Understanding the behavior of fixed composition CdSe_xTe_1-x(CST) solar cells [Abstract]

Cadmium selenide (CdSe) plays a vital role to achieving the high short-circuit current density (J_SC ) and passivating the defects in the absorber layer for CdTe photovoltaics necessary to reach high efficiency. Incorporation of CdSe into devices can be done either by fabricating a CdSe/CdTe bilayer or directly depositing the CdSe_xTe_1-x (CST). While the bilayer results in better device performance, the intrinsic properties of the CST suggest it should be the better absorber material. Here, we fabricated and investigated the structural and opto-electronic properties of fixed composition CST films for varying Se concentrations and report device parameters. The films were produced by leveraging our multisource evaporation chamber, allowing a wide range of Se compositions to be investigated without modification to the system. For fixed compositions CST absorber layers, the minority carrier lifetime is improved with higher Se content though the grain sizes are slightly smaller for higher Se content. Note that all these samples (pure CdTe and CST) have undergone same CdCl₂ treatment. The device efficiency for fixed composition CST absorber layer observed is as high as 12.2% while for pure CdTe device (no Se) is 7%. The short circuit current density is high (28 mAcm⁻² ), but CST devices suffer from low open circuit voltage (V_oc) and fill factor (FF). For comparison, CdSe/CdTe bilayer devices also fabricated using this system were able to reach efficiency up to 17.7% (V_oc 839 mV, J_sc 29.0 mAcm⁻² , FF 72.6%), indicating the system produces good material. We will discuss the material properties of CST and correlate these values to the device performance.