Defect characterization of polycrystalline silicon layers obtained by aluminum-induced crystallization and epitaxy
- PDF / 1,211,049 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 164 Views
0989-A16-03
Defect characterization of polycrystalline silicon layers obtained by aluminum-induced crystallization and epitaxy Dries Van Gestel, Ivan Gordon, Lodewijk Carnel, Guy Beaucarne, and Jef Poortmans IMEC, Leuven, B-3001, Belgium
ABSTRACT In order to reduce the harmful influence of grain boundaries in thin-film polycrystalline Si solar cells we form absorber layers on foreign substrates with columnar grains with a grain width much larger than the layer thickness. Such layers with a grain size in the range of ~1-100 µm can be obtained by aluminum-induced crystallization and epitaxy. Until now however, the open-circuit voltage of solar cells made from these layers was quasi-independent of the grain size. To understand this fact, defect etching combined with electron microscopy, as well as Electron Backscattered diffraction (EBSD) measurements were performed to investigate the crystallographic defects. A very large density (~ 109 cm-2) of intra-grain defects (IGD) was found. Room temperature Electron Beam Induced Current (EBIC) measurements were carried out to localize and investigate the electrically active defects. The intra-grain defects found with defect etching showed a strong recombination activity. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of our pc-Si solar cells is due to the presence of numerous electrically active intra-grain defects. INTRODUCTION Thin-film polycrystalline silicon (pc-Si) solar cells (grain size ~ 0.1 ñ 100 µm) on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminuminduced crystallization (AIC) in combination with epitaxy is a possible way to obtain pc-Si layers with columnar grains on foreign substrates [1]. Over the last few years at IMEC we have achieved with this approach an increase in absolute efficiency of ~1.5 % per year, the best efficiency being 8% at the moment [2]. To maintain this trend, further research is necessary to e.g. enhance the layer quality. Because there is a high minority carrier recombination probability at grain boundaries, control over the grain size distribution is important. The AIC seed layer formed on the foreign substrate before epitaxial growth allows us to influence this distribution of the absorber layer [3]. However, we recently showed that solar cells made from pc-Si layers with very small grains of 0.2 µm had almost the same open-circuit values (Voc) as solar cells made from AIC-based pc-Si layers with grain diameters of up to 50 µm [4,5]. Since the Voc of thin-film solar cells is a measure for the electronic quality of the absorber material, the quasi-independence of the opencircuit voltage on the grain size indicates that at the moment grain boundaries and grain size distribution are not the only factors limiting the electronic quality of our pc-Si layers. It is known that intra-grain dislocations can be harmful for the electrical layer quality, and different models have already been proposed to describe their effect on the minority
Data Loading...
