Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 250

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Metal rear contact Metal rear contact

n-type n-type

i-type i-type a-SiGe:H a-SiGe:H

TCO TCO

Metal rear contact Metal rear contact

p-type p-type

p-type p-type

n-type n-type n-type n-type

i-type i-type a-Si:H a-Si:H

i-type i-type a-Si:H a-Si:H i-type i-type a-Si:H a-Si:H

n-type n-type

p-type p-type p-type p-type

TCO TCO TCO TCO

Steel substrat Steel substrat

Glass superstrate Glass superstrate Glass superstrate Glass superstrate

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Fig. 6.10 Layer sequence of various p-i-n cell structures of amorphous silicon (a-Si:H)
(left: stainless steel substrate cell, centre: superstrate cell on glass, right: tandem cell made
of a-Si:H and a-SiGe:H on glass superstrate; for all cells the deposition sequence starts with
the bottom layer and ends with the top layer) (TCO transparent conductive oxide)

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Within small consumer electronics applications, amorphous silicon has gained
an undefeated monopoly position worldwide (watches, calculators etc.). However,
due to the poor stability of its physical properties, it is inappropriate for applica-
tions where a higher installed power is needed (like in grid-connected photo-
voltaic systems). When applied outdoors, electrical efficiencies are in some cases
considerably reduced within the first months of operation. To date, efficiency
reduction is still clearly above one forth so that the efficiency is markedly reduced
to below 10 % (degradation; Staebler-Wronski effect) /6-20/. Nevertheless it is
necessary to note, that all long-term monitoring programs show a saturation of the
degradation effect, latest after two years of operation. Power ratings of amorphous
silicon cells are thus typically based on the stabilised power after degradation.
Therefore customers can compare the power related costs on a fair basis. How-
ever, in the medium term, more stable amorphous silicon cells are expected to be
manufactured; latest findings have shown that efficiencies of 14.6 % (tandem cell
with three p-i-n-layers) can be achieved and that degradation comes to a standstill
at 13 % /6-21/.

Thin film solar cells based on chalcogenides and chalcopyrits, particularly CdTe
and CuInSe 2 ("CIS"). The advantages of the thin layer technology in case of hy-
drogen-passivated amorphous silicon (a-Si:H) are counteracted by relatively low
electrical cell efficiencies, when compared to crystalline silicon. In contrast poly-
crystalline thin films made of direct semiconductors, such as cadmium telluride
(CdTe) and copper indium di-selenium (CuInSe 2 ), have at least on a laboratory