Some investigations on photovoltaic mechanism in silicon ribbon

Abstract

Laser scanning technique combined with microwave contactless lifetime measuring method was used to investigate characteristics of ribbon cells which were prepared by the capillary method. Lasers of two wavelengths, 633 nm (He-Ne) and 1060 nm (YAG) were used to examine local variation in characteristics. Short circuit current (Isc) was measured during traverse of ribbon stage. Isc excited by He-Ne laser (I633) represented surface information of ribbon while Isc excited by YAG laser (I633) represented that of bulk. Areas were classified in accordance with the nature of Isc into the following four categories: 1) high (I633), high I633, 2) high I633, low I1060, 3) low I633, high I1060 and 4) low I633, low I1060. Samples were then subjected to lifetime measurement where bulk lifetime (τB) and surface recombination velocity (S) were separated from observed lifetime (τM) by calculating the deviation from the exponential curve. According to the natures of τB and S, areas were also classified into four classes as follows: A) high τB, small S, B) high τB, large S, C) low τB, small S and D) low τB, large S. Correspondence between the lifetime classification and photoresponse categories were discussed concerning the identification of crystallographic defects. A model was proposed to explain the influence of inclusions on cell efficiency. Characterisitics calculated from this model are consistent with the results obtained in the present investigation and with experience in ribbon cell manufacturing.