ON THE TRANSPORT-PROPERTIES OF MICROCRYSTALLINE SILICON

To determine the charge collection mechanism in hydrogenated microcrys talline silicon (mu c-Si:H) solar cells, we have measured the electron ic transport properties of mu c-Si:H by time-of-flight and by ac capac itance and conductance on a unique 5.6 mu m thick sample. We found the electron drift mobility mu(D) -2.8 +/- 0.2 cm(2) V-1 s(-1), thermally activated with E-A = 0.14 +/- 0.1 eV. Evidence for field inhomogeneit y was observed as an initial maximum of the photocurrent transients an d as an increase of capacitance over the geometrical value. The freque ncy dependence of the capacitance exhibits marked differences from a-S i:H and is proposed as a tool for studying the effects of microstructu re on electronic properties. Changes of the sample capacitance with te mperature and illumination were observed. As a consequence of the inho mogeneity of the material, several different activation energies were found: 0.14 eV for electron drift mobility, 0.29 eV for ac conductivit y, 0.4 eV for steady state dark conductivity and finally greater than or equal to 0.8 eV for the photocapacitance relaxation. (C) 1998 Elsev ier Science B.V. All rights reserved.