ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating
We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm2 and the active area 26.4 cm2, we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as s ubstrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.