Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 244

Already in the sixties of the last century, a multitude of research and develop-
ment activities have been conducted to develop cost-efficient thin film solar cells
(see /6-14/). For this purpose "direct" semiconductors are required. This substance
category mainly includes II-VI, III-V and I-III-VI 2 compounds. Also amorphous
silicon (a-Si), discovered in the 1970's within the scope of photovoltaic projects,
is a direct semiconductor. It is characterised by good absorption properties and
seemed suitable as base material for thin film solar cells.
Yet, due to still unresolved problems with regard to the competing semiconduc-
tor materials or technologies, crystalline silicon (includes crystalline and poly-
crystalline technologies) will continue to be predominantly used as base material
within the years to come.
Since solar cells are (still) relatively expensive, there is a tendency to concen-
trate solar radiation and thus to reduce the required surface of the photovoltaic
cells. Furthermore, efficiencies of photovoltaic cells tend to increase with in-
creased irradiance - if cell temperature remains constant. For concentrating sys-
tems, more expensive but more efficient solar cell technologies may be applied
cost-efficiently. For instance, mirror and lens systems are used to concentrate
solar radiation. But under these circumstances tracking systems are additionally
needed, helping to enhance the energy yield per unit surface. Such concentrating
systems are most suitable for direct radiation (only direct radiation can be fo-
cused) and thus for regions throughout the world where the solar radiation is de-
termined by direct irradiance (like in deserts). Therefore their application in Mid-
dle Europe, with a ratio of direct to diffuse radiation of approximately One, based
on the annual radiation (see Fig. 2.9), is not appropriate in most cases.
State-of-the-art technologies in terms of solar cell development, with regard to
laboratory research and manufacturing, have been summarised in Table 6.1. In the
following the various solar cell type technologies are briefly outlined.
Solar cells from crystalline silicon. This cell technology is mainly based on proc-
esses applied within the semiconductor industry (see /6-7/, /6-8/, /6-9/, /6-15/).
With regard to crystalline cell manufacturing three steps are distinguished:
− production of high-purity silicon as base material,
− manufacture of wafers or thin films and
− solar cell production.
Silica sand (SiO 2 ) serves as base material for high-purity silicon. By means of a
specific reduction method (melting electrolysis) silica sand is transformed into
"metallurgical grade silicon" characterised by a maximum purity of 99 %. How-
ever, this purity is still insufficient for solar cell production.
Thus, further expensive purification steps are required for the production of
silicon used by the semiconductor industry, since for semiconductor silicon (semi-
conductor grade silicon; SeG-Si) the impurity content must not exceed 10 -9 . The
required silicon purification is worldwide in most cases performed by the Siemens
process. This purification method starts with the conversion of metallurgical grade
silicon into trichlorosilane using hydrochloric acid. The subsequent fractional