Silicon-based solar cells Characteristics and production processes ...

Silicon-based solar cells – characteristics and production processes
Rear Locally diffused). Although it is made on a 4 cm 2 plate, its basic parameters, such
as: V oc = 0,706 V, J sc = 42,2 mA/cm 2 i FF = 0,828, determine the level of values to
which one should refer their results in the field of silicon cell construction. The cell,
manufactured in the photolithographic technology, has such excellent parameters
due to the following factors: a very small value of the reflection coefficient ~ 2 %,
obtained as a result of selective etching of the front surface, a high resistance of the
n-layer between the front contacts – above 200 Ω/□, an additional donor impurity of
the n + -type under the front contacts and an additional acceptor impurity of the p + -
type under the local rear contacts. One of the more interesting concepts are the cells
with buried contacts, type BCSC (Buried Contact Solar Cell), which have been
produced since the beginning of the 1990s, under license of the English company
BP Solar [65].
Fig. 30. Cross-section of a mono-crystalline silicon solar cell with buried contacts (BCSC).
The highest conversion efficiency, 22 %, was obtained for a mono-crystalline
BCSC cell, type FZ. The basis for achieving such a high efficiency is the application
of the front electrode contacts made of a nickel-copper-silver layer, deposited by
means of a chemical bath in the ~ 30 ÷ 50 μm deep laser-cut grooves. Applying a
second donor diffusion through the mask makes it possible to create a strongly doped
n +- type area under the contacts, with the laminar resistance at the level of 20 Ω/□,
which affects the reduction of the series resistance. The high-resistant emitter R ρ
~ 120 ÷ 200 Ω/□, between the thin leading electrodes, reduces the disadvantageous
effect of the Auger recombination process and allows for a significant improvement
of the cell’s current density.
Another cell type, combining the mono-crystalline silicon with the amorphous
one, is the HIT-type solar cell (Heterojunction with Intrinsic Thin layer), whose
construction scheme is presented in Fig. 31.
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