Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Since the first publication of our amphoteric interface recombination model [3] and its first presentation
at the NUMOS workshop [4], we applied it effectively for emitter and back surface field layer stack
optimization [5]. As an example, Fig. 5 shows the measured (symbols) symmetric passivation including
fits with our model (lines) of the same wafer, first with thicker intrinsic amorphous silicon (grey),
then with the emitter layer stack (blue) and finally with the back surface field layer stack (yellow and
red) (see again Fig. 1a to know the heterojunction solar cell structure). From Fig. 5 it can be seen that
the back surface field layer stack induced initially (yellow) a high density of interface defects. Improved
layer growth then leads to low interface defect densities while keeping the high field effect passivation
of the former layer (red).
Fig. 5: Intrinsic, emitter and back surface field layer stack lifetime curves. Their interpretation serves
for fast device development.
Textured crystalline silicon minimizes front surface reflection losses by geometrical light trapping, resulting
in a potential current gain of almost 15%. However, while for flat crystalline silicon wafers it is
sufficient to just remove the native oxide on top of them before amorphous silicon layer growth, textured
crystalline Si wafers have to be cleaned before. Shortly after this project started we succeeded
in cleaning our textured wafers in another IMT research group’s laboratory. Fig. 6 shows how the initially
achieved textured heterojunction solar cell results could in the meanwhile be improved, first by
adapting the back surface field layer stacks growth conditions to the surface texture and then by modifying
the surface herself, such as to reach an open-circuit voltage of 700 mV [6]. Except for the company
Sanyo, these are among the first cells fabricated in research laboratory exceeding the 700 mV
on textured wafers.
Fig. 6: Improvement of textured silicon heterojunction solar cells.
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