ITO-free inverted polymer solar cell on metal substrate with top-illumination

• Al–Au alloy as the cathode achieves an indium tin oxide (ITO)-free.
• Al–Au alloy is spontaneously formed during the annealing of the active layer.
• Al–Au alloy has low enough work function and high oxygen stability.
• ITO-free solar cell shows PCE of 2.3% on metal substrate using blade coating.

High-performance polymer solar cells are conventionally fabricated on the rigid and costly indium-tin-oxide (ITO) glass substrates, which are incompatible with the roll-to-roll process with low cost. This study shows that polymer solar cells can be made on ITO-free flexible stainless steel with only a moderate compromise in power conversion efficiency. A blend of poly (3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) was used as the active material for the inverted structure. The bottom electrode was an alloy of Al and Au spontaneously formed from a bi-layer during thermal annealing of the active layer. This alloy had a low work function of 3.9 eV, and a high resistance to oxide formation throughout the entire process. Semi-transparent Ag was used as the top electrode. These solar cells on stainless steel had an efficiency of 2.6%. The majority of the loss came from the top electrode transmission at approximately 60%. This all-metal approach makes roll-to-roll polymer cell fabrication possible.

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