Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells

Simultaneous and locally resolved determination of the mechanical stress variation and the free hole concentration using Raman spectroscopy is demonstrated in laser crystallized amorphous silicon layers. Such layers are often used for the fabrication of thin film solar cells, e. g., on borosilicate glass substrates. The combined effects of stress and doping on the Raman signal can be separated based on the use of three wavelengths in the visible. The results show that the free hole concentration in the samples investigated varies between 1 x 10(18) and 1.3 x 10(19) cm(-3). Stress as well as the free hole concentration vary substantially within the sample. The stress level varies between 575 and 850 MPa (+/- 12 MPa). Cross-sectional transmission electron microscopy images show the presence of extended lattice defects such as dislocations and grain boundaries in the crystallized Si layer which could account for the lateral stress variations detected by Raman spectroscopy. The impact of film inhomogeneity in terms of stress and doping on the performance of a solar cell will be discussed. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3319654