Faculté des sciences

Growth, microstructure and electrical performances of thin film microcrystalline silicon solar cells

Bailat, Julien ; Shah, Arvind (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2005 ; 1804.

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    The goal of this thesis work is to improve the understanding of the growth of microcrystalline silicon solar cells and the relationships between their microstructure and their electrical performances. For that purpose, the work was divided into three different parts: - microstructure and growth of the microcrystalline layer and devices - growth simulations - electronic quality of the intrinsic layer (i-layer) and open-circuit voltage (Voc) Microstructure of µc-Si:H The microstructure was first fully characterized by TEM. The main questions were how the microstructure of the intrinsic layer (i-layer) is influenced by the deposition parameters and the nature of the substrate. It was shown that the substrate surface chemistry has a strong influence on the nucleation, whereas the surface roughness plays a minor role. The silane concentration (SC), one of the main deposition parameters, has a strong effect on the nucleation and crystallinity of the whole intrinsic layer. Growth simulations An innovative model for simulating the growth of microcrystalline and amorphous silicon was developed in order to understand in a better way the formation of the microstructure. It is based on two simple selection rules and three intuitive simulation parameters (i.e. critical size of nuclei, number of crystallographic orientations and desorption probability). This model is able to reproduce the main characteristics of the growth dynamics and microstructure of microcrystalline silicon: conical shape of the grains, thickness transition from amorphous to crystalline material, amorphous to crystalline transition with respect to deposition parameter (SC) and surface roughness evolution with respect to layer thickness. This simple model gives the keys to understanding the growth dynamics and microstructure of microcrystalline silicon layers. Electronic quality of i-layer and Voc The electronic quality of the i-layer was evaluated by the optical absorption of the defects of the i-layer as measured with the help of Fourier transform photocurrent spectroscopy (FTPS). Defects in the i-layer were artificially created by proton irradiation in order to demonstrate that FTPS is able to monitor the defect related absorption of the i-layer within complete thin film silicon solar cells In order to observe the influence of SC on the defect density, two series of cells deposited with different SC were fabricated. The FTPS measurements demonstrated that the defect related absorption of the i-layer decreases as SC is raised. Finally, variation of the defect density either by stepwise annealing of a sample irradiated by a proton beam or by variation of SC of i-layer was shown to influence the Voc significantly. From the above observations and conclusions one speculates that further tailoring of the structure of the solar cells could lead to a better open-circuit voltage for µc-Si:H solar cells.