The diffusion of holes in undoped amorphous Si :H

Abstract

The diffusion length of holes in undoped a-Si : H has been measured by a modified surface-photovoltage method. The a-Si : H was deposited from a d.c. glow discharge in silane. Both field-free and field-assisted hole transport were observed, and could be distinguished by varying the width of the surface depletion layer using different illumination levels. Field-free (thermal) diffusion lengths vary from about 0·3 μm to about 1·1 μm, being a function of substrate temperature during film growth and of the details of the deposition procedure. The hole diffusion length increases with temperature in the range from -30°C to 60°C in the form of a Boltzmann factor with a characteristic energy of 0·22 eV for a film grown at 330°C and of 0·28 eV for a film grown at 250°C. Our results suggest that analyses of a-Si : H solar cells ought not to ignore field-free hole transport.