Fabrication Of Novel Silicon Heterojunction For Photovoltaic Application

Traditional silicon（Si） solar cells with high power conversion efficiency（PCE） and long-term stability of more than 20 years have already been commercialized. However, the Si solar cells still suffered from the high fabrication cost, high temperature processing and low absorption coefficient. In recent years, heterojunction structure and surface texture technique were proposed to solve the problems. The center of these methods is how to optimize the interfacial energy level alignment and reduce the interfacial recombination. In this thesis, we develop Si surface passivation technique by using methyl group（-CH₃）,and apply it to two types of silicon heterojunction structure: Si/PEDOT:PSS hybrid structure and Si/transition metal oxide（TMO） heterostructure. The effects of interfacial properties on the photovoltaic performance are systematically investigated. The thesis contents the following three parts:（1） We fabricate Si nanowires array by metal assisted chemical etching; and Si pyramids array by anisotropic etching. CH₃ passivation was developed to passivate the dangling bond on the surface of Si. By using X-ray photoelectron emission spectroscopy, we showed that CH₃ passivation provide better surface stability in air than that by H-passivation.（2） Si/PEDOT:PSS hybrid solar cells were fabricated by using a p-type organic material（PEDOT:PSS） as a hole-transporting layer. The device performance was found to depend on the coverage of PEDOT:PSS. The interfacial energy-level alignment of Si/PEDOT:PSS heterojunction was measured using Kelvin probe force microscopy（KPFM）. The light absorption was calculated by COMSOL. We found that the coverage effect results from the position difference between the interfacial electrical field and absorption areas. The bottom coverage morphology gives the optimized device performance because of the position overlapping of the junction electrical field and absorption area.（3） The Si/TMO(MoO_3-x, WO_3-x, and V₂O_5-x photovoltaic devices were fabricated by depositing an ultrathin TMO layer on the hydrogenated or methylated Si surfaces through a thermal evaporation method. The passivation effects of CH₃ and H on the devices performance were systematically investigated. We found that CH₃ passivation was more effective to suppress the interfacial reaction between Si and TMO. By investigate the interfacial energy level alignment and the recombination process, we figure out the deposition of MoO＿(_3-x) causes less interfacial oxidation and provides superior energy channel for hole collection. Therefore, the best performance is found in CH₃-Si/MoO＿(_3-x) device, with PCE of 9.4%.