On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

Abstract

Recently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

Device grade microcrystalline silicon owing to reduced oxygen contamination

“…Thus, the purifier technique results in a substantial enhanced device performance and in even higher solar cell efficiencies than the earlier microdoping approach: 9 meanwhile, a single-junction cell efficiency of 7.7% has been achieved. 16 The microdoping approach could therefore even more than successfully be replaced by the purifying technique.…”

The “Micromorph” cell: a new way to high-efficiency-low-temperature crystalline silicon thin-film cell manufacturing?

“…VHF-GD deposited µc-Si:H shows a pronounced absorption edge; thereby a gap-energy as low as 1 eV could be deduced (8). As a surprising fact, the overall absorption within the whole near-infrared range is higher than that obtained with monocystalline silicon.…”

“…Apart from few isolated papers (3-5), µc-Si:H was in the past generally not seriously taken into account as a candidate for becoming an active semiconductor for solar cells. The novel inputs that have lead to the successful use of µc-Si:H in solar cells as reported in recent work (1,(6)(7)(8)(9), can be reduced to two main characteristics:…”

“…The purifying technique: A gas purifier (oxygen getter) installed close to the reactor removes oxygen-containing impurities (8). Thereby, low dark conductivities and µc-Si:H single-junction solar cells with 7.7% efficiency could be obtained (6,11).…”

“…crystalline silicon columns throughout the thickness of the cell would be ideal. Meier et al (8) have shown, using SEM and TEM micrographs and X-ray measurements, that VHF-GD µc-Si:H layers possess, indeed, a pronounced columnar structure in the (220) direction. At present there are no experimental tools at hand that allows one to quantify the characteristics c)-d) in a microscopic way giving thereby the possibility to "tune" the deposition conditions for obtaining an "ideal" µc-Si:H as sketched above.…”