High-efficiency polycrystalline CdTe thin-film solar cells with an oxygenated amorphous cds (a-CdS:O) window layer

In the conventional CdS/CdTe device structure, the poly-CdS window layer has a bandgap of ∼2.4 eV, which causes absorption in the short-wavelength region. Higher short-circuit current densities (J_sc) can be achieved by reducing the CdS thickness, but this can adversely impact device open-circuit voltage (V_oc) and fill factor (FF). Also, poly-CdS film has about 10% lattice mismatch related to the CdTe film, which limits the improvement of device V_oc and FF. In this paper, we report a novel window material: oxygenated amorphous CdS film (a-CdS:O) prepared at room temperature by rf sputtering. The a-CdS:O film has a higher optical bandgap (2.5-3.1 eV) than the poly-CdS film and an amorphous structure. The preliminary device results have demonstrated that J_sc of the CdTe device can be greatly improved while maintaining higher V_oc and FF. We have fabricated a CdTe cell demonstrating an NREL-confirmed J_sc of 25.85 mA/cm² and a total-area efficiency of 15.4%.