Study on the nano-crystalline Si embedded ZnO thin films for solar cell application

The characteristics of nano-crystalline silicon (nc-Si) embedded in Si-based dielectric matrix have been investigated extensively. However, these Si-based dielectric materials have the highly-resistive nature and difficulty in building up the built-in electric field, which limit the performances of nc-Si thin films for solar cell (SC) application. In this study, we propose to use ZnO as a new matrix material for the nc-Si thin films with better optoelectronic properties because of the unique characteristics of ZnO. We successfully demonstrate the formation of nc-Si embedded in ZnO thin films using a ZnO/Si multilayer (ML) structure by radio-frequency (RF) magnetron sputtering method. From the high-resolution transmission electron microscope (HRTEM) images, we clearly observe the amorphous Si (a-Si) nano-clusters after deposition and high density of nano-crystalline clusters after annealing in the ZnO/Si ML structure. From atomic force microscope (AFM) images, significant variations on the surface morphologies are observed under different Si sputtering powers (P_Si) after deposition. The larger surface roughness and clearer formation of a-Si nano-clusters are observed for P_Si higher than 75 W. Combined with the Raman spectra and X-ray diffraction patterns, the results indicate that the sputtered Si atoms with higher P_Si have more kinetic energy to aggregate together and are easier to form a-Si nano-clusters during deposition. Such morphology is helpful for the nc-Si formation and the better crystallization of the ZnO matrix during annealing. Thus, high density of nano-crystalline clusters is observed in the HRTEM images after annealing. Our experimental results show the nc-Si embedded ZnO thin film is certainly achievable, and a high conversion efficiency SC integrating nc-Si thin film with ZnO matrix can be expected.