14% CdS/CdTe Thin Film Cells with ZnO:Al TCO

Abstract

An Al-doped ZnO front contact was successfully used for the first time for the fabrication of high efficiency CdS/CdTe thin-film solar cells. The ZnO:Al films were deposited on aluminosilicate glass by RF sputtering from a ZnO:Al₂O₃ target. The ZnO:Al film has ∼95% average transmission in the visible spectrum with ∼3 ohm/square sheet resistance. The CdS and CdTe thin films were also deposited by RF sputtering and devices were completed with a vapor CdCl₂ treatment and evaporated Cu/Au back contacts. The highest processing temperature was 387°C, reached during the vapor CdCl₂ treatment. The devices were tested at NREL with efficiency of 14.0% which is a record for an all-sputtered CdS/CdTe solar cell. The ZnO-based cell had J_SC of 23.6 mA/cm² compared to 20.7 mA/cm² for our recent NREL-tested 12.6% cell on a commercial soda-lime-glass/SnO₂:F substrate. Other parameters of the 14% ZnO based cell are: FF = 73.25% and V_OC = 814 mV. The improved performance is almost entirely due to higher current because of better optical and electrical properties of ZnO:Al TCO. We report also on relative stability between devices on SnO₂:F and ZnO:Al TCO, under one-sun light soak at V_OC.