Thin-film polycrystalline silicon solar cells. Final report, September 11, 1978-September 10, 1979

Abstract

The effect of grain boundaries on the performance of solar cells fabricated on Wacker Silso polycrystalline silicon substrates has been studied. Efficiencies of 8% (no AR coating) were obtained (11.2% with AR coating). This is 20% lower than the efficiency obtained for identically fabricated single-crystal cells. This reduction can be accounted for as follows: 4% reduction in V/sub oc/ due to grain boundaries, 8% reduction in J/sub sc/ due to grain boundaries, and 8% reduction in J/sub sc/ due to a lower minority carrier diffusion length inside the grains. Quantum efficiency measurements were made with a 150-..mu..m-diameter light spot inside grains and across grain boundaries, permitting a determination of L/sub n/ inside a grain (L/sub n/ = 75 ..mu..m for Wacker Silso). Laser scans were obtained at lambda = 0.633 ..mu..m and at lambda = 1.15 ..mu..m. Some grain boundaries are shown to be electrically inactive. Active grain boundaries were also observed by a novel liquid-crystal technique. Deep traps were studied by DLTS. The crystallographic orientation of individual grains was measured by an SEM technique called electron channeling patterns. Two silicon bicrystals were grown with well defined grain boundaries ((111) growth directions, 30/sup 0/ and 5/sup 0/ misorientation between themore » (110) directions).« less