Investigation of the Causes and Variation of Leakage Currents in Amorphous Silicon P-I-N Diodes
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Investigation of the Causes and Variation of Leakage Currents in Amorphous Silicon P-I-N Diodes Todd R. Johnson*, Gautam Ganguly, George S. Wood, and David E. Carlson BP Solar, North American Technology Center, Toano, Virginia *Princeton University, Princeton, New Jersey ABSTRACT Excess leakage currents under reverse bias (known as shunting) and spontaneous reductions of this excess leakage under increased reverse bias (known as curing) were investigated in hydrogenated amorphous silicon (a-Si:H) based single junction p-i-n type diodes. An increase in the frequency of shunting was observed when the front contacts were switched from tin oxide to zinc oxide, most likely due to defects in the previously deposited zinc oxide coated glass was observed. Storage in the dark and light soaking up to 100 hours were both observed to independently increase the leakage current in previously leaking diodes. Models for the distribution of shunt-causing defects within a given cell area were considered. Comparing the measured frequency of shunting using cells of varying area (1 to 16 mm2) to the models’ predictions indicate a distribution of point defects separated by relatively large average distances that are slightly larger for tin oxide (5-6 mm) than for zinc oxide (4 mm). INTRODUCTION The commercialization of a-Si:H based thin film solar cells has been going on for some time. [1,2] However, the consequent shunting in a-Si:H thin film solar cells has been an issue for manufacturers because the higher leakage currents in reverse bias results in a loss of power (efficiency). It has previously been observed that subjecting some cells to a higher voltage can “cure” these shunts and repair the cells to normal working order [3]. However, curing is not effective on all shunts and sometimes the shunts reappear or increase in leakage after some period of time [4]. In some cases, large shunts are caused by tin oxide particles [4]. The mechanisms of both shunting and curing have not been definitively explained and so systematic prevention of shunts in manufactured cells has not been possible. In this study, we looked to determine the distribution of the shunts themselves: whether they were grouped together in clusters, randomly distributed single point defects, or something else. Microscope observations were made of the cells at different stages of deposition and before and after curing to see if visible changes or shunt causing defects could be seen. We have also compared various front and back contacts to shed light on the causes and find possible cures for shunting. EXPERIMENT a-Si:H based solar cells were fabricated by DC plasma enhanced chemical vapor deposition (PECVD) on transparent conducting oxide coated glass substrates. Some of the glass substrates were coated with smooth or textured tin oxide (~700 nm thick) by a commercial vendor while others were RF magnetron sputter coated with ~100nm of zinc oxide, in house. The back contact consisted of a zinc oxide layer of ~100 nm and /or ~200 nm of aluminum or silver. The deposi
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