Hydrogen Passivation of Thin-film Polysilicon Solar Cells
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0989-A18-11
Hydrogen Passivation of Thin-film Polysilicon Solar Cells Lode Carnel, Ivan Gordon, Dries Van Gestel, Guy Beaucarne, and Jef Poortmans mcp ssc, imec, kapeldreef, leuven, 3001, Belgium
ABSTRACT In this work we characterized fine-grained polysilicon layers with a grain size of only 0.2 µm before and after passivation. Plasma hydrogenation led to a higher hydrogen concentration in the first micron of the layer than nitride passivation. The highest efficiency of 5.0 % was reached when nitride passivation was followed by plasma passivation. INTRODUCTION Over the last decade, the production of photovoltaic modules has increased with an impressive annual growth rate of more than 30 %. This resulted in a significant shortage of solar grade Si material and a sharp increase of the Si feedstock price. To reduce the cost price of photovoltaic energy, other technologies need to be developed that use less silicon. The ultimate silicon technology could be a thin (~ 5 µm) polycrystalline silicon layer deposited from the gas phase onto a foreign carrier substrate such as an alumina ceramic or a glass-ceramic substrate [1]. Such a technology could lead to a sharp cost reduction while maintaining the efficiency and the stability of a fully crystalline Si absorber. However, current efficiencies are still well below those achieved for bulk silicon solar cells, mainly due to the large number of defects both at the grain boundaries and in the grain itself [2]. To passivate these defects, hydrogen is introduced into the layers. EXPERIMENTAL DETAILS In this work we studied the hydrogen passivation of defects using fine-grained (~ 0.2 µm grains) polysilicon. The p-type doped fine-grained polysilicon layers were obtained by direct deposition of silicon on top of an oxidized silicon wafer. The deposition is done by thermal chemical vapour deposition at temperatures above 1000 ºC and at atmospheric pressure. The passivation is carried out using two different methods: a hydrogen plasma treatment and a hightemperature anneal (firing) of a hydrogenated silicon-nitride (a-SiNx:H) layer. Plasma hydrogenation offers an infinite amount of hydrogen, while the a-SiNx:H is a solid source with only a finite amount of hydrogen. The hydrogen incorporation in the layers was measured with secondary ion mass spectroscopy (SIMS) using deuterium as an easily traceable isotope of hydrogen. The passivated layers were studied electronically by mobility, temperature-dependent resistivity and spreading resistance profiling (SRP) measurements. Finally, solar cells were made to compare both passivation methods at device level.
RESULTS
p-type layer characterization Figure 1 plots the D-profiles of the first two microns of identical polysilicon layers after both the plasma and the a-SiN:D passivation. The influence of the presence of a diffused emitter on the D-profiles is also shown for both passivation techniques.
plasma w/o emitter plasma with emitter a-SiN:H w/o emitter a-SiN:H with emitter
10
[D] (10
18
-3
cm )
100
1
0.1 0
0.5
1
1.5
2 d
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