High rate growth of device grade silicon thin films for solar cells
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High rate growth of device grade silicon thin films for solar cells
M. Kondo, S. Suzuki, Y. Nasuno and A. Matsuda Research Initiative for Thin Film Silicon Solar Cells National Institute of advanced Industrial Science and Technology Umezono, Tsukuba, Ibaraki 305-0035, Japan ABSTRACT
We have developed a plasma enhanced chemical vapor deposition (PECVD) technique for high-rate growth of µc-Si:H at low temperatures using hydrogen diluted monosilane source gas under high-pressure depletion conditions.
It was found that
material qualities deteriorate, e.g. crystallinity decreases and defect density increases with increasing growth rate mainly due to ion damage from the plasma.
We have
found that deuterium dilution improves not only the crystallinity but also defect density as compared to hydrogen dilution and that deuterium to hydrogen ratio incorporated in the film has a good correlation with crystallinity.
The advantages of the deuterium
dilution are ascribed to lower ion bombardment due to slower ambipolar diffusion of deuterium ion from the plasma.
Further improvement of material quality has been
achieved using a triode technique where a mesh electrode inserted between cathode and anode electrodes prevents from ion bombardment. In combination with a shower head cathode, the triode technique remarkably improves the crystallinity as well as defect density at a high growth rate.
As a consequence, we have succeeded to obtain much
better crystallinity and uniformity at 5.8 nm/s with a defect density of 2.6x1016cm-3. We also discuss the limiting factors of growth rate and material quality for µc-Si solar cells.
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Solar cells are one of the most important devices in this 21st century because of
the increasing demands of CO2 suppression and secured power generation. Taking into account natural abundance, environmental safety of materials and mass production, thin film silicon solar cells are considered to be the most promising candidate.
A4.3.1
In the last
two decades, amorphous silicon based solar cells have been extensively developed and successfully industrialized by many companies [1]. A remaining issue, however, is photo-degradation, i.e., the Staebler-Wronski effect [2], which limits the efficiency even in the stacked solar cell.
On the other hand, microcrystalline silicon solar cells has
been developed rapidly since the Neuchatel group demonstrated an efficiency of 7.7% by a µc-Si single junction cell and an initial efficiency of 13.1% by a tandem cell [3]. The advantage of µc-Si solar cell is the absence of the degradation phenomena in spite of the inclusion of the appreciable amount of the amorphous phase.
As a consequence
of the further development of the µc-Si solar cell, an efficiency of about 10% is achieved for a single junction cell [4], and further improvement is under progress particularly in Voc.
From an industrial point of view, cost reduction is a key issue for
solar cells to be widely used as a power generation source, and therefore the R&D of a low cost and h
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