Figure 13.16 (b) shows a typical band diagram of such a micromorph solar cell,
which is also called a tandem solar cell. On the left-hand side the electronic band diagram
of the amorphous silicon top cell is shown; on the right-hand side the electronic band
diagram of the nanocrystalline silicon bottom cell is shown. The blue and green shortwavelength light is absorbed in the top cell, where electron-hole pairs are generated.
Similarly, the red and infrared long-wavelength light is absorbed in the bottom cell, where
electron-hole pairs are also generated. Let us take a closer look at the two electron-hole
pairs excited in the top and bottom cells, respectively. The hole generated in the
amorphous top cell moves to the top p layer and the electron excited in the bottom cell
drifts to the bottom n layer. Both can be collected at the front and back contacts. However,
the electron excited in the top cell drifts to the top n layer and the hole generated in the
bottom cell drifts to the p layer. Just as for the III-V multi-junction devices, the electrons
and holes have to recombine at a tunnel recombination junction between the n layer of the
top cell and the p layer of the bottom cell. Often a very thin and defect-rich layer is used
for this purpose. Just as for the III-V multi-junction solar cells, the total current density is
equal to that of the junction with the lowest current density. Therefore, for an optimized
multi-junction cell all current densities in the various subcells have to be matched in order
to achieve the best spectral utilization.
Figure 13.17 shows the J-V curves of a single junction a-Si:H solar cell and of a
single junction nc-Si:H solar cell. Let us assume that the V oc of the high bandgap a-Si:H
top cell has an open circuit voltage of 0.9 V and a relatively low short circuit density of 15
mAcm
−2
, whereas the low bandgap material of nc-Si:H has a lower open circuit voltage of
0.5 V and a higher short circuit current density of 25 mAcm
−2
. If we make a tandem of
both junctions, the resulting current density of the double junction is lower than the
currents in both bottom cells. Because the open circuit voltage is approximately
proportional to ln(J ph /J 0 ), the open circuit voltages of the junctions in a tandem cell will be
slightly lower than for similar single junction cells. The total current utilization of the
tandem cell is determined by the bottom cell, i.e. 25 mAcm
−2 . Given the examples of the
single junctions here, the best current density matching of both cells would deliver 12.5
mAcm
−2
. The current record tandem cell has an efficiency of 12.3% and was manufactured
by the Japanese company Kaneka [47].
which is also called a tandem solar cell. On the left-hand side the electronic band diagram
of the amorphous silicon top cell is shown; on the right-hand side the electronic band
diagram of the nanocrystalline silicon bottom cell is shown. The blue and green shortwavelength light is absorbed in the top cell, where electron-hole pairs are generated.
Similarly, the red and infrared long-wavelength light is absorbed in the bottom cell, where
electron-hole pairs are also generated. Let us take a closer look at the two electron-hole
pairs excited in the top and bottom cells, respectively. The hole generated in the
amorphous top cell moves to the top p layer and the electron excited in the bottom cell
drifts to the bottom n layer. Both can be collected at the front and back contacts. However,
the electron excited in the top cell drifts to the top n layer and the hole generated in the
bottom cell drifts to the p layer. Just as for the III-V multi-junction devices, the electrons
and holes have to recombine at a tunnel recombination junction between the n layer of the
top cell and the p layer of the bottom cell. Often a very thin and defect-rich layer is used
for this purpose. Just as for the III-V multi-junction solar cells, the total current density is
equal to that of the junction with the lowest current density. Therefore, for an optimized
multi-junction cell all current densities in the various subcells have to be matched in order
to achieve the best spectral utilization.
Figure 13.17 shows the J-V curves of a single junction a-Si:H solar cell and of a
single junction nc-Si:H solar cell. Let us assume that the V oc of the high bandgap a-Si:H
top cell has an open circuit voltage of 0.9 V and a relatively low short circuit density of 15
mAcm
−2
, whereas the low bandgap material of nc-Si:H has a lower open circuit voltage of
0.5 V and a higher short circuit current density of 25 mAcm
−2
. If we make a tandem of
both junctions, the resulting current density of the double junction is lower than the
currents in both bottom cells. Because the open circuit voltage is approximately
proportional to ln(J ph /J 0 ), the open circuit voltages of the junctions in a tandem cell will be
slightly lower than for similar single junction cells. The total current utilization of the
tandem cell is determined by the bottom cell, i.e. 25 mAcm
−2 . Given the examples of the
single junctions here, the best current density matching of both cells would deliver 12.5
mAcm
−2
. The current record tandem cell has an efficiency of 12.3% and was manufactured
by the Japanese company Kaneka [47].