Page 200 - A Comprehensive Guide to Solar Energy Systems

202 A COmPrehenSIVe GUIDe TO SOlAr enerGy SySTemS

FIGURE 9.22 The HJT (HIT) cell structure.

however, the SmartWire contacting technique (Section 9.5.2.2) can open possibilities of
mass production in the near future [34].
The hJT cells have higher V OC values (0.72–0.73 V), an efficiency of over 22%, and of-
−1
fer a lower temperature coefficient (−0.2 to −0.3% K ) which can be almost half that of a
standard c-Si cell [35]; this may be very important for some applications.
The potential of the hJT technology has recently been highlighted with the report that
silicon heterojunction solar cells with interdigitated back contacts have been shown to
have efficiencies of 26.7% [36].

9.4.3 Si Wafer-Based Multijunction Cells
Further improvements in efficiency may be reached by creating a tandem structure of a
high-efficient c-Si cell with a thin film cell [36]. Also, further development involving inex-
pensive, high-quality Si wafers could provide ideal templates for the overgrowth of thin,
crystalline, wider bandgap cells, boosting the efficiency potential of Si wafer-based mul-
tijunction cells. however, there are many problems in overcoming lattice mismatch at the
interfaces [11]. Amorphous upper cells would remove the need for lattice matching, but
it is also necessary to match PV-generated current densities of the c-Si bottom cell and
the amorphous top cell. A structure of the c-Si cell overlaid with a thin film cell is shown
−2
in Fig. 9.23. There is one limiting factor—current densities over 30 mA cm at STC are
necessary for a considerable efficiency increase that has not yet been reached by most of
thin film structures. With the emergence of perovskite cells, the promising combination of
perovskites/c-Si can be considered with the possibility of efficiencies reaching 35% [37].
The perovskite deposition is a relatively low-cost process and it could result in both a de-
crease of cost per watt and an increase in efficiency. however, there are questions about