Recent advances in polymer/silicon heterojunction solar cells

Figure 4: Stable BackPEDOT solar cell with metal foil on the rear.
To keep any humidity away from the PEDOT:PSS layer, we have developed a novel rear-side metallization scheme based on placing a metal-foil onto the entire cell rear, which also has the potential of significantly simplifying the cell processing (see also Section 5). The metal foil itself is – in contrast to the evaporated metal – blocking any humidity. Figure 4 shows a direct comparison of the efficiency evolution of a standard BackPEDOT solar cell without metal foil (green circles) and a BackPEDOT cell with metal foil on the rear (red triangles) during the first 600 hours after processing. Evaporated silver (as applied in our standard BackPEDOT cells) shows a very high permeability against humidity, whereas the metal foil effectively blocks any humidity leading to stable cell efficiencies.
The demonstration of stable cell efficiencies can be considered an important milestone in the development of polymer/silicon heterojunction cells.
5 INDUSTRIAL-TYPE BACKPEDOT CELLS


Figure 5: Possible process sequences for an industrialtype (a) PERC and (b) BackPEDOT solar cell.
One attractive option to implement the PEDOT:PSS/c-Si junction into industrial solar cells is
sketched in Fig. 5. It is widely believed that the nextgeneration industrial silicon solar cell generation is based on the passivated emitter and rear cell (PERC) concept. A typical process sequence for an industrial-type PERC solar cell with screen-printed metal contacts is shown in
Fig. 5 (a). The rear surface is passivated with an Al2O3/SiNx stack which is locally opened by laser ablation. Finally, the rear is covered with a screen-printed Aluminum metal paste and fired. The process steps of Al2O3 deposition, SiNx deposition, laser contact opening and the Al screen-printing could be replaced in an industrial-type BackPEDOT cell by just depositing the
PEDOT:PSS onto the entire rear and attaching a solderable metal foil to it. This process sequence would reduce the number of process steps and hence has the potential of significantly reducing the process costs compared to a PERC process. An example of such a process sequence is shown in Fig. 5 (b). In our lab, we have fabricated the first batch of solar cells with screenprinted front and PEDOT:PSS/c-Si at the rear.