solar cells gave efficiencies of up to 14.8 and 17.0%, respectively, which were lower than the reference best cell efficiencies of up to 19.5% for CdS and 18.2% for Zn1–xSnxOy (ZTO). ZGO was, however, able to shift its Ec further up than ZTO, making it a potent ESL for high-band-gap absorbers. Based on the results, we listed a few key properties that are required for a good ACIGS solar cell ESL and gave a few suggestions on how they are linked to the previous success of ZTO.