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 (JSC ) 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 CdSexTe1-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 (Voc) and fill factor (FF). For comparison, CdSe/CdTe bilayer devices also fabricated using this system were able to reach efficiency up to 17.7% (Voc 839 mV, Jsc 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.</p

Cadmium Selenide (CdSe) as an active absorber layer for solar cells with Voc approaching 750 mV

Cadmium Selenide (CdSe) is a semiconductor material with a band gap (1.74 eV) suitable for top cell for the fabrication of tandem devices. Here we explore the optoelectronic properties of evaporated CdSe and the subsequent device performance. The as-deposited CdSe film (thickness ∼800 nm) has small grains t ∼ 200–500 nm) that grow to the order of several microns after cadmium chloride (CdCl2) treatment. In addition, the CdCl2 treatment yielded enhanced photoluminescence (PL) response and long carrier lifetime. However, in addition to a significant band edge PL, we observe a wide peak at energies below the bandgap, suggesting defect states in the absorbance affecting the recombination in the device. The CdSe material was used as an active layer in photovoltaic devices (device structure SnO2/CdSe/HTLs/Au) and achieved a device efficiency of 2.6% with Voc exceeding 750 mV, FF of 56%, and Jsc of 6.1 mAcm-2 when illuminated through the thin Au (front) side. The device efficiency can be improved by replacing gold (Au, 10 nm) which has relatively poor transmittance and sheet resistance. We will discuss the comprehensive evaluation of CdSe films and devices for the photovoltaic application.</p