Development of 2D Structured CuS-Incorporated rGO Back Contact Layer for CdTe Solar Cells

The unique CuS-rGO composite back contacts were developed for CdS/CdTe heterojunction solar cells by combining sonochemical and spin-coating methods. The incorporation of CuS into the rGO matrix tunes the electrical properties of rGO and reduces the migration of Cu2+ into the CdTe absorber layer. In this study, the CuS, rGO, and CuS-rGO nanocomposite were synthesized by precipitation, the modified Hummer’s method, and the sonochemical method, respectively. Three different composites were prepared by varying the CuS-to-rGO ratio. The structural, phase crystallization, optical, morphological, compositional, chemical, electrical, and electronic properties of bare CuS, rGO, and CuS-rGO composites were analyzed. The quantitative band alignment interpretation carried out using ultraviolet photoelectron spectroscopy (UPS) exhibited near-ohmic interface formation at the CuS-rGO composite/CdTe interface, with positive valence band (ΔEVB) and conduction band (ΔECB) offset values of 0.40 and 0.33 eV, respectively, for the CR1 sample with a CuS-to-rGO ratio of 1:1. The back contacts were prepared by spin coating the CuS, rGO, and CuS-rGO nanocomposites on top of the FTO/CdS/CdTe structure, and the performance of the cells was tested and compared with a reference cell without the back contact layer (FTO/CdS/CdTe/Ag). Among the tested cells, the cell with composite CR1 as the back contact produced the best performance, with open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) values of 0.760 V, 22.25 mA/cm, 57.35%, and 9.70%, respectively, with good reproducibility. To the best of our knowledge, this is the initial study on the CuS-rGO nanocomposite back contact layer for thin-film photovoltaics.