Use of graphene as an interlayer in CZTS-based thin film solar cell employing various substrate temperatures

In this study, the effect of substrate temperature applied during the film deposition process and post-annealing in sulfur atmosphere on properties of CZTS thin films produced using a single-target CZTS by sputtering system and the impact of using a CVD-grown single layer graphene film, as an interlayer in CZTS thin film solar cells (CZTS/ Gr/CdS) were investigated. All CZTS films were deposited at 200, 250, 300 and 350 degrees C substrate temperatures, and some were exposed to post-annealing in a sulfur atmosphere using a Rapid Thermal Processing (RTP) system. Both as-deposited and post-annealed CZTS samples exhibited a Cu-poor composition. Moreover, all samples prepared by applying substrate temperatures showed a Zn-poor composition. XRD patterns revealed dominant peaks corresponding to the tetragonal kesterite CZTS phase with a preferred (112) orientation across all samples. Structural analysis identified the most promising sample as the one synthesized at 350 degrees C without applying postannealing in sulfur atmosphere, with a crystallite size of 58.29 nm, micro-strain of 6.21 x 10-4, and dislocation density of 2.94 x 10-4. The Raman spectra of this CZTS thin film exhibited dominant peaks around 338 cm-1 , attributed to the kesterite CZTS phase. SEM images showed that the film produced at 350 degrees C without postannealing had the most homogeneous structure. The optical band gap of all CZTS films ranged between 1.42 eV and 1.72 eV. The Raman spectra and AFM image of the graphene film synthesized on copper (Cu) foil confirmed the synthesis of single layer graphene. Optical transmission and electrical measurements showed 96 % optical transparency and 782 ohm/square sheet resistance values, respectively. The solar cells constructed employing CZTS sample produced at 350 degrees C without applying post-annealing in sulfur atmosphere (CZTS-350-AD) without graphene interlayer (Glass/Mo/CZTS-350-AD/CdS/i-ZnO/AZO/Al) and with graphene interlayer (Glass/Mo/ CZTS-350-AD/Gr/CdS/i-ZnO/AZO/Al) structures exhibited 6.39 % and 7.59 % conversion efficiency, respectively. The improvement in cell efficiency, achieved by attaining higher JSC and FF values in the cell, can be attributed to the high conductivity and carrier mobility of graphene, which passivates defects at the interface and facilitates more efficient charge collection by reducing non-radiative recombination, thereby enhancing the overall cell efficiency.