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Fig. 6.29 Line positions in
the spectra of Fig. 6.28
substitutional sites, jumping via vacancies. The theoretical treatment [ 42 ] is,
however, beyond this tutorial level.
57 Mn/ 57 Fe Experiments on Si Solar-Cells
6.5.3
Multi-crystalline silicon is widely used for solar cells, but contains different lattice
defects and metallic impurities such as iron atoms, which produce carrier trapping
centers, and therefore, degrade the energy conversion efficiency of solar cells. No
direct observation on the charge states of Fe atoms has been achieved in multi-crys-
talline Si solar cells during operation, i.e. under light illumination. The Fermi Level
will be shifted by injecting the excess carriers, and consequently different charge states
of interstitial and substitutional Fe atoms are expected to appear in the on-line
Mössbauer spectra of 57 Mn/ 57 Fe in mc-Si under light illumination [ 43 ]. Figure 6.30
shows the top and the back surfaces of mc-Si solar cell. Ag electrodes on an anti-
reflection Si-N layer can be seen and the 57 Mn implantation was performed through
this top surface. During a Mössbauer spectral measurement under dark condition, I-V
characteristic of the p-n junction was measured every one hour, in order to control
the defect accumulation due to 57 Mn implantation. The I-V curve, however, did not
change with increasing the implantation dose of 57 Mn.
The spectrum of 57 Mn/ 57 Fe in the p-region of the p-n junction, i.e., solar cell,
was measured at 400 K under light illumination. In Fig. 6.31 a this spectrum (red
points) was compared with that in p-type multi-crystalline (mc)-Si (Black points).
The ''black spectrum'' consists of two components, as is shown with red and green
components in Fig. 6.31 a. They are assigned to interstitial and substitutional Fe in
Si matrix with the isomer shifts of 0.8 and -0.06 mms -1 , respectively. On the
other hand, the ''red spectrum'' of the solar cell is very broad, and therefore, is
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