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

Silicon-based solar cells – characteristics and production processes
temperature contact metallization, then all these elements can be the ways for an
easy leak of the already separated pairs of electrons and holes. This causes a drop in
the photovoltage, especially for the multicrystalline silicon-based cells. A large drop
of R sh can take place on the edges of the cell, and this results from the easy access for
outer metallic impurities. The series resistance is the sum of the resistances of
particular components of the cell. According to Figure 8, this is the resistance of: the
Si base material’s contact, type p, to the rear contact R 1 , the base material R 2 , the Si
emitter, type n, between the front electrodes R 3 , the emitter’s contact to the front
electrode R 4 and the front electrode R 5 . The resistance of the rear contact, made of
a full metallic layer, should be considered as negligibly small. The resistance value of
the contact R 4 , which is fundamentally important for the efficiency of the cell and
which depends on the paste type, the base resistance and the temperature of the
metallization process, can be experimentally determined, e.g. with the use of the
transmission line method [44]. For instance, for a cell with the surface area of 100
cm 2 , characterizing in its fill factor value FF above 0,75 {the mentioned FF is discussed
in Chapter 2.5, dependence (23)}, the value of R s must be below 10 mΩ , and the
value of R sh – above 10 Ω.
Fig. 8. Cross-section of a Si-based cell and the series resistance components.
Table 8 compiles the basic differences in the contact parameters of the front
collecting electrode of a crystalline silicon-based solar cell, produced with the method
of screen printing (SP), photolithography (PL) and chemical bath (CB), applied in BCSC
cells with a buried contact (Chapter 2.6). The data refers to the resistivity value of the
contact (R 4 ), which has a fundamental effect on the series resistance R s of the cell,
and also the resistivity value of the contact material, which affects R 5 .
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