Photovoltaic mechanisms in polycrystalline thin film solar cells. Quarterly technical progress report No. 3, April 1-June 30, 1979

The vertical junction effect in polysilicon solar cells (due to dopant penetration along grain boundaries) is investigated using a cylindrical grain model. The model shows that for small grain radii the short circuit current can increase above the single crystal value if the grain boundaries collect carriers and that th short-circuit current approaches the single crystal value as the grain size increases. The spatial dependence of the dark I-V characteristic is investigated by fabricating small diodes on a polysilicon wafer. Dark I-V characteristics are measured and illuminated characteristics are projected for each of the diodes. The diode characteristics are given together with SEM micrographs of the diodes and the effect of areal inhomogemety is investigated. The relative orientations of adjacent crystal grains and the nature of the boundaries between grains are studied by x-ray diffraction, optical microscopy and SEM methods. Orientation data is presented along with electron channelling patterns obtained from individual grains. A conclusion is drawn on the origin of sinuous grain boundaries