Investigation of n-ZnO/p-Si and n-TiO2/p-Si Heterojunction Solar Cells: TCAD + DFT

This paper focuses on exploring new materials and structure as a means to increase the efficiency of solar cells. Since silicon is widespread on earth, it is desirable to study heterojunction solar cells made mainly of silicon and new materials. Therefore, ZnO/Si and TiO2/Si heterojunction solar cells were studied in this paper. First, the electrical and optical properties of ZnO and TiO2 were determined using the Perdew-Burke-Ernzerhof (PBE), PBE functional revised for solids (PBESol) and Perdew-Wang (PW91) functionals of the Generalized gradient approximation (GGA) in Density Functional Theory (DFT). The obtained results in various functionals are assessed and analyzed. It was found that geometric optimized structures of TiO2 and ZnO is mechanical stable. Accordingly, in all functionals, the effective mass of the electron in ZnO and TiO2 proved to be smaller than that of the hole. The mobility of electrons and holes in ZnO was calculated to be 430.72 cm
$^{2}\text{V}^{-1}\text{s}^{-1}$
and 5.25 cm
$^{2}\text{V}^{-1}\text{s}^{-1}$
respectively. In TiO2, it was 355.27 cm
$^{2}\text{V}^{-1}\text{s}^{-1}$
and 46.38 cm
$^{2}\text{V}^{-1}\text{s}^{-1}$
. When PW91, PBESol, PBE functionals were used, the dielectric constant was determined to be 11, 11.5, 8.5 for ZnO and 9.5, 10, 9 for TiO2, respectively. According to the DFT results, it was determined that ZnO and TiO2 are transparent and mainly n-type direct semiconductors. According to device simulation, the maximum short-circuit current of ZnO/Si and TiO2/Si heterojunction solar cells is 18 mA/cm2 at a thickness of 80 nm and 15.3 mA/cm2 at a thickness of 40 nm. Finally, the average fill factor of ZnO/Si and TiO2/Si solar cells was 0.73 and 0.76 respectively. So TiO2 can be used as a transparent contact and ZnO as an emitter layer in a silicon-based solar cell.