Bonded Hydrogen In Nanocrystalline Silicon Photovoltaic Materials: Impact On Structure And Defect Density

In the 21st century, shortage of traditional energy and deterioration of environment become the major global crises threatening survival and development of human being. Therefore, exploitation of clean and renewable energy resources becomes extremely urgent. As an inexhaustible and green energy, solar energy turn out to be one of the most promising candidates. Solar cell is an equipment that convert sunlight to electricity. Although the traditional crystalline silicon solar cells still account for the vast majority of market share in the global photovoltaic industry, novel thin film solar cells, especially the silicon-based thin film solar cells, have attracted great attention due to their advantages including low manufacturing cost, accessibility of large area fabrication, low temperature coefficient, and short time of energy payback.For a long time, amorphous silicon thin film solar cell, as one of the silicon-based thin film solar cells with quite mature technology, still has some unavoidable disadvantages, i.e., low energy conversion efficiency, low conductance, and light induced degradation of cell performance (so-called Staebler-Wronski effect). However, the emergency of hydrogenated nanocrystalline silicon (nc-Si:H) revitalize the development of Si thin film solar cells. Hydrogenated nanocrystalline silicon is a mixed-phase material consisting of nanocrystals embedded in amorphous tissue. Extensive optical and electrical investigations of nc-Si thin films have been carried out. Strong optical absorption and high photocurrent are found in nc-Si films and attributed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains. There are numbers of attempts to realize high efficiency and good stability single-junction and tandem nc-Si thin film solar cells.We have carried out a detailed structural and optical investigation of nc-Si:H thin films prepared by plasma enhanced chemical vapor deposition. The microstructural properties of the nc-Si:H thin films are extensively characterized and physically interpreted based on the growth mechanism. Careful infrared spectroscopic analysis reveals that the bonding hydrogen in a platelet-like configuration which is considered to be located in grain boundaries greatly affects the oxygen incorporation of the nc-Si:H thin films, while the later is linked to the introduction of dangling bond defects from electron spin resonance observation. Consequently, we propose that in nc-Si:H films a high content of bonding hydrogen in the grain boundaries is of great importance to form a hydrogen-dense amorphous issue around the small crystalline grains, i.e., a compact grain boundary structure with good passivation, thus effectively prevent the post-deposition oxidation of the grain boundary surface which possibly leads to the formation of the dangling bond defects. This work was supported by the National Major Basic Research Project of 2010CB933702 and Natural Science Foundation of China under contract of 11074169.